Approaches To

For a CT examination to allow for comprehensive urinary tract imaging, it must be able to adequately detect urinary tract calculi, renal masses, and renal collecting system, bladder, and ureteral abnormalities. Given the already widely acknowledged ability of CT to visualize renal masses and urolithiasis, recent modifications in

CT technique have centered around the creation of a protocol allowing for detailed evaluation of the renal collecting systems, ureters, and bladder.

There are several different philosophies concerning how to perform CTU. The approach that has existed the longest (but which is used by only a minority of investigators) involves combining precontrast and contrast-enhanced axial CT imaging with direct coronal imaging of the kidneys, ureters, and bladder, after injected contrast material have been excreted into the renal collecting systems (5). The coronal images were initially obtained with plain radiography. To do this the patient was removed from the CT scanner after the initial unenhanced and contrast-enhanced CT scans were obtained, and then conventional radiographs were taken at 10 to 15 minutes. Using this approach, a combined CT-excretory urogram was performed, with the latter portion of the study allowing for a detailed assessment of the urothelium.

Unfortunately, the radiographic images from these combined studies are not always of the best quality. This is because it is not always easy to ensure that once a patient is removed from a CT scanner, he or she will have prompt access to a radiography room. If there is too long a delay between the CT scanning and conventional X-ray acquisition, then the renal collecting system and ureteral distention and opacification will not be optimal at the time that the radiographs are obtained.

As a response to this potential timing difficulty, radiologists at the Mayo Clinic in Rochester, Minnesota, installed X-ray equipment in a CT scanning room (6). By doing this, they could obtain conventional radiographs at any time after CT images are acquired. This solution is probably not practical for most institutions, because such CT room modification is costly and the use of installed radiography equipment is restricted almost entirely to those patients referred for combined CT-EU. There is a significant cost of slowing down patient throughput in such a modified CT room. Obviously, regular CT examinations cannot be performed while the conventional abdominal radiographs are being obtained, processed, and reviewed.

As an alternative, investigators at the Mayo Clinic and others (7) explored the option of obtaining scan projection radiographs or digital scout radiographs instead of conventional radiographs after completion of the first two series of CT images (unenhanced scans evaluating patients for urolithiasis and enhanced scans evaluating patients for renal masses). This technique offers the advantage of requiring only a CT scanner rather than both a CT scanner and standard radiography equipment; however, it has an important limitation: the resolution of a digital scout radiograph is inferior to that of a conventional radiograph. Thus, the renal collecting systems, ureters, and bladder cannot be evaluated as accurately. In response to this problem, software has been developed that has been shown to significantly improve the quality of the scan projection radiograph (8). Using this software, so-called "enhanced" scan projection radiographs can be obtained.

At least two studies have compared the quality of standard and enhanced digital scout radiographs with that of standard film-screen radiographs (7,9). Both have determined that conventional radiography allows for superior imaging of the urinary tract compared with standard digital scout or scan projection radiography. In particular, intrarenal collecting system detail, interface, and bone image quality are significantly inferior on standard scan projection radiographs than on conventional radiographs. In contrast, enhanced scan projection radiographs appear to be similar to plain films at least in some aspects, such as in detecting urinary tract calculi (7).

Most researchers evaluating CTU have performed this study in a very different fashion (8,10-15). They have chosen, instead, to rely exclusively on the acquisition of an additional series of axial images obtained after renal excretion of injected contrast material has occurred. These excretory-phase axial images are used for a detailed evaluation of the renal collecting systems, ureters, and bladder (instead of relying on conventional radiography or CT scan projection radiography). Although there are a number of variations in the reported techniques for this excretory-phase image acquisition, all rely upon the same principle: a series of axial images are obtained using extremely thin sections (usually 2.5 mm thick or less) obtained at a delay far beyond that employed for standard abdominal and pelvic CT or renal mass CT. In CTU, the last set of images is obtained anywhere from 5 to 15 minutes after the initiation of the contrast material injection. Because most institutions utilizing this technique perform their scans on multidetector-row helical CT equipment, many refer to this technique as multidetector CTU or MDCTU. Although these images can also be obtained on single-detector helical scanners or even nonhelical scanners, they are much more quickly acquired and more easily processed when MDCTU is used.

At most centers where MDCTU is performed, thick-section three-dimensional (3-D) reconstructed images are also created. This is done, in part, because thick-section 3-D images more closely resemble the excretory urographic images with which radiologists and urologists are most familiar. To maximize postprocessed image quality when such reconstructions are obtained, the source axial images are usually reconstructed at overlapping intervals (usually with 50% overlap). It must be emphasized, however, that reliance solely on standard postprocessed 3-D images is not appropriate at the present time, because this will lead to many interpretative errors (most commonly false negative studies). The axial images have to be reviewed in addition to these 3-D images (although this can be done using another set of axial images reconstructed at contiguous rather than overlapping intervals).

In contrast, preliminary work has suggested that it may be possible to use thin-section (3 mm or less) 3-D or reformatted images instead of axial images to evaluate the renal collecting systems, ureters, and bladder without sacrificing sensitivity or specificity in diagnosing urinary tract pathology. In a recent study performed by Feng et al. (16), false negative and false positive diagnoses of uro-thelial cancer were identical for thin-section axial and thin-section coronal reformatted images (the latter actually being 2 mm thick 3-D images reconstructed in the coronal plane). The reformatted images offered the advantage of allowing reviewers to completely evaluate the urinary tract using a smaller number of images (about 100 rather than more than 500) in less time (three minutes rather than four minutes or longer).

Several preliminary studies have suggested that of the two CT-only urographic approaches, excretory-phase axial image acquisition is more sensitive (even when comparison is made with the enhanced scan projection radiographs). In one series that included 26 preselected patients with 97 urinary tract abnormalities detected on axial CT images, only 61 (63%) of these abnormalities could be prospectively identified on simultaneously obtained enhanced scan projection radiographs, 17 of which could not even be seen in retrospect (17). In this study, reviewers also subjectively graded urinary tract visualization to be inferior on digital scout radiographs, and they also rated the vast majority of detected abnormalities as more conspicuous on 3-D reconstructions than on the enhanced digital scouts. Other studies have also suggested that the accuracy in detecting urinary tract abnormalities is far superior if the MDCTU approach is utilized (8,12). It may be for this reason that some researchers who previously advocated the use of the digital scout radiograph have also begun to use axial CT image acquisition to assess the urinary tract.

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