Renal volume measurements: Accuracy and repeatability of US compared with that of MR imaging

PURPOSE: To determine the accuracy and repeatability of ultrasonography (US ) with the ellipsoid formula in calculating the renal volume. MATERIALS AND METHODS: The renal volumes in 20 volunteers aged 19-51 years were determined by using US with the ellipsoid formula and magnetic resonan ce (MR) imaging with the voxel-count method by two independent observers fo r each modality. The observers performed all measurements twice, with an in terval between the first and second examinations. The voxel-count method wa s the reference standard. Repeatability was evaluated by calculating the SD of the difference (method of Bland and Altman). RESULTS: Renal volume was underestimated with US by 45 mL (25%) on average. A comparable underestimation was found when the ellipsoid formula was appl ied to MR images. This indicates that the inaccuracy of US renal volume mea surements (a) occured because the kidney does not resemble an ellipsoid and (b) was not primarily related to the imaging modality. Intra- and interobs erver variations in US volume measurements were poor; the SD of the differe nce was 21-32 mL. For comparison, the SD of the difference in reference-sta ndard measurements was 5-10 mL. CONCLUSION: Use of US with the ellipsoid formula is not appropriate for acc urate and reproducible calculation of renal volume.