Increased Sensitivity and Signal-to-Noise Ratio in Diffusion-Weighted MRI using Multi-Echo Acquisitions

Abstract

Post-mortem diffusion MRI (dMRI) enables acquisitions of structural imaging data with otherwise unreachable resolutions -at the expense of longer scanning times. These data are typically acquired using highly segmented image acquisition strategies, thereby resulting in an incomplete signal decay before the MRI encoding continues. Especially in dMRI, with low signal intensities and lengthy contrast encoding, such temporal inefficiency translates into reduced image quality and longer scanning times. This study introduces Multi Echo (ME) acquisitions to dMRI on a human MRI system -a time-efficient approach, which increases SNR (Signal-to-Noise Ratio) and reduces noise bias for dMRI images. The benefit of the introduced ME dMRI method was validated using numerical Monte Carlo simulations and showcased on a post-mortem brain of a wild chimpanzee. The proposed Maximum Likelihood Estimation echo combination results in an optimal SNR without detectable signal bias. The combined strategy comes at a small price in scanning time (here 30% additional) and leads to a substantial SNR increase (here up to 1.9× which is equivalent to 3.6 averages) and a general reduction of the noise bias. Diffusion | dMRI | Multi-Echo | Segmented EPI | Post-Mortem | Relaxometry | Noise | SNR | ChimpanzeeCorrespondence: ceichner @cbs.mpg.de Eichner et al. | bioRχiv | March 4, 2020 | 1-9

A 48-Channel Receive Array Coil for Mesoscopic Diffusion-Weighted MRI of Humanex vivoBrain Imaging on the 3T Connectome Scanner

“…Reducing scan time using parallel imaging techniques is not strictly essential when constraints on acquisition time are lifted for ex vivo examinations. On the other hand, 2D acquisitions are still often used despite their SNR inefficiency per unit time [11,22]. For example, mapping tissue microstructural features such as axon diameter throughout the whole human brain involves measurements at multiple b-values [50], and protocol optimization may be facilitated by 2D scans acquired at resolutions on the order of 0.8 to 1 mm isotropic.…”

“…Connectome Project was the development of human scanners with ultra-high gradients, which allow high b-values to be achieved without loss of SNR [21]. Initial results have already shown the advantages of a 300 mT m −1 gradient system for imaging whole post-mortem human brains at 0.6 mm isotropic resolution [20], or smaller, non-human primate brain samples at 0.8 mm isotropic resolution [22]. Those results were obtained with an in vivo head coil.…”