RADIOLOGY OF THE URINARY TRACT: INTRODUCTION

Radiology imaging has advanced dramatically since the first edition of this text. Technological advances and other innovations have greatly modified imaging of the urinary tract, with the dominant change being an increasing emphasis on cross-sectional modalities, especially CT. The result has been improved accuracy and earlier diagnoses of urinary tract disorders.

This chapter introduces the basic concepts in imaging of the urinary tract. The available imaging modalities and principles of their interpretation are discussed, especially in terms of its anatomy and its variants. The importance of choosing the most appropriate study for a given clinical scenario cannot be overemphasized and the next section of the chapter reviews technique selection. Clinical exercises and case examples are used to demonstrate important imaging concepts and diseases of the urinary tract. Finally, a bibliography of suggested readings is provided at the end of the chapter.

TECHNIQUES AND NORMAL ANATOMY

This section introduces the common radiologic techniques used in evaluation of the urinary tract. Emphasis is on a detailed description of each technique as it applies to the urinary tract. Also, a discussion of normal anatomy and some important fundamental concepts of interpretation is included. A basic knowledge of the gross anatomy is assumed with emphasis placed on the radiographic anatomic considerations.

Abdominal Radiography

Conventional radiographs (plain films) can occasionally provide important clues to diseases of the urinary tract. Radiographs of the abdomen when used to evaluate the urinary tract are often referred to as KUBs (kidney, ureter, and bladder). KUBs may serve a role as preliminary films (scouts) prior to an examination such as an intravenous urography, or they may be used as a general evaluation of the abdomen or the urinary tract. As stated, abnormalities of the urinary tract may be suggested on conventional radiographs and, among other things, the bones and soft tissues should be evaluated and abnormal densities, especially calcifications, should be sought. "Gas, mass, bones, stones" can be used as a reminder of main areas to examine on the KUB (Fig. 9–1). Soft tissue masses can occasionally be detected and suggest renal or pelvic lesions. Sclerotic bony lesions can suggest metastatic prostate cancer and lytic bony lesions can be seen with disseminated renal cell carcinoma. Additionally, the bony changes of renal osteodystrophy (diffuse bony sclerosis) may be identified on plain radiographs. Vertebral anomalies are associated with congenital malformations of the urinary tract.

In the setting of trauma, fractures of the lumbar transverse processes suggest possible renal injuries and pelvic fractures raise concern for coexistent bladder or urethral trauma. Air and calcifications should be specifically sought over the urinary tract. Emphysematous pyelonephritis, a urologic emergency with high mortality, is the result of a renal infection by gas-producing organisms and may be diagnosed on plain films by mottled or linear collections of air within the renal parenchyma. If emphysematous pyelonephritis is suspected, emergency computed tomography (CT) should be performed to delineate the extent of involvement and immediate urologic consultation obtained.

Finally, radiographs are useful for detecting and evaluating urinary tract calculi. It has been reported that 90% of calculi are radiopaque and can be identified on conventional radiographs. However, recent studies suggest that no more than 40% to 60% of urinary tract stones are detected and accurately diagnosed on plain radiographs. The sensitivity for detection of stones is limited when the calculi are small, of lower density composition, or when overlapping stool, bony structures, or air is obscuring the stones. Additionally, the specificity of conventional radiography is somewhat limited because a multitude of other calcifications occur in the abdomen, including arterial vascular calcifications, pancreatic calcifications, gallstones, leiomyomas, and many more. (More than 200 causes of calcification in the abdomen have been described.) Phleboliths, which are calcified venous thromboses, are especially problematic because they frequently overlap the urinary tract and are difficult to differentiate from distal ureteral stones. Lucent centers are a hallmark of phleboliths, whereas renal calculi are often most dense centrally. Additionally, oblique films and tomograms may be useful to differentiate true renal calculi from densities within the tissues anterior or posterior to the kidney.

Intravenous Urography

Intravenous urography (IVU), also known as intravenous pyelography (or more commonly, the IVP), has dominated imaging of the urinary tract for more than 50 years. Although recent advances in other techniques have substantially reduced its role, IVU remains an important study for some urinary tract disease processes. More importantly, however, decades of use of the IVU have established the fundamentals of imaging evaluation of the urinary tract. An understanding of these principles forms a foundation for radiologic interpretation of the urinary tract with the IVU or other more "advanced" imaging modalities. Thus, the IVU technique is explained with interspersed discussion of anatomy, normal variants, and, most importantly, some fundamentals of interpretation.

Although an urgent examination should not be delayed to prepare a patient for an IVU, overlying stool can obscure important detail on an intravenous urogram and therefore a mild bowel preparation of clear liquids and laxatives before an elective study is recommended. The study should always begin with a scout KUB. This has several purposes including detection of calcifications (which may be obscured after contrast material is injected), assurance of proper technique (patient positioning, exposure parameters) prior to contrast administration, and exclusion of contraindications to the study (retained barium, etc.). The scout film should encompass the area from the adrenals to the symphysis pubis, and sometimes more than one film may be required.

Intravenously injected iodinated contrast is excreted primarily by glomerular filtration in the kidney, opacifying the urinary tract as it progresses from the kidney through the ureter and to the bladder. Capturing this sequential "opacification" on radiographs is the fundamental basis of the IVU. There are many variations in the filming sequence for the urogram that are acceptable as long as it optimizes visualization of specific anatomy of the urinary tract during maximum contrast opacification.

Optimal visualization of the kidney is accomplished very early in the examination. Within 1 to 3 minutes after injection, the contrast bolus is filtered by the glomeruli and fills the nephron, resulting in intense opacification of the renal parenchyma; this phase of contrast opacification is called the nephrogram. Evaluation of the kidneys during the nephrographic phase is often enhanced with tomograms (nephrotomograms) (Fig. 9–2). The kidneys should be evaluated for their position, orientation, size, contour, and radiographic density. The kidneys are typically located at the level of the upper lumbar spine with the right kidney slightly lower than the left. They generally lie with their axes along the psoas muscles with the upper pole slightly more medial than the lower. Alterations in position and orientation of the kidneys may be related to congenital anomalies such as pelvic kidneys or may be secondary to mass effect from an adjacent lesion.

The size of the kidneys is somewhat variable depending on age and sex of the patient, but on the intravenous urogram the kidneys normally range from 11 to 14 cm. The right kidney is typically slightly smaller than the left. Measurement of renal size is also dependent on the examination. For example, on the IVU the kidneys appear artificially larger due to magnification. To account for this as well as other parameters such as overall body size, a generalization is that the kidneys should measure between three and four lumbar vertebra lengths. Additionally, the kidneys should be symmetric in size with a discrepancy greater than 2 cm requiring an explanation. There are a number of causes of abnormal renal size, ranging from incidental anomalies such as congenital renal hypoplasia to significant conditions such as renal artery stenosis (small kidney) or infiltrating renal neoplasm (large kidney).

The kidneys should have a reniform shape and a smooth contour. Embryologically, the kidney is composed of lobes that smoothly fuse to create the kidney; however, not uncommonly, small residual clefts remain where the lobes fail to completely fuse, a condition referred to as persistent fetal lobation. This must be distinguished from true renal scarring, which most often results from chronic vesicoureteral reflux/chronic bacterial pyelonephritis or from renal infarcts. The clefts of fetal lobation occur between lobules, i.e., the cortex between calyces, whereas scarring typically occurs in the cortex over the calyx. Additionally, the calyces are generally distorted and rounded with chronic reflux disease. Bulges to the renal contour are of more concern because they raise suspicion for a mass. A key concept in evaluation of a possible mass is parenchymal thickness as measured from calyces to edge of the kidney. A fairly common normal variant is the dromedary hump, which is a bulge created along the lateral mid aspect of the left kidney related to splenic impression on the kidney. This bulge is differentiated from a mass by a typical calyx that extends out toward the bulge, keeping the parenchymal thickness similar to the rest of the kidney. A true mass results in increased parenchymal thickness or even mass effect on the adjacent calyces, which are displaced away from the bulge. The radiographic density of the kidneys following contrast injection is related to arterial supply, renal function and excretion, and venous outflow. Alterations in any of these parameters may result in abnormalities of one or both of the nephrograms. For example, ureteral obstruction results in a delayed and increasingly dense nephrogram.

Soon after the nephrographic phase, contrast begins filling the intrarenal collecting system including the calyces and renal pelvis. This portion of the study is termed the pyelographic phase (Fig. 9–3). Several films are used to evaluate the collecting system (intrarenal collecting system and ureter) beginning at our institution with a KUB obtained 5 minutes after contrast injection. Evaluation of the intrarenal collecting system is improved by placing a compression device over the lower abdomen, thereby compressing the ureters on the sacrum, resulting in increased distention and improved visualization of the proximal ureters and renal collecting system. Compression is contraindicated in several settings, including ureteral obstruction, abdominal aortic aneurysm, and recent abdominal surgery. Typically, compression is applied after the 5-minute KUB has been obtained and evaluated by the radiologist. A film of the kidneys is performed after 10 minutes, allowing for the compression of the ureter to result in proximal distention. The intrarenal collecting system consists of calyces, infundibula, and the renal pelvis. Normally, each kidney consists of 7 to 14 evenly distributed calyces. The individual renal calyx, from the Latin for "chalice," is a delicate appearing cup-shaped structure. Not uncommonly, partial fusion of the calyces occurs, especially in the renal poles, creating the compound calyx. Other calyceal variants occur, including variants of number (polycalycosis, unicalyx kidney) and size (megacalycosis, microcalyx) and must be differentiated from true pathology. The calyces may not be visualized if compressed or may be deviated by masses. The normal delicate, cup-like appearance can be distorted or irregular in conditions such as papillary necrosis, tuberculosis, or transitional cell carcinoma. Subtle rounding or ballooning of the calyces is one of the earliest signs of urinary tract obstruction. Diverticula may arise from the calyces, creating a haven for stone formation, recurrent infection, or even transitional cell malignancy. The renal pelvis is also quite variable in appearance. A common variation is the so-called "extrarenal" pelvis, where the pelvis lies outside the renal sinus. In this setting the pelvis tends to be more prominent and rounded, mimicking hydronephrosis. This can be differentiated from true obstruction by normal appearing calyces. The renal pelvis should be evaluated for filling defects and mass effect.

Release of compression results in a bolus of contrast material entering the ureters, which is evaluated with a KUB, and often with oblique films, obtained immediately after release of the device at 15 minutes (Fig. 9–4). Occasionally, fluoroscopy may be utilized to visualize suspicious areas of the ureter not seen on the conventional films. The ureter extends from the ureteral pelvic junction to the ureteral vesicle junction. Proximally, the ureter passes over the psoas muscle and should generally lay just lateral to the lumbar spine. The midportions of the ureters course over the lateral sacrum with the distal portion gently curving laterally in the pelvis before entering the bladder. The ureter is an actively peristalsing structure that is not normally seen in total on the IVU. In fact, complete visualization of the ureter may suggest distal obstruction. The ureter should be inspected for filling defects, which can be caused by stones or tumor, and should be symmetric in size. Evaluation of the ureteral course is important. Typically, the ureter should be no more lateral than the tips of the lumbar transverse processes and no more medial than the lumbar pedicles. Deviations of the normal ureter generally suggest extrinsic diseases, such as mass lesions. However, in patients with large psoas muscles the ureters may be displaced laterally as an incidental result.

Finally, the bladder is opacified last on the study beginning around 5 minutes after injection. Early filling films, later distended films, and postvoiding images complete the evaluation of the bladder (Fig. 9–5). The bladder is an oval to rounded structure that normally lies just above the pubic symphysis on the IVU. Not uncommonly, especially on early filling films, some extrinsic compression of the bladder can be seen due to the sigmoid colon. In women, the dome of the bladder may normally be indented by the uterus. These normal findings must be differentiated from abnormal extrinsic mass effects. Bladder wall thickness can sometimes be visualized and assessed, especially if thickened. Additionally, the bladder mucosa should be scrutinized for irregularity or filling defects that may suggest a mass.

Films in addition to the typical IVU sequence may be used, taking advantage of the greater density of contrast material than that of urine, and include prone and upright films as well as delayed films as needed. Regardless of the filming routine, the best IVU is the one monitored by the radiologist and tailored for the patient based on the study indication.

Retrograde Pyelography/Cystography/ Urethrography

Direct injection of water-soluble iodinated contrast material is a useful method of examining various regions of the urinary tract. The advantage of this method of evaluation is the direct control over the contrast injection rather than reliance on secondary excretion from the kidney. Retrograde pyelography, often carried out in conjunction with cystoscopy, is performed by placing a small catheter into the distal ureter. Contrast material is then injected through this catheter into one or both ureters. Fluoroscopy and conventional radiographs should then be obtained. This study usually results in excellent evaluation of the ureter and intrarenal collecting system. The ureter is typically seen in its entirety, which rarely occurs with other imaging studies. Interpretation is similar to that of the IVU with the caveat that the contrast within the collecting system is under greater pressure than physiologic conditions and mild ballooning of the calyces as well as occasional extravasation can occur normally.

Imaging of the bladder is performed with a cystogram, for which a catheter is placed into the bladder and contrast material is then injected. The contrast material is optimally injected under fluoroscopic observation but occasionally is performed with only static conventional radiographs, such as in the trauma setting. Anatomic considerations and evaluation are similar to the IVU with a few caveats. One advantage to cystography is that vesicoureteral reflux can be evaluated during the conventional cystogram unlike during IVU. Recently, CT cystography, in which after contrast instillation CT imaging is utilized instead of conventional films, has been used, especially in the setting of trauma to evaluate for bladder injury.

The urethra may be evaluated with contrast material via two methods. In one, the urethra is evaluated during voiding, often following a cystogram (voiding cystourethrogram or VCUG). Alternatively, a retrograde study may be performed (retrograde urethrogram). The urethra in the male consists of four portions, including the prostatic, membranous, bulbous, and penile portions. During voiding, the urethra is fairly uniformly distended and tubular in appearance (Fig. 9–6). On a retrograde study, the more posterior urethra (prostatic and membranous) is often contracted and seen as a thin wisp of contrast. The female urethra appears as a short, slightly funnel-shaped tubular structure during voiding (Fig. 9–7) (Note that a special catheter is required for evaluation of the female urethra in a retrograde fashion.) The urethra in males is generally evaluated for injuries but may also be examined for filling defects, masses, strictures, and fistula. The female urethra is most commonly examined for diverticula.

Ultrasonography

Ultrasonography is a useful technique for evaluation of the urinary tract, made especially attractive by its ease of use and lack of complications (no contrast material or ionizing radiation). The kidneys are generally well seen in all but the largest of patients (Fig. 9–8). The renal medulla is hypoechoic (darker) relative to renal cortex and can be identified in most normal adults as cone-shaped central structures. (Occasionally, this corticomedullary distinction is not visible.) The renal cortex is isoechoic or slightly hypoechoic compared with the echogenicity of the adjacent liver. Renal echogenicity exceeding that of the liver is abnormal and requires explanation. Most commonly hyperechoic kidneys are seen in the setting of medical renal disease, such as end-stage hypertensive glomerulosclerosis.

In addition to echogenicity, the kidneys should be assessed for size, location, and symmetry. Scarring and masses can be evaluated. Unlike the IVU, where masses are often nonspecific, ultrasonography allows a more detailed evaluation including the ability to confidently diagnose the most common renal mass—the simple cyst. Solid masses, however, remain nonspecific and generally require further evaluation. Like the IVU, there are normal variants that can mimic mass lesions including dromedary humps and persistent collections of normal renal tissue within the substance of the renal parenchyma referred to as persistent columns of Bertin. Additionally, the parenchyma near the renal hila may appear prominent as well, occasionally mimicking a mass. Each of these lesions may be distinguished by their echogenicity being equal to surrounding tissue, lack of mass effect, and characteristic location. Occasionally, additional imaging may be required in equivocal cases.

The renal sinus is the area engulfed by the kidney medially, harboring the renal pelvis, arteries, veins, nerves, and lymphatics that enter and exit the kidney, all contained within a variable amount of fat. Fat is typically brightly echogenic on ultrasound, and fat within the renal sinus dominates the ultrasonographic appearance, creating what is known as the central echo complex. The size of the central echo complex is variable, often more prominent in the elderly and minimal in the child. Absence of the central echo complex may suggest a mass such as a transitional cell carcinoma replacing the normal fat. Alternatively, the complex may be very prominent in the benign condition of renal sinus lipomatosis. Calcifications are characteristic on ultrasound, being brightly echogenic and resulting in shadowing posteriorly as the sound waves are attenuated. Renal stones or calcifications may be detected within the renal parenchyma or in the intrarenal collecting system. The echogenicity of the normal renal sinus, however, creates difficulty because sometimes it obscures or mimicks small stones.

Ultrasonography is also excellent for detecting hydronephrosis with the distended collecting system being easily recognized within the central echo complex. The ureters are not normally seen on ultrasound due to obscuring overlying tissue and their small size. Evidence of their patency may be verified by Doppler detection of urine rapidly entering the bladder from the distal ureters, i.e., distal ureteral jets (Fig. 9–9). The bladder is seen as a rounded or oval anechoic (fluid) structure in the pelvis. The bladder may demonstrate mass lesions, such as transitional cell carcinoma, or stones. The urethra is not typically seen on an ultrasound image although urethral diverticula may occasionally be demonstrated.

Computed Tomography

CT is now the dominant radiologic imaging modality for evaluation of the urinary tract. The advent of multidetector spiral CT has further propelled CT to the forefront of urologic imaging. Several factors make CT effective in assessing the urinary tract. The high contrast resolution and spatial resolution afforded by CT allow detection and evaluation of subtle differences in very small structures. Mathematical calculations of the attenuation of the CT x-ray beam allow quantitative evaluation of the relative density of structures (i.e., their Hounsfield units), and it is through these "CT numbers" that much unique diagnostic information of the urinary tract is gained. Examinations can be performed amazingly fast because thin-slice CT scans of the entire urinary tract are now obtainable in just a few seconds. Finally, the wide availability and relative safety of CT furthers its appeal.

CT scans of the urinary tract may be performed with and/or without intravenous iodinated contrast material depending on the indications. Noncontrast studies may be performed to evaluate stone disease and other calcifications. Additionally, noncontrast views of the kidneys serve as a baseline to evaluate for lesion enhancement after contrast administration, a critical factor in mass evaluation. On noncontrast examinations the kidneys are homogeneous and have a density similar to most soft tissue (Fig. 9–10). In all but the thinnest adults, fat is seen surrounding the kidneys and extending into the renal sinus. Contrast-enhanced studies of the kidneys are best performed with a mechanical power injector.

With rapid scanning and contrast bolus timing, several sequential phases of opacification within the kidney can be delineated by CT including corticomedullary, nephrographic, and excretory phases. The corticomedullary phase can be seen if scanning is performed during the first 20 to 90 seconds after contrast administration and represents the early preferential blood flow to the renal cortex (Fig. 9–11); however, small masses could be missed during this phase, being obscured within the unenhanced renal medulla. Subsequently, contrast begins to pass into the distal collecting tubules within the renal medulla, resulting in a more homogeneous opacification of the renal parenchyma, termed theCT nephrographic phase. This generally occurs around 2 to 4 minutes after contrast medium injection. Finally, the excretory phase is seen when contrast opacifies the collecting system (Fig. 9–12). Each different phase of opacification may better demonstrate different disease processes and thus various scanning protocols are used to evaluate the kidneys depending on the indication.

One of the major recent advances in imaging has been the ability to noninvasively evaluate the vascular system, and thin-section early CT images accurately demonstrate the main arterial and venous structures of the kidney (Fig. 9–13). Just as with IVU or any modality, the kidneys should be evaluated for position, orientation, size, and radiographic density. Unlike IVU, however, CT provides much greater specificity regarding renal disease, including mass lesions. The ubiquitous simple cyst is generally easily diagnosed and differentiated from the more concerning solid mass. Fat within a solid mass generally allows the diagnosis of the benign angiomyolipoma. The solid, non-fat-containing mass in the adult should be considered a renal cell carcinoma until proven otherwise. CT is sensitive in detecting renal masses and, although not always supplying a specific diagnosis, typically provides important information allowing for appropriate patient management.

CT is also useful in staging renal neoplasms. Non-neoplastic renal disease, such as trauma and complicated infections, is accurately demonstrated on CT images, which provide specific information regarding the extent and severity of the process. Unlike conventional radiographs, IVU, or even ultrasound, CT provides a thorough evaluation of the adrenal glands, which appear as small inverted Y-shaped structures above the kidneys (Fig. 9–14). Additionally, the remainder of the retroperitoneum, containing fat, the normal occupants of the retroperitoneum (kidneys, adrenals, pancreas, duodenum, and parts of the colon), and vascular structures, is well seen by CT, and diseases such as inflammation, infection, and tumor are easily demonstrated.

Until recently, the ureters and their disorders were the domain of the IVU or retrograde pyelogram. However, since the mid-1990s, spiral CT has become the study of choice for evaluating suspected ureteral stones. Two factors were key in this transition. First, the invention of spiral CT technique allowed for continuous coverage of the entire ureter without skip areas. Second, virtually all stones are dense and conspicuous on CT. There are many other advantages to using CT to evaluate suspected ureteral stones including speed of the examination, identification of alternative explanations for the pain (appendicitis, divertculitis, aneurysm, etc.), and elimination of intravenous contrast complications (because the study is performed without contrast). Scans no thicker than 5 mm are performed from the top of the kidneys to the symphysis pubis. The ureter can be visualized and followed from the renal pelvis to the bladder in most cases and appears as a tubular 2- to 3-mm fluid structure surrounded by retroperitoneal fat (Fig. 9–15). Stones can be diagnosed by their high density and location within the ureter. Secondary signs of obstruction have been described, including dilation of the proximal ureter, hydronephrosis, renal enlargement and stranding in the fat surrounding the kidney. As on the IVU, phleboliths can prove troubling due to their frequent close approximation with the distal ureter; however, their central lucency, lack of surrounding inflamed ureteral wall, and lack of secondary signs of obstruction usually allow their distinction.

When contrast material has been administered, the ureters appear as dense, rounded structures in the retroperitoneum (Fig. 9–16). On CT, the bladder appears as a rounded water or contrast density structure in the pelvis. One pitfall is that the first few images of contrast beginning to enter the bladder may mimic a bladder mass. The bladder wall should be evaluated for thickening and irregularity, which may suggest hypertrophy, inflammation, or carcinoma. Stones may be detected within the bladder. The urethra is not normally seen on CT.

Magnetic Resonance Imaging

Just like CT, technical advances in magnetic resonance (MR) imaging have led to increasing use in urinary imaging. Fast-scanning techniques that allow breath-hold imaging, combined with the spectacular tissue contrast of MR imaging and the ability to directly image in any plane, make this an attractive modality for evaluating the urinary tract. Lack of ionizing radiation adds to its appeal, but cost, availability, claustrophobia, and the contraindication of certain materials including pacemakers remain major drawbacks. Finally, MR imaging of the kidney is performed with gadolinium as the contrast agent, not iodinated contrast material. In renal imaging one of the main advantages of gadolinium versus iodine is the virtual lack of nephrotoxicity at clinical doses. On MR imaging, the kidneys appear to be of variable signal intensity, depending on the imaging factors and, like CT, contrast-enhanced phases of imaging (arterial, corticomedullary, nephrographic, and excretory) are all visible (Fig. 9–17).

Specific imaging sequences are designed to manipulate imaging factors to allow for optimum evaluation of the particular clinical concern. The ability to image in any plane creates a unique advantage for MR imaging (Fig. 9–18). The kidneys should be evaluated in a fashion similar to that of other modalities. Recently, two techniques have been developed to allow the ureters to be evaluated. In one method, the high signal intensity of water (urine) is utilized to make the ureters conspicuous compared to other tissues. In the other technique, the MR imaging contrast agent gadolinium is given and the ureters opacify similar to IVU or contrast-enhanced CT. The bladder is well visualized, similar to CT. Finally, the adrenal glands are well seen, as in CT, and the normal shape is the same as that described for CT and the signal intensity depending on particular imaging parameters. The ureters, bladder, and adrenals are evaluated in a fashion similar to that used for CT.

Nuclear Medicine

The basic technique of a nuclear medicine study is discussed in Chapter 1; here, we briefly examine the more specific role in evaluation of the urinary tract. In general, the value of nuclear imaging in the urinary tract is several-fold: Functional information related to quantifiable collected data is obtained, the radiation dose is lower than that for traditional radiographic techniques, and the incidence of complications is very low.

Renal evaluation is typically performed by intravenous bolus injection of renal-specific agents such as technetium-labeled mercaptoacetyltriglycine (^99mTc-MAG₃). Images are acquired every few seconds that demonstrate renal blood flow, with additional images obtained over several minutes that show renal uptake and excretion. Recall that the recorded data can be used to produce images, but are also quantifiable and employed to generate time–activity curves (Fig. 9–19).

Information about renal perfusion, morphology, relative function of each kidney, and excretion can be extremely useful in evaluation of conditions such as renovascular hypertension, obstruction, and renal transplant examination. Although anatomically oriented data can be obtained with other radioisotopes that aggregate more in the renal parenchyma, in general, nuclear medicine renal studies suffer from fairly low spatial resolution and, therefore, are often used in conjunction with other imaging studies.

Radionuclide cystography is another useful test used to diagnose and monitor vesicoureteral reflux. Here, technetium pertechnetate is mixed with saline and infused into the bladder with subsequent images obtained over the urinary tract. This study is quite sensitive for the detection of significant reflux but at a considerably lower radiation dose than conventional cystography. Another important study is the radioactive iodine labeled metaiodobenzylguanidine (MIBG) examination. MIBG collects in adrenal medullary tissue and is useful in diagnosis and evaluation of pheochromocytoma. Additional radioisotopes are available for urinary tract imaging, usually in fairly specific roles.

Angiography

The role of angiography as a diagnostic tool continues to diminish with the increasing accuracy of noninvasive techniques to evaluate the vascular system. The renal arteriogram is performed after puncture of a more peripheral vessel such as the common femoral artery, with advancement of a catheter into the renal artery origin. Contrast material is injected via the catheter and rapid, typically digital, conventional radiographic images are obtained. The renal arterial vessels are well demonstrated, along with nephrographic images of the kidney and views of the venous drainage (Fig. 9–20). Delayed images may be obtained to demonstrate the renal collecting system. The still superior spatial resolution of angiography permits detailed evaluation of the renal arterial supply and has a small but important diagnostic role in evaluating the small vessels of the kidney for such diseases as vasculitis and fibromuscular dysplasia. The angiogram plays little role in diagnostic evaluation of the renal parenchyma, having been supplanted by cross-sectional imaging techniques. The main role for angiography today, as discussed later, is aiding and guiding interventional techniques.

TECHNIQUE SELECTION

No one ideal technique is yet available for the comprehensive evaluation of the urinary tract. Each technique has strengths and weaknesses that affect their thoroughness and accuracy in evaluating urinary tract diseases and also patient complaints. Importantly, imaging techniques are not necessarily exclusive and in some circumstances are complementary—taken alone they may not provide enough information but together allow a correct clinical diagnosis. A knowledge of which tests are most appropriate for a given clinical question is paramount for physicians involved with the treatment of urinary tract disease. Issues of cost, complications, and time are consequences of an injudicious study choice. However, most importantly, the diagnosis of a patient's condition may not be made unless the appropriate test has been used to evaluate the condition.

The plain radiograph (KUB) has fairly limited use for evaluating the GU tract. Although abnormal "stones, bones, gases and masses" may be demonstrated by the KUB, the utility of the study is limited by its lack of sensitivity and specificity. The KUB can be used effectively to follow radiographically visible stone disease such as assessing stone burden or ureteral stone passage; it is also used to assess stent position, especially ureteral stents. The KUB is also essential as a screening image prior to an IVU or other studies.

Several factors have made the IVU central to evaluating the urinary tract for many decades. The IVU is able to assess both function and morphology, to evaluate the entire urinary tract, and has high spatial resolution allowing for subtle lesion detection, especially of the collecting system. Its lack of sensitivity and specificity for many disorders, however, has always been a shortcoming. For example, less than 50% of renal masses less than 3 cm will be detected on an IVU. Moreover, even mass lesions detected are nonspecific and require further evaluation with additional modalities. However, due to the strengths of the study as discussed earlier, the IVU is still a useful test in certain circumstances. The IVU can be used to evaluate the urinary tract for congenital anomalies, to assess obstruction, and to evaluate for mucosal lesions such as transitional cell carcinoma of the upper tracts or papillary necrosis. As technology has advanced, the IVU has been supplanted in many indications by more modern modalities. The IVU is no longer generally indicated for assessing renal masses, urinary tract infection, trauma, ureteral colic, and vascular diseases, such as renovascular hypertension.

The conventional radiographic techniques that utilize direct contrast injection (retrograde pyelogram, cystogram, and urethrogram) maintain specific roles. For example, the voiding cystogram for evaluating ureteral reflux and the retrograde male urethrogram in suspected urethral injuries remain studies of choice.

Nuclear medicine sustains its role in functional evaluation of the urinary tract, especially the kidney. Renal scintigraphy is an important tool in the assessment of renal function and can be useful in evaluating obstruction, renovascular hypertension, renal transplants or an occasional problematic pseudomass. The ability to quantitatively assess relative renal function is frequently an important issue for the surgeon, determining whether nephrectomy or attempted renal sparring surgery is most appropriate. The nuclear medicine cystogram, due to its high sensitivity and lower radiation dose, remains a key tool in evaluating and following ureteral reflux. The MIBG study plays a unique role regarding the pheochromocytoma. The diagnosis of pheochromocytoma is generally made with a classic clinical history combined with confirmatory biochemical evidence, with MIBG providing a confirmatory diagnosis. More important is the role for MIBG in detecting metastatic or recurrent disease or in locating extra-adrenal lesions. Molecular imaging promises to revolutionize radiology and may play a future role in the urinary tract, especially in oncologic imaging.

The safety and ease of ultrasound solidify the utility of this modality, especially in pediatric imaging. Ultrasound is useful in evaluating the kidney for masses, scarring, and hydronephrosis, especially in children. For example, it is used to exclude postobstructive (hydronephrosis) etiologies of acute renal failure, to evaluate for sequelae (scarring) of vesicoureteral reflux in children, and to diagnose simple renal cysts. Ultrasound is generally the study of choice in evaluating the renal transplant as well. However, the relatively small and sometimes technically limited (in large patients) field of view, lack of visualization of the ureters, and lack of functional assessment limit the use of ultrasound in some circumstances. For instance, in the setting of obstruction ultrasound may demonstrate hydronephrosis, but often the etiology of the obstruction, such as ureteral stone or mass, is not identified. Additionally, solid renal masses are nonspecific on ultrasound and require further imaging, usually with CT. Finally, the ultrasound has only moderate sensitivity for detecting renal stone disease.

As stated in the discussion of techniques, the diagnostic role of conventional angiography continues to diminish as noninvasive CT and MR angiography develop. Patient comfort, speed of examination, diminished complications, and reduced cost all favor noninvasive vascular imaging. However, two main factors allow for a persistent important role for the angiogram. Compared to CT and MR, conventional angiography still has superior resolution for small-vessel evaluation. Thus, diagnostic angiography may play a role in diagnosis of small-vessel renal disease such as polyarteritis nodosa. More importantly, unlike CT and MR, catheter angiography allows for the ability to simultaneously treat abnormalities diagnosed at the time of angiography. For example, although many modalities are used to evaluate for renovascular hypertension, angiography alone allows for treatment at the time of diagnosis as in the patient with fibromuscular dysplasia whose hypertension may be cured with transluminal angioplasty at the time of arteriography. The angiogram is similarly used in acute renal hemorrhage, acute arterial obstruction, and occasional renal mass management. The important and wide-ranging role of the interventional radiologist in the management of urinary tract disease is beyond the scope of this chapter.

MR imaging continues to grow in utility for evaluating the urinary tract and is the study of choice in certain instances. Like CT, MR imaging has excellent spatial and contrast resolution and can evaluate the renal vasculature and renal and adrenal anatomy, characterize lesions, and evaluate the bladder and prostate. Fluid/contrast-enhancing techniques are beginning to allow evaluation of the ureters and the remainder of the collecting system. MR imaging is thus an excellent choice to screen for renovascular hypertension, to stage renal cell carcinoma, to problem solve difficult renal masses, and to evaluate the adrenal mass, and it is gaining favor in evaluation of certain ureteral and bladder conditions. Finally, unlike CT, MR imaging does not use iodinated contrast material and is especially useful in the setting of chronic renal insufficiency when an iodinated contrast agent is contraindicated. However, high cost, limited availability, and contraindications such as pacemakers and claustrophobia limit the widespread use of MR imaging.

Finally, CT is now the examination of choice for urinary tract imaging. From the adrenal glands to the prostate, the CT scan is the preferred study for many GU conditions including trauma, complicated infections, renal and adrenal masses, neoplastic conditions, retroperitoneal disease, renovascular hypertension, and ureteral colic. CT may be the sole study needed or may serve as an adjunct to other studies. With the advent of thinner slices and faster scans, the ability of CT to evaluate the urinary tract mucosa will emerge, and the CT urogram will almost certainly completely replace one of the last indications for the IVU. As discussed previously, CT benefits from wide availability, speed, high contrast and spatial resolution, and patient ease. CT is limited when iodinated contrast is contraindicated or radiation exposure is of special concern such as in pregnancy.

In summary, there is as yet no one comprehensive best imaging examination for the urinary tract; each has its advantages and disadvantages and their value depends on indications of the study. The 25-year-old pregnant woman with hematuria is quite different from the 75-year-old man with the same symptom, and the issue of which imaging modality is best will vary with these considerations. The physician must combine evidence-based knowledge of the accuracy and utility of various studies with the art of medicine, combining science with finesse to ultimately result in the best possible evaluation and care of the individual patient. Finally, although the requesting physician should be well informed about the utility, accuracy, strengths, and weaknesses of available tests, the best care, especially in the fast changing field of imaging, is provided by close consultation between the physician and radiologist.

EXERCISE 9-1: ADRENAL MASSES

Clinical Histories:

Case 9-1. A 52-year-old male patient presents with vague right abdominal pain. A CT scan of the upper abdomen is shown in Fig. 9–21.

Case 9-2. A 47-year-old patient with newly diagnosed lung cancer presents to the emergency room for right flank pain. A CT scan without contrast of the upper abdomen is shown in Fig. 9–22.

Case 9-3. A 39-year-old male patient presents with refractory hypertension and episodes of headaches and palpitations. A CT scan with contrast of the abdomen was performed (Fig. 9–23).

Questions:

Radiologic Findings:

9-1. In Fig. 9–21, a predominantly fat-containing 5-cm mass (arrow) is seen within the right upper abdomen. The mass lies just medial to the right lobe of the liver and is seen to arise from the posterior aspect of the adrenal gland. (B is incorrect.) Although subtle, the thin rim of tissue surrounding the lesion demarcates the mass and differentiates it from normal adjacent retroperitoneal fat. The fatty nature of the lesion is confirmed by the low-density tissue within the mass, similar to that of adjacent normal retroperitoneal and subcutaneous fat. Fat is rare within adrenal metastasis. (A is incorrect.) Although the retroperitoneal sarcoma is a consideration for a retroperitoneal fatty mass, the adrenal origin of the lesion as well as the fatty nature make adrenal myelolipoma the most likely diagnosis. (C is the correct answer to Question 9-1.)

9-2. In this case, the diagnosis or exclusion of metastatic disease is one of the most important issues facing the radiologist in daily practice. The diagnosis of metastatic disease allows appropriate therapy for the patient including the possible prevention of unnecessary surgery. Perhaps more importantly, misdiagnosing a benign lesion as metastatic disease may mistakenly prevent potentially curative therapies such as surgery. In Fig. 9–22, there is a small 2-cm homogeneous mass (arrow) arising from the medial limb of the right adrenal gland. Recall that the density of a lesion can be quantitated on CT with Hounsfield unit measurements (although not shown, the Hounsfield unit measurements of the mass was 8 HU). Acute adrenal hematomas are high-density masses on noncontrast CT scan measuring between 50 to 90 HU. (D is incorrect.) Adrenal carcinomas are typically large heterogeneous lesions and are quite rare. (C is incorrect.) The distinction between adrenal metastasis and adenoma is a critical one. Although there can be overlap in their appearances, certain imaging characteristics of adrenal adenomas allow a confident diagnosis in the vast majority of cases as we will see. Even with a known primary malignancy, however, statistically the most likely etiology of a small adrenal mass is benign adrenal adenoma. (B is the correct diagnosis to Question 9-2.)

9-3. In this case, the CT scan of Fig. 9–23 demonstrates a 4-cm solid mass (arrow) appearing just anterior to the left kidney. Note the fat plane that clearly shows the mass to not be arising from the kidney. No specific characteristics are seen such as fat. The lesion is denser than surrounding muscle, making a cyst unlikely. (C is incorrect.) Adrenal lymphoma is typically bilateral and usually shows diffuse enlargement of the adrenal glands and is typically accompanied by retroperitoneal adenopathy. (D is incorrect.) Although metastatic disease can have variable appearances and cannot be radiographically excluded, the lesion is also typical for a pheochromocytoma and, given the clinical history, is the most likely diagnosis. (A is the correct answer to Question 9-3.)

Discussion:

The adrenal mass is a common problem for the radiologist and is being incidentally diagnosed more often with the increased use of cross-sectional imaging, especially CT and MR imaging. In fact, the term adrenal incidentaloma has been coined for the small adrenal mass identified on imaging studies obtained for other reasons. Although there are many causes of adrenal masses, the most common include benign adenomas, metastatic disease, adrenal carcinoma, and myelolipomas.

The most common adrenal mass is the adrenal adenoma. Although they can be hyperfunctioning and result in clinical syndromes, the majority of adrenal adenomas are not functioning and are diagnosed incidentally. Distinguishing these incidentalomas from more significant pathology is critical. Fortunately, most adenomas have specific characteristics that allow a confident diagnosis. Adenomas are similar to normal adrenal cortical tissue in that they contain a high proportion of cellular lipid material. This results in a low-density appearance and Hounsfield measurements on unenhanced CT. MR imaging can be used to demonstrate the same characteristic by using special imaging sequences that reveal intracellular lipid. This feature along with other characteristics of adenomas, including small size and uniform appearance, allow a confident imaging diagnosis in most patients. The adrenal gland is a common site of metastatic disease, with breast and lung carcinoma being most common sources. The imaging characteristics of metastatic disease are quite variable. Lesions may be unilateral or bilateral, homogeneous or heterogeneous in appearance (Fig. 9–24). The larger the metastatic lesion, generally, the more necrosis and hemorrhage and the more heterogeneous the lesion appears. Smaller lesions tend to be more uniform. Fortunately, metastatic disease does not contain high intracellular lipid-like adenomas and thus do not show the lipid-type imaging changes that characterize adenomas. However, metastatic disease can be indistinguishable from other adrenal pathology, and histologic confirmation with biopsy may be necessary. Pheochromocytomas are an unusual catecholamine-producing tumor that most commonly arises in the adrenal medulla, although in 10% of cases they may arise in an extra-adrenal location. Most tumors arise sporadically although a small percentage occur in certain syndromes. Most pheochromocytomas produce a constellation of symptoms referable to their catecholamine production including hypertension and episodic headaches and palpitations. Most pheochromocytomas appear as a nonspecific adrenal mass on CT. Many of these lesions are fairly homogeneous solid masses. However, necrosis, calcification and cystic formation all occur. On MR imaging, the diagnosis may be suggested by the fairly specific finding of a very bright adrenal mass on T₂-weighted images. Finally, MIBG, which collects in adrenal medullary-type tissue, can provide important information about these tumors. Although they may be used to confirm the diagnosis of a suspected adrenal pheochromocytoma, a more important role for MIBG imaging is in the evaluation of metastatic disease or recurrent tumor or for the localization of extra-adrenal lesions. The MIBG scan typically shows a brightly intense area of activity at the site of the lesion (Fig. 9–25).

EXERCISE 9-2: RENAL MASS

Clinical Histories:

Case 9-4. A 35-year-old woman with recurrent urinary tract infections underwent an IVU. A single view of the kidneys obtained during this study (Fig. 9–26) reveals another finding.

Case 9-5. A 45-year-old woman presents for a right upper quadrant ultrasound to evaluate for gallbladder disease. An image of her right kidney obtained during this study is displayed in Fig. 9–27A . A subsequent CT scan of the lesion is shown in Fig. 9–27B .

Case 9-6. A 55-year-old man presents with a history of right flank pain and hematuria. A CT with contrast of the abdomen is obtained (Fig. 9–28).

Case 9-7. A 60-year-old man presents with left flank pain and hematuria. A CT scan of the abdomen is shown in Fig. 9–29.

Questions:

Radiologic Findings:

9-4. In this case, the nephrotomogram (Fig. 9–26) shows a well-defined, rounded lesion (arrows) that arises from the midportion of the right kidney and causes slight smooth distortion of the renal calyceal morphology. This lesion has no distinguishable wall and has a distinct interface with the adjacent renal parenchyma. The lesion appears lucent compared with normal enhancing renal parenchyma suggesting it may be water density. Therefore, C is the correct answer to Question 9-4.

9-5. In this case, the ultrasound image (Fig. 9–27A ) reveals a hyperechoic lesion with echogenicity similar to that of adjacent perirenal fat. However, this appearance on ultrasound is nonspecific and requires further evaluation and a CT should generally be obtained. The CT (Fig. 9–27B ) shows that this lesion (arrow) does indeed contain fat. The presence of definitive fat within a renal mass is virtually pathognomonic for the diagnosis of angiomyolipoma, which is a benign lesion containing fat, blood vessels, and smooth muscle. Therefore, D is the correct answer to Question 9-5.

9-6. In this case, the lesion seen in Fig. 9–28 is an inhomogeneous soft tissue mass (arrow) arising from the right kidney, which proved to be a renal cell carcinoma. It displays many of the common CT characteristics of renal cell carcinoma including a somewhat rounded shape with irregular margins, enhancement with IV contrast, and inhomogeneity (which can be due to hemorrhage, proteinaceous debris, and even calcifications). Renal cell carcinomas almost never contain fat, making C the correct choice.

9-7. In this case, the lesion has imaging and clinical characteristics indistinguishable from renal cell carcinoma. However, the lesion (Fig. 9–29, arrows) is an oncocytoma, a benign tumor arising from the distal tubule or collecting ducts. Classically, oncocytoma is often associated with a characteristic central stellate scar. However, scarring can be seen in a renal cell carcinoma as well and these lesions cannot be reliably distinguished by imaging alone, making Cthe correct answer for Question 9-7.

Discussion:

These cases demonstrate examples of the most common renal masses, both benign and malignant. In general, all of these renal masses expand and displace normal renal parenchyma and normal collecting system structures. They are distinguished from infiltrating processes (such as infiltrating neoplasms, infections, and infarctions), which tend to preserve normal renal morphology. Expansile or exophytic renal masses may be seen by plain film, IVU, and cross-sectional imaging (ultrasonography, CT, and MR imaging). In many, but not all cases, the characteristics revealed by various imaging modalities can provide accurate diagnoses and/or determine the need for further follow-up imaging and/or tissue diagnosis.

The simple cyst is the most common renal mass, present in up to 50% of the population older than age 50. They are almost always asymptomatic and discovered incidentally. Although they occasionally may become infected, hemorrhage, or cause pain, their main importance lies in differentiating the lesions from renal tumors. Cysts can be single or multiple, unilateral or bilateral, and vary greatly in size. Pathologically they are thought to be acquired lesions arising from blocked collecting ducts or tubules. They have thin fibrous capsules lined with epithelial cells and contain clear serous fluid. Only the largest of renal cysts may be evident on plain radiographs. On the IVU a renal cyst classically is well defined, lucent, and with imperceptible walls. Although these findings suggest a cyst, even the most characteristic of lesions are still nonspecific and require further evaluation with additional studies. On cross-sectional imaging modalities, cysts are sharply demarcated from adjacent parenchyma, homogeneous in appearance, rounded with imperceptible walls, and do not enhance with the administration of contrast material. By ultrasonography, a clearly anechoic lesion with no distinguishable wall, a sharp interface with adjacent parenchyma, and enhanced through-transmission of sound can be diagnosed as a simple cyst (Fig. 9–30). By CT, cysts measure near water density and show no enhancement or associated solid components. Lesions meeting the criteria for simple cysts do not require follow-up. Cystic appearing lesions that do not meet the above criteria, such as those with thick enhancing walls, those containing internal debris, or those with calcifications, can represent cystic neoplasms and must be evaluated further by serial imaging and/or by histological diagnosis.

Solid renal masses are of even greater concern. One such lesion, the angiomyolipoma, or AML, is most easily distinguished from other renal masses by the presence of internal fat. These lesions are hamartomous tumors of mesenchymal origin that are usually well differentiated and benign. In addition to fat, they contain sheets of smooth muscle and thick-walled blood vessels. They most commonly occur in middle-aged females. Although these are usually asymptomatic, they are predisposed to spontaneous hemorrhage, especially when large. They can occur as sporadic solitary lesions or in association with tuberous sclerosis, in which case multiple angiomyolipomas are often present. As stated, the demonstration of fat in a renal mass by CT is essentially diagnostic of angiomyolipoma. Although these lesions are benign, they are often removed when greater than 4 to 5 cm due to the increased risk of hemorrhage, and for this reason smaller angiomyolipomas require follow-up to monitor the lesion for growth.

Another benign renal neoplasm that deserves comment is the oncocytoma, which originates from the epithelium of the distal tubules or collecting ducts. A characteristic, although nonspecific, pathologic feature of these lesions is a central stellate scar. They are typically asymptomatic and discovered incidentally, although they may occasionally be associated with a flank mass, pain, or hematuria. On IVU, they present as a solid renal mass, requiring further evaluation. On cross-sectional imaging studies, an oncocytoma appears as a well-defined renal mass. The diagnosis of oncocytoma may be suggested by a central stellate scar. However, even when classic, the imaging characteristics described above cannot be used to reliably differentiate them from malignant renal cell carcinoma and excision is generally indicated. Note that biopsy is generally not recommended because the cytologic appearance of oncocytoma and renal cell carcinoma (RCC) may be indistinguishable.

RCC is the most common primary renal malignancy, originating from the epithelium of the proximal tubule, having a male predominance, and a peak incidence in adults in their 50s. Any renal mass lesion that cannot be definitively identified as one of the benign entities mentioned above must be assumed to be renal cell carcinoma until proven otherwise, most often by tissue diagnosis. Classically, RCC is associated with the clinical triad mentioned above of flank pain, a flank mass, and hematuria, although all three are present in less than 10% of cases. More commonly, these lesions are being discovered incidentally before symptoms have developed. They have the IVU characteristics of a solid renal mass and may demonstrate calcifications in up to 30% of cases. On ultrasound, a nonspecific renal mass is seen. Note that these lesions may be hyperechoic and mimic angiomyolipomas or have central necrosis mimicking the central scar of oncocytomas. By CT, they tend to be rounded, soft-tissue masses, enhancing after the administration of intravenous contrast agent. When small, they are often homogeneous, although when larger they are more heterogeneous frequently with necrosis and often with calcifications. One important role for imaging beyond detecting renal cell carcinoma is evaluating the extent of tumor spread. Renal cell carcinoma can extend locally and invade adjacent soft tissues, especially when large and extensive. In addition, RCC has a propensity to spread into the renal veins and beyond and the extent of this must be delineated prior to surgery. Evidence of enlarged lymph nodes and spread to liver, lung, bones, and other areas, suggesting metastatic disease, should be sought. Surgical excision is the treatment of choice for resectable lesions, making accurate staging to determine surgical candidacy all the more important.

EXERCISE 9-3: STONE DISEASE

Clinical Histories:

Case 9-8. A 36-year-old female presents with acute right flank pain. A CT scan without intravenous contrast is obtained (Fig. 9–31).

Case 9-9. A 41-year-old female presents with a history of vague flank pain and recurrent urinary tract infections. An ultrasound of the right kidney is shown in Fig. 9–32.

Questions:

Radiographic Findings:

9-8. In this case, a CT scan (Fig. 9–31) of the abdomen was obtained without intravenous contrast. Stranding in the fat planes can be seen on the right in the retroperitoneum. Stranding within the fat planes on a CT is a nonspecific finding resulting from many conditions. In general, the stranding can be related to inflammation such as recent surgery, infection, or abnormal fluid collections such as blood or urine. Thus, the stranding seen in this case could result from any of the first three listed possible answers. However, within the right ureter is a high-density rounded structure consistent with a ureteral calculus (arrow). (B is the correct answer to Question 9-8.) Two main parameters that should be noted are the size and location of a stone, because these two factors are directly related to the likelihood of stone passage. Additionally, once the diagnosis of a ureteral stone is made, the radiologist must continue to evaluate the remainder of the scan because additional abnormalities may also exist.

9-9. In this case, a renal ultrasound (Fig. 9–32) demonstrates rounded highly echogenic areas throughout the central parenchyma of the kidney. Several important additional observations include strong uniform shadowing posteriorly from the echogenic areas consistent with sound attenuation and suggesting calcification. Although attenuation of the ultrasound beam occurs with air, such as might occur with emphysematous pyelonephritis, the shadowing in those cases is often "dirty" in appearance, being somewhat inhomogeneous. (D is incorrect.) Also, the calcifications are located in the medullary area of the kidney, unlike the cortical location of cortical nephrocalcinosis. (A is the correct answer to Question 9-9.)

Discussion:

Suspected stone disease is a common indication for urinary tract imaging. Calcifications occurring in the kidney can be dystrophic, related to abnormal tissue such as within tumors, cysts, or infection. This type of calcification is to be distinguished from nephrocalcinosis and nephrolithiasis. Nephrocalcinosis refers to the development of calcification within the renal parenchymal, generally unrelated to an underlying renal pathology. Furthermore, nephrocalcinosis should be distinguished from nephrolithiasis, which are stones within the collecting system. Note that nephrocalcinosis and nephrolithiasis may coexist.

Nephrocalcinosis is additionally subdivided into two categories depending on location. That which occurs in the renal cortex is termed cortical nephrocalcinosis and that within the medulla is called medullary nephrocalcinosis. Cortical nephrocalcinosis is less common and occurs most frequently in the setting of chronic glomerulonephritis or acute cortical necrosis, the latter condition being seen as a result of toxic ingestions such as ethylene glycol or related to acute hypotensive events. Cortical nephrocalcinosis may be detected on plain radiographs or cross-sectional imaging modalities such as CT or ultrasonography. The diagnosis is usually made by demonstrating thin linear bands of calcification at the extreme periphery of the kidney that may extend into the columns of Bertin but should not involve the renal medulla. Medullary nephrocalcinosis is more frequently observed than cortical disease and is most often due to hypercalcemic states such as hyperparathyroidism, renal tubular acidosis, or medullary sponge kidney. On plain films and CT studies, medullary nephrocalcinosis appears as speckled or dense calcifications within the renal medulla, sparring the cortex. In medullary sponge kidney, an anatomic condition of abnormally dilated collecting tubules, the condition may be unilateral or even focal, although medullary nephrocalcinosis from other causes is typically bilateral and diffuse. On ultrasound examination, shadowing echogenic foci are noted within the renal medulla.

Nephrolithiasis (better known to the public as kidney stones) is much more common than nephrocalcinosis. In fact, urinary tract calculi occur in as many as 12% of the population of the United States. Although there are clearly definable causes in some cases (hereditary conditions, metabolic diseases such as gout, certain urinary tract infections, and predisposing anatomic conditions), the vast majority of cases are labeled idiopathic. Many small stones that are located within the intrarenal collecting system are asymptomatic; however, stones that pass into the ureter (ureterolithiasis) may obstruct the urinary tract and result in excruciating pain. Additionally, stones may cause hematuria or be a nidus for recurrent infection. Conventional radiographs have long been used to evaluate stone disease; in fact, the first description of urinary calculi was in April 1896, only a few months after the discovery of the x-rays by Roentgen.

Stones appear as calcific densities on plain radiograph overlying the urinary tract (Fig. 9–33). Urinary tract calculi are variably opaque and visible depending on their size, composition, and location. The accuracy of conventional radiographs for detecting stones has long been overestimated. Perhaps only 50% of stones are identified prospectively and one can never be certain that an individual calcification on an isolated plain radiograph is within the urinary tract or simply overlies it. Confusing calcifications are many, including phleboliths, arterial calcifications, calcified lymph nodes, and other calcified masses.

The IVU has also long been used to evaluate stone disease. The IVU is used to confirm the location of a calcification in the urinary tract, to identify underlying predisposing issues (diverticula, anomalies), and finally to assess obstruction by stones in the ureter. Stones on ultrasound appear as brightly echogenic structures and often with posterior shadowing. However, not all stones shadow, and because there are many small noncalcific echogenic foci (vessels, fat) normally within the kidney, the accuracy for detecting renal calculi is only moderate with ultrasound. Additionally, ultrasound suffers from its inability to visualize only the most proximal and distal ureters and must rely on nonspecific indirect signs such as hydronephrosis and absent ureteral jets to suggest ureteral stones and obstruction. CT has now moved to the forefront in the imaging evaluation of stone disease. Virtually all urinary tract stones are dense on CT and show up as bright foci. Even stones as small as 1 mm are usually visible with current CT scanners. Additionally, the entire urinary tract can be visualized on CT without overlapping or obscuring structures. In patients who present acutely with flank pain and are suspected of having ureteral stones, CT has become the study of choice. The diagnosis is confirmed by directly identifying a stone within the ureter. Secondary findings of obstruction may also be identified on CT, helping to confirm the diagnosis. Renal enlargement, perinephric stranding, and dilation of the ureter and intrarenal collecting system are frequently present in ureteral obstruction. One major additional advantage to CT is the ability to identify alternative explanations for the cause of a patient's acute flank pain. In fact, as many as one-third of all patients originally felt to have ureteral stones are shown by CT to have an alternative diagnosis (Fig. 9–34). At this point, MR imaging performs little role in the evaluation of stone disease.

Bladder stones may occur secondary to transport from the ureter or arise de novo. Most cases of bladder stones are secondary to urinary stasis such as occurs with bladder outlet obstruction from neurogenic bladders or prostatic enlargement. The diagnosis of bladder stones is similar to those in the upper urinary tract. Finally, urethral stones occur and in males the vast majority are present as a result of passage from the bladder or above. In women, urethral stones are most frequently the result of urethral diverticula, which can result in urinary stasis and stone formation.

EXERCISE 9-4: BLADDER MASS

Clinical Histories:

Case 9-10. A 65-year-old male presents with hematuria. A plain film from a urogram is shown in Fig. 9–35.

Case 9-11. A 65-year-old male presents with microscopic hematuria. A coned-down view of the bladder from an IVU is shown in Fig. 9–36.

Questions:

Radiographic Findings:

9-10. In this case, the coned film of the bladder shows a filling defect (arrow) in the upper leftward aspect of the urinary bladder. The lesion appears to arise from the bladder mucosa, whereas blood clots and bladder stone have intraluminal locations. (B and D are incorrect.) Although squamous cell carcinomas are the second most common bladder mucosal neoplasm, they are much less common than transitional cell carcinoma. Additionally they often are associated with calcifications and recurrent infections. (A is incorrect.) Thus, the lesion is highly suggestive of transitional cell carcinoma, the most common malignancy of the urinary tract. (C is the correct answer to Question 9-10.) Therefore, further evaluation is required. Cystoscopy, which allows for direct visualization and biopsy, remains the gold standard for evaluation of bladder masses.

9-11. In this case, the radiograph shows symmetrical enlarged prostate with calcifications (arrow) indenting the bladder base. This is commonly seen in elderly men with benign prostatic hypertrophy. (D is the correct answer to Question 9-11.)

Discussion:

Transitional cell carcinoma (TCC) is the most common neoplasm of the urinary collecting system and represents up to 90% of all neoplasms of the bladder itself. Although they can occur anywhere that there is transitional epithelium, from the renal collecting system to the urethra, they are most commonly found in the urinary bladder. This is felt to be due to several factors, including the large surface area of the bladder. In addition, it has been well documented that TCC is associated with numerous chemical carcinogens as well as cigarette smoking. Also, because the bladder acts as a temporary storage site prior to excretion, carcinogens remain in contact with the epithelium of the bladder for a longer period of time than they do with that of the remainder of the urinary tract. Bladder TCC usually presents with hematuria. A transitional cell carcinoma can obstruct the vesicoureteral junction and cause obstructive symptoms as well. Transitional cell carcinoma of the bladder spreads by local invasion and by lymphatic and hematogenous spread. Most are superficial at presentation, with only about 1 in 4 displaying muscle invasion and 1 in 20 having distant metastases at the time of diagnosis.

Plain radiographs are most often unremarkable in TCC of the bladder, with less than 1% displaying some stippled calcifications. Transitional cell carcinomas can be seen as filling defects in a contrast-filled bladder, particularly when greater than 1 cm in size. Filling defects within the bladder on IVU or cystogram are somewhat nonspecific with considerations including tumor, radiolucent stones, fungus balls, and blood clots. However, transitional cell cancers have a characteristic stippled and frond-like appearance. It is important to note that IVU is fairly insensitive for detecting bladder TCC and a negative study does not exclude the lesion. Ultrasound can show exophytic soft-tissue lesions within the bladder. CT is useful for evaluation of possible bladder masses, because the size of the mass itself, as well as the extent of invasion through the bladder wall into adjacent pelvic structures, can be evaluated. Also important is evaluation of abdominal and pelvic lymph nodes, and post-treatment examination for tumor recurrence. CT generally demonstrates a soft-tissue mass with occasional calcification arising from the bladder wall (Fig. 9–37). Although early, MR imaging may prove useful for evaluating bladder tumor invasion. While the above imaging findings strongly suggest the diagnosis of transitional cell carcinoma, cystoscopy is important to confirm the histologic diagnosis and evaluate for other, smaller lesions not visible on imaging.

Other tumors can be seen in the bladder as well, including malignancies such as squamous cell carcinoma and adenocarcinoma, uncommon benign lesions, and some inflammatory processes may appear mass-like. Pheochromocytomas can rarely be seen (Fig. 9–38) arising from the bladder wall. Despite the rarity, the classic symptomatology of hypertension, headache, and flushing occurring during micturiction should suggest the diagnosis of a catecholamine-producing tumor. On imaging, these are reported to have characteristic thick, circumferential calcifications.

Benign prostatic hypertrophy is seen in elderly men older than 60 years of age. The hypertrophied prostatic lobes extend upward and impress on the bladder base. However, prostatic hypertrophy can be present without uplifting of the bladder base. During IVU, the enlarged prostate may also elevate the interureteric ridge, causing a "J-shaped" appearance of the distal ureters. However, an enlarged prostate indenting the bladder base is nonspecific because prostate cancer must be included in the differential diagnosis.

Dalrymple NC. Pearls and pitfalls in the diagnosis of ureterolithiasis by unenhanced helical CT. Radiographics. 2000;20:439–447. [PMID: 10715342]

Dunnick NR, Korobkin M. Imaging of adrenal incidentalomas: current status. AJR. 2002;179:559–568. [PMID: 12185019]

Dyer RB, Chen MY, Zagoria RJ. Abnormal calcifications in the urinary tract. Radiographics. 1998;16:123–142.

Dyer RB, Chen MY, Zagoria RJ. Intravenous urography: technique and interpretation. Radiographics. 2001;21:799–824. [PMID: 11452054]

Ornstein DK, Arcangeli CF, Andriole GL. Renal masses: urologic management. MRI Clin North Am. 1997;5:1. [PMID: 8995121]

Zagoria RJ. Imaging of small renal masses: a medical success story. AJR. 2000;175:945. [PMID: 11000140]