A novel NMR instrument for the in-situ monitoring of the absolute polarization of laser-polarized 129Xe

Abstract

A new fully digital and home-built NMR (Nuclear Magnetic Resonance) spectrometer working at very-low magnetic field (4.5 mT) is presented. This spectrometer was initially dedicated for the in situ measurement of the absolute polarization of hyperpolarized 129Xe. It allows detection and acquisition of NMR signals of proton (1H) at 190 kHz and of hyperpolarized xenon-129 (HP 129Xe) at 50 kHz. In this new NMR instrument, we replaced as much analog electronics as possible by digital electronic and software. Except for the power amplifier and the preamplifier, the whole system is digital. The transmitter is based on the use of a Direct Digital Synthesizer (DDS) board. The receiving board allows direct digitalization of the NMR signals thanks to an 8-bits analog-to-digital converter (ADC) clocked at 100 MHz. Decimation is preformed to dramatically improve the ADC resolution so as the final achieved effective resolution could be as high as 14-bits at 5 MHz sampling frequency. NMR signals are then digitally downconverted (DDC). Low-pass decimation filtering is applied on the base-band signals (I/Q) to enhance much more the dynamic range. The system requires little hardware. The transmitter and the receiver are controlled using Labview environment. It is a versatile, flexible and easy-to-replicate system. This was actually one of underlying ideas behind this development. Both 1H and hyperpolarized 129Xe NMR signals were successfully acquired. The system is used for the measurement of the absolute polarization of hyperpolarized 129Xe in hyperpolarizing experiments for the brain perfusion measurements. The high degree of flexibility of this new design allows its use for a large palette of other potential applications.