A smart switching system to enable automatic tuning and detuning of metamaterial resonators in MRI scans

Abstract

We present a radio-frequency-activated switching system that can automatically detune a metamaterial resonator to enhance magnetic resonance imaging (MRI) performance. Local sensitivity-enhancing metamaterials typically consist of resonant components, which means that the transmitted radio frequency field is spatially inhomogeneous. The switching system shows for the first time that a metamaterial resonator can be detuned during transmission and tuned during reception using a digital circuit. This allows a resonating system to maintain homogeneous transmit field while maintaining an increased receive sensitivity. As a result, sensitivity can be enhanced without changing the system-provided specific absorption rate (SAR) models. The developed digital circuit consists of inductors sensitive to the transmit radio-frequency pulses, along with diodes acting as switches to control the resonance frequency of the resonator. We first test the automatic resonator detuning on-the-bench, and subsequently evaluate it in a 1.5 T MRI scanner using tissue-mimicking phantoms. The scan results demonstrate that the switching mechanism automatically detunes the resonator in transmit mode, while retaining its sensitivity-enhancing properties (tuned to the Larmor frequency) in receive mode. Since it does not require any connection to the MRI console, the switching system can have broad applications and could be adapted for use with other types of MRI scanners and field-enhancing resonators.

Improving Magnetic Resonance Imaging with Smart and Thin Metasurfaces

“…Compared to the existing solutions for detunable MRI MTMs 29,30 , our smart metasurfaces exhibit a larger penetration depth and slower decrease of the SNR enhancement factor while maintaining a similar or even better SNR enhancement performance. The results clearly show that the SNR in MRI can be substantially improved with smart yet thin MTMs.…”

“…So far, only very few tunable MRI MTMs have been presented in general, and there are even less proposals that achieve the tuning depending on the imaging phase without the need of active intervention. 29,30 Non-linear MTMs 29 are introduced as a promising solution since they can be sensitive to the imaging phase but also MRI-compatible additional battery-powered electronic sensing circuits 30 can be used to take care of the state-switching. However, in all known cases the suggested MTMs are bulky structures (such as helix-shaped configurations or 3d arrangements of conducting bars), putting patient-specific design, patient comfort, and flexible or conformal manufacturing benefits beyond reach.…”

Adaptive Cylindrical Wireless Metasurfaces in Clinical Magnetic Resonance Imaging

“…Several investigations have been conducted to realize the nonlinear properties of metasurfaces to solve this problem. [ 24,34 ] Although a nonlinear planar metasurface was suggested to avoid interference in the transmitting field, [ 24 ] it has a few drawbacks. The proposed “intelligent metasurface” was illustrated by the proximal image quality (SE image of an onion) in a qualitative rather than quantitative manner, that is, from the field or the real FA.…”

“…Another study suggested detuning the metasurface in the transmitting period, but their design was planar in configuration. [ 34 ] Moreover, the design of the switching system was too complicated, and its nonlinearity could not be controlled and was not quantitatively characterized from the field.…”

Improving B1 homogeneity in abdominal imaging at 3 T with light and compact metasurface

“…This effect locally increased the transmit efficiency and Signal-to-Noise Ratio (SNR) of the BC. The water-filled wire MS was followed by several modifications proposed for the same type of resonant focusing operation [21,22,23,24,25,26,27,28], and some alternative MS-based coupled resonators were proposed (e.g., [29,30]).…”