Ex-vivo High-resolution Magnetic Resonance Imaging Of Intracranial Atherosclerotic Plaque With Histological Validation And In-vivo Evaluation Of Culprit Plaque

Stroke is the leading cause of mortality and morbidity worldwide. In China, due to the rapid population ageing and poorly controlled risk factors of cerebrovascular disease,the incidence of stroke is increasing rapidly year by year. Cerebrovascular diseases not only endanger health, but also bring a huge economic burden to society.Atherosclerotic disease is the maojor cause to stroke, including carotid atherosclerotic disease（CAD） and intracranial atherosclerotic disease（ICAD）. In the past few decades, the studies of coronary and carotid atherosclerotic disease have reported that morphological and compositional characteristics of plaque are the basic of plaque rupture which leads to stroke. However, traditional angiography methods, such as DSA,MRA, CTA, etc. cannot meet the demand for diagnosis of the wall and plaque.High-resolution magnetic resonance imaging（HR-MRI） as an in vivo non-invasive diagnostic imaging method can display plaque, including qualitative and quantitative assessment. And the correlation between plaque features and occurrence of cerebrovascular events has been well confirmed in CAD.In recent years, with the development of HR-MRI, ICAD becomes a research hotspot. HRMRI can not only provide as accurate stenosis assessment like DSA, CTA,MRA, but also may explain the patient’s clinical symptoms according to morphologic and compositional features of plaque and predict the likelihood of recurrent stroke.However, due to the difficulties to obtain specimens of intracranial vessels in vivo, these inferences have not been confirmed by pathology. The interpretation of intracranial wall imaging is still based on the experience of CAD. Thus, we carried out this study.The study consists of two chapters: the first chapter is about in vitro HRMRI of intracranial arterial plaque with pathology validation. We report the signal features and relaxation time of different plaque components on MRI, which provides a reference for intracranial plaque imaging in vivo; the second chapter is about comparing the characteristics of culprit plaque and non-culprit plaque in middle cerebral artery（MCA）and basilar artery（BA）. We aims to explore the characteristics of high risk plaque and the underlying mechanism in different intracranial arteries.Chapter 1Ex-vivo High-resolution Magnetic Resonance Imaging of Intracranial Atherosclerotic Plaque with histological validationPart 1 Identification and Quantitative Assessment of Different Components of Intracranial Atherosclerotic Plaque by ex vivo 3 T High-resolution Multicontrast MRIBackground and Purpose: Intracranial atherosclerotic disease（ICAD） is recognized as a major cause of stroke. Recent development of high resolution MRI techniques have enabled imaging of intracranial atherosclerotic plaque in vivo. However,identifying plaque composition remains challenging given the small size and the lack of histological validation. Although few studies have attempted to characterize intracranial atherosclerotic plaques by ex-vivo MRI, most based on high-field（7 T）, there are still many issues to be resolved. This study aims to qualitatively and quantitatively evaluate intracranial plaque components ex vivo at 3 T with histological validation.Methods: 53 intracranial arterial sections with atherosclerotic plaques from 20cadavers（11 males, age 73.8±10.9） were imaged by 3 T MRI using multi-contrast fast-spin echo sequences（T₁ , T₂, proton-density weighted and short time inversion recovery） sequences. The specimens were sent for histological processing following imaging acquisition. The MRI slices and histological sections were matched carefully according to the location. For histology-MRI comparative analysis, the signal characteristics of plaque components were qualitatively evaluated（i.e., isointense,hyperintense and hypointense） and the signal intensitties were quantified. For Histology-MRI Blinded Analysis, radiologists and pathologists independently classified plaque and quantified plaque components. The percentage of areas of the main plaque components, the ratio of fibrous cap thickness to maximum wall thickness and plaque burden were calculated and compared between MRI and histology.Results: 207 MRI and histology slices were matched. 1. histology-MRI comparative analysis: Signal intensity of plaque components was distinct in all sequences（P<0.05）. Signal intensity of lipid core was significantly lower than fibrous cap in T₂, PD and STIR sequences（P<0.01）, and was comparable in T₁ -weighted images（P=1.00）. The best contrast between lipid core and fibrous cap was found on T₂-weighted images.Calcification showed the lowest signal intensity in T₂, PD and T₁ sequences（P<0.001）.Sensitivity and specificity were 90.53% and 69.64% for fibrous cap and lipid core, and83.33% and 100% for calcification. Cohen’s kappa（κ） values were 0.59 for fibrous cap and lipid core, and 0.90 for calcification. 2. MRI reliably classified plaque type compared with histology（κ=0.69） with an overall accuracy of 80.7%. The sensitivity and specificity using MRI to identify fibro-lipid atheroma（type IV-V） was 94.8% and 77.1%,respectively. Inter-observer agreement was excellent（κ=0.77）. 3. Plaque composition was comparable between MRI and histology: fibrous tissue（81.86 ± 10.59% vs. 81.87 ±11.59%, P=0.999）, lipid core（19.51±10.76% vs. 19.86 ± 11.56%, P=0.863）, calcification（9.68 ± 5.21% vs. 8.83 ± 5.67%, P=0.463） and fibrous cap（31.10 ± 11.28% vs. 30.83 ±8.51%, P=0.903）. However MRI overestimated plaque burden（65.18 ± 9.01% vs. 52.71± 14.58%, P<0.001）. The intraobserver and intraobserver reproducibility were good to excellent for the percentage of areas and thicknesses and plaque burden（ICC=0.779 to0.985）.Conclusion : Intracranial plaque components have distinct and different signal intensities at 3 T. Ex vivo 3 T high-resolution multicontrast MRI can accurately classify plaque types and quantitatively assess intracranial atherosclerotic plaque components.Our study provides direct reference for in vivo intracranial plaque imaging.Part 2 Ex-vivo Quantitative Imaging of Intracranial Atherosclerotic Plaque at 3 T MRI with histological validationBackground and Purpose: Recent development of high resolution MRI techniques have enabled imaging of intracranial atherosclerotic plaque in vivo. However, identifying plaque composition remains challenging given the small size and the lack of histological validation. This study aims to quantify the relaxation times of intracranial plaque components ex vivo at 3 T with histological validation.Methods: A total of 44 intracranial arteries with atherosclerotic plaques from 13 cadavers（9 male, age 79.7±10.4） were excised. Quantitative T₁ /T₂/T₂^* mapping sequences were acquired. The specimens were sent for histological processing following imaging acquisition. The MRI slices and histological sections were matched carefully according to the location. Plaque components including: fibrous cap, lipid core, fibrous tissue, calcification, and healthy wall were segmented on histology, and their relaxation times were derived from quantitative images.Results: Relaxation times of plaque components were distinct and different（P<0.05）. T₁ , T₂ and T₂^* values of lipid core were lower than fibrous cap（519±149ms vs833±238,P<0.001; 69±51ms vs 99±76ms, P=0.026; 28±11ms vs 41±15ms, P<0.001）. T₁ relextion time of fibrous cap were comparable with fibrous tissue and healthy wall（P<0.001; P=0.002）, but T₂ values were similar among them（P>0.05）. T₁ relaxation times of lipid core was shorter than those of fibrous tissue and normal wall（519±149ms vs 596±146ms, P=0.002; 519±149ms vs 632±123ms, P<0.001）. In all, T₁ , T₂ and T₂^*relaxation time of fibrous cap were the longest and Calcification were the shortest.Conclusion: Intracranial plaque components have distinct and different relaxation times at 3 T. This results can be translated to in vivo intracranial vessel wall MR imaging by developing sequences based on the NMR tissue properties of the identified atherosclerotic plaque components, eventually resulting in a dedicated MRI protocol for in vivo imaging of different intracranial plaque components.Chapter 2MR imaging of intracranial culprit plaque in vivoBackground and Purpose: Intracranial atherosclerotic disease（ICAD） is an major cause of ischemic stroke. Luminal stenosis alone cannot fully assess the severity of lesions. High-resolution magnetic resonance imaging has the potential to display intracranial wall and plaque characteristics. The study aims to compare morphological and compositional characteristics between culprit and non-culprit plaques in middle cerebral arteries（MCA） and basilar arteries（BA）, providing a reference for identifying high risk patients.Methods: From January 2014 to August 2015, 95 patients with MCA atherosclerosis and 55 patients with BA atherosclerosis were recruited. All patients underwent high-resolution magnetic resonance imaging including T₁ , T₂-weighted 2D "black blood" fast spin echo sequences, STIR sequence and T₁ enhanced sequence. A culprit plaque was defined as a lesion arising on the ipsilateral side to an ischemic stroke on neuro-imaging with accompanying clinical symptoms, whilst a non-culprit plaque was defined as either a plaque occurring in a contralateral artery of a symptomatic patient or one in asymptomatic controls. Plaque morphology characteristics, including luminal stenosis, minimum lumen area, and plaque burden and remodelling index, were compared between culprit and non-culprit plaques in middle cerebral arteries and basilar arteries, as well as the signal intensities on multiple sequences.Results: 1. T₁ signal intensities and enhancement ratio of the culprit plaque in MCA were greater than non-culprit plaques（P=0.011; P=0.003）. Culprit plaques showed smaller minimum lumen areas（P<0.001）, but the plaque burden and stenosis of culprit plaques were higher than non-culprit plaques（P<0.001）; logistic regression analysis showed T₁ signal intensities, plaque enhancement ratio, minimum lumen area, stenosis rate, plaque burden, and patterns of enhancement were risk factors of cerebrovascular events; 2. The outer wall area and wall area of culprit plaques in BA were bigger than the non-culprit plaques（P=0.017; P=0.005）, but other morphological and signal indicators showed no statistically significant difference; logistic regression analysis showed outer wall area and wall area were risk factors of cerebrovascular events; 3. The remodelling index of MCA plaque was smaller than that of the BA plaque（P=0.041）, but the degree of stenosis and plaque burden were more serious（P<0.001; P=0.004）.Conclusion: The morphological and compositional characteristics were different between culprit and non-culprit plaques in intracranial arteries, especially MCA. The characteristics of culprit plaques between MCA and BA were different, which indicated different mechanisms in different intracranial arteries. The results can be used to evaluate high risk patients in clinical practices and help make the individual therapy decision.