Quantification of urinary stone volume: attenuation threshold-based CT method--a technical note.

Purpose: To compare two threshold-based computed tomographic (CT) methods for the quantification of urinary stone volume; to assess their accuracy and precision at varying tube voltages, tube currents, and section thicknesses in a phantom; and to determine interobserver agreement with each of these methods in a pilot clinical study.

Materials And Methods: After institutional review board approval, written informed consent was waived. The study was HIPAA compliant. Thirty-six calcium oxalate stones were scanned in an anthropomorphic phantom. For the fixed threshold method, stones were segmented with 0.6-mm-thick sections by using attenuation thresholds of 130 and 575 HU (equal to half of mean attenuation of all stones). For the variable threshold method, stones were segmented at an attenuation threshold equal to half of the attenuation of each stone and at variable section thicknesses (0.6, 1, and 3 mm), tube currents (150, 100, and 50 mAs [reference]), and tube voltages (100 and 80 kVp). Normalized Bland-Altman analysis was used to assess the bias and precision of the two CT methods compared with that of the fluid displacement method (reference standard). Two independent readers retrospectively measured stone volumes in 17 patients (male-to-female ratio, 1.4; mean age, 55 years), and interobserver agreement was assessed by using Bland-Altman limits of agreement.

Results: The variable threshold method was more accurate and precise than the fixed threshold method with an attenuation threshold of 130 HU (P < .0001). Thinner sections (0.6 and 1 mm) resulted in more accurate (P < .05) and precise (P < .0001) stone volume measurements than 3-mm-thick sections. With the variable threshold method, no significant difference was seen in the accuracy and precision of stone volume measurements at various tube currents and tube potentials. Interobserver agreement was high with the fixed and variable threshold methods (r > 0.97).

Conclusion: An attenuation threshold-based CT method can be used to quantify urinary stone volume even at low radiation doses. The most accurate and precise method utilizes variable attenuation derived from the attenuation of each stone and thin sections.