- Email: [email protected]
Applications of spiral CT in genitourinary imaging
New Developments in Medicine
Applications of Spiral CT in Genitourinary Imaging' Sachiko T. Cochran, MD
Spiral computed tomography (CT), which is also called helical or volume-acquisition CT, is performed by advancing the patient on the gantry during simultaneous tube rotation with continuous x-ray exposure. This represents the latest technologic advance in CT imaging (1). The 1-second tube rotations coupled with continuous forward movement of the table permits volumetric acquisition of patient data during a single breath hold. Spiral CT has advantages over conventional, incremental CT for imaging of the genitourinary tract. In the evaluation of the renal parenchyma for masses and stones, the elimination of respiratory misregistration and the ability to reconstruct overlapping images at arbitrary intervals improves visualization. In patients with symptoms of acute urinary tract obstruction, short acquisition times allow us to replace excretory urography with unenhanced spiral CT of the abdomen and pelvis for evaluation. Contrast material enhancement can be optimized by appropriate timing of the spiral scanning to coincide with peak contrast enhancement, thus enabling new applications such as CT angiography (2). This article describes and discusses the new imaging applications that are now possible with spiral CT technology.
The ability of spiral CT to acquire volume data provides an alternative modality for vascular imaging. In the genitourinary tract, spiral CT angiography has been shown to be useful in quantifying the number of arteries (3). Acad Radiol 1998; 5:380-389 1From the Department of Radiological Sciences, UCLA Medical Center for Health Sciences, 10833 Le Conte Ave, Los Angeles, CA 900241721. Received August 8, 1997; revision requested August 25; revision received September 8; a c c e p t e d September 12, Address reprint requests to SE.C, ©AUR, 1998
380
Living Renal Donor Evaluation Advances in surgical technique, tissue typing, and immunosuppressive agents over the past decade have improved the 1-year survival rate of patients with renal transplants to more than 80%. Because there is an inadequate supply of cadaver kidneys, living, related donor transplantation programs have expanded to help meet the need. The traditional imaging procedures used to evaluate potential living renal donors have been excretory urography and renal arteriography. Spiral CT angiography, including conventional radiography during the excretory phase, has been shown to be a suitable replacement for these two imaging studies (3,4). In this application of spiral CT angiography, opacification of the bowel is not necessary. A total of 125 mL of contrast material is injected intravenously at a rate of 4-5 mL/sec. After a 20-second delay, imaging is performed. Because most donors are relatively young and healthy, we have not found it necessary to give a test injection to generate a time-attenuation curve. The donor hyperventilates and then holds his or her breath for 30 seconds while the images are acquired. Spiral CT is performed with 3-mm collimation at a table speed of 5 or 6 mm/sec (pitch, 1.7 or 2) at 210 mA and 120 kV. At 8-10 minutes after injection of the contrast material, conventional radiographs (anteroposterior abdomen, oblique as needed, and postvoid views) are obtained. CT data are reconstructed at 2-mm increments. Multiplanar reformatting can be used to display vessels in a second plane. The data set can be used to generate three-dimensional reconstructions (Fig 1). Excretory urography has traditionally been used to identify the abnormalities such as benign and malignant parenchymal masses, congenital anomalies, double ureters, calculi, and obstructions that are encountered in 12%-13% of donors. The increased sensitivity of CT over excretory urography and angiography in depicting renal masses represents an important improvement in screening
a.
b.
Figure 1. Spiral CT angiograms in a 32-year-old male donor with a single renal artery bilaterally. (a) Axial image through the region of the origin of single renal arteries from the aorta. (b) Three-dimensional maximum intensity projection reconstruction of volume data displayed in coronal projection.
Figure 2. Spiral CT angiogram in a 23-year-old female donor with unsuspected scars from bilateral chronic pyelonephritis. Image reveals cortical scars in the lower pole of both kidneys (arrows), as well as in the upper pole of the right kidney (not shown).
Figure 4. Spiral CT angiogram in a 50-year-old female donor with an incidentally discovered 1~/2-cm cystic mass in the body of the pancreas (arrow).
Figure 3. Spiral CT angiogram in a 50-year-old female donor with an incidentally discovered 1-cm vascular mass in the left kidney (arrow). It is unlikely that this mass would have been depicted at conventional excretory urography.
(Figs 2, 3). In addition, because other organs are simultaneously imaged, incidental findings that may preclude donation are also depicted, even when these disorders are outside the urinary tract (Fig 4). Conventional arteriography has been used to identify the number of renal arteries and early prehilar branches, as well as vascular disease. Given that 29% of kidneys are supplied by more than one renal artery, this is an important presurgical screening examination. We have demonstrated that the overall accuracy rate in determining the number of renal arteries with spiral CT angiography is comparable to that with conventional arteriography (3). In a group of 40 patients who underwent spiral CT angiography, conventional arteriography, and donor nephrectomy, there was agreement between the results of spiral CT angiography and surgery in 90% and
381
O.
b.
Figure5. Spiral CT angiograms in a 23-year-old man with left UPJ obstruction. (o) Renal vein (arrow) is anterior to the UPJ. (b) Second renal artery (arrow) (more superior artery not shown) also courses anterior to the ureter and renal vein. S = stent in ureter. agreement between results of conventional angiography and surgery in 87% of the cases. While conventional angiography has often been considered to be the standard, it depicts 10% fewer renal arteries than are found at autopsy (5). Because these small (smaller than 1 mm diameter) accessory arteries supply blood to an insignificant portion of renal parenchyma, their preservation during surgery is not critical. Spiral CT angiography, likewise, does not identify these tiny accessory vessels. We no longer perform renal arteriography in potential renal donors. We have replaced excretory urography and angiography with a single study that includes spiral CT angiography and coiaventional radiography. The advantages of a combined study include reduced exposure to contrast material, improved patient comfort, less morbidity, and less radiation exposure. We have estimated that the combined study costs 35%-50% less than excretory urography and angiography (3).
Evaluation of Crossing Vessel at Ureteropelvic Junction Spiral CT angiography can be used to depict vessels that cross the ureteropelvic junction (UPJ), information that is valuable in planning surgical management in patients with obstruction (Fig 5). Using endoscopic tools, urologists now treat UPJ obstruction by endoluminally incising the UPJ longitudinally with a cutting device that is introduced percutaneously or retrograde to the obstruction (ie, endopyelotomy) (6,7). Endopyelotomy, which
382
Figure6. Spiral CT angiogram obtained in a 27-year-old w o m a n because of renal artery stenosis. The left kidney is 2 cm smaller than the right. On the basis of this image, fibromuscular dysplasia is suspected (note irregularity in the wall of the ]eft renal artery).
has become a standard procedure for the treatment of UPJ obstruction, has a success rate of 80%-90%. Patients with posterior or posterolateral vessels are not candidates for endopyelotomy and should be treated laparascopically or with open surgical pyelotomy. Vascular complications such as hematoma, pseudoaneurysm, and arteriovenous fistula occur if retroperitoneal vessels caused the UPJ obstruction and are incised. This is not unexpected, as 75% of those with UPJ obstruction have vessels near the UPJ. The vessels are usually located anterior to the UPJ, although they have been found posterolateral to it in 5%-10% of patients (8,9). Quillin
Vol 5, No 5, May 1998
et al (10) found that of 24 patients with UPJ obstruction who were evaluated with spiral CT angiography, 11 (46%) had crossing vessels and three of the vessels were posterior to the UPJ. The presence of a ureteral stent was helpful, but not essential, in determining the relationship of the ureter to the vasculature. Evaluation of Renal Artery Stenosis Spiral CT angiography is a good screening procedure for renal artery stenosis when the study includes the primary transaxial data set (Fig 6). In a study of 52 patients, Prokop et al (11) found that normal spiral CT angiography findings rule out a hemodynamically significant (>50%) renal artery stenosis with a sensitivity of 100%, specificity of 92%, and negative predictive value of 100%. In a study of 31 patients, Rubin et al (12) found that a three-dimensional rendering technique of maximum intensity projection based On volumetric spiral CT angiography data had a sensitivity of 92% and a specificity of 83% for the depiction of 70% or greater stenoses. Shaded surface display had a sensitivity of 59% and a specificity of 82%1 These results reinforce the importance of the primary data set in the evaluation of renal artery stenosis. Unlike renal donors and persons with crossing vessels at the UPJ, patients suspected of having renal artery stenosis often have cardiovascular compromise, which can affect the Circulation time. To determine the optimum time for imaging, a 20-mL bolus of contrast material is injected at a rate of 5 mL/sec. After a delay of 10 seconds, serial axial images at a single level just superior tO the renal arteries are obtained every 2 seconds. CT numbers within the aorta are obtained, resulting in a time-attenuation curve. The dec lay time for the CT angiography is selected at the peak of the calculated time-attenuation curve.
When a patient presents in the emergency department with sudden onset of flank pain and microscopic hematuria, an acutely obstructing ureteral calculus is suspected, and excretory urography is the usual examination performed to confirm the clinical suspicion, Excretory urography, however, has certain limitations in that it requires intravenous injection of contrast material, it may take hours to identify the level of obstruction in cases of high-grade obstruction, and ureteral calculi may not be visible on conventional radiographs so that the cause of obstruction may be elusive at excretory urography. It is
SPIRAL CT IN GENITOURINARY IMAGING
estimated that 25% of stones passing through the ureters are not seen at excretory urography, especially if the stone is not causing obstruction at the time of examination. A substitute for excretory urography, unenhanced spiral CT of the kidney, ureter, and bladder, which was first described by Smith et al (13), is becoming widely used (Fig 7). It is faster to perform and more accurate than excretory urography in the evaluation of flank pain, and it does not require intravenous injection of contrast material. At our institution, the patient charge for this examination is equivalent to that for excretory urography. Spiral scanning should be performed contiguously through the kidneys down to the bladder. Five-millimeter collimation with a pitch of 1.5 to 1.8 is used, and the data are reconstructed at 3-5-mm intervals. No oral or intravenous contrast medium is administered. When the positive findings of dilatation of the calyces, hydroureter, and a stone in the ureter are present, the diagnosis of obstructing ureteral Calculus is made and no further testing is necessary. The limit of resolution for a 2-ram stone is 5-mm Collimation. Two-millimeter stones are not detectable with 7- or 10-mm collimation, even with 3-ram section reconstructions. Three-millimeter stones are consistently depicted with 10-ram collimation (14). When a stone is identified in the ureter, spiral CT can precisely depict its size and location. The location of the stone may be important in planning treatment with extracorporeal shock wave lithotripsy or ureteroscopic manipulation. Most stones that are 4 mm or smaller in diameter pass spontaneously. Secondary signs seen at spiral CT are also helpful, particularly if a stone is not identified with certainty. Ureteral dilatation, perinephric fat stranding (Fig 8), dilated intrarenal collecting system, periureteric stranding, or ureterovesical junction edema indicate a stone. If these signs are present, however, and a ureteral stone is not seen, the stone may have recently passed. One area in which the spiral CT scan is not helpful is in distinguishing a distal ureteral calculus from a phlebolith. Edema within the wall of the ureter caused by acute obstruction may cause a circumferential rim of soft-tissue attenuation around a calculus (rim sign) (Fig 7). This sign has been reported in 45%-77% of studies in which ureteral calculi were present and 6%-8% of studies in which phleboiiths were present. The absence of a rim sign is not helpful (15,16). Spiral CT has a sensitivity of 91%97% for detection of urinary tract calculi, a specificity of 96%-98%, and an accuracy of 94%-98% (17-19). In the presence of cysts in the renal sinus or
383
a.
b.
Figure 8. Unenhanced spiral CT scans in a 37-year-old man with left abdominal pain. (a) Image through the kidneys shows left perinephric fluid and stranding and dilated calyx and proximal ureter. Note the nonobstructing stones in the right kidney, (b) image through the ureterovesical junction shows a stone.
a.
b.
Figure 7. Unenhanced spiral CT scans obtained with 5-mm section thickness in a 52-year-old man with acute onset of left flank pain, (a) image through the kidneys demonstrates pelvicalyceal dilatation on the left. (b) Image through the region of the midureter demonstrates dilatation of the left ureter (arrow) with mild stranding around the ureter. (c) Image through the ureterovesical junction demonstrates a stone in the left ureter with surrounding ureter wall edema (rim sign) (arrow).
C.
384
a.
b.
Figure 9. Spiral CT scans o b t a i n e d in a 46-year-old man with episodic left a b d o m i n a l pain and microscopic hematuria and in w h o m u n e n h a n c e d spiral CT findings were negative. (a) CT urogram demonstrates d e l a y in excretion on the left. (b) Delayed scan demonstrates w e d g e - s h a p e d areas of contrast material retention consistent with a c u t e pyelonephritis. The u n e n h a n c e d scans (not shown) h a d shown no e v i d e n c e of obstruction a n d no perinephric stranding.
a.
b.
Figure 10. Images of a 43-year-old man with a c u t e onset of left flank pain and in w h o m u n e n h a n c e d spiral CT findings were negative. (a) Unenhanced scan shows no e v i d e n c e of hydronephrosis or perinephric stranding and no stone in the course of the ureter. (b) Contrast-enhanced spiral CT scan demonstrates lack of n e p h r o g r a m in a segment of the left kidney, which is most consistent with infarct. The size a n d shape of the kidney are normal.
hydrocalyces from a previous obstruction, spiral CT scans may at first glance appear to be positive. However, an obstructing stone is not seen. False-negative results of spiral CT occur in 5% of cases. Spiral CT results may be normal in patients with acute pyelonephritis, renal vein thrombosis, and renal artery emboli, but the nephrogram may be abnormal when contrast medium is administered (Figs 9, 10). To facilitate diagnosis, we continue the examination by intravenously administering contrast material if the spiral CT results are negative or equivocal
(15%-25% of cases) (20,21). As many as 20%-25% of patients with symptoms of renal colic actually have diseases outside the urinary tract; these include acute appendicitis (Fig 11), acute cholecystitis, diverticulitis, biliary obstruction, pancreatitis (Fig 12), leaking abdominal aortic aneurysm, torsed adnexal mass, and dermoid cyst. Most of these abnormalities are not apparent at conventional excretory urography. The ability of spiral CT to depict other disease processes unrelated to the urinary tract is of great advantage.
385
Q.
b.
Figure 11, Unenhanced spiral CT scans in 34-year-old man with right-sided flank pain. (a) Image shows no evidence of hydronephrosis. (b) Image shows no evidence of hydroureter, Soft-tissue mass (arrow) with stranding is seen in the region of the appendix, it was treated conservatively with antibiotics, and the symptoms resolved.
Reformatted images that are generated by using curved planar reformatting can be helpful in determining whether a calcification is in the urinary tract (22). Phleboliths and arterial calcifications, which may be confused with ureteral stones, often can be identified with this technique. Coronal reconstructions can be used to determine stone size. Analysis of CT attenuation of a calcification may be helpful in distinguishing ureteral calculi from ph!eboliths. A stone would be expected to be homogeneous in attenuation, whereas a phlebolith may have a central or eccentric region of low attenuation. On occasion, it may help to change the position of the patient during the examination when there is a question as to whether a calcification is in the ureterovesical junction or bladder.
Spiral CT scanning provides better depiction of the kidneys than conventional excretory urography with linear tomography with respect to both parenchymal masses and renal calculi. It has been demonstrated that a mass in the renal parenchyma must be 3 cm in diameter before it can be reliably recognized at excretory urography, whereas a mass 1 cm in diameter can be consistently identified at spiral CT (23). Because of the relatively long scanning time of conventional CT, is has not been practical to substitute CT scanning of the renal parenchyma for the linear tomography portion of excretory urography. With the advent of spiral CT, however, it has become more feasible
386
Figure 12. Unenhanced spiral CT scan in a 44-year-old man with right-sided flank pain, Note swelling of the h e a d of the pancreas with fluid and stranding. Images through the interpolar region of the kidneys (not shown) showed no evid e n c e of hydronephrosis or hydroureter, The patient was treated for acute pancreatitis, and symptoms resolved.
to replace linear nephrotomography of the kidneys in the nephrogram phase of the examination with spiral CT of the renal parenchyma during the equilibrium phase (at 90-120 seconds). Conventional overhead anteroposterior and oblique abdominal radiographs are still obtained to evaluate the urothelium during the excretory phase of the study (at 8-10 minutes). The combined imaging study is referred to as spiral CT urography.
a.
Figure 13. (a) Spiral CT urogram in a 78-year-old man with microscopic hematuria shows a subtle mass in the right renal pelvis. (b) Conventional pyelogram unequivocally demonstrates a filling defect in the pelvis. b.
Spiral CT is performed before administration of contrast material, and the kidneys are imaged as necessary to identify calculi or characterize a mass. One hundred milliliters of contrast material is then injected at a rate of 2 3 mL/sec. Spiral CT (10-mm sections) through the kidneys is performed 90-120 seconds after contrast medium injection. Conventional radiographs are obtained at 8-10 minutes to evaluate the urothelium. No oral contrast medium is administered. When a mass is identified at excretory urography, linear nephrotomographic images do not reliably characterize the mass. In these cases, another imaging study such as ultrasound or CT is required for further evaluation. Substituting spiral CT through the kidneys for conventional linear nephrotomography eliminates the need for further imaging and allows for a definitive diagnosis of masses that are 1.5 cm or larger in most instances. Furthermore, with spiral CT it is possible to refocus the image so that it is in the epicenter of the mass, thus reducing partial voluming effects. We have shown that the spiral CT portion of CT urography contributes additional information about the urinary tract in 41% of patients (24). Additional important information in the urinary tract was recognized only with conventional radiography in 8% of studies (eg, duplicated collecting system, Fraley syndrome, filling defects in the
collecting system [Fig 13] and bladder). Thus, conventional radiography is still needed for a complete evaluation of the urinary tract. Another advantage of spiral CT urography is its depiction of disease outside the urinary tract that may explain symptoms that have been erroneously attributed to the urinary tract (eg, testicular cancer [Fig 14], appendicitis, lymphoma, aortic aneurysm), as well as incidental disease processes that are unrelated to the patient's symptoms (eg, pancreas carcinoma [Fig 15], adrenal adenoma). In our study, comorbid conditions outside the urinary tract were seen at spiral CT urography in 23% of the cases (24). However, these incidental findings changed therapy or management in only 6% of the cases. In 7% of the studies, the additional findings outside the urinary tract were depicted with conventional radiography only. Evaluation of a renal mass is a valuable function of spiral CT urography. Conventional, thin-section (3-5mm) CT scans are obtained before and after the administration of contrast material to characterize a renal mass. The purpose of the precontrast study is to depict fat within a mass, which would indicate a diagnosis of angiomyolipoma. In addition, calcium within a mass and renal calculi can be visualized. With spiral CT, respiratory misregistration can be avoided and lesions can be recon-
387
Figure 14. (a) Spiral CT urogram in a 28-year-old m a n with a c u t e onset of flank pain is n e g a t i v e for obstruction but demonstrates a cystic mass in the retroperitoneum. (b) Ultrasound scan reveals an inhomogeneous, hypoe c h o i c mass in the left testicle, Radical o r c h i e c t o m y rev e a l e d a seminoma.
structed to the center (ie, refocused) to obtain the most accurate Hounsfield unit readings. Imaging can be tailored as needed by varying the scan delay after administration of contrast material. Scans acquired during the excretory phase show greater renal mass conspicuity compared with scans acquired during the arterial or corticomedullary phase, where the lack of medullary opacification can lead to missed identification or mischaracterization of lesions (Fig 16) (25-27). Enhancement of renal neoplasms is time dependent, and the lesions may not be apparent during the early corticomedullary phase (28). Images obtained during the early nephrogram phase, when there is cortical-medullary differentiation, may not depict a medullary mass. The corticomedullary phase of the nephrogram occurs 40.60 seconds after contrast matefial administration, the equilibrium phase at 90-150 seconds, and the pyelographic phase at 3-4 minutes. These times may vary with cardiac output or circulation time. Renal masses are depicted and characterized at a time later than that which is used for general abdominal CT with contrast material enhancement. If a suspicious renal parenchymal lesion is noted at general abdominal spiral CT scanning during the corticomedullary phase, acquisition through the renal parenchyma should be repeated during a later phase. In addition, because of the intensity of the nephrogram, optimal window settings for viewing the renal parenchyma differ. A window width of 500-700 and level of 150-230 may be more appropriate (29).
388
b.
Figure 15. Spiral CT urogram in a 75-year-old man 6 months after extracorporeal shock w a v e lithotripsy for a renal stone. The kidneys were normal. There is a mass in the tail of the pancreas (arrows) and retroperitoneal a d e n o p a t h y and soft-tissue stranding from pancreatic cancer. Note the splenic lesion.
a.
b.
Figure 16. Spiral CT urograms d e m o n s t r a t e i m p o r t a n c e of i m a g i n g in equilibrium p h a s e to d e t e c t renal masses. (a) Spiral CT scan o b t a i n e d during c o r t i c o m e d u l l a r y p h a s e of n e p h r o g r a m looks normal. (b) On i m a g e o b t a i n e d during equilibrium phase, a 2-cm mass in t h e right kidney is a p p a r e n t . P a t h o l o g i c diagnosis was a n g i o m y o l i p o m a , w h i c h d i d n o t c o n t a i n fat,
'.EFERENCE~
1. Heiken JP, Brink JA, Vannier MW. Spiral (helical) CT. Radiology 1993; 189:647-656, 2, Prokop M, Schaefer-Prokop C, Galanski M. Spiral CT angiography of the abdomen. Abdom Imaging 1997; 22:143-153. 3. Cochran ST, Krasney RM, Danovitch GM, et al. Spiral CT angiography for living renal donor evaluation. A JR 1997; 168;1569-1573. 4. Platt JF, EllisJH, Korobkin M, Reige KA, Konnak JW, Leichtman AB. Potential renal donors: comparison of conventional imaging with helical CT. Radiology 1996; 198:419-423. 5. Boijsen E. Angiographic studies of the anatomy of single and multiple renal arteries. Acta Radial 1959; 183(suppl):1-135. 6, Nakada SY, Pearle MS, Clayman RV. Acucise endopyelotomy: evolution of a less-invasive technology. J Endouro11996; 10:133139. 7. Meretyk I, Meretyk S, Clayman RV. Endopyelotomy: comparison of ureteroscopic retrograde and antegrade percutaneous tech' niques, J Uro11992; 148:775-783. 8. Sampaio FJB, Aragao AHM. Anatomical relationship between the intrarenal arteries and the kidney collecting system. J Urol 1990; 143:679. 9. Sampaio FJB, Favorite LA. Ureterepelvic junction stenosis: vascular anatomical background for endopyelotomy. J Uro11993; 150:1787-1791. 10. Quillin SP, Brink JA, Heiken JP, Siegel CL, McClenna BL, Clayman RV. Helical (spiral) CT angiography for identification of crossing vessels at the ureteropeMc junction. A JR 1996; 166:1125-1130. 11. Prokop M, Galanski MW, Chavan A, Schaefer-Prokop CM. CT angiography of the renal arteries: an update (abstr). Radiology 1994; 193(P);215. 12. Rubin GD, Dake MD, Napel S, et al. Spiral CT of renal artery stenosis: comparison of three-dimensional rendering techniques. Radiology 1994; 190:181-189. 13. Smith RC, Rosenfield AT, Choe KA, et al. Acute flank pain: comparison of non-contrast-enhanced CT and intravenous urography. Radiology 1995; 194:789-794, 14. Silberzweig JE, Mitty HA, Waterhouse RL Jr. Helical CT for the detection of ureteral calculi (abstr). Radiology 1996; 201 (P):219. 15. Kawashima A, Sandier CM, Boridy IC, Takahashi N, Benson GS, GoIdman SM. Unenhanced helical CT of uretolithiasis: value of the "tissue-rim" sign. A JR 1996; 168:997-1000.
16. Heneghan JP, 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. 17. Smith RC, Verga M, McCarthy S, Rosenfield AT. Diagnosis of acute flank pain: value of unenhaneed helical CT. A JR 1996; 166:97-101. 18. Ronaghi AH, Cochran ST, Application of nonenhanced helical CT in diagnosis of suspected ureteral stones (abstr). A JR 1997; 168(suppl):66. 19. Sheley RC, Semonsen KG, Quinn SF. Renal colic CT: first 200 patients (abstr). Radiology 1996; 201 (P):219. 20. Talner L, Schmutz F, Carpenter K, Bush W. Helical CT in patients with acute flank pain (abstr). A JR 1996; 166(suppl):66. 21. Huynh TV, Liu C, Casola G. Value of heIical CT in the diagnosis of acute renal colic (abstr). Radiology 1996; 201 (P):219. 22, Sommers FG, Jeffrey RB Jr, Rubin GD, et al. Detection of ureteral calculi in patients with suspected renal colic: value of reformatted noncontrast helical CT. A JR 1995; 165:509-513. 23. Warshauser DM, McCarthy SM, Street L, et al. Detection of renal masses: sensitivities and specificities of excretory urography/linear tomography, US and CT. Radiology 1988; 169:363-365. 24. Barbaric ZL, Deseran M, Schwartz R, Litwer C, Cochran ST. CT urography (CTU). Presented at the European Society of Uroradiology, Florence, Italy, September 14, 1994. 25. Cohan RH, Sherman LS, Korobkin M, BassJC, Francis IR. Renal masses: assessment of corticomedullary-phase and nephrographic-phase CT scans. Radiology 1995; 196:445-451. 26, Zeman R, Zeiberg A, Hayes WS, Silverman PM, Cooper C, Garra BS. Hellcat CT of renal masses: the value of delayed scans. A JR 1996; 167:771-776. 27. Szolar DH, Kammerhuber F, AItziebler S, et al. Multiphasic helical CT of the kidney: increased conspicuity for detection and characterization of small (<3-cm) renal masses. Radiology 1997; 202:211-217. 28. Birnbaum BA, Jacobs JE, Ramchandani P. Multiphasic renal CT: comparison of renal mass enhancement during the corticomedullary and nephrographic phases. Radiology 1996; 200:753758. 29. O'Malley ME, Gazit IE, Eutace S, Clarke P. Evaluation of dedicated renal windows for contrast-enhanced spiral CT (abstr). A JR 1996; 166(suppl):65.
389
Applications of Spiral CT in Genitourinary Imaging' Sachiko T. Cochran, MD
Spiral computed tomography (CT), which is also called helical or volume-acquisition CT, is performed by advancing the patient on the gantry during simultaneous tube rotation with continuous x-ray exposure. This represents the latest technologic advance in CT imaging (1). The 1-second tube rotations coupled with continuous forward movement of the table permits volumetric acquisition of patient data during a single breath hold. Spiral CT has advantages over conventional, incremental CT for imaging of the genitourinary tract. In the evaluation of the renal parenchyma for masses and stones, the elimination of respiratory misregistration and the ability to reconstruct overlapping images at arbitrary intervals improves visualization. In patients with symptoms of acute urinary tract obstruction, short acquisition times allow us to replace excretory urography with unenhanced spiral CT of the abdomen and pelvis for evaluation. Contrast material enhancement can be optimized by appropriate timing of the spiral scanning to coincide with peak contrast enhancement, thus enabling new applications such as CT angiography (2). This article describes and discusses the new imaging applications that are now possible with spiral CT technology.
The ability of spiral CT to acquire volume data provides an alternative modality for vascular imaging. In the genitourinary tract, spiral CT angiography has been shown to be useful in quantifying the number of arteries (3). Acad Radiol 1998; 5:380-389 1From the Department of Radiological Sciences, UCLA Medical Center for Health Sciences, 10833 Le Conte Ave, Los Angeles, CA 900241721. Received August 8, 1997; revision requested August 25; revision received September 8; a c c e p t e d September 12, Address reprint requests to SE.C, ©AUR, 1998
380
Living Renal Donor Evaluation Advances in surgical technique, tissue typing, and immunosuppressive agents over the past decade have improved the 1-year survival rate of patients with renal transplants to more than 80%. Because there is an inadequate supply of cadaver kidneys, living, related donor transplantation programs have expanded to help meet the need. The traditional imaging procedures used to evaluate potential living renal donors have been excretory urography and renal arteriography. Spiral CT angiography, including conventional radiography during the excretory phase, has been shown to be a suitable replacement for these two imaging studies (3,4). In this application of spiral CT angiography, opacification of the bowel is not necessary. A total of 125 mL of contrast material is injected intravenously at a rate of 4-5 mL/sec. After a 20-second delay, imaging is performed. Because most donors are relatively young and healthy, we have not found it necessary to give a test injection to generate a time-attenuation curve. The donor hyperventilates and then holds his or her breath for 30 seconds while the images are acquired. Spiral CT is performed with 3-mm collimation at a table speed of 5 or 6 mm/sec (pitch, 1.7 or 2) at 210 mA and 120 kV. At 8-10 minutes after injection of the contrast material, conventional radiographs (anteroposterior abdomen, oblique as needed, and postvoid views) are obtained. CT data are reconstructed at 2-mm increments. Multiplanar reformatting can be used to display vessels in a second plane. The data set can be used to generate three-dimensional reconstructions (Fig 1). Excretory urography has traditionally been used to identify the abnormalities such as benign and malignant parenchymal masses, congenital anomalies, double ureters, calculi, and obstructions that are encountered in 12%-13% of donors. The increased sensitivity of CT over excretory urography and angiography in depicting renal masses represents an important improvement in screening
a.
b.
Figure 1. Spiral CT angiograms in a 32-year-old male donor with a single renal artery bilaterally. (a) Axial image through the region of the origin of single renal arteries from the aorta. (b) Three-dimensional maximum intensity projection reconstruction of volume data displayed in coronal projection.
Figure 2. Spiral CT angiogram in a 23-year-old female donor with unsuspected scars from bilateral chronic pyelonephritis. Image reveals cortical scars in the lower pole of both kidneys (arrows), as well as in the upper pole of the right kidney (not shown).
Figure 4. Spiral CT angiogram in a 50-year-old female donor with an incidentally discovered 1~/2-cm cystic mass in the body of the pancreas (arrow).
Figure 3. Spiral CT angiogram in a 50-year-old female donor with an incidentally discovered 1-cm vascular mass in the left kidney (arrow). It is unlikely that this mass would have been depicted at conventional excretory urography.
(Figs 2, 3). In addition, because other organs are simultaneously imaged, incidental findings that may preclude donation are also depicted, even when these disorders are outside the urinary tract (Fig 4). Conventional arteriography has been used to identify the number of renal arteries and early prehilar branches, as well as vascular disease. Given that 29% of kidneys are supplied by more than one renal artery, this is an important presurgical screening examination. We have demonstrated that the overall accuracy rate in determining the number of renal arteries with spiral CT angiography is comparable to that with conventional arteriography (3). In a group of 40 patients who underwent spiral CT angiography, conventional arteriography, and donor nephrectomy, there was agreement between the results of spiral CT angiography and surgery in 90% and
381
O.
b.
Figure5. Spiral CT angiograms in a 23-year-old man with left UPJ obstruction. (o) Renal vein (arrow) is anterior to the UPJ. (b) Second renal artery (arrow) (more superior artery not shown) also courses anterior to the ureter and renal vein. S = stent in ureter. agreement between results of conventional angiography and surgery in 87% of the cases. While conventional angiography has often been considered to be the standard, it depicts 10% fewer renal arteries than are found at autopsy (5). Because these small (smaller than 1 mm diameter) accessory arteries supply blood to an insignificant portion of renal parenchyma, their preservation during surgery is not critical. Spiral CT angiography, likewise, does not identify these tiny accessory vessels. We no longer perform renal arteriography in potential renal donors. We have replaced excretory urography and angiography with a single study that includes spiral CT angiography and coiaventional radiography. The advantages of a combined study include reduced exposure to contrast material, improved patient comfort, less morbidity, and less radiation exposure. We have estimated that the combined study costs 35%-50% less than excretory urography and angiography (3).
Evaluation of Crossing Vessel at Ureteropelvic Junction Spiral CT angiography can be used to depict vessels that cross the ureteropelvic junction (UPJ), information that is valuable in planning surgical management in patients with obstruction (Fig 5). Using endoscopic tools, urologists now treat UPJ obstruction by endoluminally incising the UPJ longitudinally with a cutting device that is introduced percutaneously or retrograde to the obstruction (ie, endopyelotomy) (6,7). Endopyelotomy, which
382
Figure6. Spiral CT angiogram obtained in a 27-year-old w o m a n because of renal artery stenosis. The left kidney is 2 cm smaller than the right. On the basis of this image, fibromuscular dysplasia is suspected (note irregularity in the wall of the ]eft renal artery).
has become a standard procedure for the treatment of UPJ obstruction, has a success rate of 80%-90%. Patients with posterior or posterolateral vessels are not candidates for endopyelotomy and should be treated laparascopically or with open surgical pyelotomy. Vascular complications such as hematoma, pseudoaneurysm, and arteriovenous fistula occur if retroperitoneal vessels caused the UPJ obstruction and are incised. This is not unexpected, as 75% of those with UPJ obstruction have vessels near the UPJ. The vessels are usually located anterior to the UPJ, although they have been found posterolateral to it in 5%-10% of patients (8,9). Quillin
Vol 5, No 5, May 1998
et al (10) found that of 24 patients with UPJ obstruction who were evaluated with spiral CT angiography, 11 (46%) had crossing vessels and three of the vessels were posterior to the UPJ. The presence of a ureteral stent was helpful, but not essential, in determining the relationship of the ureter to the vasculature. Evaluation of Renal Artery Stenosis Spiral CT angiography is a good screening procedure for renal artery stenosis when the study includes the primary transaxial data set (Fig 6). In a study of 52 patients, Prokop et al (11) found that normal spiral CT angiography findings rule out a hemodynamically significant (>50%) renal artery stenosis with a sensitivity of 100%, specificity of 92%, and negative predictive value of 100%. In a study of 31 patients, Rubin et al (12) found that a three-dimensional rendering technique of maximum intensity projection based On volumetric spiral CT angiography data had a sensitivity of 92% and a specificity of 83% for the depiction of 70% or greater stenoses. Shaded surface display had a sensitivity of 59% and a specificity of 82%1 These results reinforce the importance of the primary data set in the evaluation of renal artery stenosis. Unlike renal donors and persons with crossing vessels at the UPJ, patients suspected of having renal artery stenosis often have cardiovascular compromise, which can affect the Circulation time. To determine the optimum time for imaging, a 20-mL bolus of contrast material is injected at a rate of 5 mL/sec. After a delay of 10 seconds, serial axial images at a single level just superior tO the renal arteries are obtained every 2 seconds. CT numbers within the aorta are obtained, resulting in a time-attenuation curve. The dec lay time for the CT angiography is selected at the peak of the calculated time-attenuation curve.
When a patient presents in the emergency department with sudden onset of flank pain and microscopic hematuria, an acutely obstructing ureteral calculus is suspected, and excretory urography is the usual examination performed to confirm the clinical suspicion, Excretory urography, however, has certain limitations in that it requires intravenous injection of contrast material, it may take hours to identify the level of obstruction in cases of high-grade obstruction, and ureteral calculi may not be visible on conventional radiographs so that the cause of obstruction may be elusive at excretory urography. It is
SPIRAL CT IN GENITOURINARY IMAGING
estimated that 25% of stones passing through the ureters are not seen at excretory urography, especially if the stone is not causing obstruction at the time of examination. A substitute for excretory urography, unenhanced spiral CT of the kidney, ureter, and bladder, which was first described by Smith et al (13), is becoming widely used (Fig 7). It is faster to perform and more accurate than excretory urography in the evaluation of flank pain, and it does not require intravenous injection of contrast material. At our institution, the patient charge for this examination is equivalent to that for excretory urography. Spiral scanning should be performed contiguously through the kidneys down to the bladder. Five-millimeter collimation with a pitch of 1.5 to 1.8 is used, and the data are reconstructed at 3-5-mm intervals. No oral or intravenous contrast medium is administered. When the positive findings of dilatation of the calyces, hydroureter, and a stone in the ureter are present, the diagnosis of obstructing ureteral Calculus is made and no further testing is necessary. The limit of resolution for a 2-ram stone is 5-mm Collimation. Two-millimeter stones are not detectable with 7- or 10-mm collimation, even with 3-ram section reconstructions. Three-millimeter stones are consistently depicted with 10-ram collimation (14). When a stone is identified in the ureter, spiral CT can precisely depict its size and location. The location of the stone may be important in planning treatment with extracorporeal shock wave lithotripsy or ureteroscopic manipulation. Most stones that are 4 mm or smaller in diameter pass spontaneously. Secondary signs seen at spiral CT are also helpful, particularly if a stone is not identified with certainty. Ureteral dilatation, perinephric fat stranding (Fig 8), dilated intrarenal collecting system, periureteric stranding, or ureterovesical junction edema indicate a stone. If these signs are present, however, and a ureteral stone is not seen, the stone may have recently passed. One area in which the spiral CT scan is not helpful is in distinguishing a distal ureteral calculus from a phlebolith. Edema within the wall of the ureter caused by acute obstruction may cause a circumferential rim of soft-tissue attenuation around a calculus (rim sign) (Fig 7). This sign has been reported in 45%-77% of studies in which ureteral calculi were present and 6%-8% of studies in which phleboiiths were present. The absence of a rim sign is not helpful (15,16). Spiral CT has a sensitivity of 91%97% for detection of urinary tract calculi, a specificity of 96%-98%, and an accuracy of 94%-98% (17-19). In the presence of cysts in the renal sinus or
383
a.
b.
Figure 8. Unenhanced spiral CT scans in a 37-year-old man with left abdominal pain. (a) Image through the kidneys shows left perinephric fluid and stranding and dilated calyx and proximal ureter. Note the nonobstructing stones in the right kidney, (b) image through the ureterovesical junction shows a stone.
a.
b.
Figure 7. Unenhanced spiral CT scans obtained with 5-mm section thickness in a 52-year-old man with acute onset of left flank pain, (a) image through the kidneys demonstrates pelvicalyceal dilatation on the left. (b) Image through the region of the midureter demonstrates dilatation of the left ureter (arrow) with mild stranding around the ureter. (c) Image through the ureterovesical junction demonstrates a stone in the left ureter with surrounding ureter wall edema (rim sign) (arrow).
C.
384
a.
b.
Figure 9. Spiral CT scans o b t a i n e d in a 46-year-old man with episodic left a b d o m i n a l pain and microscopic hematuria and in w h o m u n e n h a n c e d spiral CT findings were negative. (a) CT urogram demonstrates d e l a y in excretion on the left. (b) Delayed scan demonstrates w e d g e - s h a p e d areas of contrast material retention consistent with a c u t e pyelonephritis. The u n e n h a n c e d scans (not shown) h a d shown no e v i d e n c e of obstruction a n d no perinephric stranding.
a.
b.
Figure 10. Images of a 43-year-old man with a c u t e onset of left flank pain and in w h o m u n e n h a n c e d spiral CT findings were negative. (a) Unenhanced scan shows no e v i d e n c e of hydronephrosis or perinephric stranding and no stone in the course of the ureter. (b) Contrast-enhanced spiral CT scan demonstrates lack of n e p h r o g r a m in a segment of the left kidney, which is most consistent with infarct. The size a n d shape of the kidney are normal.
hydrocalyces from a previous obstruction, spiral CT scans may at first glance appear to be positive. However, an obstructing stone is not seen. False-negative results of spiral CT occur in 5% of cases. Spiral CT results may be normal in patients with acute pyelonephritis, renal vein thrombosis, and renal artery emboli, but the nephrogram may be abnormal when contrast medium is administered (Figs 9, 10). To facilitate diagnosis, we continue the examination by intravenously administering contrast material if the spiral CT results are negative or equivocal
(15%-25% of cases) (20,21). As many as 20%-25% of patients with symptoms of renal colic actually have diseases outside the urinary tract; these include acute appendicitis (Fig 11), acute cholecystitis, diverticulitis, biliary obstruction, pancreatitis (Fig 12), leaking abdominal aortic aneurysm, torsed adnexal mass, and dermoid cyst. Most of these abnormalities are not apparent at conventional excretory urography. The ability of spiral CT to depict other disease processes unrelated to the urinary tract is of great advantage.
385
Q.
b.
Figure 11, Unenhanced spiral CT scans in 34-year-old man with right-sided flank pain. (a) Image shows no evidence of hydronephrosis. (b) Image shows no evidence of hydroureter, Soft-tissue mass (arrow) with stranding is seen in the region of the appendix, it was treated conservatively with antibiotics, and the symptoms resolved.
Reformatted images that are generated by using curved planar reformatting can be helpful in determining whether a calcification is in the urinary tract (22). Phleboliths and arterial calcifications, which may be confused with ureteral stones, often can be identified with this technique. Coronal reconstructions can be used to determine stone size. Analysis of CT attenuation of a calcification may be helpful in distinguishing ureteral calculi from ph!eboliths. A stone would be expected to be homogeneous in attenuation, whereas a phlebolith may have a central or eccentric region of low attenuation. On occasion, it may help to change the position of the patient during the examination when there is a question as to whether a calcification is in the ureterovesical junction or bladder.
Spiral CT scanning provides better depiction of the kidneys than conventional excretory urography with linear tomography with respect to both parenchymal masses and renal calculi. It has been demonstrated that a mass in the renal parenchyma must be 3 cm in diameter before it can be reliably recognized at excretory urography, whereas a mass 1 cm in diameter can be consistently identified at spiral CT (23). Because of the relatively long scanning time of conventional CT, is has not been practical to substitute CT scanning of the renal parenchyma for the linear tomography portion of excretory urography. With the advent of spiral CT, however, it has become more feasible
386
Figure 12. Unenhanced spiral CT scan in a 44-year-old man with right-sided flank pain, Note swelling of the h e a d of the pancreas with fluid and stranding. Images through the interpolar region of the kidneys (not shown) showed no evid e n c e of hydronephrosis or hydroureter, The patient was treated for acute pancreatitis, and symptoms resolved.
to replace linear nephrotomography of the kidneys in the nephrogram phase of the examination with spiral CT of the renal parenchyma during the equilibrium phase (at 90-120 seconds). Conventional overhead anteroposterior and oblique abdominal radiographs are still obtained to evaluate the urothelium during the excretory phase of the study (at 8-10 minutes). The combined imaging study is referred to as spiral CT urography.
a.
Figure 13. (a) Spiral CT urogram in a 78-year-old man with microscopic hematuria shows a subtle mass in the right renal pelvis. (b) Conventional pyelogram unequivocally demonstrates a filling defect in the pelvis. b.
Spiral CT is performed before administration of contrast material, and the kidneys are imaged as necessary to identify calculi or characterize a mass. One hundred milliliters of contrast material is then injected at a rate of 2 3 mL/sec. Spiral CT (10-mm sections) through the kidneys is performed 90-120 seconds after contrast medium injection. Conventional radiographs are obtained at 8-10 minutes to evaluate the urothelium. No oral contrast medium is administered. When a mass is identified at excretory urography, linear nephrotomographic images do not reliably characterize the mass. In these cases, another imaging study such as ultrasound or CT is required for further evaluation. Substituting spiral CT through the kidneys for conventional linear nephrotomography eliminates the need for further imaging and allows for a definitive diagnosis of masses that are 1.5 cm or larger in most instances. Furthermore, with spiral CT it is possible to refocus the image so that it is in the epicenter of the mass, thus reducing partial voluming effects. We have shown that the spiral CT portion of CT urography contributes additional information about the urinary tract in 41% of patients (24). Additional important information in the urinary tract was recognized only with conventional radiography in 8% of studies (eg, duplicated collecting system, Fraley syndrome, filling defects in the
collecting system [Fig 13] and bladder). Thus, conventional radiography is still needed for a complete evaluation of the urinary tract. Another advantage of spiral CT urography is its depiction of disease outside the urinary tract that may explain symptoms that have been erroneously attributed to the urinary tract (eg, testicular cancer [Fig 14], appendicitis, lymphoma, aortic aneurysm), as well as incidental disease processes that are unrelated to the patient's symptoms (eg, pancreas carcinoma [Fig 15], adrenal adenoma). In our study, comorbid conditions outside the urinary tract were seen at spiral CT urography in 23% of the cases (24). However, these incidental findings changed therapy or management in only 6% of the cases. In 7% of the studies, the additional findings outside the urinary tract were depicted with conventional radiography only. Evaluation of a renal mass is a valuable function of spiral CT urography. Conventional, thin-section (3-5mm) CT scans are obtained before and after the administration of contrast material to characterize a renal mass. The purpose of the precontrast study is to depict fat within a mass, which would indicate a diagnosis of angiomyolipoma. In addition, calcium within a mass and renal calculi can be visualized. With spiral CT, respiratory misregistration can be avoided and lesions can be recon-
387
Figure 14. (a) Spiral CT urogram in a 28-year-old m a n with a c u t e onset of flank pain is n e g a t i v e for obstruction but demonstrates a cystic mass in the retroperitoneum. (b) Ultrasound scan reveals an inhomogeneous, hypoe c h o i c mass in the left testicle, Radical o r c h i e c t o m y rev e a l e d a seminoma.
structed to the center (ie, refocused) to obtain the most accurate Hounsfield unit readings. Imaging can be tailored as needed by varying the scan delay after administration of contrast material. Scans acquired during the excretory phase show greater renal mass conspicuity compared with scans acquired during the arterial or corticomedullary phase, where the lack of medullary opacification can lead to missed identification or mischaracterization of lesions (Fig 16) (25-27). Enhancement of renal neoplasms is time dependent, and the lesions may not be apparent during the early corticomedullary phase (28). Images obtained during the early nephrogram phase, when there is cortical-medullary differentiation, may not depict a medullary mass. The corticomedullary phase of the nephrogram occurs 40.60 seconds after contrast matefial administration, the equilibrium phase at 90-150 seconds, and the pyelographic phase at 3-4 minutes. These times may vary with cardiac output or circulation time. Renal masses are depicted and characterized at a time later than that which is used for general abdominal CT with contrast material enhancement. If a suspicious renal parenchymal lesion is noted at general abdominal spiral CT scanning during the corticomedullary phase, acquisition through the renal parenchyma should be repeated during a later phase. In addition, because of the intensity of the nephrogram, optimal window settings for viewing the renal parenchyma differ. A window width of 500-700 and level of 150-230 may be more appropriate (29).
388
b.
Figure 15. Spiral CT urogram in a 75-year-old man 6 months after extracorporeal shock w a v e lithotripsy for a renal stone. The kidneys were normal. There is a mass in the tail of the pancreas (arrows) and retroperitoneal a d e n o p a t h y and soft-tissue stranding from pancreatic cancer. Note the splenic lesion.
a.
b.
Figure 16. Spiral CT urograms d e m o n s t r a t e i m p o r t a n c e of i m a g i n g in equilibrium p h a s e to d e t e c t renal masses. (a) Spiral CT scan o b t a i n e d during c o r t i c o m e d u l l a r y p h a s e of n e p h r o g r a m looks normal. (b) On i m a g e o b t a i n e d during equilibrium phase, a 2-cm mass in t h e right kidney is a p p a r e n t . P a t h o l o g i c diagnosis was a n g i o m y o l i p o m a , w h i c h d i d n o t c o n t a i n fat,
'.EFERENCE~
1. Heiken JP, Brink JA, Vannier MW. Spiral (helical) CT. Radiology 1993; 189:647-656, 2, Prokop M, Schaefer-Prokop C, Galanski M. Spiral CT angiography of the abdomen. Abdom Imaging 1997; 22:143-153. 3. Cochran ST, Krasney RM, Danovitch GM, et al. Spiral CT angiography for living renal donor evaluation. A JR 1997; 168;1569-1573. 4. Platt JF, EllisJH, Korobkin M, Reige KA, Konnak JW, Leichtman AB. Potential renal donors: comparison of conventional imaging with helical CT. Radiology 1996; 198:419-423. 5. Boijsen E. Angiographic studies of the anatomy of single and multiple renal arteries. Acta Radial 1959; 183(suppl):1-135. 6, Nakada SY, Pearle MS, Clayman RV. Acucise endopyelotomy: evolution of a less-invasive technology. J Endouro11996; 10:133139. 7. Meretyk I, Meretyk S, Clayman RV. Endopyelotomy: comparison of ureteroscopic retrograde and antegrade percutaneous tech' niques, J Uro11992; 148:775-783. 8. Sampaio FJB, Aragao AHM. Anatomical relationship between the intrarenal arteries and the kidney collecting system. J Urol 1990; 143:679. 9. Sampaio FJB, Favorite LA. Ureterepelvic junction stenosis: vascular anatomical background for endopyelotomy. J Uro11993; 150:1787-1791. 10. Quillin SP, Brink JA, Heiken JP, Siegel CL, McClenna BL, Clayman RV. Helical (spiral) CT angiography for identification of crossing vessels at the ureteropeMc junction. A JR 1996; 166:1125-1130. 11. Prokop M, Galanski MW, Chavan A, Schaefer-Prokop CM. CT angiography of the renal arteries: an update (abstr). Radiology 1994; 193(P);215. 12. Rubin GD, Dake MD, Napel S, et al. Spiral CT of renal artery stenosis: comparison of three-dimensional rendering techniques. Radiology 1994; 190:181-189. 13. Smith RC, Rosenfield AT, Choe KA, et al. Acute flank pain: comparison of non-contrast-enhanced CT and intravenous urography. Radiology 1995; 194:789-794, 14. Silberzweig JE, Mitty HA, Waterhouse RL Jr. Helical CT for the detection of ureteral calculi (abstr). Radiology 1996; 201 (P):219. 15. Kawashima A, Sandier CM, Boridy IC, Takahashi N, Benson GS, GoIdman SM. Unenhanced helical CT of uretolithiasis: value of the "tissue-rim" sign. A JR 1996; 168:997-1000.
16. Heneghan JP, 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. 17. Smith RC, Verga M, McCarthy S, Rosenfield AT. Diagnosis of acute flank pain: value of unenhaneed helical CT. A JR 1996; 166:97-101. 18. Ronaghi AH, Cochran ST, Application of nonenhanced helical CT in diagnosis of suspected ureteral stones (abstr). A JR 1997; 168(suppl):66. 19. Sheley RC, Semonsen KG, Quinn SF. Renal colic CT: first 200 patients (abstr). Radiology 1996; 201 (P):219. 20. Talner L, Schmutz F, Carpenter K, Bush W. Helical CT in patients with acute flank pain (abstr). A JR 1996; 166(suppl):66. 21. Huynh TV, Liu C, Casola G. Value of heIical CT in the diagnosis of acute renal colic (abstr). Radiology 1996; 201 (P):219. 22, Sommers FG, Jeffrey RB Jr, Rubin GD, et al. Detection of ureteral calculi in patients with suspected renal colic: value of reformatted noncontrast helical CT. A JR 1995; 165:509-513. 23. Warshauser DM, McCarthy SM, Street L, et al. Detection of renal masses: sensitivities and specificities of excretory urography/linear tomography, US and CT. Radiology 1988; 169:363-365. 24. Barbaric ZL, Deseran M, Schwartz R, Litwer C, Cochran ST. CT urography (CTU). Presented at the European Society of Uroradiology, Florence, Italy, September 14, 1994. 25. Cohan RH, Sherman LS, Korobkin M, BassJC, Francis IR. Renal masses: assessment of corticomedullary-phase and nephrographic-phase CT scans. Radiology 1995; 196:445-451. 26, Zeman R, Zeiberg A, Hayes WS, Silverman PM, Cooper C, Garra BS. Hellcat CT of renal masses: the value of delayed scans. A JR 1996; 167:771-776. 27. Szolar DH, Kammerhuber F, AItziebler S, et al. Multiphasic helical CT of the kidney: increased conspicuity for detection and characterization of small (<3-cm) renal masses. Radiology 1997; 202:211-217. 28. Birnbaum BA, Jacobs JE, Ramchandani P. Multiphasic renal CT: comparison of renal mass enhancement during the corticomedullary and nephrographic phases. Radiology 1996; 200:753758. 29. O'Malley ME, Gazit IE, Eutace S, Clarke P. Evaluation of dedicated renal windows for contrast-enhanced spiral CT (abstr). A JR 1996; 166(suppl):65.
389