Mitochondrial oxidative phosphorylation capacity in skeletal muscle measured by ultrafast Z ‐spectroscopy ( UFZ ) MRI at 3T

Abstract Purpose To investigate the feasibility of rapid CEST MRI acquisition for evaluating oxidative phosphorylation (OXPHOS) in human skeletal muscle at 3T, utilizing ultrafast Z‐spectroscopy (UFZ) combined with MRI and the Polynomial and Lorentzian line‐shape Fitting (PLOF) technique. Methods UFZ MRI on muscle was evaluated with turbo spin echo (TSE) and 3D EPI readouts. Five healthy subjects performed in‐magnet plantar flexion exercise (PFE) and subsequent changes of amide, PCr, and partial PCr mixed Cr (Cr + ) CEST dynamic signals post‐exercise were enabled by PLOF fitting. PCr/Cr CEST signal was further refined through pH correction by using the ratios between PCr/Cr and amide signals, named PCAR/CAR, respectively. Results UFZ MRI with TSE readout significantly reduces acquisition time, achieving a temporal resolution of <50 s for collecting high‐resolution Z‐spectra. Following PFE, the recovery/decay times (τ) for both PCr and Cr in the gastrocnemius muscle of the calf were notably longer when determined using PCr/Cr CEST compared to those after pH correction with amideCEST, namely = 87.1 ± 15.8 s and = 98.1 ± 20.4 s versus = 32.9 ± 19.7 s and = 43.0 ± 13.0 s, respectively. obtained via 31 P MRS ( = 50.3 ± 6.2 s) closely resemble those obtained from pH‐corrected PCr/Cr CEST signals. Conclusion The outcomes suggest potential of UFZ MRI as a robust tool for non‐invasive assessment of mitochondrial function in skeletal muscles. pH correction is critical for the reliable OXPHOS measurement by CEST.