Amplification of the effects of magnetization exchange by ³¹P band inversion for measuring adenosine triphosphate synthesis rates in human skeletal muscle

Purpose The goal of this study was to amplify the effects of magnetization exchange between γ-adenosine triphosphate (ATP) and inorganic phosphate (Pi) for evaluation of ATP synthesis rates in human skeletal muscle. Methods The strategy works by simultaneously inverting the ³¹P resonances of phosphocreatine (PCR) and ATP using a wide bandwidth, adiabatic inversion radiofrequency pulse followed by observing dynamic changes in intensity of the noninverted Pi signal versus the delay time between the inversion and observation pulses. This band inversion technique significantly delays recovery of γ-ATP magnetization; consequently, the exchange reaction, Pi 虠 γ-ATP, is readily detected and easily analyzed. Results The ATP synthesis rate measured from high-quality spectral data using this method was 0.073 ± 0.011 s^-1 in resting human skeletal muscle (N = 10). The T₁ of Pi was 6.93 ± 1.90 s, consistent with the intrinsic T₁ of Pi at this field. The apparent T₁ of γ-ATP was 4.07 ± 0.32 s, about two-fold longer than its intrinsic T₁ due to storage of magnetization in PCR. Conclusion Band inversion provides an effective method to amplify the effects of magnetization transfer between γ-ATP and Pi. The resulting data can be easily analyzed to obtain the ATP synthesis rate using a two-site exchange model. Magn Reson Med 74:1505-1514, 2015.