Stim-CODE: PNS and CNS Constraint Optimized Diffusion-Encoding for Neuroimaging on 200 mT/m Whole-Body Gradients

Ultra-high-performance whole-body MRI gradients ( $G_{\max }$ $\ge$ 200mT/m, $S_{\max }$ $\ge$ 200 T/m/s) can reduce echo times (TE) in diffusion-weighted imaging (DWI), improving SNR. However, full hardware utilization is limited by cardiac (CNS) and peripheral (PNS) nerve stimulation thresholds. This work demonstrates Stimulation Constraint Optimized diffusion-encoding (Stim-CODE) for neuro diffusion.

Stim-CODE uses the gradient optimization (GrOpt) toolbox to optimally design diffusion-encoding gradient waveforms subject to a CNS or PNS "constraint envelope" based on other gradient events in the sequence. Stim-CODE was evaluated in simulations and in vivo protocols with in-plane resolutions of 1.5 $\times$ 1.5 mm ${}^2$ and 1.0 $\times$ 1.0 mm ${}^2$ across G ${}_{\max }$ = [45, 80, 200] mT/m and $S_{\max }=200$ T/m/s. Healthy volunteers (N = 10) underwent neuro DWI at b = 1000 s/mm ${}^2$ with Monopolar, Stim-CODE- $M_0$ , Bipolar, and Stim-CODE- $M_1$ encoding. SNR, apparent diffusion coefficient (ADC) maps, and ADC uncertainty were quantified. One volunteer underwent DTI with the same encoding schemes acquired at $b$ = [1000, 3000]s/mm ${}^2$ .

Stim-CODE reduced TEs across all $b$ -values, with larger gains for higher resolutions, greater G ${}_{\max }$ , and $M_1$ -nulled waveforms. Phantom and in vivo experiments confirmed equivalent diffusion-weighting with TE savings and SNR improvements. Stim-CODE- $M_1$ had significant SNR gains over Bipolar scans, but some differences in global ADC were observed. DTI metrics were similar across scans, with Stim-CODE showing lower uncertainty.

Stim-CODE maximizes hardware capabilities to achieve time-optimized diffusion-encoding while respecting CNS and PNS, enabling similar diffusion-weighting as conventional approaches with reduced TE and higher SNR.