Significance of Defect Severity in Technetium-99m-MIBI SPECT at Rest to Assess Myocardial Viability: Comparison with Fluorine-18-FDG PET

The pathophysiological significance of 99mTc-MIBI uptake at rest for assessing myocardial viability in patients with coronary artery disease (CAD) is still controversial. Therefore, we studied the relationship of 99mTc-MIBI uptake at rest and preserved or absent uptake of 18FDG as assessed with PET in 111 consecutive patients after overnight withdrawal of their antianginal medication. Each ventricle was evaluated in 13 segments derived from 25 regions of interest (ROIs) in short-axis cuts and 18FDG uptake was normalized to the intraindividual normal reference ROI (ROI with maximal = 100% 99mTc-MIBI uptake). Segments with a normalized 18FDG uptake > 70% were defined as viable while segments with a 18FDG uptake < 50% were defined as nonviable. Five to 11% of segments with 99mTc-MIBI uptake at rest < or = 30% of peak activity were viable and 80%-84% nonviable. Of moderate to severe 99mTc-MIBI defects at rest (31%-70% of peak), 13%-61% were viable. Segmental 99mTc-MIBI uptake and normalized 18FDG uptake were linearly correlated (r = 0.61, n = 1443, p < 0.001). In segments revealing severely reduced 99mTc-MIBI uptake (< or = 50% of peak) the correlation was considerably lower (r = 0.44, n = 295, p < 0.001). In patients with CAD, 99mTc-MIBI uptake underestimates myocardial viability in comparison to 18FDG-PET. Myocardial 99mTc-MIBI uptake therefore appears to reflect myocardial blood flow rather than myocardial viability. Patients with moderate and severe 99mTc-MIBI defects at rest may benefit from additional metabolic PET imaging prior to final therapeutic decisions.