Systematic Difference of Extrarenal Background in Left and Right Kidney Regions at Renography

Abstract

The problem of whether the extrarenal sodium [¹³¹I]iodohippurate background activity at renography is higher in the right kidney region than in the left was studied by analyzing data from 711 consecutive renographies. The 3 conical collimators had an aperture diameter of 78 mm, an internal diameter at crystal surface of 50 mm, and a skin-to-crystal surface distance of 115 mm, thus allowing the radioactivity of almost the entire kidney to be registered with equal efficiency. The fractional function of the right kidney was calculated using three different modes of background subtraction: 1) direct subtraction of thorax background curve; 2) background subtraction using mean correction factors; and 3) like 2, but after correction of mean correction factors for the inaccuracy of [¹³¹I]-human serum albumin. The mean fractional function of right kidney in these three modes was 53.7% (mode 1), 51.7% (mode 2), and 50.4% (mode 3). Data from the literature suggest that the mean fractional function of the right kidney is slightly below 50%, in the range of 49%. The results indicate that the extrarenal sodium [¹³¹I]-iodohippurate background at renography is significantly higher in the right than in the left kidney region and, accordingly, that the direct subtraction of the thorax background curve is incorrect. The magnitude of the problem depends on the kidney region volume defined by the collimators. To avoid systematic errors of background subtraction, each renographist should investigate the system used and determine background subtraction correction factors.

Key Words:

• Background subtraction
• fractional renal function
• renography