Rapid and simultaneous measurement of phosphorus metabolite pool size ratio and reaction kinetics of enzymes in vivo

Purpose: The metabolites phosphocreatine (PCr), adenosine triphosphate (ATP), and in-organic phosphate (Pi) are biochemically coupled. Their pool sizes, assessed by their magnetization ratios, have been extensively studied and reflect bioenergetics status in vivo. However, most such studies have ignored chemical exchange and T₁ relaxation effects. In this work, we aimed to extend the T₁ ^nom method to simultaneously quantify the reaction rate constants as well as phosphorus metabolite pool size ratios under partially relaxed conditions. Materials and Methods: Modified Bloch-McConnell equations were used to simulate the effects of chemical exchanges on T₁ relaxation times and magnetization ratios among PCr, γ-ATP, and Pi. The T₁ ^nom method with iteration approach was used to measure both reaction constants and metabolite pool size ratios. To validate our method, in vivo data from rat brains (N = 8) at 9.4 Tesla were acquired under two conditions, i.e., approximately full relaxation (TR = 9 s) and partial relaxation (TR = 3 s). We compared metabolite pool size ratios and reaction constants before and after correcting the chemical exchange and T₁ relaxation effects. Results: There were significant errors in underestimation of PCr/γATP by 12 % (P = 0.03) and overestimation of ATP/Pi ratios by 14 % (P = 0.04) when not considering chemical exchange effects. These errors were minimized using our iteration approach, resulting in no significant differences (PCr/γATP, P = 0.47; ATP/Pi, P = 0.81) in metabolite pool size ratios and reaction constants between the two measurements (i.e., short versus long TR conditions). Conclusion: Our method can facilitate broad biomedical applications of ³¹P magnetization saturation transfer spectroscopy, requiring high temporal and/or spatial resolution for assessment of altered bioenergetics. Level of Evidence: 2. Technical Efficacy: Stage 1. J. Magn. Reson. Imaging 2018;47:210–221.