Ultra-low-dose limited renal CT for volumetric stone surveillance: advantages over standard unenhanced CT.

Purpose: To describe and validate a novel CT approach using volumetric analysis for renal stone surveillance.
Materials and Methods: This prospective trial consisted of a standard low-dose non-contrast CT (SLD) of the abdomen and pelvis, immediately followed by an ultra-low-dose non-contrast CT (ULD) with reconstruction limited to the kidneys. A novel dedicated software tool was applied that automates stone volume, density, and maximum linear size. Manual linear stone size was measured by a radiology fellow and urology resident for comparison. CT dose and clinical charges were considered.
Results: Twenty-eight stones in 16 patients were analyzed. Mean effective dose of ULD CT was 0.57 mSv, an average 92% lower than the SLD CT dose. For SLD, mean size ± SD (range) (mm) was 7.9 ± 6.2 (2.6-30.5) for Reader 1, 7.3 ± 6 (2.4-30.7) for Reader 2, and 9.3 ± 6.4 (3.7-33.1) for the automated software. For ULD, mean size ± SD (range) (mm) was 7.3 ± 6 (2.5-30.5) for Reader 1, 7.2 ± 6.1 (2.1-30.7) for Reader 2, and 9.1 ± 6.4 (4.2-32.8) for the automated software. Automated stone diameters were larger than manual diameters for 27/28 stones (mean difference, 23%); difference was ≥ 2 mm in 30%. Average variability between manual measurements was 8.6% (SLD) and 7.8% (ULD), but was 0% for the automated technique. Our institutional charge for ULD renal CT is slightly less than renal US, and > 4× less than SLD CT. The Medicare global fee for the ULD renal CT is less than the SLD CT of the abdomen and pelvis.
Conclusions: This focused stone surveillance CT protocol is lower cost and lower dose compared to the standard CT approach. Automated assessment of stone burden provides improved reproducibility over manual linear measurement and offers the advantages of 3D measurements and volumetry. We now offer and perform this protocol in routine clinical practice for stone surveillance.