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-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.
Highlights
Diffusion MRI is a powerful, non-invasive technique for imaging axonal orientations as well as characterizing white and gray matter microstructure (Conturo et al, 1999; Lagana et al, 2010; McNab et al, 2009; Mori and Zhang, 2006; Okano and Mitra, 2015)
The basic premise of Diffusion MRI (dMRI) in the human brain is that the diffusion of water molecules in white matter is anisotropic, and that its preferential direction is aligned with the orientation of the underlying fibers (Mori and Zhang, 2006)
The geometrical decoupling of nearest neighbors was S21 measured with an average value of −16 dB and ranged from −14 dB to −18 dB
Summary
Diffusion MRI (dMRI) is a powerful, non-invasive technique for imaging axonal orientations as well as characterizing white and gray matter microstructure (Conturo et al, 1999; Lagana et al, 2010; McNab et al, 2009; Mori and Zhang, 2006; Okano and Mitra, 2015). The basic premise of dMRI in the human brain is that the diffusion of water molecules in white matter is anisotropic, and that its preferential direction is aligned with the orientation of the underlying fibers (Mori and Zhang, 2006). Advanced dMRI sampling schemes may require images to be acquired with multiple b-values. Satisfying all these requirements would lead to acquisition times that are prohibitive for in vivo imaging in the absence of any image acceleration. Motion artifacts, which are exacerbated by long acquisitions, and distortions near tissue-air interfaces further degrade the effective resolution that is achievable in vivo
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