Triple repetition time saturation transfer (TRiST) ³¹P spectroscopy for measuring human creatine kinase reaction kinetics

Human cardiac phosphorus MR saturation transfer experiments to quantify creatine kinase forward rate constants (k_f) have previously been performed at 1.5 T. Such experiments could benefit from increased signal-to-noise ratio (SNR) and spectral resolution at 3 T. At 1.5 T, the four-angle saturation transfer method was applied with low-angle adiabatic pulses and surface coils. However, low-angle adiabatic pulses are potentially problematic above 1.5 T due to bandwidth limitations, power requirements, power deposition, and intrapulse spin-spin relaxation. For localized metabolite spin-lattice relaxation time (T₁) measurements, a dual repetition time approach with adiabatic half-passage pulses was recently introduced to solve these problems at 3 T. Because the saturation transfer experiment requires a T₁ measurement performed while one reacting moiety is saturated, we adapt the dual repetition time approach to measure k_f using a triple repetition time saturation transfer (TRiST) method. A new pulsed saturation scheme with reduced sensitivity to static magnetic field inhomogeneity and compatibility with cardiac triggering is also presented. TRiST measurements of k_f are validated in human calf muscle against conventional saturation transfer and found to agree within 3%. The first 3-T TRiST measurements of creatine kinase k_f in the human calf (n = 6), chest muscle, and heart (n = 8) are 0.26 ± 0.04 ^s-1, 0.23 ± 0.03 ^s-1, and 0.32 ± 0.07 ^s-1, respectively, consistent with prior 1.5 T values.