Metabolic scar assessment with18F-FDG -PET: Correlation to ischemic VT substrate and successful ablation sites

Abstract

Background_ Functional/molecular imaging characteristics of ischemic ventricular tachycardia (VT) substrate are incompletely understood. Objective: Compare regional 18F-FDG - PET tracer uptake with detailed electroanatomic maps (EAM) in a more extensive series of post-infarction VT patients to define metabolic properties of the VT substrate/successful ablation sites. Methods_ 3D metabolic left ventricular (LV) reconstructions were created from perfusion-normalized 18F-FDG images in consecutive patients undergoing VT ablation. Metabolic defects were defined as severe (<50% uptake) or moderate (50-70% uptake) referenced to the maximal 17-segmental uptake. Color-coded PET scars reconstructions were co-registered with corresponding high-resolution 3D EAM. Results_ All 56 patients had ischemic cardiomyopathy (EF=29±12%). Severe PET defect (<50%) was larger than EAM voltage scar (<0.5mV) with 63.0±48.4cm2 vs. 13.8±33.1cm2 (p<0.001). Similarly, moderate/severe PET defect (≤70%) was larger than areas with abnormal voltage (≤1.5mV) measuring 105.1±67.2cm2 vs. 56.2±62.6cm2 (p<0.001). Analysis of bipolar voltage (n=23,389 mapping-points) showed decreased voltage among PET <50% (n=10,364; 0.5±0.3mV) to PET 50-70% (n=5,243; 1.5±0.9mV, p<0.01) with normal voltage among PET normal areas >70% (n=7,782, 3.2±1.3mV, p<0.001). Eighty-eight percent of VT channel/exit sites (n=44) were metabolically abnormal (PET <50%: 78%; PET 50-70%: 10%), while 12% (n=6) were in metabolically normal areas (PET>70%). Metabolic channels (n=26) existed in 45% (n=25) of patients with average length/width of 17.6±12.5mm/10.3±4.2mm. Metabolic channels were oriented apex/base (86%) predominantly, harboring VT channel/exit sites in 31%. Metabolic Rapid Transition Areas (RTA: >50% change of 18F-FDG tracer uptake/15mm) were detected in 59% (n=33) co-localizing to VT channels/exit sites (15%) or its proximity (85%, 12.8±8.5mm). Metabolism-voltage mismatches (MVM) with PET<50%/voltage>1.5mV) were seen in 21% (n=12) harboring VT channel/exit sites in 41% of patients. Conclusions_ Abnormal 18F-FDG uptake categories can be detected using incremental 3D step-up reconstructions. They predicted decreasing bipolar voltages and VT channel/exit sites in ~90%. Additionally, functional imaging allowed detecting novel molecular tissue characteristics within the ischemic VT substrate such as metabolic channels, RTA, and MVM demonstrating intra-substrate heterogeneity and providing possible targets for imaging-guided ablation.