Myocardial perfusion imaging by single photon emission computed tomography (SPECT)

Abstract

Single Photon Emission Computed Tomography (SPECT) is superior to the planar imaging method because it provides a higher image contrast resolution and allows separation of overlapping myocardial regions. Several computerized methods are now available and are being developed for quantitative analysis of SPECT myocardial perfusion by Tl-201 or the new Tc-99m labeled myocardial perfusion agents. In patient studies, all short axis and apical portions of vertical long axis TI-201 SPECT images are quantified by dividing each myocardial slice into 60 equal sectors and displaying the maximal count per sector as a linear profile. The best threshold for defining normal limits was developed in a pilot group of 45 patients. After comparing patients’ profiles with normal limits, abnormal and normal portions of the patients’ profiles are plotted on a 2-dimensional polar map which is divided into specific coronary artery territories based on a scheme developed in a group of patients with disease of different coronary arteries. ROC analysis for defect size showed that the optimal thresholds for a definite perfusion defect were 12 % for the LAD and LCX and 8 % for the RCA territories. These criteria were prospectively applied to an additional 138 patients which yielded respective sensitivity, specificity and normalcy rates for overall detection of CAD of 96 %, 56 % and 86 % with high sensitivity and specificity for identification of CAD in individual coronary arteries. The accuracy of this quantitative SPECT technique was further assessed in a multicenter trial consisting of 318 patients whose SPECT images were obtained by various cameras, computers and operators. The results indicated that the quantitative SPECT method, utilizing standard normal limits developed at Cedars-Sinai Medical Center, can be applied at other institutions with similar accuracies. In 66 patients with prior myocardial infarction, new quantitative criteria were developed by ROC analysis that took into consideration contiguity of defects with the infarct zone. The new defect thresholds (40 % for LCX, 20 % for RCA and 12 % for LAD) were 86 % accurate for detection of patients with multivessel coronary disease after myocardial infarction. SPECT is superior to the planar imaging method in detecting patients without prior myocardial infarction, and those with moderate or single vessel coronary disease. SPECT is increasingly being used in conjunction with Tc-99m labeled myocardial perfusion agents. The results of qualitative analysis of SPECT Tc-99m SestaMIBI studies in a multicenter study have been similar to those of TI-201 SPECT for detection of perfusion defects and evaluation of the patterns of defect reversibility. Using an approach similar to that used for quantitation of TI-201 SPECT studies, a quantitative method has recently been developed for the interpretation of exercise-rest Tc-99m SestaMIBI images. Furthermore, methods are being developed for the analysis of same day rest-stress protocols and for the absolute quantification of myocardial perfusion by performing attenuation and scatter correction on Tc-99m SestaMIBI myocardial perfusion images. Myocardial perfusion SPECT images are being quantified with respect to the extent of myocardial perfusion deficit which holds promise for assessing percent infarcted and jeopardized myocardium as important prognostic indicators in coronary artery disease.