Correctly accounting for radiofrequency spillover in saturation transfer experiments: application to measurement of the creatine kinase reaction rate in human forearm muscle.

Measurements based on transfer of magnetization have been widely used to calculate reaction rates in systems with slow chemical exchange. In these experiments, correction for direct irradiation of the observed resonance is in general accomplished by replacing the equilibrium magnetization in the Bloch equations by a magnetization obtained in an experiment in which a low power radio-frequency (RF) pulse is placed contralaterally, that is, at an equal distance from the observed resonance as the saturated one but on the opposite side. However, the magnetization observed during contralateral irradiation is affected not only by direct irradiation but also by chemical exchange. We demonstrate here that including this effect leads to a substantial improvement in the accuracy of reaction rate determination in steady-state saturation transfer experiments and, if incomplete saturation is present, also in transient saturation transfer experiments. In steady-state saturation transfer experiments performed in vivo at 1.9 T, correct interpretation of the contralateral saturation transfer experiment resulted in a correction of the creatine kinase reaction rate by approximately 11% on average.