Various methods including MRI plaque imaging have been used to evaluate carotid artery plaques, but ultrasonography, which can be performed readily and non-invasively at the bedside, is excellent for follow-up examination and rapid diagnosis in emergency situations. Conventionally, ultrasound examination of carotid artery plaques has focused on vascular stenosis due to plaques and emphasized quantitative aspects such as the degree of stenosis. Clinically, however, some plaques resist treatment and repeatedly cause ischemic events despite a relatively low degree of stenosis. In such cases, qualitative diagnosis based on the evaluation of plaque vulnerability is important. In addition to the items that have been conventionally evaluated, such as the plaque echogenicity, ulceration, and mobility, evaluation of diverse aspects of plaques has become possible due to the advent of new ultrasound techniques. Plaque neovascularization can be evaluated by contrast-enhanced ultrasound (CEUS), visualization of slow blood flows has become possible without the use of a contrast agent by superb micro-vascular imaging (SMI), and detailed morphology and volume can now be examined using a 3D probe. Moreover, because of the excellent portability of ultrasound devices, the evaluation of plaque properties is occasionally useful for the planning of the therapeutic strategy in the acute phase of cerebral infarction. Thus, ultrasonography can provide a wide range of diagnostic information.

Carotid artery plaque diagnosis using CT is useful for detecting ulcer formation or calcified lesions through the spatial-resolution-based rearrangement of multiplanar images. Quantitative assessment, in which the plaque volume (mm³) is calculated, and qualitative assessment, in which plaque is classified into vulnerable or calcified plaque using the Hounsfield Unit (HU), are possible. Calcified plaque is clearly visualized on CT, with a high HU, and the grade of calcification can be evaluated. Carotid artery stenting (CAS) for carotid artery stenosis with marked calcification may not lead to sufficient dilation, and CT is useful for preoperative assessment. On the other hand, vulnerable plaque may show a low HU, and fresh post-CAS infarction/restenosis more frequently appear when the volume of plaque with a low HU is larger. However, in the presence of a hematoma in the site of vulnerable plaque with a low HU, the HU may increase; therefore, qualitative assessment is limited. Furthermore, the limitations of CT include renal toxicity related to the use of contrast medium, radiation exposure, and artifacts. However, CT is more advantageous than MRI from the viewpoints of exposure-time shortening and availability in case of emergency.

MRI is able to visualize the wall and vessel lumen and has good soft tissue contrast which allows for the assessment of morphologic and compositional features of carotid artery plaques. It has been reported that presence of vulnerable plaque is related with an increased incidence of cerebral ischemic events. It is also reported that unstable carotid plaques which contain large amount of lipid-rich necrotic core (LRNC) and intraplaque hemorrhage (IPH) are related with an increased incidence of ischemic complications during or after carotid artery stenting (CAS). MRI assessment of LRNC and IPH has a good sensitivity and specificity. Also, it enables a volumetric analysis. In this review, we describe the current understanding of the magnetic resonance (MR) carotid plaque imaging and its clinical application for CAS.

The preoperative plaque diagnosis and evaluation of plaque protrusion (PP) into the stent lumen after stent placement or postdilatation are very important for the prevention of ischemic complications in carotid artery stenting (CAS). The usefulness of intravascular ultrasound (IVUS), which makes these evaluations during CAS possible, is discussed.

Optical coherence tomography (OCT) is an intravascular imaging tool. Its high resolution facilitates detailed examination of intravascular morphological characteristics. In the coronary field, OCT systems are applied as practical clinical diagnostic method. In the carotid artery field, OCT is not covered by health insurance, and it is used as a research tool. We have published many reports on the morphological characteristics of carotid artery lesions on OCT or its application for carotid artery stenting. In this article, we review the application of OCT for carotid artery plaque and its usefulness.

With advances in medical engineering, endoscopy using an optical fiber has become thin with high-image quality and high resolution, and it is used as angioscopy in the vascular disease. Angioscopy has been used in the coronary artery disease since the 1980s and contributed to the clarification of the pathology of coronary artery disease, but fewer studies have been reported in the carotid artery disease. Angioscopy is covered by national health insurance for the coronary artery disease, but it is not covered in the carotid artery disease. After approval by the Ethics Committee and obtaining consent from patients, I diagnose plaque by angioscopy before and after placing a stent in carotid artery stenting (CAS) and observe the blood vessel after stenting, for which safe application of angioscopy is necessary. Using the modified Parodi method + distal filter protection, blood is removed by flashing heparinized saline through a parent catheter and the vascular lumen is observed. When the plaque was examined before placing a stent, the plaque was whitish and indistinguishable from the normal blood vessel in patients treated with CAS in the subacute and chronic phases, but intimal hyperplasia and hemorrhage were noted in plaque in only one patient treated with CAS in the acute phase. Accumulation of acute phase cases may be important to elucidate the pathology of carotid artery plaque on angioscopy.

Positron emission tomography (PET) with ¹⁸F-2-deoxy-2-fluoro-D-glucose (FDG) provides valuable metabolic information regarding arteriosclerotic lesions and may be applied for the detection of vulnerable plaque. When FDG, as a glucose analog, is phosphorylated in the intracytoplasmic area, its metabolism stops and remains, so-called intracellular metabolic trapping, and images are made utilizing this characteristic. Pathological examination of extirpated plaque confirmed that FDG accumulation was strongly correlated with the distribution and density of inflammatory cells, especially macrophages. Images suggestive of plaque inflammation and therefore its vulnerability were obtained. Even in patients with relatively slight stenosis, FDG accumulation was often observed; it was not always correlated with the percent stenosis, but quantitative values of FDG uptake or its changes may reflect the progression of arteriosclerosis. FDG is also utilized as a surrogate marker of drug efficacy, such as statin. Some studies indicated its association with vascular events, but no long-term, large-scale, prospective study excluding cancer-bearing patients has been conducted. In the future, data must be further accumulated. Concerning carotid artery lesions, there are no data on the onset of asymptomatic lesions or prediction of treatment-related risks, and future studies may provide promising information.