Genitourinary System and Retroperitoneum

Congenital Abnormalities

Congenital abnormalities of the urinary tract occur quite frequently, and you should be aware of the most common variants. Embryologically, the ureter buds and grows superiorly from the bladder to meet and connect with the renal parenchyma. The ureter can divide as it ascends, causing a person to have two partially duplicated or completely separate collecting systems for one kidney. If complete ureteral duplication is found, the ureter that supplies the upper half of the kidney often becomes obstructed ( Fig. 7.4 ). If the upper pole of the kidney is completely obstructed, all that is visualized is the normally draining lower pole collecting system, and it looks like a drooping lily. You should also be aware that the duplicated ureter that supplies the upper pole may often have an ectopic insertion into the bladder, urethra, or vagina.

A number of other anomalies occur in the course of the normal embryologic ascent of the kidneys out of the bony pelvis. These anomalies include one kidney rising normally and the other kidney remaining in the pelvis. Remember, it is rare to have a unilateral kidney, and thus if you see only one kidney in its normal position, you should look elsewhere for an ectopic kidney ( Fig. 7.5 ). Another common variant is fusion of the inferior aspect of both kidneys (a horseshoe kidney). This is relatively easy to identify because the axis of the kidneys is abnormal, with the superior aspect of the kidneys tilted outward instead of inward ( Fig. 7.6 ).

Renal Cysts

Renal cysts are quite common, and their incidence increases with age. Most persons older than 60 years have one or more simple renal cysts. These are often found incidentally on ultrasound or CT scans ordered for other reasons. Ultrasound is a good and inexpensive initial test to characterize a renal cyst. The margins of a benign simple cyst should be well defined, and increased echoes should be seen on the posterior aspect of the cyst because of good transmission of sound through the fluid in the cyst ( Fig. 7.7A ). If a cyst has septa or internal echoes on ultrasound, a short-term follow-up ultrasound or CT scan is usually ordered to exclude a cystic neoplasm.

On a CT scan, renal cysts are typically hypodense relative to the renal parenchyma; however, occasionally they can be hyperdense due to blood or protein content (see Fig. 7.7B ). Hyperdense renal cysts should not enhance more than about 10 Hounsfield units after contrast administration and should have a smooth outline and be internally homogeneous.

Currently renal cysts or cystic masses are classified according to the Bosniak criteria. Bosniak I lesions are benign cysts that are round or oval with a thin wall, no septations, Hounsfield CT units of 0 to 20, and no enhancement with intravenous contrast. Follow-up is not usually needed. Bosniak II lesions are the same but with a few septations, a few thin calcifications, or both. These are often followed up in 6 to 12 months, but both Bosniak I and II lesions have a very small chance of being malignant. A Bosniak III lesion has any or all of the following: a thick wall, calcifications, Hounsfield density of 0 to 20, and no enhancement of nodules. These lesions have about a 60% chance of being malignant. Bosniak IV lesions have a thick wall, thick septations, coarse calcifications, Hounsfield density of more than 20, and enhancement with intravenous contrast. All these lesions should be considered to be malignant and require further workup.

Polycystic renal disease presents a difficult imaging problem. In the adult form of this heritable disorder, often progressive renal failure occurs. A CT scan will demonstrate very lumpy kidneys, but the cysts may not be well defined because hemorrhage often is found within the cyst. Cysts also are usually identified in the liver and sometimes in the pancreas ( Fig. 7.8 ).

Hematuria

Hematuria can be traumatic or nontraumatic and visible or microscopic. Microscopic hematuria is defined as three or more red blood cells (RBCs) per high-power field (HPF) on microscopic evaluation of urinary sediment from two of three properly collected urine specimens. In cases of trauma and visible hematuria, a CT scan is indicated. In cases of trauma, microscopic hematuria, and little suspicion of injury to other organs, many physicians do not do any imaging but rather wait 48 hours to see if the hematuria clears. Young women with a clinical picture of simple cystitis and other patients whose microscopic hematuria completely and permanently resolves after rest or therapy are unlikely to benefit from imaging.

Of patients with nontraumatic visible hematuria, about 25% have cancer, 25% have infection, and 15% have calculi. About 5% of patients with nontraumatic microscopic hematuria (5 RBCs/HPF) have a urologic cancer. Obviously, the presence of unilateral flank pain suggests calculi, discussed later. If RBC casts are seen in urine with associated proteinuria, a renal biopsy is usually performed. Neither CT nor an ultrasound examination can completely exclude a urologic malignancy because CT and ultrasound are poor for detection of small urothelial tumors and evaluation of bladder tumors. The best evaluation of painless nontraumatic hematuria often begins with cystoscopy and possibly retrograde pyelogram at the same time. If this is negative, a CT scan is indicated.

Renal Stone Disease

Calcification can occur within the substance of the kidney or within the collecting system. Calcification within the substance of the kidney (nephrocalcinosis) may be cortical (near the periphery of the kidney) or medullary (near the ends of the calyces). Cortical calcification can be due to chronic glomerulonephritis, cortical necrosis, or acquired immunodeficiency syndrome (AIDS)-related nephropathy. Medullary calcification may be idiopathic or caused by papillary necrosis, medullary sponge kidney, or other hypercalcemic states (including hyperparathyroidism and osteoporosis; Fig. 7.9 ).

The vast majority of renal calculi are radiopaque and appear dense (or white) on a routine x-ray ( Fig. 7.10A ). Occasionally, renal stones become very large and essentially fill the collecting system of the kidney. These are referred to as staghorn calculi (see Fig. 7.10B ). If calculi are radiopaque, overlie the kidneys, or are within the course of a ureter, they are usually fairly easy to see. Sometimes it can be difficult to visualize a small stone in the region where the ureter passes anterior to the sacrum. Remember that a large number of vascular calcifications occur low within the bony pelvis and to the sides of the bladder. These phleboliths typically can be recognized because they are round, have a lucent (dark) center, and are more lateral and lower in the pelvis than the normal course of the ureter.

The most common clinical presentation of stone disease is intense flank pain with hematuria. If the patient is having a first presentation of renal stone disease, a spiral noncontrast CT scan is indicated, even if no calculus is seen on the plain x-ray of the abdomen. Only about 50% of renal calculi are seen on a plain film, whereas detection with CT is about 95%. CT scanning is exquisitely sensitive for the detection of even tiny calcifications and provides additional detail about other potential causes of pain, and the size of the stone can be accurately measured. Ultrasound can visualize large renal stones and hydronephrosis but is poor for the detection of small renal or any size ureteral stones ( Fig. 7.11 ).

If a history of renal stones is known, imaging is not always needed. Imaging may be reserved for patients who have pain uncontrolled by medication, those with continued flank pain for more than 5 days, those who have continued hematuria 2 weeks after passing a stone, those with microscopic hematuria for more than 1 month, and those who have acute flank pain and are known to have a solitary kidney or pelvic tumor.

The size of a stone is inversely proportional to its passage. A stone 5 mm or smaller has a 68% probability of spontaneous passage, whereas a 10-mm stone is very unlikely to pass spontaneously. Stones typically become lodged in the ureter at the ureteropelvic junction, iliac vessels, or ureterovesicular junction.

On a CT urogram, obstruction of the ureter by a stone may cause delayed visualization of the affected kidney and ureter. When it is visualized, the ureter is usually dilated and the renal calyces are blunted. On delayed images, whereas the normal kidney is completely clear of contrast, the affected kidney and ureter will be seen retaining contrast. Delayed images are often necessary to determine the exact level of the ureteral obstruction.

Usually a kidney with an acute problem due to renal calculi will have associated perirenal edema in the adjacent soft tissues ( Fig. 7.12A ). Benign age-related perinephric stranding is usually bilateral and occurs mostly in persons older than 55 years. If a stone is lodged in the distal ureter, a dilated proximal ureter can often be seen (see Fig. 7.12B ). Sometimes on a CT scan it is difficult to differentiate ureteral calculi from atherosclerotic vascular calcifications or vascular phleboliths. One way is to look for a soft tissue rim around the calculus (see Fig. 7.12C ). Phleboliths are most common in the lower pelvis below the middle portion of the femoral heads, whereas the ureters almost always enter the bladder at a more cephalad level.

Occasionally, the back pressure caused by an obstructing ureteral stone can rupture a renal calyx or renal pelvis. When this occurs, extravasation of urine and contrast outside the kidney into the perirenal space is seen. When an obstructing lesion of the ureter is present, the urologist may perform cystoscopy and then place a little tube into the distal ureter and inject contrast (retrograde pyelogram). The ureter, renal pelvis, and calyces are usually visualized. Because pressure is being exerted during the injection, minimal blunting of the calyces is normal under these circumstances. A retrograde pyelogram is useful for looking at small lesions within the collecting system, such as a transitional cell carcinoma. Occasionally, air bubbles will be inadvertently injected along with the contrast, which may give the appearance of filling defects. The key to differentiating these entities is that tumors will not move around on different views; many calculi also have sharp or geometric borders ( Fig. 7.13 ). Air bubbles move and are completely round.

Renal Failure

Imaging is indicated in patients with unexplained oliguria or new onset of renal insufficiency or failure (serum creatinine level > 2 mg/dL). Many imaging studies of the kidneys rely on normal function. The most common clinical question is whether renal failure is due to obstruction or to medical renal disease. Because the intravenous contrast material used for a CT or MRI scan can reduce renal function, the imaging examination of choice for acute or chronic renal insufficiency is ultrasonography. Normally, the cortex of the kidney has the same ultrasound echo density as the liver or has fewer echoes than the substance of the liver. In cases of medical renal disease, more echoes are found within the renal cortex than within the liver. This is probably the result of fibrosis and scarring ( Fig. 7.14 ). For the use of intravenous contrast with a CT or MRI scan, an estimated glomerular filtration rate (eGFR) of 60 mL/min per 1.73 m ² or greater is acceptable, an eGFR of 30 to 60 mL/min per 1.73 m ² usually requires hydration of the patient, and a value of less than 30 mL/min per 1.73 m ² is a significant contraindication.

Pyelonephritis and Renal Infections

The clinical findings of pyelonephritis include fever, flank pain, and pyuria. Most patients with pyelonephritis have no discernible findings on imaging studies. Imaging studies are usually not warranted in an uncomplicated patient unless there are repetitive episodes or there is persistent worsening pain after 3 days of appropriate antibiotic therapy. CT scanning with and without intravenous contrast is appropriate in patients who are complicated (e.g., have diabetes, are immunocompromised, had prior renal surgery or history of stones). Sometimes, in patients who have severe acute pyelonephritis, enough edema of the renal parenchyma is present that the swelling causes compression of the calyces or renal pelvis, and there is poor visualization of the collecting system on a CT urogram. Occasionally, with acute pyelonephritis, focal areas of edema can be seen. The real purpose of ordering CT scans in these cases should be to look for a renal parenchymal or perirenal abscess.

In patients with chronic pyelonephritis, the kidney is usually shrunken and has an irregular outer margin. Typically, the cortex also is thinned. The irregularities of the outer cortical margin are fairly characteristic, with dimpling or scarring directly over a calyx. If there is dimpling or a defect in the margin of the kidney between two calyces, it is more likely to be the result of a focal infarct.

Persons with diabetes are particularly prone to an unusual form of acute pyelonephritis called emphysematous pyelonephritis . In these patients, gas is generated by the bacteria within the parenchyma of the kidney. Usually the kidney is nonfunctional, and a dark radiating striated gas pattern is seen where you would normally expect to find a kidney ( Fig. 7.15 ).

Occasionally, inflammatory abnormalities can cause enlargement of both kidneys, particularly in acute glomerulonephritis. The differential diagnosis for bilaterally enlarged kidneys includes bilateral obstruction, leukemia, glycogen storage diseases, lymphoma, and polycystic disease, as well as a number of other entities ( Fig. 7.16 ).

Infections of the kidney can progress to the stage at which the kidney is essentially nonfunctional. In an entity known as xanthogranulomatous pyelonephritis , a nonfunctional kidney is seen, with some calcification visible within it ( Fig. 7.17 ). The kidney is removed surgically. However, on the basis of any imaging study, it is difficult to differentiate xanthogranulomatous pyelonephritis from a renal tumor.

Renal tuberculosis can affect the kidneys, ureter, and bladder; the infection typically begins in the kidneys, and you should look there first. In the early stages, narrowing or amputation of the infundibulum is seen between a renal calyx and the renal pelvis. In late stages, a nonfunctional shrunken kidney with clumps of calcification is found. A number of fungal infections can affect the kidney in diabetic and immunosuppressed patients. Fungal infections often cause large fungal clumps or balls within the collecting system that can obstruct the kidney ( Fig. 7.18 ).

Renal Trauma

Blunt trauma, particularly during a motor vehicle accident, can cause a number of renal abnormalities. Significant kidney trauma should be suspected when a fracture of the 12th rib or fractures of the transverse processes of the lumbar vertebrae are found. Another useful sign on the plain x-ray is nonvisualization of the psoas margin on one side. In a renal contusion, the kidney is intact, but interstitial edema may lead to reduced blood flow. Lacerations and intrarenal hematomas can be incomplete (not extending into the calyceal system), whereas complete lacerations are usually accompanied by significant hemorrhage and urine extravasation ( Fig. 7.19 ). Minor renal injuries are usually treated conservatively, and even kidneys that have large lacerations may ultimately heal. Surgical intervention may not be required unless there is major blood loss. Occasionally, avulsion of the renal vascular pedicle occurs, with disruption of the blood supply ( Fig. 7.20 ).

The advantage of CT scanning in cases of renal trauma or penetrating abdominal injury is that you can assess other organs, such as the liver and spleen, for concomitant injuries and the peritoneal and retroperitoneal areas for hematomas. Although it is not generally appreciated, lithotripsy (ultrasound used to fragment renal stones) can cause significant renal trauma. Postlithotripsy hemorrhage generally resolves without intervention, and imaging studies are not usually ordered.

Renal Tumors

Among the occasional benign tumors of the kidneys is an angiomyolipoma. It is easily identified on CT scanning by the presence of fat within the mass ( Fig. 7.21 ). Other tumors include fibromas, oncocytomas, and hamartomas.

Renal cell carcinoma constitutes about 85% of all primary renal malignancies. It usually occurs in the sixth decade, and men are affected twice as often as women. The classic clinical triad consists of gross hematuria, flank pain, and a flank mass, although this triad is seen in only approximately 10% of patients. Typically, gross or microscopic hematuria raises the suspicion of a urinary malignancy. Large mass lesions within the kidney will displace the collecting system and produce an irregular contour of the kidney on a CT urogram, but very small tumors or pedunculated neoplasms can be difficult to appreciate. Although renal cancer can be visualized with ultrasound, CT scanning with and without intravenous contrast (with thin cuts through the kidneys) is now the imaging procedure of choice in an older patient with persistent painless hematuria and normal cystoscopy ( Fig. 7.22 ). MRI also can be used to evaluate renal masses ( Fig. 7.23 ).

Most renal cell carcinomas are relatively solid, but some are cystic, and you may have some difficulty differentiating a cystic neoplasm from a benign renal cyst. On a CT scan, the finding of a thickened or irregular wall, inhomogeneous texture, a mural mass within a cystic abnormality, and large size are criteria for malignancy. On the CT scan, also look for potential extension into the renal vein and inferior vena cava, as well as into the nearby nodes ( Fig. 7.24 ). Metastases from renal cell carcinoma are present initially in 10% of patients and tend to go (in descending order) to the lung, bone, retroperitoneal and mediastinal nodes, liver, and brain. On the chest x-ray, the metastases are usually nodules ranging from 0.5 cm to several centimeters. When the metastases are in bone, they tend to be very aggressive, expansile, and lytic (destructive) ( Fig. 7.25 ).

Renal Artery Stenosis

Fewer than 5% of patients with hypertension have renal artery stenosis as the cause. Imaging is usually reserved for patients whose hypertension is uncontrolled by two medications or uncontrolled by medication and with increasing levels of serum blood urea nitrogen or creatinine. Imaging studies for suspected renal artery stenosis and renovascular hypertension are discussed in Chapter 5 .

Renal Transplant Evaluation

Renal transplants are typically located anteriorly in the pelvis (more commonly on the right). The causes of dysfunction include acute tubular necrosis, rejection, drug toxicity, vascular abnormalities, and rejection. Pelvic ultrasound with Doppler is the initial imaging examination for evaluation in both the acute and chronic periods.