Accelerated diffusion and relaxation‐diffusion MRI using time‐division multiplexing EPI.

Purpose: To develop a time‐division multiplexing echo‐planar imaging (TDM‐EPI) sequence for approximately two‐ to threefold acceleration when acquiring joint relaxation‐diffusion MRI data with multiple TEs. Methods: The proposed TDM‐EPI sequence interleaves excitation and data collection for up to 3 separate slices at different TEs and uses echo‐shifting gradients to disentangle the overlapping echo signals during the readout period. By properly arranging the sequence event blocks for each slice and adjusting the echo‐shifting gradients, diffusion‐weighted images from separate slices can be acquired. Therefore, we present 2 variants of the sequence. A single‐TE TDM‐EPI is presented to demonstrate the concept. Next, a multi‐TE TDM‐EPI is presented to highlight the advantages of the TDM approach for relaxation‐diffusion imaging. These sequences were evaluated on a 3 Tesla scanner with a water phantom and in vivo human brain data. Results: The single‐TE TDM‐EPI sequence can simultaneously acquire 2 slices with a maximum b value of 3000 s/mm2 and 2.5 mm isotropic resolution using interleaved readout windows with TE ≈ 78 ms. With the same b value and resolution, the multi‐TE TDM‐EPI sequence can simultaneously acquire 2 or 3 separate slices using interleaved readout sections with shortest TE ≈ 70 ms and ΔTE ≈ 30 ms. Phantom and in vivo experiments have shown that the proposed TDM‐EPI sequences can provide similar image quality and diffusion measures as conventional EPI readouts with multiple echoes but can reduce the overall relaxation‐diffusion protocol scan time by approximately two‐ to threefold. Conclusion: TDM‐EPI is a novel approach to acquire diffusion imaging data at multiple TEs. This enables a significant reduction in acquisition time for relaxation‐diffusion MRI experiments but without compromising image quality and diffusion measurements, thus removing a significant barrier to the adoption of relaxation‐diffusion MRI in clinical research studies of neurological and mental disorders. [ABSTRACT FROM AUTHOR]