A simple objective method to assess the radiopacity of urinary calculi and its use to predict extracorporeal shock wave lithotripsy outcomes.
We describe an objective method to evaluate kidney stone radiopacity for use in selection of cases suitable for ESWL. We recruited 76 adult patients with a solitary 1 to 2 cm renal pelvic stone. All patients underwent routine plain x-ray of the urinary tract but an aluminum step wedge (Gammex) was adapted to the cassette before x-ray exposure. This x-ray was then digitized and analyzed by histogram to calculate the gray level of the stone and of each step of the aluminum step wedge. This allowed radiographic stone density to be expressed in mm aluminum equivalent. All patients also underwent abdominopelvic computerized tomography and then ESWL was started. Stone density on plain x-ray was 1.83 to 5.93 mm aluminum equivalent. There was a positive correlation between these values and stone attenuation values on computerized tomography (r(2) 0.83, p <0.005). The 12 patients in whom ESWL failed were found to have stones of significantly higher density than stones in patients with complete stone fragmentation (mean +/- SD 4.8 +/- 0.74 vs 3.35 +/- 0.88 mm aluminum equivalent, p <0.005). There was also a positive correlation between stone radiopacity in mm aluminum equivalent and the total number of shock waves required to achieve complete fragmentation (r(2) 0.66, p <0.005). The aluminum step wedge with plain x-ray of the urinary tract provides a good reference for objectively assessing the radiopacity of renal calculi.