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Editorial |
1 From the Department of Radiology, Albert Einstein College of Medicine and Montefiore Medical Center, 111 E 210th St, Bronx, NY 10467. Received July 29, 1999; accepted August 9. Address reprint requests to the author (e-mail: amis@aecom.yu.edu).
Index terms: Editorials • Kidney, CT, 81.12113, 81.12115 • Ureter, calculi, 82.811 • Ureter, CT, 82.12111, 82.12115 • Ureter, stenosis or obstruction, 82.811 • Urography, 80.1221
The excretory urogram is terminally ill. It survives only because of clinicians and radiologists who do not fully appreciate that more accurate delineation of the urinary tract can be gained with other techniques. Over my 31 years of learning and practicing urology (initially) and radiology, the time-honored urogram served well for demonstrating the anatomy and, to a more limited degree, the function of the urinary tract. It is now time to move on to different techniques.
The advent of cross-sectional imaging methods and their increasing sophistication over the quarter century of their existence has prompted uroradiologists to question the continued effectiveness of the urogram. Pollack and Banner (1) wrote on the status of excretory urography in 1985, concluding that the technique was useful for many urinary tract conditions and that the time had not yet arrived to write its epitaph. In 1992, Choyke (2) questioned whether rumors of the death of the urogram were premature. His opinion was written in response to a report on the combination of ultrasonography and kidneys, ureter, bladder, or KUB, radiography as essentially equivalent to excretory urography with regard to the diagnosis of acute renal colic. Choyke concluded that urography remained the examination of choice. This conclusion, however, came into question in 1995 with publication of the seminal article by Smith and colleagues (3), who compared nonenhanced computed tomography (CT) and intravenous urography for the examination of patients with acute flank pain. They eliminated the possible presence of ureteral stone as one of the two major remaining indications for excretory urography, the other being the work-up in patients with hematuria.
Over the past 4 years, the results of multiple studies have confirmed the superiority of nonenhanced helical CT for help in detecting urinary tract calculi. The single remaining substantive issue with regard to calculi, which caused many urologists to favor urography over CT, was the inability of nonenhanced CT to demonstrate the degree of obstruction. This issue has now been resolved by Boridy and colleagues (4) who, in the current issue of Radiology, show that the extent of perinephric edema seen on nonenhanced helical CT images can be used to predict the degree of ureteral obstruction in patients with ureterolithiasis, with an accuracy of 94%.
Writing from the same institution as Boridy et al a year earlier, investigators (5) were alerted to this potentially useful sign when they found that perinephric stranding was more prominent in patients whose stones passed spontaneously. They concluded that the presence of perinephric stranding and fluid were useful ancillary signs for help in predicting the likelihood of spontaneous passage of small stones. Increased pressure in the collecting system is considered to be the most important factor that moves a stone down the ureter, as well as a cause of fluid extravasation into the perinephric space, which explains the positive correlation between an increased degree of perinephric edema and an increased rate of spontaneous stone passage. Results from at least one other published study corroborate the findings of Boridy et al: In 1996, Regan and colleagues (6) reported that magnetic resonance urography demonstrated perirenal fluid in 20 (87%) of 23 acutely obstructed kidneys.
Despite these findings, CT has not yet realized its full potential. A message from the Society of Uroradiology in 1998 regarding the future directions of imaging research suggested that much of the information available at CT (eg, stone size, attenuation, shape, location) is not yet fully used (7). These data could be integrated into treatment modalities such as extracorporeal shockwave lithotripsy, so that CT could be used to aid not only in diagnosis but also in treatment. Further, virtual endoscopy will most likely allow a "fly-through" view of the ureter and help differentiate stones from phleboliths and even help detect small urothelial neoplasms.
Some results have already been reported (8,9) on the capability of CT for in vitro determination of stone composition. These results indicate that it may eventually be possible to differentiate among uric acid, struvite, and calcium oxylate stones (9). Although almost all urinary tract calculi, regardless of composition, are visible at nonenhanced CT, there is one group of patients in whom the obstructing calculus is not visible: In approximately 4% of patients infected with human immunodeficiency virus (HIV) who are undergoing indinavir therapy, a commonly used HIV-protease inhibitor, precipitated crystals can form and lead to ureteral obstruction (10).
Given the substantial capabilities of CT with regard to diagnosis of urinary tract stones, how is CT currently faring in comparison with excretory urography for imaging in patients with acute flank pain? A recent survey (11) of 203 academic radiology departments and 515 private radiology practices resulted in a 29% overall response rate. Nonenhanced helical CT scans were obtained in the majority of patients with acute flank pain at 44% of academic practices, as compared with at 13% of private practices. According to the responding radiologists, urologists who preferred excretory urography did so because they were less comfortable examining and making management decisions on the basis of CT images. Recent results published in the emergency medicine and the urology literature (12–14), however, indicate increasing acceptance of the technique by clinicians. I believe it is but a matter of time until CT becomes the standard method used to evaluate patients with acute flank pain.
The remaining major indication for excretory urography is hematuria. Patients with hematuria require evaluation of both the renal parenchyma and the urothelium. The traditional evaluation has been with cystoscopy and excretory urography (supplemented with retrograde ureteropyelography as needed). At many institutions, however, it has become common to examine such patients with excretory urography, as well as with cross-sectional imaging, to help evaluate the kidneys for the presence of masses. The need to evaluate the renal parenchyma with modalities other than excretory urography is based on the fact that findings from high-quality excretory urography with nephrotomography can be used to identify only 85% of lesions 3 cm or larger in diameter, with CT as the reference standard (15). With diminishing size of a renal mass (cystic or solid), the sensitivity of excretory urography decreases dramatically. The need to supplement urography with additional examinations to exclude renal masses has prompted a rethinking of how patients with hematuria should be evaluated. The term CT urography has been coined for an examination that involves CT evaluation of the renal parenchyma and some method for visualizing the collecting system, whether with reformatted contrast material–enhanced coronal CT images, a CT scout image obtained several minutes after intravenous administration of contrast material, or one or more abdominal radiographs at the conclusion of contrast-enhanced CT.
In 1996, Perlman and colleagues (16) followed conventional excretory urography with CT of the kidneys within 2 hours to take advantage of the contrast material injected for urography. This technique allowed a reasonably good demonstration of the renal parenchyma. Reconstructed CT ureteropyelography was subsequently reported (17) to be effective for evaluation of patients after kidney transplantation. Another study of reformatting found reliable demonstration of the collecting system, comparable with the opacification seen at excretory urography (18); this study raised the possibility of CT diagnosis of urothelial lesions, although clinical validation studies are still required to evaluate the ability of CT to help diagnose subtle urothelial lesions.
At its annual scientific assembly in 1999, the Society of Uroradiology debated how best to perform CT urography for evaluation of hematuria. Not surprisingly, as with excretory urography, there were multiple viewpoints. The consensus was that a typical examination would involve nonenhanced helical CT of the abdomen and pelvis to exclude calculi. This would be followed (if no stone is seen) by biphasic (early and delayed) contrast-enhanced CT of the renal parenchyma to determine the presence of masses. Thereafter, opinion diverges as to whether acquisition of CT scout images or of abdominal radiographs (one or more, as needed) is the more appropriate method for depicting the urothelium, with recognition that cystoscopy remains the reference standard for diagnosis of bladder lesions. As with the reformatted CT images of the collecting system, it is obvious that further studies are needed to validate the sensitivity of CT scout images for depicting fine urothelial detail. CT urography, as it undergoes further study and refinement, will combine the exquisite diagnostic capabilities of CT and excretory urography.
If nonenhanced helical CT is becoming the procedure of choice in patients with acute flank pain and CT urography would be the optimal examination for patients with hematuria, is there sufficient CT capacity in the United States to handle the load? It is estimated that there are 6,500 CT systems currently in operation (19). Recent information from a major vendor of CT equipment (GE Medical Systems, Milwaukee, Wis) indicates that, at present, approximately 50% of CT scanners in this country have helical technology, and it is estimated that by 2005, 95% of all CT units in this country will be helical units (Fox SH, oral communication, 1999). Thus, it seems that in all but the most isolated rural practices or hospitals, an appropriate CT unit will be available for evaluation of patients with urinary tract disease.
In the near future, excretory urography will be replaced by nonenhanced helical CT to evaluate for stones and by CT urography to evaluate for hematuria and other genitourinary conditions. CT urography should be viewed simply as an evolution of urography, which survived for 7 decades as one of the mainstays in imaging of the urinary tract. The transition will take a few years, and, even beyond that, there will probably be relatively rare instances where excretory urography remains an appropriate examination for untangling complicated congenital anomalies, demonstrating surgical reconstructions of the urinary tract, and following up patients with a history of transitional cell carcinoma.
Footnotes
See also the article by Boridy et al (pp 663–667 ) in this issue.
References
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