tients often undergo CT scan without undergoing formal plain radiography.8 –10 A review with bone windows allows one to make a subjective distinction between the stone and stent/tube simply with a click of the computer mouse. Quantitative measurement using pixel density can be more timeconsuming, requires assistance of a radiologist, and multiple measurements at different levels of the collecting system are required. Additionally, depending on the reconstruction interval or slice thickness selected for imaging, a tiny stone/fragment may be partially in or out of the arbitrary slice and this may introduce error in the objective measurement of pixel density.11,12 Because the difference in the densities of stones and stents/tubes, is wide, the visual distinction— by way of windowing—works very well. CONCLUSIONS Although stents and stones have the same CT appearance in routine abdomen window settings, appropriate use of the bone window allows for the visual distinction between stent and stone. This distinction is attributable to differences in pixel densities. Clearly, it is much simpler to click the computer mouse to adjust the view to bone windows and see a visual difference between the stent and stone than it is to move the cursor along each part of the density that may be either stent or stone. This observation allows for a more definitive diagnosis of renal and ureteral calculi in patients in whom a nephrostomy tube or ureteral stent is present. REFERENCES 1. Waldmann TB, Lashley DB, and Fuchs EF: Unenhanced computerized axial tomography to detect retained calculi after percutaneous ultrasonic lithotripsy. J Urol 162: 312– 314, 1999. 2. Nakada SY, Hoff DG, Attai S, et al: Determination of stone composition by non-contrast spiral computed tomography in the clinical setting. Urology 55: 816 –819, 2000. 3. Joseph P, Mandal AK, Singh SK, et al: Computerized tomography attenuation value of renal calculus: can it predict successful fragmentation of the calculus by extracorporeal shock wave lithotripsy? A preliminary study. J Urol 167: 1968 –1971, 2002. 4. Newhouse JH, Prien EL, Amis ES Jr, et al: Computed tomographic analysis of urinary calculi. AJR Am J Roentgenol 142: 545–548, 1984. 5. Williams JC, Paterson RF, Kopecky KK, et al: High resolution detection of internal structure of renal calculi by helical computerized tomography. J Urol 167: 322–326, 2001. 6. Fuchs T, Kachelriess M, and Kalender WA: Technical advances in multi-slice spiral CT. Eur J Radiol 36: 69 –73, 2000. 7. Bushberg JT, Seibert JA, Leidholdt EMJ, et al: X-ray computed tomography, in Passano WM (Ed): The Essentials of Medical Imaging. Baltimore, Williams & Wilkins, 1994, pp 239 –289. 826
8. Smith RC, Rosenfield AT, Choe KA, et al: Acute flank pain: comparison of non-contrast-enhanced CT and intravenous urography. Radiology 194: 789 –794, 1995. 9. Fielding JR, Steele GS, Fox LA, et al: Spiral computerized tomography in the evaluation of acute flank pain: a replacement for excretory urography. J Urol 157: 2071–2073, 1997. 10. Smith RC, and Coll DM: Helical computed tomography in the diagnosis of ureteric colic. BJU Int 86: 33–41, 2000. 11. Colistro R, Torreggiani WC, Lyburn ID, et al: Unenhanced helical CT in the investigation of acute flank pain. Clin Radiol 57: 435–441, 2002. 12. Hamm M, Wawroschek F, Weckermann D, et al: Unenhanced helical CT in the evaluation of acute flank pain. Eur Urol 39: 460 –465, 2001.
EDITORIAL COMMENT After shock wave lithotripsy or endourologic intervention for urinary calculi, the urologist is occasionally faced with the difficulty of determining whether the patient is truly stone free and whether it is safe to remove a ureteral stent or nephrostomy tube. Depending on stone composition, location, and patient body habitus, distinguishing an in situ tube from stone may be problematic using standard imaging formats. This article describes the use of non-contrast-enhanced CT imaging using bone windows instead of conventional abdominal windows to distinguish urinary stones from in situ stents or nephrostomy tubes in 3 patients. The authors, through in vitro testing, first performed CT scans on ureteral stents and various nephrostomy catheters and determined the attenuation coefficients or Hounsfield units (HU) were in the 1600 to 2600 range. Previous investigations have demonstrated that for urinary stones, HU are consistently less than 1600 HU. Despite the differences in the attenuation coefficients between stones and tubes, using abdominal windows, which employ a narrow range of HU, these differences are not visibly apparent. However, when viewed through bone windows, which by convention use a much wider range of HU, the different attenuation coefficients may become apparent to the naked eye and stones may appear less dense than stents or nephrostomy tubes. Placing the cursor on the area of interest to measure the pixel density can also allow quantitative HU determination. From a practical perspective, no modification to current CT scanning technique is required nor is the patient subjected to additional radiation. The electronic image manipulation to produce the bone window images is performed at a computer work station. Also, using bone window sample measurements does not affect calculation of stone size. The questions not answered in this report due to the small cohort are will these observations hold up for stones of all compositions? (The stones identified in this report were located in the renal pelvis and ureterovesical junction.) How effective will the technique be for stones in other portions of the ureter? What is the effect of interobserver variability on HU measurements using the technique? As most clinically significant sized radiopaque stones would be visible on a KUB (kidneys, ureter, bladder) film, this technique seems potentially most relevant for patients with radiolucent stones, obese patients, or for those with stones overlying the bony sacrum. Evaluation of these patient populations should be conducted. Moreover, the reproducibility of results and the clinical advantages need to be evaluated using a larger sample size. In summary, this report provides preliminary evidence that a simple modification in CT imaging technique may be clinically useful. For those urologists who are now digitally linked to their radiology departments, this simple technique is now readily available. The technique holds promise as a more acUROLOGY 63 (5), 2004
curate method of assessing stone-free status than conventional CT or plain film imaging in those patients with in situ stents or tubes. H. Razvi, M.D. Division of Urology University of Western Ontario London, Ontario, Canada doi:10.1016/j.urology.2003.12.046 © 2004 ELSEVIER INC. ALL RIGHTS RESERVED
REPLY BY THE AUTHORS For a number of years, our hospital has placed computer work stations on the clinical wards and at our office desks. During the past 3 years, we have used the described “bone window” technique to search for stones of various sizes that may be buried in the coils of an indwelling stent or nephros-
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tomy tube or that lie half-hidden alongside the ureteral portion of a stent. We have discovered many residual stones that we would otherwise have missed: in the renal pelvis, in the calices (distinguishing it from the tip of a stent), and in the ureter. These stones have had a variety of compositions, including uric acid, calcium, and cystine. As noted in the Editorial Comment, interobserver reliability will be significant for both the HU measurement and distinguishing stones from a partial cut of a stent. Our own diagnostic accuracy has improved with experience. We do not really know how many we still miss, but it is certainly fewer than before. Additional scientific studies will help clarify this issue. It is fair to say that to evaluate for residual stones in patients with indwelling stents, the standard clinical practice in our hospital is to use both the abdominal and—with a click of the computer mouse—the bone window. Cigdem Tanrikut, M.D. doi:10.1016/j.urology.2003.12.047 © 2004 ELSEVIER INC. ALL RIGHTS RESERVED