ABSTRACT Purpose To evaluate the feasibility of High‐Temporal‐Resolution (HTR) Ultrafast Z‐spectrum (UFZ) CEST MRI for monitoring the dynamic fluctuations of phosphocreatine (PCr) and creatine (Cr) in human skeletal muscle (SM) at 3 T during and after exercise. Methods HTR UFZ‐CEST MRI of SM was achieved using a 1D spatial and spectral encoding with a 2D turbo spin‐echo readout. Five healthy subjects performed in‐magnet plantar flexion exercise, and dynamic changes in Amide, PCr, and partial PCr‐mixed creatine (Cr + ) CEST signals were quantified using the Polynomial and Lorentzian Line‐shape Fitting (PLOF) method. This approach enables monitoring of PCr/Cr CEST dynamics throughout exercise and recovery. Results Dynamic CEST contrasts with 7.6 s temporal resolution revealed PCr depletion (−76.9% ± 17.7%) and concomitant Cr elevation (469.0% ± 116.3%) during exercise, followed by exponential PCr recovery and Cr decay during recovery. The post‐exercise recovery constants for Cr and PCr CEST were 61.5 ± 45.6 s and 73.3 ± 66.1 s, respectively. Using two‐step Bloch‐McConnell simulation based on the experimental lineshape and CEST changes, we estimated an exchange rate of 150 ± 70 s −1 for CrCEST and 185 ± 105 s −1 for PCrCEST in the resting state. Conclusion These results establish UFZ‐CEST as a rapid, noninvasive method for mapping Cr/PCr dynamics in human SM and provide a foundation for clinical assessment of mitochondrial function.
Ju et al. (2026) studied this question.