Increased extra-cardiac background uptake on immediate and delayed post-stress images with 99Tcm sestamibi: determinants, independence, and significance of counts in lung, abdomen and myocardium.

Extra-cardiac activity on stress scans with 99Tcm sestamibi (MIBI) may influence scan interpretation. Lung uptake represents a potential sign of severe disease, whereas abdominal uptake may interfere with visualization of myocardial defects. We assessed myocardial, lung and infradiaphragmatic abdominal activity on images at 4 min (IMM) and 1 h (DEL) post-stress in 1800 consecutive studies. Potential variation among patients in organ activity was reduced with a weight-based dosing protocol. Multifactorial analysis was used to compare organ activity, and background ratios, i.e lung/heart or abdomen/heart, on stress images to (1) result of tomography, (2) peak workload, and (3) protocol (same-day versus separate-day rest/stress). Lung/heart ratios were primarily related to tomographic abnormalities, and abdomen/heart ratios to low stress workload; neither was related to protocol, and the two measurements appeared independent. Elevated lung/heart ratios (compared to angiographic normals), present on 16% and 10% of IMM and DEL images respectively, were determined primarily by increases in regional lung activity. Lung activity (normalized for dose) was higher in cases with disease than in normals (P<0.0001 as assessed by tomography, P<0.05 in the subset with correlating angiography). Increased abdominal/heart ratios derived primarily from increased abdominal activity, but were partially dependent on reciprocal decreases in myocardial activity. Elevated MIBI lung uptake ratios on abnormal scans can be attributed primarily to increased lung persistence of the radiotracer, and would thus be consistent with their use as a sign of left ventricular failure. Elevated abdominal background is associated with both higher splanchnic activity and lower myocardial activity, and is a non-specific finding related to suboptimal exercise intensity.