Effects of background oversubtraction on quantitative analysis of thallium scans

The authors investigated the effects of the oversubtraction that can occur in quantitative thallium analysis using background interpolation, by developing a model relating background interpolation-based myocardial washout, Wbi, to true myocardial washout, Wt . Wbi/Wt = 1 - [(Cd/Bti - Bdi/Bti)/(Ci/Bti - Bii/Bti)]/1 - [(Cd/Bti - Bdi/Bii)/(Ci/Bti - 1)], where Ci = initial cardiac activity, Cd = delayed cardiac activity, Bti = initial true background activity Bii = initial interpolated background activity, and Bdi = delayed interpolated background activity. The model was tested with phantoms and applied to data from 100 patients. With use of background interpolation, and depending on the relationship between cardiac and pulmonary thallium clearance, myocardial washout in ischemic and infarcted regions could be substantially over- or underestimated, leading to serious errors in the diagnosis of coronary artery disease