¹H and ³¹P MRS Interleaved With High Time Resolution Reveals Closely Matching Creatine CH₂ and PCr Dynamics During...

The rate of intramuscular phosphocreatine (PCr) depletion and recovery in response to exercise estimated from ³¹P MRS is an established measure for oxidative capacity. The creatine CH₂ resonance in ¹H MRS is known to exhibit a similar pattern. So far, repeating the exercise for consecutive ¹H and ³¹P experiments posed limitations on the interpretation. Acquiring both datasets in a single time-resolved experiment allows for direct quantitative comparison of creatine-CH₂ and PCr kinetics. This can help answer to what extent creatine-CH₂ 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/m², age = 35.7 ± 9.3 years) or higher BMI (n = 11, BMI = 34.1 ± 3.6 kg/m², age = 35.0 ± 6.6 years) were measured on a 7 T MR system and MR-compatible ergometer. Localized ¹H and ³¹P 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-CH₂ signal, while the CH₃ resonance remained stable. Neither the recovery nor exercise-on-kinetics time constants were significantly different when quantified from ¹H or ³¹P MR spectra in each group (recovery, lower BMI: τ_PCr-recovery = 35 ± 12 s vs. τ_{Cr-CH2-recovery} = 36 ± 11 s, higher BMI: τ_PCrrecovery = 65 ± 30s vs. τ_{Cr-CH2-recovery} = 60 ± 11 s, and exercise-on, lower BMI: τ_{PCr-on-kinetics} = 39 ± 14 s vs. τ_{Cr-CH2-on-kinetics} = 38 ± 15 s, higher BMI: τ_{PCr-on-kinetics} = 77 ± 53 s vs. τ_{Cr-CH2-on-kinetics} = 70 ± 53 s). Significantly different time constants between the groups distinguished by BMI were detected, likewise with ¹H and ³¹P MRS. Interestingly, though, creatine-CH₂ 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 ¹H and ³¹P MRS. The strong correlation between τ_PCr and τ_Cr-CH₂ and preserved intergroup differences suggests that quantifying Cr-CH₂ by ¹H MRS might, within limitations, serve as a surrogate for the estimation of oxidative capacity via PCr from ³¹P MR spectra. The results contribute to the discussion on NMR visibility of myocellular creatine pools.