Delayed FDG Imaging Improves Myocardial/Blood Pool Contrast In some patients with CAD, imaging of the myocardium with F-18 deoxyglucose (FDG) at 30-60 min post injection by PET produces images with low contrast between myocardium and blood pool. We hypothesized that imaging at a later time might improve the quality of FDG images both by increasing myocardial uptake and clearance of FDG from the blood. We tested this hypothesis in 7 pts with CAD: 6 men, 1 woman, age 66+/-11; 3 diabetics. PET images were acquired both at 30-60 min post injection of 5-10 mCi of FDG (1 hr after glucose load) and at 135-165 min. For each pt, the entire LV was divided into 41 approximately equal segments. Both absolute (nCi/cc/mCi) and normalized (to peak thallium activity on stress) FDG uptake were analyzed. All data were decay corrected to time of injection. Normalized early and late FDG values were highly correlated (slope = 1.26, intercept =-0.25, r=0.92, SEE =0.05), implying early and late images provide similar clinical information. Of 215 segments, 208 (97%) were concordant for viability between early and late imaging. Absolute decay corrected myocardial FDG uptake increased on late imaging (77+/-27 early vs 116+/- 57 late, p<0.05). In all pts, late imaging was associated with decreased blood pool activity (34+/-6.6 early vs 18+/-7.1 late, p<0.01), resulting in improved late myocardium/blood pool contrast ratio (2.3 early vs 7.1 late, p<0.01). This improvement in myocardial contrast was similar for diabetics (from 2.1 to 6.5) and non diabetics (from 2.5 to 7.6), and was consistent with marked visual improvement. These data suggest that despite a slight decrease in myocardial counts due to decay, the increased myocardial FDG uptake combined with decreased blood pool activity improves late image quality. The good early-late FDG correlation suggests that the improvement does not alter the FDG distribution clinically, but this needs to be verified in a larger number of patients.
