Add it up: Quantifying myocardial perfusion reserve w/ SPECT viable

Researchers are developing a method of quantitative myocardial perfusion imaging that analyzes myocardial perfusion reserve (MPR) using SPECT with advanced cadmium zinc telluride crystal detectors to unlock extent of coronary artery disease (CAD), according to a study published April 11 by The Journal of Nuclear Medicine.

Simona Ben-Haim, MD, from the Institute of Nuclear Medicine at University College London Hospitals, and colleagues tested a kinetic model of quantifying MPR with solid-state detector SPECT and Tc-99m sestamibi in order to uncover arterial disease hidden in multiple vessels. 

“Although the standard semiquantitative approach remains a sensitive means for diagnosing or ruling out the presence of obstructive CAD in individual patients, it often reveals only the coronary territory subtended by the vessel with the most severely flow-limiting stenosis, underestimating the extent of disease elsewhere in the coronary tree,” wrote Ben-Haim et al. “This limitation is due to the fact that coronary vasodilator reserve is diffusely abnormal in patients with extensive CAD, leading to the so-called balanced reduction in myocardial perfusion, reduced flow heterogeneity, and underestimation of the extent of underlying obstructive CAD. Absolute quantification of myocardial flow reserve has been proposed as a potential approach to overcoming this limitation.”

For this prospective study, 95 patients with a median age of 67 years old were recruited and underwent dynamic SPECT imaging performed both at rest and at peak vasodilator stress using either adenosine or dipyridamole, and subsequently standard gated myocardial perfusion imaging. The technique involved acquisition of serial images and gated resting imaging, both in list mode. Reconstruction of dynamic imaging resulted in 6070 frames, 36 seconds per frame, implementing ordered subsets and a factor analysis to estimate blood-pool and time-activity curves. These were applied as input functions for the study’s two-compartment kinetic model for obtaining an MPR index as a ratio of stress and rest values.

Results showed that global MPR index was higher in the 51 patients with normal myocardial perfusion imaging. Sixteen patients indicated for invasive coronary angiography were shown to have 20 vessels with stenosis at or exceeding 50 percent. The MPR index was 1.11 in obstructed arteries in contrast with the 1.30 index for unobstructed vessels. A multivariable regression analysis revealed significant correlations between age, smoking history, global stress and regional stress.

“Dynamic tomographic imaging and quantification of a retention index describing global and regional perfusion reserve are feasible using a solid-state camera,” concluded the authors. “Preliminary results show that the MPR index is lower in patients with perfusion defects and in regions supplied by obstructed coronary arteries. Further studies are needed to establish the clinical role of this technique as an aid to semiquantitative analysis of MPI.”

The only challenge to the technique was a possible underestimation of high-flow rate retention estimates due to a nonlinear relationship between myocardial blood flow and radiotracer extraction. The researchers recommended further studies to identify normal limits of MPR index measurements when using Tc-99m sestamibi.