Motion‐compensated diffusion encoding gradients for segmented thick‐slab 3D magnetic resonance diffusion‐weighted imaging of the brain

Abstract Background Routine clinical magnetic resonance diffusion‐weighted imaging (DWI) is generally performed with 2D echo planar sequences. A single thick‐slab 3D approach could offer higher signal‐to‐noise ratio and better slice resolution. This has not been adopted due to the difficulty to avoid motion‐induced phase error that interfere with multi‐shot spatial encoding. Purpose To introduce a new approach for 3D brain DWI: rather than relying on navigator echoes for phase correction, moment‐nulled diffusion encoding gradients are used to minimize phase variations at the source. Methods A standard 2D echo planar imaging sequence was modified to incorporate moment‐nulled diffusion encoding gradients and a second phase encoding gradient for spatial multi‐shot encoding along the slice select direction. The single thick‐slab 3D diffusion‐weighted imaging sequence was evaluated with brain scans in healthy volunteers on a 3 Tesla scanner. Results Incorporation of both first and second order moment nulling achieved substantial, albeit not comprehensive, reduction of motion‐related ghosting artifacts. Without such motion compensation or with first order moment nulling only, motion‐related artifacts were consistently more severe. Even though the approach comes with a penalty in echo time—at a diffusion weighting of 1000 s/mm 2 , 119 ms for the moment‐nulled 3D acquisition versus 82 ms for the conventional 2D acquisition—the measured ratio between SNR 3D and SNR 2D for a 92‐slice scan was 0.99. Conclusions This proof‐of‐concept shows that first and second order moment nulling may be a viable avenue for enabling 3D diffusion imaging. At higher slice numbers the SNR 3D is expected to clearly surpass corresponding SNR 2D . However, further investigation into echo time reduction and correction of residual phase variations is needed before the 3D approach is viable for translation into a clinical setting. Specifically, with higher gradient strength shorter echo times can be realized. Moreover, this reduces third and higher order gradient moments and associated residual phase shifts.