Pre-excitation gradients for eddy current nulled convex optimized diffusion encoding (Pre-ENCODE)

Abstract

Purpose: To evaluate the use of pre-excitation gradients for eddy current-nulled convex optimized diffusion encoding (Pre-ENCODE) to mitigate eddy current-induced image distortions in diffusion-weighted MRI (DWI).

Methods: DWI sequences using monopolar (MONO), ENCODE, and Pre-ENCODE were evaluated in terms of the minimum achievable echo time (TE ${}_{\min }$ ) and eddy current-induced image distortions using simulations, phantom experiments, and in vivo DWI in volunteers ( $N=6$ ).

Results: Pre-ENCODE provided a shorter TE ${}_{\min }$ than MONO (71.0 $\pm$ 17.7ms vs. 77.6 $\pm$ 22.9ms) and ENCODE (71.0 $\pm$ 17.7ms vs. 86.2 $\pm$ 14.2ms) in 100 $\%$ of the simulated cases for a commercial 3T MRI system with b-values ranging from 500 to 3000 s/mm ${}^2$ and in-plane spatial resolutions ranging from 1.0 to 3.0mm ${}^2$ . Image distortion was estimated by intravoxel signal variance between diffusion encoding directions near the phantom edges and was significantly lower with Pre-ENCODE than with MONO (10.1 $\%$ vs. 22.7 $\%$ , $p=6^{-5}$ ) and comparable to ENCODE (10.1 $\%$ vs. 10.4 $\%$ , $p=0.12$ ). In vivo measurements of apparent diffusion coefficients were similar in global brain pixels (0.37 [0.28,1.45] $\times 10^{-3}$ mm ${}^2$ /s vs. 0.38 [0.28,1.45] $\times 10^{-3}$ mm ${}^2$ /s, $p=0.25$ ) and increased in edge brain pixels (0.80 [0.17,1.49] $\times 10^{-3}$ mm ${}^2$ /s vs. 0.70 [0.18,1.48] $\times 10^{-3}$ mm ${}^2$ /s, $p=0.02$ ) for MONO compared to Pre-ENCODE.

Conclusion: Pre-ENCODE mitigated eddy current-induced image distortions for diffusion imaging with a shorter TE ${}_{\min }$ than MONO and ENCODE.

Keywords: diffusion; eddy currents; time-optimal.