'Volumes and surface areas of 45 kidneys were determined ultrasonographically in vivo before autopsy and in a water bath phantom after autopsy by means of both the ellipsoid and the stepped section methods.Comparison of results revealed that renal volume may be determined by the simplest method, the ellipsoid method, with sufficient accuracy for clinical use. Results also revealed that renal mass expressed in grams may be directly obtained from renal volume expressed in milliliters, but mass in grams was found to correlate better with renal surface area than with renal volume. A formula relating renal mass to both volume and surface area was developed from regression analysis of the data and was found to provide a more precise e stimate of renal mass than does mass computed from either volume or surface area alone. (Key words: renal volume, renal surfilce area, renal mass) Changes in renal mass accompanied by changes in renal architecture are easily detected by vis ual inspection of ultrasonographic image s. Diseases that result in alterations of renal mass without significant alterations in architecture require a quan• titative method of detection. Renal volume has been used to detect such conditions. This study compares two popular method:; of volume determination and provides a method for computing renal mass. MATERIALS AND METHODSVolumes of 45 kidneys were determined ul· trasonographically by means of the ellipsoid method and the stepped section method.The ellipsoid method assumes that the shape of the structure studied approximates the shape of a prolate ellipsoid ( fig. 1). Volume (V) was determined from the greatest measurements of length (L), width (W), and thickness (T) as related by the following formula: Each linear measurement was performed three times, and the average value used for computation.The s tepped section method requires multiple parallel sections (fig. 2). The area of the structure of interest in each plane was determined by u manual planimeter and multiplied hy the section interval, which was 0.5 em in this study. The total volume wns the sum of the individual section volumes. Each area measurement was performed three times, and the average was used. Volumes were calculated from ultrasonograms obtained in longitudinal and transverse planes, and an average was obtained.Ultrasonograms were obtained in vivo before autopsy and in a water bath phantom following autopsy. Renal volumes obtained by each method were compared with each other and with renal masses. Assuming ellipsoid geometry, surface area varies as the two-thirds power of volume; thus, an effective renal surface area (SA) was defined as follows:where L, W, and T are greatest measurements of renal length, width, and thickness in three orthogonal planes. Regression analysis was performed on the data to determine the best method to compute renal mass. 151