Urinary Calculi: Radiation Dose Reduction of 50% and 75% at CT-Effect on Sensitivity

Purpose: To retrospectively determine the effect of 50% and 75% dose reduction on sensitivity and specificity of computed tomography (CT) for the detection of urinary calculi. Materials and Methods: This HIPAA compliant study was institutional review board approved; informed consent was waived. Raw CT data from 47 consecutive patients (24 male patients, 23 female patients; mean age, 46.0 years) who underwent unenhanced CT for flank pain were collected. Original CT examinations were performed by utilizing an automated dose modulation algorithm. Reconstructions of raw CT data were performed at 100%, 50%, and 25% of the original tube current by using simulation software; tube currents averaged 177, 88, and 44 mA, respectively. All reconstructed examinations were randomized and evaluated by two radiologists blinded to the presence, number, location, and size of calculi. The opinion of an unblinded radiologist who separately reviewed the original examination and report and the electronic medical record served as the reference standard. One hundred eight calculi (85 renal, 21 ureteral, and two in the bladder) were present in 32 of 47 patients. Calculus diameter ranged from 0.14 to 1.32 cm (mean, 0.34 cm). The decrease in sensitivity was assessed between doses and was independently evaluated for all calculi and separately for calculi greater than 3 mm in diameter by using the McNemar test, adjusted for clustered data. Results: For all calculi, the blinded readers demonstrated combined sensitivities of 91.7%, 83.3%, and 67.1% for the 100%, 50%, and 25% tube current reconstructions, respectively. For stones greater than 3 mm, combined sensitivities were 97.7%, 93.0%, and 91.9%, respectively, for the 100%, 50%, and 25% reconstructions. There was no significant difference between the 100% examinations and the 50% and 25% reconstructions for detection of stones greater than 3 mm (P = .106 and .099, respectively). Conclusion: There were no significant differences between the 100% examinations and the 50% and 25% examinations for the detection of calculi greater than 3 mm. (c) RSNA, 2009