Measuring energy diffusion: phosphocreatine in human skeletal muscle.

Introduction The creatine kinase (CK) reaction plays a central role in energy transfer in the myocyte, shuttling high-energy phosphate (HEP) created in the mitochondria, to sites of utilization in the myofibrils [1,2]. At the mitochondria, the reverse reaction produces phosphocreatine (PCr) from adenosine triphosphate (ATP) and creatine (Cr). PCr diffuses to the myofibrils to regenerate ATP from adenosine diphosphate to fuel muscular contraction. The PCr energy transfer shuttle has been widely modeled. Knowledge of the diffusion coefficient of PCr, DPCr, is key, but ill-defined in humans. Localized P MRS is uniquely able to access and quantify PCr and forward CK flux noninvasively [3]. The aim of this study is to develop and optimize a P MRS protocol for measuring DPCr, and to characterize PCr energy transport in human muscle in relation to CK flux. We report DPCr in 8 healthy subjects as a function of diffusion time (tdiff), and that PCr diffusion in muscle is anisotropic. Methods A 3-pulse, stimulated echo sequence was chosen to permit DPCr measurements with long tdiff’s (>>T2) (tdiff =TM+32ms, TM is the mixing time). Diffusion encoding was provided via gradient lobes placed after the first and third RF pulses. Spectra were localized to a plane parallel to a 17-cm/8cm transmit/receive P coil set using depth-resolved surface coil spectroscopy (DRESS) [4], with the first pulse slice-selective (TR/TE= 8000/80ms; bandwidth =500Hz). DPCr was measured at tdiff’s corresponding to TM of 50,100,150,200,400,700, and 1000 ms, by acquiring one spectrum without diffusion gradients, and three spectra with diffusion gradients applied on the x, y, and z axes. The b-value was chosen to optimize precision of the DPCr measurement (1000≤b≤2000 s/mm, b= [γ.G.δ].tdiff, with γ is the gyromagnetic ratio, G is the diffusion gradient strength, and δ is the diffusion gradient duration). Averaging was adjusted to maintain adequate and comparable signal-to-noise ratio for each acquisition. Peak areas were fitted using CFIT [5]. DPCr was obtained in the three directions (Dxx, Dyy, Dzz) and the mean diffusivity Dav was calculated as Dav= (Dxx+Dyy+Dzz)/3. The calf muscles of eight healthy subjects who gave informed consent were scanned on a Philips Achieva 3.0T system.