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CURRENT PRACTICE ISSUES |
1 Department of Radiology, University of Michigan Medical Center, Ann Arbor, Michigan
Indications for EU have decreased dramatically over the past decade, since it has become clear that CT has much greater sensitivity in detecting renal and ureteral calculi (1) and in detecting and characterizing renal masses (2). The remaining role for EU has been for the evaluation of the renal collecting systems and ureters. Now, with the recent emergence of CT urography, even this role is being challenged. As Kawashima and colleagues discuss in the preceding article, two CT urography approaches have been evaluated: hybrid CT-EU CT urography and CT-only CT urography. The authors have provided excellent examples of the diagnostic capabilities of both of these approaches.
Hybrid CT-EU CT urography, a combination of CT and EU performed with a modified CT scanner (ceiling-mounted overhead x-ray tube over a CT scanner) or with the innovative CT SPR technique described in the article, combines the already well-known capabilities of both CT and EU into a single study. This option provides the comfort of familiarity. As a result of years of experience interpreting these separate studies, it is very easy for radiologists and urologists to embrace such a combination approach. In contrast, adoption of the CT-only CT urography technique requires that the renal collecting systems and ureters be evaluated in a less familiar way: with thin-section excretory phase axial images, preferably acquired on a multi–detector row helical CT scanner (since this machine can obtain high-resolution images using very thin section thickness [
1 mm]) (3–5).
Because radiologists are not used to looking at the renal collecting systems and ureters on axial CT images and at the subsequently generated 3D reformatted images, CT-only CT urography may be less easily accepted than hybrid CT-EU CT urography. However, CT-only CT urography does have several distinct advantages: (a) It can be performed by a well-trained CT technologist in a CT room used exclusively for performing CT examinations. (b) Physician monitoring of the examination is not needed. (c) Patient preparation is not required. (d) Extrinsic structures are more easily determined to be outside of the renal collecting systems and ureters and not as likely to interfere with urinary tract assessment. (e) It allows assessment of the urothelium as well as the lumen within the urothelium, thereby potentially demonstrating more abnormalities than conventional or scanned projection radiographs.
Many radiology departments are staffed by a variety of different technologists, with some being trained as general radiology technologists and others subspecialized in various areas, including CT, US, MR imaging, nuclear radiology, and angiography. This is done as a result of the large number of currently available imaging modalities and the need to have knowledgeable technologists involved with each modality. A technologist performing hybrid CT-EU CT urography may need special training, as this approach requires that technologists be knowledgeable about performing both CT and EU.
Another problem is the inherent inefficiency of using a room in which a CT scanner has been installed for obtaining a series of plain film or scanned projection radiographs. It takes time to acquire these images. During this time, an expensive CT scanner essentially remains idle. Further, a radiologist should evaluate the hybrid CT-EU CT urography study as it is being performed, similar to the way in which EU has traditionally been tailored by a supervising radiologist. Additional urographic images such as prone projection or delayed views or even excretory phase enhanced CT images may be needed in order to clarify any detected abnormalities. A CT scanner will also be unused during the time required for review of previously obtained images, including the conventional or scanned projection radiographs. This approach may not be practical for radiology groups needing to maximize patient throughput on their CT scanners.
In comparison, the CT-only CT urography protocol can be performed easily without the direct supervision of a radiologist. A large number of axial CT images can be rapidly acquired. These images can be sent promptly to an independent workstation, where CT technologists can create standardized coronal 3D and 2D reformatted images within 10 minutes, if desired, during which time the CT scanner can be used to image other patients.
Kawashima and colleagues note that patients must have minimal bowel preparation prior to undergoing a hybrid CT-EU CT urography examination, since otherwise bowel gas and stool may interfere with assessment of the urinary tract on the conventional or scanned projection radiographs. With CT-only CT urography, such preparation is not needed. Although mild bowel preparation seems like a small intervention, in our experience, patients overwhelmingly prefer undergoing CT-only CT urography over EU for this reason.
In addition, a number of abnormalities of the collecting systems, ureters, and bladder can often be better evaluated at CT. For example, in some instances, intraluminal defects such as stones, blood clots (Fig 1), or tumors can be distinguished more easily from pseudodefects such as overlying bowel gas or vascular impressions when CT-only CT urography is performed instead of EU or hybrid CT-EU CT urography. Also, at hybrid CT-EU CT urography, extrinsic structures such as vascular calcifications, gallstones, or other high-contrast objects such as foreign bodies may obscure visualization of the urinary tract on EU or conventional or scanned projection radiographs and may even be mistaken as intraluminal disease. These objects are easily distinguished as being extraluminal on axial CT images.
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Although we have found that the 3D images are occasionally helpful (for example, facilitating identification of the course of the ureters in patients with congenital abnormalities or in detecting intrarenal collecting system disease, such as papillary necrosis or renal tubular ectasia), reliance on axial CT images is essential if subtle urothelial abnormalities are to be detected. However, it is hoped that as we gain more experience with isotropic imaging, one day 3D images will be generated that can play a larger diagnostic role and may even be able to replace the large number of axial images that must be reviewed currently.
Kawashima et al mention several limitations of CT-only CT urography. One of these concerns the fact that the distal ureters are often incompletely opacified (up to one-third of the time) when this technique is used. It is important to note that this has been a challenge for EU as well and would likely also be a problem for hybrid CT-EU CT urography, as it is difficult to image a urinary tract when it is completely distended and opacified due to normal physiologic ureteral peristalsis.
A more important limitation noted by Kawashima and colleagues is the greater radiation exposure with CT-only CT urography. This incremental radiation exposure has probably been overstated in the past and will probably not be a problem in the future. We have found that the radiation dose of our current three-phase CT-only imaging protocol is comparable to that of a routine abdomen-pelvis CT scan and an EU examination. These are two studies that likely would have been performed in many of our patients had CT urography not been available. Still, owing to the high radiation dose, to date we have generally restricted use of CT-only CT urography to patients who have already been evaluated by a urologist and in whom there is a high clinical suspicion of urinary tract disease, particularly urothelial malignancy. Even in these patients, it is important to weigh the risks of radiation against the possible risk of not detecting a possible urothelial tumor. Regardless, it is also likely that new modifications in CT urography technique and ongoing developments in CT software and hardware will allow radiation dose reduction in the near future.
In conclusion, CT urography is a powerful and emerging tool that is useful for revealing urinary tract anatomy and disease and will likely serve to make the standard EU obsolete in the very near future. Although both hybrid CT-EU CT urography and CT-only CT urography are able to demonstrate a wide array of urinary tract abnormalities, we believe CT-only CT urography has already shown greater promise, primarily due to greater ease and efficiency of use and probable greater sensitivity in detecting some urinary tract disease. By combining the known ability of CT in evaluating patients with suspected urolithiasis and in detecting and characterizing renal masses with what we have already found to be an excellent ability to demonstrate even small urothelial lesions, CT-only CT urography has become the only examination needed for thorough evaluation of the urinary tract.
Financial Interest: Both authors have no financial relationships to disclose.
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Department of Radiology, Mayo Clinic, Rochester, Minnesota
We would like to thank Drs Caoili and Cohan for their commentary on our article. In the preceding article, we described two of the leading techniques developed for clinical implementation of CT urography in the evaluation of patients with hematuria and suspected urologic disease. Both of these approaches to CT urography represent the evolutionary application of widely available CT technology principally to detect and define small and uncommon abnormalities of the urothelium.
Our comprehensive CT urography approach combining CT with EU, described in the article as installation of a ceiling-mounted x-ray tube and a modified CT tabletop to acquire EU film images, was created to meet our clinical demands (1). This unique CT technique also was developed to enable us to evaluate the potential use of standard and modified CT SPR images as a replacement for traditional EU film images as well as to compare the usefulness of conventional film images, CT SPR images, and multiplanar 2D and 3D reformatted CT images advocated by those physicians performing CT-only CT urography. In our practice, the uroradiologists perform online monitoring of each CT urography examination and optimize the technical aspects of the examination depending on both the initial urologic indications and the EU and axial CT findings identified during the examination. Optional delayed CT scans and EU film images can be obtained as needed. This approach combines the strengths of the complementary modalities EU and CT to detect urinary and extraurinary abnormalities.
The clinical introduction of multi–detector row CT and the parallel development of sophisticated workstations capable of manipulating large volume isotropic data sets enabled the creation of CT-only CT urography, as detailed in the preceding commentary. Drs Caoili and Cohan nicely detail their success with this CT-only CT urography approach, which has evolved through multiple steps in their practice from variable multiphasic acquisitions to a variety of reformation algorithms and projection techniques. Unsolved problems with the CT-only CT urography approach include the evaluation of nondistended ureters in the delayed acquisition phase, the physician time commitment to viewing the large CT data set at multiple window settings, and the potentially inadequate nature of nonmonitored examinations performed according to protocols in patients with unexpectedly complex disease presentations. Just as traditional EU techniques were never standardized universally, it will be determined over time which CT uroradiologic approach is more accurate and efficient.
We believe this discussion soon will develop into a question of using a hybrid technique based on the next generation of CT technology versus the widespread adoption of rapidly evolving MR urography. Our practice will certainly change as long-term, high-volume studies of CT urography and MR urography techniques, results, and their clinical implications are published. Regardless of the format used to create the diagnostic images, the essence of our article is the presentation of general CT urography principles based on current techniques, which mandate the development of additional interpretative skills by radiologists.
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