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

Aktham Asfour

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DOI: 10.4236/jbise.2010.311143                

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.

Keywords

NMR; Very-Low Field; Digital System; 1H; Hyperpolarized 129Xe; Absolute Polarization; Brain Perfusion Measurements

Conflicts of Interest

The authors declare no conflicts of interest.

References

[1]	Duhamel, G., et al. (2001). Xenon-129 MR imaging and spectroscopy of rat brainusing arterial delivery of hyperpolarized xenon in alipid emulsion. Magnetic Resonance in Medicine (MRM), 46, 208-212.
[2]	Duhamel, G., et al. (2002) Global and regional cerebral blood flow measurements using NMR of injected hyperpolarized xenon-129. Academic Radiology, 9(2), 498- 500.
[3]	Kilian, W., et al. (2004) Dynamic NMR spectroscopy of hyperpolarized 129Xe in human brain analyzed by an uptake model. Magnetic Resonance in Medicine (MRM), 51, 843-847.
[4]	Venkatesh, A.K., et al. (2001) Using dynamic hyperpolarized xenon MR to measure brain perfusion, Proceedings of the International Society of Magnetic Resonance in Medicine, 9, 951.
[5]	Swanson, S.D., et al. (1997) Brain MRI with laser- polarized 129Xe. Magnetic Resonance in Medicine (MRM), 38, 695-698.
[6]	Asfour, A. (2010) Design and Development of a New Dedicated RF Sensor for the MRI of the Rat Brain, Jour- nal of Biomedical Science and Engineering (JBISE), 3, 167-180.
[7]	Happer, W., et al. (1984) Polarization of the nuclear spins of noble-gas atoms by spin exchange with optically pumped alkali-metal atoms. Physical Review A, 29, 3092-3110.
[8]	Raoof, K. Asfour, A. and Fournier, J.-M. (2002) A complete digital magnetic resonance imaging (MRI) system at low magnetic field (0.1 T). Proceedings of the IEEE Instrumentation and Measurement Technology Conference IMTC 2002, Anchorage, Alaska, 20-23 May 2002.
[9]	Shen J., et al. (2005) Home-built magnetic resonance imaging system (0.3 T) with a complete digital spectrometer, Review of Scientific Instruments, 76, 105101- 105108.
[10]	Michal, C.A., Broughton, K. and Hansen, E. (2002) A high performance digital receiver for home-built nuclear magnetic resonance spectrometers. Review of Scientific Instruments, 73(2), 453-458.
[11]	Saam, B.T. and Conradi, M.S. (1998) Low frequency NMR polarimeter for hyperpolarized gases. Journal of Magnetic Resonance (JMR), 134, 67-71.
[12]	Pobell, F. (1991) Matter and methods at low temperatures. Spring, Berlin.