DOI: 10.1148/rg.24si045509
RadioGraphics 2004;24:S247-S280
© RSNA, 2004
Classic Signs in Uroradiology1
Raymond B. Dyer, MD,
Michael Y. Chen, MD and
Ronald J. Zagoria, MD
1 From the Department of Radiology, Wake Forest University School of Medicine, Medical Center Blvd, Winston-Salem, NC 27157. Recipient of a Certificate of Merit award for an education exhibit at the 2003 RSNA scientific assembly. Received February 17, 2004; revision requested March 9 and received March 29; accepted May 12. All authors have no financial relationships to disclose. Address correspondence to R.B.D. (e-mail: rdyer@wfubmc.edu).
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Abstract
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The language of radiology is rich with descriptions of imaging findings, often metaphorical, which have found common usage in the day-to-day practice of genitourinary radiology. These "classic signs" give us confidence in our diagnosis. Some of the signs have become so familiar to us that they are referred to as an "Aunt Minnie." When the sign is invoked, or an Aunt Minnie is recognized, it often brings an impression of the image to mind, and it may have specific diagnostic and pathologic implications. The article uses classic signs accumulated from the literature to review a variety of pathologic conditions in the urinary tract.
© RSNA, 2004
Index Terms: Genitourinary system, CT, 80.1211 • Genitourinary system, MR, 80.12141 • Genitourinary system, radiography, 80.122, 80.123 Genitourinary system, US, 80.1298
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LEARNING OBJECTIVES FOR TEST 6
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After reading this article and taking the test, the reader will be able to:
- Recognize radiologic signs associated with urinary tract disease.
- Describe the pathophysiologic characteristics associated with the radiologic findings.
- Apply the illustrated signs across imaging modalities.
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Introduction
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Descriptions of observations made from images are the radiologist’s stock in trade. Because we "see what we know," any device that aids in the recognition and interpretation of imaging findings is useful. The article reviews "classic signs," often referred to as an "Aunt Minnie," encountered in the urinary tract.
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Staghorn and Related Signs
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A renal stone described as a staghorn implies a branched renal calculus that resembles the antlers of a stag (Fig 1). It is usually composed of struvite; but less commonly, it is formed from cystine or uric acid. In its most common form, a staghorn renal stone is associated with recurrent urinary tract infections from bacterial pathogens that produce alkaline urine. As such, it is the only type of renal stone that is more commonly seen in women (1).
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Figure 1a. (a) Staghorns. (b) On a scout image obtained before excretory urography, a calculus fills nearly the entirety of a bifid right renal collecting system, giving it a branched appearance that resembles the antlers of a stag.
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Figure 1b. (a) Staghorns. (b) On a scout image obtained before excretory urography, a calculus fills nearly the entirety of a bifid right renal collecting system, giving it a branched appearance that resembles the antlers of a stag.
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The staghorn configuration can be disrupted when infection complicates obstruction related to the stone. Renal enlargement from pyonephrosis or xanthogranulomatous pyelonephritis may produce a fragmented staghorn (2). In addition to the obstructing stone, which may be fragmented, renal enlargement and nonexcretion of contrast material from the involved kidney constitute the classic excretory urographic triad of xanthogranulomatous pyelonephritis (Fig 2). A stone associated with a nonfunctioning kidney may also be seen with pyonephrosis or long-standing hydronephrosis. CT of xanthogranulomatous pyelonephritis will also illustrate this classic triad. The replacement of the renal parenchyma by the indolent infectious process in the diffuse form of xanthogranulomatous pyelonephritis produces hypoattenuating masses arranged in a "hydronephrotic" pattern, which replaces the renal parenchyma. There may be enhancement in the margins of these masses after contrast material administration. This appearance on CT scans has been described as the bear paw sign (Fig 3) (3).
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Figure 2a. (a) Scout radiograph obtained before excretory urography demonstrates disruption of the elements of a staghorn calculus—a fragmented staghorn—in an enlarged right kidney. (b) Excretory urogram shows no evidence of contrast material excretion from the right kidney. Renal enlargement, presence of an obstructing stone, and absence of excretion are considered the classic imaging triad of xanthogranulomatous pyelonephritis.
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Figure 2b. (a) Scout radiograph obtained before excretory urography demonstrates disruption of the elements of a staghorn calculus—a fragmented staghorn—in an enlarged right kidney. (b) Excretory urogram shows no evidence of contrast material excretion from the right kidney. Renal enlargement, presence of an obstructing stone, and absence of excretion are considered the classic imaging triad of xanthogranulomatous pyelonephritis.
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Figure 3a. (a) A bear’s paws. (Photograph entitled "Bad Boys of the Arctic" reprinted with permission from Thomas D. Mangelsen, Inc.) (b) Contrast material-enhanced CT scan (same patient as in Fig 2) demonstrates a centrally obstructing stone with replacement of the renal parenchyma by low-attenuation collections in a "hydronephrotic" pattern. Note the lack of dilatation of the renal pelvis and infundibula. (c) CT scan obtained at a slightly lower level shows the fragments of a staghorn calculus within the parenchymal collections, which exhibit marginal enhancement. The pattern seen at CT resembles a bear’s paw.
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Figure 3b. (a) A bear’s paws. (Photograph entitled "Bad Boys of the Arctic" reprinted with permission from Thomas D. Mangelsen, Inc.) (b) Contrast material-enhanced CT scan (same patient as in Fig 2) demonstrates a centrally obstructing stone with replacement of the renal parenchyma by low-attenuation collections in a "hydronephrotic" pattern. Note the lack of dilatation of the renal pelvis and infundibula. (c) CT scan obtained at a slightly lower level shows the fragments of a staghorn calculus within the parenchymal collections, which exhibit marginal enhancement. The pattern seen at CT resembles a bear’s paw.
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Figure 3c. (a) A bear’s paws. (Photograph entitled "Bad Boys of the Arctic" reprinted with permission from Thomas D. Mangelsen, Inc.) (b) Contrast material-enhanced CT scan (same patient as in Fig 2) demonstrates a centrally obstructing stone with replacement of the renal parenchyma by low-attenuation collections in a "hydronephrotic" pattern. Note the lack of dilatation of the renal pelvis and infundibula. (c) CT scan obtained at a slightly lower level shows the fragments of a staghorn calculus within the parenchymal collections, which exhibit marginal enhancement. The pattern seen at CT resembles a bear’s paw.
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Jack Stone and Other Stone Configurations
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Urinary tract stones composed of calcium oxalate dihydrate can assume a spiked configuration, resembling a child’s toy jack. Although most commonly seen in the bladder, jack stones may occasionally form in the kidney (Fig 4). A stone with less well-developed spikes, giving rise to a mamillated appearance, is sometimes referred to as a mulberry stone (Fig 5). The loose crystalline lattice of calcium oxalate dihydrate allows these stones to be easily fragmented with various forms of lithotripsy, despite their formidable appearance (1).
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Renal Parenchymal Calcification
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Thin rims of dystrophic calcification may be deposited at the inner and outer margins of the renal cortex as a result of a major vascular insult that produces cortical necrosis, or rarely, as a consequence of glomerulonephritis, hyperoxaluria, and Alport syndrome, with the development of cortical nephrocalcinosis. The pattern of parenchymal calcification is said to resemble a tramline or railroad tracks (4).
Medullary nephrocalcinosis is most commonly caused by medullary sponge kidney, renal tubular acidosis, and hyperparathyroidism (1). Medullary sponge kidney is the term applied to the development of ectatic tubules occurring in the medullary pyramids (Fig 6) (5,6). As a result of stasis and the occasionally associated condition of hypercalciuria, stones may form in the cavities, producing medullary nephrocalcinosis. The growing calculus sign refers to the apparent enlargement of stones between the preliminary image and images obtained after contrast material administration, as contrast material fills the ectatic tubules harboring the stones (6).
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Figure 6a. (a) "Sponge" kidney, made from a sponge! (b) Scout image from excretory urography demonstrates calcifications clustered in the medullary portion of the left kidney. (c) After contrast material administration, numerous cavities are identified within the renal papilla in the patient with medullary sponge kidney. Some of the calcifications appear to grow, as contrast agent fills the entire cavity containing the stone.
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Figure 6b. (a) "Sponge" kidney, made from a sponge! (b) Scout image from excretory urography demonstrates calcifications clustered in the medullary portion of the left kidney. (c) After contrast material administration, numerous cavities are identified within the renal papilla in the patient with medullary sponge kidney. Some of the calcifications appear to grow, as contrast agent fills the entire cavity containing the stone.
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Figure 6c. (a) "Sponge" kidney, made from a sponge! (b) Scout image from excretory urography demonstrates calcifications clustered in the medullary portion of the left kidney. (c) After contrast material administration, numerous cavities are identified within the renal papilla in the patient with medullary sponge kidney. Some of the calcifications appear to grow, as contrast agent fills the entire cavity containing the stone.
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Perirenal Cobwebs
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Perirenal cobwebs were initially attributed to collateral vessels seen in the perinephric space in patients with renal vein thrombosis (7). As our ability to image the perinephric space with CT improved, it became clear that a number of disease processes were manifest by development of prominent perinephric structures (8). Kunin (9) formalized our understanding by describing several types of septa that compartmentalize the perirenal space and that may confine, or act as a conduit for, extension of a disease process (Fig 7). Currently, perirenal cobwebs (visualization of perirenal septa) are most frequently encountered during the CT evaluation of urinary tract obstruction from stone disease. Perirenal stranding, occurring in the setting of flank pain from ureteral colic, is an exaggeration of the visibility of these septations due to edema and fluid extravasation, and is an important secondary sign of acute ureteral obstruction from stones (10). Perirenal stranding in the asymptomatic patient is often a nonspecific finding and may be seen in benign and malignant conditions.
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Figure 7a. (a) Cobwebs. (b) CT scan obtained after extracorporeal lithotripsy reveals a subcapsular hematoma and exaggeration of Kunin septa on the left. Renofascial and renorenal septa are especially well identified. (c) On another CT scan obtained at the lower aspect of the left kidney, fasciofascial septa are nicely seen, and the cobweb appearance is particularly well developed.
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Figure 7b. (a) Cobwebs. (b) CT scan obtained after extracorporeal lithotripsy reveals a subcapsular hematoma and exaggeration of Kunin septa on the left. Renofascial and renorenal septa are especially well identified. (c) On another CT scan obtained at the lower aspect of the left kidney, fasciofascial septa are nicely seen, and the cobweb appearance is particularly well developed.
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Figure 7c. (a) Cobwebs. (b) CT scan obtained after extracorporeal lithotripsy reveals a subcapsular hematoma and exaggeration of Kunin septa on the left. Renofascial and renorenal septa are especially well identified. (c) On another CT scan obtained at the lower aspect of the left kidney, fasciofascial septa are nicely seen, and the cobweb appearance is particularly well developed.
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Soft-Tissue Rim Sign
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The CT evaluation of stone disease has given rise to new signs. The soft-tissue rim sign is caused by edema of the ureteral wall surrounding a stone at its site of impaction (Fig 8) (11). The importance of the sign lies in the fact that it may help to distinguish a stone in the ureter from a phlebolith in an adjacent vein, because the occurrence of a soft-tissue rim around a phlebolith is uncommon. It should be noted, however, that the soft-tissue rim sign may be absent with stones larger than 4 mm or when a stone is impacted at the ureterovesical junction.
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Figure 8a. (a) One kind of rim (with thanks to Christopher R. Dyer for his assistance with this photograph.) (b) CT scan shows a thin soft-tissue rim (arrow) surrounding a stone impacted in the middle of the left ureter. The rim represents edema of the ureteral wall. The presence of a tissue rim sign allows a confident diagnosis of a stone within the ureter.
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Figure 8b. (a) One kind of rim (with thanks to Christopher R. Dyer for his assistance with this photograph.) (b) CT scan shows a thin soft-tissue rim (arrow) surrounding a stone impacted in the middle of the left ureter. The rim represents edema of the ureteral wall. The presence of a tissue rim sign allows a confident diagnosis of a stone within the ureter.
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Comet Sign
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The comet sign (Fig 9) has also been used at CT to aid in the differentiation of a phlebolith from a stone in the ureter, especially in the anatomic pelvis (12). The calcified phlebolith represents the comet nucleus and the adjacent, tapering, noncalcified portion of the vein is the comet tail. The reliability of the comet sign is not as great as that for the soft-tissue rim sign, however.
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Figure 9a. (a) Comet. (Photograph entitled "Comet Hyakutake" reprinted with permission from Bill and Sally Fletcher.) (b) CT scan shows a calcification (the comet nucleus) (arrow) with a soft-tissue tail that represents a pelvic vein (arrowhead). Together, this appearance constitutes the comet sign. Note the stone at the left ureterovesical junction.
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Figure 9b. (a) Comet. (Photograph entitled "Comet Hyakutake" reprinted with permission from Bill and Sally Fletcher.) (b) CT scan shows a calcification (the comet nucleus) (arrow) with a soft-tissue tail that represents a pelvic vein (arrowhead). Together, this appearance constitutes the comet sign. Note the stone at the left ureterovesical junction.
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Other Rims
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A very important rim sign with an entirely different cause is that associated with major vascular compromise in the kidney. This sign is most commonly seen with renal artery obstruction from thrombosis, embolus or dissection. At contrast- enhanced CT or MR imaging, a 1- to 3-mm rim of subcapsular enhancement, paralleling the renal margin, can be seen as a result of preserved perfusion of the outer renal cortex by capsular perforating vessels. The finding may be partial or total depending on the level of vascular occlusion, and there may be an abrupt termination of contrast material in the renal artery referred to as the arterial cut-off sign (Fig 10) (13,14). The rim sign of vascular compromise has also been described with renal vein thrombosis and acute tubular necrosis (13).
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Figure 10a. Rim sign of vascular compromise. (a) Enhanced CT scan of a motor vehicle accident victim demonstrates no perfusion in the majority of the right kidney. The right renal artery abruptly terminates; this is the arterial cut-off sign (arrowhead). (b) Repeat contrast-enhanced CT scan, obtained 72 hours after a, demonstrates a thin marginal rim of preserved subcapsular enhancement in the left kidney, typical of the rim sign of renovascular compromise. Vascular compromise in this case was caused by intimal injury and thrombosis of the main renal artery.
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Figure 10b. Rim sign of vascular compromise. (a) Enhanced CT scan of a motor vehicle accident victim demonstrates no perfusion in the majority of the right kidney. The right renal artery abruptly terminates; this is the arterial cut-off sign (arrowhead). (b) Repeat contrast-enhanced CT scan, obtained 72 hours after a, demonstrates a thin marginal rim of preserved subcapsular enhancement in the left kidney, typical of the rim sign of renovascular compromise. Vascular compromise in this case was caused by intimal injury and thrombosis of the main renal artery.
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The reverse rim sign refers to a hypoattenuating renal cortex visualized at CT, seen against a background of intact medullary perfusion after contrast material is given. This sign also implies severe derangement of cortical blood flow with development of cortical necrosis (Fig 11) (15). Cortical necrosis may develop as a consequence of obstetric complications, shock from numerous causes, transfusion reaction or other causes of intravascular hemolysis, toxins, and rejection in the transplanted kidney (16).
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Figure 11. A reverse rim. CT scan, obtained to exclude a large retroperitoneal hematoma in a patient with sustained hypotension for 1 hour after cardiac catheterization and subsequent cardiac arrest, shows a hypoattenuating renal cortex (arrow) compared with the medullary enhancement. No additional contrast material was given after the catheterization. The patient rapidly developed multiorgan failure that led to her death.
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A different type of rim sign can be seen in association with chronic hydronephrosis. After contrast material is administered, enhancement occurs in the residual, but markedly atrophic, renal parenchyma, surrounding the dilated calices and renal pelvis. The inner margin of this hydronephrotic rim is concave toward the renal hilum, and enhancement of the cortical columns between the dilated collecting system elements may be seen. This type of rim has been observed in all forms of contrast-enhanced imaging of the obstructed kidney (Fig 12). Unopacified urine in the dilated collecting system may produce a negative pyelogram. Depending on the degree of residual excretory function, delayed imaging may show pools of contrast material or a urine–contrast material level within the distended collecting system (17).
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Figure 12. Hydronephrotic rim. CT scan, obtained in a patient with hematuria after minimal trauma, reveals a rim of enhancement surrounding a markedly dilated right renal pelvis and collecting system, findings consistent with congenital ureteropelvic junction obstruction. Note the variable thickness of the enhancing tissue rim (in contrast to the rim sign of vascular compromise [cf Fig 10]), as well as enhancement within cortical columns (arrow).
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Spotted Nephrogram
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Irregular, patchy enhancement in the renal parenchyma, referred to as the spotted nephrogram, may occur as a result of small vessel occlusion, which can be seen with necrotizing vasculitis (periarteritis nodosa), scleroderma, and hypertensive nephrosclerosis (18). Although first observed on angiograms (Fig 13), the abnormal perfusion pattern can be identified on CT (Fig 14) and MR images after contrast material administration (19).
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Figure 13a. Spotted nephrogram. (a) Spotted cat. (Courtesy of Russell I. Poole, Mishawaka, Ind.) (b) Late image from midstream aortography demonstrates patchy perfusion in both kidneys, giving the parenchyma a spotted appearance: the spotted nephrogram. (c) Late image from selective right renal arteriography in the same patient demonstrates small vessel occlusion and multiple areas of parenchymal infarction (arrow) with islands of preserved perfusion. The patient proved to have periarteritis nodosa.
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Figure 13b. Spotted nephrogram. (a) Spotted cat. (Courtesy of Russell I. Poole, Mishawaka, Ind.) (b) Late image from midstream aortography demonstrates patchy perfusion in both kidneys, giving the parenchyma a spotted appearance: the spotted nephrogram. (c) Late image from selective right renal arteriography in the same patient demonstrates small vessel occlusion and multiple areas of parenchymal infarction (arrow) with islands of preserved perfusion. The patient proved to have periarteritis nodosa.
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Figure 13c. Spotted nephrogram. (a) Spotted cat. (Courtesy of Russell I. Poole, Mishawaka, Ind.) (b) Late image from midstream aortography demonstrates patchy perfusion in both kidneys, giving the parenchyma a spotted appearance: the spotted nephrogram. (c) Late image from selective right renal arteriography in the same patient demonstrates small vessel occlusion and multiple areas of parenchymal infarction (arrow) with islands of preserved perfusion. The patient proved to have periarteritis nodosa.
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Figure 14. Spotted nephrogram. CT scan of another patient with periarteritis nodosa demonstrates the CT correlate of the angiographic findings, with patchy perfusion of the kidneys caused by multiple areas of infarction. (Case courtesy of N. Reed Dunnick, MD, University of Michican Health System, Ann Arbor, Mich.)
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Crescent Sign
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Abstract
LEARNING OBJECTIVES FOR TEST...
