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

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Abstract

Purpose

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 77:1409–1418, 2017. © 2016 International Society for Magnetic Resonance in Medicine

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