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

Abstract

The low frequency plateau in the frequency response of an untuned micro-resonator permits broadband radio-frequency reception, albeit at the expense of optimal signal-to-noise ratio for a particular nucleus. In this contribution we determine useful figures of merit for broadband micro-coils, and thereby explore the parametric design space towards acceptable simultaneous excitation and reception of a microfluidic sample over a wide frequency band ranging from 13C to 1H, i.e., 125–500 MHz in an 11.74 T magnet. The detector achieves 37% of the performance of a comparably sized, tuned and matched resonator, and a linewidth of 17 ppb using standard magnet shims. The use of broadband detectors circumvents numerous difficulties introduced by multi-resonant RF detector circuits, including sample loading effects on matching, channel isolation, and field distortion.

Any‐nucleus distributed active programmable transmit coil

“…As Mandal et al note for their approach, "it is possible to extend the technique to higher frequencies if smaller coils are used." Although this approach works and is similar to the approach taken by ultra-broadband micro-spiral coils for multinuclear NMR, [16][17][18] too many coils would be needed to scale to human-scale high-field MRI.…”

“…Devices capable of handling higher voltages tend to be slower, and currently, there are no devices capable of directly interfacing to the coil without a resonant circuit at frequencies up to 127.7 MHz ( 1 H 3 T Larmor frequency) in a reasonable way. As Mandal et al note for their approach, “it is possible to extend the technique to higher frequencies if smaller coils are used.” Although this approach works and is similar to the approach taken by ultra‐broadband micro‐spiral coils for multinuclear NMR, 16–18 too many coils would be needed to scale to human‐scale high‐field MRI.…”

“…Using variable circuit elements to tune coils to different nuclei between scans has also been attempted, [13][14][15] but they have limited tuning ranges and may require mechanical changes for better performance. In NMR, where the length scales are small, untuned micro-spirals [16][17][18] and transmission line coils with resistive terminations [19][20][21][22] have been reported, but these would not efficiently scale to macro-scale MRI. Here, we depart altogether from the concept of tuned macro-scale coils and present the Any-nucleus Distributed Active Programmable Transmit Coil (ADAPT Coil), an inexpensive and untuned, yet scalable coil capable of efficiently transmitting at arbitrary frequencies using a unique architecture composed of many high-frequency semiconductor power switches integrated directly into the coil structure.…”

Sample-centred shimming enables independent parallel NMR detection

“…Conventional NMR is primarily conducted in a time-inefficient sequence that can include one or more of the following steps: sample loading, coil tuning and matching, B 0 field shimming to an acceptable B 0 distortion level, and radiofrequency (RF) excitation and reception at the target NMR frequencies, and long recovery time further restrict the speed of acquisition. Altogether the operations are time-consuming and reduce sample throughput.Current approaches from a hardware perspective, to boost up experimental speed and address the low throughput problem in current magnet systems, include automated sample loading 6-8 , multinuclear RF detectors [9][10][11] , and automated tuning and matching systems 12,13 . For instance, an automated flow-through system, integrated with liquid sample sensing, has proven that a large batch of sequential fluidic samples can be handled, each within 2.4 s, with sufficiently high NMR resolution 7 .…”

“…Current approaches from a hardware perspective, to boost up experimental speed and address the low throughput problem in current magnet systems, include automated sample loading 6-8 , multinuclear RF detectors [9][10][11] , and automated tuning and matching systems 12,13 . For instance, an automated flow-through system, integrated with liquid sample sensing, has proven that a large batch of sequential fluidic samples can be handled, each within 2.4 s, with sufficiently high NMR resolution 7 .…”