Background Correction and Measurement of Injected Activity in Quantification of Renal Function

We have compared four regions of interest frequently used for kidney background correction during renography: left and right circular (C), semilunar (S), below (I), and between both kidneys (B), in patients with four types of kidneys: bilateral normal, bilateral subnormal, bilateral abnormal, and left abnormal/right subnormal. C and S give similar results, I overestimates the function of both kidneys, and B overestimates right kidney function because of the lack of liver activity correction. To test measurements of injected activity, we measured pre- and postinjection activity in the syringe at the collimator surface, with and without depth correction, in a syringe holder at 24 cm from the collimator, and in a phantom containing a known activity, immersed in water and separated from the collimator surface by perspex plates at a distance representing kidney depth. We found a high correlation between the phantom and syringe methods: the phantom, however, most closely approaches the in vivo situation.