# Diffusion-Weighted Imaging of the Macaque Brain Using Diffusion-Prepared Turbo Spin Echo in MRI
###### Abstract  
Magnetic resonance imaging (MRI) integrates a static magnetic field, a time-varying gradient magnetic field at kHz and a radio-frequency (RF) magnetic field for non-invasive and real-time imaging; meanwhile, diffusion MRI (dMRI) pushes a further and closer dimension to the scale of neural fibers through sensitizing the gradient field to recognize water molecular displacement over distances of 1-20 \(\upmu\)m along fibers. Contemporary dMRI approaches face challenges of magnetic field inhomogeneity as well as sequence-associated distortion and signal loss, the common remedies of which are repeated scans and post-reconstruction algorithms. In this study, over an anesthetized macaque with a customized head coil on 3 T MRI, we have proposed and implemented a monopolar diffusion-prepared module for turbo spin echo  
For common clinical uses, a sampling size (voxel size) of the normal imaging paradigm (MR sequence) in MRI human scans (\(\leq\)3 T) of \(\geq\)1 mm in size is considered sufficient to diagnose tissue-based diseases. However, the performance of submillimeter-scale research in non-human primates (NHPs) [4] and human _ex vivo_[5] research reveals more information of mesoscale organization in brain, which obviously is an approach for tracing the root of brain diseases. Submillimeter-scale dMRI can be achieved, but is more difficult with commonly available MRI machines (1.5 T or 3 T) than ultra-high-field MRI (UHF, 7 T and higher). As a compromise, submillimeter-scale dMRI from 3 T acquired by prevailing EPI-based (echo-planar imaging) sequence usually utilizes means of more steps, such as averages of repetition scans, calibration and smooth-phase information and further reconstruction for correcting artifacts [6], which therefore may cost more time but obtain images of lower quality than those from UHF.  
EPI-based dMRI methods (e.g., diffusion-weighted EPI, DW-EPI) are the most popular sequences for diffusion acquisition, with traits of rapid acquisition speed to reduce motion-related artifacts. The diffusion gradients could generate geometric distortion caused by susceptibility-induced off-resonance fields [7] and then exacerbate those distortion induced by the EPI acquisition method and B\({}_{0}\) inhomogeneity [8]; therefore, turbo spin echo (TSE)-based dMRI techniques were introduced, with minimal sensitivity to field inhomogeneity and the capability for high-quality (SNR, contrast, etc.), high-resolution images, albeit with long scan time, high SAR (specific absorption rate), and sensitivity to motion [9]. A monopolar diffusion-prepared module for TSE sequence (DP-TSE) has been proposed as an alternative method to achieve distortion-free and high-resolution diffusion images with improved SNR, and the approach has been evaluated over phantom and human subjects, with promising potential in pushing for high resolutions and minimal subject motion [10]; moreover, quantitative measures on phantoms indicated that the DP-TSE sequence was able to acquire accurate and consistent ADC values with different echo-train-lengths (ETLs). However, we also observed apparent image blurring in human brain imaging due to inevitable head motions caused by respiration, the heartbeat, etc. Therefore, for the present study, our purpose was to evaluate the sequence's performance over anesthetized macaques with their heads fixed to stereotaxic apparatus, and through diffusion-weighted imaging of the macaque brain, we aimed to investigate whether the proposed approach was suitable for NHP brain studies at 1 mm and even finer spatial resolutions.