Effect of the size of regions of interest on the estimation of differential renal function in children with congenital hydronephrosis.

The estimation of differential renal function from dynamic renography affects clinical decisions. The estimation requires the kidneys to be delineated with regions of interest. However, in the presence of unilateral hydronephrosis it is not unusual for the affected kidney to be enlarged so that the regions of interest required to delineate the normal and abnormal kidneys are themselves dissimilar in size. The question, which then arises is, will this difference in the sizes of the regions of interest affect the resultant estimation of differential renal function? Eighteen children aged 1 month to 7 years, with prenatal ultrasound diagnosis of unilateral hydronephrosis where the affected kidney was larger than the normal kidney, underwent a total of 57 diuretic renograms using 99mTc-mercaptoacetyltriglycine. The renographic data were retrieved from optical disc and re-analysed. Regions of interest were generated which enclosed each kidney plus a region of interest of equal size to the abnormal large kidney was placed over the normal kidney. The consensus report from the Scientific Committee of Radionuclides in Nephrology described the Rutland-Patlak plot and integral methods for the estimation of differential renal function from dynamic renography. These two methods were used to analyse renal curves with and without background subtraction. Evaluation of the results suggest that technical factors, including the size of the region of interest, may affect differential renal function, and may, in part, explain the reports of the super-normal kidney in the literature.