DOI: 10.1148/rg.24si045513
RadioGraphics 2004;24:S35-S54
© RSNA, 2004
CT Urography1
Akira Kawashima, MD, PhD,
Terri J. Vrtiska, MD,
Andrew J. LeRoy, MD,
Robert P. Hartman, MD,
Cynthia H. McCollough, PhD and
Bernard F. King, Jr, MD
1 From the Department of Radiology, Mayo Clinic, 200 First St SW, Rochester, MN 55905. Presented as an education exhibit at the 2003 RSNA scientific assembly. Received March 1, 2004; revision requested March 26; final revision received June 4; accepted June 7. All authors have no financial relationships to disclose. Address correspondence to A.K. (e-mail: kawashima.akira@mayo.edu).
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Abstract
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With the recent introduction of multi–detector row helical computed tomography (CT), the radiologic evaluation of patients with urologic disease has changed rapidly. Two major approaches to CT urography have been developed. The first approach combines axial CT with timed excretory urography (EU) performed by using conventional radiography, digital radiography, or CT scanned projection radiography (SPR). This approach produces traditional projection urograms, and the timed imaging technique is familiar to radiologists and clinicians. Additional excretory phase CT can be performed when the EU findings are positive or indeterminate. Improved CT SPR processing technology produces radiographlike images, thus eliminating patient transportation between the CT and urography suites or the necessity for a CT suite with a ceiling-mounted x-ray tube and a modified CT tabletop for performance of EU. The second approach to CT urography combines axial CT with thin-section excretory phase CT. The near-isotropic volume data set enables creation of high-resolution two- and three-dimensional reformatted images. However, the increased amount of radiation and the time required for data manipulation are concerns. Further studies evaluating large numbers of patients with various urothelial abnormalities will be necessary to determine the optimal CT urography technique for clinical practice.
© RSNA, 2004
Index Terms: Genitourinary system, CT, 80.12113 • Genitourinary system, neoplasms, 80.32 • Hematuria, 80.899 • Urography, 80.1221
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LEARNING OBJECTIVES
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After reading this article and taking the test, the reader will be able to:
- Discuss the current role of urologic imaging in evaluation of patients with hematuria.
- Describe the two main techniques of CT urography and the strengths and weaknesses of each technique.
- Identify the key imaging features of urothelial disease at CT urography.
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Introduction
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With the recent introduction of multi–detector row helical computed tomography (CT), the uroradiologic evaluation of patients with common and complex disease is changing rapidly (1,2). Excretory urography (EU) has been the initial modality for upper tract imaging in patients with hematuria, flank pain, and other urologic diseases for the past 5 decades (3,4). However, EU is less sensitive in detecting renal masses than ultrasonography (US), CT, or magnetic resonance (MR) imaging. EU with nephrotomography allows identification of only 21%, 52%, and 82% of masses less than 2 cm in diameter, 2–3 cm in diameter, and 3 cm or larger in diameter, respectively, when CT is used as the standard of reference (5). Moreover, when a mass is detected with EU, further characterization with cross-sectional imaging is necessary because EU does not allow reliable differentiation of solid masses from cysts.
In 1995, Smith et al (6) reported the value of unenhanced CT in evaluating patients with acute flank pain in comparison with that of EU. Since that time, many imaging centers have replaced EU with unenhanced CT for evaluation of patients with acute renal colic in the emergency setting. CT has also become the imaging method of choice in evaluating patients with renal inflammatory disease and traumatic injuries (7–9). The remaining major indication for EU is hematuria. Patients with hematuria require evaluation of both the renal parenchyma and the urothelium. Despite its deficiencies in evaluating the parenchyma, EU has continued to be the initial imaging modality of choice for assessing the upper tract urothelium (4).
CT has evolved from single–detector row scanners into multi–detector row helical volumetric acquisition techniques, and these advances have had a significant impact on imaging of the urinary tract. Application of multi–detector row CT to evaluation of the urinary tract has been termed CT urography (10). The concept of CT urography is attractive since both the renal parenchyma and urothelium can be evaluated with a single comprehensive examination. This primary use of CT urography potentially also allows shortening the overall duration of the patient’s schedule for diagnostic evaluation. In some medical centers, CT urography is becoming the definitive study for patients with hematuria.
Although there continues to be a lack of large-scale research on the cost-effectiveness of various uroradiologic imaging strategies in the evaluation of patients with hematuria and other urologic indications, sufficient information exists to define a reasonable approach to patients with hematuria. The objectives of this article are (a) to describe the key elements in the evaluation of hematuria, (b) to review basic concepts of urinary tract imaging, (c) to review two major approaches to CT urography—combined (hybrid) CT and EU CT urography and CT-only CT urography—and (d) to describe the CT urography protocol at our institution. At our center, combined CT and EU CT urography was performed in over 7,000 cases in 4 years. Adjunct thin-section, excretory phase enhanced CT was performed in a subset of those patients. The key imaging findings of urothelial neoplasms and other important urothelial diseases demonstrated with the two major CT urography techniques in the same patients are presented with representative cases.
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Evaluation of Patients with Hematuria
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Hematuria is common and can originate from any site in the urinary tract. The presence of gross hematuria usually prompts patients to seek medical attention, and a thorough urologic investigation is warranted to determine its cause. In contrast, the etiology, diagnosis, and management of asymptomatic microhematuria are controversial. Asymptomatic microscopic hematuria is often not a sign of underlying surgical urologic disease. Some degree of hematuria is identified in 9%–18% of normal individuals (11,12). Routine screening of adults for microscopic hematuria with dipstick testing is not currently recommended because hematuria associated with significant urologic disease may be intermittent. However, once asymptomatic microscopic hematuria is documented, the patient should be evaluated. The definition of microscopic hematuria recently recommended by the American Urological Association is three or more red blood cells per high-power field at microscopic evaluation of the urinary sediment from at least two of three properly collected urinalysis specimens (11).
Patients with risk factors for significant urologic disease should be considered for urologic evaluation after one episode of properly documented microscopic hematuria. Risk factors for significant urologic disease include gross hematuria, irritable voiding symptoms, history of smoking, history of occupational exposure to chemicals or dyes (benzenes or aromatic amines), all adults older than 40 years, history of urologic disorder or disease, history of recurrent urinary tract infection despite appropriate use of antibiotics, history of pelvic irradiation, analgesic abuse (eg, phenacetin), cyclophosphamide exposure, and hereditary nonpolyposis colorectal cancer (HNPCC) syndrome (Lynch syndrome) (13,14). Patients with proteinuria, red blood cell casts, elevated serum creatinine level, and dysmorphic red blood cells (
80%) in urine are recommended to undergo a complete medical evaluation to exclude primary diffuse renal parenchymal disease (13,14). In a study of 1,000 consecutive adults with asymptomatic gross or microscopic hematuria by Mariani et al (15) in 1989, 9% of patients were found to have life-threatening abnormalities, and an additional 23% had lesions requiring at least observation. Microscopic hematuria associated with systemic anticoagulation therapy frequently is precipitated by significant urologic disease and therefore prompt evaluation is required.
Urologic evaluation for hematuria should involve a thorough examination of the upper and lower urinary tract. Optimal, comprehensive upper tract imaging studies should allow detection of renal cell carcinoma, transitional cell carcinoma, urolithiasis, and renal infection. As previously noted, many studies have demonstrated increased sensitivity and specificity in detection of renal parenchymal abnormalities for cross-sectional imaging studies compared to EU. The single major unresolved issue when these cross-sectional techniques are used is whether CT urography can depict urothelial abnormalities of the intrarenal collecting systems and ureters with sensitivities equal or superior to that of conventional EU. While gross bladder disease can be detected with imaging studies, cystourethroscopy continues to be recommended for complete evaluation of the lower urinary tract to exclude subtle urothelial abnormalities.
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Basic Concepts of Urinary Tract Imaging
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Comprehensive upper tract imaging must include (a) unenhanced axial CT of the kidneys, (b) enhanced CT of the abdomen and pelvis, and (c) excretory phase enhanced images of the urinary tract obtained with projection urography and/or axial CT images.
Unenhanced CT scans and plain abdominal radiographs (kidney, ureter, and bladder [KUB], scout) are primarily used for the evaluation of stone disease (Figs 1, 2), renal parenchymal calcifications, precontrast attenuation measurements of renal masses, and exclusion of hemorrhagic changes. These precontrast images are followed by contrast material–enhanced imaging, essential for complete evaluation of the urinary tracts. Nephrographic phase enhanced images are useful for the evaluation of the renal parenchyma, especially in the detection and evaluation of renal neoplasms, parenchymal scarring, and renal inflammatory disease. Corticomedullary differentiation nephrographic phase CT scans obtained 30–70 seconds after the start of intravenous contrast material injection provide information about the renal vasculature and perfusion, although small renal masses located in the medullary portions of the kidneys may not be appreciated in comparison with homogeneous nephrographic enhanced CT scans (16). Homogeneous nephrographic enhanced CT scans are typically obtained 90–180 seconds after initiation of intravenous contrast material administration. Homogeneous nephrographic enhanced CT scans are more sensitive for detection and characterization of renal masses than corticomedullary differentiation nephrographic phase enhanced CT scans (16). Multiphasic enhanced CT scans tend to be more helpful in characterizing renal masses than single-phase enhanced imaging (17).
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Figure 1. Combined CT and EU CT urography technique used at the authors’ institution. Photograph shows a multi-detector row helical CT scanner and an overhead radiographic x-ray tube (arrowhead). The modified CT tabletop (curved white arrow) is made of carbon fiber material. A screen-film cassette (large black arrow) in a stationary slip-in grid (small black arrow) is inserted into the opening (straight white arrow) of the auxiliary CT tabletop. The auxiliary CT tabletop moves so that a patient can be accurately positioned underneath the overhead radiographic x-ray tube.
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Figure 2a. Standard CT urography image sequence for the combined CT and EU CT urography technique used at the authors’ institution. (a) Abdominal radiograph shows a small opaque calculus projected over the right kidney (arrow). (b) Unenhanced CT scan shows normal renal parenchyma and the calculus in the interpolar portion of the right kidney. (c, d) Contrast-enhanced CT scans obtained during the homogeneous nephrographic (c) and pyelographic (d) phases show the normal nephrographic progression. On the pyelographic phase image (d), the calculus is obscured in the contrast material-filled calix. (e) Eight-minute excretory urogram obtained with application of external ureteral compression (arrows). (f) Ten-minute excretory urogram obtained after release of external ureteral compression.
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Figure 2b. Standard CT urography image sequence for the combined CT and EU CT urography technique used at the authors’ institution. (a) Abdominal radiograph shows a small opaque calculus projected over the right kidney (arrow). (b) Unenhanced CT scan shows normal renal parenchyma and the calculus in the interpolar portion of the right kidney. (c, d) Contrast-enhanced CT scans obtained during the homogeneous nephrographic (c) and pyelographic (d) phases show the normal nephrographic progression. On the pyelographic phase image (d), the calculus is obscured in the contrast material-filled calix. (e) Eight-minute excretory urogram obtained with application of external ureteral compression (arrows). (f) Ten-minute excretory urogram obtained after release of external ureteral compression.
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Figure 2c. Standard CT urography image sequence for the combined CT and EU CT urography technique used at the authors’ institution. (a) Abdominal radiograph shows a small opaque calculus projected over the right kidney (arrow). (b) Unenhanced CT scan shows normal renal parenchyma and the calculus in the interpolar portion of the right kidney. (c, d) Contrast-enhanced CT scans obtained during the homogeneous nephrographic (c) and pyelographic (d) phases show the normal nephrographic progression. On the pyelographic phase image (d), the calculus is obscured in the contrast material-filled calix. (e) Eight-minute excretory urogram obtained with application of external ureteral compression (arrows). (f) Ten-minute excretory urogram obtained after release of external ureteral compression.
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Figure 2d. Standard CT urography image sequence for the combined CT and EU CT urography technique used at the authors’ institution. (a) Abdominal radiograph shows a small opaque calculus projected over the right kidney (arrow). (b) Unenhanced CT scan shows normal renal parenchyma and the calculus in the interpolar portion of the right kidney. (c, d) Contrast-enhanced CT scans obtained during the homogeneous nephrographic (c) and pyelographic (d) phases show the normal nephrographic progression. On the pyelographic phase image (d), the calculus is obscured in the contrast material-filled calix. (e) Eight-minute excretory urogram obtained with application of external ureteral compression (arrows). (f) Ten-minute excretory urogram obtained after release of external ureteral compression.
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Figure 2e. Standard CT urography image sequence for the combined CT and EU CT urography technique used at the authors’ institution. (a) Abdominal radiograph shows a small opaque calculus projected over the right kidney (arrow). (b) Unenhanced CT scan shows normal renal parenchyma and the calculus in the interpolar portion of the right kidney. (c, d) Contrast-enhanced CT scans obtained during the homogeneous nephrographic (c) and pyelographic (d) phases show the normal nephrographic progression. On the pyelographic phase image (d), the calculus is obscured in the contrast material-filled calix. (e) Eight-minute excretory urogram obtained with application of external ureteral compression (arrows). (f) Ten-minute excretory urogram obtained after release of external ureteral compression.
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Figure 2f. Standard CT urography image sequence for the combined CT and EU CT urography technique used at the authors’ institution. (a) Abdominal radiograph shows a small opaque calculus projected over the right kidney (arrow). (b) Unenhanced CT scan shows normal renal parenchyma and the calculus in the interpolar portion of the right kidney. (c, d) Contrast-enhanced CT scans obtained during the homogeneous nephrographic (c) and pyelographic (d) phases show the normal nephrographic progression. On the pyelographic phase image (d), the calculus is obscured in the contrast material-filled calix. (e) Eight-minute excretory urogram obtained with application of external ureteral compression (arrows). (f) Ten-minute excretory urogram obtained after release of external ureteral compression.
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Contrast material typically appears in the renal collecting systems 3 minutes after the start of bolus intravenous contrast material administration. The intrarenal collecting systems and ureters are usually well distended by 8–10 minutes following intravenous contrast material injection, and their appearance both with and without external ureteral compression can be studied. Images of the collecting system obtained during the excretory phase are essential for assessing subtle urothelial abnormalities including urothelial tumors, papillary necrosis, caliceal deformity, ureteral stricture, and inflammatory changes of the renal collecting systems, ureters, and bladder. The ureters are generally well visualized on 10-minute decompression images. The bladder is seen best on 20-minute and postvoiding images. Additional urograms including prone and oblique projection images and further delayed images are useful in imaging obstructing ureteral lesions as well as bladder lesions.
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CT Urography Techniques
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CT urography techniques are still evolving but generally use one of two approaches (10,18,19). One approach combines the use of axial CT images with some form of traditional timed projection radiography using conventional screen-film abdominal radiographs, computed digital radiographs, or CT scanned projection radiographic (SPR) images following intravenous contrast material administration. This combined CT and EU CT urography method combines the strengths of both CT and EU into one comprehensive examination. A second approach to CT urography combines conventional axial CT with the addition of thin-section axial CT images obtained during the excretory phase of enhancement and reviewed as two-dimensional (2D) and three-dimensional (3D) reformatted images. This CT-only CT urography method is attractive because this method potentially replaces the technology needed for conventional EU. Earlier studies have suggested that CT-only CT urography is feasible and allows detection of many abnormalities. However, a formal comparison of EU and CT-only CT urography has not yet been performed, to our knowledge.
Combined CT and EU CT Urography
EU remains the standard of reference for noninvasive visualization of intraluminal filling defects in the collecting systems and urothelial abnormalities. Previous studies examining patients with hematuria have been based on traditional EU techniques. Combined CT and EU CT urography methods incorporate the strength of each modality into a single comprehensive examination. The advantage of this technique is that it is immediately available in a timed display form familiar to radiologists and referring clinicians and no CT postprocessing is necessary.
Perlman et al (20) described the concept of CT urography in 1996. Conventional EU film images were obtained in a urography suite followed by patient transfer to a CT room for adjunct CT without additional intravenous contrast material injection, with scans limited to the kidneys and any additional localized abnormality identified at EU. Lesions in the urinary collecting systems were detected only at EU in 27 of 30 patients (20). This technique provided a reasonable evaluation of renal masses; however, the advantage of unenhanced CT for detecting urinary calculi was lost because the CT acquisition followed the EU.
In other centers, CT urography was developed with an abdominal radiograph obtained initially. Helical CT scans of the abdomen and pelvis without and with intravenous contrast material administration were next acquired to evaluate urolithiasis and the renal parenchyma. The patient was then immediately transferred to a urography suite to complete the urographic film portion of the study. This approach combines the advantages of EU with those of CT, allowing one comprehensive imaging study. However, movement of the patient between procedure rooms requires additional time and can result in scheduling and staffing conflicts. The patient transfer also may adversely affect the level of pyelocaliceal distention during the urographic portion of the examination.
A unique alternative to this approach is the acquisition of conventional radiographs with a ceiling-mounted overhead x-ray tube while the patient is lying on the CT table for multiphasic CT acquisitions (21) (Figs 1, 2). This method allows high-spatial-resolution EU film images (
4 line pairs per millimeter [lp/mm] for screen-film radiography) to be obtained at various times before and after the CT acquisitions and without the need to transfer the patient between the radiography and CT suites. Conventional screen-film radiographs are obtained by using a screen-film radiographic combination (Insight film, Min R Medium front screen, Lanex Medium back screen, M8 processor; Eastman Kodak, Rochester, NY) and a high-line-density (152 lines per inch) stationary, focused slip-on grid (12:1 grid ratio) (Mitaya Manufacturing, Tokyo, Japan) (22). This technique requires the use of an auxiliary, radiolucent CT tabletop that does not introduce artifacts in the CT image (23). The tabletop has an opening to allow insertion of a screen-film radiographic system beneath the patient.
This approach has been accepted as a comprehensive urologic imaging study at our institution. In our clinical experience, urothelial abnormalities that are better or only seen on conventional screen-film urograms rather than on the axial CT scans comprise 10% of all the abnormalities detected in CT urography examinations (21). This underscores the need for high-quality radiographic images in CT urography. With this technique, excretory phase enhanced CT scans can be acquired to correlate with positive or inconclusive urographic findings anytime during the physician-monitored CT urography examination. With our method, conventional screen-film cassettes combined with stationary grids can be replaced simply for digital film management and interpretation with indirect storage phosphor digital radiographic imaging plates combined with the same stationary grid.
An alternative method of obtaining projection images without moving the patient off of the CT table is the use of the CT SPR technique (Fig 3). A CT SPR image, which is referred to as a scout view (GE Healthcare, Milwaukee, Wis), topogram (Siemens Medical Systems, Iselin, NJ), or scanogram, is typically designed for prescan anatomic localization. The spatial resolution of CT SPR (approximately <1 lp/mm) when performed at 80 kV and 300 mA is inferior to that of conventional radiography, while the contrast resolution of opacified structures is similar to that of conventional radiography (22). Use of 80 kVp is preferred to 120 kVp for diagnostic projection radiographic imaging because iodine has a K shell binding energy of 32 keV, similar to the conventional EU film technique.
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Figure 3a. Different types of projection radiographic images used in urography. (a) Conventional 10-minute excretory urogram. (b) Corresponding CT SPR image obtained at 80 kVp and 300 mA shows a dark band along the margin of the iodinated contrast material (arrows). (c) Modified CT SPR image, obtained after reprocessing of the original CT SPR image data in b with clinically optimized algorithms, shows substantial minimization of the artifacts and appears similar to the conventional urogram (a).
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Figure 3b. Different types of projection radiographic images used in urography. (a) Conventional 10-minute excretory urogram. (b) Corresponding CT SPR image obtained at 80 kVp and 300 mA shows a dark band along the margin of the iodinated contrast material (arrows). (c) Modified CT SPR image, obtained after reprocessing of the original CT SPR image data in b with clinically optimized algorithms, shows substantial minimization of the artifacts and appears similar to the conventional urogram (a).
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Figure 3c. Different types of projection radiographic images used in urography. (a) Conventional 10-minute excretory urogram. (b) Corresponding CT SPR image obtained at 80 kVp and 300 mA shows a dark band along the margin of the iodinated contrast material (arrows). (c) Modified CT SPR image, obtained after reprocessing of the original CT SPR image data in b with clinically optimized algorithms, shows substantial minimization of the artifacts and appears similar to the conventional urogram (a).
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However, current conventional CT SPR imaging uses simple edge enhancement image filters, which result in dark and bright band artifacts along the margin of high-attenuation objects (eg, iodinated contrast media) (22). These objectionable artifacts around high-contrast objects can be substantially minimized in improved CT SPR with reprocessing techniques that use clinically optimized contrast enhancement algorithms while maintaining adequate high-contrast spatial resolution (Applied Science Laboratory; GE Healthcare) (24–26). This CT urography approach, the combination of helical CT scans and enhanced CT SPR urographic images, is attractive because modification of the CT tabletop and installation of a ceiling-mounted x-ray tube are unnecessary and this technique therefore can be performed on any multi–detector row CT equipment. As with reformatted CT images of the collecting system, further scientific studies are needed to validate the sensitivity of optimally enhanced CT SPR images in large numbers of patients with urothelial lesions.
The enhanced CT SPR image is characterized by four unique features: (a) adaptive enhancement, (b) overlapped data sampling, (c) optimization of the filter, and (d) use of a deconvolution filter (Applied Science Laboratory; GE Healthcare). The first important feature of the enhanced CT SPR image is the manner in which it is enhanced. Enhancement of the CT SPR image is performed by using adaptive enhancement instead of simple edge enhancement (eg, unsharp masking). The algorithm first calculates directional gradients (signal changes) along different orientations, the information from which will determine the direction and the magnitude of the SPR enhancement. By doing this, overshoot and undershoot near the high-contrast objects can be minimized. The second feature of the enhanced CT SPR image is the use of overlapped data samples. This ensures a more precise and accurate design of the filter characteristics. When overlapped samples are used, the flexibility of the filter design is significantly increased. The third feature of the enhanced CT SPR image is subjective filter optimization of the final image in such a way that it closely matches the appearance of a conventional radiograph. The fourth feature of the enhanced CT SPR image is introduction of a deconvolution filter, which improves the z-axis high-contrast spatial resolution by approximately 20%–30% (1.2–1.3 lp/mm for an enhanced CT SPR image). It takes a few minutes to automatically reprocess the original CT SPR data to generate an enhanced CT SPR image after selecting the original CT SPR image. No additional radiation is given to create the enhanced CT SPR images. With a newer version of the software, enhanced CT SPR images will prospectively be generated when the enhanced CT SPR imaging mode is chosen.
A bowel preparation that uses a mild laxative such as 35 g (1
oz) of extract of senna prior to CT and EU CT urography stimulates a bowel movement in order to reduce gas and fecal material in the colon so that the kidneys and ureters can be visualized more clearly. Diuresis caused by the ingestion of fluid or mild diuretics such as coffee or tea will decrease the degree of concentration of contrast material excreted into the urinary tract, limiting optimal opacification of the collecting systems. This diuresis and swallowing of excessive gas may be prevented by instructing the patient to take "nothing by mouth" for several hours prior to the CT urography examination. The instructions to take nothing by mouth prior to the examinations are very similar to those of standard CT examinations. In our practice, we believe the benefits outweigh the slight inconvenience to the patient. This minimal bowel preparation should not add any risk for the patient.
The evaluation of the entire urinary collecting system requires review of a composite of urographic images (3,4,27). Visualizing the intrarenal collecting system and ureter with EU requires optimal opacification and distention. After the intravenous contrast material injection, abdominal compression is applied unless the patient has contraindications including abdominal aortic aneurysm, recent abdominal surgery, severe abdominal pain, suspected renal trauma, and renal transplantation (4). This use of external ureteral compression is important to ensure adequate pyelocaliceal distention, especially when low-osmolar iodinated contrast material is used (4). The intrarenal collecting system and proximal ureters are well distended on 8-minute compression film images, and their appearance with and without compression is reviewed (3). The ureters are generally well visualized on 10-minute decompressed film images. Twenty-minute and postvoiding film images are optional but may be useful for bladder evaluation.
An external ureteral compression device should be placed around the patient with the anterior superior iliac spine used as the landmark. Fully inflated, compression balloons should be symmetrically placed at the level of the most anterior portion of the ureters. The patient should also be instructed not to tense the abdominal muscles. The degree of abdominal compression should not exceed the patient’s acceptance.
Radiation Exposure: Consideration of radiation exposure is important with clinical introduction of these new techniques. One abdominal radiograph obtained with a stationary grid in a 21-cm anteroposterior diameter patient delivers an effective skin exposure (ESE) level of 412 mR (106.3 µC/kg) and effective dose of 0.5 mSv (22). One CT SPR image obtained at 300 mA delivers an ESE level of 330 mR (85.1 µC/kg) and effective dose of 0.54 mSv (22). The ESE and effective doses of conventional radiography must increase with patient thickness to maintain comparable image noise, approximately doubling for each additional 4–5 cm of patient thickness, whereas the same technique with 300 mA and 80 kVp may be used to obtain the CT SPR images regardless of the body habitus.
One abdominal-pelvic helical CT scan delivers an ESE level of 2,500 mR (645 µC/kg) and effective dose of approximately 11 mSv (22). Estimated skin doses and effective dose for combined CT and EU CT urography are considered to be less than what a separate CT study and conventional EU with nephrotomograms would deliver. The total effective dose of the "standard" CT urography examination, which comprises five film images (scout image, 8-, 10-, and 20-minute delayed images, and postvoiding image) and CT scans of the abdomen and pelvis with and without intravenous contrast material, is estimated to be 24.5 mSv.
CT-Only CT Urography
This approach relies exclusively on the acquisition of unenhanced and enhanced CT scans of the abdomen and pelvis, including the essential acquisition of thin-section helical CT scans of the urinary tracts during the excretory phase of enhancement. Multiplanar 2D and 3D reformation images can be generated on workstations from axial source images obtained during the excretory enhanced phase. No bowel preparation is necessary for this type of CT urography examination; however, the risk of aspiration of solid food from vomiting can be lessened by withholding oral intake for several hours prior to the examination. In some patients, metallic hip prostheses may result in beam hardening artifacts and make assessment of the distal ureters and bladder difficult.
In a study by McNicholas et al (28) using a single–detector row helical scanner, excretory phase enhanced CT scans were obtained with a section thickness of 5 mm, pitch of 1.5, and section increment of 2.5 mm. Multi–detector row CT scanners allow single breath-hold acquisition of scans of the abdomen and pelvis with narrow collimation to achieve high spatial resolution. Several researchers have used multi–detector row CT with section thickness of 2.5–3 mm and section increment of 1–1.25 mm for CT urography (29–32). More recently, that technique has been modified to a section thickness of 1–1.25 mm to generate a single volume data set with near-isotropic voxels (33,34). By using the large data set generated, 2D and 3D reformatted images can be displayed with improved spatial resolution in nontransverse planes on workstations (Figs 4, 5).
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Figure 4a. Obstructing transitional cell carcinoma of the ureter. Different types of reformatted images generated from CT images are compared with a conventional EU image. (a) Twenty-minute excretory urogram shows left pyelocaliectasis and ureterectasis to the level of the iliac crest, where the dilated ureter abruptly terminates (straight arrow). There is extravasation of urinary contrast material around the left renal pelvis and proximal ureter (curved arrows) secondary to forniceal rupture. (b) Excretory phase CT scan obtained with 1.25-mm section thickness shows a soft-tissue attenuation mass in the left ureter with associated periureteral soft-tissue thickening (arrow). No lymphadenopathy was identified. (c) Curved reformatted image shows the soft-tissue attenuation mass with resulting ureteral obstruction (arrow). The left ureter distal to the lesion is not dilated. (d-f) Three-dimensional maximum intensity projection (MIP) (d), average intensity projection (e), and perspective volume rendered (f) reformatted images show findings similar to those on the conventional urogram (a). The diagnosis was invasive grade 3 (of three grades) urothelial carcinoma with invasion into the periureteral fat plane.
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Figure 4b. Obstructing transitional cell carcinoma of the ureter. Different types of reformatted images generated from CT images are compared with a conventional EU image. (a) Twenty-minute excretory urogram shows left pyelocaliectasis and ureterectasis to the level of the iliac crest, where the dilated ureter abruptly terminates (straight arrow). There is extravasation of urinary contrast material around the left renal pelvis and proximal ureter (curved arrows) secondary to forniceal rupture. (b) Excretory phase CT scan obtained with 1.25-mm section thickness shows a soft-tissue attenuation mass in the left ureter with associated periureteral soft-tissue thickening (arrow). No lymphadenopathy was identified. (c) Curved reformatted image shows the soft-tissue attenuation mass with resulting ureteral obstruction (arrow). The left ureter distal to the lesion is not dilated. (d-f) Three-dimensional maximum intensity projection (MIP) (d), average intensity projection (e), and perspective volume rendered (f) reformatted images show findings similar to those on the conventional urogram (a). The diagnosis was invasive grade 3 (of three grades) urothelial carcinoma with invasion into the periureteral fat plane.
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Figure 4c. Obstructing transitional cell carcinoma of the ureter. Different types of reformatted images generated from CT images are compared with a conventional EU image. (a) Twenty-minute excretory urogram shows left pyelocaliectasis and ureterectasis to the level of the iliac crest, where the dilated ureter abruptly terminates (straight arrow). There is extravasation of urinary contrast material around the left renal pelvis and proximal ureter (curved arrows) secondary to forniceal rupture. (b) Excretory phase CT scan obtained with 1.25-mm section thickness shows a soft-tissue attenuation mass in the left ureter with associated periureteral soft-tissue thickening (arrow). No lymphadenopathy was identified. (c) Curved reformatted image shows the soft-tissue attenuation mass with resulting ureteral obstruction (arrow). The left ureter distal to the lesion is not dilated. (d-f) Three-dimensional maximum intensity projection (MIP) (d), average intensity projection (e), and perspective volume rendered (f) reformatted images show findings similar to those on the conventional urogram (a). The diagnosis was invasive grade 3 (of three grades) urothelial carcinoma with invasion into the periureteral fat plane.
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Figure 4d. Obstructing transitional cell carcinoma of the ureter. Different types of reformatted images generated from CT images are compared with a conventional EU image. (a) Twenty-minute excretory urogram shows left pyelocaliectasis and ureterectasis to the level of the iliac crest, where the dilated ureter abruptly terminates (straight arrow). There is extravasation of urinary contrast material around the left renal pelvis and proximal ureter (curved arrows) secondary to forniceal rupture. (b) Excretory phase CT scan obtained with 1.25-mm section thickness shows a soft-tissue attenuation mass in the left ureter with associated periureteral soft-tissue thickening (arrow). No lymphadenopathy was identified. (c) Curved reformatted image shows the soft-tissue attenuation mass with resulting ureteral obstruction (arrow). The left ureter distal to the lesion is not dilated. (d-f) Three-dimensional maximum intensity projection (MIP) (d), average intensity projection (e), and perspective volume rendered (f) reformatted images show findings similar to those on the conventional urogram (a). The diagnosis was invasive grade 3 (of three grades) urothelial carcinoma with invasion into the periureteral fat plane.
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Figure 4e. Obstructing transitional cell carcinoma of the ureter. Different types of reformatted images generated from CT images are compared with a conventional EU image. (a) Twenty-minute excretory urogram shows left pyelocaliectasis and ureterectasis to the level of the iliac crest, where the dilated ureter abruptly terminates (straight arrow). There is extravasation of urinary contrast material around the left renal pelvis and proximal ureter (curved arrows) secondary to forniceal rupture. (b) Excretory phase CT scan obtained with 1.25-mm section thickness shows a soft-tissue attenuation mass in the left ureter with associated periureteral soft-tissue thickening (arrow). No lymphadenopathy was identified. (c) Curved reformatted image shows the soft-tissue attenuation mass with resulting ureteral obstruction (arrow). The left ureter distal to the lesion is not dilated. (d-f) Three-dimensional maximum intensity projection (MIP) (d), average intensity projection (e), and perspective volume rendered (f) reformatted images show findings similar to those on the conventional urogram (a). The diagnosis was invasive grade 3 (of three grades) urothelial carcinoma with invasion into the periureteral fat plane.
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Figure 4f. Obstructing transitional cell carcinoma of the ureter. Different types of reformatted images generated from CT images are compared with a conventional EU image. (a) Twenty-minute excretory urogram shows left pyelocaliectasis and ureterectasis to the level of the iliac crest, where the dilated ureter abruptly terminates (straight arrow). There is extravasation of urinary contrast material around the left renal pelvis and proximal ureter (curved arrows) secondary to forniceal rupture. (b) Excretory phase CT scan obtained with 1.25-mm section thickness shows a soft-tissue attenuation mass in the left ureter with associated periureteral soft-tissue thickening (arrow). No lymphadenopathy was identified. (c) Curved reformatted image shows the soft-tissue attenuation mass with resulting ureteral obstruction (arrow). The left ureter distal to the lesion is not dilated. (d-f) Three-dimensional maximum intensity projection (MIP) (d), average intensity projection (e), and perspective volume rendered (f) reformatted images show findings similar to those on the conventional urogram (a). The diagnosis was invasive grade 3 (of three grades) urothelial carcinoma with invasion into the periureteral fat plane.
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Figure 5a. Superficially invasive papillary urothelial carcinoma of the intrarenal collecting system, grade 2 (of three grades). (a) Eight-minute excretory urogram shows amputation of the upper left renal infundibulum (straight arrow) with distortion of upper pole calices (curved arrow). (b) Excretory phase CT scan obtained with 1.25-mm section thickness shows a soft-tissue attenuation mass in the central portion of the upper left kidney (straight arrow) with associated renal parenchymal loss and caliceal distortion (curved arrow). Neither lymphadenopathy nor extrarenal tumor extension was identified. (c) Coronal reformatted image generated from thin-section axial CT scans obtained during the excretory phase (b) shows the relationship of the amputated infundibulum to the mass. (d) Three-dimensional MIP image shows a left pyelocaliceal system that appears similar to that seen on the conventional urogram (a) with an amputated infundibulum (arrow).
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Figure 5b. Superficially invasive papillary urothelial carcinoma of the intrarenal collecting system, grade 2 (of three grades). (a) Eight-minute excretory urogram shows amputation of the upper left renal infundibulum (straight arrow) with distortion of upper pole calices (curved arrow). (b) Excretory phase CT scan obtained with 1.25-mm section thickness shows a soft-tissue attenuation mass in the central portion of the upper left kidney (straight arrow) with associated renal parenchymal loss and caliceal distortion (curved arrow). Neither lymphadenopathy nor extrarenal tumor extension was identified. (c) Coronal reformatted image generated from thin-section axial CT scans obtained during the excretory phase (b) shows the relationship of the amputated infundibulum to the mass. (d) Three-dimensional MIP image shows a left pyelocaliceal system that appears similar to that seen on the conventional urogram (a) with an amputated infundibulum (arrow).
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Figure 5c. Superficially invasive papillary urothelial carcinoma of the intrarenal collecting system, grade 2 (of three grades). (a) Eight-minute excretory urogram shows amputation of the upper left renal infundibulum (straight arrow) with distortion of upper pole calices (curved arrow). (b) Excretory phase CT scan obtained with 1.25-mm section thickness shows a soft-tissue attenuation mass in the central portion of the upper left kidney (straight arrow) with associated renal parenchymal loss and caliceal distortion (curved arrow). Neither lymphadenopathy nor extrarenal tumor extension was identified. (c) Coronal reformatted image generated from thin-section axial CT scans obtained during the excretory phase (b) shows the relationship of the amputated infundibulum to the mass. (d) Three-dimensional MIP image shows a left pyelocaliceal system that appears similar to that seen on the conventional urogram (a) with an amputated infundibulum (arrow).
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Figure 5d. Superficially invasive papillary urothelial carcinoma of the intrarenal collecting system, grade 2 (of three grades). (a) Eight-minute excretory urogram shows amputation of the upper left renal infundibulum (straight arrow) with distortion of upper pole calices (curved arrow). (b) Excretory phase CT scan obtained with 1.25-mm section thickness shows a soft-tissue attenuation mass in the central portion of the upper left kidney (straight arrow) with associated renal parenchymal loss and caliceal distortion (curved arrow). Neither lymphadenopathy nor extrarenal tumor extension was identified. (c) Coronal reformatted image generated from thin-section axial CT scans obtained during the excretory phase (b) shows the relationship of the amputated infundibulum to the mass. (d) Three-dimensional MIP image shows a left pyelocaliceal system that appears similar to that seen on the conventional urogram (a) with an amputated infundibulum (arrow).
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Detection of urothelial abnormalities with excretory phase enhanced CT requires visualization of the optimally distended and opacified collecting system as traditionally seen at EU. It may be difficult to obtain a single set of images on which the collecting systems are completely opacified due to normal physiologic peristalsis of the intrarenal collecting systems and ureters. Several studies have shown that CT acquisition performed with abdominal compression improves opacification of the collecting system when compared to CT scans obtained without compression (28,35). Caoili et al (33) used two CT acquisitions of the entire urinary tract during the excretory phase of enhancement: one with abdominal compression and the other after release of abdominal compression. McNicholas et al (28) showed that excretory phase CT scans obtained with patients in a proneposition also improved opacification of the distal ureters compared to CT scans obtained in supine patients without abdominal compression.
Alternative techniques for achieving optimal visualization of the collecting systems include supplemental use of normal saline infusion and diuretic injection. McTavish et al (34) reported that supplemental infusion of 250 mL of physiologic saline immediately after injecting intravenous contrast material significantly improved opacification of the distal ureters. Nolte-Ernsting et al (31) reported that intravenous injection of low-dose diuretics (10 mg of furosemide) before intravenous contrast material injection also permitted less dense, homogeneous opacification of the collecting systems compared to supplemental infusion of 300 mL of normal saline. Because CT has inherent contrast resolution superior to that of conventional radiography, dilution of the contrast material does not affect substantially the perception of contrast material opacification of the collecting systems and may minimize beam hardening artifacts associated with dense contrast material in the intrarenal collecting system (31,34).
In a recent study, Caoili et al (36) compared the effects of abdominal compression, intravenous saline infusion, and two different imaging delays (5 and 7.5 minutes) on upper tract distention and opacification during CT urography. The 7.5-minute delayed excretory phase enhanced CT acquisition technique resulted in the most significantly increased distention of the intrarenal collecting system and proximal ureter, followed by the saline infusion technique. The application of abdominal compression did not improve distention or opacification of the urinary tracts when compared to the saline infusion technique in their series.
An alternative CT urography approach consists of two CT acquisitions (unenhanced and enhanced CT acquisitions) and an alternative contrast material injection technique (30). In this technique, after the initial unenhanced CT, sequential administrations of a split dose of intravenous contrast material are performed, and both nephrographic and excretory phase images are obtained during the second acquisition (30). However, this method requires two contrast material injections and two CT examinations separated by 15 minutes. Chow and Sommer (29) reported obtaining nephrographic and excretory phase enhanced CT scans of the kidneys and proximal ureters with abdominal compression by scanning 90 seconds after the second dose of a two-phase injection of iodinated contrast material temporally separated by 2 minutes. Immediately after release of compression, the excretory phase enhanced CT scan is continued to cover the distal ureters and bladder. Further studies are needed to develop methods for consistent, optimal distention and opacification of the distal ureters.
Assessment of source axial CT scans, displayed by using wide window settings similar to the bone window settings, is essential for accurate diagnosis. The acquisition of large CT data sets (400–800 axial source images) requires interactive softcopy viewing on a workstation rather than film review. Postprocessing techniques including multiplanar reformation and 3D reformatted images can be generated from excretory phase enhanced axial CT scans, displaying the urothelial anatomy and disease in a traditional coronal display format (37). Initial urologist acceptance of these techniques may be enhanced by display of the reformatted images, which are similar to the conventional film urogram in appearance. Multiplanar reformatted images in orthogonal coronal or oblique (en face) planes help define the location and extent of the lesions shown on axial CT images (Fig 4).
Maximum intensity projection (MIP), average intensity projection (AIP), and perspective volume rendered reformatted images from thin (5–20 mm) and thick (35–90 mm) slabs can be generated from the original axial data set. Thick-slab 3D reformatted images provide an overview of the collecting systems and mimic conventional excretory urograms, but assessment of urothelial wall thickness and small intraluminal filling defects is difficult. Thin-slab reformatted images have the advantage of covering a considerably longer range than standard multiplanar reformation and have the ability to demonstrate small filling defects and thickened urothelial walls, which may be obscured by adjacent contrast material in the collecting system and periureteral soft tissues with thick-slab reformation. The workstation window settings must be adjusted to optimally display the contrast material–filled urinary tracts, varying with the type of MIP, AIP, and volume rendered reformation and the degree of concentration of contrast material in the urinary tract. Even with the same volume data set, the window settings are not the same for optimal display between the various types of reformation. Labor-intensive to perform, curved planar reformation provides a single image to outline the course of ureterectasis to the point where an obstructing process such as a calculus or tumor is present (Fig 4) (29).
A comparison study of the clinical performance of 3D reformation imaging with that of CT SPR imaging has not yet been performed, to our knowledge. However, for comparable z-axis resolution in a 3D CT image, the z-axis width of each image would need to be about 0.5 mm or less, which is at the very limit of CT technology. Such 3D images require computationally intensive processing and much higher patient radiation doses.
Radiation Exposure.—
In a study by McTavish et al (34), estimated skin doses from CT urography performed with the three-phase CT scan protocol were similar to those of standard EU, while the total effective doses from CT urography were approximately two times higher than those of EU. In a study by Caoili et al (33), estimated effective doses from a four-phase CT urography protocol ranged from 25 to 30 mSv. Estimated effective doses from an abdominal-pelvic CT examination and conventional EU with nephrotomography range from 10 to 15 mSv and from 5 to 10 mSv, respectively (33). The researchers concluded that multiphase CT urography exposes a patient to an amount of radiation similar to what would be experienced during a combination of standard EU and CT of the abdomen and pelvis. Because of the higher radiation dose, this type of CT urography is indicated for the evaluation of hematuria only in patients with a high risk of malignancy (37). Continued efforts are needed to reduce radiation exposure.
Cost.—
CT urography takes longer than standard abdominal-pelvic CT and requires interactive communication between the CT technologist and the monitoring radiologist during the examination acquisition. At our institution, combined CT and EU CT urography is charged as a combination of CT of the abdomen and pelvis with and without intravenous contrast material and a limited EU study. Postprocessing of the axial CT data into multiplanar reformation or 3D display images is more labor-intensive and can result in an additional reformation charge.
CT Urography Protocol at Our Institution —
The standard CT urography protocol at our institution is summarized in the Table. Precontrast images consist of plain abdominal radiographs and unenhanced CT scans (LightSpeed QX/i; GE Healthcare). Contrast-enhanced CT scans then are obtained from the dome of the liver to the pubic symphysis at 5-mm section thickness 90 seconds after starting intravenous administration of 100–140 mL of iodinated contrast material. These typically demonstrate homogeneous nephrograms (the uniform or homogeneous nephrographic phase). CT scans obtained during the corticomedullary differentiation phase of enhancement are optional and are obtained when more vascular and perfusion information would be needed. These optional corticomedullary differentiation phase enhanced CT scans are obtained 40 seconds after starting intravenous contrast material administration. Abdominal compression is applied after completion of early enhanced CT scans.
The intrarenal collecting systems and ureters are typically well distended by 8 minutes following intravenous contrast material injection, and their appearance both with and without compression can be studied on the urographic images. The ureters are generally well visualized on the 10-minute decompression image. Although the opacified bladder is seen best on 20-minute and postvoiding images, these images are optional and can be obtained in any radiographic suite. Additional urographic images including prone and oblique projection and delayed views may be obtained when necessary. Optional excretory phase enhanced CT scans at 1.25–5-mm section thickness may be obtained before or after release of abdominal compression at the radiologist’s discretion. The excretory phase CT images are typically obtained to clarify abnormalities demonstrated on the urographic images or to provide additional information with regard to poorly distended collecting systems and ureters.
Original CT SPR images are obtained at 80 kVp and 300 mA including precontrast, 8-minute compression, and 10-minute decompression images. Enhanced CT SPR images are generated after reprocessing the original raw data of CT SPR images by using the enhanced algorithm.
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Case Presentations
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Urothelial Tumors
Most primary tumors of the collecting system are malignant with transitional cell carcinoma being the most common. Transitional cell carcinomas of the renal pelvis account for 7% of primary malignancies of the kidneys. Primary carcinoma of the ureter is rare and accounts for 1% of all cancers of the upper urinary tract. Hematuria is present in 72% of patients with transitional cell carcinoma of the urinary tract. When urothelial carcinoma of the urinary tract is found, careful scrutiny of the entire upper tracts for additional subtle filling defects is essential since urothelial tumors tend to be multiple and can occur synchronously or metachronously. Hereditary nonpolyposis colorectal cancer (HNPCC) syndrome (Lynch syndrome) is an autosomal dominant disorder and characterized by familial predisposition to colorectal carcinoma and extracolonic cancers including transitional cell carcinoma of the urinary tracts and cancers of the endometrium, ovary, stomach, small bowel, pancreas, hepatobiliary tract, and brain (38).
Lowe and Roylance (39) reported five major patterns of renal pelvic transitional cell carcinomas at EU. The urographic findings include (a) single or multiple filling defects in the renal pelvis and calices, which are characteristically irregular and in continuity with the wall of the collecting system (35%); (b) filling defects within dilated calices secondary to partial or complete obstruction of the infundibulum (26%); (c) caliceal amputation (19%); (d) absent or decreased excretion without renal enlargement caused by long-standing obstruction of the ureteropelvic junction and atrophy (13%); and (e) hydronephrosis with renal enlargement caused by tumor obstruction of the ureteropelvic junction (6%) (Figs 5, 6) (39). Early-stage pelvicaliceal transitional cell carcinomas frequently appear as discrete renal pelvic masses and less frequently as focal or diffuse thickening of the calix or renal pelvis (40,41). The urographic findings of ureteral transitional cell carcinoma include (a) a nonfunctioning kidney secondary to high-grade urinary obstruction (46%), (b) hydronephrosis with or without hydroureter (34%), (c) single or multiple ureteral filling defects with or without hydroureter and hydronephrosis (19%), and (d) fixation of the ureter with irregular narrowing of the lumen and nontapering (Figs 4, 7) (42,43).
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Figure 6a. Caliceal urothelial carcinoma in a 60-year-old woman with hereditary nonpolyposis colorectal cancer syndrome (Lynch syndrome) whose history included bladder carcinoma, endometrioid ovarian carcinoma, endometrial carcinoma, and colonic polyps. (a) Eight-minute excretory urogram shows a filling defect in the upper renal calix (arrow). Bowel gas projected over the right kidney and renal collecting system obscures detail. (b, c) Oblique axial (b) and coronal (c) reformatted images generated from pyelographic phase CT scans obtained with 1.25-mm section thickness show a polypoid tumor (arrow), which is confined to the intrarenal collecting system with no obliteration of the peripelvic fat plane. No lymphadenopathy is identified. (d) Three-dimensional MIP image shows distortion of the upper renal calices (arrow). Ureteroscopic biopsy revealed grade 2 (of three grades) urothelial carcinoma, which was treated with endourologic laser ablation.
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Figure 6b. Caliceal urothelial carcinoma in a 60-year-old woman with hereditary nonpolyposis colorectal cancer syndrome (Lynch syndrome) whose history included bladder carcinoma, endometrioid ovarian carcinoma, endometrial carcinoma, and colonic polyps. (a) Eight-minute excretory urogram shows a filling defect in the upper renal calix (arrow). Bowel gas projected over the right kidney and renal collecting system obscures detail. (b, c) Oblique axial (b) and coronal (c) reformatted images generated from pyelographic phase CT scans obtained with 1.25-mm section thickness show a polypoid tumor (arrow), which is confined to the intrarenal collecting system with no obliteration of the peripelvic fat plane. No lymphadenopathy is identified. (d) Three-dimensional MIP image shows distortion of the upper renal calices (arrow). Ureteroscopic biopsy revealed grade 2 (of three grades) urothelial carcinoma, which was treated with endourologic laser ablation.
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Figure 6c. Caliceal urothelial carcinoma in a 60-year-old woman with hereditary nonpolyposis colorectal cancer syndrome (Lynch syndrome) whose history included bladder carcinoma, endometrioid ovarian carcinoma, endometrial carcinoma, and colonic polyps. (a) Eight-minute excretory urogram shows a filling defect in the upper renal calix (arrow). Bowel gas projected over the right kidney and renal collecting system obscures detail. (b, c) Oblique axial (b) and coronal (c) reformatted images generated from pyelographic phase CT scans obtained with 1 | |
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Abstract
LEARNING OBJECTIVES
