Effects of extra-renal background subtraction and kidney depth correction in the measurement of GFR by gamma camera renography

99Tcm-DTPA gamma camera renography was performed in 83 patients with a wide range of renal function. An uptake index (UI) proportional to the single kidney glomerular filtration rate (GFR) was calculated from the renograms according to two different algorithms, one based on a blood background taken from a region of interest (ROI) over the heart (method I), and the other a modification of that procedure including an extra-renal background subtraction (method II). The extra-renal background was defined by a ROI closely surrounding the kidney, making the separation between the extra-renal background and intra-renal blood background possible. Method II was further evaluated by comparing the results without and with correction for kidney depth (method III), the skin-kidney distance being measured by sonography. Calculated UI from the renograms was compared with measured 51Cr-EDTA plasma clearance as the reference method. Method I overestimated GFR at low kidney function. Addition of the extra-renal background subtraction (method II) eliminated that error and correction for kidney depth improved the accuracy (the standard error of the estimate was reduced from 18.2 min-1 to 12.3 min-1). It is concluded that the measurement of GFR by gamma camera renography is improved by these additional operations.