## 6 Magnetic Resonance Imaging and Artificial Intelligence  
Magnetic resonance imaging (MRI) is a powerful technique for the non-invasive morphologic characterization of atherosclerotic plaques in vivo. Indeed, while other conventional instrumental technologies, such as tomographic computed angiography and ultrasonography, can detect only the size of the lesions and the alterations of the blood flow, MRI can identify the components of the plaques, allowing the vulnerability of the lesion to be estimated based on the prevalence of fibrotic and lipidic compounds and intralesional complications [134; 135; 136; 137; 138; 139]. There are several methods that can be used to analyze carotid plaque using MRI. The most commonly used methods are T1-weighted imaging, T2-weighted imaging, diffusion-weighted imaging, and magnetization transfer imaging.  
Each of the four methods has its strengths and weaknesses for imaging carotid plaque. The choice of imaging method should depend on the specific clinical question being addressed. MRI T1-weighted imaging provides important information for risk stratification, allowing the lipid-rich and fibrous components of the plaque to be differentiated [140]. However, T1-weighted imaging has a limited ability to detect intraplaque hemorrhage. Instead, T2-weighted imaging provides high contrast between hemorrhages and fibrous tissue within the plaque, but it has a scarce ability to differentiate lipid components. Detecting intraplaque hemorrhage represents an important predictor of plaque vulnerability [141]. Diffusion-weighted imaging (DWI) is a technique that can detect changes in the movement of water molecules within the tissue and thus has been used to assess the microstructural integrity of the plaque. In fact, DWI can detect the presence of microstructural alteration such as necrosis or fibrosis [142]. However, it has a limited ability to detect intraplaque hemorrhage. Magnetization transfer imaging (MTI) has been used to study changes in the interaction between protons within the tissue. MTI can be used to assess the extracellular matrix of the plaque, such as collagen and elastin degradation, but this technique has a limited ability to detect intraplaque hemorrhage [143].  
Compared with other techniques, high-resolution MRI has some other advantages in plaque evaluation: the high-soft tissue contrast; the possibility of combining multiple contrast weightings for more valuable information; the lack of radiation; and operator independence.  
High-risk plaques (HRP) are defined as plaques with luminal surface disruption or a lipid-rich necrotic core occupying >40% of the wall or intraplaque hemorrhage [144]. These characteristics most affect the risk of stroke compared to vascular lumen constriction. In this concern, recent studies have ascribed the etiology of a percentage of cryptogenic strokes to the vulnerability of carotid plaques with non-significant stenosis [58; 145].  
The detection of peculiar features in vessel wall-high-resolution magnetic resonance imaging (VW-HRMRI) study allows the definition of the stability of atherosclerotic plaques. Vulnerable plaques are characterized by inhomogeneous hyperintensity and irregular surfaces in the pre-enhanced MRI-T1 WI sequence and hypointensity signals in the T2 WI-SPAIR image, while post-enhanced T1 WI vulnerability characteristics are the discontinuous thin linear enhancement on the plaque surface and hyperintensity in the plaque [146].  
Recently, MRI-derived images have been used to create new software based on the use of AI to diagnose and characterize the atherosclerotic disease.