Differential renal function (DRF) calculation with Tc-diethylenetriamine-pentaacetic acid (DTPA) or mercaptoacetyltriglycine (MAG3) is gaining clinical importance in the evaluation of patients with ureteropelvic junction obstruction with hydronephrosis (HN). Although many decisions regarding surgery are based on the functional changes detected by serial diuretic renography, the reliability of DRF estimation has been questioned. Factors such as the correction of nonrenal activity included in the renal region of interest (background subtraction) and immature function resulting in poor renal extraction may cause unusual elevation in the differential function. The aim of this study is to evaluate the impact of the radiopharmaceutical and positioning of the background on the DRF.
Patients and methods
A prospective study was undertaken in 83 patients (48 male, 35 female, median age 5 years) with unilateral or bilateral ureteropelvic junction obstruction with or without HN. Forty-five and 38 renograms were performed with 99mTc-DTPA and 99mTc-MAG3, respectively. DRF was calculated using two different regions of interest (subrenal and perirenal) for background subtraction. Of the 83 patients, 37 also had dimercaptosuccinic acid scans. The difference between DRF based on background selection, age, radiopharmaceutical, and correlation with dimercaptosuccinic acid uptake was analyzed. The difference in DRF between two background (BG) drawings (sebrenal and perirenal) and 95% confidence intervals were calculated.
For 83 renograms, the mean difference was 4.82 (range −7 to +25, SD 5.26). The 95% confidence intervals were −3.67 to 5.97. Analysis of the data revealed that greater spread of DRF between the techniques was seen in patients with right HN (mean 7.96, P<0.001). In patients with right HN, when lower pole background activity was used, the right renal function was overestimated by an average of 4.67%. When MAG3 was used, all patients had statistically significant DRF for the different BG subtraction techniques, whereas the DRF values were only statistically different in those with right HN when DTPA was used.
The current study supports the existence of BG-related variation in the DRF calculations. We found that the average counts in the perirenal BG more precisely represent nonrenal activity and recommend the use of semilunar-perirenal background region of interest to reduce the overlapping hepatic activity.