No abstract

To extend the coverage of brain coil arrays to the neck and cervical-spine region to enable combined head and neck imaging at 7 Tesla (T) ultra-high field MRI. Methods:The coil array structures of a 64-channel receive coil and a 16-channel transmit coil were merged into one anatomically shaped close-fitting housing.Transmit characteristics were evaluated in a B 1 + -field mapping study and an electromagnetic model. Receive SNR and the encoding capability for accelerated imaging were evaluated and compared with a commercially available 7 T brain array coil. The performance of the head-neck array coil was demonstrated in human volunteers using high-resolution accelerated imaging. Results: In the brain, the SNR matches the commercially available 32-channel brain array and showed improvements in accelerated imaging capabilities. More importantly, the constructed coil array improved the SNR in the face area, neck area, and cervical spine by a factor of 1.5, 3.4, and 5.2, respectively, in regions not covered by 32-channel brain arrays at 7 T. The interelement coupling of the 16-channel transmit coil ranged from −14 to −44 dB (mean = −19 dB, adjacent elements <−18 dB). The parallel 16-channel transmit coil greatly facilitates B 1 + field shaping required for large FOV neuroimaging at 7 T. Conclusion:This new head-neck array coil is the first demonstration of a device of this nature used for combined full-brain, head-neck, and cervical-spine imaging at 7 T. The array coil is well suited to provide large FOV images, which potentially improves ultrahigh field neuroimaging applications for clinical settings. K E Y W O R D S7 Tesla (7T), head and neck, MRI, neuroimaging, array coil, ultrahigh field (UHF)

Diffusion magnetic resonance imaging (dMRI) of whole ex vivo human brain specimens enables three‐dimensional (3D) mapping of structural connectivity at the mesoscopic scale, providing detailed evaluation of fiber architecture and tissue microstructure at a spatial resolution that is difficult to access in vivo. To account for the short T2 and low diffusivity of fixed tissue, ex vivo dMRI is often acquired using strong diffusion‐sensitizing gradients and multishot/segmented 3D echo‐planar imaging (EPI) sequences to achieve high spatial resolution. However, the combination of strong diffusion‐sensitizing gradients and multishot/segmented EPI readout can result in pronounced ghosting artifacts incurred by nonlinear spatiotemporal variations in the magnetic field produced by eddy currents. Such ghosting artifacts cannot be corrected with conventional correction solutions and pose a significant roadblock to leveraging human MRI scanners with ultrahigh gradients for ex vivo whole‐brain dMRI. Here, we show that ghosting‐correction approaches that correct for either polarity‐related ghosting or shot‐to‐shot variations in a separate manner are suboptimal for 3D multishot diffusion‐weighted EPI experiments in fixed human brain specimens using strong diffusion‐sensitizing gradients on the 3‐T Connectom MRI scanner, resulting in orientationally biased dMRI estimates. We apply a recently developed advanced k‐space reconstruction method based on structured low‐rank matrix (SLM) modeling that handles both polarity‐related ghosting and shot‐to‐shot variation simultaneously, to mitigate artifacts in high‐angular resolution multishot dMRI data acquired in several fixed human brain specimens at 0.7–0.8‐mm isotropic spatial resolution using b‐values up to 10,000 s/mm2 and gradient strengths up to 280 mT/m. We demonstrate the improved mapping of diffusion tensor imaging and fiber orientation distribution functions in key neuroanatomical areas distributed across the whole brain using SLM‐based EPI ghost correction compared with alternative techniques.

Purpose Functional magnetic resonance imaging (fMRI) during infancy poses challenges due to practical, methodological, and analytical considerations. The aim of this study was to implement a hardware‐related approach to increase subject compliance for fMRI involving awake infants. To accomplish this, we designed, constructed, and evaluated an adaptive 32‐channel array coil. Methods To allow imaging with a close‐fitting head array coil for infants aged 1‐18 months, an adjustable head coil concept was developed. The coil setup facilitates a half‐seated scanning position to improve the infant’s overall scan compliance. Earmuff compartments are integrated directly into the coil housing to enable the usage of sound protection without losing a snug fit of the coil around the infant’s head. The constructed array coil was evaluated from phantom data using bench‐level metrics, signal‐to‐noise ratio (SNR) performances, and accelerated imaging capabilities for both in‐plane and simultaneous multislice (SMS) reconstruction methodologies. Furthermore, preliminary fMRI data were acquired to evaluate the in vivo coil performance. Results Phantom data showed a 2.7‐fold SNR increase on average when compared with a commercially available 32‐channel head coil. At the center and periphery regions of the infant head phantom, the SNR gains were measured to be 1.25‐fold and 3‐fold, respectively. The infant coil further showed favorable encoding capabilities for undersampled k‐space reconstruction methods and SMS techniques. Conclusions An infant‐friendly head coil array was developed to improve sensitivity, spatial resolution, accelerated encoding, motion insensitivity, and subject tolerance in pediatric MRI. The adaptive 32‐channel array coil is well‐suited for fMRI acquisitions in awake infants.

In vivo diffusion-weighted magnetic resonance imaging is limited in signal-to-noise-ratio (SNR) and acquisition time, which constrains spatial resolution to the macroscale regime. Ex vivo imaging, which allows for arbitrarily long scan times, is critical for exploring human brain structure in the mesoscale regime without loss of SNR. Standard head array coils designed for patients are sub-optimal for imaging ex vivo whole brain specimens. The goal of this work was to design and construct a 48-channel ex vivo whole brain array coil for high-resolution and high b -value diffusion-weighted imaging on a 3T Connectome scanner. The coil was validated with bench measurements and characterized by imaging metrics on an agar brain phantom and an ex vivo human brain sample. The two-segment coil former was constructed for a close fit to a whole human brain, with small receive elements distributed over the entire brain. Imaging tests including SNR and G-factor maps were compared to a 64-channel head coil designed for in vivo use. There was a 2.9-fold increase in SNR in the peripheral cortex and a 1.3-fold gain in the center when compared to the 64-channel head coil. The 48-channel ex vivo whole brain coil also decreases noise amplification in highly parallel imaging, allowing acceleration factors of approximately one unit higher for a given noise amplification level. The acquired diffusion-weighted images in a whole ex vivo brain specimen demonstrate the applicability and advantage of the developed coil for high-resolution and high b -value diffusion-weighted ex vivo brain MRI studies.