Time-Efficient (Seg-)EPI Dixon Abdominal APT/Cest MRI with Distortion-Quantified Water-Only Mapping Under SAR Constraints
Abstract
Purpose
To quantify how SAR-constrained saturation design, Dixon water–fat separation, and distortion/motion correction affect quantitative abdominal APT/CEST using a time-efficient (seg-)EPI acquisition, and to define a robust operating range for water-only APT mapping.
Methods
A dual-echo (seg-)EPI Dixon-CEST sequence was implemented to enable water-only Z-spectrum quantification. Respiratory motion was mitigated using gating with inter-offset registration. Geometric distortion was corrected using field-map unwarping/topup and quantified in millimeters relative to a distortion-insensitive reference. Saturation parameters were evaluated under Normal Operating Mode whole-body SAR constraints (≤2.0 W/kg) across B1rms 1–3 μT and total saturation time 1–3 s, centered at B1rms 2.0 μT and Tsat 2.0 s. A compact 24-offset sampling scheme emphasized amide (~3.5 ppm), rNOE (~−3.5 ppm), and interleaved B0 references. Quantification used Lorentzian amplitude fitting (optionally multi-pool fitting) with B0 correction via WASSR or interleaved references. Validation comprised Bloch–McConnell simulation plus geometric and fat-mimic/protein phantoms.
Results
EPI-based CEST supports a target scan time of ~3 minutes for 24 offsets including overhead. Distortion correction motivates a primary endpoint of reducing maximum geometric error by several millimeters (e.g., ~9 mm to ~4 mm) while preserving spatial fidelity for registration. Dixon water-only processing motivates a target reduction of fat-related APT bias on the order of ~0.6% in fat-mimic settings and improved repeatability across repeated acquisitions. Expected abdominal APT magnitude is several percent (~4–6%), providing a reference scale for sensitivity, uncertainty, and repeatability metrics. The planned analysis will report the SAR-feasible operating region that balances APT sensitivity with robustness to B0/B1 variations.
Conclusion
Fast Dixon water-only (seg-)EPI APT/CEST with distortion quantified in millimeters and SAR-limited saturation selection provides physics-based endpoints for robust abdominal quantitative mapping and a translation-ready framework for protocol optimization.