A Fast Pre-Polarization Method for NMR in Moving State

NMR measurement in continuously moving state is an essential feature of NMR techniques in highly time-constrained conditions, such as NMR logging [1] , downhole [2] and on-line [3] NMR analysis of flowing fluids . Pre-polarization needs additional magnets; a fine design also reduces NMR sensor's length and improves its mobility. It's also of significant to introduce pre-polarization to shorten the waiting time, then to increase measurement efficiency in these NMR applications.
In this work, a fast three-step pre-polarization method was proposed based on the discussion of theoretical mechanism and design principle of NMR pre-polarization. As shown in Fig.1a, the proposed static B₀ fields for pre-polarization consist of an over-polarization field (B_a), an under-polarization field (B_b) and a field (B_c) for polarization adjustment. By employing a numerical simulation procedure based on Bloch equations, the individual fields in this scheme is optimized and improved under a given movement velocity condition (v=3cm/s). Efficiency and applicability analysis results demonstrated that, this new pre-polarization design and its corresponding optimization method can save the waiting time significantly for a wider T₁ components range. The polarization ratio (M_z/M₀) is 0.9933~1.0040 for T₁ components between 0.0 and 4.5s after a waiting time (t) of 12.49s (Fig.1b). The results provided new insights and supports for moving and scanning NMR sensor and pulse sequences design.

Fig.1 (a) Pre-Polarization scheme; (b) Efficiency for different T1 components