CT of Common Renal Disorders
Author: R. Brooke Jeffrey, MD
Objectives: Upon the completion of this CME article, the reader will be able to:
- Discuss the CT characteristics of renal cysts and the various causes for multiple cysts including adult polycystic kidney disease, tuberous sclerosis, von Hippel-Lindau disease and multilocular cystic lymphoma.
- Explain the concern over identifying solid masses in the kidney and the staging of renal carcinoma.
- Describe the use of CT in evaluating pyelonephritis and related infectious conditions of the kidney.
- Discuss the use of CT in diagnosing renal stones and the issues involved regarding the likelihood of passage versus the need for intervention.
In the human body, the kidneys have several functions including (but not limited to) the maintenance of fluid volume, the acid-base status of the bloodstream, and the excretion of certain substances. When the body is low on fluid (dehydrated) the kidneys conserve water. Likewise, if the body has an excess amount of fluid, it increases its excretion to decrease the overload. In addition, many substances, such as drugs (metabolized and un-metabolized), are excreted by the kidneys as a mechanism of ridding them from the bloodstream. In fact, if the kidneys are not functioning normally, the amount of drug taken should be altered. Finally, the kidneys are involved with the body’s acid-base status through their excretion and absorption of certain solutes and electrolytes. Therefore, because of these important bodily functions, it is imperative that a good foundation is obtained regarding some of the abnormalities that may develop in the renal system.
Computed Tomography (CT) is an ideal method for evaluating structural abnormalities of the urinary tract and renal parenchyma. While non-contrast studies are essential for the detection of renal calculi (stones), the use of intravenous contrast agents enables the detection of renal parenchymal masses and assessment of the renal collecting system. This chapter will focus on the role of CT in diagnosing common renal disorders such as renal cysts and tumors, renal infections, and calculous disease.
Renal Mass Evaluation – Cystic Lesions
One of the most frequent indications for CT of the kidneys is to evaluate a renal mass detected during sonography or excretory urography. Computed tomography is often the pivotal exam in guiding the clinical management and indicating whether there is a need for surgery. CT is able to characterize most renal masses based on an analysis of attenuation values and pattern of contrast enhancement.
Cysts are the most commonly encountered renal masses. Simple cysts typically require no therapy in the absence of hemorrhage or infection. Benign simple cysts must however, be distinguished from cystic renal cell carcinoma by a combination of morphologic and enhancement criteria. In general, simple renal cysts do not have perceptible walls and have attenuation values < 20 Hounsfield units (HU). Following the administration of intravenous contrast, there should be no enhancement of the cyst. On occasion, simple cysts may undergo internal hemorrhage or calcification. Thin areas of calcification (2 mm or less) are usually indicative of a minimally complicated benign cystic lesion, requiring no further therapy. Cysts that have solid mural elements or enhancing septae are worrisome for cystic renal cell carcinomas and are most often surgically resected.
Although it is not uncommon to have scattered multiple simple cysts in adult patients, the presence of large numbers of cysts on CT may indicate cystic diseases of the kidney including adult polycystic kidney disease, acquired cystic disease of dialysis, and cysts associated with either tuberous sclerosis or von Hippel-Lindau disease.
Adult polycystic kidney disease is a genetic disorder that is inherited in an autosomal dominant fashion. This means that the individual had one parent who also had the disorder. Likewise, people with adult polycystic kidney disease have a 50% chance that every child they conceive will also be affected. Therefore, the offspring of people who have this disorder should undergo periodic evaluations of their kidneys to look for the development of cysts. Because, the abnormal gene has been found on chromosome number 16, another option for screening is to perform genetic testing. Patients are usually asymptomatic until they reach their 20’s to 30’s. The most common symptoms (if symptoms develop) are flank pain, hematuria (blood in the urine), and proteinuria (protein in the urine). Adult polycystic kidney disease accounts for about 10% of end-stage renal failure. The renal cysts are usually numerous, thin-walled, and can be found in the cortex and medulla of the kidney ranging in size from millimeters to centimeters. Cysts are also common in the liver, developing in about 30% of cases. Though rare, cysts may also occur in the spleen, pancreas, lungs, ovaries, testes, and thyroid. Finally, about 2% of patients may have intracranial aneurysms, however, routine cerebral arteriography is not warranted.
Adult polycystic kidney disease is a different disorder from infantile polycystic kidney disease. Infantile polycystic kidney disease is also genetic in inheritance but is autosomal recessive and manifests itself much earlier in the person’s life. The cysts, however, are very small, usually less than a millimeter in size. If this disorder manifests itself in utero, it usually results in a very low amount of amniotic fluid (oligohydramnios) leading to the birth of child than does not survive the newborn period. If it manifests after birth, the than lifespan of the individual depends upon when the kidneys fail.
In patients with tuberous sclerosis, angiomyolipomas occur in over half of the cases. A definitive diagnosis of angiomyolipoma can be made with CT by detecting the presence of fat within the lesion. Fat on CT has negative attenuation values typically less than 20 HU. Tuberous sclerosis is also a genetic disorder that is inherited in an autosomal dominant fashion, however, 85% of cases are new mutations. This means that there is no family history in the majority of cases. The abnormal gene for this disorder has been found on chromosome number 9. Besides the kidneys, the angiomyolipomas are commonly found in the brain (often leading to the development of seizures) and also in the skin, though they can actually develop in almost any organ. These patients can also have white moles and patches of skin that lack pigment called "ash leaf spots".
Von Hippel-Lindau disease (another autosomal dominant disorder) typically has angiomas of the cerebellum and retina; however, the angiomas can also be found in the kidney, liver, and pancreas. The abnormal gene of this genetic disorder has been found on chromosome number 3. Patients with von Hippel-Lindau disease who have angiomas of the kidney are prone to developing carcinoma in up to 45% of cases. Therefore, careful follow-up with periodic CT imaging is essential for the early detection of renal cell cancer.
Other cystic lesions that can be identified with CT include the rare benign neoplasm known as multilocular cystic lymphoma. This appears as a cystic mass containing multiple septations with a well-defined capsule. Fibrous septae separate the different locules of the cystic mass. In many instances, it is not possible to distinguish this entity from cystic renal cell carcinoma and, therefore, most lesions are removed surgically.
Renal Mass Evaluation – Solid Lesions
Renal cell carcinoma is the most common solid tumor of the kidney and represents well over 90% of solid renal masses. The remaining solid masses include lymphoma, metastatic tumors, sarcomas, and other rare lesions like oncocytoma. Approximately 20,000 new cases of renal cell carcinoma are diagnosed each year in the United States and it accounts for 8,000 deaths annually. Though it can actually develop at almost any age, the peak incidence is between the ages of 50 and 60. It is more common in men with a ratio of 2 to 1 compared to women. Environmental factors that have been implicated as potential risk factors include cigarette smoking and exposure to cadmium.
Many patients with renal cell cancer do not have symptoms until the tumor has reached a significant size. Symptoms, when they occur, include flank pain, fever of unknown origin, night sweats, weight loss, hematuria, and a palpable mass.
Computed tomography is often the imaging method of choice for evaluating any suspected solid renal mass based on prior imaging studies such as ultrasound or urography. On CT, renal cell carcinomas extend exophytically from the kidney contour as they typically arise from the cortex
(figure 1). On non-contrast CT scans, renal cell carcinomas typically are iso-attenuating with the remainder of the normal kidney. Following intravenous contrast, renal cell carcinomas enhance, but are typically lower in attenuation than the remainder of the kidney during the nephrographic or excretory phases. Bolus scans during an early arterial phase may reveal a hypervascular lesion. Heterogeneity within a renal cell carcinoma, including either necrosis and or areas of calcifications, is common on CT. An accurate local staging of renal cell carcinoma is essential for optimal clinical management. The key criteria for staging include:
Stage I = identification of a tumor confined to the kidney and renal capsule.
Stage II = tumors that have spread into the perinephric space.
Stage IIIA = renal vein invasion.
Stage IIIB = lymph node involvement
Stage IV = direct adjacent organ invasion and distant metastases.
The prognosis is good for Stage I lesions with a five-year survival of about 75% following treatment. The five-year survival for Stage II drops to about 50% to 60% and for Stage IIIA is about 30% to 40%. The five-year survival for stage IIIB is poor at 10% and is less than 5% for stage IV carcinomas. Therefore, as depicted, an earlier diagnosis may result in a lesser stage and a better prognosis.
The most common infection of the kidney is pyelonephritis. Though many different microorganisms can infect the urinary tract, gram-negative bacteria are the most common. For the gram-negative bacteria, Escherichia coli (E. coli) accounts for about 80% of the infections. Other common bacteria include Proteus, Klebsiella, and Enterobacter. Proteus can produce a substance called urease and Klebsiella can produce extracellular polysaccharides, which can predispose to stone formation. Therefore, these infections are more often seen in the presence of calculi.
Under normal conditions, bacteria that gain access to the bladder are diluted and then flushed as the person voids. In addition, for most people, urine has antibacterial properties that can often prevent an infection from developing. However, there can be circumstances that develop, which predispose and individual to becoming infected. To begin with, these infections are more common in women than in men. In fact, over 5 million office visits by women for bladder infections occur each year in the United States. This is partly due to the proximity of the urethra to the anus, making it particularly prone to becoming colonized with gram-negative bacteria of the gastrointestinal tract in women. Other risk factors for developing bladder infections (which increase the risk for pyelonephritis) include pregnancy (which increases urinary stasis), obstruction, vesicoureteral reflux (which is a reflux of urine into the ureter and sometimes into the kidney), and genetic factors (that may influence a person’s susceptibility).
Pyelonephritis is most often an ascending infection from the bladder. In most instances, the diagnosis of pyelonephritis is established clinically on the basis of flank pain, pyuria (significant numbers of white cells in the urine), fever, and chills. In patients with a classic presentation and positive urine cultures, no imaging is required if there is prompt clinical response to appropriate antibiotic therapy. If there is clinical doubt about the diagnosis, or if there is a poor response to antibiotic therapy, CT may be helpful in evaluating the kidney for focal or diffuse bacterial infection.
Intravenous contrast is essential in evaluating areas of renal infection. Acute pyelonephritis often has a lobar distribution on contrast CT appearing as single or multiple wedge-shaped zones of decreased attenuation
(figure 2). Not infrequently there is soft tissue stranding involving the adjacent perinephric space. If pyelonephritis is not adequately treated, it may evolve into a liquefied abscess, which appears on CT as a rounded water-density mass within an area of pyelonephritis. The treatment of choice for renal abscesses larger than 3 cm is percutaneous drainage. The success rate is comparable to surgery and is much less invasive and expensive. In patients with extensive perinephric abscesses, surgical drainage may be required particularly if the abscess is multilocular.
Diabetic patients are particularly prone to severe forms of renal infection such as emphysematous pyelonephritis. This entity is characterized by diffuse gas formation within the kidney produced by either E. coli or other gram-negative organisms
(figure 3). Unfortunately, antibiotic therapy is often not effective and therefore, nephrectomy is usually required. In addition, mortality rates over 50% have been reported.
Infection associated with obstruction ("pyonephrosis") is generally a urologic emergency requiring either percutaneous nephrostomy or ureteral stenting to provide drainage of the obstructed and infected kidney. On CT there is almost always marked dilatation of the renal collecting system. The cause of the obstruction is often a large ureteral stone that is also readily visualized by computed tomography.
Although relatively rare, renal tuberculosis is still a problem in the United States and it has a variety of CT manifestations. Tuberculosis can produce calcifications and strictures, which may result in diffuse renal atrophy with end-stage calcification of the kidney. Renal abscesses or areas of calyceal obstruction are also common in patients infected with tuberculosis.
Renal Colic and Stone Disease
Kidney stones are a common cause for flank pain and in general are most often seen in men compared to women. The most common stone is calcium based accounting for 75% to 80% of all stones. Most individuals who develop a stone will eventually form another (on average, one stone about every 2 to 3 years). Many patients who develop stones will have a family history suggesting that the disorder is often familial in origin (or possibly inherited).
CT has largely replaced excretory urography as the diagnostic method of choice in patients presenting with acute flank and suspected ureteral or renal calculi. Because CT is exquisitely sensitive for calcifications, even very small stones (2 to 3 mm in size) can be diagnosed with confidence
(figure 4). Helical CT has proven to be extremely accurate for the detection of renal stones including those that by conventional urographic techniques are considered to be "non-opaque". These include uric acid stones, xanthine stones, and cystine stones.
Uric acid stones are also more commonly formed in men than in women and half the patients have gout. These stones develop when the urine becomes supersaturated with undissociated uric acid. Other disorders that may lead to the development of these stones are myeloproliferative syndromes (i.e. chronic myelogenous leukemia and polycythemia vera) and chemotherapy treatment of malignant tumors. Cystine stones essentially only form in patients that have cystinuria. Cystinuria is the most common inborn error of amino transport and is inherited in an autosomal recessive fashion (meaning both parents are normal but carry the recessive gene). Likewise, xanthine stones primarily only develop in people who have a high urinary concentration of xanthine. A common cause for this abnormality is another autosomal recessive genetic disorder called xanthine oxidase deficiency.
One unusual form of calculus, called a struvite stone, is more common in women and essentially only develops as a result of chronic infection (with a bacteria called Proteus). These stones can become large and fill the entire renal pelvis producing a "staghorn" appearance. Struvite is a substance called ammoniomagnesium phosphate and can only form in the presence of a high urinary ammonium level. The urease enzyme produced by the bacteria Proteus is able to degrade urea into ammonia leading to the development of these stones.
In the clinical management of a patient with a ureteral calculus, the two key features determining the need for urologic intervention are the size and the location of the stone. Stones 4 mm or less in size pass about 75% of the time. However, stones that are 5 to 7 mm in size have only a 60% chance of spontaneous passage and stones greater than 7 mm in size have only a 40% chance of passage. The location of the stone is also critical in determining the need for intervention. About 70% of distal ureteral stones
(figure 5) pass spontaneously, compared to only 45% for mid-ureteral stones.
In addition to finding the calcification along the course of the ureter, CT may detect other secondary findings that are helpful in corroborating the diagnosis. These include renal enlargement, hydronephrosis, periureteral and perinephric stranding (due to forniceal rupture) and soft tissue thickening around the ureter adjacent to the stone (the "tissue rim-sign"). In a small percentage of cases, phleboliths (stones within a vein) located in the vessels of the renal pelvis or in the gonadal vein may be problematic and can be confused with ureteral calculi. In these patients intravenous contrast may be invaluable to rule-in or exclude the diagnosis.
In summary, computed tomography in many ways has become the preferred modality in diagnosing renal disorders from masses, to complications of infection, to the evaluation of individual with a renal calculus.
- Typical Renal Carcinoma by Contrast CT – Note the heterogeneous mass
(arrow) extending from left renal cortex.
- Multifocal Pyelonephritis of Right Kidney – Note multiple wedge-shaped zones of decreased attenuation.
- Emphysematous Pyelonephritis – Note areas of gas forming infection in left kidney (arrow).
- Renal Calculi – Note multiple high attenuation stones within left kidney (arrows).
- Distal Ureteral Stone – Note high density stone in the distal right ureter (arrow).
References or Suggested Reading:
- Kenney PJ, McClennan BL. The Kidney from Computed Body Tomography with MRI Correlation. 2nd ed. Philadelphia: Lippincott-Raven Press. 1998.
- Smith RC, Levine J, Dalrympie NC, et al. Acute flank pain: a modern approach to diagnosis and management. Semin in Ultrasound CT MR 1999,20:108-135.
- Diel J, Perlmutter S, Venkataramanan N, et al. Unenhanced helical CT using increased pitch for suspected renal colic: an effective technique for radiation dose reduction? J Comput Assist Tomogr 2000;24:795-801.
- Yamamoto S, Nakamura K, Kawanami S, et al. Renal angiomyolipoma: evolutional changes of its internal structure on CT. Abdom Imaging 2000;25:651-4.
- Takebayashi S, Hidai H, Chiba T, et al. Renal cell carcinoma in acquired cystic kidney disease: volume growth rate determined by helical computed tomography. AM J Kidney Dis 2000;36:759-66.
- Abramson S, Walders N, Applegate KE, et al. Impact in the emergency department of unenhanced CT on diagnostic confidence and therapeutic efficacy in patients with suspected renal colic: a prospective survey. AJR AM J Roentgenol 2000;175:1689-95. (2000 ARRS President’s Award. American Roentgen Ray Society)
- Tsuboi N, Horiuchi K, Kimura G, et al. Renal masses detected by general health checkup. Int J Urol 2000;7:404-8.
- Liu W, Esler SJ, Kenny BJ, et al. Low-dose nonenhanced helical CT of renal colic: assessment of ureteric stone detection and measurement of effective dose equivalent. Radiology 2000;215:51-4.
Dr. R. Brooke Jeffrey is currently a Professor of Radiology and is the Chief of Abdominal Imaging at Stanford University School of Medicine. He is active in clinical practice and is a member of various professional organizations, including the American College of Radiology.
Dr. Jeffrey is a well-known speaker and he has lectured on many different radiology topics at numerous conferences around the country. He is active in research and has authored several publications in peer-review medical journals.