A no-tune no-match wideband probe for nuclear quadrupole resonance spectroscopy in the VHF range

Abstract

Nuclear quadrupole resonance (NQR) spectroscopy is a method for the characterization of chemical compounds containing so-called quadrupolar nuclei. Similar to nuclear magnetic resonance (NMR), the sample under investigation is irradiated with strong radiofrequency (RF) pulses, which stimulate the emission of weak RF signals from the quadrupolar nuclei. The signals are then amplified and Fourier transformed so as to obtain a spectrum. In principle, narrowband NQR spectra can be measured with NMR spectrometers. However, pure NQR signals require the absence of a static magnetic field and several special applications require the characterization of a substance over a large bandwidth, e.g. 50–100% of the central frequency, which is hardly possible with standard NMR equipment. Dedicated zero-field NQR equipment is not widespread and current concepts employ resonating probes which are tuned and matched over a wide range by using mechanical capacitors driven by stepper motors. While providing the highest signal to noise ratio (SNR) such probes are slow in operation and can only be operated from dedicated NMR consoles. We developed a low-cost NQR wideband probe without tuning and matching for applications in the very high frequency (VHF) range below 300 MHz. The probe coil was realized as part of a reactive network which approximates an exponential transmission line. The input reflection coefficient of the two developed prototype probe coils is ≤ 20 dB between 90–145 MHz and 74.5–99.5 MHz, respectively. Two wideband NQR spectra of published test substances were acquired with an SNR of better than 20 dB after sufficient averaging. The measured signals and the SNR correspond very well to the theoretically expected values and demonstrate the feasibility of the method. Because there is no need for tuning and matching, our probes can be operated easily from any available NMR console.

The MR Cap: A single‐sided MRI system designed for potential point‐of‐care limited field‐of‐view brain imaging

“…Several approaches have been proposed for effectively creating a coil with a more uniform frequency response than the typical simple LC circuit without using a series resistor, as we did. These include quasi‐transmission line coils, coupled resonant structures, use of a low‐impedance preamplifier, and inductively coupled negative feedback mechanisms . However, it must be verified that any employed Q‐broadening technique not only creates a wide‐bandwidth receive chain, but actually does so in a lossless way and thus improves the SNR across that bandwidth.…”

Untuned broadband spiral micro-coils achieve sensitive multi-nuclear NMR TX/RX from microfluidic samples

“…measuring 1 H and 13 C at 11.7 T, central frequency is 315 MHz and bandwidth is 375 MHz). A widely studied subcategory of wideband detectors are delay and transmission lines with lumped and distributed elements, respectively 31 – 34 . In this case, the detector is a two-port element, in a form of - or T-network of inductors and capacitors, and is terminated to its characteristic impedance at both ports.…”

Untuned broadband spiral micro-coils achieve sensitive multi-nuclear NMR TX/RX from microfluidic samples

“…While the above-mentioned topologies rely on resonating detection schemes, several alternative approaches have considered the challenge of multi-nuclear NMR detection in a continuous, wide bandwidth [31][32][33][34][35][36] . These coils offer a detection bandwidth comparable to the central frequency of the detection band (e.g.…”

“…measuring 1 H and 13 C at 11.7 T, central frequency is 315 MHz and bandwidth is 375 MHz). A widely studied subcategory of wideband detectors are delay and transmission lines with lumped and distributed elements, respectively [31][32][33][34] . In this case, the detector is a two-port element, in a form of π-or T-network of inductors and capacitors, and is terminated to its characteristic impedance at both ports.…”