Definition of Vascular Territories on Myocardial Perfusion Images by Integration with True Coronary Anatomy: A Hybrid PET/CT Analysis

For interpretation of myocardial perfusion studies, tissue segments are usually assigned to coronary vascular territories based on general assumptions about the most frequent vascular distribution pattern. These assumptions may be inaccurate because of interindividual variability of coronary anatomy. This limitation may be overcome by hybrid imaging through the individual integration of coronary anatomy with myocardial tissue regions. Methods: We studied 71 consecutive patients who underwent 82Rb perfusion PET/CT, including CT angiography, for work-up of coronary artery disease on a 64-slice PET/CT scanner. Coronary vessels as defined by CT were assigned to each of 17 myocardial segments for PET analysis using fusion images. Reassigned segmental maps were compared with standard assignment as proposed by the American Heart Association model, without knowledge of individual anatomy. The validity of segmental assignment was tested in 6 dogs by comparison of PET/CT with ex vivo dye staining of coronary territories. Results: Dog studies showed excellent agreement between PET/CT-defined segments and ex vivo-stained territories (kappa, 0.80). In patients, 72% (51/71) demonstrated differences from the standard assignment in at least 1 myocardial segment; 112 of 1,207 segments were reassigned to nonstandard vascular territories. Most frequently, standard right coronary segments were reassigned to the left circumflex territory (39% of reassigned segments), standard circumflex segments were reassigned to the left anterior descending territory (30%), and standard left anterior descending segments were reassigned to either circumflex or right coronary (12% and 11%, respectively). In 27 studies with a myocardial perfusion defect, relative uptake in the vascular territory with the defect was significantly lower after CT-based reassignment and was higher in remote territories, resulting in better separation (ratio of defect to remote, 0.75 +/- 0.13 vs. 0.81 +/- 0.12 before reassignment; P = 0.0014). Conclusion: Standard assumptions about vascular territory distribution in myocardial perfusion analysis are frequently inaccurate because of morphologic variability of the coronary tree. If hybrid imaging has been used to study coronary anatomy and myocardial tissue perfusion, then localization of perfusion abnormalities should be based on CT-derived anatomy. This may bring about more accurate assignment to culprit vessels and thus improved guidance and monitoring of targeted therapy.