Simultaneous 18-FDG PET and MR imaging in lower extremity arterial disease

Abstract

Background: Simultaneous positron emission tomography (PET) and magnetic resonance imaging (MRI) is a novel hybrid imaging method integrating the advances of morphological tissue characterization of MRI with the pathophysiological insights of PET applications.

Aim: This study evaluated the use of simultaneous 18-FDG PET/MR imaging for characterizing atherosclerotic lesions in lower extremity arterial disease (LEAD).

Methods: Eight patients with symptomatic stenoses of the superficial femoral artery (SFA) under simultaneous acquisition of 18-FDG PET and contrast-enhanced MRI using an integrated whole-body PET/MRI scanner. Invasive plaque characterization of the SFA was performed by intravascular imaging using optical coherence tomography. Histological analysis of plaque specimens was performed after directional atherectomy.

Results: MRI showed contrast enhancement at the site of arterial stenosis, as assessed on T2-w and T1-w images, compared to a control area of the contralateral SFA (0.38 ± 0.15 cm vs. 0.23 ± 0.11 cm; 1.77 ± 0.19 vs. 1.57 ± 0.15; p-value <0.05). On PET imaging, uptake of 18F-FDG (target-to-background ratio TBR > 1) at the level of symptomatic stenosis was observed in all but one patient. Contrast medium-induced MR signal enhancement was detected in all plaques, whereas FDG uptake in PET imaging was increased in lesions with active fibroatheroma and reduced in fibrocalcified lesions.

Conclusion: In this multimodal imaging study, we report the feasibility and challenges of simultaneous PET/MR imaging of LEAD, which might offer new perspectives for risk estimation.

Keywords: FDG PET = F-18 fluorodeoxyglucose positron emission tomography; atherectomy; magnetic resonance imaging (MRI); optical coherence tomography; peripheral arterial disease.

Figure 1

Representative coregistered images of PET/MR imaging and corresponding histology and optical coherence tomography. The upper part of the figure shows representative images of target stenotic lesion (arrowhead) in patients with different uptake patterns of 18F-FDG along the superficial femoral artery in simultaneous PET/MR imaging. The mid part of the figure shows zoomed axial view MR images of the target stenotic lesion (arrowhead) detailing plaque characterized by positive remodeling on T2-w MR. The plaque shows Gd-based contrast agent enhancement on T1-w post-contrast images of MR. MR and PET images are inherently coregistered and fused due to the simultaneous acquisition. In the lower part of the figure, plaque histology shows fibrous plaques and interspersed inflammatory cells as depicted by CD68 positive macrophages (arrow) and corresponding OCT frames showing the target stenotic area of the SFA. Signal-poor fibrous plaques with diffuse borders and little or no backscattering are delineated in green, whereas fibro-calcific plaques are shown in red. CD68 = antibody to CD68 (24).

Figure 2

MR imaging results. (A) MR plaque size measurements based on T2-w axial images showed a significant larger plaque size in the diseased target lesion compared to a contralateral SFA lesion. (B) MR contrast agent enhancement was higher in the diseased leg than in a contralateral SFA lesion on T1-w axial images. (C,D) Correlation of MR imaging parameters showed a significant correlation of PSV with the degree of stenosis and PSV with MR contrast enhancement in the plaque.

Figure 3

PET imaging results. Coronal views of 18F-FDG PET maximum intensity projections of the entire study population demonstrated different FDG uptake patterns among patients. A cross indicates the symptomatic side. Quantitative analysis [target-to-background ratio normalized to the muscle ROI signal (TBRm)] showed uptake (TBRm > 1) for all patients, except for patient ID 2.