The Field-Frequency Lock for Fast Field Cycling Magnetic Resonance: From NMR to MRI

G Galuppini,G Ferrante,L Magni

doi:10.3389/fphy.2021.688479

Abstract

Magnetic field stability plays a fundamental role in Nuclear Magnetic Resonance (NMR) and Magnetic Resonance Imaging (MRI) experiments, guaranteeing accuracy and reproducibility of results. While high levels of stabilization can be achieved for standard NMR techniques, this task becomes particularly challenging for Fast Field Cycling (FFC) NMR and MRI, where the main magnetic field is switched to higher or lower levels during the pulse sequence, and field stabilization must be guaranteed within a very short time after switching. Recent works have addressed the problem with rigorous tools from control system theory, proposing a model based approach for the synthesis of magnetic field controllers for FFC-NMR. While an experimental proof of concept has underlined the correctness of the approach for a complete FFC-NMR setup, the application of the novel, model based Field-Frequency Lock (FFL) system to a FFC-MRI scanner requires proper handling of field encoding gradients. Furthermore, the proof of concept work has also stressed how further advances in the hardware and firmware could improve the overall performances of the magnetic field control loop. The main aim of this perspective paper is then discussing the key challenges that arise in the development of the FFL system suitable for a complete MRI scanner, as well as defining possible research directions by means of preliminary, simulated experiments, with the final goal of favoring the development of a novel, model based FFL system for FFC-MRI.

Highlights

Fast Field Cycling (FFC) Nuclear Magnetic resonance (NMR) and Magnetic Resonance Imaging (MRI) are two high-end techniques that exploit the dependence of the spin-lattice relaxation rate R1 1/T1 on the B0 magnetic field experienced by the sample, to highlight information about molecular dynamics otherwise invisible to standard NMR or MRI
The phase Locked Loop (PLL) control scheme is only effective for the compensation of slow magnetic field drifts, such as thermal drift effects, and does not allow for the rapid field stabilization [5] required in the FFC context
2.1.1 Receiver Hold Procedure As previously introduced, the main step allowing to overcome the use of PLLs and improve the FieldFrequency Lock (FFL) performance is the generation of a continuous NMR signal, acting as magnetic field disturbance measure

Summary

Introduction

Fast Field Cycling (FFC) Nuclear Magnetic resonance (NMR) and Magnetic Resonance Imaging (MRI) are two high-end techniques that exploit the dependence of the spin-lattice relaxation rate R1 1/T1 on the B0 magnetic field experienced by the sample, to highlight information about molecular dynamics otherwise invisible to standard NMR or MRI. In standard FFC setups, a magnetic field control loop, based on direct magnetic field measurements, takes care of the tracking of the field reference profile, but may not provide the desired field regulation and disturbance rejection performances during the acquisition phase [3, 4]. The NMR FieldFrequency Lock (FFL) is another common approach to reduce magnetic field oscillations in NMR experiments [5, 6, 9, 10, 11, 12, 13, 14]. The standard implementation of the FFL is the phase Locked Loop (PLL) [8, 10, 18]: the lock signal is first processed to extract its main frequency, which is compared with the reference one to generate an error signal. The control action can be computed and applied to the system only at a relatively low frequency, resulting in long closed-loop settling times and poor high frequency disturbance rejection capabilities [19, 20]

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