Band Inversion Amplifies 31P-31P Nuclear Overhauser Effects: Relaxation Mechanism and Dynamic Behavior of ATP in the Human Brain by 31P MRS at 7 T

Purpose: To develop an improved method to measure the P-31 nuclear Overhauser effect (NOE) for evaluation of adenosine triphosphate (ATP) dynamics in terms of correlation time (tc), and contribution of dipole-dipole (DD) and chemical shift anisotropy (CSA) mechanisms to T-1 relaxation of ATP in human brain. Methods: The NOE of ATP in human brain was evaluated by monitoring changes in magnetization in the beta-ATP signal following a band inversion of all downfield P-31 resonances. The magnetization changes observed were analyzed using the Bloch-McConnell-Solomon formulation to evaluate the relaxation and motion dynamic parameters that describe interactions of ATP with cellular solids in human brain tissue. Results: The maximal transient NOE, observed as a reduction in the beta-ATP signal, was 24 +/- 2% upon band inversion of gamma and alpha-ATP, which is 2-3-fold higher than achievable by frequency-selective inversion of either gamma-or alpha-ATP. The rate of P-31-P-31 cross relaxation (0.21 +/- 0.02 s(-1)) led to a tau(c) value of (9.1 +/- 0.8) x 10(-8) s for ATP in human brain. The T-1 relaxation of b-ATP is dominated by CSA over the DD mechanism (60%: 40%). Conclusions: The band inversion method proved effective in amplifying P-31 NOE, and thus facilitating ATP tau(c) and relaxation measurements. This technique renders ATP a potentially useful reporter molecule for cellular environments. (C) 2016 International Society for Magnetic Resonance in Medicine