The use of kidney-ureter-bladder radiography (KUB) ( eFig. 24.1 ) and intravenous urography ( Fig. 24.1 ) for the routine imaging of the kidneys has been predominantly superseded by US and CT, which permit more detailed evaluation of the kidneys and surrounding structures.

Intravenous urography: excretory phase.

This image was obtained 10 minutes after the injection of the contrast material. The kidneys are well visualized; contrast material outlines the calyces, pelvis, ureters, and bladder.

Plain radiograph of the abdomen: kidneys, ureters, and bladder.

The kidneys lie posteriorly in the retroperitoneum in the upper abdomen. They are surrounded by fat. The ribs overlie the kidney, and bowel gas is visible in the right upper quadrant. The psoas muscles are also well visible because retroperitoneal fat abuts them.

US is a frequently used diagnostic examination for the evaluation of the kidneys and urinary tract. It is noninvasive, uses no ionizing radiation, and requires minimal patient preparation. It is the first-line examination in azotemic patients to assess renal size and the presence or absence of hydronephrosis and obstruction. It is used to assess the vasculature of native and transplanted kidneys. US is also used to evaluate renal structure and to characterize renal masses. It is the primary modality of imaging for evaluation of a transplanted kidney. It is also the most commonly used modality for imaging guidance for a kidney biopsy. The use of point-of-care ultrasound (POCUS) is highlighted in Chapter 26 .

In diagnostic US, high-frequency sound waves are used to evaluate various organs. In the abdomen, 2.5- to 4.0-mHz sound waves are generally employed, and these sound waves are sent and received by a transducer. A piezoelectric crystal within the transducer converts electrical energy into high-frequency sound waves that are transmitted through the patient’s body. The transducer then converts the returning, reflected waves back into electrical energy that is processed by the ultrasound unit and displayed as an image. The speed of the sound waves traveling through the body depends on the properties of the tissues being imaged.

Different tissues and the interface between these tissues have different acoustic impedance. As the sound wave travels through different tissues, part of the wave is reflected back to the transducer. The depth of the tissue interface is determined by the time the sound wave requires to return to the transducer. A grayscale image is produced by the measured reflected sound, in which the intensity of the pixels (picture elements) is proportional to the intensities of the reflected sound ( Fig. 24.2 ). Certain interfaces produce strong echoes and are known as specular reflectors (e.g., the renal capsule and bladder wall). Nonspecular reflectors generate echoes of lower amplitude, such as in the renal parenchyma. Strong reflection of sound by bone and air results in little or no information from the tissues beneath; this appearance is known as shadowing. Lack of acoustic impedance as observed in fluid-filled structures, such as the urinary bladder lumen and renal cysts, allows the sound waves to penetrate further, which results in a relative increase in intensity deep to the structures; this is known as increased through transmission. All of these features are used to help characterize various lesions. Real-time US, which provides sequential images at a rapid frame rate, allows the demonstration of motion of organs.

Renal ultrasonography: normal kidney.

The central echogenic structure represents the vascular elements, calyces, and renal sinus fat. The peripheral cortex is noted to be smooth and regular. Renal pyramids may be depicted as hypoechoic structures between the central echo complex and the cortex.

Doppler US, based on the Doppler frequency shift of the sound wave caused by moving objects, can be used to assess venous and arterial blood flow. ^, The movement of blood cells in blood vessels is used to generate Doppler information, and this is used to derive diagnostic information. Spectral Doppler is a technique that presents blood flow measurements graphically, displaying flow velocities over time. In the kidney, spectral Doppler is usually performed using a technique called pulsed wave Doppler, which allows evaluation of flow at a particular location. Graphically depicted flow waveforms produced with pulsed wave Doppler can be evaluated both qualitatively and quantitatively to assess for vascular abnormalities ( eFig. 24.2A ). One commonly used quantitative parameter is the resistive index, which is a measure of the resistance to blood flow caused by the distal microvascular bed. This serves as a nonspecific indicator of disease in both native and transplanted kidneys. In general, a normal resistive index is between 0.50 and 0.70 ( eFig. 24.2B ). An increased resistive index is a nonspecific indicator of disease and a sign of increased peripheral vascular resistance. On the other hand, an abnormally low resistive index is usually due to flow limitation upstream from the measurement, as in renal artery stenosis (RAS).

Spectral Doppler ultrasound of the normal kidney.

(A) Normal waveform. (B) Calculation of the resistive index (RI). V1, Peak systolic velocity (S); V2 , end-diastolic velocity (D). RI = (S-D)/S.

With Doppler color-flow US, the image is encoded with colors assigned to the pixels representing the direction and volume of flow within vessels ( Fig. 24.3 ). In power Doppler US, the amplitude of the signal, without any directional information, is used to produce a color map of the intrarenal vasculature and flow within the kidneys ( eFig. 24.3 ). Continued progress in Doppler technology has led to the development of microvascular imaging, an advanced Doppler technique that uses special wall filters to increase sensitivity for flow in small vessels ( Fig. 24.4 ). ^,

Doppler color-flow ultrasonography: normal kidney.

The red echogenic areas represent arterial flow (flow toward the transducer), and blue echogenic areas represent venous flow (flow away from the transducer).

Microvascular imaging, an advanced Doppler technique with superior ability to demonstrate small vessels throughout the kidney.

Contrast-enhanced ultrasound demonstrating a large renal cell carcinoma (papillary subtype) in the upper pole of the kidney as a hypoenhancing mass, compared with avidly enhancing cortex.

Power Doppler ultrasonography: normal kidney.

The color image represents a summation of all flow—arterial and venous—within the kidney.

Elastography is a technique by which the mechanical properties of a target tissue are assessed. It is able to measure the stiffness of a particular tissue, and its role in the evaluation of chronic parenchymal disease is being evaluated. The change in the elasticity of a particular tissue is assessed by changes in the propagation velocity of ultrasound waves through that tissue. Elastography is being studied for use in both chronic diseases of native kidneys and in the evaluation of transplant kidneys, and the most common technique being used currently is shear wave elastography.

There has been an increase in the use of intravenous contrast agents with US for the evaluation of the kidneys and renal masses, a technique called contrast-enhanced US. The contrast agents are microbubbles of a high-molecular-weight gas such as perfluorocarbons that are stabilized by a thin capsule of lipid or protein. The microbubbles are about half the size of a red blood cell and thus remain intravascular, acting as a blood pool agent. The advantage of these agents is that they are excreted by pulmonary ventilation, and therefore they can be used in patients with poor renal function. These contrast agents have expanded the role of US and have given an additional option in the imaging of patients with compromised renal function. The development of these US-specific contrast agents enables excellent display of microvasculature and dynamic enhancement patterns.

Uses for contrast-enhanced US in the kidney include the following: 1. characterization of focal renal lesions: accurate Bosniak classification of complex renal cysts, increase in diagnostic confidence in the evaluation of solid renal masses, and differentiation of variant normal anatomy (e.g., hypertrophied column of Bertin); ^, 2. evaluation of vascular complications in the transplanted kidney such as vascular thrombosis and parenchymal ischemia; and 3. follow-up of renal trauma. CEUS can also identify and characterize postintervention complications.

US is the most common imaging technique in guiding a renal biopsy. Its advantages include the lack of radiation and mobility, thus allowing bedside procedures. US guidance is also used for other interventional procedures such as percutaneous nephrostomy placement and renal mass ablations.

Ultrasonography: Normal Anatomy

US images of the kidneys are generally obtained in the longitudinal, transverse, and parasagittal planes. The appearance of the perinephric fat varies from slightly less echogenic to highly echogenic in comparison with the renal cortex. The renal capsule is visible as an echogenic line surrounding the kidney. The centrally located renal sinus and hilum, containing renal sinus fat, vessels, and the collecting system, are usually echogenic because of the presence of fat (see Fig. 24.2 ). The amount of renal sinus fat generally increases with age. Tubular structures corresponding to vessels and the collecting system may be visible in the renal hilum. Doppler color-flow US may be used to differentiate the vessels from the collecting system.

The normal renal cortex is less echogenic (i.e., it appears darker) than the liver and spleen. The renal medullary pyramids are hypoechoic, and their triangular shape points to the renal hilum. The renal cortex is peripheral, and its separation from the medulla is usually demarcated by an echogenic focus attributable to the arcuate arteries along the corticomedullary junction. Columns of Bertin have the same echogenicity as the renal cortex and separate the renal pyramids. On occasion, a large column of Bertin may simulate a mass. Even when a column of Bertin is large or prominent, its echogenicity is the same as the remainder of the cortex, the vascular pattern observed on power Doppler images is the same, and it will be isoenhancing to cortex on CEUS.

Renal size is accurately measured by US. Normal kidneys range from 9 to 13 cm, depending on the age, sex, and body habitus of the patient. The contours of the kidneys should be smooth; occasionally some mild, regular nodularity is present as a result of persistent developmental fetal lobulation. The renal arteries and veins may be visible extending from the renal hilum to the aorta and inferior vena cava (IVC). The veins lie anterior to the arteries. The renal arterial branching pattern within the kidneys may be visible on Doppler color-flow US (see Fig. 24.3 ). The resistive indices of the main, intralobar, and arcuate vessels may be calculated (see eFig. 24.2B ). With power Doppler US, the intrarenal vasculature may be assessed, demonstrating greater flow to the cortex than to the medulla, reflecting the normal flow to the kidney (see eFig. 24.3 ). ^, The renal calyces are not typically visible with US unless distension caused by obstruction or reflux is present. When visible, the collecting systems are anechoic structures in the renal sinus fat, connecting together at the renal pelvis. The urinary bladder is visible in the pelvis as a fluid-filled anechoic structure. The entrances of the ureters into the bladder at the trigone may be visualized on Doppler color-flow US as ureteral jets ( eFig. 24.4 ).

Ureteral jet.

Color-flow image of urine entering the bladder.

When a kidney is not identified in its normal location, the remainder of the abdomen and pelvis should be assessed. Ectopic kidneys may lie lower in the abdomen or within the pelvis and may also be located on the opposite side; the kidneys may even be fused (e.g., horseshoe kidneys). Horseshoe kidneys tend to lie lower in the retroperitoneum, and their axes may be different from those of normal kidneys.

CT allows comprehensive anatomic evaluation of the kidneys and urinary tract and is often complementary to other imaging modalities. CT is the imaging modality of choice for the evaluation of renal colic, renal stone disease, renal trauma, renal infection and abscess, renal mass, hematuria, and urothelial abnormalities. Hardware and software developments have enabled improved CT performance including radiation dose reduction, faster study acquisition over longer distances, better image quality, optimized image display (e.g., three-dimensional printing and augmented reality), and application of newer techniques (e.g., dual-energy spectral CT).

CT is the computer reconstruction of a radiographically generated image that depicts a slice through the area being studied in the body. The x-ray tube produces a highly collimated fan beam and is mounted opposite an array of electronic detectors. This system rotates in tandem around the patient. The detector system collects hundreds of thousands of samples representing the attenuation of the x-ray along the line formed from the x-ray source to the detector as the rotation occurs. This data set is transferred to a computer, where the image is reconstructed. The CT image is made up of pixels (picture elements), each corresponding to a CT density number (Hounsfield unit [HU]) that represents the number of x-rays absorbed by the patient at a particular point in the cross-sectional image. These pixels represent a two-dimensional display of a three-dimensional object or volume element (voxel). The third dimension is the slice thickness or depth. Thus the HU is the average attenuation of x-rays of all the tissues within a specific voxel, which is then used to create the individual image. The images are then displayed on a computer monitor for reviewing and analysis.

The HU of water is 0. Tissues that attenuate more x-rays than water have positive HU, and those with less x-ray attenuation than water have negative HU. Different shades of gray on a scale of white to black are assigned to HU (the highest number is depicted as white, the lowest as black). The image of each slice is thus created on a display monitor, and this image may be manipulated on viewing monitors to accentuate the regions being imaged. The advantage of this digital image set is that by using various tools, such as window levels and widths and different summation and reconstruction techniques, images can be optimized to evaluate a particular organ or region.

The initial CT scanners were relatively slow because the technology required a point-and-shoot process. This initial generation of body CT scanner led to a scan of the abdomen that took up to 2 to 4 minutes or more to complete. In 1990, helical/spiral technology was introduced in which the x-ray tube and detector system continuously rotated around the patient, and the patient moved continuously through the gantry. Scan time through the abdomen was significantly reduced. After helical/spiral CT, a two-detector system was introduced that produced two slices for every 360-degree rotation of the x-ray tube and detector system. Multidetector CT systems with 640 detectors are in use primarily for advanced applications, such as CT angiography for the coronary arteries. Currently, most CT systems varying from 64 to 320 detectors are used in abdominal and pelvic scanning. With multidetector CT, each 360-degree rotation results in the number of slices equal to the number of detectors (i.e., a 64-detector system produces 64 slices in one 360-degree rotation). These technologic advances have led to a dramatic increase in the speed of scans (4–10 seconds), a routine use of thin slices or collimation (1–2 mm thick), and a marked improvement in spatial resolution (ability to display small objects clearly).

As a result of the faster scanning times, the use of enhancement by intravenous contrast material has improved and become more widely used. For example, the kidneys can be scanned in the arterial, corticomedullary, nephrographic, and delayed phases, which allows for a more complete assessment. Current, state-of-the-art multidetector CT acquires data as a volumetric study, and the slice thickness has been reduced to the point that sagittal, coronal, oblique, and off-axis images may be displayed with no loss of resolution. The data acquisition may also be displayed as a three-dimensional volumetric display with the regions of interest highlighted. The kidneys are well suited for assessment with multidetector CT.

More recent technical developments include dual-energy and spectral-energy scanners that offer the ability to image an organ at different energy strengths. Tissues behave differently at different energies, and this fact is used in techniques such as characterization of calculi and obtaining virtual unenhanced images and iodine maps in different organs ( eFig. 24.5 ).

Dual-energy computed tomography (CT) for stone characterization in two different patients.

(A) Postprocessed color-coded axial CT image in a 54-year-old patient obtained after dual-energy CT scanning shows a left renal calculus coded blue, indicating a nonuric acid stone. (B) Postprocessed color-coded coronal CT image in a 66-year-old patient obtained after dual-energy CT scanning shows a right distal ureteric calculus color-coded red, indicating a uric acid stone. Technique: An initial routine stone protocol multidetector CT scan is performed for detection and localization of the calculi along the renal pelvicalyceal system. Subsequently, a focused dual-source CT scan is performed in the region of the stone. Dual-energy CT is performed on the dual-source CT scanner (Somatom Definition) with the following technique: 80 kV/350–380 mAs and 140 kV/80-98 mAs, 14 × 1.2 mm/64 × 0.6 mm. The postprocessing is performed using a three-material decomposition algorithm on the dual-energy software on the scanner console (Syngo.via).

Image courtesy Avinash Kambadakone, M.D., Massachusetts General Hospital, Boston, MA.

Most clinical questions require the use of intravenous contrast, and a tailored CT urography protocol is a way to provide a comprehensive examination of the urinary tract. CT urography may be used to assess the kidney as a whole (anatomic), the vascular tree (function and perfusion), and the excretory (urothelial) patterns. Noncontrast scans enable assessment of renal calculi, high-density cysts, and contour abnormalities. Early-phase scans (12–15 seconds) enable arterial assessment (e.g., evaluation for RAS). Scanning at 25 to 30 seconds (corticomedullary phase) results in clear corticomedullary differentiation, recommended for renal mass evaluation. At 90 to 100 seconds, true nephrographic-phase imaging of the kidneys is obtained. Delayed imaging, typically at 5 to 10 minutes, enables the evaluation of the urothelium (calyces, renal pelvis, ureters, and bladder) in the excretory phase. Axial images, multiplanar reconstructions, maximum-intensity projection images, and three-dimensional volumetric displays complement each other in CT urography. CT urography is superior to intravenous urography. Not all the phases are required for all clinical situations; therefore the examination should be tailored to a specific clinical question. Urothelial lesions can often be evaluated by combining the nephrographic and delayed phases into a single phase by using a split–contrast bolus technique. This involves giving the iodinated contrast intravenously in two separate boluses but scanning only once.

Another technical innovation is the development of spectral CT or multienergy CT. This involves the ability to scan a particular tissue with two or more different energies. Knowing how a target tissue attenuates at different energies can provide further details of the tissue composition. For example, spectral CT can help in differentiating types of renal calculi and characterizing renal masses. It also provides the ability to generate virtual unenhanced data sets, as well as increasing detection rates of iodine-containing tissues.

Work on reducing radiation exposure is also being aggressively pursued. Because of the development of new reconstruction algorithms, such as iterative reconstruction and model-based reconstruction, there has been a significant (50%–70%) reduction in the resultant radiation dose without any change in the quality of the study. Further advances in technology will result in incremental dose reduction.

Computed Tomography: Normal Anatomy

The retroperitoneal anatomy is easily viewed with CT ( eFigs. 24.6-24.8 and Figs. 24.6–24.8 ). The kidneys lie in the retroperitoneum, surrounded by Gerota fascia in the perinephric space. Perinephric fat surrounds the kidneys with the liver anterosuperior on the right, the spleen superior on the left, and the spine, aorta, and IVC central to each kidney. The abdominal intraperitoneal contents lie anteriorly. Renal arteries are easily seen on both arterial and venous phases, generally located posterior to the venous structures. The adrenal glands are found in a location superior to the upper poles of the kidneys. In corticomedullary phase imaging, it is easy to distinguish the renal cortex from the medulla. Cortical thickness and medullary appearance may be readily assessed. The nephrographic phase should demonstrate symmetric enhancement of each kidney. At 7 to 10 minutes in the excretory phase, the calyces should be well depicted with sharp fornices, a cupped central section, and a narrow, smooth infundibulum leading to the renal pelvis. Coronal images in slab maximum-intensity projection three-dimensional volumetric reformations help in displaying anatomic details. The excretory phase images delineate the ureters from the renal pelvis to the bladder. A curved reformatted series of images or three-dimensional display is needed to display the ureters in their entirety. Proper tailoring of the examination to the diagnostic problem provides guidance for the correct imaging acquisition. ^{, ,}

Computed tomographic scan: normal corticomedullary phase.

Axial slice (A) and coronal image (B) demonstrate the dense enhancement of the cortex in relation to the medulla containing the renal pyramids.

Renal computed tomographic angiogram: normal findings.

The aorta and exiting renal arteries on the right and left are visible in this volume-rendered reconstruction. The kidneys are visible peripherally with the branching renal arteries.

Computed tomographic scan: normal excretory phase.

The calyces and renal pelvis are now easily noted because they are opacified by the excreted contrast material. This scan is obtained 5 to 10 minutes after the injection of contrast material.

Noncontrast computed tomographic scan through the midportion of normal kidneys.

The kidneys lie in the retroperitoneum with the lumbar spine and psoas muscles more centrally. The liver is anterolateral to the right kidney, and the spleen anterolateral to the left kidney.

Computed tomographic scan: normal nephrographic phase.

The axial image ( A) and the coronal image (B) demonstrate the homogeneous appearance of the kidneys, with the cortex and medulla no longer differentially enhanced. These images are typically obtained 80 to 120 seconds after the injection of contrast material.

Computed tomographic urogram: normal findings.

The maximum-intensity projection (MIP) image (A) and the volume-rendered image (B) demonstrate the calyces, renal pelvis, ureters, and bladder. The MIP image is a slab, 15-mm thick, in the coronal plane. The volume-rendered image was taken as the extraneous tissues adjacent to the kidneys were removed, and it highlights the genitourinary tract.

In modern clinical practice, iodinated contrast media for intravascular use are nonionic and either low-osmolar or iso-osmolar. Contrast media have a plasma half-life of 1 to 2 hours in patients with normal renal function. Virtually all the administered contrast media are excreted by the kidneys within 24 hours. In patients with renal impairment, contrast media may be excreted via alternate routes including the biliary system or gastrointestinal tract.

Allergic-like adverse reactions to any intravascular contrast media may occur. The exact mechanism of these reactions is unclear. It is often associated with the activation of mast cells and basophils, as well as subsequent release of histamine and other mediators. Immunoglobulin E–mediated allergic anaphylaxis causing contrast media reaction is rare but could explain the ineffectiveness of premedication in some patients.

The incidence of adverse reactions to modern iodinated contrast is low (overall 0.48%–0.82%; severe 0.01%). Mild acute allergic-like reactions (limited urticaria, cutaneous edema, limited itchy throat, nasal congestion, and sneezing) and physiologic reactions (limited nausea and vomiting, transient flushing, headache, and vasovagal reactions) are typically self-limiting and require symptomatic or no treatment. Moderate-to-severe allergic-like reactions (extensive urticaria, bronchospasm, laryngeal edema, hypotension, hypertensive crisis, pulmonary edema, seizures, and hypoglycemia) can be potentially fatal and require treatment.

In general, intradermal skin testing has not been shown to be useful in predicting the likelihood of allergic-like reactions owing to their idiosyncratic nature. Premedication does not prevent all adverse reactions (breakthrough rate 2%–19%). Most breakthrough reactions are similar in severity to the index reaction. ^, Nevertheless, many experts believe that premedication can mitigate the likelihood of an allergic-like reaction in high-risk patients.

The efficacy of current premedication regimens for allergic-like contrast reactions in high-risk patients is unclear. ^, One study showed no difference in the rate of repeat reaction in patients who were and were not premedicated with steroids and who received the same contrast agent that caused the index reaction. Changing the contrast agent within the same class in high-risk patients was associated with a reduced risk of recurrent reaction. One study found using an alternative contrast agent within the same class with or without steroid premedication was more effective than steroid premedication and using the same contrast agent.

Although second-generation antihistamines show no significant sedative side effects, there are no current studies directly comparing the efficacy of diphenhydramine with a second-generation antihistamine in premedication regimens for acute contrast reactions.

Like CT, MRI is a computer-based, multiplanar imaging modality. Instead of ionizing radiation, however, electromagnetic radiation is used in MRI. MRI is an alternative to contrast-enhanced CT, especially in patients with allergy to iodinated contrast material and in patients for whom reduction of radiation exposure is desired, such as pregnant women and children. MRI routinely allows detailed tissue characterization of the kidney and surrounding structures. The properties of physics underlying MRI are complex and are addressed only briefly.

Clinical MRI is based on the interaction of hydrogen ions (protons) and radiofrequency waves in the presence of a strong magnetic field. The strong magnetic field, called the external magnetic field, is generated by a large-bore, high-field strength magnet. Most magnets in clinical use are superconducting magnets. The magnet strength is measured in teslas (T) and can range from 0.2 to 3 T for clinical imaging and up to 15 T for animal research. Renal imaging is performed best on high-field magnets (1.5–3 T) that allow for higher spatial resolution and faster imaging.

Images of the patient are obtained through a multistep process of energy transfer and signal transmission. When a patient is placed in the magnet, the mobile protons associated with fat and water molecules align longitudinal to the external magnetic field. No signal is obtained unless a resonant radiofrequency pulse is applied to the patient. The radiofrequency pulse causes the mobile protons within the patient to move from a lower, stable energy state to a higher, unstable energy state (excitation). When the radiofrequency pulse is removed, the protons return to the lower-energy steady state while emitting frequency transmissions or signals (relaxation). In radiologic terms, an external radiofrequency pulse “excites” the protons, causing them to “flip” to a higher energy state. When the radiofrequency pulse is removed, the protons “relax” with emission of a “radio signal.” The signals produced during proton relaxation are separated from one another with applied magnetic field gradients. The emitted signals are captured by a receiving coil and reconstructed into images through a complex computerized algorithm: the Fourier transform.

Different tissues have different relaxation rates that lead to different levels of signal production or signal intensity. The signal intensity of each tissue is determined by three characteristics:

• 1.

  Proton density of the tissue. The greater the number of mobile protons, the greater the signal produced by the tissue. For example, a volume of urine has more mobile protons than does the same volume of renal tissue; therefore urine produces more signal than do the kidneys. Stones have far fewer mobile protons per unit volume and therefore produce little signal.

• 2.

  T1 relaxation time. The T1 time is how quickly a proton returns to the preexcitation energy state. The shortest T1 times (rapid relaxation) produce the strongest signal.

• 3.

  T2 relaxation time. The T2 time is how quickly the proton signal decays as a result of nonuniformity of the magnetic field. A nonuniform field accelerates signal decay and leads to signal loss.

In MRI, multiple pulse sequences are obtained. A pulse sequence is a set of defined radiofrequency pulses and timing parameters used to obtain image data. These sequences include, but are not limited to, spin echo, gradient echo, inversion recovery, and steady-state free precession. The data are obtained in volumes (voxels), reconstructed as two-dimensional pixels, and displayed in relation to variations in tissue signal intensity (tissue contrast). Tissue contrast, like signal intensity, is determined by proton density and relaxation times. T1 weighting is related to the rate of T1 relaxation and the time allowed for relaxation, also known as the pulse repetition time (TR). T2 weighting is related to the rate of T2 relaxation and the time at which the “radio signal” is sampled by the receiver coil, also known as the echo time (TE). TR and TE are programmable parameters that can be altered to accentuate T1 and T2 weighting with contrast media. For the general observer, T1-weighted sequences have short TR and TE and show simple fluid as black. T2-weighted sequences have long TR and TE and show simple fluid as white ( Fig. 24.9 ).

Normal signal characteristics of simple fluids on magnetic resonance imaging.

Urine appears dark on T1-weighted sequences (A) and bright on T2-weighted sequences (B).

Many programmable parameters other than TR and TE are used to optimize imaging. These include, but are not limited to, choice of pulse sequence, coil types and gradients, slice orientation and thickness, field of view and matrix, gating to reduce motion, and use of intravenous contrast material. Although many pulse sequences are used in clinical MRI, ultrafast sequences are preferred for renal imaging. These fast sequences can be obtained in <30 seconds while the patients hold their breath. The benefits of rapid acquisition include improvement in image quality, as a result of reduction of motion artifact, reduction of total scan time, and the ability to perform dynamic imaging.

MRI is not possible for patients who have certain implanted medical devices, such as most pacemakers, ferromagnetic aneurysm clips, and ferromagnetic stapedial implants. Not all implants or devices cause problems, but knowledge of the type of device is crucial for determining whether the patient can safely enter the magnet. Regularly updated information regarding patient safety and the compatibility of a medical device in the MRI environment may be found at Dr. Shellock’s MRIsafety.com .

Diagnostic Magnetic Resonance Imaging Technique

Routine MRI evaluation of the kidneys includes axial and coronal T1-weighted and T2-weighted sequences. Dynamic contrast media–enhanced T1-weighted sequences with fat suppression are also routinely obtained. Due to excellent tissue differentiation provided by MRI, the renal cortex and medullary pyramids are easily differentiated on sequences that are not enhanced by contrast media. On T1-weighted sequences, the renal cortex has higher signal intensity than do the medullary pyramids. On T2-weighted sequences, the renal cortex has lower signal intensity than do the medullary pyramids ( eFig. 24.9 ). With kidney injury, this corticomedullary differentiation disappears. ^, Urine, like water, normally appears black on T1-weighted sequences and white on T2-weighted sequences ( Figs. 24.9 and 24.10 ).

Normal appearance of corticomedullary differentiation on magnetic resonance imaging.

Coronal (A) and axial (B) T2-weighted images demonstrate decreased signal intensity of the renal cortex in relation to the medullary pyramids. Axial T1-weighted image (C) demonstrates increased signal intensity of the renal cortex in relation to the medullary pyramids.

Paramagnetic effects of gadolinium on urine.

(A) Coronal T1-weighted image from a magnetic resonance urogram (MRU) demonstrates enhancement of the urine in the collection system. (B) Coronal T2-weighted image from an MRU demonstrates low signal intensity of urine in the collecting system secondary to effects of gadolinium. (C) Axial T1-weighted, delayed image after contrast medium enhancement demonstrates layering of contrast material. The denser, more concentrated gadolinium is dark (arrow). The less concentrated gadolinium is brighter and layers above (arrowhead).

Contrast-enhanced MRI allows for dynamic evaluation of the kidneys and surrounding structures. Serial acquisitions are obtained after bolus injection of gadolinium contrast (0.1–0.2 mmol per kilogram of body weight) at 2 mL/sec. ^, The injection should be administered by means of an automatic, MRI-compatible power injector to ensure accuracy of the timed bolus including volume and rate of injection. ^, The corticomedullary-arterial phase (approximately 20 seconds after injection) is best for evaluating the arterial structures and corticomedullary differentiation. In the nephrographic phase (70–90 seconds after injection), tumor detection is maximized and the renal veins and surrounding structures are best demonstrated ( eFig. 24.10 ). Imaging can be performed in any plane, but the coronal plane is used most frequently for dynamic imaging because it allows imaging of the kidneys, ureters, vessels, and surrounding structures in the fewest number of images. The characteristics of parenchymal enhancement are similar to those observed on contrast-enhanced CT.

Magnetic resonance appearance of a normal kidney after bolus injection of gadolinium contrast material at 20 seconds (A), 50 seconds (B), and 80 seconds (C) after the start of the injection.

Blood vessels can be variable in signal intensity on routine MRI that is not enhanced by contrast media, ranging from white to black. This is due to many factors including, but not limited to, flow-related parameters, location and orientation of the imaged vessel, and choice of pulse sequence. By taking advantage of some of these factors, diagnostic angiography and venography may be performed without the use of intravenous contrast; these sequences are sometimes called “bright-blood” sequences. Although contrast-enhanced MR angiography remains the preferred method of vascular imaging, high-quality MR angiography that is not enhanced by contrast media has regained popularity because of the advancement in MR hardware and imaging sequences, as well as the risk of nephrogenic systemic fibrosis in patients with poor renal function. MR angiography not enhanced by contrast media is particularly attractive for evaluating the renal arteries in patients with severe renal dysfunction or those with a relative contraindication for contrast-enhanced MR angiography. The most robust sequences are based on inversion recovery and balanced steady-state free precession techniques. ^, Older, less robust techniques include time-of-flight MR angiography, which is based on flow-related enhancement, and phase-contrast MR angiography, which is based on velocity and direction of flow. Phase-contrast MR angiography can be used in conjunction with contrast-enhanced MR angiography to detect turbulent flow and high velocities associated with stenoses. Unlike MR angiography that is not enhanced by contrast media, contrast-enhanced MR angiography minimizes flow-related enhancement and motion. The success of contrast-enhanced MR angiography depends on the T1-shortening properties of gadolinium, which allow for faster imaging, increased coverage, and improved resolution. ^, Accurate timing of the bolus injection is critical in contrast-enhanced MR angiography. The time at which the bolus arrives at the renal arteries may be determined with a bolus injection of 1 mL of gadolinium-based contrast agent, followed by a saline flush. A three-dimensional T1-weighted gradient-echo MRI pulse sequence is then obtained in the coronal plane during the injection of approximately 15 to 20 mL of gadolinium-based contrast agent at 2 mL/sec, timed to capture the arterial phase. ^, Sequential three-dimensional sequences are obtained to capture the venous phase (magnetic resonance venography). The data sets can be postprocessed into multiple formats, improving ease and accuracy of interpretation ( Fig. 24.11 ).

Magnetic resonance angiogram reconstructed with three-dimensional software.

(A) Visualization of a small accessory right renal artery (arrow) is excellent. (B) The accessory artery is depicted in a way to make more accurate luminal measurements.

MR urography consists of protocols tailored to the evaluation of the renal collecting system and the disease found there. MR urography can be performed with heavily T2-weighted sequences, in which urine provides the intrinsic contrast, or with contrast media–enhanced T1-weighted sequences, which mimic CT urography. Heavily T2-weighted sequences are most useful in patients with dilated collecting systems, in whom all water-filled structures are bright ( eFig. 24.11 ), and in patients with impaired renal excretion, in whom contrast media–enhanced urography is most limited. Unfortunately, without adequate distension of the collecting system, T2-weighted evaluation is limited. Although a good morphologic examination, T2-weighted urography is ultimately limited by a lack of functional information. For example, T2-weighted urography cannot reliably differentiate between an obstructed system and an ectatic collecting system ( eFig. 24.12 ) . Contrast media–enhanced T1-weighted urography in the excretory phase is superior to T2-weighted urography because both structure and function can be evaluated.

Duplicated collecting system.

(A and B) Contrast material–enhanced magnetic resonance urograms demonstrate a duplicated collecting system on the right with delayed excretion of the upper pole moiety. (C) Obstruction of the upper pole moiety is confirmed on intravenous urogram.

Bilateral hydronephrosis secondary to bladder tumor.

(A and B) Heavily T2-weighted magnetic resonance urograms (MRUs) demonstrate bilateral hydronephrosis and hydroureter caused by bladder mass (arrow). (C) Contrast medium–enhanced MRU in the nephrographic phase demonstrates asymmetric enhancement of the kidneys. (D) MRU in the excretory phase demonstrates asymmetric excretion of gadolinium. There is no excretion on the right as demonstrated by unenhanced (dark) urine within the collecting system.

(A and B) Coronal T2-weighted images demonstrate right renal atrophy and dilation of the right collecting system in a patient who had undergone bladder resection and ilioconduit reconstruction (arrow).

On these static images, it is difficult to differentiate between an obstructed system and a nonobstructed system. The patient had pelvocaliectasis without obstruction, demonstrated on the contrast medium–enhanced portion of the examination.

T2-weighted and contrast media–enhanced T1-weighted sequences are complementary and are frequently obtained together as part of a complete MR urography examination. In patients with nondilated systems, both techniques require hydration and furosemide for adequate distension of the renal collecting system. ^, Typical MR urography starts with a coronal, heavily T2-weighted sequence in which simple fluid (urine, cerebrospinal fluid, ascites) is bright and all other tissues are dark (see eFig. 24.11 ). This rapid breath-hold sequence takes <5 seconds to obtain and is presented as a urogram-like image. The T2-weighted sequence is used as an initial survey of fluid within the collecting system. Low-dose furosemide (0.1 mg per kilogram of body weight; maximum dose, 10 mg) may be administered intravenously in selected cases, 30 to 60 seconds before the intravenous administration of gadolinium-based contrast agent (0.1 mmol/kg). ^, Furosemide is given to increase urine volume and dilute the gadolinium-based contrast agent within the collecting system. ^, Coronal, three-dimensional, contrast media–enhanced T1-weighted sequences are obtained with the same technique as in renal contrast-enhanced MR angiography, in the corticomedullary-arterial phase, nephrographic phase, and excretory phase (see eFig. 24.11 ). Additional sequences may be obtained in any plane to better evaluate suspected pathologic conditions.

By combining renal MRI and MR urography, the clinician can obtain a comprehensive morphologic and functional evaluation of the urinary tract. MR urography helps to accurately evaluate the upper urinary tract and is useful in the evaluation of anatomic anomalies including duplications, ureteropelvic obstruction, anomalous crossing vessels, and ureteroceles ( eFig. 24.12 ). ^, Obstructive disease is well evaluated regardless of whether the cause is intrinsic or extrinsic to the collecting system.

Scintigraphy offers imaging-based diagnostic information on renal structure and function. Many single-photon radiotracers have long been in routine clinical use in renal scintigraphy; they are tailored to provide physiologic information complementing primarily anatomic and structural-based imaging modalities, such as US, CT, and MRI. With the rapid expansion of PET and hybrid structural-functional imaging systems, such as PET-CT and PET-MRI, additional unprecedented opportunities have developed for quantitative imaging evaluation of renal diseases in clinical medicine and research. Scintigraphy including PET makes a unique contribution to the imaging evaluation of renal structure and function. Common radiopharmaceuticals used in renal scintigraphy are described first.

GFR and effective RPF may be assessed by means of dynamic quantitative nuclear imaging techniques. The GFR quantifies the amount of filtrate formed per minute (normal, 125 mL/min in adults). Only 20% of RPF is filtered through the semipermeable membrane of the glomerulus. The filtrate is protein free and almost completely reabsorbed in the tubules. Filtration is maintained over a range of arterial pressures with autoregulation. The ideal agent for the determination of GFR is inulin, which is only filtered but is neither secreted nor reabsorbed. ^,

In these studies, ^99m Tc-DTPA is often used to demonstrate renal perfusion and assess glomerular filtration, although 5% to 10% of injected ^99m Tc-DTPA is protein bound and 5% remains in the kidneys after 4 hours. A typical imaging protocol includes posterior 5-second flow images for 1 minute, followed by 1-minute-per-frame images for 20 minutes. The GFR may be obtained through the Gates method, in which images of renal uptake are obtained during the second and third minutes after ^99m Tc-DTPA administration. Regions of interest are drawn over the kidneys, and background activity correction is applied. A standard dose is counted by the gamma camera for normalization. Depth photon attenuation is corrected according to a formula relating body weight and height. A split GFR can be obtained for each kidney, which is not possible with the creatinine clearance method. ^, This is useful in assessing relative function of each kidney if nephrectomy is planned, for example.

The effective RPF (normal, 585 mL/min in adults) can be obtained with ¹³¹ I-ortho-iodohippurate and ^99m Tc-MAG3 imaging. However, ¹³¹ I-ortho-iodohippurate has been largely replaced by ⁹⁹ Tc-MAG3 because MAG3 has better imaging characteristics and dosimetry (when radiolabeled with ^99m Tc). Currently ⁹⁹ Tc-MAG3 is the renal imaging agent of choice primarily because of the combined renal clearance of ⁹⁹ Tc-MAG3 by both filtration and tubular extraction, which enables clinicians to obtain relatively high-quality images even in patients with impaired renal function. The imaging protocol includes posterior 1-second images for 60 seconds (flow study), followed by 1-minute images for 5 minutes and then 5-minute images for 30 minutes. The relative tubular function may be obtained by drawing renal regions of interest, corrected for background activity. ^, A renogram is constructed to depict the renal tracer uptake over time. The first portion of the renogram has a sharp upward slope occurring approximately 6 seconds after peak aortic activity (phase I); the upward slope represents perfusion. This is followed by extension to the peak value, which represents both renal perfusion and early renal clearance (phase II), which can be dependent on body position. The next phase (phase III) is depicted by a downward slope, which represents excretion. Normal perfusion of the kidneys is symmetric (50% ± 5%). The peak of the renogram occurs at approximately 2 to 3 minutes (vs. 3–5 minutes with DTPA) in normal adults, and by 30 minutes, more than 70% of the tracer is cleared and present in the urinary bladder ( Fig. 25.13 ). ^, Assessment of renal function is further discussed in Chapter 23 .

Normal-appearing renogram with technetium 99m-labeled mercaptoacetyltriglycine (MAG3).

(A) Planar posterior images. Left: Static image with whole kidney regions of interest (ROI; white outlines ) and background ROIs (circular dashed lines) inferiorly. Right upper series: Sequential flow images at 5-second intervals show prompt symmetric uptake of tracer in both kidneys. Right lower series: Sequential functional images at 1-minute intervals show normal excretion of tracer with visualization of the ureters and urinary bladder. (B) Time-activity curves for each kidney, corrected for background, collected from the ROIs in A, showing the excretory phase. Less than 30% of peak activity is retained in each kidney after 20 minutes, which is normal. At 20 minutes, intravenous furosemide (Lasix) was given to demonstrate normal washout. CTS/SEC, Radioactivity counts per second.

Renal cortical structure can be imaged with ^99m Tc-DMSA; the appearance of these images is strongly correlated with differential GFR and differential renal blood flow. Imaging is started 90 to 120 minutes after administration of the tracer and can be obtained up to 4 hours later. Planar images are obtained in the anterior, posterior, left anterior oblique/right anterior oblique, and right posterior oblique/left posterior oblique projections. SPECT is also often performed. In a scan with normal results, renal cortical uptake is evenly distributed. Normal variations include dromedary hump (splenic impression on the left kidney), fetal lobulation, horseshoe kidney, crossed fused ectopy, and hypertrophied column of Bertin. The renal images also allow accurate assessment of the relative renal size, position, and axis. ^,

When a patient presents with previously undiagnosed renal failure, the question is whether the kidney disease is acute or chronic. US is the most helpful initial imaging study for evaluating the patient with an elevated creatinine level of unknown duration. US can help separate chronic, end-stage kidney failure (ESKF) from potentially reversible AKI or CKD by defining renal size, echogenicity, and presence or absence of hydronephrosis and cystic disease. This is easily achieved using grayscale US. A thin rim of decreased echogenicity may surround the kidneys in patients experiencing acute kidney injury, reflecting perirenal edema. Small, echogenic kidneys indicate preexisting CKD; however, acute reversible components must still be searched for. Acute, reversible components that can be diagnosed on imaging are few but include hydronephrosis and hypertension caused by RAS. If no acute process is found on US, no further imaging workup is necessary (according to the ACR appropriateness criteria for renal failure: kidney disease of unknown duration). Normal renal size, with or without increased echogenicity, typically requires more extensive evaluation for acute causes because grayscale US may not be accurate in the minimally dilated obstructive situation.

Many causes of AKI are encountered in the hospital setting. Prerenal and renal causes include hypotension or dehydration resulting in hypoperfusion of the kidneys and nephrotoxic drugs and account for more than 90% of all cases. Typically, prerenal and renal causes are diagnosed clinically, not with imaging. Although postrenal causes for AKI are less common, when they are identified and treated, the AKI is often rapidly reversible.

US is more than 95% accurate in detecting hydronephrosis (i.e., dilation of the collecting systems and renal pelvis) ^, ; however, the cause of the hydronephrosis may not be seen. If US cannot determine the cause of obstruction, noncontrast CT or MRI is the appropriate next imaging study. The typical US findings of hydronephrosis are a dilated, anechoic, fluid-filled renal pelvis and calyces. Hydronephrosis is generally graded according to the extent of calyceal dilation and the degree of cortical thinning. ^, In mild (grade I) hydronephrosis, the pelvicalyceal system is filled with fluid, which causes slight separation of the central renal sinus fat ( eFig. 24.13 ). The calyces are not distorted, and the thickness of the renal cortex appears normal. In moderate (grade II) hydronephrosis, the pelvicalyceal system appears more distended with greater separation of the central echo complex. The contour of the calyces is rounded, but the cortical thickness is unaltered ( Fig. 24.14 ). With moderate-to-severe (grade III) hydronephrosis, the calyces are more distended and cortical thinning is recognized. In severe (grade IV) hydronephrosis, the calyceal system is markedly dilated ( eFig. 24.14 ). The calyces appear as large, ballooned, fluid-filled structures with a dilated renal pelvis of variable size. Cortical loss is evident, with the dilated calyces approaching or reaching the renal capsule. In general, the length and overall size of a hydronephrotic kidney is increased. Long-standing obstruction may, however, result in renal parenchymal atrophy, and the kidney may be somewhat small, with marked cortical thinning. The degree of hydronephrosis is not always correlated with the amount of obstruction.

Moderate (grade II) hydronephrosis: ultrasonography.

Longitudinal image (A) and transverse image (B). The dilated calyces are rounded and filled with urine. The renal pelvis is dilated as well. Again, note the connection between the calyces and renal pelvis. The cortex remains relatively normal in thickness, and the renal border is smooth.

Mild (grade I) hydronephrosis: ultrasonography.

The central echo complex is separated by the mildly distended calyces and renal pelvis. Notice the connection between the calyces and the renal pelvis. The thickness of the cortex is preserved, and the renal border remains smooth.

Severe (grade IV) hydronephrosis: ultrasonography.

Longitudinal image of the right kidney demonstrates a large fluid-filled sac; no normal elements of the kidney remain visible. The cortex is almost gone, but the outer border of the kidney remains smooth.

Although hydronephrosis is usually easily diagnosed with US, it must not be confused with renal cystic disease. In hydronephrosis, the dilated calyces have visibly direct communication with the renal pelvis, which is also dilated. In cystic disease, the round fluid-filled cysts have walls and no direct communication is evident between each calyx and the renal pelvis. Peripelvic cysts are frequently misdiagnosed as dilated components of the renal collecting system. Renal artery aneurysm may also be confused with a dilated renal pelvis but can be diagnosed correctly with added Doppler color-flow US.

The presence of hydronephrosis on US does not always indicate obstruction. ^, Grade I hydronephrosis and possibly more severe grades may be observed in patients in whom no obstructive cause is found. Nonobstructive causes of hydronephrosis include increased urine production and flow, acute and chronic infection, vesicoureteral reflux, papillary necrosis, congenital megacalyces, overdistended bladder, and postobstructive dilation. In patients with repeated episodes of intermittent or partial obstruction, the calyces become quite distensible or compliant, which causes the appearance of hydronephrosis to vary, depending on the state of hydration and urine production. Patients with vesicoureteral reflux also demonstrate distensible pelvicalyceal systems.

US is also used in patients with CKD. Cortical echogenicity may be increased in both acute and chronic renal parenchymal disease ( Fig. 24.15 ). The pattern should be bilateral in CKD, and the degree of cortical echogenicity is correlated with the severity of the interstitial fibrosis, global sclerosis, focal tubular atrophy, and number of hyaline casts per glomerulus. Similar correlation is observed with decreasing renal size. These findings, however, are nonspecific, and kidney biopsy may be required for diagnosis. The normal corticomedullary differentiation is lost with increasing cortical echogenicity. Cortical echogenicity may also be increased in some patients with AKI, such as in glomerulonephritis and lupus nephritis. Sequential studies over time may be used to assess the progression of disease by monitoring the renal size and cortical echogenicity.

Adulthood-acquired polycystic kidney disease.

Computed tomographic scan, axial image without contrast material (A) and axial image after administration of contrast material (B). The kidneys are small bilaterally with multiple 1-cm cysts primarily in the cortex.

The key to the diagnosis of renal parenchymal disease is renal cortical biopsy and resulting histopathologic study. US facilitates the performance of kidney biopsy by demonstrating the kidney and proper location for biopsy. US may also be used to evaluate for complications associated with biopsy, such as perirenal hematoma and arteriovenous fistula.

When US demonstrates hydronephrosis, but not the cause, it is usually followed by CT. Noncontrast CT will demonstrate the dilated pelvicalyceal systems in the kidney. The parenchymal thickness can be visualized in relation to the dilated collecting systems; the urine-filled calyces and pelvis are less dense than the surrounding parenchyma. The course of the dilated ureters may be followed distally to the bladder and prostate to establish the site of obstruction. The cause of obstruction is frequently visible and may include pelvic tumors, distal ureteral stones, and retroperitoneal adenopathy or mass. For patients in whom chronic long-standing obstruction is the cause of kidney injury, CT generally demonstrates large, fluid-containing kidneys with little or no cortex remaining.

In advanced CKD without obstruction, CT typically demonstrates small, contracted kidneys, which may also show evidence of adulthood-acquired polycystic disease if the patient is on dialysis (see Fig. 24.15 ). In general, the overall size and thickness of the renal parenchyma appear to decrease with age. Other causes for CKD may be demonstrated on imaging, including autosomal dominant polycystic kidney disease (ADPKD) ( Fig. 24.16 ); the kidneys are enlarged and contain innumerable cysts. Frequently, some of the cyst walls may contain thin rims of calcification. The density of the internal contents of the cysts may also vary as a result of hemorrhage or proteinaceous debris. For patients undergoing regular dialysis, iodinated contrast may be given, if necessary, for CT scans.

Autosomal dominant polycystic kidney disease: computed tomographic (CT) scan without contrast material.

This CT image demonstrates the markedly enlarged kidney bilaterally with multiple low-density cysts throughout both kidneys. The little remaining renal parenchyma is noted by the sparse, higher-density material squeezed by the cysts.

Like CT, MRI is accurate in demonstrating renal structure, as well as prerenal and postrenal causes of kidney injury. MRI is sensitive for the detection of renal parenchymal disease, but the renal parenchymal causes of injury have nonspecific features and biopsy is generally required. Noncontrast MRI routinely allows for detailed tissue characterization of the kidney and surrounding structures. Both iodinated contrast and gadolinium-based contrast agents should be avoided if possible in patients with AKI and CKD stage 4 and 5. Newer MRI sequences, such as diffusion-weighted and bright-blood techniques, provide a way to increase the detection of neoplastic and vascular causes of renal failure without the use of intravenous contrast agents. ^,

In acute kidney injury, glomerular and tubular dysfunctions are reflected by abnormal findings on renal scintigraphy and renography. Renal uptake of ^99m Tc-MAG3 is prolonged, with tubular tracer stasis and little or no excretion. In patients with AKI, if ^99m Tc-MAG3 has more renal activity than hepatic activity 1 to 3 minutes after injection, recovery is likely, whereas when renal uptake is less than the hepatic uptake, dialysis may be needed. In CKD, renal perfusion, cortical tracer extraction, and tracer excretion are diminished. However, this imaging pattern is nonspecific and must be interpreted in the clinical context.

If US cannot determine the cause of obstruction, CT is the next imaging modality of choice due to rapid speed of acquisition and accuracy. Contrast-enhanced CT and, more specifically, CT urography are most useful in assessing the patient with unilateral obstruction. Small differences in the enhancement pattern of the kidneys are well demonstrated with contrast-enhanced CT ( eFig. 24.16 ). Differences in excretion patterns by kidneys are also sensitively depicted on contrast-enhanced CT. ^, The urine-filled or contrast material-filled ureters point to the obstruction with demonstration of both intraureteral and extraureteral causes of the obstruction ( Fig. 24.17 ). MRI demonstrates similar findings and may be used when contrast-enhanced CT is contraindicated.

Unilateral obstruction: contrast material-enhanced computed tomographic scan.

The coronal image demonstrates the difference in enhancement between the two kidneys, with the moderately dilated renal pelvis and calyces on the right. The large heterogeneous pelvis mass is the source of the obstruction: recurrent rectal carcinoma.

Nuclear medicine assessment by means of diuretic renography may also be used to evaluate for obstructive uropathy. Scintigraphy with ^99m Tc-MAG3 is often employed. Furosemide (Lasix) is administered intravenously (1 mg/kg; higher dose in cases of renal insufficiency) when the renal pelvis and ureter are maximally distended. Regions of interest are drawn around each renal pelvis, with the background regions as crescent shapes lateral to each kidney. After furosemide administration, in cases of dilation without obstruction, the collecting system empties rapidly, with a subsequent steep decline in the renogram curve. Obstruction can be ruled out if the clearance half-time of the renal pelvic emptying is <10 minutes. A curve that reaches a plateau or continues to rise after administration of furosemide is indicative of obstruction, with a clearance half-time of more than 20 minutes ( Fig. 24.18 ). A slow downward slope after furosemide administration may be indicative of partial obstruction. An apparent poor response to furosemide may also occur in patients with severe pelvic dilation (reservoir effect). Other pitfalls include poor injection technique of either the diuretic or the radiotracer, impaired renal function, and dehydration, in which delayed tracer transit and excretion may not be overcome by the effect of a diuretic. Kidneys in neonates (<1 month of age) may be too immature to respond to furosemide, and neonates are thus not suitable candidates for diuretic renal scintigraphy.

Abnormal findings on renogram with technetium 99m-labeled mercaptoacetyltriglycine, demonstrating obstructive urinary kinetics with a poor response to furosemide.

(A) Static and timed images in the same format as in Fig. 24.13 . (B) Individual time-activity curves for each kidney. Intravenous furosemide (Lasix) was given at 15 minutes. CTS/SEC , Radioactivity counts per second.

End-stage kidney failure: ultrasonography.

(A and B) The kidneys are highly echogenic in relation to the adjacent liver. No normal renal structures are visible, but the kidneys remain smooth in overall contour. Note the two small hypoechoic renal cysts in the surface in A.

Unilateral hydronephrosis: contrast material–enhanced computed tomographic scan.

Axial (A) and coronal (B) nephrographic-phase images of an obstructed left kidney. The right kidney is in the nephrographic phase, whereas the left (obstructed) kidney is still in the corticomedullary phase; this is apparent with differential enhancement. In the excretory phase image (C), the right kidney has contrast material within the collecting system and the renal pelvis. The left kidney has no contrast material in the pelvicalyceal system and contains only nonopacified urine. The patient had lymphoma with retroperitoneal lymph nodes, which caused the obstruction more distally.

Various protocols in relation to the timing of furosemide administration have also been reported. In the F0 method, furosemide is injected simultaneously with ^99m Tc-MAG3 administration. A 17-year clinical experience at one institution proved that this protocol is useful for patients of all ages and for all indications. Taghavi and colleagues compared diuresis renographic protocols with injection of furosemide 15 minutes before (F−15) and 20 minutes after (F+20) administration of ^99m Tc-MAG3. In this comparative study of 21 patients with dilation of the pelvicalyceal system, the F−15 protocol produced fewer equivocal results than did the F+20 method and therefore was considered the preferable protocol. Further experience is needed to determine the optimal timing interval between furosemide and ^99m Tc-MAG3 injections in diuresis renography.

Calcifications may occur in many regions of the kidney. Nephrolithiasis or renal calculi are the most common and occur in the pelvicalyceal system. Nephrocalcinosis refers to diffuse or punctate renal parenchymal calcification occurring in either the medulla or cortex, usually bilaterally. Some patients with nephrocalcinosis may also develop nephrolithiasis. Calcifications also occur in vascular structures, particularly in patients with diabetes and advanced atherosclerotic disease. Rimlike calcifications may occur in simple renal cysts and polycystic disease. Patients with renal carcinomas may exhibit variable calcifications as well. All types of calcification are best demonstrated on noncontrast CT.

Cortical calcification is most often associated with cortical necrosis from any cause. The calcifications are dystrophic and tend to resemble tram tracks and to be circumferential. Other entities in which cortical calcification is found include hyperoxaluria, Alport syndrome, and, in rare cases, chronic glomerulonephritis. The stippled calcifications of hyperoxaluria may be found in both the cortex and the medulla, as well as in other organs, such as the heart. In Alport syndrome, only cortical calcifications are found.

Calcifications in the medulla are much more common than cortical calcifications. The most common cause of medullary nephrocalcinosis is primary hyperparathyroidism. The distribution appears to be within the renal pyramid and may be either focal or diffuse and either unilateral or bilateral. Nephrocalcinosis occurs in other diseases in which hypercalcemia or hypercalciuria occurs, such as hyperthyroidism, sarcoidosis, hypervitaminosis D, immobilization, multiple myeloma, and metastatic neoplasms. These calcifications are nonspecific and punctate in appearance and are usually medullary in location.

In 70% to 75% of cases of renal tubular acidosis, there is evidence of nephrocalcinosis. The calcifications tend to be uniform and distributed throughout the renal pyramids bilaterally. With medullary sponge kidney and renal tubular ectasia, small calculi form in the distal collecting tubules, probably because of stasis. The appearance varies from involvement of only a single calyx to involvement of both kidneys throughout. Calcifications are small, round, and within the peak of the pyramid adjacent to the calyx. Medullary sponge kidney is also associated with nephrolithiasis because the small calculi in the distal collecting tubules may pass into the collecting systems and ureters, resulting in renal colic.

Calcifications that occur in renal tuberculosis are typically medullary in location and may mimic other forms of nephrocalcinosis. Calcification occurs in the pyramids as part of the healing process. With overwhelming involvement of the kidney, the entire kidney may be destroyed; this results in diffuse, heavy calcification throughout the entire kidney, which becomes small and scarred. Medullary calcifications are also visible in patients with renal papillary necrosis. With necrosis of the papilla, the material is sloughed into the calyces. Retained tissue fragments may calcify and have the appearance of medullary nephrocalcinosis.

Nephrolithiasis is a common clinical entity. In the United States, the prevalence of kidney stones is 10.9% in men and 9.5% in women. Most urinary tract stones are composed of calcium salts of either oxalate or phosphate or a combination of the two. This composition accounts for the dense appearance on imaging. Stasis contributes to the formation of stones in the urinary tract. Renal colic or flank pain is the most common presenting symptom. Most patients also have hematuria, although it may be absent if a ureter is completely obstructed by the stone. The pain that occurs with a passing renal stone is probably caused by the distension of the tubular system and renal capsule of the kidney and by the peristalsis associated with ureteral contractions as the stone moves distally.

Most urinary calculi that are 4 mm or smaller pass with conservative treatment. The larger the stone, the more likely other measures will be necessary to treat the stone and associated obstruction.

Noncontrast CT of the abdomen and pelvis is the imaging modality of choice for the evaluation of patients with suspected renal stone disease. Two meta-analyses evaluating the diagnostic performance of low-dose CT for detecting uroliths found pooled sensitivity of 93% to 97% and pooled specificity of 95% to 97%. ^, Low-dose CT and ultra-low-dose CT have been shown to be similar in efficacy as standard-dose CT for the detection of renal stones but may not be as effective in detecting stones <3 mm in size or in obese patients. The radiation dose from low-dose CT is approximately 20% to 25% of the standard CT, and the radiation dose from ultra-low-dose CT is about one-half of low-dose CT. Noncontrast CT allows evaluation for complications of urolithiasis and procedurally relevant anatomy, but it also provides an alternative diagnosis other than urolithiasis for flank pain in 7% to 14% of patients. CT may also be used for stone disease follow-up. The KUB is useful for following stone disease only when a stone is densely calcified and large enough to be visible ( eFig. 24.17 ).

Renal stone: plain radiograph of the kidneys, ureters, and bladder.

A large laminated stone is visible in the renal pelvis of the right kidney. The outline of the normal left kidney can be seen with no calcifications overlying it. The right kidney outline cannot be seen.

Noncontrast CT is performed from the top of the kidneys to below the pubic symphysis. No patient preparation is needed. Intravenous contrast material is rarely needed. The studies are performed with 3-mm collimation or less, and the slices are reconstructed to be contiguous or slightly overlapping. Virtually all renal stones are denser than the adjacent soft tissues ( eFig. 24.18 ); exceptions are renal stones associated with indinavir (a first-generation HIV-protease inhibitor) and tiny uric acid stones (<1–2 mm in diameter). As expected, calcium oxalate and calcium phosphate stones are the most dense. Matrix stones, which are rare, may also be relatively low in density, but they usually contain calcium impurities that make them visible. Dual-energy imaging with CT has demonstrated the ability to distinguish different types of stones (see eFig. 24.5 ). ^,

Renal stones: noncontrast computed tomographic scan.

Axial image (A) and coronal image (B) demonstrate 4- to 5-mm stones in the upper and lower poles of the left kidney. There are no signs of obstruction.

Calculi may be visible in all parts of the collecting system and the urinary tract. Small punctate calcifications (≈1 mm) are occasionally observed just at the tip of the renal pyramid. These may represent the calcification noted in Randall plaques. Obstruction occurs most commonly at the ureteropelvic junction, at the pelvic brim, where the ureters cross over the iliac vessels, and at the ureterovesical junction. The diagnosis is made on the noncontrast CT by demonstrating the calcified stone within the urine-filled ureters ( eFig. 24.19 ). Secondary signs may be present to assist in the diagnosis. Hydronephrosis and hydroureter to the point of the stone may be visible. Asymmetric perinephric and periureteral stranding may also be related to forniceal rupture and urine leak ( eFig. 24.20 ). The involved kidney may be less dense than the normal kidney because of increased interstitial fluid and edema. ^, The affected kidney may also be larger than the normal kidney. At the point of obstruction, the stone may be visible within the ureter, with soft tissue thickening of the ureteral wall at that level. This thickening is probably caused by edema and inflammation associated with the passage of the stone.

Ureteral stone: noncontrast computed tomographic scan.

(A) A 5- to 6-mm stone is noted in the midportion of the right ureter. (B) Axial image of the midportion of the kidneys reveals the urine-filled right renal pelvis and a right kidney that is slightly less dense than the left. These are signs of obstruction.

Ureteral stone: noncontrast computed tomographic scan.

Axial images of the kidneys show perinephric and peripelvic stranding and fluid on the right (A) caused by forniceal rupture and leakage of urine as a result of the distal obstructing stone at the right ureterovesical junction (B). Note the phlebolith on the right posterior to the bladder and lateral to the seminal vesicle; phleboliths are commonly confused with distal ureteral stones.

Noncontrast CT has the additional advantage of assessing the overall stone burden of the patient, not just the passing stone. Also, the size may be accurately measured, which enables clinicians to make treatment decisions. Distal ureteral stones are occasionally confused with phleboliths, which are common in the pelvis (see eFig. 24.20 ). Images reconstructed in the coronal plane along the course of the ureters down to the level of the stone may be helpful. Also, close inspection of phleboliths frequently reveals a small, soft tissue tag leading to the calcification: the “comet tail” sign. CT urography is occasionally necessary in confusing or difficult cases, as well as in complicated cases in which the patient is febrile and pyelonephritis or pyohydronephrosis is suspected.

US is commonly used as a screening examination in pregnant patients and children presenting with flank pain and suspicion of stone disease because US is sensitive for diagnosing hydronephrosis and does not expose the patient or fetus to ionizing radiation. ^, It is a fast, cost-effective, and dynamic modality. The sensitivity and specificity of US for the detection of uroliths are 54% to 57% and 73% to 91%, respectively, when compared with CT. ^, A study in a pediatric cohort found US showed a sensitivity of 67% and specificity of 97% for the detection of renal stones when compared with CT. Unilateral hydronephrosis may be observed, although the examination may be normal early in the passage of a renal stone. Renal stones may be visualized within the kidney as hyperechoic foci with distal acoustic shadowing or reverberation artifacts ( eFig. 24.21 ). ^, Ureteral stones are rarely seen because of overlying bowel gas. Distal ureteral stones near the ureterovesical junction may be visualized through the urine-filled bladder transabdominally. US may demonstrate an absent ureteral jet in the bladder on the side in which a stone is being passed.

Renal stone: ultrasonography.

Longitudinal image (A) and Doppler color-flow image (B) demonstrate an echogenic focus at the corticomedullary junction. Not all stones show shadowing, but in this case, reverberation artifact (REVERB) is visible on the Doppler color-flow image, which helps establish the diagnosis

In the evaluation of acute stone disease, MRI or MR urography is not the examination of first choice, but it is a suitable alternative for selected patients, especially those in whom reduction of radiation exposure is desired (pediatric and pregnant patients). ^, Stones are difficult to identify in nondilated systems, even in retrospect. When stones are observed on MRI, they are visible as black foci on both T1- and T2-weighted sequences. Stones become more conspicuous in a dilated collecting system ( Fig. 24.19 ); however, a nonenhanced filling defect is a nonspecific finding. Blood, air, or debris may have the same appearance. If stones or other calcifications are a concern, noncontrast CT is the examination of choice for improved conspicuity.

Renal stones.

Calcification (arrowhead) well viewed on computed tomography (A) is difficult to demonstrate on magnetic resonance imaging (B) (arrow), even in retrospect. (C) A stone (arrowhead) is more conspicuous when it is located within a mildly dilated collection system.

When the use of iodinated contrast material is contraindicated, or when reduction of radiation exposure is desired, MR urography can be used to determine the cause and location of an obstructing process ( Fig. 24.20 ). MR urography is highly accurate in demonstrating obstruction, regardless of whether the process is acute or chronic. Acute obstruction may be associated with perinephric fluid, which is well demonstrated on T2-weighted sequences. However, perinephric fluid is a nonspecific finding and can be found in association with other renal disease. MRI is useful in evaluating the patient who has recently undergone surgery for renal stone disease. MRI has been reported as being more accurate than CT in differentiating perirenal and intrarenal hematomas ( eFigs. 24.22 and 24.23 ). Contrast-enhanced MRI can also demonstrate damage to the collecting system and areas of ischemia without the risk of nephrotoxicity. Stone disease is discussed further in Chapter 40 .

Magnetic resonance urographic reconstructions demonstrating a nonoccluding distal ureteral stone (arrow).

(A to C) Three-dimensional postrenal processing techniques are used to mimic intravenous urography. (D) Postcontrast axial imaging demonstrates a stone within the lumen of the distal ureters.

Subcapsular hematoma after lithotripsy.

Coronal T2-weighted sequence (A) demonstrates high-signal intensity blood contained by left renal capsule (arrowheads). Axial T1-weighted image (B) and gadolinium-enhanced T1-weighted image (C) show mass effect on left kidney (arrowheads) caused by a subcapsular hematoma. The signal intensity is consistent with the presence of intracellular methemoglobin.

Hematoma status after surgical removal of staghorn calculus.

T2-weighted axial image (A), T1-weighted axial image (B), and postcontrast T1-weighted axial image (C) show an intrarenal hematoma (arrows) at the site of incision plane. This extends into the renal pelvis. No urine extravasation was demonstrated.

Acute pyelonephritis is typically a diagnosis made clinically. Most cases of acute pyelonephritis occur by the ascending route from the bladder and are caused by gram-negative bacteria. Vesicoureteral reflux may contribute, although the ascent of the bacteria up the ureter also occurs in its absence. This is due to the presence of the adhesin P fimbriae and powerful endotoxins that appear to inhibit ureteral peristalsis creating a functional obstruction. The bacteria are transported to the renal pelvis, where intrarenal reflux occurs and the bacteria traverse the calyceal system to the ducts and tubules within the renal pyramid. Enzyme release results in destruction of tubular cells with subsequent bacterial invasion of the interstitium. As the infection progresses, it spreads throughout the pyramid and to the adjacent parenchyma. The inflammatory response leads to focal or more diffuse swelling of the kidney. Without adequate treatment, necrosis of the involved regions and microabscess formation occur. These microabscesses may coalesce into larger macroabscesses, which tend to be surrounded by a rim of granulation tissue. Perinephric abscess results from the rupture of an intrarenal abscess through the renal capsule or the leak from an infected and obstructed kidney (pyonephrosis). The overall distribution in the kidney is usually patchy or lobar, but sometimes it is diffuse. Subsequent scarring of the kidney after treatment reflects the magnitude of the infection and tissue destruction that occurred.

Pyelonephritis may also occur by hematogenous spread of bacteria to the cortex of the kidney and eventual involvement of the medulla. The pattern of involvement is usually round lesions, peripheral, and frequently multiple. Blood-borne infection is less common than ascending infection and is usually observed in intravenous drug abusers, immunocompromised patients, or patients with a source of infection outside the kidney, such as heart valves or teeth.

Imaging is rarely used or needed in uncomplicated pyelonephritis, and most patients respond to therapy within 72 hours. Imaging should be reserved for patients who are not responding to conventional antibiotic treatment, patients with an unclear diagnosis, patients with coexisting stone disease and possible obstruction, patients with diabetes and poor antibiotic response, and patients who are immunocompromised. Imaging is used to assess complications of acute pyelonephritis including renal and perinephric abscess, emphysematous pyelonephritis, and xanthogranulomatous pyelonephritis. All of these entities are imaged best with cross-sectional imaging techniques, specifically CT.

US results are normal in the majority of patients with acute pyelonephritis. When the examination results are abnormal, the findings are often nonspecific. US is performed to look for a cause for acute pyelonephritis, such as obstruction or renal calculi in adults or renal anomalies and hydronephrosis in children, and to search for complications. Altered parenchymal echogenicity is the most frequent finding with loss of the normal corticomedullary differentiation. The echogenicity is usually decreased or heterogeneous in the affected area ( eFig. 24.24 ). There may be focal or generalized swelling of the kidney. Power Doppler imaging may improve sensitivity in demonstrating focal hypoperfusion, but this is nonspecific. Tissue harmonic US imaging may be more sensitive in demonstrating focal or segmental, patchy, hypoechoic areas extending from the medulla to the renal capsule.

Acute pyelonephritis: renal ultrasonography.

The hypoechoic region in the upper pole represents an area affected by acute pyelonephritis. The surrounding parenchyma is somewhat distorted, with loss of the normal corticomedullary junction.

Contrast-enhanced CT is the most sensitive and specific imaging study in patients with acute pyelonephritis. ^, The nephrographic phase of CT is best for imaging in patients with acute pyelonephritis ( Fig. 24.21 ). Wedge-shaped areas of decreased density extending from the renal pyramid to the cortex are most characteristic. The nephrogram may be streaky or striated in either a focal or global manner ( eFig. 24.25 ). There may be focal or diffuse swelling of the kidney. The areas of involvement may appear almost masslike (see Fig. 24.21 ). The changes in the nephrogram are related to decreased concentration of contrast media in the tubules with focal ischemia. Tubular destruction and obstruction with debris are also present. There is usually a sharp demarcation between diseased tissue and normal parenchyma, which continues to enhance normally in the nephrographic phase. Soft tissue stranding and thickening of Gerota fascia are caused by the adjacent inflammatory process (see eFig. 24.25 ). The walls of the renal pelvis and proximal ureter may be thickened. The calyces and renal pelvis may be effaced. Mild dilation is also occasionally noted. With hematogenous-related pyelonephritis, the early findings tend to be multiple, round cortical regions of hypodensity that become more confluent and involve the medulla with time. These findings may persist for weeks despite successful treatment with antibiotics.

Acute pyelonephritis.

Contrast material–enhanced computed tomographic scan, axial (A) and coronal (B) images. The left kidney shows multiple areas of involvement. The hypodense region in the midportion of the kidney appears almost masslike (A and B). A nephrogram is striated in the region of involvement in the upper pole (B).

Acute pyelonephritis: contrast material–enhanced computed tomographic (CT) scan.

The heterogeneous CT nephrogram shows the diffuse involvement of the right kidney. Stranding and some fluid are visible in the perinephric space (arrow) with thickening of Gerota fascia (arrowhead).

MRI is comparable with contrast-enhanced CT for the evaluation of pyelonephritis. The enhancement characteristics of acute pyelonephritis on MRI are similar to those on CT. On non contrast sequences, the affected area has increased T2 signal intensity and decreased T1 signal intensity in relation to the normal renal parenchyma.

The American College of Radiology proposes that ^99m Tc-DMSA may have a role in evaluating children with atypical or recurrent urinary tract infections but acknowledges there is disagreement among experts. The United Kingdom National Institute for Health and Care Excellence guidelines recommend ^99m Tc-DMSA imaging at 4 to 6 months following atypical urinary tract infection in children younger than 3 years old or recurrent urinary tract infection in children younger than 16 years old to detect renal parenchymal defects. Cortical imaging with ^99m Tc-DMSA has been shown to be highly sensitive for detecting acute pyelonephritis in the appropriate clinical setting. ^, In acute pyelonephritis, segmental regions of decreased tracer uptake are demonstrated in oval, round, or wedge patterns. There may also be diffuse generalized decrease in renal uptake, which, in association with a normal or slightly enlarged kidney, is suggestive of an acute infectious process. The pathophysiologic basis for decline in ^99m Tc-DMSA cortical uptake in infection is related to diminished delivery of the tracer to the infected area and to direct infectious injury to the tubular cells, which compromises their function and tracer uptake. A wedge-shaped cortical defect with regional decrease in renal size is compatible with postinfectious scarring. Renal infarcts may also have similar appearance. ^, Attention to ^99m Tc-DMSA image processing and quality is paramount to achieving high interreader agreement. ^, There may also be a role for FDG PET-CT in the imaging evaluation of renal infection.

Renal abscess results from severe pyelonephritis and occurs two to three times more frequently in patients with diabetes. Abscesses are more common with hematogenous infection than with ascending infection. Contrast-enhanced CT characteristics of renal abscess include a reasonably well-defined mass with a low-density central region and a thick, irregular wall or pseudocapsule ( eFig. 24.26 ). Enhancement adjacent to the abscess is variable, depending on the amount of inflammation. Mature abscesses may demonstrate a more sharply demarcated border with peripheral rim enhancement. Gas may be visible within the abscess. MRI is comparable with contrast-enhanced CT for the evaluation of renal abscess. The central region of the abscess can have a variable appearance, but generally it is of decreased T1 and increased T2 signal intensity. The wall enhancement characteristics are also similar to those on contrast-enhanced CT ( eFig. 24.27 ).

Renal abscess: contrast material–enhanced computed tomographic scan.

(A) Axial image demonstrates the hypodense abscess in the right kidney with extension into the perinephric space and the right flank. (B) Axial image with the patient in the decubitus position reveals the method of diagnosis: needle aspiration. A drainage catheter was subsequently placed for treatment.

Renal abscess.

A mass in the upper pole of the left kidney demonstrates intermediate to low signal intensity (arrow) on the sagittal T2-weighted image (A) and heterogeneous but predominantly peripheral enhancement (arrow) on the sagittal postcontrast T1-weighted image (B). On biopsy, this mass was found to be Aspergillus infection.

Renal parenchymal infections can extend into the perinephric space with resulting abscess formation. CT and MRI best reveal the involvement of the perinephric and paranephric spaces within the retroperitoneum. In general, inflammatory changes and heterogeneous fluid-density or signal-intensity collections may be identified. Associated gas is best identified on CT.

In patients with preexisting cystic disease, suspected infection can be best evaluated using US, CT, or MRI. The presence of enhancement and documentation of changes compared with prior imaging is crucial.

Emphysematous pyelonephritis is a severe necrotizing infection of the renal parenchyma, usually caused by gram-negative bacteria (Escherichia coli, Klebsiella pneumoniae, Proteus mirabilis). Of patients with emphysematous pyelonephritis, 90% have uncontrolled diabetes. Emphysematous pyelonephritis is characterized by severe acute pyelonephritis, urosepsis, and hypotension. The gas found in the renal parenchyma is believed to form as a result of the high levels of glucose in the tissue by fermentation with the production of CO ₂ . The gas may also be observed in the pelvicalyceal system or perinephric space (or both). If the gas is extensive enough, it may be visible on plain radiographs or KUB images. The gas is usually mottled, bubbly, or streaky in appearance and may be observed in the areas over the kidneys. US may suggest the diagnosis of emphysematous pyelonephritis by demonstrating gas within the kidney. With gas present, there is acoustic shadowing in the involved region. CT is the most specific and sensitive modality for the identification of renal gas. The gas dissects through the parenchyma in a linear focal or global manner, radiating along the pyramid to the cortex. It may extend into the perinephric space. There is generally extensive parenchymal destruction with streaks or mottled collections of gas within the kidney ( Fig. 24.22 ). Little or no fluid is seen. Emphysematous pyelitis represents gas within the pelvicalyceal system without parenchymal gas. The distinction is important because emphysematous pyelitis carries a less grave prognosis.

Emphysematous pyelonephritis: contrast material–enhanced computed tomographic scan.

A noncontrast image (A) and a contrast material–enhanced image (B) demonstrate gas in the left renal parenchyma with extension into the perinephric space. The nephrogram is striated throughout. Global involvement of the kidney is frequent.

Xanthogranulomatous pyelonephritis is an end-stage condition resulting from chronic obstruction with long-standing infection, usually with Proteus mirabilis or E. coli. The renal parenchyma is destroyed and replaced by vast amounts of lipid-laden macrophages. The kidney is usually barely functional or nonfunctional. The destruction is typically global, but it may involve only a portion of the kidney. A staghorn calculus may be seen on KUB. On US the kidney appears enlarged with loss of identifiable landmarks. A large calculus or staghorn calculus usually fills the renal pelvis, with debris filling adjacent hypoechoic regions ( Fig. 24.23 ). CT defines the extent and adjacent organ involvement best. The findings on CT include an enlarged but generally reniform mass filling the perinephric space. ^, Calcification is found in 75% of cases, excretion is absent or markedly decreased in 85% of cases, and the involved region appears as a mass in more than 85% of cases. The process is focal in <15% of cases. There is frequent perinephric extension. Fistulas may occur in adjacent structures, with adenopathy noted in the retroperitoneum. MRI may show many similar findings when compared with CT, although calcifications are less conspicuous ( eFig. 24.28 ).

Xanthogranulomatous pyelonephritis.

Contrast material–enhanced computed tomographic scan. A large staghorn calculus fills the renal pelvis and collecting systems in the left kidney. Much of the remainder of the kidney is replaced by hypodense material—the xanthogranulomatous infection—within the calyces and parenchyma; some minimal enhancement of the cortex remains.

Xanthogranulomatous pyelonephritis with staghorn calculus.

(A) Axial T2-weighted image demonstrates a stone of low signal intensity within the right renal pelvis (arrow) that is associated with increased renal size and replacement of the medullary pyramids and calyces with material of high-signal intensity. (B) Axial postcontrast T1-weighted image demonstrates asymmetric enhancement and hydronephrosis.

Malacoplakia is a rare inflammatory condition that most commonly involves the bladder but may also involve the ureter and kidney. Typically, the kidney is affected by obstruction from the lower urinary tract. When the kidney is directly involved, it is a multifocal process that may appear similar to xanthogranulomatous pyelonephritis on imaging.

Renal tuberculosis occurs by hematogenous spread. The genitourinary tract is the second most common extrapulmonary site of involvement. Evidence of previous pulmonary tuberculosis is found in <50% of patients with genitourinary tuberculosis. Only 5% may have active tuberculosis. Renal involvement is bilateral; the findings are determined by the extent of the infection, the stage of the infection, and the host’s response. Calcified granuloma may be found within the cortex or medulla, papillary necrosis may be visible ( eFig. 24.29 ), and hydrocalyx with infundibular strictures may develop ( eFig. 24.30 ). The kidney may become focally or globally scarred as the disease progresses. There may be areas of nonfunction with dystrophic calcifications. In the end stage, the kidney may be small and scarred with bizarre calcifications; this condition is the so-called autonephrectomy. ^,

Renal tuberculosis.

(A and B) T2-weighted images demonstrate asymmetric cortical thinning and focal areas of increased signal intensity in the distribution of the medullary pyramids. (C) Postcontrast T1-weighted image shows absence of enhancement, which is consistent with the presence of granulomas with caseous necrosis. (D) T2-weighted image after treatment shows distorted, dilated calyces containing debris. Right-sided hydronephrosis is present as a result of a distal ureteral stricture.

Renal tuberculosis: contrast material–enhanced computed tomographic scan.

Axial (A) and coronal (B) images show the destruction of the right kidney as a result of renal tuberculosis. Parenchymal calcifications are present with dilated calyces as a result of the attenuation and truncation of the renal pelvis and ureter.

Chronic pyelonephritis is usually associated with vesicoureteral reflux that occurs in childhood. One or both kidneys may be involved. An affected kidney has focal scars that are associated with calyceal dilation. The scarring is often separated by normal regions of the kidney and normal-appearing calyces. When involvement is global, the kidney may be small. With US, the kidneys have irregular outlines with regions of cortical loss. Underlying dilated calyces may be visible. The regions of scarring may be echogenic in comparison with the adjacent normal kidney. CT and MRI demonstrate the abnormal architecture of the affected kidney. ^, Nephrographic-phase images reveal the regions of cortical loss; the involved dilated calyces extend to the capsular surface. Dilation of the calyces is variable. Chronic pyelonephritis may be unilateral or bilateral. Excretory phase images best delineate the extent of involvement, especially in the coronal format.

In the patient with AIDS, urinary tract infections are quite common. ^, The infections are frequently hematogenous with unusual organisms such as Pneumocystis jiroveci, cytomegalovirus, and Mycobacterium avium-intracellulare . The infections may also be seen in other abdominal organs—liver, spleen, and adrenals. ^, In patients with AIDS, renal involvement may be detected in US, demonstrating increased cortical echogenicity and loss of the corticomedullary differentiation ( eFig. 24.31 ). Renal size is also increased, and it is a bilateral process. Infections of the urinary tract are discussed further in Chapter 38 .

Acquired immunodeficiency syndrome-related nephropathy: ultrasonography.

Longitudinal image of the right kidney. The size of the kidney is normal to slightly increased. The corticomedullary distinction is lost with diffuse increased cortical echogenicity.

Each imaging modality has its advantages and disadvantages in renal mass evaluation depending on ease of access, cost, and sensitivity. US is helpful in determining the cystic nature of a lesion. CT is currently the method of choice for characterizing renal masses but is unable to differentiate benign from malignant solid renal lesions with high accuracy in all cases. MRI has sensitivities and specificities similar to those of CT but is generally reserved for cases in which the patient has a contraindication to iodinated contrast medium or in which radiation dose must be limited; as a problem-solving tool to further characterize indeterminate lesions on CT; to evaluate for venous involvement; and to distinguish vessels from retroperitoneal lymph nodes. Contrast-enhanced US is emerging as another method for renal lesion characterization, especially in whom CT or MRI contrast is contraindicated, but currently this application in the United States is an off-label use of ultrasound contrast agents ( eFig. 24.32 ). ^,

A Bosniak category IV cyst in a 58-year-old woman.

(A) Grayscale ultrasonogram reveals a complex cyst with a solid nodular component (arrow). (B) Power Doppler image reveals flow within the nodular component of the lesion (arrow), confirming its vascularized nature. (C) Composite image including a contrast material–enhanced image on the left and a grayscale image on the right. The nodule (arrow) reveals dense arterial phase enhancement with heterogeneous washout. The findings were consistent with a neoplastic cyst. The lesion was subsequently resected and was found to be a clear cell carcinoma, Fuhrman grade 2.

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