Measurement of Renal Depth in Dynamic Renal Scintigraphy Using Ultralow-Dose CT

Abstract

Renal depths predicted using predefined formulas are commonly used for camera-based evaluation of renal function. We investigated the feasibility and utility of renal depth measurement using ultralow-dose CT images acquired in conjunction with dynamic renal scintigraphy. Dynamic renal scintigraphy with Tc-MAG3 was performed in 117 patients (225 kidneys) using a SPECT/CT scanner, and ultralow-dose CT (estimated effective dose of 0.17 mSv) was performed during free breathing immediately before tracer injection. The clarity of the renal contour on the CT images was evaluated visually. The renal depths were measured by 2 methods and compared with depths predicted by 2 previously reported methods. The accuracy of camera-based clearance using predicted and measured depths was evaluated using a single-sample method as a standard. The clarity of the renal contour was poor in 18 of 225 kidneys, and 12 of 117 patients were considered ineligible for depth measurement. The measurement for eligible patients showed excellent intraobserver and interobserver repeatabilities. Although mean depths were similar among the 2 CT measurement methods and 2 prediction methods, absolute differences of more than 1 cm were observed in approximately 20% of kidneys between CT measurement and prediction. CT measurement of renal depth failed to improve the accuracy of camera-based clearance evaluation. Ultralow-dose CT allowed measurement of renal depth in most patients. Substantial differences in renal depth between prediction and CT measurement indicated potential usefulness of CT measurement, although no actual improvement in the accuracy of clearance estimation was demonstrated in this study.