A pulsed current-mode class-D low-voltage high-bandwidth power amplifier for portable NMR systems.

Low-field NMR has seen growing interest in recent years, especially for portable applications. The lower homogeneity magnets used for portable applications require short RF pulses to ensure enough transmit bandwidth to excite the sample volume and also support short echo periods. Furthermore, the preferred use of a high-Q coil to improve signal-to-noise ratio (SNR) prolongs the pulse transients. Thus, at such low Larmor frequencies, the excitation pulse transients become comparable or longer than the pulse length, such that the transmit bandwidth begins to limit measurement SNR. This paper describes the design of a pulsed current-mode class-D power (PCMCD) transmitter that addresses this issue by generating high power in a tuned sample coil while maintaining short transients, thus resulting in high output bandwidth. The transmitter also uses a charge recycling mechanism to maximize power efficiency for RF train excitation, which also results in faster pulse repetition rate and reduces allowable echo time. Experimental results from a small form-factor PCMCD transmitter are presented. This design generates a peak RF power of 240 W into a 9.16 µH coil at 4 MHz while operating off a single 12 V power supply. NMR measurement results using the transmitter are also described, showing minimum achievable echo time of 70 µs and 25 µs depending on the transmitter mode of operation.

Opening the Blood Brain Barrier with an Electropermanent Magnet System.

Opening the blood brain barrier (BBB) under imaging guidance may be useful for the treatment of many brain disorders. Rapidly applied magnetic fields have the potential to generate electric fields in brain tissue that, if properly timed, may enable safe and effective BBB opening. By tuning magnetic pulses generated by a novel electropermanent magnet (EPM) array, we demonstrate the opening of tight junctions in a BBB model culture in vitro, and show that induced monophasic electrical pulses are more effective than biphasic ones. We confirmed, with in vivo contrast-enhanced MRI, that the BBB can be opened with monophasic pulses. As electropermanent magnets have demonstrated efficacy at tuning B0 fields for magnetic resonance imaging studies, our results suggest the possibility of implementing an EPM-based hybrid theragnostic device that could both image the brain and enhance drug transport across the BBB in a single sitting.

Detecting Dye-Contaminated Vegetables Using Low-Field NMR Relaxometry.

Dyeing vegetables with harmful compounds has become an alarming public health issue over the past few years. Excessive consumption of these dyed vegetables can cause severe health hazards, including cancer. Copper sulfate, malachite green, and Sudan red are some of the non-food-grade dyes widely used on vegetables by untrusted entities in the food supply chain to make them look fresh and vibrant. In this study, the presence and quantity of dye-based adulteration in vegetables are determined by applying 1H-nuclear magnetic resonance (NMR) relaxometry. The proposed technique was validated by treating some vegetables in-house with different dyes and then soaking them in various solvents. The resulting solutions were collected and analyzed using NMR relaxometry. Specifically, the effective transverse relaxation time constant, T2,eff, of each solution was estimated using a Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence. Finally, the estimated time constants (i.e., measured signatures) were compared with a library of existing T2,eff data to detect and quantify the presence of unwanted dyes. The latter consists of data-driven models of transverse decay times for various concentrations of each water-soluble dye. The time required to analyze each sample using the proposed approach is dye-dependent but typically no longer than a few minutes. The analysis results can be used to generate warning flags if the detected dye concentrations violate widely accepted standards for food dyes. The proposed low-cost detection approach can be used in various stages of a produce supply chain, including consumer household.

Analytical models of probe dynamics effects on NMR measurements.

This paper provides a detailed analysis of three common NMR probe circuits (untuned, tuned, and impedance-matched) and studies their effects on multi-pulse experiments, such as those based on the Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence. The magnitude of probe dynamics effects on broadband refocusing pulses are studied as a function of normalized RF bandwidth. Finally, the probe circuit models are integrated with spin dynamics simulations to design hardware-specific RF excitation and refocusing pulses for optimizing user-specified metrics such as signal-to-noise ratio (SNR) in grossly inhomogeneous fields. Preliminary experimental results on untuned probes are also presented.

Electropermanent magnets for variable-field NMR.

In this work, a dynamically tunable B0 field is used to perform variable-field NMR. The system consists of an array of electropermanent AlNiCo-5 magnets whose magnetizations are individually programmed using pulse-power control. This design allows the field strength to be varied for field-dispersion measurements. An ultra-broadband front-end is utilized that maintains efficient power transmission over a broad range of frequencies for robust operation without probe tuning. We perform T1-T2 correlation measurements at various B0 field strengths (0.5-2 MHz) and demonstrate discrimination of different dairy products. We observe variation in the frequency dependence of the proton spin-lattice relaxation for the different products as a function of the degree of protein hydration. This variable-field technique provides a low-cost alternative to fast field-cycling NMR and could open possibilities for novel contrast measurements and spatial encoding in magnetic resonance imaging.

An autonomous, highly portable NMR spectrometer based on a low-cost System-on-Chip (SoC).

This paper describes the development of a portable and self-optimizing NMR spectrometer based on a miniaturized custom analog front-end and a System-on-Chip (SoC)-based digital back-end. The SoC integrates a field-programmable gate array (FPGA) fabric with a hard processor running a Linux operating system, thus enabling fully-autonomous operation without the need for an external computer. In the proposed approach, data captured by the FPGA fabric during regular operation is transported to the hard processor using an integrated on-chip bus for further processing. The processed results are then used to automatically estimate parameter values that optimize a suitable cost function, such as signal-to-noise ratio (SNR) per unit time. Finally, the optimized values of both electrical and NMR-related tuning parameters (e.g., preamplifier gain and frequency response, pulse length and amplitude, operating frequency, etc.) are programmed back into the front-end and back-end hardware. Experimental NMR results from various samples in a ∼0.1 T permanent magnet are presented to verify the operation of the proposed spectrometer. These demonstrate on-board Laplace inversion and automated frequency tuning to compensate for temperature changes. Preliminary 14N NQR results are also presented.

Vascular Graft Pressure-Flow Monitoring Using 3D Printed MWCNT-PDMS Strain Sensors.

Real-time monitoring of arteriovenous graft blood flow would provide early warning of graft failure to permit interventions such as angioplasty or graft replacement to avoid catastrophic failure. We have developed a new type of flexible pulsation sensor (FPS) consisting of a 3D printed elastic cuff wrapped around a graft and thus not in contact with blood. The FPS uses multi-walled carbon nanotubes (MWCNTs) dispersed in polydimethylsiloxane (PDMS) as a piezoresistive sensor layer, which is embedded within structural thixotropic PDMS. These materials were specifically developed to enable sensor additive manufacturing via 3D Bio-plotting, and the resulting strain sensor is more compliant and has a wider maximum strain range than graft materials. Here, we analyze the strain transduction mechanics on a vascular graft and describe the memristive properties of MWCNT-PDMS composites, which may be mitigated using AC biasing. In vitro testing of the FPS on a vascular graft phantom showed a robust, linear sensor output to pulsatile flows (170-650 mL/min) and pressures (62-175 mmHg). The FPS showed an RMS error when measuring pressure and flow of 7.7 mmHg and 29.3 mL/min, with a mean measurement error of 6.5% (pressure) and 8.0% (flow).