Abstract
Atherosclerosis is a systemic disease that occurs in the coronary, carotid and peripheral arteries, as well as in the aorta. It is the main contributor to the progression of cardiovascular disease, which is the leading cause of death and illness in developed countries. The condition is characterized by a thickening of the arterial wall and narrowing of the vessel lumen as a result of accumulation of fatty materials.
The available imaging modalities to assess atherosclerotic vessels are both non-invasive and invasive and commonly identify the lumen diameter or stenosis, wall thickness and plaque volume. Imaging of carotid atherosclerosis can be performed with many different techniques. The imaging modalities vary in invasiveness, cost and type of parameters that can be investigated. Though most techniques discussed can investigate degree of stenosis and in most cases also vessel wall thickness, it is plaque composition, as a marker of plaque vulnerability, which is usually not assessed. However, ultrasonography has been reported to give information on plaque components, since this technique can differentiate between heterogenous and fatty, and homogeneous and fibrous plaque. However ultrasound is operator-, patient- and position dependent. Multicontrast MRI on the other hand is able to identify and visualize most plaque components. Unfortunately, it does not perform well in detecting the lipid-rich necrotic core, though contrast enhancement can aid in delineation of plaque components.
Diffusion Weighted Imaging has emerged as a promising candidate for carotid plaque imaging as it can identify lipid core and fibrous cap, facilitating the distinction between stable and vulnerable plaque. Still, plaque segmentation proves difficult due to limited resolution, complicating assessment of plaque vulnerability through measurement of lipid core size. The diffusion properties of tissue can be used to assess plaque type by relating average ADC within plaque to lipid content.
Diffusion Tensor imaging can improve the quality of ADC-maps by measuring diffusion in several directions as well as provide information about the anisotropy of the diffusion. Possibly, low average diffusion anisotropy is related to degradation of the fibrous cap, which is typical for vulnerable, fatty plaque. Therefore DTI seems to be a good candidate to determine the degree of plaque vulnerability by investigating lipid core size and degradation of the fibrous cap. However, this technique still has to prove its worth by continued research.