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 ³¹P nuclear Overhauser effect (NOE) for evaluation of adenosine triphosphate (ATP) dynamics in terms of correlation time (τ_c), 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 τ_c 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 τ_c and relaxation measurements. This technique renders ATP a potentially useful reporter molecule for cellular environments. Magn Reson Med 77:1409–1418, 2017.