Quantifying the Area at Risk in Reperfused ST-Segment-Elevation Myocardial Infarction Patients Using Hybrid Cardiac Positron Emission Tomography-Magnetic Resonance Imaging

Abstract

Background: Hybrid positron emission tomography and magnetic resonance allows the advantages of magnetic resonance in tissue characterizing the myocardium to be combined with the unique metabolic insights of positron emission tomography. We hypothesized that the area of reduced myocardial glucose uptake would closely match the area at risk delineated by T2 mapping in ST-segment-elevation myocardial infarction patients.

Methods and results: Hybrid positron emission tomography and magnetic resonance using (18)F-fluorodeoxyglucose (FDG) for glucose uptake was performed in 21 ST-segment-elevation myocardial infarction patients at a median of 5 days. Follow-up scans were performed in a subset of patients 12 months later. The area of reduced FDG uptake was significantly larger than the infarct size quantified by late gadolinium enhancement (37.2±11.6% versus 22.3±11.7%; P<0.001) and closely matched the area at risk by T2 mapping (37.2±11.6% versus 36.3±12.2%; P=0.10, R=0.98, bias 0.9±4.4%). On the follow-up scans, the area of reduced FDG uptake was significantly smaller in size when compared with the acute scans (19.5 [6.3%-31.8%] versus 44.0 [21.3%-55.3%]; P=0.002) and closely correlated with the areas of late gadolinium enhancement (R 0.98) with a small bias of 2.0±5.6%. An FDG uptake of ≥45% on the acute scans could predict viable myocardium on the follow-up scan. Both transmural extent of late gadolinium enhancement and FDG uptake on the acute scan performed equally well to predict segmental wall motion recovery.

Conclusions: Hybrid positron emission tomography and magnetic resonance in the reperfused ST-segment-elevation myocardial infarction patients showed reduced myocardial glucose uptake within the area at risk and closely matched the area at risk delineated by T2 mapping. FDG uptake, as well as transmural extent of late gadolinium enhancement, acutely can identify viable myocardial segments.

Keywords: F-flurodeoxyglucose; ST-segment–elevation myocardial infarction; T2 mapping; area at risk; cardiovascular magnetic resonance imaging; hybrid PET-MR imaging; infarct size; positron emission tomography; viability.

Figure 1. The hybrid PET-MR cardiac imaging protocol

(FDG: ¹⁸F-fluorodeoxyglucose; PET: Positron emission tomography; AC: Attenuation correction; EGE: Early gadolinium enhancement; LGE: Late gadolinium enhancement).

Figure 2. Hybrid PET-MR cardiac imaging of transmural infarction

A representative study patient with a distal LAD occlusion. Top panel: LGE-MR shows a transmural infarction confined to the LV apex and apical septum (red arrows); Upper middle panel: Corresponding FDG-PET images show areas of reduced FDG uptake matching those of infarction (white arrows); Lower middle panel: Corresponding T2 mapping MR images showing areas of increased T2 values matching the areas of reduced FDG uptake (black arrows); Bottom panel: Corresponding fused LGE-MR and FDG-PET images showing co-localization of the area of infarction and reduced FDG uptake.

Figure 3. Hybrid PET-MR cardiac imaging of subendocardial infarction

Three study patients with subendocardial myocardial infarction (red arrows) with significant myocardial salvage. The areas of reduced FDG uptake (black arrows) and the areas of edema on T2 maps (the AAR) (white arrows) extended both transmurally and radially beyond the areas of LGE.

Figure 4. Quantifying the area of reduced FDG uptake using different thresholding techniques

* denotes statistical significance.

Figure 5. Correlation and agreement between reduced FDG uptake and the AAR delineated by T2-mapping (a & b using 2SD for reduction in FDG uptake quantification and c & d using a threshold of 50% on the polar maps)

Figure 6. Comparison of FDG uptake and LGE between the acute and 12 month follow-up hybrid cardiac PET-MR scans

* denotes statistical significance

Figure 7. FDG uptake in acute and follow-up hybrid cardiac PET-MR scans

Representative LV short axis slices of FDG-PET and LGE-MR images in 2 STEMI patients with differing degrees of myocardial salvage. Patient A presented with an anterior STEMI and transmural myocardial infarction (red arrow) on the acute scan. On the follow-up scan there is no significant change in the area of reduced FDG uptake confirming non-viable myocardium in the infarct zone. Patient B presented with an inferior subendocardial myocardial infarction (red arrow). The area of reduced FDG uptake is substantially larger than the area of LGE indicating significant myocardial salvage. On the follow-up scan there is a significant reduction in the area of reduced FDG uptake which is now restricted to the infarct zone.

Figure 8. Reduced myocardial glucose uptake in remote myocardium

One study patient with a mid-right coronary artery STEMI had an area of reduced FDG uptake (white arrow) matching the inferior infarct by LGE (red arrow) and the AAR in the inferior LV wall by T2-mapping (black arrow). There was an additional area of reduced FDG uptake in the anterior and antero-septal walls (yellow arrows) in remote myocardium. The follow-up scan showed improvement in FDG uptake in the anterior and antero-septal walls of the remote myocardium.