(Radiographics. 2000;20:439-447.)
© RSNA, 2000
Pearls and Pitfalls in the Diagnosis of Ureterolithiasis with Unenhanced Helical CT1
(CME available in print version and on RSNA Link)
Neal C. Dalrymple, MD ,
Brant Casford, MD ,
David P. Raiken, MD ,
Kelcey D. Elsass, MD and
Rafael A. Pagan, MD
1 From the Department of Diagnostic Radiology, Wilford Hall Medical Center (59 MDW/MTRD), 2200 Bergquist Dr, Suite 1, Lackland Air Force Base, TX 78236-5302. Presented as a scientific exhibit at the 1998 RSNA scientific assembly. Received April 8, 1999; revisions requested April 20 and received May 26; accepted May 26. Address reprint requests to N.C.D. (e-mail: ncdalrymple@earthlink.net).
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Abstract
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Several signs to assist interpretation of unenhanced helical computed tomographic (CT) scans obtained for suspected ureterolithiasis have been described. Because signs such as perinephric stranding are not always readily apparent, a methodical approach to interpretation of CT studies is important in determining the presence or absence of ureterolithiasis. Evaluation of the poles of the kidneys is helpful in detecting subtle stranding of the perinephric fat. Inspection of the intrarenal collecting system within the poles of the kidneys is helpful in identifying subtle collecting system dilatation and can help prevent mistaking an extrarenal pelvis for hydronephrosis. Careful inspection of the ureter throughout its course is the most reliable method of distinguishing between ureteral stones and phleboliths. However, when the ureter cannot be followed antegrade, the pelvic portion can often be identified in a retrograde fashion. When secondary signs of obstruction are present but no stone is present, differential diagnostic considerations include a recently passed stone, pyelonephritis, urinary tract obstruction unrelated to stone disease, and protease inhibitor deposition disease.
Index Terms: Ureter, calculi, 82.81 • Ureter, CT, 82.12115 • Ureter, stenosis or obstruction, 82.843
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Introduction
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Unenhanced helical computed tomography (CT) has proved to be an accurate, safe, and rapid examination used to diagnose and treat patients presenting with acute flank pain (1,2). Secondary signs of urinary tract obstruction and other signs such as the soft-tissue rim sign have been described that aid in interpretation of the CT examination. However, most studies have described the presence or absence of signs such as perinephric stranding without providing methods for identifying them. This article assists the reader in identifying signs of ureterolithiasis and proposes a methodical approach to interpretation of CT scans based on the review of over 400 unenhanced helical CT examinations performed in the evaluation of acute flank pain. Herein, we describe the CT technique, outline an evaluation of the secondary signs of obstruction as well as a method for characterizing calcifications, and discuss protease inhibitor deposition.
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Technique
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The CT examination performed for suspected ureterolithiasis is conducted without intravenous or enteric administration of contrast material. Because most ureteral stones are small, narrow collimation of 5 mm is recommended. Most institutions perform helical CT with a pitch of 1, although there is no documented disadvantage to increasing the table speed. Multiplanar reconstructions are seldom necessary, but they can be used to illustrate stone location or other anatomic relationships.
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Secondary Signs of Urinary Tract Obstruction
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Interpretation of the CT scans begins with inspection for secondary signs of urinary tract obstruction. The secondary signs of obstruction first described include asymmetric stranding of the perinephric fat, dilatation of the intrarenal collecting system, hydroureter, and unilateral renal enlargement (3). Subsequently, unilateral absence of the white pyramid was described as an additional secondary sign.
Stranding of the perinephric fat is a common finding, particularly among older patients. The stranding likely represents fluid that collects within the bridging septa of the perinephric fat as a result of increased lymphatic pressure. Fluid collections may become asymmetric in the presence of unilateral obstruction or pyelonephritis. More focal, nonlinear perinephric fluid collections associated with obstruction likely represent extravasated urine as a result of forniceal rupture and should be reported, since many clinicians treat such patients with prophylactic antibiotics.
Although unilateral or asymmetric perinephric stranding is often obvious (Fig 1), the findings may be difficult to discern. When the difference in stranding is subtle, it may be apparent only at the upper or lower poles of the kidneys. These differences become more conspicuous when the most superior and most inferior sections of each kidney are compared directly with those of the contralateral kidney (Fig 2). Asymmetric stranding of the perinephric fat may be seen as a loss of definition of the fat-kidney interface or a very fine linear stranding compared with that of a well-defined interface around the contralateral kidney. Subtle perinephric stranding may be obscured unless the window and level of the CT image is such that the fat is readily visible as gray rather than black.
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Figure 1. Readily apparent perinephric stranding. Axial CT scan shows stranding of the fat surrounding the left kidney and proximal left ureter. In addition, the left kidney is enlarged, with dilatation of the intrarenal collecting system.
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Figure 2. Subtle perinephric stranding. On an axial CT scan, the interface between the lower pole of the left kidney with the surrounding fat is indistinct, indicating mild stranding of the perinephric fat. In contrast, margins of the lower pole of the right kidney are well defined.
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Evaluation for collecting system dilatation should focus on the renal sinus in the upper and lower poles. Because the presence of an extrarenal pelvis may lead to an erroneous diagnosis of urinary tract obstruction, apparent dilatation of the central portions of the renal collecting system must be viewed with caution (Fig 3). There is considerably less variation in the intrarenal portions of the collecting system. For this reason, collecting system dilatation is best identified in the upper and lower poles. Dilated calices and infundibula appear as rounded fluid-filled structures that partially obliterate the renal sinus fat compared with the contralateral kidney (Fig 4). These structures may become mildly enlarged before significant enlargement of the renal pelvis occurs.
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Figure 3a. Extrarenal pelvis. (a) Axial CT scan shows apparent dilatation of the left renal pelvis. (b) Axial CT scan obtained at a slightly lower level shows that the extrarenal pelvis tapers rapidly to a normal proximal ureter that is not dilated.
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Figure 3b. Extrarenal pelvis. (a) Axial CT scan shows apparent dilatation of the left renal pelvis. (b) Axial CT scan obtained at a slightly lower level shows that the extrarenal pelvis tapers rapidly to a normal proximal ureter that is not dilated.
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Figure 4. Collecting system dilatation. Axial CT scan shows dilated, fluid-filled structures that partially obliterate the renal sinus fat in the lower pole of the left kidney. Smaller, rounded structures in the renal sinus of the right kidney represent the normal collecting system.
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When ureteral dilatation is present, it is usually readily identified, although continuity with the renal pelvis should be verified to avoid mistaking a thrombosed or enlarged gonadal vein for the ureter.
Recently, unilateral absence of the white pyramid was described as an additional secondary sign of urinary tract obstruction (4). Bilateral high-attenuation renal pyramids are an occasional incidental finding. Ureteral obstruction may result in tubular hydronephrosis, decreasing the attenuation of the medullary pyramid on the obstructed side so that the pyramids have high attenuation on only the unobstructed side (Fig 5). Although the sensitivity and specificity of this sign are yet to be proved, the finding of high-attenuation medullary pyramids in only one kidney suggests the presence of obstruction in the contralateral kidney.
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Figure 5a. Unilateral absence of the white pyramid as a sign of urinary tract obstruction. (a) Axial CT scan of a patient with a stone in the distal left ureter shows the renal pyramids, which appear hyperattenuating in the right kidney (arrows) and isoattenuating relative to renal parenchyma in the left kidney. (b) CT image of another patient reveals hyperattenuating pyramids in the left kidney and isoattenuating pyramids in the obstructed right kidney, which has a horizontal axis.
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Figure 5b. Unilateral absence of the white pyramid as a sign of urinary tract obstruction. (a) Axial CT scan of a patient with a stone in the distal left ureter shows the renal pyramids, which appear hyperattenuating in the right kidney (arrows) and isoattenuating relative to renal parenchyma in the left kidney. (b) CT image of another patient reveals hyperattenuating pyramids in the left kidney and isoattenuating pyramids in the obstructed right kidney, which has a horizontal axis.
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Characterizing Calcifications
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After the evaluation for secondary signs of obstruction, the ureters are inspected for calcifications. This evaluation should begin by following the course of each ureter from the renal pelvis to the bladder. One of the most common difficulties is distinguishing distal ureteral stones from phleboliths. Although some authors have attempted to characterize phleboliths and stones according to attenuation characteristics (5,6), no reliable criteria have been established as yet. There is no substitute for establishing continuity of a calcification within the ureteral lumen on sequential images. This assessment can be accomplished by identifying the ureter on each image from the renal pelvis to the ureteropelvic junction or, when viewing scans at a workstation, by following the course of the ureter in cine mode. However, even the meticulous observer who is knowledgeable about ureteral anatomy cannot follow the ureter throughout its course in some patients, particularly when there is a paucity of retroperitoneal fat.
Confusion most often arises on following the course of the ureter into the pelvis where the ureter may be obscured between bowel loops and the iliac vessels. The problem may be compounded when the CT examination consists of two or more helical acquisitions that are not perfectly contiguous. Fortunately, most phleboliths occur in the more inferior portions of the pelvis, and the ureter can often be traced in a retrograde fashion from the bladder. The ureterovesical junction is often identified as a high-attenuation, thickened region in the midline of the posterior bladder wall. From this region, the distal ureter can be traced back so that continuity of the calcification with the ureter can be either verified or excluded (Figs 6, 7).
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Figure 6. Normal ureterovesical junctions. Axial CT scan shows bilateral hyperattenuating areas of focal thickening of the posterior bladder wall (arrows), an appearance that represents normal ureterovesical junctions.
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Figure 7a. Retrograde identification of the ureters. (a) Axial CT scan shows the region of the ureterovesical junctions, which are visualized as focal thickening of the posterior bladder wall (arrows). (b) A more cranial image shows a calcification without a soft-tissue rim posterior in the right pelvis. The ureters can be traced back from the ureterovesical junctions (arrows) and are separate from the calcification. (c) On another more proximal image, the distal ureters that could not be followed from above are now readily identified (arrows). The right ureter is clearly anterior to the pelvic calcification, which can be definitively characterized as a phlebolith.
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Figure 7b. Retrograde identification of the ureters. (a) Axial CT scan shows the region of the ureterovesical junctions, which are visualized as focal thickening of the posterior bladder wall (arrows). (b) A more cranial image shows a calcification without a soft-tissue rim posterior in the right pelvis. The ureters can be traced back from the ureterovesical junctions (arrows) and are separate from the calcification. (c) On another more proximal image, the distal ureters that could not be followed from above are now readily identified (arrows). The right ureter is clearly anterior to the pelvic calcification, which can be definitively characterized as a phlebolith.
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Figure 7c. Retrograde identification of the ureters. (a) Axial CT scan shows the region of the ureterovesical junctions, which are visualized as focal thickening of the posterior bladder wall (arrows). (b) A more cranial image shows a calcification without a soft-tissue rim posterior in the right pelvis. The ureters can be traced back from the ureterovesical junctions (arrows) and are separate from the calcification. (c) On another more proximal image, the distal ureters that could not be followed from above are now readily identified (arrows). The right ureter is clearly anterior to the pelvic calcification, which can be definitively characterized as a phlebolith.
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The soft-tissue rim sign can be used when the ureter cannot be confidently identified by any of the above methods (7). Although not all ureteral stones have this sign, approximately 77% of them are surrounded by a rim of soft tissue (Figs 8, 9). Less than 10% of phleboliths have a soft-tissue rim. Therefore, the absence of the rim sign does not have a high negative predictive value, but its presence is highly predictive of a ureteral stone.
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Figure 8a. Soft-tissue rim sign in the ureter. (a) Axial CT scan shows a rim of soft tissue surrounding a stone in the midportion of the left ureter. (b) On an axial CT scan of a different patient, no soft tissue is present around a gonadal vein phlebolith. The normal left ureter was followed from above and identified as separate from the calcification (arrow).
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Figure 8b. Soft-tissue rim sign in the ureter. (a) Axial CT scan shows a rim of soft tissue surrounding a stone in the midportion of the left ureter. (b) On an axial CT scan of a different patient, no soft tissue is present around a gonadal vein phlebolith. The normal left ureter was followed from above and identified as separate from the calcification (arrow).
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Figure 9. Soft-tissue rim sign in the pelvis. Axial CT scan shows a rim of soft tissue surrounding a stone in the distal left ureter. A phlebolith in the posterior right pelvis is surrounded by fat.
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Another pitfall may be encountered in the diagnosis of a passed stone. Although the soft tissue of the ureterovesical junction is often seen surrounding distal stones, the tissue may be so thin as to be imperceptible. This misleading appearance may lead to the conclusion that the stone has passed into the bladder. The presence of both secondary signs of obstruction and a stone that appears to be within the bladder may lead to the misdiagnosis of a passed stone. In such cases, rescanning the patient in the prone position helps one differentiate between passed stones that fall anteriorly and stones within the ureterovesical junction that do not (Fig 10) (8).
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Figure 10a. Differentiation of a ureterovesical junction stone from a stone that has passed into the bladder. (a) Axial CT scan of a patient with right flank pain, obtained with the patient supine, shows a stone adjacent to the right posterior bladder wall. Differential diagnosis includes a passed versus ureterovesical junction stone. (b) On a scan obtained with the patient placed prone, the stone does not fall anteriorly, indicating the stone is still within the ureterovesical junction. (c) Axial CT scan of another patient shows a stone posteriorly in the bladder when the patient is supine. (d) On a scan obtained with the second patient placed prone, the stone moves freely within the bladder and rests on the anterior bladder wall.
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Figure 10b. Differentiation of a ureterovesical junction stone from a stone that has passed into the bladder. (a) Axial CT scan of a patient with right flank pain, obtained with the patient supine, shows a stone adjacent to the right posterior bladder wall. Differential diagnosis includes a passed versus ureterovesical junction stone. (b) On a scan obtained with the patient placed prone, the stone does not fall anteriorly, indicating the stone is still within the ureterovesical junction. (c) Axial CT scan of another patient shows a stone posteriorly in the bladder when the patient is supine. (d) On a scan obtained with the second patient placed prone, the stone moves freely within the bladder and rests on the anterior bladder wall.
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Figure 10c. Differentiation of a ureterovesical junction stone from a stone that has passed into the bladder. (a) Axial CT scan of a patient with right flank pain, obtained with the patient supine, shows a stone adjacent to the right posterior bladder wall. Differential diagnosis includes a passed versus ureterovesical junction stone. (b) On a scan obtained with the patient placed prone, the stone does not fall anteriorly, indicating the stone is still within the ureterovesical junction. (c) Axial CT scan of another patient shows a stone posteriorly in the bladder when the patient is supine. (d) On a scan obtained with the second patient placed prone, the stone moves freely within the bladder and rests on the anterior bladder wall.
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Figure 10d. Differentiation of a ureterovesical junction stone from a stone that has passed into the bladder. (a) Axial CT scan of a patient with right flank pain, obtained with the patient supine, shows a stone adjacent to the right posterior bladder wall. Differential diagnosis includes a passed versus ureterovesical junction stone. (b) On a scan obtained with the patient placed prone, the stone does not fall anteriorly, indicating the stone is still within the ureterovesical junction. (c) Axial CT scan of another patient shows a stone posteriorly in the bladder when the patient is supine. (d) On a scan obtained with the second patient placed prone, the stone moves freely within the bladder and rests on the anterior bladder wall.
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Protease Inhibitor Deposition
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Although virtually all stones previously considered radiolucent on plain radiographs, such as uric acid stones, are readily identified on CT scans, the recent use of protease inhibitors to treat human immunodeficiency viral (HIV) disease has led to an increasing prevalence of urinary tract obstruction caused by deposition of crystals that are nonopaque on CT scans (9). The presence of secondary signs of obstruction on the symptomatic side in the absence of an identifiable calculus typically prompts a differential diagnosis of a passed stone, pyelonephritis, or obstruction unrelated to stone disease. However, in a patient undergoing therapy with the protease inhibitor indinavir (Crixivan; Merck, Rahway, NJ), the same constellation of findings should suggest the diagnosis of indinavir crystal deposition (Fig 11). Because demonstration of indinavir crystals requires gas chromatography, which is not available in most hospital laboratories, intravenous urography or retrograde urography should be used as they provide a rapid way to confirm the diagnosis of crystal deposition disease suspected on the basis of CT findings and clinical history.
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Figure 11a. Protease inhibitor deposition in a 35-year-old man with HIV disease taking indinavir who presented with right flank pain radiating to the right groin. (a) Axial CT scan of the kidneys shows dilatation of the right renal collecting system, as well as mild stranding of the perinephric fat medial to the lower pole of the right kidney. (b) A more inferior image shows dilatation of the right ureter (arrow). (c) Another more inferior scan shows that the right ureter remains dilated to the ureterovesical junction (arrow). No calcification was identified. (d) Subsequent retrograde ureterogram shows multiple filling defects in the distal right ureter. Stricture of the ureter distal to the indinavir fragments is likely caused by recent stone impaction near the ureterovesical junction.
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Figure 11b. Protease inhibitor deposition in a 35-year-old man with HIV disease taking indinavir who presented with right flank pain radiating to the right groin. (a) Axial CT scan of the kidneys shows dilatation of the right renal collecting system, as well as mild stranding of the perinephric fat medial to the lower pole of the right kidney. (b) A more inferior image shows dilatation of the right ureter (arrow). (c) Another more inferior scan shows that the right ureter remains dilated to the ureterovesical junction (arrow). No calcification was identified. (d) Subsequent retrograde ureterogram shows multiple filling defects in the distal right ureter. Stricture of the ureter distal to the indinavir fragments is likely caused by recent stone impaction near the ureterovesical junction.
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Figure 11c. Protease inhibitor deposition in a 35-year-old man with HIV disease taking indinavir who presented with right flank pain radiating to the right groin. (a) Axial CT scan of the kidneys shows dilatation of the right renal collecting system, as well as mild stranding of the perinephric fat medial to the lower pole of the right kidney. (b) A more inferior image shows dilatation of the right ureter (arrow). (c) Another more inferior scan shows that the right ureter remains dilated to the ureterovesical junction (arrow). No calcification was identified. (d) Subsequent retrograde ureterogram shows multiple filling defects in the distal right ureter. Stricture of the ureter distal to the indinavir fragments is likely caused by recent stone impaction near the ureterovesical junction.
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Figure 11d. Protease inhibitor deposition in a 35-year-old man with HIV disease taking indinavir who presented with right flank pain radiating to the right groin. (a) Axial CT scan of the kidneys shows dilatation of the right renal collecting system, as well as mild stranding of the perinephric fat medial to the lower pole of the right kidney. (b) A more inferior image shows dilatation of the right ureter (arrow). (c) Another more inferior scan shows that the right ureter remains dilated to the ureterovesical junction (arrow). No calcification was identified. (d) Subsequent retrograde ureterogram shows multiple filling defects in the distal right ureter. Stricture of the ureter distal to the indinavir fragments is likely caused by recent stone impaction near the ureterovesical junction.
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Summary
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The use of unenhanced helical CT offers the radiologist and clinician a rapid method for evaluating patients presenting with acute flank pain without the risks associated with the use of intravenously administered contrast media. Although the findings may be readily apparent in many cases, a methodical approach to the interpretation of the CT scans can help the radiologist identify subtle signs of urinary tract obstruction and to resolve confusion between ureteral stones and phleboliths. Careful inspection of the perinephric fat surrounding the upper and lower renal poles allows identification of subtle stranding. Identification of the renal collecting system within the upper and lower poles of the kidneys allows detection of subtle signs of dilatation and helps prevent mischaracterization of an extrarenal pelvis as hydronephrosis. Although following the course of the ureter from one image to the next in either antegrade or retrograde fashion is the most specific method for characterizing calcifications, the soft-tissue rim sign can be helpful in difficult cases. Ureterovesical stones can be differentiated from stones that have recently passed into the bladder by rescanning the patient in the prone position. Finally, although the presence of secondary signs of obstruction without identification of a stone should prompt a differential diagnosis including a recently passed stone, pyelonephritis, or obstruction unrelated to stone disease, protease inhibitor deposition should also be considered in patients with HIV disease undergoing treatment with indinavir.
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Footnotes
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Abbreviation: HIV = human immunodeficiency virus
LEARNING OBJECTIVES After reading this article and taking the test, the reader will be able to:
• Identify secondary signs of urinary tract obstruction, even when subtle, on unenhanced helical CT scans.
• List several techniques for tracing the ureter to allow differentiation between ureteral stones and phleboliths.
• Provide a concise differential diagnosis for the presence of secondary signs of urinary tract obstruction on unenhanced CT scans when no stone is identified.
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References
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Smith RC, Essenmacher KR, Rosenfield AT, Choe KA. Acute flank pain: comparison of non–contrast-enhanced CT and intravenous urography. Radiology 1995; 194:789-794.[Abstract/Free Full Text]
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Dalrymple NC, Verga M, Anderson KR, et al. The value of unenhanced helical computerized tomography in the management of acute flank pain. J Urology 1998; 159:735-740.[Medline]
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Smith RC, Verga M, Dalrymple N, McCarthy S, Rosenfield AT. Acute ureteral obstruction: value of secondary signs on helical unenhanced CT. AJR Am J Roentgenol 1996; 167:1109-1113.[Abstract/Free Full Text]
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Roychowdhury A, Makris J, Colby JM, et al. Unilateral absence of the "white pyramid" sign on non-contrast CT: a sign of tubular hydronephrosis?; Presented at the 98th Meeting of the American Roentgen Ray Society, Scientific Session 33, San Francisco, Calif, April 26–May 1, 1998..
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Bell TV, Fenlon HM, Davison BD, Ahari HK, Hussan S. Unenhanced helical CT criteria to differentiate distal ureteral calculi from pelvic phleboliths. Radiology 1998; 207:363-367.[Abstract/Free Full Text]
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Traubici J, Neitlich JD, Smith RC. Distinguishing pelvic phleboliths from distal ureteral stones on unenhanced helical CT: is there a radiolucent center?. AJR Am J Roentgenol 1999; 172:13-17.[Abstract/Free Full Text]
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Heneghan JH, Dalrymple NC, Verga M, Rosenfield AT, Smith RC. Soft-tissue "rim" sign in the diagnosis of ureteral calculi with use of unenhanced helical CT. Radiology 1997; 202:709-711.[Abstract/Free Full Text]
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Levine J, Neitlich J, Smith RC. The value of prone scanning to distinguish ureterovesical junction stones from ureteral stones that have passed into the bladder: leave no stone unturned. AJR Am J Roentgenol 1999; 172:977-981.[Abstract/Free Full Text]
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Blake SP, McNicholas MMJ, Raptopoulos V. Nonopaque crystal deposition causing ureteric obstruction in patients with HIV undergoing indinavir therapy. AJR Am J Roentgenol 1998; 171:717-720.[Abstract/Free Full Text]
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Abstract
Introduction
