CT of Common Renal Disorders
Author:
R. Brooke Jeffrey, MD
Objectives: Upon the completion of this CME article, the reader will be able to:
1.
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.
2.
Explain the concern over identifying solid masses in the kidney and the staging of
renal carcinoma.
3.
Describe the use of CT in evaluating pyelonephritis and related infectious conditions
of the kidney.
4.
Discuss the use of CT in diagnosing renal stones and the issues involved regarding
the likelihood of passage versus the need for intervention.
Introduction
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 unmetabolized), 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 followup 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.
Renal Infection
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 waterdensity 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 rimsign”). 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.
Figures:
1
Typical Renal Carcinoma by Contrast CT – Note the heterogeneous mass
(arrow) extending from left renal cortex.
2
Multifocal Pyelonephritis of Right Kidney – Note multiple wedge-shaped zones of
decreased attenuation.
3
Emphysematous Pyelonephritis – Note areas of gas forming infection in left kidney
(arrow).
4
Renal Calculi – Note multiple high attenuation stones within left kidney (arrows).
5
Distal Ureteral Stone – Note high density stone in the distal right ureter (arrow).
References or Suggested Reading:
1.
Kenney PJ, McClennan BL. The Kidney from Computed Body Tomography with
MRI Correlation. 2nd ed. Philadelphia: Lippincott-Raven Press. 1998.
2.
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.
3.
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.
4.
Yamamoto S, Nakamura K, Kawanami S, et al. Renal angiomyolipoma: evolutional
changes of its internal structure on CT. Abdom Imaging 2000;25:651-4.
5.
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.
6.
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;175:168995. (2000 ARRS President’s Award. American Roentgen Ray Society)
7.
Tsuboi N, Horiuchi K, Kimura G, et al. Renal masses detected by general health
checkup. Int J Urol 2000;7:404-8.
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.
About the Author
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 ultrasound
and radiology topics at numerous conferences around the country. He is active in research
and has authored several publications in peer-review medical journals.
Examination:
1.
In general, simple renal cysts do not have perceptible walls and have attenuation
values that are
A.
< 20 Hounsfield units (HU).
B.
> 20 Hounsfield units (HU).
C.
< 40 Hounsfield units (HU).
D.
> 40 Hounsfield units (HU).
E.
> 60 Hounsfield units (HU).
2.
Areas of calcification that are ________ are usually indicative of a minimally
complicated benign cystic lesion requiring no further therapy.
A.
thin at 6 mm or less
B.
thin at 2 mm or less
C.
thin at 4 mm or less
D.
thick at 4 mm or more
E.
thick at 6 mm or more
3.
Regarding adult polycystic kidney disease,
A.
it is a genetic disorder that is inherited in an autosomal recessive fashion.
B.
the abnormal gene has been found on chromosome number 9.
C.
D.
E.
patients are usually asymptomatic until they reach theirs 60’ to 70’s.
it accounts for about 10% of end-stage renal failure.
can be associated with cysts in the liver in 3% of cases.
4.
Numerous renal cysts that are thin-walled and can be found in the cortex and
medulla ranging in size from millimeters to centimeters is most consistent with
A.
tuberous sclerosis
B.
von Hippel-Lindau disease
C.
multilocular cystic lymphoma
D.
cystic renal cell carcinoma
E.
adult polycystic kidney disease
5.
Regarding tubular sclerosis,
A.
angiomyolipomas occur in about 10% of patients.
B.
it is a genetic disorder that is inherited in an autosomal recessive fashion and
few cases are new mutations.
C.
the abnormal gene has been found on chromosome number 3.
D.
when angiomyolipomas are found in the brain, they rarely cause seizures.
E.
patients can have white moles and patches of skin that lack pigment called
“ash leaf spots”.
6.
Detecting the presence of fat by CT within the lesion in the kidney is most indicative
of
A.
tuberous sclerosis
B.
von Hippel-Lindau disease
C.
multilocular cystic lymphoma
D.
cystic renal cell carcinoma
E.
adult polycystic kidney disease
7.
In which of the following disorders, if present in the kidney, does a 45% chance for
developing renal carcinoma exist?
A.
tuberous sclerosis
B.
von Hippel-Lindau disease
C.
multilocular cystic lymphoma
D.
cystic renal cell carcinoma
E.
adult polycystic kidney disease
8.
Which of the following benign disorders has the highest likelihood for not being able
to be distinguished from carcinoma and thus most lesions are removed surgically?
A.
tuberous sclerosis
B.
von Hippel-Lindau disease
C.
multilocular cystic lymphoma
D.
acquired cystic disease of dialysis
E.
adult polycystic kidney disease
9.
The most common solid tumor of the kidney is
A.
lymphoma
B.
metastatic tumors
C.
D.
E.
renal cell carcinoma
sarcoma
oncocytoma
10.
Regarding the CT evaluation of renal cell carcinoma,
A.
the tumor extends exophytically from the kidney contour as they typically
arise from the medulla.
B.
on non-contrast CT scans, renal cell carcinomas typically are less attenuating
with the remainder of the normal kidney.
C.
following intravenous contrast, renal cell carcinomas enhance, but are
typically higher in attenuation than the remainder of the kidney during the
nephrographic or excretory phases.
D.
bolus scans during an early arterial phase may reveal a hypervascular lesion.
E.
heterogeneity within a renal carcinoma is uncommon but may include either
necrosis and or areas of calcifications.
11.
Identify renal cell carcinoma that has spread into the perinephric space is consistent
with
A.
Stage I disease
B.
Stage II disease
C.
Stage IIIA disease
D.
Stage IIIB disease
E.
Stage IV disease
12.
A five-year survival for renal cell carcinoma that is in the 30% to 40% range is
consistent with
A.
Stage I disease
B.
Stage II disease
C.
Stage IIIA disease
D.
Stage IIIB disease
E.
Stage IV disease
13.
Regarding pyelonephritis, in patients with a classic presentation and positive urine
cultures,
A.
no imaging is required if there is prompt clinical response to appropriate
antibiotic therapy.
B.
patients should be evaluated by ultrasonography as the initial approach.
C.
patients should be evaluated by IVP as the initial approach.
D.
patients should be evaluated by CT as the initial approach.
E.
patients should be evaluated by MRI as the initial approach.
14.
Regarding the CT evaluation of the infected kidney,
A.
intravenous contrast is rarely needed.
B.
acute pyelonephritis often has a lobar distribution appearing as single or
multiple wedge-shaped zones of increased attenuation.
C.
if pyelonephritis is not adequately treated, it may evolve into a liquefied
abscess, which appears on CT as a rounded water-density mass.
D.
A & B above.
E.
B & C above.
15.
Pyonephrosis is an infectious disorder
A.
that is related to tuberculosis.
B.
that is caused by gas formation within the kidney.
C.
that describes abscess formation.
D.
associated with obstruction.
E.
that describes the wedge-shaped appearance seen on CT.
16.
Renal tuberculosis (TB) is still a problem in the United States and has a variety of
common CT manifestations that may include all of the following except
A.
calcifications
B.
strictures
C.
abscesses
D.
areas of calyceal obstruction
E.
gas formation
17.
Regarding kidney stones,
A.
they are most often seen in women compared to men.
B.
the most common stone is calcium based accounting for 45% to 50% of all
stones.
C.
most individuals who develop a stone will rarely form another.
D.
many patients who develop stones will have a family history suggesting that
the disorder is often familial (or possibly inherited).
E.
stones that become large and fill the entire renal pelvis are called xanthine
stones.
18.
Regarding the CT evaluation of kidney stones,
A.
Excretory urography is still the diagnostic method of choice in patients
presenting with acute flank and suspected ureteral or renal calculi.
B.
CT is exquisitely sensitive for calcifications, even very small stones (2 to 3
mm in size) can be diagnosed with confidence.
C.
Helical CT has proven to be extremely accurate for the detection of renal
stones that contain calcium but is not good in detecting “non-opaque”
stones.
D.
A & B above.
E.
B & C above.
19.
In the clinical management of patients with a ureteral calculus, which of the
following statements is true?
A.
The only key feature determining the need for urologic intervention is the
size of the stone.
B.
Stones 4 mm or less in size pass about 40% of the time.
C.
Stones that are greater than 7 mm in size have only a 10% chance of passage.
D.
About 95% of distal ureteral stones pass spontaneously.
E.
Only 45% of mid-ureteral stones pass spontaneously.
20.
For renal 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 including all of the following except
A.
the presence of phleboliths
B.
hydronephrosis
C.
periureteral and perinephric stranding
D.
soft tissue thickening around the ureter adjacent to the stone.
E.
renal enlargement