18F-Fluoride PET can be used to investigate calcification activity in aortic atheroma, coronary artery disease and aortic stenosis with major clinical potential. While PET-MR has advantages over PET-CT, including tissue characterisation and reduced radiation exposure and motion correction, the two modalities have not been directly compared for cardiovascular application.
11 patients with a recent myocardial infarction and 7 with mild/moderate aortic stenosis underwent 18F-Fluoride cardiac PET-CT followed immediately by cardiac PET-MR. Uptake was evaluated semi-quantitatively by an expert reader over the aortic valve, ascending aorta, coronary arteries and areas of myocardial infarction. A novel free-breathing MR-based attenuation correction was compared against the standard Dixon breath-held approach, and both maps validated against PET-CT. Results were expressed in standardised uptake values (SUV) and tissue-to-background ratios (TBR’s).
The pattern of uptake on the aortic valve and ascending aorta was visually similar between PET-CT and both PET-MR AC techniques. Coronary uptake, clearly appreciated on PET-CT was not always easily identified on free breathing PET-MR. This appeared to be due to PET dropout secondary to metallic stents (figure 1). Employing standard breath-held PET-MR corrected this problem. Overall, image quality was significantly improved by novel free-breathing attenuation correction and fluoride uptake in areas of LGE were greater than that of remote healthy myocardium on both PET-MR and PET-CT scans.
The results of this study validate PET-MR as a reliable method of imaging microcalcification activity on the aortic valve, aorta, coronary arteries and myocardium. The novel free-breathing PET-MR AC map improves image quality but is susceptible to PET dropout in stented arteries. Further work on tissue classification for implanted metallic material is now needed to refine this promising technique.