Original contributionEx vivo study of carotid endarterectomy specimens: quantitative relaxation times within atherosclerotic plaque tissues
Introduction
Atherosclerosis is a major contributor to morbidity and mortality with the associated cardio- and cerebrovascular ischemia ranking as leading causes of death [1], [2]. Carotid atherosclerosis is a major contributor to ischaemic stroke [3]. The ability to determine to what extent an individual patient will benefit from a particular therapy, such as carotid endarterectomy or pharmacotherapy, is of paramount importance in addressing the disease. To date, clinicians predominantly rely upon the degree of luminal stenosis, as determined by ultrasound or angiography, to guide treatment decisions [4], [5], [6]. Despite these guidelines, uncertainty exists concerning the relevance of stenosis in dictating therapy [7], [8], and current evidence suggests that factors such as plaque inflammation, fibrous cap thickness, and presence of lipid-rich necrotic core play a role in developing ischaemic symptoms [9]. Within the research domain a large number of studies have exploited the inherent soft tissue contrast provided by magnetic resonance imaging (MRI) to identify plaque composition and improve risk stratification [10], [11].
A multitude of studies have applied quantitative MRI (qMRI) to characterize the normal range of healthy tissues and quantify change in tissues due to pathophysiology [12]. qT2 values have previously been reported on carotid endarterectomy specimens using both small bore ultra high-field (9.4T) systems (13–16) and clinical (1.5 T) systems (13,17–19). One such study reported the effect of temperature and formaldehyde fixation on T2 relaxation times [13]. A separate study compared in vivo and ex vivo T2 relaxation times and reported similar distributions; shortening of T2 relaxation times within lipid core was also reported [14].
High resolution ex vivo studies which report relaxation times at field strengths utilized within clinical examinations have the potential to enable future studies to optimize the relative contrast between tissues. Determining ideal sequences to assess the presence of lipid components is particularly important as the presence of a lipid core is known to increase the chances of subsequent stroke [15], [16], [17]. The ability to detect and measure fibrous cap is similarly important as fibrous cap thickness has been prospectively identified as a risk factor for cerebrovascular events [17].
This study aims to image carotid endarterectomy specimens from patients with advanced atherosclerotic disease on a clinical 1.5-T system in combination with a custom-built receive-only solenoid coil for micro-imaging. By segmenting plaque components on the basis of histological classification, the specific T1, T2 and T2⁎ relaxation times for each plaque component are computed. This study aims to investigate if the differences in relaxation times between plaque components are statistically significant.
Section snippets
Experiment
Nine patients scheduled for carotid endarterectomy were recruited and gave informed written consent prior to surgery. The study design was approved by the local research ethics committee. One patient had bilateral disease and both plaques were removed (giving us a total sample size of n = 10). The patients had the following demographics [7/9 male, age: 72±6.9 years, luminal stenosis: 78.3±13.5%, 8/9 hypertensive, 7/9 hyperlipidemic, 1/9 diabetic]. Stenoses were measured using previous clinical
Results
The total number of demarcated ROIs, for quantitative T1, T2 and T2⁎, respectively, were as follows (fibrous cap: n=16, 10 and 7; connective tissue: n=29, 22 and 8; lipid/necrotic core: n=17, 13 and 8). The total numbers varied and was dependent on the number of matching slice locations with spatially matched histology, acceptability of curve fitting and variations in spatial resolution between pulse sequences.
Our study observed no significant difference in T1 relaxation between plaque tissues (
Discussion
Identification of atherosclerotic plaque components with greater risk of thromboembolic activity is paramount in establishing the clinical utility of carotid MRI. A recent review speculates that carotid MRI is poised for multi-centre trial evaluation to assess if the technique can prospectively improve patient outcome [18]. Existing limitations, which have confined the technique within the research domain, include the necessity to have a dedicated in vivo coil, total length of scan time and the
Acknowledgments
This study was supported by a National Institute of Health Research, Cambridge Biomedical Research Centre grant.
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