Metabolic Scar Assessment with18F-FDG PET: Correlation to Ischemic Ventricular Tachycardia Substrate and Successful Ablation Sites

Abstract

The functional and molecular imaging characteristics of ischemic ventricular tachycardia (VT) substrate are incompletely understood. Our objective was to compare regional ¹⁸F-FDG PET tracer uptake with detailed electroanatomic maps (EAMs) in a more extensive series of postinfarction VT patients to define the metabolic properties of VT substrate and successful ablation sites. Methods: Three-dimensional (3D) metabolic left ventricular reconstructions were created from perfusion-normalized ¹⁸F-FDG PET images in consecutive patients undergoing VT ablation. PET defects were classified as severe (defined as <50% uptake) or moderate (defined as 50%-70% uptake), as referenced to the maximal 17-segment uptake. Color-coded PET scar reconstructions were coregistered with corresponding high-resolution 3D EAMs, which were classified as indicating dense scarring (defined as voltage < 0.5 mV), normal myocardium (defined as voltage > 1.5 mV), or border zones (defined as voltage of 0.5-1.5 mV). Results: All 56 patients had ischemic cardiomyopathy (ejection fraction, 29% ± 12%). Severe PET defects were larger than dense scarring, at 63.0 ± 48.4 cm² versus 13.8 ± 33.1 cm² (P < 0.001). Similarly, moderate/severe PET defects (≤70%) were larger than areas with abnormal voltage (≤1.5 mV) measuring 105.1 ± 67.2 cm² versus 56.2 ± 62.6 cm² (P < 0.001). Analysis of bipolar voltage (23,389 mapping points) showed decreased voltage among severe PET defects (n = 10,364; 0.5 ± 0.3 mV) and moderate PET defects (n = 5,243; 1.5 ± 0.9 mV, P < 0.01), with normal voltage among normal PET areas (>70% uptake) (n = 7,782, 3.2 ± 1.3 mV, P < 0.001). Eighty-eight percent of VT channel or exit sites (n = 44) were metabolically abnormal (severe PET defect, 78%; moderate PET defect, 10%), whereas 12% (n = 6) were in PET-normal areas. Metabolic channels (n = 26) existed in 45% (n = 25) of patients, with an average length and width of 17.6 ± 12.5 mm and 10.3 ± 4.2 mm, respectively. Metabolic channels were oriented predominantly in the apex or base (86%), harboring VT channel or exit sites in 31%. Metabolic rapid-transition areas (>50% change in ¹⁸F-FDG tracer uptake/15 mm) were detected in 59% of cases (n = 33), colocalizing to VT channels or exit sites (15%) or near these sites (85%, 12.8 ± 8.5 mm). Metabolism-voltage mismatches in which there was a severe PET defect but voltage indicating normal myocardium were seen in 21% of patients (n = 12), 41% of whom were harboring VT channel or exit sites. Conclusion: Abnormal ¹⁸F-FDG uptake categories could be detected using incremental 3D step-up reconstructions. They predicted decreasing bipolar voltages and VT channel or exit sites in about 90% of cases. Additionally, functional imaging allowed detection of novel molecular tissue characteristics within the ischemic VT substrate such as metabolic channels, rapid-transition areas, and metabolism-voltage mismatches demonstrating intrasubstrate heterogeneity and providing possible targets for imaging-guided ablation.

Keywords: 18F-FDG PET imaging; VT channel or exit sites; functional imaging; ventricular tachycardia substrate.

Graphical abstract

FIGURE 1.

Workflow of ¹⁸F-FDG reconstructions. (A) Raw ¹⁸F-FDG PET DICOM showing left ventricle with central region of interest (purple). (B) Resulting 3D LV (purple) and right ventricle (blue) reconstruction. (C) Same slice as shown in A, with color map according to value of ¹⁸F-FDG uptake in each segment, showing decreased uptake in inferolateral scar area. (D) Generated 3D color PET reconstruction from short-axis slices with inferolateral scar based on ¹⁸F-FDG uptake percentage.

FIGURE 2.

Metabolic channel. (A) EAM showing inferior scar with 0.5- to 1.5-mV setting and VT channel or exit site (white point and arrow). (B) Corresponding PET 3D reconstruction showing metabolic channel (dashed lines). (C) Coregistration of EAM and PET 3D reconstruction showing VT channel or exit (arrow) within metabolic channel (dashed lines). Additional example is shown in supplemental materials.

FIGURE 3.

RTA. (A) EAM showing apical and inferior scars with 0.5- to 1.5-mV setting and VT channel or exit site (white point and arrow). (B) PET 3D reconstruction demonstrating RTA (circle, change of ≥50% uptake/15 mm [red to blue color shift]). (C) Coregistration showing VT channel or exit within RTA; arrow points to VT channel or exit site within RTA (circle). Additional example is shown in supplemental materials.

FIGURE 4.

MVM. (A) EAM showing apical and inferior scars with 0.5- to 1.5-mV setting and VT channel or exit site (white point and arrow). (B) PET 3D reconstruction demonstrating larger severe PET defect (red area = MVM). (C) Coregistration of 3D PET reconstruction and EAM demonstrating VT channel or exit within MVM (arrow). Additional example is shown in supplemental materials.

FIGURE 5.

Scatterplot showing average bipolar voltage of severe PET defect, moderate PET defect, and normal PET areas. There is stepwise increase in bipolar voltage with each step in ¹⁸F-FDG PET uptake.