Factors affecting the measurement of renal length.

The variation in kidney length between the 1-minute radiograph (centered over the kidney) and the 5-minute study (centered over the mid-abdomen) was evaluated on 100 random excretory urograms. When both kidneys were measurable, the change in kidney size was discordant in 40%. Most such changes are due to changes in geometry, differences in centering, or changes in the angle of the kidney with respiration. These alterations are of much greater magnitude than the reported swelling of the kidney due to contrast medium. Since there is practically no relation between the time of injection and changes in renal image size, it makes no difference whether the 1-minute or 5-minute radiograph is selected for measurement.