H and P MRS Interleaved With High Time Resolution Reveals Closely Matching Creatine CH₂ and PCr Dynamics During Exercise.

The rate of intramuscular phosphocreatine (PCr) depletion and recovery in response to exercise estimated from P MRS is an established measure for oxidative capacity. The creatine CH resonance in H MRS is known to exhibit a similar pattern. So far, repeating the exercise for consecutive H and P experiments posed limitations on the interpretation. Acquiring both datasets in a single time-resolved experiment allows for direct quantitative comparison of creatine-CH and PCr kinetics. This can help answer to what extent creatine-CH mimics PCr dynamics and provide data on the visibility of myocellular creatine. Twenty-seven volunteers, assigned to a group with lower BMI (n = 16, BMI = 22.1 ± 3.2 kg/m, age = 35.7 ± 9.3 years) or higher BMI (n = 11, BMI = 34.1 ± 3.6 kg/m, age = 35.0 ± 6.6 years) were measured on a 7 T MR system and MR-compatible ergometer. Localized H and P MR spectra were acquired interleaved during a single 5-min submaximal exercise effort and recovery, with 6 s time resolution. Exercise led to reduced creatine-CH signal, while the CH resonance remained stable. Neither the recovery nor exercise-on-kinetics time constants were significantly different when quantified from H or P MR spectra in each group (recovery, lower BMI: τ = 35 ± 12 s vs. τ = 36 ± 11 s, higher BMI: τ = 65 ± 30s vs. τ = 60 ± 11 s, and exercise-on, lower BMI: τ = 39 ± 14 s vs. τ = 38 ± 15 s, higher BMI: τ = 77 ± 53 s vs. τ = 70 ± 53 s). Significantly different time constants between the groups distinguished by BMI were detected, likewise with H and P MRS. Interestingly, though, creatine-CH and PCr depletion differed, correlating positively. Closely matching Cr-CH₂ and PCr kinetics was confirmed for the first time in single time-resolved experiments, using interleaved H and P MRS. The strong correlation between τ and τ and preserved intergroup differences suggests that quantifying Cr-CH₂ by H MRS might, within limitations, serve as a surrogate for the estimation of oxidative capacity via PCr from P MR spectra. The results contribute to the discussion on NMR visibility of myocellular creatine pools.