Simultaneous estimation of T1 and the flip angle in hyperpolarized NMR experiments using acquisition at non-regular time intervals

Abstract

In NMR spectroscopy of the liquid state T1 is typically measured using an inversion recovery pulse sequence; but with hyperpolarized spins use is made of a sequence of multiple small radiofrequency (RF) induced nutations, α. Depending on the values of α and τ, the time interval between the pulses, the estimate of T1 can be artifactually smaller than the real value; so without knowing the value of α the estimate of T1 can be incorrect. Thus, we propose a method that involves a series of pulses with timing governed by a geometric sequence (or in general, any mathematically specified non-uniformly spaced sequence). This approach enables the simultaneous estimation of both the intrinsic T1 value and α. The method was successfully applied to obtain T1=(44.9±0.3)s and α=(4.0±0.2)° (n=3) for a sample of hyperpolarized 13C-urea in solution, matching with the inversion recovery pulse sequence estimate of T1=44±2s using non-hyperpolarized 13C-urea in solution.