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
To develop an improved method to measure the 31P nuclear Overhauser effect (NOE) for evaluation of adenosine triphosphate (ATP) dynamics in terms of correlation time (τc), and contribution of dipole-dipole (DD) and chemical shift anisotropy (CSA) mechanisms to T1 relaxation of ATP in human brain.Methods:
The NOE of ATP in human brain was evaluated by monitoring changes in magnetization in the β-ATP signal following a band inversion of all downfield 31P 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 β-ATP signal, was 24 ± 2% upon band inversion of γ- and α-ATP, which is 2–3-fold higher than achievable by frequency-selective inversion of either γ- or α-ATP. The rate of 31P–31P cross relaxation (0.21 ± 0.02 s−1) led to a τc value of (9.1 ± 0.8) × 10−8 s for ATP in human brain. The T1 relaxation of β-ATP is dominated by CSA over the DD mechanism (60%: 40%).Conclusions:
The band inversion method proved effective in amplifying 31P NOE, and thus facilitating ATP τc and relaxation measurements. This technique renders ATP a potentially useful reporter molecule for cellular environments. Magn Reson Med, 2016. © 2016 Wiley Periodicals, Inc.