PET/MRI of atherosclerosis

Abstract

Myocardial infarction and stroke are the most prevalent global causes of death. Each year 15 million people worldwide die due to myocardial infarction or stroke. Rupture of a vulnerable atherosclerotic plaque is the main underlying cause of stroke and myocardial infarction. Key features of a vulnerable plaque are inflammation, a large lipid-rich necrotic core (LRNC) with a thin or ruptured overlying fibrous cap, and intraplaque hemorrhage (IPH). Noninvasive imaging of these features could have a role in risk stratification of myocardial infarction and stroke and can potentially be utilized for treatment guidance and monitoring. The recent development of hybrid PET/MRI combining the superior soft tissue contrast of MRI with the opportunity to visualize specific plaque features using various radioactive tracers, paves the way for comprehensive plaque imaging. In this review, the use of hybrid PET/MRI for atherosclerotic plaque imaging in carotid and coronary arteries is discussed. The pros and cons of different hybrid PET/MRI systems are reviewed. The challenges in the development of PET/MRI and potential solutions are described. An overview of PET and MRI acquisition techniques for imaging of atherosclerosis including motion correction is provided, followed by a summary of vessel wall imaging PET/MRI studies in patients with carotid and coronary artery disease. Finally, the future of imaging of atherosclerosis with PET/MRI is discussed.

Keywords: Atherosclerosis; PET/MRI; hybrid imaging; vascular; vulnerable plaque.

Figure 1

Coronary images from an oncology patient showing the left anterior descending artery and right coronary artery with motion correction (MC), only translational motion correction, and translational motion correction and non-rigid motion correction. Improvements in the visualization of the vessels are observed when applying translational MC, and further improvements are observed with translation and non-rigid MC. [Images reproduced with permission from Munoz et al. (94)].

Figure 2

Cross-sectional images of the myocardium for an oncology patient showing images with non-motion correction (NMC), gated, and motion-corrected PET images, alongside with profiles across the myocardium. Motion correction improves the sharpness of the myocardium compared to NMC and reduces noise compared to gated images [Images reproduced with permission from Munoz et al. (94)].

Figure 3

Co-registered (A) T1w TFE, (B) TOF, (C) T2w TSE, (D) pre- and (E) post-contrast T1w TSE images of a plaque in the internal carotid artery; (F) displays the delineation of plaque components and the inner and outer vessel wall: red = lumen; green = outer vessel wall; yellow = LRNC; orange = ring of calcifications; remaining vessel wall area = fibrous tissue. Hyperintense signal in A, and high signal in B (with respect to surrounding muscle tissue) in the bulk of the plaque is indicative for IPH (asterisk in T1w TFE and TOF images). A ring of hypointense signal, indicative for calcifications, is observed in all weightings [orange in (F)]. The lipid-rich necrotic core (yellow contour) can be identified as a region within the bulk of the plaque which does not enhance on black blood T1w MRI [red asterisk in (E)]. The fibrous cap can be recognized as a region with signal enhancement on the post-contrast images between the lipid-rich necrotic core and the lumen. This region with enhancement is interrupted, indicating a thin or ruptured fibrous cap [arrow in (E)]. [Images reproduced with permission from Kwee et al. (22)].

Figure 4

Maps generated from images acquired 1 week apart of a 70-year-old male. Parametric maps are overlaid on anatomic MR images, and voxel K^trans values which reflect microvascular flow, permeability, and surface area, are color coded from 0 to 0.6 min^-1. The parametric maps that are acquired on two different days are very similar indicating a high scan-rescan reproducibility. The necrotic core exhibits low K^trans values at the bulk of the plaque, while the highly vascularized adventitia at the outer rim demonstrates high K^trans values. Another region of higher K^trans values is observed near the inner rim of the plaque. [Reproduced, with permission, from Gaens et al. (110)].

Figure 5

(A) ¹⁸F-FDG PET image of the neck (left) of a patient with a carotid plaque; (B) a colour overlay of the PET image as displayed in (A) on the corresponding MR image. Anatomical information from MRI confirms ¹⁸F FDG uptake in the symptomatic carotid plaque (arrow), while hardly any uptake is shown in the contralateral asymptomatic carotid artery.