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| Electromagnetic Interference Suppression Method for an Unshielded Portable Ultra-Low Field Magnetic Resonance Imaging Scanner |
| Yang Lei1, He Wei1, He Yucheng2,3, Wu Jiamin2,4, Xu Zheng1 |
1. School of Electrical Engineering Chongqing University Chongqing 400044 China;
2. Shenzhen Academy of Aerospace Technology Shenzhen 518057 China;
3. School of Life Northwestern Polytechnical University Xi’an 710072 China;
4. School of Mechatronics Engineering Harbin Institute of Technology Harbin 150001 China |
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Abstract The ultra-low field (ULF) magnetic resonance imaging (MRI) scanner features much lower cost, much lighter weight, much smaller footprint, and fewer safety concerns than high-filed ones. However, the intrinsic low signal strength of ULF MRI scanners makes them more easily impacted by electromagnetic interference (EMI). The conventional electromagnetic shielding methods passively block the EMI signal but restrict the mobility of ULF MRI scanners. Therefore, the passive electromagnetic shielding should be discarded. The reference channel-based active EMI suppression methods outperform others for better suppression rate and electromagnetic environment adaptation. However, the present deep-learning reference channel-based methods have drawbacks of demanding additional scan time and modification of pulse sequences.
This paper proposes a signal-analysis reference channel-based active EMI suppression method with the following features. (1) The EMI detecting coils with appropriate locations avoid sample MR signal. (2) The EMI-detecting coils of reference channels and RF-receiving coils of the MR channel acquire the reference EMI signal and contaminated MR signals synchronously. (3) The reference EMI signals estimate EMI signals in the MR channel (primary EMI signal). (4) The denoised MR signal is obtained by subtracting the estimated primary EMI signal from the contaminated MR signal. The EMI transmission paths are analyzed, and human-body coupling is the main path. The ‘ring’ shaped EMI pickup coil is designed to sample human body coupled EMI signal. The ‘ring’ shaped EMI pickup coil, along with one surface coil and one solenoid coil, constitutes three reference channels.
It is found that reference EMI signals acquired in a limited time interval also contribute to estimating EMI signals in the MR channel precisely. Then, the concept termed related window is proposed. A data point at a specific time in the k-space of the MR channel is estimated by the data points ink-spaces of all reference channels within the related window. The periphery data of the k-spaces of each channel are selected as calibration data to calculate the transfer matrix for reference EMI signals without the calibration measurement and modification of pulse sequences. The calibration target matrix is constructed from the calibration data of the MR channel, and the calibration source matrix is formed by the calibration data of reference channels in the related window. The two matrixes are decomposed into multiple layers by wavelet analysis. The least-square method calculates the transfer matrix in each layer. The reconstruction source matrix is constructed from k-spaces of reference channels within related windows and decomposed into the same layers. The primary EMI signal is estimated by the reconstruction source matrix and transfer factors in each layer. The denoised MR signal is acquired by subtracting the estimated EMI signal of the MR channel from the contaminated MR signal.
The active EMI suppression system is fitted to a home-built portable unshielded 50 mT MRI scanner. The one-dimensional signal analysis and in-vivo scan are carried out in a building located in an industrial park without EMI restrictions. The image results show that the signal-to-noise rate (SNR) is improved by 3.9-fold, and the active EMI suppression system ensures that the ULF MRI scanner works normally without shielding.
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Received: 10 January 2023
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2024, Vol. 39 
