British Journal of Radiology (2003) 76, 199-209
© 2003 British Institute of Radiology
doi: 10.1259/bjr/26360633
CT urography and virtual endoscopy: promising imaging modalities for urinary tract evaluation
J K Kim, MD
and
K-S Cho, MD
Department of Radiology, Asan Medical Center, University of Ulsan, 388-1 Poongnap-dong, Songpa-gu, Seoul 138-736, Korea
Correspondence: Kyoung-Sik Cho
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Abstract
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CT urography and virtual endoscopy images are generated from dedicated multislice helical CT data sets and various three-dimensional reconstruction techniques. These imaging techniques can provide external and endoscopic images of the urinary tract and also provide high spatial resolution images helping overcome some of the limitations of intravenous urography and ultrasound. This pictorial review presents clinical applications of CT urography and virtual endoscopy in various urinary tract abnormalities.
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Introduction
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Intravenous urography (IVU) and ultrasound, which until now have been used as the first step in evaluating urinary tract, have limitations such as a low sensitivity for small lesion detection. Recent advances in CT including software developments have led to the use of three-dimensional (3D) imaging reconstruction techniques and allow CT urography and virtual endoscopy to be used in daily practice.
In order to produce optimal CT urography and virtual endoscopy, high-resolution and thin-slice CT data are necessary. Furthermore, dedicated CT protocols and the application of adequate 3D reconstruction techniques are necessary. In this pictorial review, we present various clinical applications of CT urography and virtual endoscopy.
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Multislice CT data acquisition and 3D reconstruction
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We use a four-channel multislice CT scanner (LightSpeed QX/i; GE Medical Systems, Milwaukee, WI). Patients receive 120 ml of intravenous contrast material (Iopamiro 300 [iopamidol]; Bracco, Milano, Italy) injected via a mechanical injector at 3 ml sec-1. Unenhanced CT scans are routinely acquired, and those of vascular and delayed phases are additional acquisitions. Scans in the vascular phase are obtained when a vascular abnormality is suspected on review of the unenhanced CT images, and delayed scanning may be performed in order to achieve opacification of a dilated urinary tract.
Various 3D reconstruction techniques can be used for CT urography: maximum intensity projection (MIP); curved multiplanar reformation (cMPR); volume rendering; and virtual endoscopy. MIP selects and displays the maximum voxel value along a line of the viewer's projection through a given volume (Figure 1
). This technique is widely used for angiography and urography because of its ability to clearly visualize high attenuation structures high intensity. Unfortunately, high-intensity material such as calcification may obscure structures of interest [1]. Volume rendering utilizes the entire volume of data, summing the contribution of each voxel along a line from the viewer's eye through the data set, displaying the resulting composite for each pixel (Figure 2). Much more powerful computers are required for this technique and image quality is dependent according to the setting [1]. cMPR displays all voxel values on a curve randomly drawn by the operator (Figure 3); the most important advantage of this technique is visualization of both opacified and unopacified materials. Thus it can be used in the case of severe urinary obstruction. For virtual endoscopy, polarization of the voxel data into air and solid is necessary; voxel values under the given threshold are merged into air and those above the threshold into solid; enabling this technique to demonstrate the surface between the air and solid (Figure 4).
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Figure 1. Maximum intensity projection. (a) This technique selects and displays the maximum voxel value along a line of viewer's projection through a given volume. (b) An example of normal CT urography using maximum intensity projection.
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Figure 2. Volume rendering. (a) This technique takes the entire volume of data and displays each pixel according to the equation in the lower portion of the diagram. (b) An example of normal CT urography using volume rendering.
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Figure 3. Curved multiplanar reformation. (a) This technique displays all voxel values on the curve randomly drawn by the operator. (b) CT urography using this technique can show the entire urinary tract regardless of opacification.
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Figure 4. Virtual endoscopy. (a) This technique polarizes all the voxel value into air and solid. Voxel values under the given threshold are merged into air and those above the threshold into solid. Thereafter, the surface between the air and solid is displayed. (b) An example of virtual endoscopy for normal collecting system.
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When the urinary tract is well opacified, MIP or volume rendering techniques can be applied. If contrast material does not adequately fill the dilated urinary tract in the case of severe obstruction, cMPR may be helpful. Virtual endoscopy can provide intraluminal evaluation of the opacified urinary tract.
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Clinical application in the upper urinary tract
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Urolithiasis
Unenhanced helical CT has recently been shown to be of value in detecting urolithiasis with a high sensitivity up to 97% [2]. The additional use of MIP or cMPR images can help stone location (Figure 5) and raise the sensitivity for stone detection on abdominal radiographs used after CT in managing patients with ureterolithiasis [3].
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Figure 5. Lateral view of CT urography using maximum intensity projection shows a stone (arrow) in the calyceal diverticulum (arrowheads) and a stone (arrow) in the proximal ureter.
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Crossing vessels at the ureteropelvic junction obstruction
Crossing vessels at the ureteropelvic junction accounts for 25% to 39% of patients with hydronephrosis secondary to pelviuretric junction obstruction [4]. The presence or absence of crossing vessel at the ureteropelvic junction is of importance because the success rate of endopyelotomy is only 42% in patients with crossing vessels as opposed to greater than 80% in patients without crossing vessels [5]. CT angiography is useful for identifying crossing vessels in patients with ureteropelvic junction obstruction (Figure 6).
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Figure 6. CT angiography using maximum intensity projection shows the dilated pelvicalyceal system (asterisk) of the right kidney and a branch of the renal artery (arrow) crossing the ureteropelvic junction.
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Congenital anomalies: ureteral duplication
Ureteral duplication is a relatively common congenital anomaly of the urinary tract. In evaluating patients with this disease, demonstration of the entire course of the upper and lower moiety ureters are important. CT urography can depict not only opacified ureters but also unopacified ureters not shown on IVU. Furthermore, CT urography can identify the location of ectopic ureteral insertion (Figure 7).
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Figure 7. CT urographic images using (a) maximum intensity projection and (b) volume rendering shows complete duplication of the left urinary tract. The dilated upper moiety ureter (arrows) inserts into the vagina (arrowhead).
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Primary urothelial neoplasms
Primary urothelial neoplasms usually present as intraluminal filling defects with/without urinary obstruction. In patients with suspected urothelial tumours, the entire urothelium must be examined because of the multicentric nature of the tumour.
Compared with IVU and ultrasound, CT urography and virtual endoscopy have advantages including a higher detection rate for small lesions, visualization of the urinary tract regardless of opacification by using cMPR and intraluminal evaluation, and the demonstration of surrounding structures by referring to the source images (Figure 8).
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Figure 8. Multiple urothelial carcinomas involving the collecting system and proximal ureter. (a) Intravenous urography shows a filling defect (arrow) in the proximal ureter. Although abrupt cut-offs of the collecting systems (arrowheads) are noted, the presence or absence of a mass is unclear. (b) CT urography using maximum intensity projection directly visualizes masses (arrowheads) in the collecting systems as well as in the proximal ureter (arrow). (c) Virtual endoscopy at the level of the proximal ureter shows an intraluminal mass (arrows). (d) Virtual endoscopy at the level of the renal pelvis shows a mass (arrows) filling the infundibulum and protruding into the pelvis. Note a normal infundibular lumen (asterisk).
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Metastatic and direct extension in the urinary tract
Metastases to the urinary tract may occur by (1) the urinary route, (2) haematogeneous spread and (3) direct invasion from a nearby tumour. Metastatic spread via urinary or haematogeneous routes usually manifest as multifocal mucosal nodules or wall thickening, whereas direct invasion generates a short or long stricture [6]. CT urography and virtual endoscopy can demonstrate the causes of strictures as well as mucosal nodules or wall thickening (Figures 9 and 10).
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Figure 9. Recurrent cervical cancer invading the right distal ureter. (a) CT urography using curved multiplanar reformation shows a dilated right urinary tract (asterisk) due to a mass in the pelvic cavity (arrows). (b) Conventional axial CT image at the level of the right ureterovesical junction demonstrates the recurrent cervical mass (arrows) invading the right distal ureter (arrowhead).
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Figure 10. Ureteral metastasis from gastric cancer. (a) CT urography using maximum intensity projection shows irregular wall thickening (arrows) of the right proximal ureter. (b) Virtual endoscopy shows irregular lumen with multiple nodules (arrows) and ulcers (arrowheads).
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Renal mass evaluation
CT scanning identifies most renal masses missed on IVU or ultrasound. Furthermore, in patients scheduled to undergo partial nephrectomy or enucleation of a cortical mass, the relationship between the mass and the collecting system can be precisely evaluated by CT urography (Figures 11 and 12).
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Figure 11. CT urography using maximum intensity projection shows a renal mass (arrowheads) invading the collecting system (arrow).
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Figure 12. Renal cell carcinoma which can be removed by partial nephrectomy. (a) CT urography using maximum intensity projection shows a renal mass (M) that is separated from the collecting system and therefore can be removed as sparing the adjacent collecting system. (b) Conventional axial CT image also demonstrates that the mass can be separated from the renal sinus fat (arrows).
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Clinical application in the urinary bladder
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Focal lesion detection
In patients with haematuria, careful evaluation of the urinary bladder is of great importance. IVU and ultrasound are insensitive in the detection of small lesions. Virtual cystoscopy with a contrast-filled bladder, first reported by Merkle et al [7], may be of use in bladder evaluation, and can identify lesions as small as 3 mm in diameter (Figure 13) [8].
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Figure 13. Bladder carcinoma. (a) Virtual cystoscopy looking at the left wall of the bladder shows a 4 mm sized polyp (arrow) near the left ureteral orifice (white arrowhead), confirmed as transitional cell carcinoma. Black arrowhead=urethral orifice. (b) Conventional cystoscopic image depicts a polyp protruding into the bladder lumen.
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Artefacts of CT urography and virtual endoscopy
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The most frequently encountered problem producing artefact is patient motion resulting in pseudolesions and image distortion (Figure 14).
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Figure 14. Motion artefact distorts CT urography image (a) and causes false luminal irregularity on virtual endoscopy image (b).
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In virtual cystoscopy, inadequate mixing of contrast material and urine may causes both pseudolesions and obscure true abnormalities. To minimize this pitfall repeated positional change and ambulation are required to produce adequate mixing of contrast and urine. The application of different threshold levels for voxel categorization may be of value (Figure 15).
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Figure 15. Importance of adjusting threshold in case of inadequate mixing of the contrast and urine. (a) Source CT image shows a fluid–fluid level (arrows) due to adequate mixing of the contrast and urine. (b) Virtual cystoscopy image with threshold of 190 HU shows artefact in the anterior wall of the bladder. (c) On virtual cystoscopy image with threshold of 100 HU, the artefact disappears and a small nodule (arrow), confirmed as transitional cell carcinoma, is noted.
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Conclusion
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The development of multislice CT, increased computing power and new software has enabled novel techniques to be used to evaluate urinary tract disease.
Received for publication January 22, 2002.
Revision received July 30, 2002.
Accepted for publication October 21, 2002.
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References
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Abstract
Introduction
