On neglecting chemical exchange when correcting in vivo (31)P MRS data for partial saturation: commentary on: "Pitfalls in the measurement of metabolite concentrations using the one-pulse experiment in in Vivo NMR"

This article replies to Spencer et al. (J. Magn. Reson. 149, 251--257, 2001) concerning the degree to which chemical exchange affects partial saturation corrections using saturation factors. Considering the important case of in vivo (31)P NMR, we employ differential analysis to demonstrate a broad range of experimental conditions over which chemical exchange minimally affects saturation factors, and near-optimum signal-to-noise ratio is preserved. The analysis contradicts Spencer et al.'s broad claim that chemical exchange results in a strong dependence of saturation factors upon M(0)'s and T(1) and exchange parameters. For Spencer et al.'s example of a dynamic (31)P NMR experiment in which phosphocreatine varies 20-fold, we show that our strategy of measuring saturation factors at the start and end of the study reduces errors in saturation corrections to 2% for the high-energy phosphates.