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| Signal-to-Noise Ratio Enhancement Method for Ultra-Low Field Magnetic Resonance Imaging Based on Low-Frequency Surface Metamaterials |
| Kong Xiaohan1, Zhang Yana1, Wu Jiamin2,3, He Yucheng2, Xu Zheng1 |
1. School of Electrical Engineering Chongqing University Chongqing 400044 China;
2. Shenzhen Academy of Aerospace Technology Shenzhen 518057 China;
3. School of Mechatronics Engineering Harbin Institute of Technology Harbin 150001 China |
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Abstract Ultra-low field magnetic resonance imaging (ULF-MRI) offers the benefits of compact size and lightweight. However, the imaging signal-to-noise ratio (SNR) is lower due to limitations in the main magnetic field strength. Thus, a resonant coil array is used for localized signal enhancement in ULF-MRI at 54.6 mT (at a frequency of 2.32 MHz).
Given the significant frequency contrast with high-field MRI, the application and design approach for resonant coils in ULF-MRI is presented. The principle is explained using an equivalent circuit. The resonant coil is positioned on the surface of the sample, coaxially aligned with the radio frequency (RF) receiving coil for maximum enhancement. It's tuned to the resonant frequency with an additional capacitor. During RF reception, the coil amplifies the local RF magnetic field, enhancing the received magnetic resonance signals and improving the SNR of the image. A matching method for the receiving coil with the resonance coil is introduced. Due to the strong coupling between the resonant coil and the receiving coil, traditional methods of matching the receiving coil become ineffective. An appropriate deviation of the matching capacitor for the resonant coil is required to maximize the transfer factor from the receiving coil to the preamplifier. The optimization design of the resonant coil is then carried out using a combination of the partial element equivalent circuit (PEEC) method and unitary design. The PEEC method allows rapid calculation of the AC resistance, while the uniform design method facilitates the selection of optimal coil structures under multiple constraints and design objectives. Finally, an overlapped decoupling approach minimizes the coupling between individual coils within the resonant coil array, and the optimum center-to-center distance between the coils is given.
For the single resonant coil, simulation and imaging results show that the SNR enhancement decreases with distance. The overall SNR remains higher than in the absence of resonant coils. Clear trends in the imaging results for a 5.5 cm diameter water phantom demonstrate this effect, with a maximum SNR enhancement of up to 8.4 times. Resonant coil arrays are also used in imaging experiments with a large water phantom of 110 mm diameter, and a maximum SNR enhancement of 2.7 times is observed. Although the SNR enhancement decreases compared to single coils, the array provides a larger and more uniform enhancement region. Arm imaging experiments verify the performance of the array in human imaging by wrapping the resonant array around the subject's forearm. After the array is applied, the maximum SNR is improved by a factor of 4.9. The SNR improvement is slightly higher than for the large water phantom due to the smaller diameter of the forearm.
The effectiveness of localized enhancement using resonant coils has been confirmed. Compared to conventional surface coils, they offer lower cost, a wireless design for flexible placement, and eliminate the need for additional RF channels. Resonant coil arrays are suitable for enhancing the local SNR in different human body regions and can be freely configured into arrays based on imaging requirements. In future work, resonant coil arrays can be designed specifically for other body regions to achieve optimal enhancement effects.
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Received: 10 January 2023
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2024, Vol. 39 
