High-Resolution Black-Blood MRI of the Carotid Vessel Wall Using Phased-Array Coils at 1.5 and 3 Tesla1

Abstract

Purpose-To investigate the magnetic field dependence of the signal-to-noise ratio (SNR) for carotid vessel wall magnetic resonance imaging (MRI) using phased-array (PA) surface coils by comparing images obtained at 1.5 T and 3 T, and to determine to what extent the improved SNR at the higher field can be traded for improved spatial resolution.Materials and methods-Two pairs of dual-element PA coils were constructed for operation at the two field strengths. The individual elements of each PA were matched to 50 Ω impedance on the neck and tuned at the respective frequencies. The coils were evaluated on a cylindrical phantom positioned with its axis parallel to the main field, and with the coils placed on either side of the phantom parallel to the sagittal plane. In-vivo MR images of the carotid arteries were obtained in five subjects at both field strengths with a fast spin-echo double-inversion black-blood pulse sequence with fat saturation. SNR was measured at both field strengths using standard techniques.Results-At a depth corresponding to the average location of the carotid arteries in the study subjects, mean phantom SNR for the two coils was higher at 3 T by a factor of 2.5. The greater than linear increase is due to only partial coil loading of these relatively small coils. The practically achievable average SNR gain in vivo was 2.1. The lower in vivo SNR gain is attributed to a reduction in T 2 and prolongation of T 1 at the higher field strength and, to a lesser extent, the requirement for reduced refocusing pulse flip angle to operate within specific absorption ratio limits. The superior SNR at 3 T appears to provide considerably improved vessel wall delineation.Conclusions-Carotid artery vessel wall MRI using phased-array surface coils provides a considerable increase in SNR when field strength is raised from 1.5 T to 3 T. This increase can be traded for enhanced in-plane resolution.

Assessment of the carotid artery by MRI at 3T: A study on reproducibility

“…The field strength at which images are acquired is another factor that can influence the precision of the measurements. In a recent study (35) a considerable increase in SNR was observed when the field strength was increased from 1.5T to 3T. This increase could be traded for a better in-plane resolution, which in turn could improve the accuracy of the measurements.…”

Signal-to-noise ratio increase in carotid atheroma MRI: a comparison of 1.5 and 3 T

“…The improvement factor was somewhat higher for the four-channel carotid array, at 2.47 for the multiple-image method and 2.39 for the noise-only method. This has been seen by other groups [11] and is related to the small coil area, which leads to a higher proportion of the noise arising from the coil element itself, rather than from the imaging volume. It is higher than seen in vivo in part because of the correction for the relaxation time of the phantom, which was not applied in vivo, but also because of the lower conductivity of the phantom as compared with tissue.…”

“…One of the early studies, in 2005, used a double inversion recovery (DIR) prepared turbo spin echo (TSE) sequence to image a phantom and healthy volunteers [11]. The phantom experiments demonstrated an SNR increase of a factor of 2.5 for 3 T over 1.5 T, compared with 2.1 in vivo.…”

Accelerated 3D carotid MRI using compressed sensing and parallel imaging

“…Imaging of the carotid artery with black-blood MRI can be used to identify plaques that are vulnerable for rupture [1,2]. 3D imaging is particularly interesting to overcome the SNR and volumetric coverage limitations of 2D multislice techniques.…”