3D MERMAID: 3D Multi-shot enhanced recovery motion artifact insensitive diffusion for submillimeter, multi-shell, and SNR-efficient diffusion imaging.

Abstract

Purpose: To enhance SNR per unit time of diffusion MRI to enable high spatial resolution and extensive q-sampling in a feasible scan time on clinical scanners.

Methods: 3D multi-shot enhanced recovery motion-insensitive diffusion (MERMAID) consists of a whole brain nonselective 3D multi-shot spin-echo sequence with an inversion pulse immediately before the excitation pulse to enhance the recovery of longitudinal magnetization. The excitation flip angle is reduced to the Ernst angle. The sequence includes a trajectory using radially batched internal navigator echoes (TURBINE) readout, where a 3D projection of the FOV is acquired at a different radial angle in every shot. An image-based phase-correction method combined with compressed sensing image reconstruction was developed to correct phase errors between shots. The performance of the 3D MERMAID sequence was investigated using Bloch simulations as well as phantom and human scans at 3 T and then compared to a typical multi-slice 2D spin-echo sequence.

Results: Improvements in SNR per unit time of 70%-240% were observed in phantom and human scans when using 3D MERMAID compared to a single-slice 2D spin-echo sequence. This SNR per unit time improvement allowed scans to be acquired at a nominal isotropic resolution of 0.74 mm and a total of 112 directions across four shells (b = 150, 300, 1000, 2000 s/mm²) in 37 min on a clinical scanner.

Conclusion: The 3D MERMAID sequence was shown to significantly improve SNR per unit time compared to multi-slice 2D and 3D diffusion sequences. This SNR improvement allows for shorter scan times and higher spatial and angular resolutions on clinical scanners.