- Email: [email protected]
Multidetector CT urography: techniques, clinical applications, and pitfalls
Multidetector CT Urography: Techniques, Clinical Applications, and Pitfalls Syed A. Akbar, Koenraad J. Mortele, Kathy Baeyens, Maka Kekelidze, and Stuart G. Silverman For two decades, computed tomography (CT) has challenged intravenous urography (IVU) in the evaluation of urinary tract abnormalities. Compared with IVU, CT is more sensitive and specific in the detection and characterization of a variety of urinary tract disorders, including renal masses and urolithiasis. The last purported advantage of IVU has been its ability to depict subtle and mucosal abnormalities of the urothelium. Now, using multidetector CT (MDCT), this challenge has been overcome. In this article, we review the current role of MDCT urography in the evaluation of the urinary tract. © 2004 Elsevier Inc. All rights reserved.
C
OMPUTED TOMOGRAPHY (CT) has become the radiologic test of choice in the evaluation of the kidneys and urinary collecting system. CT has proven superior to intravenous urography (IVU) for several clinical indications, including the evaluation of renal masses,1-3 infection,4,5 trauma,6,7 and urinary calculi.8-10 IVU fails to detect 25-50% of small (⬍3 cm) renal masses and 15% of larger masses.2 Furthermore, IVU lacks sufficient specificity for accurately characterizing any renal mass as benign.11,12 A historic advantage of IVU has been its ability to depict subtle urothelial abnormalities due to its high spatial resolution. This has been the single most important reason why IVU has been used to evaluate patients with hematuria rather than CT. With the introduction of multidetector technology, CT can be used to image the kidneys, ureters, and bladder, with thin collimation during a single breath-hold. This results in the ability to image the entire urinary tract in one acquisition. MDCT creates images with an in-plane spatial resolution that is greater than what can be obtained with singledetector CT, and is close to IVU. The ability to obtain thinly collimated data has also improved out-of-plane resolution, such that the volumetric data that can be obtained with MDCT is nearly isotropic. As a result, coronal reformatted images provide IVU-like images that are able to depict subtle urothelial abnormalities, including early cancers.13,14
accomplished with one acquisition. In an attempt to solve this problem, multiple CT urography (CTU) protocols have been described. A form of CT “hybrid” urography in which an axial CT scan of the urinary tract is followed by one or more abdominal radiographs or CT topograms has been described by Perlman et al.15 Their strategy combines the advantages of CT in imaging the kidneys, with those of contrast radiography in imaging the urinary collecting system. It remains to be seen, however, whether the image quality of a CT topogram is sufficient to view the urothelium. If a conventional radiograph is obtained after the CT scan, the patient needs to undergo imaging in two different locations. Movement of the patient between procedure rooms requires additional time and resources, can cause scheduling conflicts, and may adversely affect the level of pyelocalyceal distention at the time of the radiograph. McNicholas et al16 suggested that CTU could be performed by using single-detector CT alone. They compared excretory phase single detector CT with IVU in the ability to opacify the urinary collecting system. Patients were imaged supine, prone, and supine with compression. CTU was found to be comparable to IVU in its ability to opacify the calices, pelvis, and upper and mid-ureters. Imaging prone and supine, with compression, resulted in better opacification of the collecting system than
TECHNIQUE
From the Division of Abdominal Imaging and Intervention, Brigham and Women’s Hospital, Harvard Medical School, 75 Francis Street, Boston, MA. Address reprint requests to Koenraad J. Mortele, MD, Division of Abdominal Imaging and Intervention, Brigham and Women’s Hospital, 75 Francis Street, Boston, MA 02115; e-mail: [email protected]. © 2004 Elsevier Inc. All rights reserved. 0887-2171/04/2501-0005$30.00/0 doi:10.1053/j.sult.2003.11.002
Acquisition The fundamental problem with imaging the urinary tract is that, because of peristalsis, it is difficult to image all ureteral segments in an opacified and distended state. Using IVU, multiple images can be obtained for this purpose, but with CT and its higher radiation dose, ureteral imaging must be
Seminars in Ultrasound, CT, and MRI, Vol 25, No 1 (February), 2004: pp 41-54
41
42
the supine, noncompression technique. Relative to axial images, reformatted images were judged to be of additional value in 44% of cases. A MDCT scanner configured with a special tabletop apparatus has been used to obtain both CT and radiographic images without moving the patient.17 The modified CT tabletop is capable of receiving a combined slip-on grid/standard film cassette. This system requires the installation of a ceilingmounted X-ray tube above the CT table and the attachment of an auxiliary CT tabletop that has a hollow bay under the patient surface in which to place the radiographic cassette. With this approach, the CTU protocol consists of a KUB, an unenhanced renal CT, a multiphasic contrast-enhanced renal CT scan, followed by overhead excretory urographic and post-void radiographs.17 McCullough et al18 compared the image quality and dose between KUB and CT scanned projection radiography, in their applications to CT urography. An image quality phantom was imaged with both moving and stationary grids. Entrance skin exposure; spatial, contrast, and temporal resolutions; geometric accuracy; and artifacts were assessed. KUB provided better spatial resolution than did CT scan projection radiography. However, the latter method provided improved low-contrast resolution, at exposures comparable to those used for KUB. Chow and Sommer19 described a three-phase multidetector CT urography (MDCTU) protocol with unenhanced, enhanced with abdominal compression, and post-release phases. These investigators split the contrast media bolus into two portions; the first is used to opacify the intrarenal collecting system such that, when the CT scan is performed during the nephrographic phase, both the parenchyma and the collecting system are opacified. Initially, unenhanced images are obtained from the diaphragm to the symphysis pubis. Subsequently, a 40-mL bolus on contrast media is administered at 2 mL/sec and an abdominal compression device applied. Then, the remaining 80 mL of contrast agent is administered, and CT scans are then obtained from the diaphragm to the iliac crests after a 90-sec delay (to obtain a nephrogram). A scout image is then obtained of the abdomen and pelvis, after which the compression device is released and a CT scan is immediately obtained from the iliac crests to the symphysis
AKBAR ET AL
pubis. A final scout of the abdomen and pelvis is then obtained. Caoili et al20 described 4-phase MDCTU, performed using an unenhanced scan, a nephrographic phase scan with abdominal compression, and 2 excretory-phase scans, one obtained 200 sec after the injection of contrast material with compression, and the other at 300 sec without compression. They compared CTU findings with the results of cystoscopy, ureteroscopy, and surgery in 65 patients in whom urinary tract abnormalities were strongly suspected. Their study revealed that CTU depicted many urinary tract abnormalities, including 15 of 16 uroepithelial malignancies, 5 congenital anomalies, 5 urinary tract calculi, 25 bladder abnormalities, and pelvic/ureteral abnormalities as follows:18 calyceal and/or papillary, 30 renal pelvic and/or ureteral. Our group, like others, believes that CTU is best performed by using CT alone. The optimal MDCTU protocol has not been determined, particularly with respect to the number of scans, patient position, timing of contrast media administration, and the use of compression devices. Comparing several protocols for depicting the urinary collecting system with MDCTU,14 we demonstrated that a MDCTU supplemented with an intravenous infusion of normal saline significantly improved opacification of the collecting systems and ureters, as compared with protocols not using saline. As confirmed by other investigators,16 the distal ureters were the most difficult segments to opacify, but were more reliably opacified when saline was used. Patient position was also analyzed, and no significant difference was found between prone and supine positions. We found that saline supplementation had several advantages. It is a simple protocol that is easily performed by the technologist alone. An incompletely distended collecting system limits detection of urinary tract disease, and excellent distension is achieved without use of a compression device. Furthermore, compression devices are cumbersome, of limited value in obese patients, and contraindicated in patients with acute abdominal pain or abdominal aortic aneurysms. Patients are examined with 3 CT scans: one each unenhanced, during the nephrographic phase, and during the pyelographic phases (Fig 1). We do not administer an oral bowel contrast agent. Patients are asked to void before the scan. All patients are positioned supine and receive a supplemental iv
MULTIDETECTOR CT UROGRAPHY
43
Display Methods
Fig 1. Normal MDCT urogram using three phases and supplemental saline: (a) Unenhanced phase; (b) The nephrographic phase (acquired 100 sec after administration of 100 cc of Ultravist-300); (c) Excretory phase (acquired after a 10-min delay and supplemental IV administration of 250 ml of normal saline).
infusion of normal saline. The CT technologist hangs a 250-mL bag of normal saline immediately following the injection of contrast medium and allows it to infuse by gravity. Using a 4-channel scanner, the protocol consists of the following: (1) an unenhanced phase obtained with 2.5-mm collimation; (2) a nephrographic phase, acquired also with 2.5-mm collimation 100 sec after administration of 100 cc of Ultravist-300 (Berlex Lab, Madison, NJ) at a rate of 3 mL/sec; and (3) a pyelographic phase acquired after a 10- to 15-min delay with 1-mm collimation. The same basic protocol is used with a 16-channel CT scanner, but 1.5-mm collimation is used for the unenhanced and nephrographic phases, and 0.75-mm collimation for the pyelographic phase.
Different display methods can be used to evaluate CTU datasets, including multiplanar reformatted images (MPR) generated from axial data, maximum intensity projection (MIP), and volume rendered images (Fig 2). MIP techniques use only a small fraction of the available data to generate and display images, and in our experience, are rarely useful.14 Curved multiplanar reformations, average intensity projection (AIP), and volume rendered (VR) images can be used in selected cases. Three-dimensional volume rendering retains CT data by summing the contributions from all the voxels along a line from any viewing angle through the data set. However, it is essential to review the axial images for all necessary information before 3D rendering is performed. With MPRs, pyelographic phase images should be viewed at a window center of 100 HU and a window width of 600 HU, which allows reliable visualization of the range of contrast material attenuation on the images. These window settings, which lie between those typically used for softtissue and bone viewing, are necessary to view lesions in the collecting system and ureter that could be otherwise masked. Chow and Sommer19 obtain thin-slab MIP oblique coronal images in a plane “en face” to the kidney, generated from both unenhanced and enhanced data for each kidney. Additionally, anteroposterior thick-slab MIP-images are obtained of the kidneys individually, and collectively from the enhanced compression data. Finally, a MIP-image of the distal ureters and urinary bladder is generated from the third phase data. Additional MIP, minimum intensity-projection, average intensity, and curved planar reformations are obtained as needed. CLINICAL APPLICATIONS
MDCTU is able to depict the entire urinary tract, including both the renal parenchyma and the collecting structures (ureters and bladder). As a result, a wide range of diseases can be identified. MDCTU can be used to depict anatomic variants, stone disease, inflammatory processes, and benign and malignant neoplasms. At our institution, MDCTU has replaced IVU in the evaluation of patients with hematuria.
44
AKBAR ET AL
Fig 2. Different display methods to evaluate CTU datasets: (a) Selected reformatted coronal CT image from a normal MDCT urogram, obtained with supplemental saline; (b) Maximum Intensity-Projection (MIP) image shows normal intrarenal collecting system, ureters and bladder; (c) Normal volume rendered (VR) 3-D image.
MULTIDETECTOR CT UROGRAPHY
45
Fig 3. Congenital anomalies of the collecting system and ureter: (a) Reformatted coronal CT image demonstrates renal ectopia; (b) Coronal image shows a left pelvic duplication in a patient with a history of left flank plain; (c– e) Coronal, volume rendered 3-D and MIP images show duplicated right upper ureter; (f) Coronal reformatted image shows a left-sided primary megaureter.
Normal Variants Congenital anomalies of the collecting system and ureters can be visualized better with MDCTU than with conventional CT. Congenital anomalies of renal position and number are well depicted with MDCTU, including renal ectopia, malrotation, and horseshoe kidneys (Fig 3).21 In approximately 10% of individuals, the collecting system is incompletely duplicated.22 Duplication of the collecting system may be difficult to appreciate with conventional CT, as opacification of the ureters is
usually incomplete. Coronal reconstructions with MDCTU allow the duplication to be viewed in an IVU-like fashion, and occasionally can be used to determine the extent of the duplication (Fig 3).20 Urolithiasis Previously, plain film radiography, IVU, and ultrasonography were used in the evaluation of acute flank pain or hematuria. However, IVU failed to demonstrate calculi in up to 48% of patients.23 Now, unenhanced CT has replaced IVU
Fig 4. Nephrocalcinosis and urolithiasis: (a) Reformatted coronal image shows bilateral medullary nephrocalcinosis; (b, c) Axial images with soft tissue and bone window settings show a left ureterovesical junction (UVJ) stone (arrow) causing partial obstruction; (d) Coronal image depicts the left UVJ stone (arrow). Bone windows help differentiate the small calculus from the opacified ureter; (e) Curved multiplanar reformation demonstrates an obstructing distal ureteral calculus (arrow).
MULTIDETECTOR CT UROGRAPHY
47
in the evaluation of patients presenting with ureteral colic. Many studies8-10 have documented the superior sensitivity and specificity of unenhanced helical CT when compared with IVU for the detection of renal and ureteral calculi (Fig 4). CT may also help analyze stone composition and differentiate among uric acid, struvite, and calcium oxalate stones.24 Almost all urinary tract calculi, regardless of their calcium content, are radioopaque on unenhanced CT. The rare stones that form in 4% of patients infected with HIV who are treated with indinavir therapy may not be radioopaque.25 Staghorn calculi and kidneys containing multiple stones are optimally depicted with multiplanar and 3D displays that allow planning for percutaneous approaches to stone removal. Unenhanced CT also demonstrates valuable secondary signs of obstructive uropathy, including hydronephrosis, hydroureter, renal enlargement, perinephric and periureteral stranding, and ureterovesical junction edema.26 The combination of hydronephrosis, hydroureter, and perinephric stranding has a positive predictive value of 90% for urinary obstruction.23 Differences in renal parenchymal attenuation between obstructed and nonobstructed kidneys have also been reported as a reliable secondary sign of obstruction.27 The degree of ureteral obstruction may also be predicted by the extent of perinephric edema.27 Whereas most calculi are detected with nonenhanced CT, occasionally, stones can be detected after the injection of contrast material (Fig 4). Infection MDCTU is superior to both IVU and ultrasonography in the assessment renal infection.28-36 MDCTU establishes the diagnosis in equivocal
Fig 6. Papillary necrosis in a patient with chronic pyelonephritis: (a, b) Coronal excretory phase images show papillary necrosis. Round filling defect in the interpolar region of the (a) right kidney (arrow) and in (b) the left upper pole (arrow) correspond to sloughed papillae in the collecting system. Also note the abnormal kidney contours. Overall appearance is consistent with papillary necrosis due to chronic infection.
Fig 5. Focal bacterial pyelonephritis. Axial CT nephrogram shows a low-density mass-like abnormality in the mid-pole of right kidney (arrow) due to focal bacterial pyelonephritis.
cases, assessing the extent and complications such as renal abscess formation. Gas, calculi, renal parenchymal calcifications, and hemorrhage are demonstrated well with unenhanced CT. A striated
48
AKBAR ET AL
the corticomedullary junction. Other signs of infection include focal or global enlargement of the kidney, obliteration of the renal sinus and perinephric fat planes, thickening of Gerota’s fascia, caliceal effacement caused by swelling of adjacent renal parenchyma, and thickening of the pelvicaliceal walls.4,32,33 Parenchymal abnormalities are best demonstrated on CT images obtained during the nephrographic phase (Fig 5). Delayed CT scan
Fig 7. Renal masses: (a) Coronal image shows a round solid mass in the upper pole of the left kidney (arrow). Surgical specimen yielded renal cell carcinoma (RCC); (b) Reformatted coronal image shows innumerable bilateral kidney cysts in a patient with polycystic kidney disease (APKD).
nephrogram on contrast-enhanced CT, consisting of discrete rays of alternating attenuation that extend to the capsule, is characteristic of pyelonephritis. During an acute infection, MDCTU demonstrates one or more hypodense areas within the renal parenchyma and sometimes obliteration of
Fig 8. Bladder stones and papilloma. (a) Coronal image (bone window settings) shows the presence of two large bladder stones. (b) Axial image: in the dependent portion of the bladder a triangular shaped lesion is seen. Surgical specimen yielded an inverted papilloma.
MULTIDETECTOR CT UROGRAPHY
49
Fig 9. Three-phase MDCTU in a patient with transitional cell carcinoma: (a) Unenhanced phase: a slightly hyperdense renal mass (arrow) is seen in a right lower pole calyx; (b) Nephrographic phase (acquired 100 sec after administration of 100 cc of Ultravist-300): the lesion (arrow) shows mild enhancement; (c) The pyelographic phase (acquired after a 10-min delay and supplemental administration of 250 ml of normal saline): the lesion (arrow) causes a filling defect in the contrast filled calyx; (d) Coronal reformatted CT image in the urographic phase demonstrates the location and extent of the lesion (arrow).
may help in differentiating tumor from an inflammatory mass, by showing persistent contrast enhancement in areas where there was previously diminished enhancement after contrast medium.35 CT is the modality of choice for detecting a renal abscess and emphysematous pyelonephritis.31,36 MDCTU may also be helpful in the evaluation of chronic inflammatory processes, such as renal tuberculosis, chronic pyelonephritis, and xanthogranulomatous pyelonephritis. In patients with renal papillary necrosis, MDCTU may demonstrate small kidneys, “ring shadows” in the medulla, contrast-filled clefts in the renal parenchyma, and urinary collecting system filling defects (Fig 6). Renal Masses With the widespread use of cross-sectional imaging, as many as one-half of renal cell carcinomas are discovered incidentally, and almost all are small, early-stage lesions.37 As CTU replaces IVU for urinary complaints and hematuria, the detection
small renal cancers will likely increase. The most common appearance of a small renal cell carcinoma is a noncalcified lesion with an attenuation value of 20 HU or higher, that enhances by more than 10 HU after intravenous injection of contrast medium.3 The nephrographic phase is the most accurate for detection and characterization of renal masses.3,12,37 Nephrographic and excretory phase images have previously been shown to be superior to corticomedullary phase images in the detection and characterization of renal masses. For partial nephrectomy planning, coronal reformatted images are useful in visualizing the relationship of renal lesions to adjacent collecting system anatomy (Fig 7). Trauma CT is the imaging modality of choice for the assessment of blunt abdominal injuries in general, and renal injury in particular. CT can identify renal contusions, subcapsular hematomas, lacerations,
50
AKBAR ET AL
shattered kidneys, and vascular injuries.6,7 It can also help in evaluation of the type and severity of parenchymal injury, the extent of perirenal hemorrhage, and parenchymal devascularization.38,39 Excretory phase images are needed to detect collecting system injuries, such as ureteropelvic junction disruption, and the presence of urinary extravasation. MDCTU improves the evaluation of trauma by depicting the collecting system and ureters in a fully opacified state during the coronal plane. Ureters and Bladder MDCTU distinguishes stones from soft tissue filling defects within the bladder (Fig 8). Approximately 7-8% of renal malignancies develop in the intrarenal collecting system (Fig 9). The most common malignant tumor of the ureter is transitional cell carcinoma, and 85% of these tumors are broadbased.40 Pedunculated or diffusely infiltrating tumors are less common. Sessile and infiltrating lesions behave more aggressively and are more advanced at the time of diagnosis. Infiltrating tumors are characterized by thickening and induration of the ureter wall (Fig. 10).40 Transitional cell carcinoma most commonly presents as an intraluminal mass with or without obstruction. MDCTU detects small (⬃5 mm) urothelial malignancies.20 Using axial images, 95% of the malignant foci are identified. Concentric urothelial wall thickening is identified more easily on axial CT images than on 3D reformations. However, in patients with diffuse bladder wall thickening, MDCTU cannot distinguish benign from malignant etiologies. These data suggest that MDCTU may be most sensitive for the detection of upper tract TCC. MDCTU helps evaluate patients with TCC by depicting whether the lesion has extended beyond the ureter, and by screening the rest of the urothelium for synchronous tumors (Fig. 11).41,42
PITFALLS
Factors that might lead to either misinterpreting or overlooking findings on CTU include using improper technique, confusing normal anatomic structures and variants as abnormalities, and not understanding artifacts.43,44 Technical artifacts can be caused by patient motion (respiratory or otherwise) and partial volume averaging of adjacent organs. In particular, if the patient breathes during scan acquisition, coronal reformations may appear to contain a renal contour abnormality which mimics a neoplasm (Fig 12). There are several pitfalls that may confound the diagnosis of stone disease.45 Orally administered bowel preparations containing diatrizoate meglumine and diatrizoate sodium (Gastrografin) may be excreted renally in patients with diseases involving the intestinal wall, and may mimic or obscure calculi on the unenhanced scan. When using the split dose technique, streak artifacts from contrast medium in the calyces may falsely lower the enhanced attenuation of an adjacent renal mass.46 All three phases of a MDCTU, unenhanced, nephrographic, and excretory, are essential. If the
Additional Findings As the entire abdomen and pelvis is scanned, MDCTU may demonstrate incidental and unsuspected extraurinary tract abdominal and pelvic lesions, which may have a significant impact on patient care. Such lesions include retroperitoneal tumors, abdominal aortic aneurysms, and uterine and adnexal masses.
Fig 10. Transitional cell carcinoma of the ureter. Curved planar reformatted (CPR) image shows irregular thickening of the lower right ureteral wall (arrow). Surgical specimen yielded transitional cell carcinoma (TCC). Notice also the presence of hydroureter and hydronephrosis.
MULTIDETECTOR CT UROGRAPHY
51
Fig 12. Reconstruction generated artifact may mimic a renal neoplasm. The reconstruction algorithm in this case produces deformity of the renal contours that may be mistaken for multiple masses.
corticomedullary phase was used instead of the nephrographic phase, the unenhanced renal medulla might be mistaken for a renal lesion, or a tumor might be missed because it is mistaken for a medullary pyramid. The use of nephrographic scans, the time when both the cortex and medulla are opacified, avoids these pitfalls. Embryologic renal parenchymal anomalies (persistent fetal lobation, column of Bertin, junctional parenchymal detects, and dromedary hump) are also seen well during the nephrographic phase. Postsurgical filling of renal defects by perinephric fat, following localized resection, can simulate angiomyolipomas and may pose problems if the clinical history is unknown.47 The pyelographic phase is necessary to detect urinary epithelial neoplasms, differentiate hydronephrosis from peripelvic cysts, and distinguish calyceal diverticulum from cysts. Congenital bladder anomalies [bladder diverticula (Fig 13), herniations, urachal remnants] also are best detected during the excretory phase. Finally, delayed phase imaging is helpful in opacifying ileal conduits, that if not opacified, may mimic a fluid collection. Occasional findings mimicking significant uri-
4™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™ Fig 11. Transitional cell carcinoma of the bladder extending into the ureter. (a) Axial image shows focal left bladder wall thickening (arrow). (b) Coronal image shows the irregular bladder mass. (c) Curved planar reformatted image nicely depicts extension of the bladder wall mass into the left ureter (arrow). Note also the presence of hydroureter. Surgical specimen yielded TCC.
52
AKBAR ET AL
structions.14 Therefore, MIP reconstructions should not be used alone to evaluate the urinary tract.14 RADIATION DOSE CONSIDERATIONS
A CT scan imparts a higher radiation dose than a single radiograph. Most CTU protocols utilize at least two scans; therefore, it can be expected that CTU would impart a significantly higher dose to patients than IVU. Dose measurements for our 3-scan MDCTU protocol showed that skin doses were similar to IVU, but the effective dose at MDCTU was approximately 1.5 times that of IVU.14,48 These results cannot be generalized, as the techniques for both IVU and MDCTU differ across different institutions. There are new CT scanner software programs that allow CT doses to be reduced by lowering tube current based on body width and tissue absorption.49,50 In the future, it is likely that radiation doses incurred with MDCTU will eventually be the same or even lower than those obtained in IVU. CONCLUSION Fig 13. Bladder diverticulum. Coronal image shows a left bladder wall diverticulum.
nary tract pathology include stretching of a ureter as it crosses the midline to insert into an ileal loop, mucous in the renal collecting system, and ureteral kinks. Each of these may result in unopacified segments, either leading to “blind spots” or mimicking urohelial tumors. In addition, short segments of the intrarenal collecting system and ureters that are opacified on transverse or coronal sections may appear unopacified on MIP recon-
In summary, CT has become the examination of choice for most urinary tract pathology, as it provides for “one step” evaluation of the kidneys, intrarenal collecting systems, and ureters. Although bladder pathology is detected from time to time using CT urography, cystoscopy remains the test of choice for evaluating the urinary bladder. There are several protocols for performing MDCTU, each with advantages and disadvantages. While techniques vary, each provides essentially the same diagnostic information. Whether one will be proven to be better than the others awaits further study.
REFERENCES 1. Amendola M, Bree R, Pollack HM, et al: Small renal cell carcinomas: Resolving a diagnostic dilemma. Radiology 166: 637-741, 1988 2. Warshauer DM, McCarthy SM, Street L, et al: Detection of renal masses: Sensitivities and specificities of excretory urography/linear tomography, US and CT. Radiology 169:363365, 1988 3. Silverman SG, Lee BY, Seltzer SE, et al: Small (⬃3 cm) renal masses: Correlation of spiral CT features and pathological findings. AJR Am J Roentgenol 163:597-605, 1994 4. Kawashima A, Sandler CM, Ernst RD, et al: Renal inflammatory disease: The current role of CT. Crit Rev Diagn Imaging 38:369-415, 1997
5. Pickhardt PJ, Lonergan GJ, Davis CJ Jr, et al: From the archives of the AFIP. Infiltrative renal lesions: Radiologicpathologic correlation. Armed Forces Institute of Pathology. Radiographics 20:215-243, 2000 6. Herschom S, Radomski SB, Shoskes DA, et al: Evaluation and treatment of blunt renal trauma. J Urol 146:274-276; discussion 276-277, 1991 7. Goldstein AS, Sclafani SJ, Kupferstein NH, et al: The diagnostic superiority of computerized tomography. J Trauma 25:938-946, 1985 8. Smith RC, Rosenfield AT, Choe KA, et al: Acute flank pain: Comparison of non-contrast-enhancement CT and intravenous urography. Radiology 194:789-794, 1995
MULTIDETECTOR CT UROGRAPHY
9. Sommer FG, Jeffrey RB Jr., Rubin GD, et al: Detection of ureteral calculi in patients with suspected renal colic: Value of reformatted noncontrast helical CT. AJR Am J Roentgenol 165:509-513, 1995 10. Smith RC, Verga M, McCarthy S, et al: Diagnosis of acute flank pain: Value of unenhanced helical CT. AJR Am J Roentgenol 166:97-101, 1996 11. Zagoria RJ, Dyer RB: The small renal mass: Detection, characterization, and management. Abdom Imaging 23:256265, 1998 12. Einstein DM, Herts BR, Weaver R, et al: Evaluation of renal masses detected by excretory urography: cost-effectiveness of sonography versus CT. AJR Am J Roentgenol 164:371375, 1995 13. Hu H, He HD, Foley WD, et al: Four multidetector-row helical CT: Image quality and volume coverage speed. Radiology 215:55-62, 2000 14. McTavish JD, Jinzaki M, Zou KH, et al: Multi-detector row CT urography: Comparison of strategies for depicting the normal urinary collecting system. Radiology 225:783-790, 2002 15. Perlman ES, Rosenfield AT, Wexler JS, et al: CT urography in the evaluation of urinary tract disease. J Comput Assist Tomogr 20:620-626, 1996 16. McNicholas MM, Raptopoulos VD, Schwartz RK, et al: Excretory phase CT urography for opacification of the urinary collecting system. AJR Am J Roentgenol 170:1261-1267, 1998 17. Vrtiska TJ, Rochester MN, King BF, et al: CT urography: description of a novel technique using a uniquely modified multidetector-row CT scanner (abstr). Radiology 217:225, 2000 18. McCollough CH, Bruesewitz MR, Vrtiska TJ, et al: Image quality and dose comparison among screen-film, computed, and CT scanned projection radiography: Applications to CT urography. Radiology. 221:395-403, 2001 19. Chow LC, Sommer FG: Multidetector CT urography with abdominal compression and three-dimensional reconstruction. AJR Am J Roentgenol 177:849-855, 2001 20. Caoili EM, Cohan RH, Korobkin M, et al: Urinary tract abnormalities: Initial experience with multi-detector row CT urography. Radiology 222:353-360, 2002 21. Cronan JJ, Amis ES, Zeman RK, et al: Obstruction of the upper-pole moiety in renal duplication in adults: CT evaluation. Radiology 161:17-21, 1986 22. Dunnick NR, McCallum R, Sandler C: Textbook of Uroradiology. Baltimore, MD, Williams and Wilkins, 1991, p 19 23. Fielding IR, Fox LA, Heller H, et al: Spiral CT in the evaluation of flank pain: Overall accuracy and feature analysis. J Comput Assist Tomogr 21:635-638, 1997 24. Mostafavi MR, Ernst RD, Saltzman B: Accurate determination of chemical composition of urinary calculi by spiral computerized tomography. J Urol 159:673-675, 1998 25. Blake SF, McNicholas MMJ, Raptopoulos V: Nonopaque crystal deposition causing ureteric obstruction in patients with HIV undergoing indinavir therapy. AJR Am J Roentgenol 171:717-720, 1998 26. Fielding JR, Silverman SG, Rubin GD: Helical CT of the urinary tract. AJR Am J Roentgenol 172:1199-1206, 1999 27. Georgiades CS, Moore CJ, Smith DF: Differences of renal parenchymal attenuation for acutely obstructed and unob-
53
structed kidneys on unenhanced helical CT: A useful secondary sign? . AJR Am J Roentgenol 176:965-968, 2001 28. Rauschkolb EN, Sandler CM, Patel S, et al: Computed tomography of renal inflammatory disease. J Comput Assist Tomogr 6:502-506, 1982 29. Hoddick W, Jeffrey RB, Goldberg HI, et al: CT and sonography of severe renal and perirenal infections. AJR Am J Roentgenol 140:517-520, 1983 30. June CH, Browning MD, Smith LP, et al: Ultrasonography and computed tomography in severe urinary tract infection. Arch Intern Med 145:841-845, 1985 31. Soulen MC, Pishman EK, Goldman SM, et al: Bacterial renal infection: Role of CT. Radiology 171:703-707, 1989 32. Kawashima A, Sandler CM, Goldman SM, et al: CT of renal inflammatory disease. Radiographics 17:851-866, 1997 33. Kawashima A, Sandler CM, Goldman SM: Current roles and controversies in the imaging evaluation of acute renal infection. World J Urol 16:9-17, 1998 34. Kawashima A, Fishman EK, Goldman SM, et al: Helical CT of acute pyelonephritis: Is there an ideal timing for imaging? . Radiology 201:148-149, 1996 35. Ishikawa I, Saito Y, Onouchi Z, et al: Delayed contrast enhancement in acute focal bacterial nephritis: CT features. J Comput Assist Tomogr 9:894-897, 1985 36. Ahlering TE, Boyd SD, Hamilton CL, et al: Emphysematous pyelonephritis: A 5-year experience with 13 patients. J Urol 134:1086-1088, 1985 37. Sheth S, Scatarige J, Horton K, et al: Current concepts in the diagnosis and management of renal cell carcinoma: Role of multidetector CT and three-dimensional CT. Radiographics. 21:237-254, 2001 38. Kawashima A, Sandler CM, Corl PM, et al: Imaging of renal trauma: A comprehensive review. Radiographics. 21:557574, 2001 39. Yao DC, Jeffrey RB Jr., Mirvis SE, et al: Using contrastenhanced helical CT to visualize arterial extravasation after blunt abdominal trauma: Incidence and organ distribution. AJR Am J Roentgenol 178:17-20, 2002 40. Wong-You-Cheong JJ, Wagner BJ, Davis CJ: Transitional cell carcinoma of the urinary tract: radiologic-pathologic correlation. Radiographics 18:123-142, 1998 41. Vining DJ, Zagoria RJ, Liu K, et al: CT cystoscopy: An innovation in bladder imaging. AJR Am J Roentgenol 166:409410, 1996 42. Sommer FG, Olcott EW, Chen IY, et al: Volume rendering of CT data: Applications to the genitourinary tract. AJR Am J Roentgenol 168:1223-1226, 1997 43. Akbar SA, Shirkhoda A, Jafri SZ: Normal variants and pitfalls in CT of the gastrointestinal and genitourinary tracts. Abdom Imaging 28:115-128, 2003 44. Shirkhoda A: Diagnostic pitfalls in abdominal CT. Radiographics 11:969-1002, 1991 45. Colistro R, Torreggiani WC, Lyburn ID, et al: Unenhanced helical CT in the investigation of acute flank pain. Clin Radiol 57:435-441, 2002 46. Sussman SK, Illescas FF, Opalacz JP, et al: Renal streak artifact during contrast enhanced CT comparison of high versus low osmolality contrast media. Abdom Imaging 18:180-185, 1993 47. Papanicolaou N, Harbury OL, Pfister RC: Fat filled
54
postoperative renal cortical defects: Sonographic and CT appearance. AJR Am J Roentgenol 151:503-505, 1998 48. Nawfel R, Judy P, Schleipman A, et al: Patient radiation dose during CT urography and intravenous urography. Radiology (In press). 49. Greess H, Nomayr A, Wolf H, et al: Dose reduction in
AKBAR ET AL
CT examination of children by an attenuation-based on-line modulation of tube current (CARE Dose). Eur Radiol 12:15711576, 2002 50. Tack D, De Maertelaer V, Gevenois PA: Dose reduction in multidetector CT using attenuation-based online tube current modulation. AJR Am J Roentgenol 181:331-334, 2003
C
OMPUTED TOMOGRAPHY (CT) has become the radiologic test of choice in the evaluation of the kidneys and urinary collecting system. CT has proven superior to intravenous urography (IVU) for several clinical indications, including the evaluation of renal masses,1-3 infection,4,5 trauma,6,7 and urinary calculi.8-10 IVU fails to detect 25-50% of small (⬍3 cm) renal masses and 15% of larger masses.2 Furthermore, IVU lacks sufficient specificity for accurately characterizing any renal mass as benign.11,12 A historic advantage of IVU has been its ability to depict subtle urothelial abnormalities due to its high spatial resolution. This has been the single most important reason why IVU has been used to evaluate patients with hematuria rather than CT. With the introduction of multidetector technology, CT can be used to image the kidneys, ureters, and bladder, with thin collimation during a single breath-hold. This results in the ability to image the entire urinary tract in one acquisition. MDCT creates images with an in-plane spatial resolution that is greater than what can be obtained with singledetector CT, and is close to IVU. The ability to obtain thinly collimated data has also improved out-of-plane resolution, such that the volumetric data that can be obtained with MDCT is nearly isotropic. As a result, coronal reformatted images provide IVU-like images that are able to depict subtle urothelial abnormalities, including early cancers.13,14
accomplished with one acquisition. In an attempt to solve this problem, multiple CT urography (CTU) protocols have been described. A form of CT “hybrid” urography in which an axial CT scan of the urinary tract is followed by one or more abdominal radiographs or CT topograms has been described by Perlman et al.15 Their strategy combines the advantages of CT in imaging the kidneys, with those of contrast radiography in imaging the urinary collecting system. It remains to be seen, however, whether the image quality of a CT topogram is sufficient to view the urothelium. If a conventional radiograph is obtained after the CT scan, the patient needs to undergo imaging in two different locations. Movement of the patient between procedure rooms requires additional time and resources, can cause scheduling conflicts, and may adversely affect the level of pyelocalyceal distention at the time of the radiograph. McNicholas et al16 suggested that CTU could be performed by using single-detector CT alone. They compared excretory phase single detector CT with IVU in the ability to opacify the urinary collecting system. Patients were imaged supine, prone, and supine with compression. CTU was found to be comparable to IVU in its ability to opacify the calices, pelvis, and upper and mid-ureters. Imaging prone and supine, with compression, resulted in better opacification of the collecting system than
TECHNIQUE
From the Division of Abdominal Imaging and Intervention, Brigham and Women’s Hospital, Harvard Medical School, 75 Francis Street, Boston, MA. Address reprint requests to Koenraad J. Mortele, MD, Division of Abdominal Imaging and Intervention, Brigham and Women’s Hospital, 75 Francis Street, Boston, MA 02115; e-mail: [email protected]. © 2004 Elsevier Inc. All rights reserved. 0887-2171/04/2501-0005$30.00/0 doi:10.1053/j.sult.2003.11.002
Acquisition The fundamental problem with imaging the urinary tract is that, because of peristalsis, it is difficult to image all ureteral segments in an opacified and distended state. Using IVU, multiple images can be obtained for this purpose, but with CT and its higher radiation dose, ureteral imaging must be
Seminars in Ultrasound, CT, and MRI, Vol 25, No 1 (February), 2004: pp 41-54
41
42
the supine, noncompression technique. Relative to axial images, reformatted images were judged to be of additional value in 44% of cases. A MDCT scanner configured with a special tabletop apparatus has been used to obtain both CT and radiographic images without moving the patient.17 The modified CT tabletop is capable of receiving a combined slip-on grid/standard film cassette. This system requires the installation of a ceilingmounted X-ray tube above the CT table and the attachment of an auxiliary CT tabletop that has a hollow bay under the patient surface in which to place the radiographic cassette. With this approach, the CTU protocol consists of a KUB, an unenhanced renal CT, a multiphasic contrast-enhanced renal CT scan, followed by overhead excretory urographic and post-void radiographs.17 McCullough et al18 compared the image quality and dose between KUB and CT scanned projection radiography, in their applications to CT urography. An image quality phantom was imaged with both moving and stationary grids. Entrance skin exposure; spatial, contrast, and temporal resolutions; geometric accuracy; and artifacts were assessed. KUB provided better spatial resolution than did CT scan projection radiography. However, the latter method provided improved low-contrast resolution, at exposures comparable to those used for KUB. Chow and Sommer19 described a three-phase multidetector CT urography (MDCTU) protocol with unenhanced, enhanced with abdominal compression, and post-release phases. These investigators split the contrast media bolus into two portions; the first is used to opacify the intrarenal collecting system such that, when the CT scan is performed during the nephrographic phase, both the parenchyma and the collecting system are opacified. Initially, unenhanced images are obtained from the diaphragm to the symphysis pubis. Subsequently, a 40-mL bolus on contrast media is administered at 2 mL/sec and an abdominal compression device applied. Then, the remaining 80 mL of contrast agent is administered, and CT scans are then obtained from the diaphragm to the iliac crests after a 90-sec delay (to obtain a nephrogram). A scout image is then obtained of the abdomen and pelvis, after which the compression device is released and a CT scan is immediately obtained from the iliac crests to the symphysis
AKBAR ET AL
pubis. A final scout of the abdomen and pelvis is then obtained. Caoili et al20 described 4-phase MDCTU, performed using an unenhanced scan, a nephrographic phase scan with abdominal compression, and 2 excretory-phase scans, one obtained 200 sec after the injection of contrast material with compression, and the other at 300 sec without compression. They compared CTU findings with the results of cystoscopy, ureteroscopy, and surgery in 65 patients in whom urinary tract abnormalities were strongly suspected. Their study revealed that CTU depicted many urinary tract abnormalities, including 15 of 16 uroepithelial malignancies, 5 congenital anomalies, 5 urinary tract calculi, 25 bladder abnormalities, and pelvic/ureteral abnormalities as follows:18 calyceal and/or papillary, 30 renal pelvic and/or ureteral. Our group, like others, believes that CTU is best performed by using CT alone. The optimal MDCTU protocol has not been determined, particularly with respect to the number of scans, patient position, timing of contrast media administration, and the use of compression devices. Comparing several protocols for depicting the urinary collecting system with MDCTU,14 we demonstrated that a MDCTU supplemented with an intravenous infusion of normal saline significantly improved opacification of the collecting systems and ureters, as compared with protocols not using saline. As confirmed by other investigators,16 the distal ureters were the most difficult segments to opacify, but were more reliably opacified when saline was used. Patient position was also analyzed, and no significant difference was found between prone and supine positions. We found that saline supplementation had several advantages. It is a simple protocol that is easily performed by the technologist alone. An incompletely distended collecting system limits detection of urinary tract disease, and excellent distension is achieved without use of a compression device. Furthermore, compression devices are cumbersome, of limited value in obese patients, and contraindicated in patients with acute abdominal pain or abdominal aortic aneurysms. Patients are examined with 3 CT scans: one each unenhanced, during the nephrographic phase, and during the pyelographic phases (Fig 1). We do not administer an oral bowel contrast agent. Patients are asked to void before the scan. All patients are positioned supine and receive a supplemental iv
MULTIDETECTOR CT UROGRAPHY
43
Display Methods
Fig 1. Normal MDCT urogram using three phases and supplemental saline: (a) Unenhanced phase; (b) The nephrographic phase (acquired 100 sec after administration of 100 cc of Ultravist-300); (c) Excretory phase (acquired after a 10-min delay and supplemental IV administration of 250 ml of normal saline).
infusion of normal saline. The CT technologist hangs a 250-mL bag of normal saline immediately following the injection of contrast medium and allows it to infuse by gravity. Using a 4-channel scanner, the protocol consists of the following: (1) an unenhanced phase obtained with 2.5-mm collimation; (2) a nephrographic phase, acquired also with 2.5-mm collimation 100 sec after administration of 100 cc of Ultravist-300 (Berlex Lab, Madison, NJ) at a rate of 3 mL/sec; and (3) a pyelographic phase acquired after a 10- to 15-min delay with 1-mm collimation. The same basic protocol is used with a 16-channel CT scanner, but 1.5-mm collimation is used for the unenhanced and nephrographic phases, and 0.75-mm collimation for the pyelographic phase.
Different display methods can be used to evaluate CTU datasets, including multiplanar reformatted images (MPR) generated from axial data, maximum intensity projection (MIP), and volume rendered images (Fig 2). MIP techniques use only a small fraction of the available data to generate and display images, and in our experience, are rarely useful.14 Curved multiplanar reformations, average intensity projection (AIP), and volume rendered (VR) images can be used in selected cases. Three-dimensional volume rendering retains CT data by summing the contributions from all the voxels along a line from any viewing angle through the data set. However, it is essential to review the axial images for all necessary information before 3D rendering is performed. With MPRs, pyelographic phase images should be viewed at a window center of 100 HU and a window width of 600 HU, which allows reliable visualization of the range of contrast material attenuation on the images. These window settings, which lie between those typically used for softtissue and bone viewing, are necessary to view lesions in the collecting system and ureter that could be otherwise masked. Chow and Sommer19 obtain thin-slab MIP oblique coronal images in a plane “en face” to the kidney, generated from both unenhanced and enhanced data for each kidney. Additionally, anteroposterior thick-slab MIP-images are obtained of the kidneys individually, and collectively from the enhanced compression data. Finally, a MIP-image of the distal ureters and urinary bladder is generated from the third phase data. Additional MIP, minimum intensity-projection, average intensity, and curved planar reformations are obtained as needed. CLINICAL APPLICATIONS
MDCTU is able to depict the entire urinary tract, including both the renal parenchyma and the collecting structures (ureters and bladder). As a result, a wide range of diseases can be identified. MDCTU can be used to depict anatomic variants, stone disease, inflammatory processes, and benign and malignant neoplasms. At our institution, MDCTU has replaced IVU in the evaluation of patients with hematuria.
44
AKBAR ET AL
Fig 2. Different display methods to evaluate CTU datasets: (a) Selected reformatted coronal CT image from a normal MDCT urogram, obtained with supplemental saline; (b) Maximum Intensity-Projection (MIP) image shows normal intrarenal collecting system, ureters and bladder; (c) Normal volume rendered (VR) 3-D image.
MULTIDETECTOR CT UROGRAPHY
45
Fig 3. Congenital anomalies of the collecting system and ureter: (a) Reformatted coronal CT image demonstrates renal ectopia; (b) Coronal image shows a left pelvic duplication in a patient with a history of left flank plain; (c– e) Coronal, volume rendered 3-D and MIP images show duplicated right upper ureter; (f) Coronal reformatted image shows a left-sided primary megaureter.
Normal Variants Congenital anomalies of the collecting system and ureters can be visualized better with MDCTU than with conventional CT. Congenital anomalies of renal position and number are well depicted with MDCTU, including renal ectopia, malrotation, and horseshoe kidneys (Fig 3).21 In approximately 10% of individuals, the collecting system is incompletely duplicated.22 Duplication of the collecting system may be difficult to appreciate with conventional CT, as opacification of the ureters is
usually incomplete. Coronal reconstructions with MDCTU allow the duplication to be viewed in an IVU-like fashion, and occasionally can be used to determine the extent of the duplication (Fig 3).20 Urolithiasis Previously, plain film radiography, IVU, and ultrasonography were used in the evaluation of acute flank pain or hematuria. However, IVU failed to demonstrate calculi in up to 48% of patients.23 Now, unenhanced CT has replaced IVU
Fig 4. Nephrocalcinosis and urolithiasis: (a) Reformatted coronal image shows bilateral medullary nephrocalcinosis; (b, c) Axial images with soft tissue and bone window settings show a left ureterovesical junction (UVJ) stone (arrow) causing partial obstruction; (d) Coronal image depicts the left UVJ stone (arrow). Bone windows help differentiate the small calculus from the opacified ureter; (e) Curved multiplanar reformation demonstrates an obstructing distal ureteral calculus (arrow).
MULTIDETECTOR CT UROGRAPHY
47
in the evaluation of patients presenting with ureteral colic. Many studies8-10 have documented the superior sensitivity and specificity of unenhanced helical CT when compared with IVU for the detection of renal and ureteral calculi (Fig 4). CT may also help analyze stone composition and differentiate among uric acid, struvite, and calcium oxalate stones.24 Almost all urinary tract calculi, regardless of their calcium content, are radioopaque on unenhanced CT. The rare stones that form in 4% of patients infected with HIV who are treated with indinavir therapy may not be radioopaque.25 Staghorn calculi and kidneys containing multiple stones are optimally depicted with multiplanar and 3D displays that allow planning for percutaneous approaches to stone removal. Unenhanced CT also demonstrates valuable secondary signs of obstructive uropathy, including hydronephrosis, hydroureter, renal enlargement, perinephric and periureteral stranding, and ureterovesical junction edema.26 The combination of hydronephrosis, hydroureter, and perinephric stranding has a positive predictive value of 90% for urinary obstruction.23 Differences in renal parenchymal attenuation between obstructed and nonobstructed kidneys have also been reported as a reliable secondary sign of obstruction.27 The degree of ureteral obstruction may also be predicted by the extent of perinephric edema.27 Whereas most calculi are detected with nonenhanced CT, occasionally, stones can be detected after the injection of contrast material (Fig 4). Infection MDCTU is superior to both IVU and ultrasonography in the assessment renal infection.28-36 MDCTU establishes the diagnosis in equivocal
Fig 6. Papillary necrosis in a patient with chronic pyelonephritis: (a, b) Coronal excretory phase images show papillary necrosis. Round filling defect in the interpolar region of the (a) right kidney (arrow) and in (b) the left upper pole (arrow) correspond to sloughed papillae in the collecting system. Also note the abnormal kidney contours. Overall appearance is consistent with papillary necrosis due to chronic infection.
Fig 5. Focal bacterial pyelonephritis. Axial CT nephrogram shows a low-density mass-like abnormality in the mid-pole of right kidney (arrow) due to focal bacterial pyelonephritis.
cases, assessing the extent and complications such as renal abscess formation. Gas, calculi, renal parenchymal calcifications, and hemorrhage are demonstrated well with unenhanced CT. A striated
48
AKBAR ET AL
the corticomedullary junction. Other signs of infection include focal or global enlargement of the kidney, obliteration of the renal sinus and perinephric fat planes, thickening of Gerota’s fascia, caliceal effacement caused by swelling of adjacent renal parenchyma, and thickening of the pelvicaliceal walls.4,32,33 Parenchymal abnormalities are best demonstrated on CT images obtained during the nephrographic phase (Fig 5). Delayed CT scan
Fig 7. Renal masses: (a) Coronal image shows a round solid mass in the upper pole of the left kidney (arrow). Surgical specimen yielded renal cell carcinoma (RCC); (b) Reformatted coronal image shows innumerable bilateral kidney cysts in a patient with polycystic kidney disease (APKD).
nephrogram on contrast-enhanced CT, consisting of discrete rays of alternating attenuation that extend to the capsule, is characteristic of pyelonephritis. During an acute infection, MDCTU demonstrates one or more hypodense areas within the renal parenchyma and sometimes obliteration of
Fig 8. Bladder stones and papilloma. (a) Coronal image (bone window settings) shows the presence of two large bladder stones. (b) Axial image: in the dependent portion of the bladder a triangular shaped lesion is seen. Surgical specimen yielded an inverted papilloma.
MULTIDETECTOR CT UROGRAPHY
49
Fig 9. Three-phase MDCTU in a patient with transitional cell carcinoma: (a) Unenhanced phase: a slightly hyperdense renal mass (arrow) is seen in a right lower pole calyx; (b) Nephrographic phase (acquired 100 sec after administration of 100 cc of Ultravist-300): the lesion (arrow) shows mild enhancement; (c) The pyelographic phase (acquired after a 10-min delay and supplemental administration of 250 ml of normal saline): the lesion (arrow) causes a filling defect in the contrast filled calyx; (d) Coronal reformatted CT image in the urographic phase demonstrates the location and extent of the lesion (arrow).
may help in differentiating tumor from an inflammatory mass, by showing persistent contrast enhancement in areas where there was previously diminished enhancement after contrast medium.35 CT is the modality of choice for detecting a renal abscess and emphysematous pyelonephritis.31,36 MDCTU may also be helpful in the evaluation of chronic inflammatory processes, such as renal tuberculosis, chronic pyelonephritis, and xanthogranulomatous pyelonephritis. In patients with renal papillary necrosis, MDCTU may demonstrate small kidneys, “ring shadows” in the medulla, contrast-filled clefts in the renal parenchyma, and urinary collecting system filling defects (Fig 6). Renal Masses With the widespread use of cross-sectional imaging, as many as one-half of renal cell carcinomas are discovered incidentally, and almost all are small, early-stage lesions.37 As CTU replaces IVU for urinary complaints and hematuria, the detection
small renal cancers will likely increase. The most common appearance of a small renal cell carcinoma is a noncalcified lesion with an attenuation value of 20 HU or higher, that enhances by more than 10 HU after intravenous injection of contrast medium.3 The nephrographic phase is the most accurate for detection and characterization of renal masses.3,12,37 Nephrographic and excretory phase images have previously been shown to be superior to corticomedullary phase images in the detection and characterization of renal masses. For partial nephrectomy planning, coronal reformatted images are useful in visualizing the relationship of renal lesions to adjacent collecting system anatomy (Fig 7). Trauma CT is the imaging modality of choice for the assessment of blunt abdominal injuries in general, and renal injury in particular. CT can identify renal contusions, subcapsular hematomas, lacerations,
50
AKBAR ET AL
shattered kidneys, and vascular injuries.6,7 It can also help in evaluation of the type and severity of parenchymal injury, the extent of perirenal hemorrhage, and parenchymal devascularization.38,39 Excretory phase images are needed to detect collecting system injuries, such as ureteropelvic junction disruption, and the presence of urinary extravasation. MDCTU improves the evaluation of trauma by depicting the collecting system and ureters in a fully opacified state during the coronal plane. Ureters and Bladder MDCTU distinguishes stones from soft tissue filling defects within the bladder (Fig 8). Approximately 7-8% of renal malignancies develop in the intrarenal collecting system (Fig 9). The most common malignant tumor of the ureter is transitional cell carcinoma, and 85% of these tumors are broadbased.40 Pedunculated or diffusely infiltrating tumors are less common. Sessile and infiltrating lesions behave more aggressively and are more advanced at the time of diagnosis. Infiltrating tumors are characterized by thickening and induration of the ureter wall (Fig. 10).40 Transitional cell carcinoma most commonly presents as an intraluminal mass with or without obstruction. MDCTU detects small (⬃5 mm) urothelial malignancies.20 Using axial images, 95% of the malignant foci are identified. Concentric urothelial wall thickening is identified more easily on axial CT images than on 3D reformations. However, in patients with diffuse bladder wall thickening, MDCTU cannot distinguish benign from malignant etiologies. These data suggest that MDCTU may be most sensitive for the detection of upper tract TCC. MDCTU helps evaluate patients with TCC by depicting whether the lesion has extended beyond the ureter, and by screening the rest of the urothelium for synchronous tumors (Fig. 11).41,42
PITFALLS
Factors that might lead to either misinterpreting or overlooking findings on CTU include using improper technique, confusing normal anatomic structures and variants as abnormalities, and not understanding artifacts.43,44 Technical artifacts can be caused by patient motion (respiratory or otherwise) and partial volume averaging of adjacent organs. In particular, if the patient breathes during scan acquisition, coronal reformations may appear to contain a renal contour abnormality which mimics a neoplasm (Fig 12). There are several pitfalls that may confound the diagnosis of stone disease.45 Orally administered bowel preparations containing diatrizoate meglumine and diatrizoate sodium (Gastrografin) may be excreted renally in patients with diseases involving the intestinal wall, and may mimic or obscure calculi on the unenhanced scan. When using the split dose technique, streak artifacts from contrast medium in the calyces may falsely lower the enhanced attenuation of an adjacent renal mass.46 All three phases of a MDCTU, unenhanced, nephrographic, and excretory, are essential. If the
Additional Findings As the entire abdomen and pelvis is scanned, MDCTU may demonstrate incidental and unsuspected extraurinary tract abdominal and pelvic lesions, which may have a significant impact on patient care. Such lesions include retroperitoneal tumors, abdominal aortic aneurysms, and uterine and adnexal masses.
Fig 10. Transitional cell carcinoma of the ureter. Curved planar reformatted (CPR) image shows irregular thickening of the lower right ureteral wall (arrow). Surgical specimen yielded transitional cell carcinoma (TCC). Notice also the presence of hydroureter and hydronephrosis.
MULTIDETECTOR CT UROGRAPHY
51
Fig 12. Reconstruction generated artifact may mimic a renal neoplasm. The reconstruction algorithm in this case produces deformity of the renal contours that may be mistaken for multiple masses.
corticomedullary phase was used instead of the nephrographic phase, the unenhanced renal medulla might be mistaken for a renal lesion, or a tumor might be missed because it is mistaken for a medullary pyramid. The use of nephrographic scans, the time when both the cortex and medulla are opacified, avoids these pitfalls. Embryologic renal parenchymal anomalies (persistent fetal lobation, column of Bertin, junctional parenchymal detects, and dromedary hump) are also seen well during the nephrographic phase. Postsurgical filling of renal defects by perinephric fat, following localized resection, can simulate angiomyolipomas and may pose problems if the clinical history is unknown.47 The pyelographic phase is necessary to detect urinary epithelial neoplasms, differentiate hydronephrosis from peripelvic cysts, and distinguish calyceal diverticulum from cysts. Congenital bladder anomalies [bladder diverticula (Fig 13), herniations, urachal remnants] also are best detected during the excretory phase. Finally, delayed phase imaging is helpful in opacifying ileal conduits, that if not opacified, may mimic a fluid collection. Occasional findings mimicking significant uri-
4™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™ Fig 11. Transitional cell carcinoma of the bladder extending into the ureter. (a) Axial image shows focal left bladder wall thickening (arrow). (b) Coronal image shows the irregular bladder mass. (c) Curved planar reformatted image nicely depicts extension of the bladder wall mass into the left ureter (arrow). Note also the presence of hydroureter. Surgical specimen yielded TCC.
52
AKBAR ET AL
structions.14 Therefore, MIP reconstructions should not be used alone to evaluate the urinary tract.14 RADIATION DOSE CONSIDERATIONS
A CT scan imparts a higher radiation dose than a single radiograph. Most CTU protocols utilize at least two scans; therefore, it can be expected that CTU would impart a significantly higher dose to patients than IVU. Dose measurements for our 3-scan MDCTU protocol showed that skin doses were similar to IVU, but the effective dose at MDCTU was approximately 1.5 times that of IVU.14,48 These results cannot be generalized, as the techniques for both IVU and MDCTU differ across different institutions. There are new CT scanner software programs that allow CT doses to be reduced by lowering tube current based on body width and tissue absorption.49,50 In the future, it is likely that radiation doses incurred with MDCTU will eventually be the same or even lower than those obtained in IVU. CONCLUSION Fig 13. Bladder diverticulum. Coronal image shows a left bladder wall diverticulum.
nary tract pathology include stretching of a ureter as it crosses the midline to insert into an ileal loop, mucous in the renal collecting system, and ureteral kinks. Each of these may result in unopacified segments, either leading to “blind spots” or mimicking urohelial tumors. In addition, short segments of the intrarenal collecting system and ureters that are opacified on transverse or coronal sections may appear unopacified on MIP recon-
In summary, CT has become the examination of choice for most urinary tract pathology, as it provides for “one step” evaluation of the kidneys, intrarenal collecting systems, and ureters. Although bladder pathology is detected from time to time using CT urography, cystoscopy remains the test of choice for evaluating the urinary bladder. There are several protocols for performing MDCTU, each with advantages and disadvantages. While techniques vary, each provides essentially the same diagnostic information. Whether one will be proven to be better than the others awaits further study.
REFERENCES 1. Amendola M, Bree R, Pollack HM, et al: Small renal cell carcinomas: Resolving a diagnostic dilemma. Radiology 166: 637-741, 1988 2. Warshauer DM, McCarthy SM, Street L, et al: Detection of renal masses: Sensitivities and specificities of excretory urography/linear tomography, US and CT. Radiology 169:363365, 1988 3. Silverman SG, Lee BY, Seltzer SE, et al: Small (⬃3 cm) renal masses: Correlation of spiral CT features and pathological findings. AJR Am J Roentgenol 163:597-605, 1994 4. Kawashima A, Sandler CM, Ernst RD, et al: Renal inflammatory disease: The current role of CT. Crit Rev Diagn Imaging 38:369-415, 1997
5. Pickhardt PJ, Lonergan GJ, Davis CJ Jr, et al: From the archives of the AFIP. Infiltrative renal lesions: Radiologicpathologic correlation. Armed Forces Institute of Pathology. Radiographics 20:215-243, 2000 6. Herschom S, Radomski SB, Shoskes DA, et al: Evaluation and treatment of blunt renal trauma. J Urol 146:274-276; discussion 276-277, 1991 7. Goldstein AS, Sclafani SJ, Kupferstein NH, et al: The diagnostic superiority of computerized tomography. J Trauma 25:938-946, 1985 8. Smith RC, Rosenfield AT, Choe KA, et al: Acute flank pain: Comparison of non-contrast-enhancement CT and intravenous urography. Radiology 194:789-794, 1995
MULTIDETECTOR CT UROGRAPHY
9. Sommer FG, Jeffrey RB Jr., Rubin GD, et al: Detection of ureteral calculi in patients with suspected renal colic: Value of reformatted noncontrast helical CT. AJR Am J Roentgenol 165:509-513, 1995 10. Smith RC, Verga M, McCarthy S, et al: Diagnosis of acute flank pain: Value of unenhanced helical CT. AJR Am J Roentgenol 166:97-101, 1996 11. Zagoria RJ, Dyer RB: The small renal mass: Detection, characterization, and management. Abdom Imaging 23:256265, 1998 12. Einstein DM, Herts BR, Weaver R, et al: Evaluation of renal masses detected by excretory urography: cost-effectiveness of sonography versus CT. AJR Am J Roentgenol 164:371375, 1995 13. Hu H, He HD, Foley WD, et al: Four multidetector-row helical CT: Image quality and volume coverage speed. Radiology 215:55-62, 2000 14. McTavish JD, Jinzaki M, Zou KH, et al: Multi-detector row CT urography: Comparison of strategies for depicting the normal urinary collecting system. Radiology 225:783-790, 2002 15. Perlman ES, Rosenfield AT, Wexler JS, et al: CT urography in the evaluation of urinary tract disease. J Comput Assist Tomogr 20:620-626, 1996 16. McNicholas MM, Raptopoulos VD, Schwartz RK, et al: Excretory phase CT urography for opacification of the urinary collecting system. AJR Am J Roentgenol 170:1261-1267, 1998 17. Vrtiska TJ, Rochester MN, King BF, et al: CT urography: description of a novel technique using a uniquely modified multidetector-row CT scanner (abstr). Radiology 217:225, 2000 18. McCollough CH, Bruesewitz MR, Vrtiska TJ, et al: Image quality and dose comparison among screen-film, computed, and CT scanned projection radiography: Applications to CT urography. Radiology. 221:395-403, 2001 19. Chow LC, Sommer FG: Multidetector CT urography with abdominal compression and three-dimensional reconstruction. AJR Am J Roentgenol 177:849-855, 2001 20. Caoili EM, Cohan RH, Korobkin M, et al: Urinary tract abnormalities: Initial experience with multi-detector row CT urography. Radiology 222:353-360, 2002 21. Cronan JJ, Amis ES, Zeman RK, et al: Obstruction of the upper-pole moiety in renal duplication in adults: CT evaluation. Radiology 161:17-21, 1986 22. Dunnick NR, McCallum R, Sandler C: Textbook of Uroradiology. Baltimore, MD, Williams and Wilkins, 1991, p 19 23. Fielding IR, Fox LA, Heller H, et al: Spiral CT in the evaluation of flank pain: Overall accuracy and feature analysis. J Comput Assist Tomogr 21:635-638, 1997 24. Mostafavi MR, Ernst RD, Saltzman B: Accurate determination of chemical composition of urinary calculi by spiral computerized tomography. J Urol 159:673-675, 1998 25. Blake SF, McNicholas MMJ, Raptopoulos V: Nonopaque crystal deposition causing ureteric obstruction in patients with HIV undergoing indinavir therapy. AJR Am J Roentgenol 171:717-720, 1998 26. Fielding JR, Silverman SG, Rubin GD: Helical CT of the urinary tract. AJR Am J Roentgenol 172:1199-1206, 1999 27. Georgiades CS, Moore CJ, Smith DF: Differences of renal parenchymal attenuation for acutely obstructed and unob-
53
structed kidneys on unenhanced helical CT: A useful secondary sign? . AJR Am J Roentgenol 176:965-968, 2001 28. Rauschkolb EN, Sandler CM, Patel S, et al: Computed tomography of renal inflammatory disease. J Comput Assist Tomogr 6:502-506, 1982 29. Hoddick W, Jeffrey RB, Goldberg HI, et al: CT and sonography of severe renal and perirenal infections. AJR Am J Roentgenol 140:517-520, 1983 30. June CH, Browning MD, Smith LP, et al: Ultrasonography and computed tomography in severe urinary tract infection. Arch Intern Med 145:841-845, 1985 31. Soulen MC, Pishman EK, Goldman SM, et al: Bacterial renal infection: Role of CT. Radiology 171:703-707, 1989 32. Kawashima A, Sandler CM, Goldman SM, et al: CT of renal inflammatory disease. Radiographics 17:851-866, 1997 33. Kawashima A, Sandler CM, Goldman SM: Current roles and controversies in the imaging evaluation of acute renal infection. World J Urol 16:9-17, 1998 34. Kawashima A, Fishman EK, Goldman SM, et al: Helical CT of acute pyelonephritis: Is there an ideal timing for imaging? . Radiology 201:148-149, 1996 35. Ishikawa I, Saito Y, Onouchi Z, et al: Delayed contrast enhancement in acute focal bacterial nephritis: CT features. J Comput Assist Tomogr 9:894-897, 1985 36. Ahlering TE, Boyd SD, Hamilton CL, et al: Emphysematous pyelonephritis: A 5-year experience with 13 patients. J Urol 134:1086-1088, 1985 37. Sheth S, Scatarige J, Horton K, et al: Current concepts in the diagnosis and management of renal cell carcinoma: Role of multidetector CT and three-dimensional CT. Radiographics. 21:237-254, 2001 38. Kawashima A, Sandler CM, Corl PM, et al: Imaging of renal trauma: A comprehensive review. Radiographics. 21:557574, 2001 39. Yao DC, Jeffrey RB Jr., Mirvis SE, et al: Using contrastenhanced helical CT to visualize arterial extravasation after blunt abdominal trauma: Incidence and organ distribution. AJR Am J Roentgenol 178:17-20, 2002 40. Wong-You-Cheong JJ, Wagner BJ, Davis CJ: Transitional cell carcinoma of the urinary tract: radiologic-pathologic correlation. Radiographics 18:123-142, 1998 41. Vining DJ, Zagoria RJ, Liu K, et al: CT cystoscopy: An innovation in bladder imaging. AJR Am J Roentgenol 166:409410, 1996 42. Sommer FG, Olcott EW, Chen IY, et al: Volume rendering of CT data: Applications to the genitourinary tract. AJR Am J Roentgenol 168:1223-1226, 1997 43. Akbar SA, Shirkhoda A, Jafri SZ: Normal variants and pitfalls in CT of the gastrointestinal and genitourinary tracts. Abdom Imaging 28:115-128, 2003 44. Shirkhoda A: Diagnostic pitfalls in abdominal CT. Radiographics 11:969-1002, 1991 45. Colistro R, Torreggiani WC, Lyburn ID, et al: Unenhanced helical CT in the investigation of acute flank pain. Clin Radiol 57:435-441, 2002 46. Sussman SK, Illescas FF, Opalacz JP, et al: Renal streak artifact during contrast enhanced CT comparison of high versus low osmolality contrast media. Abdom Imaging 18:180-185, 1993 47. Papanicolaou N, Harbury OL, Pfister RC: Fat filled
54
postoperative renal cortical defects: Sonographic and CT appearance. AJR Am J Roentgenol 151:503-505, 1998 48. Nawfel R, Judy P, Schleipman A, et al: Patient radiation dose during CT urography and intravenous urography. Radiology (In press). 49. Greess H, Nomayr A, Wolf H, et al: Dose reduction in
AKBAR ET AL
CT examination of children by an attenuation-based on-line modulation of tube current (CARE Dose). Eur Radiol 12:15711576, 2002 50. Tack D, De Maertelaer V, Gevenois PA: Dose reduction in multidetector CT using attenuation-based online tube current modulation. AJR Am J Roentgenol 181:331-334, 2003