High-Temporal-Resolution Monitoring of Creatine and Phosphocreatine Dynamics in Skeletal Muscle Using Ultrafast...

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.

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.

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.

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.