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. 2022 Dec 14;12(1):21604.
doi: 10.1038/s41598-022-26143-9.

Design of standalone wireless impedance matching (SWIM) system for RF coils in MRI

Sri Kirthi Kandala  1 , Sung-Min Sohn  2
Affiliations

Design of standalone wireless impedance matching (SWIM) system for RF coils in MRI

Sri Kirthi Kandala et al. Sci Rep. .

Abstract

The radio frequency (RF) power transfer efficiency of transmit coils and the signal-to-noise ratio (SNR) at the receive signal chain are directly dependent on the impedance matching condition presented by a loaded coil, tuned to the Larmor frequency. Sub-optimal impedance condition of receive coils significantly reduces coil sensitivity and image quality. In this study we propose a Standalone Wireless Impedance Matching (SWIM) system for RF coils to automatically compensate for the impedance mismatch caused by the loading effect at the target frequency. SWIM uses a built-in RF generator to produce a calibration signal, measure reflected power as feedback for loading change, and determine an optimal impedance. The matching network consists of a capacitor array with micro-electromechanical system (MEMS) RF switches to electronically cycle through different input impedance conditions. Along with automatic calibration, SWIM can also perform software detuning of RF receive coils. An Android mobile application was developed for real-time reflected power monitoring and controlling the SWIM system via Bluetooth. The SWIM system can automatically calibrate an RF coil in 3 s and the saline sample SNR was improved by 24% when compared to a loaded coil without retuning. Four different tomatoes were imaged to validate the performance of SWIM.

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Conflict of interest statement

The authors declare no competing interests.

Figures

Figure 1
Figure 1
System level block diagram of the proposed SWIM system.
Figure 2
Figure 2
SWIM system setup with fabricated receive coil and 2 m coaxial cable for remote automatic tuning and matching. (Inset) detailed images of individual printed circuit board modules and receive coil design.
Figure 3
Figure 3
Matching network array design with individual fixed capacitor values and MEMS switches displaying one of the 256 combinations as an example to show individual MEMS switch configuration, where 0-switch off and 1-switch on.
Figure 4
Figure 4
(a) Mobile application with real-time reflected power display (Inset) Oscilloscope display of the reflected power showing the three main stages of the operation (b) SWIM system algorithm describing its auto-calibration.
Figure 5
Figure 5
(a) Matching network simulation setup with 2 m coaxial cable to simulate remote tuning and matching and (b) S-parameter plot of the SWIM system modeled in ADS.
Figure 6
Figure 6
(a) Simulation setup in HFSS with 3D print housing and tube phantom (Ansys, 3D High Frequency Simulation Software, Release 21.1, https://www.ansys.com/products/electronics/ansys-hfss). Axial view of H-field inside the tube volume (b) mismatched condition due to loading and (c) retuned condition after loading, and (d) S11 plot of the HFSS simulation results for tuned and mismatched condition.
Figure 7
Figure 7
(a) Experiment setup to characterize SWIM system performance on the bench, (b) measured S11 data showing loading condition of various samples, (c) software detuning with (inset) external gating signal generated on bench, (d) impedance mismatch caused by different sized samples and complete tuning range of the SWIM system with 256 states, and (e) picture of different size tomatoes used for bench test and imaging.
Figure 8
Figure 8
MR images of saline phantom, (a) mismatched condition after loading, (b) automatic matched condition by the SWIM system with active microcontroller and wireless communication, (c) automatic matched condition by the SWIM system with microcontroller in deep-sleep mode, (d) manually matched condition without the SWIM system. MR images of various tomatoes before and after SWIM calibration, extra small tomato (e,f), small tomato (g,h), medium tomato (i,j), and large tomato (k,l). (m) Signal intensity plot of different tomatoes before and after SWIM, calculated from the region closest to the coil as shown beside.
See this image and copyright information in PMC

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