The implications of decreased performance of new generation gamma-cameras on the interpretation of 131I-Hippuran renal images.

Several patients with renal failure in whom the kidneys were more clearly imaged with 99Tcm-DTPA than 131I-Hippuran were recently observed. We have postulated that our newer generation gamma-cameras have a reduced capacity to image 131I in comparison to older generation cameras. This hypothesis was tested by comparing representative older and newer imaging systems. Full-width tenth-maximum 99Tcm linespread functions were 1.20 and 1.55 cm for the older and newer cameras, respectively. The 99Tcm count rate on the newer camera was 111% that of the older camera, and the observed kidney to background count ratios in a phantom model were virtually identical for the two cameras imaging a number of simulated 99Tcm count distributions. Full-width tenth-maximum 131I linespreads were 1.55 and 4.78 cm, respectively, on the older and newer cameras and the 131I count rate on the newer camera was 27% of the count rate on the older system. The phantom model demonstrated a consistently lower 131I perceived kidney to background count ratio on the newer gamma-camera compared to the older system. These findings should be considered in the interpretation of 131I-Hippuran studies on newer-generation equipment until revised criteria of interpretation have been established. Non-visualization of the kidney may not have the same prognostic implication using modern cameras as has been reported from studies with older generation detector systems.