Radiation Dose Reduction of Unenhanced CT Limited to the Kidneys for Follow-Up of Patients With Known Nephrolithiasis Without Symptoms.

Abstract

<b>OBJECTIVE.</b> The purpose of this study is to identify the landmarks and associated radiation dose reduction for limited CT of the kidneys of patients requiring follow-up for known nephrolithiasis. <b>MATERIALS AND METHODS.</b> This retrospective study included all adult patients who underwent CT examination type "CT abdomen + pelvis renal stone" at our institution during 2017. Several exclusion criteria were identified, including scoliosis and congenital renal abnormalities. A total of 299 patients met the inclusion and exclusion criteria. The radiation dose and <i>z</i>-axis length associated with the original CT scan were recorded. The upper and lower limits of both kidneys in relation to the vertebral body endplates were recorded, to determine the <i>z</i>-axis length for a CT scan limited to the kidneys. A commercially available radiation dose analytics software package was used to provide estimates of whole-body-and individual organ-equivalent doses for the original CT scan and the limited range CT scan. <b>RESULTS.</b> The superior endplate of T11 and the inferior endplate of L5 are landmarks that will include both kidneys on almost all scans. A limited <i>z</i>-axis range leads to a mean scan length reduction of 50%. The whole-body mean effective dose is reduced by 41.5%, and the doses to the breast and the gonadal and bladder organs are reduced by 71.7%, 73.8%, and 81.8%, respectively. <b>CONCLUSION.</b> For patients without symptoms who are undergoing CT surveillance to evaluate renal calculi growth, new stone formation, or both, a limited-range scan extending from the superior endplate of T11 to the inferior endplate of L5 results in a significant reduction in radiation dose.