Contents I. Introduction
A THEROSCLEROTIC plaque is a primary cause of ischemic stroke and is prevalent in the aorta, carotid and cerebrovascular vessels [1], [2]. Studies have shown that 46.6% of the plaques are in intracranial arteries, 27.9% are in the carotid arteries, and 25.5% are in the heart and thoracic aorta [3], [4]. However, most studies have focused on imaging the intracranial and cervical carotid arteries to diagnose plaques [5]–[8], and to a lesser extent, other studies have focused on craniocervical artery dissection [9], [10]. Early prevention and effective treatments with accurate identification of plaque structures and determination of pathologic characteristics are important to reduce cerebrovascular disease mortality and disability rates. MRI is well suited for vessel wall imaging (VWI) and is enhanced by black blood (BB) imaging technology [11], [12]. Several high-resolution 3D isotropic BB sequences for either intracranial VWI [14], [15] or extracranial carotid VWI [2], [16], [17] have potential applications for the joint intracranial and extracranial screening of atherosclerosis [13]. However, there are several challenges. First, extensive imaging coverage is required using 3D multi-contrast sequences. Second, joint intracranial and extracranial VWI requires long scan times, which are commonly 8 minutes or longer for cerebrovascular wall imaging [13], [18], [19], about 5 to 6 minutes for carotid artery wall imaging [16], [17], and more than 10 minutes for thoracic aorta imaging [20], [21]. The long acquisition times result in motion artifacts caused by slight head movements or swallowing. Third, to make a clinical diagnosis of plaques, there must be adequate image quality to delineate vessel walls, which leads to SNR and resolution challenges.To solve these problems, full coverage imaging of atherosclerotic plaques from the aortic arch to the intracranial vessels without changing the receiver coil, defined as “one-stop MR neurovascular VWI”, should be developed. Phased arrays can achieve high SNR and provide spatial sensitivities that permit parallel imaging and reduce scan times [22]–[25], which makes coil arrays highly applicable to MR VWI. The neurovascular coil system provides an opportunity to image both intracranial and extracranial arteries within a reasonable imaging time to optimally assess the association of atherosclerotic plaques and ischemic stroke occurrences [26]. These coils could greatly enhance the ability to identify and characterize plaques that contribute to the development of ischemic strokes, which is of considerable interest to clinicians who use this information to make diagnoses [27]. In this study, we aimed to build a neurovascular coil system offering a high SNR, large coverage, and time-efficient solution for one-stop MR neurovascular vessel wall imaging.However, most currently available coil arrays have been used individually to image the different regions of the neurovascular anatomy. For intracranial VWI, head coils [28]–[40] can be applied, which now commonly contain 16 to 32 elements for clinical MRI systems. For carotid artery imaging, dedicated surface coils indicate an increase in SNR [41], [42], and some phase arrays have been proposed with 4–16 elements [43]–[47].For simultaneous intracranial and extracranial arterial wall imaging, some coil designs have been developed. A 12-channel head/neck coil combined with a 4-channel carotid coil was applied for imaging the carotid and intracranial arterial wall, which achieved an isotropic spatial resolution of 0.74 mm [7]. However, for intracranial arterial wall imaging, the head coil [7] had limited channels. A 32-channel coil with 30 elements on the posterior former and two elements covering the anterior part of the neck was developed for brain and cervical spinal cord imaging [48]. However, this study mainly focused on diffusion and functional MRI, with a neck portion that had limited elements. For MR VWI of the intracranial and carotid arteries, a 32-channel coil system with a 24-channel head coil and 8-channel carotid coil was implemented, resulting in an isotropic spatial resolution of 0.6 mm [8].For MR VWI with coverage extending from the aortic arch to the intracranial vessels, a dedicated 36-channel coil system with a 23-channel posterior head-nape-back coil, an 8-channel carotid artery coil, and a 5-channel anterior neck-chest coil was designed on a 32-channel MRI system [49]. Vessel wall images could be acquired with an isotropic spatial resolution of 0.8 mm using this coil system [2]. However, the elements of this coil system had to be selected to work in three different coil modes during scanning, which was very inconvenient for one-stop MR VWI. Furthermore, limited SNR improvements were achieved due to fewer coil elements in the head portion. To achieve fast MR VWI with high spatial resolution, a new coil system with high SNR and good parallel imaging capacity, which provides full coverage imaging of atherosclerotic plaques from the aortic arch to intracranial vessels, without changing the receiver coil, should be investigated.In this study, we have designed and built a 48-channel coil system for one-stop MR VWI at 3 T, including a 32-channel head coil, 8-channel carotid coil, 4-channel chest coil and 4-channel spine coil. We also characterized coil performances by assessing SNR units, parallel imaging capabilities, and image qualities on human subjects. The coil was compared with a commercially available 36-channel coil system that included a 24-channel joint head and neck coil (17-channel head coil combined with 7-channel neck coil), 8-channel flexible small coil and 4-channel spine coil.
