Multimodality imaging of atherosclerotic plaque activity and composition using FDG-PET/CT and MRI in carotid and femoral arteries

Abstract

Purpose: To evaluate the relationship between atherosclerotic plaque inflammation, as assessed by FDG-positron emission tomography/computed tomography (FDG-PET/CT), and plaque morphology and composition, as assessed by magnetic resonance imaging (MRI), in the carotid and femoral arteries.

Materials and methods: Sixteen patients underwent FDG-PET/CT and MRI (T2-weighted (T2W) and proton density weighted (PDW)) of the carotid and femoral arteries. For every image slice, two observers determined the corresponding regions of the FDG-PET/CT and MRI image sets by matching CT and T2W axial images. Each plaque was then classified into one of three groups according to the CT appearance and T2W/PDW signal: (1) collagen, (2) lipid-necrotic core and (3) calcium. Arterial FDG uptake was measured for each plaque and normalized to vein FDG activity to produce a blood-normalized artery activity called the target to background ratio (TBR). The vessel wall thickness (VWT), the vessel wall area and the total vessel wall area were measured from the T2W MR images.

Results: The TBR value was higher in the lipid-necrotic core group compared to the collagen and calcium groups, (p<0.001). The lipid-necrotic core group demonstrated a significant TBR variation according to the median of the VWT (TBR=1.26+/-0.25 vs. 1.50+/-0.12). There was no correlation with other morphological MR parameters.

Conclusions: This study demonstrates the complementary value of non-invasive FDG-PET/CT and MR imaging for the evaluation of atherosclerotic plaque composition and activity. Lipid-rich plaques are more inflamed than either calcified or collagen-rich plaques.

Figures

Fig 1

(a) Transverse MR and FDG-PET/CT images demonstrating a collagen rich plaque of the right common carotid artery. The T2-weighted (T2W) image demonstrates the right common carotid artery (white arrow), the jugular vein (“v”), the sterno-cleido-mastoid muscle (‡) and the thyroid cartilage (†). Carotid artery wall appears hyperintense on T2W (white arrow) and on proton density weighted (PDW) images. CT confirms the absence of calcification in the artery wall. The right common carotid artery is displayed on the computed tomography (CT) and on the fused positron emission tomography/CT (PET/CT) images (white dashed circle). (b) Transverse MR images and corresponding FDG-PET/CT images indicate a carotid artery lipid-necrotic core plaque, hypointense on T2W (white arrow) and on PDW images. CT image demonstrates the absence of calcification. The white dashed circle demonstrates FDG uptake into the entire artery section on the PET/CT image.

Fig 2

Transverse MR and FDG-PET/CT images obtained at the same location in the right superficial femoral artery in two different patients. (a) The lipid-necrotic core plaque is eccentric and hypointense on T2W (white arrow) and on proton density weighted (PDW) images. The absence of calcification in the vessel wall on CT in combination with the MR findings suggests a lipid rich plaque. FDG uptake is measured inside the white dashed circle display on the fused PET/CT image. (b) Eccentric calcified plaque into the posterior wall of the superficial femoral artery, the lumen (*) is above and the superficial femoral vein (v) adjacent to the artery. The hypointense region on T2W (white arrow) and on PDW (white arrow) images is associated with a large hyperattenuated region on CT (white arrow).

Fig 3

Mean TBR values according to plaque composition. Diagram depicts the relationship between mean tissue-to-background ratio (TBR) values and plaque composition. The lipid group had a higher mean TBR than the collagen (p

Fig 4

Each group was divided according to the median of the vessel wall thickness (VWT): left below the cutoff and right above it. The threshold value is 16 mm for the collagen group, 19 mm for the lipid-necrotic core group and 16 mm for the calcium group. VWT is correlated to the mean TBR values only if the plaque belongs to the lipid-necrotic core group. Mean TBR values and VWT are not correlated in the calcified or collagen plaque groups. Error bars indicate standard derivation. TBR = tissue-to-background ratio; PET = positron emission tomography.