Flexible Multi-Turn Multi-Gap Coaxial RF Coils: Design Concept and Implementation for Magnetic Resonance Imaging at 3 and 7 Tesla.

Lena Nohava,Jacques Felblinger,Roberta Frass-Kriegl,Elmar Laistler,Andre Kuehne,Sigrun Roat,Jean-Christophe Ginefri,Michael Obermann,Raphaela Czerny

doi:10.1109/tmi.2021.3051390

Abstract

Magnetic resonance has become a backbone of medical imaging but suffers from inherently low sensitivity. This can be alleviated by improved radio frequency (RF) coils. Multi-turn multi-gap coaxial coils (MTMG-CCs) introduced in this work are flexible, form-fitting RF coils extending the concept of the single-turn single-gap CC by introducing multiple cable turns and/or gaps. It is demonstrated that this enables free choice of the coil diameter, and thus, optimizing it for the application to a certain anatomical site, while operating at the self-resonance frequency. An equivalent circuit for MTMG-CCs is modeled to predict their resonance frequency. Possible configurations regarding size, number of turns and gaps, and cable types for different B 0 field strengths are calculated. Standard copper wire loop coils (SCs) and flexible CCs made from commercial coaxial cable were fabricated as receive-only coils for 3 T and transmit/receive coils at 7 T with diameters between 4 and 15 cm. Electromagnetic simulations are used to investigate the currents on MTMG-CCs, and demonstrate comparable specific absorption rate of 7 T CCs and SCs. Signal-to-noise ratio (SNR), transmit efficiency, and active detuning performance of CCs were compared in bench tests and MR experiments. For the form-fitted receive-only CCs at 3 T no significant SNR degradation was found as compared to flat SCs on a balloon phantom. Form-fitted transmit/receive CCs at 7 T showed higher transmit efficiency and SNR. MTMG-CCs can be sized to optimize sensitivity, are flexible and lightweight, and could therefore enable the fabrication of wearable coils with improved patient comfort.

Highlights

I N MAGNETIC resonance imaging (MRI), radio frequency (RF) coils are used to excite the nuclear magnetic moments, and to receive the MR signal at the Larmor frequency which is proportional to the static magnetic field (B0)
1T1G-coaxial coils (CCs) are limited to large coils at low field and small coils at high field
An equivalent circuit is modeled; theoretical solutions for CCs are calculated for five common B0 field strengths and different coil sizes

Summary

Introduction

I N MAGNETIC resonance imaging (MRI), radio frequency (RF) coils are used to excite the nuclear magnetic moments, and to receive the MR signal at the Larmor frequency ( fLarmor) which is proportional to the static magnetic field (B0). RF coils can be operated in receive-only mode if a separate transmit coil is available. At ultrahigh field (UHF, ≥ 7 T) [1]–[3], no such whole-body transmit coils are available. Transmit/receive (Tx/Rx) coils are often employed at UHF. The target field of view (FOV) and penetration depth p determine the optimal coil size to be chosen for a certain biomedical application. A high filling factor with the RF coil placed close to the sample results in less variation in coil performance due to varying loading conditions between different subjects and/or applications. Coils form-fitted to the average anatomy, e.g. for the human head, breast, calf or finger [5]–[8], have already been implemented with rigid

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