Abstract
Accurate and precise diagnosis of disorders of kidney parenchyma, vasculature, and collecting system can be achieved by utilization of advanced imaging techniques that have rapidly evolved in the past few years. Fluoroscopic techniques and ultrasonography may be used as first-line modalities; functional analysis can be obtained by scintigraphy and magnetic resonance urography. Computed tomography is the modality of choice for stone disease and obstructive uropathy. Evaluation of focal and diffuse kidney parenchymal abnormalities can be done by magnetic resonance imaging, which also plays a key role in assessing the kidney vasculature and collecting system, both in native and transplant kidneys, in addition to parenchymal disorders and focal lesions.
Keywords
magnetic resonance imaging, magnetic resonance urography, computed tomography, ultrasonography, imaging, radiology, chronic kidney disease
There has been an impressive evolution and development of diagnostic imaging methods in recent years, expanding the array of techniques that can be used to understand and diagnose kidney diseases. Imaging modalities range from conventional fluoroscopic studies to nuclear medicine techniques to cross-sectional methods based on ultrasound, computed tomography (CT), or magnetic resonance imaging (MRI). Optimal patient care depends on an understanding of potential imaging applications and the benefits and risks related to these techniques. This is a task made more challenging because of continuing and rapid changes in the technology.
Imaging Modalities
Abdominal Radiography and Intravenous Urography
Historically, plain radiography and intravenous urography (IVU) were the preliminary tools for evaluation of kidney disease; however, these conventional methods provide limited sensitivity and specificity for most important urologic pathologies. Cross-sectional techniques have now superseded plain radiography and IVU to diagnose focal or diffuse kidney parenchymal pathologies and nephrolithiasis.
Ultrasonography
Ultrasonography (US) is a nonionizing technique that produces images in real time. It is a powerful initial kidney imaging modality that is safe, noninvasive, portable, and widely available with a lower initial test cost compared with CT or MRI. In addition, US can be performed in patients with kidney failure as iodinated contrast is not required, and it can readily differentiate between obstructive (surgical) and nonobstructive (medical) causes of kidney failure. However, image quality and detail are limited by a patient’s body habitus and the operator’s skills.
US can provide information about kidney size, cortical thickness, and echogenicity ( Fig. 6.1A–C ), and it can easily distinguish between solid and cystic kidney masses; however, characterization and differentiation between complex kidney cysts and cystic kidney tumors are limited by its soft-tissue resolution. In general, focal lesions identified on US require further imaging evaluation with CT or MRI. US is useful in detecting the presence or absence of hydronephrosis. Sensitivity for kidney calculi is generally restricted to calculi greater than 3 to 5 mm located within the renal pelvis, with relative insensitivity to ureteric calculi. Gray-scale ultrasound can be combined with color Doppler duplex imaging to assess patency and flow in kidney vasculature, particularly in the transplant kidney.
Computed Tomography
In the United States, CT is commonly used for imaging the kidneys. CT provides good spatial resolution, with detailed anatomy of the kidney parenchyma, vasculature, and the excretory system. In contrast to other imaging techniques, CT provides the highest sensitivity for detecting fine calcifications within the kidney parenchyma and throughout the collecting system, making unenhanced CT the optimal test for detecting stone disease.
Contrast-enhanced multiphase CT has been used to evaluate kidney masses. Although CT provides high spatial resolution, a relative limitation is soft-tissue contrast. This necessitates more complex diagnostic algorithms for assessing complex cystic kidney lesions that entail the measurement of density units before and after contrast administration. Multiphase CT increases the radiation dose to the patient proportionate to the number of phases used. CT for urologic indications represents the source of greatest diagnostic radiation exposures in our population; three to four CT examinations, or phases within one examination, approximate the radiation exposure determined to cause increased cancer risk in atomic bomb survivors. Contrast allergies are relatively common, and contrast-induced nephropathy (CIN) is a potential risk of iodinated CT contrast. Patients with even moderately impaired kidney function are at risk for CIN, with other risk factors contributing such as advanced age, diabetes, hypertension, heart failure, and volume depletion.
Dual-energy computed tomography (DECT) is a recent advancement in CT hardware that provides information about how substances behave at different energies. The ability to generate virtual unenhanced datasets and improved detection of iodine-containing substances on low-energy images are promising grounds for continued research and development in DECT, targeting the achievement of radiation dose reductions. Potential applications of DECT include distinguishing hyperdense kidney cysts from renal cell carcinoma (RCC), identifying kidney calculi within contrast-filled renal collecting systems, and characterizing the composition of kidney calculi, including the differentiation of uric acid stones from nonuric acid stones. Iterative reconstruction image postprocessing is another recent application in CT that may also lead to significant radiation dose reduction.
Kidney Scintigraphy
Radionuclide studies of the kidney have been used to provide imaging-based qualitative assessment of kidney function. However, these techniques suffer from low spatial resolution and do not provide detailed analysis of both structure and function. The test is based on time-resolved imaging of the kidney after intravenous administration of a radiopharmaceutical that is either filtered (technetium-diethylene triamine pentaacetic acid [Tc-DTPA]) or secreted by the tubule (technetium-mercapto acetyl triglycine [Tc-MAG3]). The use of radioactive tracers, particularly in monitoring applications where the study will be repeated, raises the concern of radiation risk, which is increased in younger patients.
Magnetic Resonance Imaging
MRI is evolving as a robust imaging technique for comprehensive evaluation of the kidneys and the collecting system ( Fig. 6.2A–C ). Excellent intrinsic tissue contrast and high spatial resolution of MRI results in more sensitive detection and specific characterization of focal and diffuse kidney pathologies, including complex cystic lesions considered problematic on CT or US. Because there is no ionizing radiation, MRI is an ideal imaging technique for assessment of kidney masses, particularly in younger patients or in patients requiring serial studies. In patients with reduced kidney function, gadolinium-chelate-based contrast agents (GBCA) have been associated with nephrogenic systemic fibrosis (NSF), which is discussed separately.
Advantages of MRI include multiplanar imaging, multiphase contrast-enhanced imaging, and acquisition of multiple types of soft-tissue contrast with an array of sequences. This ensures optimal soft tissue contrast for detection and characterization of disease. This array of image contrast may be obtained with the most up-to-date, fast imaging techniques that allow for a total scan time of less than 20 minutes yet without any concern for radiation dose accumulation as with CT.
Multiple sequences that yield specific tissue information can be used. Fluid-sensitive T2W images (half-Fourier acquisition single-shot echo train, HASTE) and T2-like sequences (true free induction with steady-state free precision, TFISP) are helpful in evaluating the collecting system because of the intrinsic high signal intensity of urine. Precontrast and dynamic postcontrast T1W three-dimensional (3D) gradient echo fat-suppressed images in arterial, capillary, venous, and delayed phases demonstrate improved spatial resolution vital for resolving masses and vascular anatomy. Arterial phase can also be optimized for evaluation of renal artery anatomy.
Functional data analysis with magnetic resonance nephrourography (MRNU) allows for evaluation of kidney physiology in a manner that was not previously possible. Both structural and functional data can be extracted in a single examination, with measurements of renal blood flow (RBF) and glomerular filtration rate (GFR).
Structural Imaging
T2W and steady-state magnetization (TFISP) imaging offers excellent structural evaluation of the kidneys and can display collecting system morphology, even in a nondistended system, because of the bright signal of native urine. Focal lesions, filling defects, and obstructive causes of urinary tract dilation can be identified even in the absence of contrast excretion.
Functional Imaging
Similar to inulin, gadolinium chelates are freely filtered at the glomerulus and are neither secreted nor absorbed by the renal tubules. Therefore the rate of gadolinium uptake in the kidney is related to the RBF. Accelerated 3D volumetric T1W gradient echo sequence (GRE) facilitates imaging of the gadolinium contrast as it arrives through the feeding renal artery and perfuses the kidney parenchyma. Rapid increase in concentration of gadolinium is seen as it enters the kidney parenchyma equivalent to the blood perfusion through the kidney. Although a portion of the perfused contrast leaves the kidney through the renal vein, another portion remains in the kidney because of glomerular filtration. Assessments of kidney parenchymal volume, and calculations of kidney perfusion in terms of RBF and GFR, are made by semiautomated methods based on mathematical modeling.
MRNU can identify the cause of obstruction in the renal collecting system, including congenital anomalies such as ureteropelvic junction obstruction ( Fig. 6.3A–C ) and acquired conditions such as stone disease, transitional cell carcinoma of the upper and lower tract, and extrinsic compression from retroperitoneal fibrosis. Preoperative assessment of kidney function using MRNU in cases of RCC may direct surgical planning toward a potential nephron-sparing procedure. Comprehensive imaging can be obtained for kidney transplant donor and recipient evaluation, and this process is discussed later in this chapter.
Risks and Benefits of Imaging Contrast in Kidney Disease
In patients with reduced GFR, including those receiving dialysis, the choice of contrast-enhanced CT or MR should be based on the expected diagnostic benefits.
NSF is a systemic fibrotic disorder that occurs in patients with advanced chronic kidney disease (CKD) or severe acute kidney injury who were exposed to one or more doses of certain GBCAs. Delayed excretion and prolonged tissue exposure to circulating free GBCA is likely a key factor in the relationship between reduced GFR and NSF. The largest subset of cases has occurred in dialysis-dependent patients (either hemodialysis or peritoneal dialysis) who had a delay between contrast exposure and dialysis. On average, patients developing NSF have been exposed to multiple GBCA doses or a large single dose, and most documented cases have been associated with gadobenate dimeglumine (Gadodiamide).
Available data from both clinical reports and animal studies support the conclusion that different GBCA formulations have different relative risks of NSF with the risk associated with the relative agent chemical stability. Recommended guidelines to minimize the risk of NSF include use of macrocyclic GBCAs, because of their relatively stable structure, or stable linear GBCAs with higher relaxivity. Relaxivity is a measure of signal generated by the GBCA, with the higher relaxivity of an agent resulting in a lower required dose. Regardless of the GBCA used, the objective is to administer the minimum dose necessary to achieve a diagnostic MRI. Hemodialysis is effective at lowering the serum concentrations of GBCAs and should be performed as soon as possible after GBCA administration. Currently, dialysis is advocated only in patients who are on dialysis before the MRI, as the mortality and morbidity risk of hemodialysis may be greater than the risk of developing NSF following the exposure to stable GBCAs.
Precautions to reduce NSF risk include use of more stable GBCA, use of lowest possible dose, and postprocedure dialysis for patients who are dialysis-dependent. Following the institution of these precautions and more judicious use of GBCA in patients with kidney disease, no new cases of NSF have been described in the United States since 2010, and the risk of NSF with more stable GBCAs appears low. Centers have more recently reported that NSF incidence has diminished after observing these precautions, while continuing to use MRI with selected GBCA enhancement even in patients who are treated with dialysis if warranted.
CIN is defined as an acute decrease in kidney function after exposure to iodinated contrast agents (ICAs) in the absence of another explanation. This is the third most common cause of hospital-acquired acute kidney injury. Multiple risk factors are associated with CIN, including reduced GFR, volume depletion, diabetes, congestive cardiac failure, advanced age, and type and dose of ICA. Evidence suggests that ICA osmolarity and viscosity may relate to CIN risk. For patients receiving dialysis, preservation of residual kidney function is associated with improved outcomes, and ICAs may jeopardize residual kidney function in these patients.
The selection of contrast, specifically GBCAs versus ICAs versus no contrast, should be individualized based on a careful risk-benefit assessment. Consideration of alternative studies, including nonenhanced examination protocols, should always occur. In general, there are more unenhanced options available for the evaluation of soft tissues and blood vessels using MRI than CT. If a contrast-enhanced CT or MRI is warranted, the test most likely to provide a result that will impact management should be performed with a goal of minimizing delayed or missed diagnoses and the need for study repetition.
Cystic Kidney Lesions
Kidney Cysts
Kidney cysts represent the most common focal kidney lesion in adults. Cysts in the kidney have been classified and characterized by Bosniak into four categories: Category I: Simple benign cysts with negligible likelihood of malignancy; Category II: Benign cystic lesions that are minimally complicated; Category IIF ( F for follow-up): Cysts less complex than category III and likely to be benign but given their complexity require follow-up studies to prove their nature; Category III: More complicated cystic lesions that require follow-up imaging and/or surgical excision; Category IV: Masses that are clearly malignant cystic carcinomas.
Imaging Features
US is the preferred method to differentiate cystic from solid lesions because of its ease of performance and lower cost. It is also accurate in differentiating simple kidney cysts ( Fig. 6.4A–D ) from complex cystic kidney masses. Although CT is widely used for characterization of cystic kidney masses, kidney lesions are best evaluated with triple-phase multidetector row CT (i.e., unenhanced, arterial phase, and nephrographic phase) with increasing radiation risk. Enhancing thick septations, solid components, or change in attenuation between 10 and 15 Hounsfield units (HU) after intravenous administration of contrast material are considered suspicious. Cystic angiomyolipomas (AMLs), oncocytomas, and infections may also show enhancement, whereas hypovascular papillary cancers may demonstrate less enhancement. MR imaging with better soft-tissue contrast resolution is 100% sensitive and 95% specific for distinguishing benign and malignant kidney cysts. Precontrast T1W images can easily identify intracyst hemorrhage in Bosniak Category IIF, which is challenging on CT. Signal intensity changes in dynamic postcontrast 3D T1-weighted GRE can delineate septae, solid elements, and nodules within a fluid-filled cyst or its wall. T2-weighted sequences also complement the 3D T1W GRE sequences for accurate characterization of kidney cysts (see Fig. 6.4A–D ).
