MR/PET Imaging of the Cardiovascular System

Abstract

Cardiovascular imaging has largely focused on identifying structural, functional, and metabolic changes in the heart. The ability to reliably assess disease activity would have major potential clinical advantages, including the identification of early disease, differentiating active from stable conditions, and monitoring disease progression or response to therapy. Positron emission tomography (PET) imaging now allows such assessments of disease activity to be acquired in the heart, whereas magnetic resonance (MR) scanning provides detailed anatomic imaging and tissue characterization. Hybrid MR/PET scanners therefore combine the strengths of 2 already powerful imaging modalities. Simultaneous acquisition of the 2 scans also provides added benefits, including improved scanning efficiency, motion correction, and partial volume correction. Radiation exposure is lower than with hybrid PET/computed tomography scanning, which might be particularly beneficial in younger patients who may need repeated scans. The present review discusses the expanding clinical literature investigating MR/PET imaging, highlights its advantages and limitations, and explores future potential applications.

Keywords: MR; PET; atherosclerosis; cardiomyopathy; hybrid imaging.

FIGURE 1. MR/PET Imaging in Cardiac Sarcoidosis

Magnetic resonance (MR) and positron emission tomography (PET) images from 4 patients with active cardiac sarcoidosis in whom characteristic patterns of myocardial late gadolinium enhancement (left column) co-localize with increased ¹⁸F-fluorodeoxyglucose uptake (fused images, right column) (50).

FIGURE 2. MR/PET Imaging in Patients With Acute Chest Pain

MR/PET imaging of a 25-year-old woman with pericarditic chest pain. (A) The late gadolinium enhancement (LGE) images demonstrate linear mid-wall LGE consistent with myocarditis. (B) Increased ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG)-PET uptake co-localized with LGE on fusion image indicating active disease, whereas (C) T2-mapping could not clearly differentiate regions of myocardial inflammation. (D) MR/PET image of a 50-year-old woman presenting with heart failure demonstrating transmural LGE in the anterior wall. (E) No increase in ¹⁸F-FDG uptake was observed in this region, consistent with an old, previously unrecognized myocardial infarction. (F) Again, T2-mapping was inconclusive (48). Abbreviations as in Figure 1.

FIGURE 3. MR/PET Imaging in Cardiac Amyloidosis

Patient with transthyretin-related amyloidosis (ATTR). (A) Short-axis fused MR/PET image demonstrating increased myocardial ¹⁸F-sodium fluoride uptake co-localizing to areas of LGE (white arrows) in the inferolateral wall. (B) PET uptake in patients with ATTR was 48% higher than in subjects with acquired monoclonal immunoglobulin light-chain (AL) amyloid and 68% higher than in control subjects (54). TBRmax = maximum tissue-to-background; other abbreviations as in Figures 1 and 2.

FIGURE 4. MR/PET Imaging of Coronary Atherosclerosis

Patient with breathlessness underwent ¹⁸F-sodium fluoride-MR/PET imaging. (A) Standard breath-held attenuation correction leads to artifacts at the diaphragm (*), heart-lung boundary (white arrowhead), and bronchus (white arrow). (B) These artifacts were corrected with a free-breathing MR sequence for attenuation correction, (C,D,F) allowing an area of increased ¹⁸F-fluoride uptake to be visualized overlying an obstructive plaque (black arrowhead) in the left anterior descending artery. Additional uptake was observed in the aortic wall and mitral valve annulus (black arrows). (E) Transmural LGE was observed in the perfusion territory of this lesion, suggesting recent plaque rupture and myocardial infarction (64). Abbreviations as in Figures 1 and 2.

CENTRAL ILLUSTRATION. Hybrid MR/PET Imaging: The Whole Is Greater Than the Sum of its Parts

Not only can the strengths of each modality shown at the base of the pyramid be achieved in a single scan, but hybrid imaging provides additional advantages, including perfect co-registration, improved motion correction, and low-radiation imaging compared with positron emission tomography (PET)/computed tomography (CT) imaging. This combination has the potential to improve the characterization of cardiovascular disease with advantages for patient diagnosis and treatment monitoring. Lower radiation is likely to be of particular value in the clinical imaging of younger patients but may also allow more complex research protocols investigating cardiovascular disease at multiple different time points with several different tracers. Magnetic resonance (MR)/PET is already being applied to the investigation of atherosclerosis and myocardial disease, although further research is required to demonstrate its repeatability, precision, and cost-effectiveness. ¹⁸F-FDG = ¹⁸F-fluorodeoxyglucose; LGE = late gadolinium enhancement.