Improved reproducibility of diffusion MRI of the human brain with a four‐way blip‐up and down phase‐encoding acquisition approach
M. Okan Irfanoglu, Neda Sadeghi, Joelle Sarlls, Carlo Pierpaoli
Abstract
Purpose
To assess the effects of blip‐up and ‐down echo planar imaging (EPI) acquisition designs, with different choices of phase‐encoding directions (PEDs) on the reproducibility of diffusion MRI (dMRI)‐derived metrics in the human brain.
Methods
Diffusion MRI data in seven subjects were acquired five times, each with five different protocols. The base design included 64 diffusion directions acquired with anterior‐posterior (AP) PED, the first and second protocols added reverse phase‐encoded 
posterior‐anterior (PA) PED images. The third one included 32 directions all with PED acquisitions with opposite polarity (AP and PA). The fourth protocol, also with 32 unique directions used four PEDs (AP, PA, right‐left (RL), and left‐right (LR)). The scan time was virtually identical for all protocols. The variability of diffusion MRI metrics for each subject and each protocol was computed across the different sessions.
Results
The highest reproducibility for all dMRI metrics was obtained with protocol four (AP/PA‐RL/LR, ie, four‐way PED). Protocols that used only for distortion correction, which are the most widely used designs, had the lowest reproducibility.
Conclusions
An acquisition design with four PEDs, including all DWIs in addition to images should be used to achieve high reproducibility in diffusion MRI studies.