Clinical impact of increasing ratio from ^99mTc-sestamibi SPECT: validation by PET-myocardial flow reserve

Myocardial flow reserve (MFR) obtained from ¹³N-ammonia PET is valuable for predicting the prognosis of patients with various heart diseases. The increase ratio (IR), a new parameter in ^99mTc-sestamibi SPECT, is an indicator of the relative increase in blood flow during stress compared to rest, and the concept is similar to that of PET-MFR. We investigated the relationship between IR and PET-MFR as well as their clinical significance. Patients who underwent ¹³N-ammonia PET and ^99mTc-sestamibi SPECT within one year of each other were enrolled and 33 patients (28 males, mean age 65.7 ± 12.9) were analyzed. Using adenosine stress and resting images of ^99mTc-sestamibi SPECT, the stress image count was divided by the resting image count, and IR was calculated. We investigated the correlation between IR and PET-MFR and validated the optimal IR cutoff value for detecting PET-MFR < 1.6 by receiver operating characteristic (ROC) analysis. IR was significantly lower than PET-MFR (mean IR, 1.198; mean PET-MFR, 1.910; p < 0.0001) and positively correlated with PET-MFR (Pearson r = 0.3567; p < 0.01). ROC analysis indicated that the optimal IR cutoff value for detecting PET-MFR < 1.6 was 1.06, yielding an area under the curve (AUC) of 0.683, with 50% sensitivity and 83% specificity. By combining summed stress score with IR, the detectability of PET-MFR < 1.6 improved the AUC of 0.798 with 83% sensitivity and 70% specificity. IR was positively correlated with PET-MFR. This suggests that IR may be useful for detecting patients with a low MFR in facilities that do not perform ¹³N-ammonia PET.