Abstract 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-ch 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 of the developed coil for high-resolution and high b -value diffusion-weighted ex vivo brain MRI studies.
Abstract Diffusion MRI (dMRI) of whole, intact, fixed postmortem human brain at high spatial resolution serves as key bridging technology for 3D mapping of structural connectivity and tissue microstructure at the mesoscopic scale. Ex vivo dMRI offers superior spatial resolution compared to in vivo dMRI but comes with its own technical challenges due to the significantly reduced T2 relaxation times and decreased diffusivity incurred by tissue fixation. The altered physical properties of fixed tissue necessitate the use of alternative acquisition strategies to preserve SNR and achieve sufficient diffusion weighting. Multi-shot or segmented 3D echo planar imaging (EPI) sequences have been used to shorten echo times (TEs) with reduced distortions from field inhomogeneity and eddy currents on small-bore MR scanners and have been adopted for high b-value dMRI of ex vivo whole human brain specimens. The advent of stronger gradients on human MRI scanners has led to improved image quality and a wider range of diffusion-encoding parameters for dMRI but at the cost of more severe eddy currents that result in spatial and temporal variations in the background magnetic field, which cannot be corrected for using standard vendor-provided ghost correction solutions. In this work, we show that conventional ghost correction techniques based on navigators and linear phase correction may be insufficient for EPI sequences using strong diffusion-sensitizing gradients in ex vivo dMRI experiments, resulting in orientationally biased dMRI estimates. This previously unreported problem is a critical roadblock in any effort to leverage scanners with ultra-high gradients for high-precision mapping of human neuroanatomy at the mesoscopic scale. We propose an advanced reconstruction method based on structured low-rank matrix modeling that reduces the ghosting substantially. We show that this method leads to more accurate and reliable dMRI metrics, as exemplified by diffusion tensor imaging and high angular diffusion imaging analyses in distributed neuroanatomical areas of fixed whole human brain specimens. Our findings advocate for the use of advanced reconstruction techniques for recovering unbiased metrics from ex vivo dMRI acquisitions and represent a crucial step toward making full use of strong diffusion-encoding gradients for neuroscientific studies seeking to study brain structure at multiple spatial scales.
Motivation: Current human MR scanners cannot resolve the full range of length scales needed to study the brain's microscopic and mesoscopic structure. Goal(s): To construct and validate the next-generation human connectomics and microstructure MRI scanner known as Connectome 2.0. Approach: The 3T Connectome 2.0 scanner incorporates a peripheral nerve stimulation-optimized asymmetric head gradient driven by dual gradient power amplifiers. Custom-built high-sensitivity 72-channel (in vivo imaging) and 64-channel (ex vivo imaging) receive coils were integrated. Results: The Connectome 2.0 scanner achieves Gmax=500 mT/m and SRmax=600 T/m/s, demonstrates 2x improved SNR for diffusion MRI over Connectome 1.0, and enables high-resolution tractography. Impact: The Connectome 2.0 scanner will allow the exploration of new microstructure properties and connectional anatomy in the living human brain with unprecedented spatial and diffusion resolution.
Long-durational diffusion weighted MRI scans with high gradient strength and high slew rate experiences in addition to the generally low signal-to-noise-ratio several problems, such as image artifacts due to eddy currents and the gradual increase of the sample temperature. Combining a high-density anatomically shaped receive coil with field monitoring and temperature control can overcome these limitations. Therefore, we designed and constructed a 64-channel whole human ex vivo brain Rx coil with integrated field monitoring and temperature control system. First SNR measurements confirm the receive capability with high SNR.
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.
Abstract Purpose To investigate whether spatiotemporal magnetic field monitoring can correct pronounced eddy current‐induced artifacts incurred by strong diffusion‐sensitizing gradients up to 300 mT/m used in high b‐value diffusion‐weighted (DW) EPI. Methods A dynamic field camera equipped with 16 1 H NMR field probes was first used to characterize field perturbations caused by residual eddy currents from diffusion gradients waveforms in a 3D multi‐shot EPI sequence on a 3T Connectom scanner for different gradient strengths (up to 300 mT/m), diffusion directions, and shots. The efficacy of dynamic field monitoring‐based image reconstruction was demonstrated on high‐gradient strength, submillimeter resolution whole‐brain ex vivo diffusion MRI. A 3D multi‐shot image reconstruction framework was developed that incorporated the nonlinear phase evolution measured with the dynamic field camera. Results Phase perturbations in the readout induced by residual eddy currents from strong diffusion gradients are highly nonlinear in space and time, vary among diffusion directions, and interfere significantly with the image encoding gradients, changing the k‐space trajectory. During the readout, phase modulations between odd and even EPI echoes become non‐static and diffusion encoding direction‐dependent. Superior reduction of ghosting and geometric distortion was achieved with dynamic field monitoring compared to ghosting reduction approaches such as navigator‐ and structured low‐rank‐based methods or MUSE followed by image‐based distortion correction with the FSL tool “eddy.” Conclusion Strong eddy current artifacts characteristic of high‐gradient strength DW‐EPI can be well corrected with dynamic field monitoring‐based image reconstruction.
The first phase of the Human Connectome Project pioneered advances in MRI technology for mapping the macroscopic structural connections of the living human brain through the engineering of a whole-body human MRI scanner equipped with maximum gradient strength of 300 mT/m, the highest ever achieved for human imaging. While this instrument has made important contributions to the understanding of macroscale connectional topology, it has also demonstrated the potential of dedicated high-gradient performance scanners to provide unparalleled in vivo assessment of neural tissue microstructure. Building on the initial groundwork laid by the original Connectome scanner, we have now embarked on an international, multi-site effort to build the next-generation human 3T Connectome scanner (Connectome 2.0) optimized for the study of neural tissue microstructure and connectional anatomy across multiple length scales. In order to maximize the resolution of this in vivo microscope for studies of the living human brain, we will push the diffusion resolution limit to unprecedented levels by (1) nearly doubling the current maximum gradient strength from 300 mT/m to 500 mT/m and tripling the maximum slew rate from 200 T/m/s to 600 T/m/s through the design of a one-of-a-kind head gradient coil optimized to minimize peripheral nerve stimulation; (2) developing high-sensitivity multi-channel radiofrequency receive coils for in vivo and ex vivo human brain imaging; (3) incorporating dynamic field monitoring to minimize image distortions and artifacts; (4) developing new pulse sequences to integrate the strongest diffusion encoding and highest spatial resolution ever achieved in the living human brain; and (5) calibrating the measurements obtained from this next-generation instrument through systematic validation of diffusion microstructural metrics in high-fidelity phantoms and ex vivo brain tissue at progressively finer scales with accompanying diffusion simulations in histology-based micro-geometries. We envision creating the ultimate diffusion MRI instrument capable of capturing the complex multi-scale organization of the living human brain - from the microscopic scale needed to probe cellular geometry, heterogeneity and plasticity, to the mesoscopic scale for quantifying the distinctions in cortical structure and connectivity that define cyto- and myeloarchitectonic boundaries, to improvements in estimates of macroscopic connectivity.
Motivation: Diffusion MRI utilizing ultra-high performance gradients for high b-value in vivo brain images still suffers from low SNR and increased eddy currents artifacts. Goal(s): To construct a high-density coil array with an integrated field monitoring system. To enhance SNR and parallel image encoding, while capturing 3rd-order field dynamics. Approach: Utilizing simulations, 3D printing technology, and radiofrequency electronics to construct a 72-channel head coil and incorporate a field monitoring system. Optimization of the combined system to operate jointly in a space-constraint MRI gradient coil environment. Results: High-resolution, high b-value diffusion in vivo imaging with greatly minimized image artefacts. Impact: The constructed 72-channel head coil along with the new Connectome 2.0 scanner will enable the investigation of new microstructure features and connectivity in the living human brain.
Quantitative susceptibility mapping (QSM) with multiple object orientations is typically limited by an appropriate examination time that is appropriate for human subjects and the small range of possible rotation angles within an MRI head coil. Ex vivo QSM can overcome these limitations supporting long acquisition times and the ability to rotate the ex vivo specimen to any position. Therefore, a 3T 26-channel ex-vivo brain array coil was developed for fixed chimpanzee brains. The coil was characterized with bench and image metrics and provides increased reception sensitivity, making it well-suited for high-resolution QSM ex vivo MRI studies.
Motivation: Ex-vivo brain DWI with long scan times poses the problem of temperature-related drift of diffusion measurement results. Goal(s): The construction of a 64-channel ex-vivo brain coil with time-course temperature stabilization for obtaining accurate DWI measurements. Approach: Combining a newly developed high-density ex-vivo brain coil array with a forced-air cooling system and a multi-channel temperature recording. Results: The air circulation system was able to maintain the ambient temperature of the coil and, thus, stabilizing the mean diffusivity values over repeated lengthy scans. Without cooling, a drift of the mean diffusivity was overserved, peaking at a 35%-offset at approximately 11 hours. Impact: Temperature-stabilized post-mortem brain samples for diffusion MRI in combination with a dedicated large channel count ex-vivo brain coil improves image quality in terms of achievable SNR and greatly reduced temperature-induced diffusivity shifts.
