13. Urologic Imaging
Apparent diffusion coefficient
Autosomal dominant polycystic kidney disease
American Urological Association
Bacillus of Calmette-Guerin
Benign prostatic hyperplasia
Bi parametric magnetic resonance imaging
Bone scan index
Color Doppler ultrasound
Contrast enhanced ultrasound
Chromophobe renal cell carcinoma
Contrast induced nephropathy
Clear cell renal cell carcinoma
Clinically significant prostate cancer
Computerized tomography urography
Dynamic contrast enhancement
Dual energy computerized tomography
Digital rectal examination
Triamine pentaacetic acid
Diffusion weighted images
European Association of Urology
Food and Drug Administration
Germ cell tumors
Grey scale ultrasound
Radiographs of the kidneys ureters and bladder
Lower urinary tract symptoms
Mecapto acetyl triglycine
Mixed epithelial stromal tumor
Multi parametric magnetic resonance imaging
multi parametric ultrasound
Magnetic resonance imaging
Magnetic resonance urography
National Comprehensive Cancer Network
Non-contrast computerized tomography
British National Institute of Health and Care Excellence
Negative predictive value
Non-seminomatous germ cell tumors.
Power Doppler ultrasound
Positron emission tomography
Prostate imaging reporting and data system
Papillary renal cell carcinoma
Prostate specific antigen
Prostate specific membrane antigen
Renal cell carcinoma
T1 weighted images
T2 weighted images
Trans abdominal ultrasound
Trans rectal US
Ureteral pelvic junction
Ureteral vesical junction
Imaging is an integral part of the evaluation of urologic patients, regardless of the organ site or disease process. Historically, the ability to image the upper tract enabled the identification and management of renal stone disease, urinary obstruction, and eventually, malignancies. In contemporary practice, imaging is a core component of the evaluation of a wide range of common urinary tract symptoms including hematuria, recurrent infection, hypertension, orchalgia, and incontinence. Almost all urologic surgeries, whether reconstructive or extirpative, are preceded by imaging—both for determining indication and planning the operation itself.
The relationship between urologist and radiologist is a critical one, as the urologist’s actions are often based upon the judgement of the radiologist. Continual communication, both to understand the image interpretation of the radiologist and to communicate the need of the urologist, is critical for good patient care, quality assurance, and maximizing patient outcomes. It is also incumbent upon the urologist to understand the indications, techniques, and strategies for general interpretation of films. By understanding imaging, and gaining the skills to interpret images in the context of urologic disease, urologists empower themselves to become better clinicians.
Multiple imaging tools are available to assess the urinary tract. Each has its own strengths and weaknesses. Knowing which to use in the correct clinical scenario will help the urologist in management decisions. Imaging tools include ultrasound (US), computerized tomography (CT), magnetic resonance imaging (MRI), radiography, and nuclear medicine studies.
US is the usual first line imaging modality as it is one that provides the most information for the least cost, both monetarily and with regard to radiation exposure. The location of the kidneys in the retroperitoneum without overlying bowel lends them well to ultrasound imaging. Ultrasound offers multiple types of imaging techniques. Grey-scale imaging is best in characterizing overall renal anatomy and renal masses to determine whether they are solid or cystic or contain calcifications. Color Doppler imaging can show flow of fluid demonstrating vascularity of a lesion or flow of urine, such as ureteral jets in the bladder. Spectral Doppler imaging investigates the specific waveforms that are produced in flow, which further investigates vascular flow, thereby confirming whether a mass is truly solid with internal vascularity or cystic, containing hemorrhage or debris . In addition, from waveform analysis, spectral Doppler imaging can elucidate functional information such as increased intrarenal pressure from venous thrombus or ureteral obstruction.
Contrast enhanced ultrasound (CEUS) utilizes microbubble agents which are foci of gas encased in polymers, lipids, or proteins. These molecules measure approximately 1–10 μm. Agents are injected intravenously and images are acquired in regions of interest, typically in dynamic imaging. The ultrasound probe detects the harmonic signals produced by the bubbles expanding and contracting in response to the ultrasound waves. The gas is exhaled and the encasing molecules are excreted, typically by the liver .
Limitations of ultrasound include the dependency on the technical ability of the end user and the body habitus of the patient. US also is only able to acquire images in a small field of view.
CT utilizes X-rays to create cross-sectional imaging with high contrast between various soft tissues, especially with the use of intravenous contrast. This is advantageous to urologic imaging as it can separate solid, enhancing lesions versus cystic lesions. It can easily detect, characterize, and localize urinary calculi. In addition, CT is able to visualize a large field of view to evaluate structures adjacent to the kidneys and urinary tract, metastasis, as well as any other abnormality that may mimic urologic pathology. Images are acquired over a few seconds, which decreases motion artifact.
CT is important for evaluation of renal lesions as it allows improved ability to detect lesions, as compared to ultrasound, especially with the administration of intravenous contrast material. Intravenous contrast is excreted through the kidneys and progressively enhances the kidneys in a time dependent predictable pattern. Images acquired 25–80 s after contrast administration are considered the corticomedullary phase in which the cortex appears hyperdense while medullary pyramids are hypodense . The nephrographic phase acquired 85–120 s after contrast administration, demonstrates homogeneous enhancement of the cortex and medullary pyramids . Finally, the urographic phase acquired 3–10 min after contrast administration images the kidneys after excretion of contrast into the collection system and bladder .
A relatively new CT technology is dual-energy CT (DECT), which utilizes or detects specific low and high kilo-voltages to discriminate between materials of different atomic numbers . Post-processing algorithms utilizing material decomposition principles have been developed to subtract iodine in contrast enhanced images to create virtual non-contrast image . This potentially allows one to gain information of a non-contrast examination without a separate CT acquisition, which significantly decreases radiation exposure to patients . The greatest downside to CT is the high radiation exposure, especially of repeated scans performed for follow-up . Iodinated intravenous contrast may also cause contrast induced nephropathy (CIN), especially in patients with renal insufficiency .
MRI utilizes a strong magnetic field and radiofrequency pulses to organize and flip protons of hydrogen atoms within the complex molecules of the human body. Different sets of pulses elicit various responses of protons and give rise to the signals which in turn translate to tissue contrast greater than CT or US. Each sequence is designed to highlight distinct attributes of the tissue under evaluation. Multiparametric MRI (mpMRI) is a combination of sequences performed in a set for in-depth characterization . The most commonly performed sequences are described below.
T2-weighted images (T2-WI) are fluid sensitive sequences in which fluid is high signal including free fluid, fluid in structures of the body such as the urinary bladder, spinal canal and gallbladder. Fluid in edema also increases signal in tissues. In addition, neoplasms are typically T2 bright, with some exceptions. Thus, T2-WI are generally anatomic sequences useful as an initial overall view of the overall condition of the patient .
Fat, blood and proteinaceous products appear bright on T1-weighted images (T1-WI). Fluid has low-signal on T1-WI. In addition, gadolinium, the intravenous contrast agent used, is also high signal on T1-WI. Thus, all contrast-enhanced images are T1-WI. Typically pre-contrast images are acquired in addition to post-contrast images so that subtraction images can then be obtained to determine which tissues are truly enhancing .
Fat Suppressed Sequences
Fat demonstrates high signal intensity on T1-WI and turbo T2-WI. A common option is the suppression of fat from these sequences in order for more problem-solving technique. Fat is suppressed on T2-WI to highlight features of fluid or malignancy. On T1-WI, fat is suppressed on contrast enhanced sequences in order to minimize signals that may confound enhancing structures, which are also T1 hyperintense.
In and Out-of-Phase
In-phase (IP) and out-of-phase (OOP) sequences take advantage of the chemical differences of water and fat molecules. The hydrogen protons in each rotate at different frequencies and align at certain times to become additive (IP) and align at other times to cancel each other (OOP). OOP images can be distinguished from IP images as all the organs are outlined in a thick black line, an artifact called “India ink.” These sequences are used together to ascertain whether lesions have fat and water in the same imaging unit, or voxel, called microscopic fat, such as in clear cell renal cell carcinoma. Lesions with fat and fluid in the same cell will be at least 10% decreased in signal intensity on OOP when compared to IP .
Free flowing water molecules typically move in random patterns called Brownian motion. Water molecules can become trapped in some clinical situations such as in highly cellular neoplasms or abscesses. Diffusion-weighted images (DWI) are acquired at specific “b-values.” Increasing b-values increases the weight of diffusion on imaging, thus DWI with high b-values only show signal of those trapped water molecules while freely diffusing water molecules lose signal. Apparent Diffusion Coefficient (ADC) maps are acquired by calculating the change of signal with multiple b-value sequences. Thus, those regions that maintain signal from low to high b-value will have low change and have low signal on ADC maps. Regions with restricted diffusion will have high signal on high b-value DWI sequences and low value on ADC maps .
Similar to CT, intravenous contrast can be administered to increase conspicuity of renal masses, infectious process, or inflammation. Because there is no radiation exposure, images of multiple different time points may be acquired after contrast administration to assess dynamic contrast enhancement. Due to angiogenesis and increased vascularity of tumors, neoplasms tend to enhance earlier. Thus, dynamic contrast enhancement can differentiate lesions from other enhancing structures. Contrast is excreted through the kidneys and delayed urogram can be acquired to evaluated the collecting system, ureters, and bladder .
MRI utilizes gadolinium based intravenous contrast. The newer, macrocyclic contrast agents have not been proven to cause nephrogenic sclerosing fibrosis, even in the setting of renal insufficiency and thus can be administered in patients with renal failure. This is advantageous compared to CT intravenous contrast, which increases risk of CIN if administered in patients with renal insufficiency. In addition, gadolinium based contrast agents have not been shown to cause contrast induced nephropathy [11–15]. However, older linear gadolinium based agents have a rare risk of causing nephrogenic systemic fibrosis in patients with renal insufficiency. If such agents are used, renal function should be assessed prior to contrast administration, Gadolinium based contrast administration is however contraindicated in pregnant patients.
For the best images, patients must be able to be still during the duration of the examination, which may last up to one hour long. They must also be comfortable within an enclosed space for long periods of time. Care must be taken to ensure any devices and implants of patients are MRI compatible.
Plain film radiographs of the kidneys ureters and bladder (KUB) utilize x-rays and have high resolution but low tissue contrast. Radiographs can identify urinary calculi however the sensitivity is not as high as CT and is better used for follow up rather than initial diagnosis. Radiographs have less of a role in urologic imaging when compared to other modalities.
Nuclear Medicine Examinations
Nuclear medicine examinations utilize radioactive particles which are attached to specific ligands. The ligands bind to molecules in the body giving specificity in assessing function and presence of certain tumors. Nuclear medicine studies such as dimercaptosuccinic acid (DMSA) evaluate renal tissue while diethylene triamine pentaacetic acid (DTPA) or mecapto acetyl triglycine (MAG-3) renal studies are excreted from the kidneys and offer information about functionality and obstruction. Positron emission tomography (PET)/CT is not typically used for renal or urinary tumors as most radiotracers are excreted into the urine, obscuring masses. There have been promising results on the use of more prostate cancer-specific radiotracers which may improve sensitivity in detecting recurrence and metastasis [16, 17].
Renal masses can be broadly divided into cystic and solid. Differentiating the two is important as solid renal masses tend to have a higher malignant potential and are typically resected while cystic masses tend to be bening (although some may have maignant potential) and can, in some cases, be managed more conservatively . Ultrasound is the first line of imaging and is important for characterizing a renal lesion. On grey-scale imaging, simple cysts are anechoic, while more complex cysts can have internal echoes, however, all cysts, including those with internal echoes, have posterior acoustic enhancement which may separate complex cysts from solid masses. Spectral imaging and dynamic CEUS imaging can also assess the interval vascularity of masses.
CT and MRI renal mass imaging require a non-contrast phase and a nephrographic phase, which is especially important as masses can occur both in the cortex and medulla. If the image is acquired too early in the corticomedullary phase, masses can be easily missed within the hypoenhancing medulla. Non-contrast imaging is important to assess true enhancement of renal masses which categorizes them into cysts and solid masses [1, 10].
The addition of a non-contrast acquisition increases the radiation exposure to patients. DECT has the ability to subtract iodine from contrast-enhanced images to create virtual non-contrast (VNC) images. There have been promising studies which have shown that Hounsfield (HU) measurements of various abdominopelvic viscera are statistically the same between acquired non-contrast CT and VNC [18, 19]. Others studies have demonstrated accurate detection of enhancing renal lesions with the use of VNC . However, not all studies support the use of VNC . One study demonstrated strong agreement between HU of renal masses measured on VNC and non-contrast images for renal lesions, especially low attenuation lesions; however, there was more discrepancy on higher attenuated lesions . Because there is no standardization of technique, and results are highly affected by technique, each institution must verify the validity of their equipment and acquisition technique in order to adopt VNC and forego a non-contrast acquisition .
Bosniak classification and management
Prevalence of malignancy (%)
Simple cyst, fluid attenuation (0–29 HU)
No follow up
Minimally complex cyst. A few thin non-enhancing septations. A few thin calcifications.
No follow up.
Mildly thickened nodular calcifications. Slight increased number of septations with perceived enhancement, <1 mm in thickness.
6 month imaging follow-up
Complicated cyst with multiple thickened, enhancing septations. Nodular, thickened calcifications.
Clear, solid nodular components
The Bosniak classification was initially developed for the CT with intravenous contrast. Contrast enhancement is a major criterion in assessing malignant potential of masses. Application of the classification can be used on MRI studies with intravenous contrast however should not be applied to studies without contrast (including US).
Benign Renal Cysts
Mixed Epithelial Stromal Tumor
Another benign multiloculated cyst is the cystic nephroma. The lesion is bimodal in age of presentation, which manifest in boys less than 4 years old and women between 40 and 60 years old . Cystic nephromas have similar histological appearances to MEST and have been suspected to be on a similar spectrum of stromal tumor. They present as encapsulated multiloculated cystic masses, at times with hemorrhagic or proteinaceous contents. Multicystic nephromas can also invaginate into the renal pelvis and have delayed enhancement of septations, which likely reflects the fibrous content of the lesion [31, 32]. There is no imaging criteria which can definitively distinguish a cystic nephroma from a cystic malignancy, and for this reason, most are resected.
Renal Cystic Disease
Autosomal Dominant Polycystic Kidney Disease
Imaging is important in screening first-degree relatives of people with ADPKD as genetic testing only identifies 70% of those with the disease . Criteria for diagnosing ADPKD is age dependent. For high-risk patients between the age of 15 and 39 years, three cysts are required for diagnosis, for high-risk patients between the age of 40 and 59 years, two cysts are required for diagnosis, and for high-risk patients 60 years or greater, four cysts are required for diagnosis .
Localized Cystic Renal Disease
Acquired Renal Cystic Disease
Patients with end-stage renal disease without an inheritable renal disease may develop acquired renal cystic disease, which is defined as at least three cysts in each kidney . The cysts are usually cortically based and small in size (<3 cm). 8–13% of patients with end-stage renal failure develop acquired renal cystic disease, which increases to 13% after 2 years of dialysis, 50% after 6 years of dialysis, 87% after 9 years of dialysis, and close to 100% after 10 years of dialysis. Patients with acquired renal cystic disease have increased risk of ureteral stones and renal cell carcinoma, particularly clear cell type (3–7% of patients develop renal malignancy) . Even after renal transplant, cysts may regress, however, risk for renal malignancy is persistently elevated. On ultrasound, kidneys are small with echogenic renal parenchyma and small cysts. CT and MRI also demonstrate similar findings. There are no consensus recommendations for screening of patients with acquired cystic kidney disease, however imaging should be considered.
Solid Renal Masses
Solid renal masses are most worrisome for renal cell carcinoma (RCC). Not all solid masses are malignant, though, and imaging can help differentiate between the two in a non-invasive fashion. Ultrasound maintains to be the first-line imaging tool, however, it does not have the soft tissue contrast to truly differentiate between solid tumors. CT offers an overall view of the lesion and surrounding structures and can detect macroscopic features of renal masses. These features are helpful for staging and detecting metastasis. mpMRI remains the best tool for distinguishing solid renal masses due to the high tissue contrast. With mpMRI, masses can be potentially categorized as benign and malignant, and even subdivided into specific types of RCC. However, even with these advanced tools, overlapping imaging features challenge diagnosis .
Image characteristics of benign and malignant solid renal tumors
Clear Cell RCC
Heterogeneous, T2 bright, Hypervascular, Intravoxel fat, Necrosis
Homogeneous, T2 dark, Hypovascular, Hemosiderin
Heterogeneous T2 signal, Heterogeneous enhancement
Infiltrative T2 intermediate to dark, strong restricted diffusion
Urothelial Renal Mass
Centered in the renal pelvis, Infiltrative
Bulk fat, vascular
Lipid Poor AML
T2 dark, hypervascular
Heterogeneous T2 signal, Stellate scar, Segmental enhancement
ccRCC subtype is the most common RCC representing 65–80% of all RCC . It is typically sporadic, but is associated with many syndromes such as von Hippel Lindau syndrome and tuberous sclerosis. Because of its potentially aggressive nature, management is also often aggressive and thus early diagnosis is important.
The second most common subtype of RCC is pRCC comprising 10–15% of RCC cases . There are two types (I and II). Type I is usually less aggressive than ccRCC, whereas type II pRCC are more aggresive. Pathologically, pRCC’s often contain necrosis and hemorrhage product . Histologically, they have papillae which are covered by thin layer of uniform cells with scant cytoplasm, hemosiderin, and foamy macrophages .
chRCC is less prevalent compared to pRCC (4–11% of all RCC). Similar to type I pRCC, chRCC have better prognosis than ccRCC with 5-year survival rate reported to be 78–93% . chRCC arise from the intercalated cells of the kidneys, much like oncocytomas. Pathologically, chRCC are well-defined lesions which present larger than other RCCs, average mass size of 7.2 cm .
Imaging features of chRCC are varied and less specific than other RCCs. They are peripherally located, homogeneous, typically demonstrate T2 intermediate to low intensity signal. They restrict diffusion and also enhance, although demonstrate intermediate enhancement, between pRCC and ccRCC. Calcifications are also often seen, which occur in 38% of such lesions. Sometimes, chRCC have a “tail” which extends towards the renal pelvis. Very rarely, chRCC may demonstrate segmental enhancement inversion, which is heterogeneous enhancement with some portions enhancing earlier and other portions demonstrating progressive enhancement . Since chRCC’s are histologically similar to oncocytomas, imaging features are similar as well. chRCC’s may exhibit an enhancing spoke wheel central scar, approximately 30–40% of time, which can also be seen in oncocytomas .
Distribution of lymphoma is wide spread and the genitourinary systems is the second most commonly affected site . Within the genitourinary system, kidneys are involved the most. Detection is typically via imaging. CT and MRI are both sensitive in diagnosis. US is likely the first line imaging technique and may be used for follow-up, however is less specific in initial diagnosis.
On imaging, lymphoma has multiple patterns of presentation. The most common presentation is multiple tumors, seen 50–60% of the time. Other presentations include single tumors, centralized to perinephric regions, and infiltrative tumors. Lymphoma may also extend into the kidney from primary retroperitoneal location .
Several secondary signs can help distinguish lymphoma from other renal malignancies. Lymphadenopathy is a systemic disease and often presents simultaneously with retroperitioneal lymphadenopathy. In addition, lymphoma is rarely associated with renal thrombus .
Intrarenal Urothelial Carcinoma
Urothelial carcinoma most commonly occurs in the bladder, less commonly in the ureters, and even more rarely the renal pelvis. When in the renal pelvis, urothelial cancers tend to grow centripetally into the renal parenchyma . These lesions can be confused with central RCC [60, 61]. It is clinically important to distinguish these two entities as management is drastically different.
Benign Solid Masses
Lipid poor AMLs, defined histologically as less than 25% fat content per high-power field, present a diagnostic dilemma as RCCs have similar appearances, especially for small masses (<3 cm) [64, 65]. On ultrasound, lipid poor AMLs are isoechoic to renal parenchyma, limiting detection and evaluation. Without fat, lipid poor AMLs have increased components of blood vessels and smooth muscle, which are both hyperattenuating compared to normal renal parenchema on CT without intravenous contrast and thus lipid poor AMLs are typically heterogeneously hyperattenuating compared to renal parenchyma, however this is not consistently true . MRI features of AML may include homogeneous T2 signal compared to renal cortex and the lack of cystic degeneration or necrosis [64, 66]. These findings are similar to that of pRCC, however AMLs tend to be more vascular. Image diagnosis of lipid poor AMLs may be difficult however should be considered as a differential, especially in patients whose demographic is atypical of RCC . If there is strong suspicion of a lipid poor AML, on the basis of genetic risk or patient age, needle biopsy with HMB45 staining can by quite diagnostic. In the absence of heightened suspicion of AML, larger solid renal masses are generally resected.
Infections in the kidneys have unique appearances. US is often the first line imaging for urinary processes, however, CT is most effective at imaging for renal infections. MRI plays less of a role in imaging as many of the secondary findings such as air and calculi are not well-visualized.
Although US is often used as first line imaging, bacterial pyelonephritis is not well characterized on grey-scale imaging and thus produces many false negative results, demonstrating abnormalities in only 24% of patients . Some findings include changes in echogenicity, decreased in the setting of edema, or increased in the setting of hemorrhage. There may be loss of corticomedullary differentiation or foci of hypoperfusion on color Doppler imaging . Use of harmonics imaging highlights patchy hypoechoic foci. Abscesses can also be seen on ultrasound as fluid collections with peripheral hyperemia which demonstrate mass effect on adjacent structures. Source for possible obstruction should be investigated, especially in the bladder (e.g. enlarged prostate).
MRI plays a smaller role in imaging bacterial pyelonephritis compared to CT as it is more expensive and time consuming without increased diagnostic benefits. Patchy edema and tubular obstruction can be seen as patchy restricted diffusion on DWI. In addition, abscesses are especially accentuated on DWI and as abscess markedly restrict diffusion. Enhancement patterns on MRI appear similar to that of CT .
Emphysematous Pyelonephritis and Pyleitis
Emphysematous pyelonephritis is a necrotizing form of infection, typically in diabetic or immunocompromised patients, resulting from obstruction by urinary calculi, neoplasm, or sloughed papilla. The disease is life-threatening with a high mortality rate, thus early diagnosis is imperative .
CT provides the most complete evaluation. The kidneys are low attenuation and may contain foci of gas in a linear pattern. Fluid from necrosis and abscesses within the kidney or in perinephric spaces are readily visible. Infection can be centered around the renal parenchyma (type 1) or more extensive connecting perinephric fluid collections with the collecting system (type 2) (Fig. 13.17) . Obstructive etiologies may also be evaluated at the same time.
Gas only within the collecting system is termed emphysematous pyelitis, and is less ominous than emphysematous pyelonephritis. On CT and ultrasound, gas can be seen layering in the renal pelvis. Care must be taken to exclude other causes of gas such as recent procedures [77, 78]. MRI is less favorable than CT as gas is not well seen on the modality; however, MRI may be used in circumstances to minimize radiation exposure or for patients with renal insufficiency.
Infection can be centered around the collecting system and is referred to as pyonephrosis. On ultrasound, there is prominence of the renal pelvis which often contains debris. On CT, a prominent renal pelvis can also be seen, and also demonstrates thickened walls (>2 mm) with adjacent fat stranding. Exclusion of obstructive etiology must be performed. Urine versus pyogenic fluid often have similar HU and may be difficult to differentiate. MRI findings are similar to CT however is able to detect debris within the collecting system .
Xanthogranulomatous pyelonephritis (XGP) is a rare chronic destructive granulomatous infection of the kidney caused by lipid-laden (foamy) macrophages. This typically occurs in women and patients with diabetes. Patients also often have obstructive renal calculi such as staghorn calculi. The most common causative organism includes Proteus mirabilis and Escherichia coli .
TB in the ureters present as multifocal strictures with ureteral wall thickening and hyperenhancement which causes hydronephrosis . The urinary bladder with TB appears with wall thickening. After time, there is increased fibrosis and shrinkage of the bladder and eventually dystrophic calcification [83, 85].
Upper Tract and Bladder Imaging
Disease of the upper urinary tract vary from urinary obstruction to urothelial cancers to infectious processes. Due to the broad nature of upper urinary tract disease, imaging plays an important role in assessment of the upper tracts in order to diagnose and treat patients as well as to follow disease progression. Multiple modalities are available to clinicians, each with their own strengths and weaknesses. Thus, understanding imaging is important to maximize the information gained while minimizing the inherent risks.
Urinary obstruction is a common entity with multiple different etiologies. Many modalities exist to assess for obstruction and cause of obstruction including various protocols of CT, MRI, US, radiography, and nuclear medicine studies. Clinical signs and symptoms are likely to point to specific causes initially, which will direct urologists to choose the highest yielding imaging study.
Urinary calculi is a common entity affecting 6% of women and 12% of men in the United States [86, 87] with incidence increasing up to the age of 60 . Urinary calculi are formed by excretion and precipitation of salts including calcium, struvite, uric acid, and cysteine into the urine. These salts may become lodged throughout the urinary tract causing pain and urinary obstruction.
The composition of urinary calculi is reflected in imaging characteristics and also have treatment implications. Calcium based calculi is the most common, representing 70–80% of calculi in the US . Within the category of calcium based calculi, calcium-based oxalate calculi are the most common, representing 60% of all calculi . When imaged with CT, calcium based urinary calculi have the highest HU measuring up to 1700 HU .
Struvite calculi, 15–20% of urinary calculi, are formed by urease-producing bacteria (e.g. Proteus, Pseudomonas, Klebsiella, and enterococci). Escherichia coli, the most common organism causing urinary tract infections, however, does not produce urease [89, 92]. Urea is broken down into carbon dioxide and ammonia which raises the pH level of urine allowing carbonate to precipitate with struvite forming calculi. These calculi typically involve at least two calyces of the renal pelvis giving the appearance of antlers reflected of its namesake staghorn calculi . HU of struvite are varied depending on percentage of struvite in the calculi and can range from 200 to greater than 1300 .
Calculi can be present throughout the entire urinary tract. Calculi within the calyces are typically non-obstructing and asymptomatic, although some non-obstructing calyceal calculi can present with renal colic and gross or microscopic hematuria. Stones that migrate into the ureteral pelvic junction (UPJ) which can cause obstruction and flank pain. Once through the UPJ, there are three anatomic locations of ureteral narrowing which calculi may lodge: just distal to the UPJ, at the crossing of the iliac vessels, and at the ureteral vesical junction (UVJ). The UVJ is quite narrow, resulting in obstruction of tiny 1–5 mm stones .
Imaging indications can be broadly divided into suspicion of stone disease versus recurrent stone disease, which are optimized with different imaging protocols. Multiple modalities are available to use including non-contrast computed tomography (NCCT), KUB, US, and MRI.
Acute Flank Pain/Suspicion of Ureteral Stone
The European Association of Urology (EAU) recommends initial evaluation with US before other diagnostic imaging as it can visualize the kidneys, collecting systems, parts of the ureters, and bladder to determine presence of calculi or urinary obstruction. US has 45% sensitivity and 94% specificity for ureteric stones and 45% sensitivity and 88% specificity for renal stones and increases to sensitivity of 77% and specificity of 93% in patients with acute flank pain [100–103]. With the utilization of US prior to CT, there was no significant difference in outcomes compared to the initial utilization of CT, however, radiation exposure was decreased .
Direct visualization of ureteral calculi is more difficult due to overlying bowel gas and retroperitoneal fat. Similar to renal calculi, ureteral calculi are seen as hyperechoic foci within the ureteral lumen and demonstrate distinct posterior shadowing (Fig. 13.21). There may also be associated ureteral wall thickening and edema. If transabdominal approach does not reveal a ureteral calculus, transvaginal or transperineal approach may be attempted to evaluate for distal ureteral calculi [104, 105].
Aside from direct visualization of calculi, US is sensitive for evaluating presence of obstructive uropathy demonstrated by hydronephrosis, hydroureter, and perinephric edema. Visualization of ureteral jets in the bladder can also confirm patency of the ureters. This may be seen on grey scale imaging as a stream of low-level echoes or as color jets on color Doppler imaging. The degree of hydration of the patient can affect the visibility of the ureteral jets and can range from less than one jet per minute to continuous flow in a healthy patient. Healthy patients can also have asymmetric jets, thus visualization of decreased jets on affected side should only be used as an adjunct tool for increased sensitivity of stone detection [106, 107].
There have been some promising studies utilizing spectral Doppler imaging and intrarenal resistive index (RI) ((peak systolic velocity-end diastolic velocity)/peak systolic velocity) to assess the hemodynamics of acute obstruction. Both absolute intrarenal RI of ≥0.70 and a difference of RI between kidneys (ΔRI) of ≥0.08 have been shown to in acute obstructive uropathy [108–110]. This is controversial, as some studies have also demonstrated less promising results in the use of RI to predict obstruction [111–113]. This accentuates the complexity of renal obstruction and renal tissue and vascular compliance and pulse pressures, which determines RI.
There are several limitations to US. The largest is that it is operator dependent [114, 115]. Evaluation can also be limited by the patient’s body habitus, decreased mobility of patients, or inability of patients to follow directions. In addition, renal vascular calcification, calcified sloughed papilla, calcified tumor, or calcified ureteral stents may be mistaken for urolithiasis. Secondary obstructive signs such as hydronephrosis can also be misdiagnosed in the setting of parapelvic cysts. Thus, confirmation of findings with other diagnostic exams such as non-contrast CT is recommended.
The advent of helical (spiral) NCCT for the evaluation of flank pain, has shown to be the exam yielding the highest sensitivity (>95%) and specificity for detecting urolithiasis . Although radiographs have higher resolution, the high contrast between tissue types in NCCT allows several facets of urolithiasis to be assessed including detection of the presence of nearly all types of calculi, size and location of calculi, and signs of obstruction. Coronal reformatted images are also available, which increase rate of detection of stones and accurate assessment of stone size [117, 118]. In addition, NCCT is a fast exam requiring only seconds to acquire a scan. Thus, NCCT is recommended after the initial evaluation with ultrasound in order to determine the extent and location of urolithiasis.
NCCT is preferred over contrast-enhanced CT because there is increased conspicuity of urothelial calculi without the obscuration by intravenous contrast, especially in more delayed phases as contrast is excreted into the collecting system and ureters. Intravenous contrast, however, can be helpful in unique situations by defining anatomy and differentiating pelvic phleboliths versus urinary calculi.
Patients should be scanned in the prone position. This allows the posteriorly located UVJ to be in a non-dependent location which can differentiate the location of calculi lodged in the UVJ versus layering calculi in the urinary bladder.
Evaluation of NCCT should include focus on the urinary collecting system, ureters, and bladder for evaluation of presence of calculi. First, location of urinary calculi should be determined, which can change prognosis. Calculi located more proximally are associated with higher need of intervention . Measurement in both axial and coronal plane should be performed to ensure maximal diameter is assessed. HU of the calculus should also be measured as this provides additional information on type of calculus as mentioned previously. Finally, investigation for signs of obstruction is important. These include hydronephrosis, perinephric fat stranding, and peri-ureteral fat stranding. Other causes of flank pain should also be assessed, especially if no calculus or findings of urinary obstruction are visible. Other causes of flank pain may include acute diverticulitis, appendicitis, rib fracture, or metastatic osseous lesions.
Conventional helical (spiral) CT uses broad range of X-ray energies. DECT is a relatively new technology which is able to assess two specific different energies of X-rays. This technology has been preliminarily shown to determine the composition of calculi by measuring the HU ratio of one energy to the other as substances absorb X-ray energies to variable degrees (Fig. 13.20) . More studies are needed to optimize this technique [121, 122].
One major concern of CT is the radiation exposure to the patient, especially to younger patients. Low dose CT has been shown to have high sensitivity (97%) and specificity (95%) in detecting urolithiasis . In addition, it has been shown that there is no significant difference in measurement of stone size on low-dose CT versus standard dose . Effort to limit scan exposure to only necessary organs is an additional method to decrease radiation exposure.
KUB can identify renal calculi, however is less sensitive (72%) than CT for calculi greater than 5 mm [122, 123]. Radiographs are better used for follow up than the evaluation for source of acute flank pain. Radiodensity overlying the region of the kidney, ureters, and bladder may indicate renal calculi. Limitations of radiography include the inability to locate the calculus in the anteroposterior plane unless lateral view is obtained. Not all types of urinary calculi can be seen on radiographs. Quality of radiographs is heavily dependent on overlying bowel contents, patient body habitus, size, location, and composition of stone. Thus, comparing to NCCT, it has been shown that radiography has decreased sensitivity (72%) for stones greater than 5 mm in diameter and less affective in the acute setting [125, 126].
Radiography exposes patients to radiation, however at a much lower dose than NCCT. The radiation exposure from multiple KUBs obtained are additive and may eventually equal that of NCCT if a large number are taken.
Although not the first line of imaging, MR urography can be helpful in assessing for secondary signs of obstruction. Hydronephrosis is readily visible on T2-WI as it is highly sensitive to fluid. The ureters are well-assessed throughout their entire course, especially if dilated. Calculi are diagmagnetic material and do not produce signal on MRI, thus are seen as signal voids on T1, T2, and gradient sequence, called “blooming,” within the urinary tract, which is at times difficult to visualize. MRI is beneficial as it provides abundant information on tissue without exposing patients to radiation, which may be more beneficial in pregnant or pediatric patients.
Urolthiasis Follow Up Imaging
Patients with a history of urolithiasis often have recurrent flank pain and stone disease. Repeat NCCTs increase patient exposure [127, 128]. If NCCT is clinically necessary, low-dose protocols should be used. Stones that are seen on the scout image of NCCT can be followed with KUB, which expose patients to less radiation . Stones that are not visualized on scout image may not be visible on KUB. Finally, ultrasound remains a radiation free and comparatively inexpensive method for imaging follow up.
Urothelial neoplasm affects the upper urothelial tract including the renal pelvis and ureters, as well as the lower tract including the urinary bladder, urachus, and urethra. The most common histological subtype of urothelial neoplasms is urothelial carcinoma, which accounts for 90% of bladder tumors  and 10–15% of renal tumors . Only 4% of urothelial carcinomas occur in the ureters . Squamous cell carcinoma is the second most common histologic type associated with recurrent urinary tract infection followed by adenocarcinoma often in the urachal remnant, both occur mostly in the bladder [133, 134].
Urothelial carcinoma is unique in that it is often multifocal and metachronous with frequent recurrence. 2–4% of people with bladder cancer will develop renal pelvis or ureteral urothelial carcinoma and 40% of those with renal pelvis or ureteral disease will develop bladder tumors . Because urothelial carcinoma can occur anywhere along the urinary tract, it is important to image the entire urinary tract for initial assessment and staging of disease and for follow-up. As such, CT and MR urography are the mainstays of imaging of urothelial carcinoma. PET/CT with fludeoxyglucose (FDG)—18F and 11C choline may also be used to assess lymph node metastases in patients with urothelial carcinoma.
Single bolus requires three different CT acquisitions, which exposes patients to much radiation. The split bolus technique decreases radiation exposure by splitting the contrast bolus into two, injecting only a portion initially and the second portion 7–8 min after initial contrast administration. CT image acquisition occurs 80–90 s after the second portion is administered. This allows for simultaneous visualization of the parenchymal and urographic phase [137, 138]. If performed on a dual energy CT scanner, a virtual non-contrast CT scan can also be obtained, thus forgoing the need for pre-contrast scan. The split bolus technique decreases radiation exposure however may obscure some subtle lesions.
Staging is often performed with CTU, which can readily detect detects metastatic disease and local invasiveness of tumor. CTU is less effective at assessing the degree of muscle invasion of lesions due to the lack of imaging contrast between muscle layers, which hinders staging of T1 versus T2 disease . However, one study showed sensitivity of 89% and specificity of 95% of detection of locally invasive disease, increasing ability to distinguish T2 versus T3 disease . Extravesical and extra-ureteral extension appear as fat stranding adjacent to the tumor. This may be confused with inflammatory or infectious changes, which have a similar appearance, though is typically more diffuse rather than focal. CTU can also detect distant metastasis in the abdomen and pelvis, such as pathologic lymphadenopathy and invasive disease into the adjacent pelvic organs .
Annual follow up with CTU for non-muscle invasive high-risk bladder tumors is recommend by the EAU . Follow up after treatment for muscle-invasive and metastatic bladder tumors are controversial and not specifically established, although regular imaging may help identify recurrent disease in a timely manner .
Follow up imaging after treatment of upper tract disease is recommended by the EAU based on invasiveness of primary disease. After radical nephroureterectomy, noninvasive tumors should be followed by CTU annually afterwards for a total of greater than 5 years. For invasive tumors, CTU should be performed every 6 months for 2 years, then annually for a total of greater than 5 years. If patients undergo kidney-sparing management, CTU should be performed at 3 months, 6 months, and then annually after resection .
MRI urography (MRU) takes advantage of its high soft tissue contrast and ability to acquire multiplanar and multiparametric sequences to serve as a powerful tool in detecting urothelial cancers. Unlike CTU, MRU does not expose patients to radiation. Thus, multiple phases of imaging after intravenous contrast administration can be obtained without harmful consequences, so single bolus technique is typically used to image urothelial tumors. The use of DWI can also further increase contrast between benign and malignant tissue . Urothelial neoplasms have similar appearance on CTU as they do on MRU; however, with the higher degree of soft tissue contrast, there is increased efficacy of staging, especially with the increased distinction of fat, which can clearly identify invasion of adjacent organs . Follow-up utilizing MRI is also useful as DWI can differentiate between recurrence and post-operative inflammation or fibrosis. Both recurrence and inflammation/fibrosis can enhance avidly, however, recurrence has been shown to restrict diffusion more . Follow up imaging after treatment may be performed with MRU in place of CTU if patients have renal failure.
Transabdominal imaging can be used to visualize larger tumors and secondary signs of obstruction such as hydronephrosis or hydroureter. The resolution of transabdominal ultrasound is not great enough to determine staging between T1 and T2 diseases. Transvesical sonography provides increased accuracy in local staging of bladder neoplasm compared to transabdominal ultrasound, however has a limited field of view and cannot assess extravesical disease effectively [147, 148]. Visualization and evaluation of the ureters is limited due to the lack of penetration of sound waves through retroperitoneal fat even on transabdominal ultrasound, thus cross-sectional imaging is needed for full assessment of urothelial carcinoma.
For evaluation of metastatic lymphadenopathy, PET/CT can be used. Detection of disease within the urinary tract is limited as it is obscured by radiotracer excreted by the kidneys, thus nuclear medicine findings must be correlated with CT findings . 18F-FDG radiotracer is typically used and can be used to evaluate for distant metastatic nodal disease, especially when renal insufficiency of a patient prevents imaging with intravenous iodinated contrast. Preliminary studies with 11C-choline and 11C-acetate, which has minimal renal excretion, has shown some promising results in identifying primary and metastatic urothelial carcinoma, although continued validation is needed .
Guidance for biopsy in the presence of an abnormal digital rectal examination or elevated PSA or a suspicious prostatic lesion detected on MRI. This includes use of Trans rectal US (TRUS) biopsy as part of the TRUS/MRI fusion technique.
Assessment of prostate volume before medical, surgical, or radiation therapy and to calculate PSA density.
Real-time guidance for the placement of brachytherapy seeds, and the planning and execution of all ablative techniques.
Assessment of lower urinary tract symptoms.
Assessment of congenital anomalies.
Assessment of Infertility.
Assessment of Hematospermia.
Assessment of Suspected recurrence in the prostatectomy bed