BACKGROUND SUBTRACTION IN TECHNETIUM-99M-MAG3 RENOGRAPHY

Correction represents a potential source of error in estimating split renal function and camera-based clearances, The purpose of this study was to determine which of five background options and four time interv als was associated with the least error for Tc-99m-mercaptoacetyltrigl ycine (MAG3), Methods: Fifteen single-kidney patients were imaged supi ne after 111-370 MBq (3-10 mCi) Tc-99m-MAG3 injection. A phantom kidne y was drawn on the 2-3-min images, approximately equal in size to the solitary kidney and used for ail time intervals, Counts in the phantom and native kidneys were calculated using manual inferior and lateral regions of interest (ROIs), automated elliptical and perirenal backgro und ROIs and no background correction at various time intervals (1-2, 1-2.5, 1.5-2.5 and 2-3 min) postinjection. With optimal background cor rection, counts and the relative function in the phantom kidney should be zero, The error was measuring by estimating both the relative func tion and absolute function expressed as the percent injected dose in t he phantom kidney. Results: The percent injected dose in the phantom k idney as well as the error in measuring relative function were signifi cantly greater than zero for the inferior background correction and th e no background correction options at all time intervals, p < 0.05, Th e percent dose in the kidney and the error associated with the lateral , elliptical and perirenal ROIs were not significantly different from zero, Conclusion: Regardless of time interval, the greatest error was associated with no background correction, The inferior ROI consistentl y underestimated the background correction and probably should not be used for Tc-99m-MAG3. There was no significant difference between erro rs generated using the lateral and automated ROIs, although automated ROIs are probably more reproducible for sequential studies.