Elsevier

Journal of Magnetic Resonance

Volume 255, June 2015, Pages 100-105
Journal of Magnetic Resonance

An ultra-low cost NMR device with arbitrary pulse programming

https://doi.org/10.1016/j.jmr.2015.02.011Get rights and content

Highlights

  • An NMR spectrometer is based on an ultra-low cost electronics board with FPGA.

  • Few auxiliary components are needed and can be assembled in a typical NMR lab.

  • Arbitrary pulse sequences with accurate timing are generated in the MHz range.

Abstract

Ultra-low cost, general purpose electronics boards featuring microprocessors or field programmable gate arrays (FPGA) are reaching capabilities sufficient for direct implementation of NMR spectrometers. We demonstrate a spectrometer based on such a board, implemented with a minimal need for the addition of custom electronics and external components. This feature allows such a spectrometer to be readily implemented using typical knowledge present in an NMR laboratory. With FPGA technology, digital tasks are performed with precise timing, without the limitation of predetermined hardware function. In this case, the FPGA is used for programming of arbitrarily timed pulse sequence events, and to digitally generate required frequencies. Data acquired from a 0.53 T permanent magnet serves as a demonstration of the flexibility of pulse programming for diverse experiments. Pulse sequences applied include a spin–lattice relaxation measurement using a pulse train with small-flip angle pulses, and a Carr–Purcell–Meiboom–Gill experiment with phase cycle. Mixing of NMR signals with a digitally generated, 4-step phase-cycled reference frequency is further implemented to achieve sequential quadrature detection. The flexibility in hardware implementation permits tailoring this type of spectrometer for applications such as relaxometry, polarimetry, diffusometry or NMR based magnetometry.

Introduction

Low cost nuclear magnetic resonance (NMR) devices are experiencing considerable publicity, both in an industrial and academic setting. A number of commercial systems are available, and in addition, published designs are readily found in the literature. These devices cater to opposing trends, for emerging routine applications where simplicity in operation is a must, as well as for ancillary characterizations in NMR or MRI experiments that are growing more and more complex. Examples for the first category include industrial quality control, for example the determination of fat or water content using spin relaxation measurements [1], [2], [3], [4], [5]. They further include the characterization of objects containing large surfaces [6], the determination of oil or water content in rock surrounding a borehole [7], or the detection of cells and biomarkers for biomedical applications [8]. Finally, a multitude of educational applications benefit from low-cost NMR devices, for teaching aspects of instrumentation, NMR theory, or molecular structure determination [9], [10], [11], [12]. In many cases, the achieved ease of use betrays an increasing sophistication of these devices. In the second category, low-cost NMR devices are supportive in the characterization of more elaborate magnetic resonance experiments. An example is the use of a dedicated spectrometer accessory for the determination of magnetic field profiles, which can be used to improve the quality of high resolution magnetic resonance images [13], [14]. Ancillary NMR spectrometers are also used in the growing field of hyperpolarized NMR, for monitoring the spin polarization that is achieved. Optical pumping experiments often include NMR devices at the pumping cell for polarimetry [15], [16]. In dissolution dynamic nuclear polarization, monitoring of NMR signals is desirable both during polarization in the solid state, as well as after dissolution in the liquid state [17].

A range of standalone NMR devices have been developed in view of these varied applications. Many of the recent implementations make use of a microcontroller for flexibility in programming [18], [19], or include parallel processing capabilities of field programmable gate arrays (FPGA) in designs similar to those of software based radios [20], [21], [22], [23], [24]. Such devices have been developed to the level, where they can serve as budget NMR consoles for high-field NMR applications at frequencies of hundreds of megahertz [25].

An increasing number of non-specialized, but pre-fabricated hardware boards that feature microprocessors or FPGA chips are becoming available in the market at ultra-low cost. The use of such boards for NMR applications is interesting, because they can allow the creation of functional NMR devices without the need for large designs of custom electronics. A spectrometer developed based on an arduino board has recently been introduced to acquire signals from earth field NMR [26]. At higher frequencies, in the range of tens to hundreds of megahertz, FPGAs are more suitable than microprocessors, because of their support for parallel processing and increased control of accurate timing. Here, we demonstrate the acquisition of relaxometry data using a spectrometer constructed from a single commercial FPGA board, which already contains required components such as digitizer, clock, memory, programming and communications interface, and for use in NMR only requires the addition of analog front-end circuits. The console is optimized for maximum flexibility by implementing most functions within the FPGA chip. With this simplicity and flexibility, a rapid and application oriented implementation of the NMR spectrometer is possible.

Section snippets

Materials and methods

The console hardware is based on an Altera Cyclone IV field programmable gate array (FPGA) with the DE0-nano evaluation board (75 × 50 × 20 mm, Terasic, Hsinchu City, Taiwan), which contains all digital signal pathways. In the FPGA, custom logic for pulse program generation and data storage is implemented (Fig. 1). Also within the FPGA, this logic is supported by an array of standard blocks performing functions such as frequency generation, communication or memory access.

The NMR pulse program is

Results and discussion

To demonstrate the performance of the FPGA-based NMR console, a set of NMR measurements was carried out, including spin-echo and spin–lattice relaxation experiments. These measurements were carried out using a 25 nL microcoil in a small permanent magnet (see Section 2). A sample of paramagnetically doped H2O was used for the experiments in order to increase repetition rate of the experiment, and at the same time to evaluate the performance of fast averaging. A spin-echo pulse sequence is shown

Conclusions

In summary, an ultra-low cost approach to an NMR spectrometer is presented. The design is based on a commercial FPGA board, which allows for frequency generation and timing accuracy that is sufficient for medium-to-high frequency NMR. An arbitrary pulse programming capability and the ability to address multiple channels from one board increase its flexibility. Through the use of a common, pre-fabricated board containing the FPGA, the design contains a minimum of external circuit components. We

Acknowledgments

This work was partially supported by a subcontract from the Los Alamos National Laboratory’s Laboratory Directed Research and Development (LDRD) program (Project No. 20110166ER).

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