Abstract
The rate of intramuscular phosphocreatine (PCr) depletion and recovery in response to exercise estimated from (31)P MRS is an established measure for oxidative capacity. The creatine CH(2) resonance in (1)H MRS is known to exhibit a similar pattern. So far, repeating the exercise for consecutive (1)H and (31)P experiments posed limitations on the interpretation. Acquiring both datasets in a single time-resolved experiment allows for direct quantitative comparison of creatine-CH(2) and PCr kinetics. This can help answer to what extent creatine-CH(2) 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(2), age = 35.7 ± 9.3 years) or higher BMI (n = 11, BMI = 34.1 ± 3.6 kg/m(2), age = 35.0 ± 6.6 years) were measured on a 7 T MR system and MR-compatible ergometer. Localized (1)H and (31)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(2) signal, while the CH(3) resonance remained stable. Neither the recovery nor exercise-on-kinetics time constants were significantly different when quantified from (1)H or (31)P 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 (1)H and (31)P MRS. Interestingly, though, creatine-CH(2) 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 (1)H and (31)P MRS. The strong correlation between τ(PCr) and τ(Cr-CH₂) and preserved intergroup differences suggests that quantifying Cr-CH₂ by (1)H MRS might, within limitations, serve as a surrogate for the estimation of oxidative capacity via PCr from (31)P MR spectra. The results contribute to the discussion on NMR visibility of myocellular creatine pools.