First-pass radionuclide angiography via pulmonary arterial catheters. A critical analysis of background components.

Residual background activity following first-pass radionuclide angiography was assessed in 44 patients after eliminating activity in the right heart, left lung, and most of the right lung. The ejection fraction (EF) was calculated in two ways: (a) assuming background activity was present only in the right heart and lungs, and (b) correcting for scattered radiation by placing a circular region of interest around the left ventricle. With the first method, contrast EF was significantly underestimated in both AP and RAO views. With the second method, correlation was close in both views and absolute values agreed well with contrast EF. These results indicate that there is a substantial and measurable background component, due mainly to Compton scatter, even when activity from the right heart and lungs is eliminated.