Date Published: March 4, 2019
Publisher: Public Library of Science
Author(s): Qinwei Zhang, Bram F. Coolen, Sandra van den Berg, Gyula Kotek, Debra S. Rivera, Dennis W. J. Klomp, Gustav J. Strijkers, Aart J. Nederveen, Quan Jiang.
The quality of carotid wall MRI can benefit substantially from a dedicated RF coil that is tailored towards the human neck geometry and optimized for image signal-to-noise ratio (SNR), parallel imaging performance and RF penetration depth and coverage. In last decades, several of such dedicated carotid coils were introduced. However, a comparison of the more successful designs is still lacking.
To perform a head-to-head comparison over four dedicated MR carotid surface coils with 4, 6, 8 and 30 coil elements, respectively.
Ten volunteers were scanned on a 3T scanner. For each subject, multiple black-blood carotid vessel wall images were measured using the four coils with different parallel imaging settings. The performance of the coils was evaluated and compared in terms of image coverage, penetration depth and noise correlations between elements. Vessel wall of a common carotid section was delineated manually. Subsequently, images were assessed based on vessel wall morphology and image quality parameters. The morphological parameters consisted of the vessel wall area, thickness, and normalized wall index (wall area/total vessel area). Image quality parameters consisted of vessel wall SNR, wall-lumen contrast-to-noise ratio (CNR), the vessel g-factor, and CNRindex ((wall–lumen signal) / (wall+lumen signal)). Repeated measures analysis of variance (rmANOVA) was applied for each parameter for the averaged 10 slices for all volunteers to assess effect of coil and SENSE factor. If the rmANOVA was significant, post-hoc comparisons were conducted.
No significant coil effect were found for vessel wall morphological parameters. SENSE acceleration affected some morphological parameters for 6- and 8-channel coils, but had no effect on the 30-channel coil. The 30-channel coil achieved high acceleration factors (10x) with significantly lower vessel g-factor values (ps ≤ 0.01), but lower vessel wall SNR and CNR values (ps ≤ 0.01).
All four coils were capable of high-quality carotid MRI. The 30-channel coil is recommended when rapid image acquisition acceleration is required for 3D measurements, whereas 6- and 8-channel coils demonstrated the highest SNR performance.
Carotid MRI is increasingly used in clinical research as a non-invasive tool for studying atherosclerotic disease [1–6]. High-resolution images with typical isotropic voxel dimensions of 0.7 mm or smaller are required to reveal the morphology of the carotid vessel wall plaque components . At the same time, adequate image signal-to-noise ratio (SNR), sufficient coverage along carotid vessels, short imaging duration, and robust reproducible imaging protocols are desired to make carotid plaque MRI feasible for routine clinical use. To meet these requirements, many dedicated carotid surface coils were developed by several companies and researchers over the past decades for improving image quality and scan efficiency above the standard vendor-provided coils which are often not specifically optimized for the neck anatomy . These advances have also proven crucial for the recent introduction of quantitative carotid imaging techniques, such as vessel wall tissue parameter mapping [9–12] and carotid flow and wall shear stress imaging [13–15]. Such techniques rely even more on high SNR and fast parallel imaging to prevent lengthy protocols and guarantee accurate parameter estimation. Moreover, achieving higher SNR and parallel imaging acceleration also increases the reproducibility of carotid imaging outcome measures for large cohort and multi-center studies [16–18].
Fig 3 shows representative in vivo non-accelerated 3D‐MERGE images from 2 volunteers with overlaid SNR contours. In the transversal orientation, the 4-, 6- and 8-channel coils had similar SNR contour areas with the exception for the 8-channel coil of volunteer #2, which had a much larger SNR-15 contour. In contrast, the 30-channel coil had less penetration at all SNR levels. In the coronal view, this difference was less obvious. For all coils, a minimal SNR of 15 was reached for the entire 100-mm carotid vessel wall section in the FH direction. The 4-channel coil had shorter SNR > 95 coverage in the FH direction compared with other coils. Additionally, all coils had SNR-15 contours larger than half of the FOV and achieved SNR > 75 in muscle tissue between the carotid artery and the coil, such as in the scalene muscles and the sternocleidomastoid (SCM) muscle.
In this study, four carotid coils achieved similar results for vessel wall morphological parameters. The choice of different SENSE settings did not affect the vessel wall morphology parameters for the 30-channel coil. In contrast, the use of SENSE acceleration slightly decreased some parameter values for the 8-channel coil, which may be attributed to less motion interference for faster SENSE measurements. For all measurements and all volunteers, mean and SD values for NWI, Awv, Tv and Th were 0.47 ± 0.04, 0.23 ± 0.03 cm2, 1.08 ± 0.10 mm and 1.04 ± 0.10 mm, respectively. Similar values were found in previous studies using comparable image resolutions [24, 27, 28]. However, these values were generally larger than those reported in a previous study using higher in-plane image resolution of 0.25 × 0.25 mm2 , as Awv and vessel wall thickness decrease with increasing resolution [17, 29]. The resolution in this study was chosen to achieve large FOV coverage in the FH direction while maintaining a reasonable scan duration.
In this study, we showed that all of the four dedicated carotid surface coils allowed local high SNR values > 20 for the chosen scanning sequence enabling high-resolution carotid artery imaging. The coils achieved similar carotid vessel wall morphology performance. Parallel imaging performance in different anatomical orientations depended on the specific coil element layout, which should be considered in study design. The high-density array 30-channel coil allowed parallel imaging acceleration factors up to 10 with only minimal penalties in g-factor noise. The coils with fewer elements (6 and 8) but bigger loop dimensions have excellent SNR performance, which is particularly attractive for 2D measurements and for SNR deprived quantitative mapping measurements.