Ring Waveguide Magneto-Inductive Detector

Lead Research Organisation: Imperial College London
Department Name: Electrical and Electronic Engineering

Abstract

The aim of this project is to develop an ultra-high sensitivity detection system for near-field dipolar radiation, for example as emitted by proton spin reversal during magnetic resonance imaging. Radiation from a central dipolar source will be synchronously coupled to a magneto-inductive wave propagating in a circular arrangement of coupled L-C loop resonators. Magneto-inductive waves are a new type of slow wave with easily controllable dispersion characteristics. The loop resonators themselves will provide signal amplification through their high Q-factor and coupling. Two techniques will be investigated to increase signals still further: parametric amplification using a second set of non-linear coupled resonators, and the use of cryogenic resonator elements. The application of the detection system to magnetic resonance imaging and spectroscopy and the location of RF signal sources in interventional procedures will be studied.

Publications

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Solymar L (2006) Rotational resonance of magnetoinductive waves: Basic concept and application to nuclear magnetic resonance in Journal of Applied Physics

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R Syms (2007) Parametrically amplified magneto-inductive ring resonators

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R Syms (2008) Flexible magneto-inductive resonators and waveguides

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Syms R (2008) Three-frequency parametric amplification in magneto-inductive ring resonators in Metamaterials

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T Floume (2009) A practical parametric magneto-inductive ring detector in Proc. 2nd Int. Cong. on Advanced Electromagnetic Materials in Microwaves and Optics, Pamplona, Spain, September 22-26

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Syms R (2010) Periodic Analysis of MR-Safe Transmission Lines in IEEE Journal of Selected Topics in Quantum Electronics

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Syms R (2017) MAGNETO-INDUCTIVE MAGNETIC RESONANCE IMAGING DUODENOSCOPE in Progress In Electromagnetics Research

 
Description The aim of this project was to develop an ultra-high sensitivity detection system for near-field dipolar radiation, for example as emitted by proton spin reversal in NMR. Radiation from a central dipolar source was to be synchronously coupled to a magneto-inductive wave propagating in a circular arrangement of coupled L-C loop resonators. The loop resonators themselves were to provide signal amplification through their high Q-factor and coupling. Two techniques were to be investigated to increase signals further: parametric amplification, and the use of cryogenic elements. Application of the detection system to magnetic resonance imaging and spectroscopy and the location of RF signal sources were to be studied.

A literature survey carried out at the start of the project highlighted the similarity of the magneto-inductive ring detector to a conventional birdcage RF detector, and allowed important judgements to be made in planning project targets. Firstly, the excellent detection uniformity offered by a birdcage suggested that it would be difficult to use any similar coil arrangement to determine the position of a RF source unambiguously, and this aim was discarded. Secondly, the known reduction in Q-factor caused by loading a cryogenic birdcage coil with tissue implied that there would be little advantage in developing a refrigerated system, since increases in performance in the coil itself would be lost in practical use, and this aim was therefore also abandoned. Instead, efforts were concentrated on the challenging task of demonstrating a parametrically amplified system, which can in principle overcome the loading due to the patient, an extremely important advantage in a detector for MRI signals.

After development of the underlying theory of magneto-inductive ring resonators in collaboration with researchers at Osnabruck University, several passive detection systems of gradually increasing size were demonstrated, using magnetically-coupled L-C resonators based on printed circuit board inductors and surface mount capacitors. These systems included flexible rings with a specially designed link mechanism that could hold the coupling coefficient between adjacent elements constant as the ring was flexed. The shapes and numbers of resonant elements were optimised and high-Q passive systems were demonstrated.

Parametrically amplified systems were then investigated, starting with two-frequency amplifiers and then moving on towards three-frequency amplifiers. In the latter case, a novel configuration based on a coupled pump and signal resonators and uncoupled idlers was demonstrated. Using this approach, signal gains of 27 dB at 63.8 Mhz (the frequency of 1H MRI in a 1.5 T magnetic field) were achieved in the laboratory, in a system with full quadrature detection. The effect of amplification in recovering the Q-factor of a loaded system was clearly demonstrated.

Work on parametric amplification of magnetic resonance signals is on-going, in collaboration with St Mary's Hospital, Paddington. Amplifiers are being trialled in a 1.5T GE Signa clinical scanner. The main difficulty identified is the need to decouple the coil during RF transmission. A number of approaches are being tried including varactor detuning and PIN diode decoupling. The latter has already been successful for fixed coils and varactor-tuned coils. At present amplification obtained using parametric coils is weak and images are noisy, mainly due to pickup on the additional cable runs required. However, good progress is now being made in eliminating noise sources.

Other possible applications of magneto-inductive devices in MRI systems including MR-safe cables for in-vivo internal imaging were also studied, and patient-safe magneto inductive cable for operation at 63.8 MHz frequency was developed, in both hybrid form (using bulk components and co-axial cable) and thin-film form (using entirely printed components).
Exploitation Route More advanced forms of detector have subsequently received funding for further development from the Wellcome Trust
Sectors Healthcare
URL http://www3.imperial.ac.uk/opticalandsemidev/metamaterials/flexiblemagnetoinductiveringmridetector
 
Description The aim of this project was to develop new forms of RF detector for magnetic resonance imaging systems, based on magneto-inductive waveguides. A ring-resonant detector and a magneto-inductive cable were both developed, and imaging trials were carried out at St Mary's Hospital Paddington. A parametrically amplified ring resonant detector was also successfully demonstrated, but is considered less promising due to the limited SNR gain achieved and the complications associated with the technology . The magneto-inductive cable was subsequently been developed into a MR-safe catheter for internal imaging of the biliary ductal system, using funding from the Wellcome Trust. This device is due to undergo early stage trials at Khon Kaen University Hospital in Thailand, where there is an epidemic of cholangiocarcinoma, in 2015.
First Year Of Impact 2005
Sector Healthcare
Impact Types Societal
 
Description Imperial College London 
Organisation Imperial College London
Country United Kingdom 
Sector Academic/University 
Start Year 2006
 
Description Magnex Scientific Ltd 
Organisation Magnex Scientific Ltd
Country United Kingdom 
Sector Private 
Start Year 2006
 
Description Osnabrueck University 
Organisation University of Osnabrück
Country Germany 
Sector Academic/University 
Start Year 2006
 
Description Regensburg University of Applied Science 
Organisation Regensburg University of Applied Science
Country Germany 
Sector Academic/University 
Start Year 2006
 
Description Siemens Magnet Technology 
Organisation Siemens AG
Department Siemens Magnet Technology
Country United Kingdom 
Sector Private 
Start Year 2006