Design and Testing of a Lightweight 3.0 T Cryogen-Free MRI System

Academic Background

Magnetic Resonance Imaging (MRI), as a non-invasive, radiation-free imaging technology, has been widely used in medical diagnostics and scientific research. Especially in the fields of small animal studies and material analysis, high-field MRI systems can provide higher spatial resolution and richer tissue contrast, offering researchers more precise imaging data. However, traditional 3.0 T MRI systems rely on liquid helium-cooled superconducting magnets, which not only incur high costs but also bring significant economic burdens and environmental impacts due to the consumption and maintenance of liquid helium. Additionally, the large size of traditional MRI systems requires substantial space for installation and operation, limiting their application in laboratories and smaller research institutions.

To address these issues, cryogen-free superconducting magnet technology has gradually become a research hotspot in recent years. This technology eliminates the dependency on liquid helium through efficient conduction cooling paths and mechanical vibration isolation techniques, significantly reducing system operation and maintenance costs. Nevertheless, cryogen-free MRI systems still face challenges in terms of magnetic field stability and homogeneity, especially under high-field conditions where minor fluctuations in the magnetic field can lead to significant degradation in imaging quality. Therefore, designing a lightweight, high-performance, and cost-effective 3.0 T cryogen-free MRI system has become an important direction in the current development of MRI technology.

Paper Source

This paper was co-authored by Jinhao Liu, Miutian Wang*, Youheng Sun, Kaisheng Lin, Wenchen Wang, Yaohui Wang, Weimin Wang, Qiuliang Wang, and Feng Liu. The research team hails from the School of Electrical Engineering at Xi’an Jiaotong University, the School of Electronics at Peking University, the Department of Biomedical Engineering at Peking University’s College of Future Technology, the School of Information Technology and Electrical Engineering at the University of Queensland, and the Institute of Electrical Engineering at the Chinese Academy of Sciences. The paper was published in the journal IEEE Transactions on Biomedical Engineering in 2017.

Research Process and Results

1. Design of the Cryogen-Free Superconducting Magnet

The research team initially designed a lightweight 3.0 T cryogen-free superconducting magnet weighing approximately 1100 kg. To ensure long-term magnetic field stability, the team optimized the magnet’s conduction cooling path, vibration isolation, mechanical damping, and structural stability. Specifically, copper thermal rings and aluminum thermal radiation shields were adopted to improve the heat transfer path from the magnet coils to the cold head, reducing magnetic field fluctuations caused by the refrigeration motor. Additionally, the team enhanced the system’s structural stability by securing the magnet base to the ground using steel screws.

2. Optimization of Magnetic Field Homogeneity and Stability

To improve magnetic field homogeneity, the research team employed both passive and active shimming techniques. The passive shimming method optimized iron piece distribution, reducing the peak-to-peak and root mean square error (RMSE) homogeneity within a 180 mm diameter spherical volume (DSV) to 22.41 ppm and 3.69 ppm, respectively. Subsequently, further improvements were achieved through active shim coils, enhancing homogeneity to 4.18 ppm and 1.02 ppm.

In terms of magnetic field stability, the research team successfully reduced the amplitude of magnetic field fluctuations from 2.168 µT to 0.004 µT—a reduction of 99.81%—through optimization of the thermal conduction path and mechanical damping techniques. Rapid Fourier Transform (FFT) analysis revealed that the optimized magnetic field fluctuations primarily concentrated around the 1 Hz frequency, consistent with the operational frequency of the cold head, with significantly reduced higher harmonic amplitudes.

3. Gradient Coil and RF Coil Design

The research team designed a dual-layer gradient coil with a peak amplitude of 200 mT/m and advanced shielding technology to confine stray magnetic fields to within 1.2 Gauss. Moreover, the team developed an 8-channel orthogonal birdcage RF coil for small animal imaging. This coil exhibited a resonant frequency of 131 MHz under no-load conditions, dropping to 127 MHz when loaded, demonstrating excellent RF field homogeneity.

4. Development of the MRI System Console

Based on the concept of Software-Defined Radio (SDR), the research team developed a custom MRI console. This console adopts an analog front-end-A/D-Field Programmable Gate Array (FPGA)-D/A-analog front-end framework, achieving functionalities such as RF waveform generation, gradient calculation, data storage, and communication. Through digital pre-emphasis techniques, the console effectively compensates for eddy current effects induced by gradient coils, further improving imaging quality.

5. Animal and Material Imaging Experiments

In mouse brain imaging experiments, the research team utilized Fast Spin Echo (FSE) sequences and Echo-Planar Diffusion-Weighted Imaging (EP-DWI) sequences, employing navigator echo correction techniques to successfully eliminate image artifacts caused by magnetic field fluctuations. Experimental results demonstrated that the system could clearly display hemorrhagic and infarcted regions in the mouse brain, with imaging resolution and quality significantly superior to traditional MRI systems.

Additionally, the research team performed high-resolution imaging of plastic gears using Single Point Imaging (SPI) technology and reduced scanning time from over 10 hours to approximately 5 hours through Compressed Sensing (CS) techniques. In petroleum core analysis experiments, the team successfully acquired T1-T2 two-dimensional spectra of the core using Inversion Recovery (IR) and Carr-Purcell-Meiboom-Gill (CPMG) sequences, providing detailed parameter analyses for oil extraction and preparation processes.

Conclusion and Significance

The highlight of this study is the successful design and testing of a lightweight, high-performance 3.0 T cryogen-free MRI system, which demonstrates outstanding imaging capabilities and stability in small animal imaging and material analysis. By optimizing magnetic field homogeneity and stability, developing advanced gradient and RF coils, and creating a custom MRI console, the research team successfully addressed technical challenges faced by cryogen-free MRI systems under high-field conditions. The application of this system not only reduces the operational and maintenance costs of MRI systems but also provides new possibilities for widespread applications in brain science research, material characterization, and industrial inspection.

Research Highlights

1. Optimization of Magnetic Field Stability: Through optimization of the thermal conduction path and mechanical damping techniques, the amplitude of magnetic field fluctuations was reduced by 99.81%, significantly improving imaging quality.
2. Enhancement of Magnetic Field Homogeneity: Adoption of passive and active shimming techniques improved magnetic field homogeneity to 4.18 ppm, meeting the requirements for high-resolution imaging.
3. Gradient Coil and RF Coil Design: The design of dual-layer gradient coils and orthogonal birdcage RF coils effectively confined stray magnetic fields and improved RF field homogeneity.
4. Development of the MRI Console: A custom console based on FPGA realized efficient RF waveform generation and gradient compensation, further enhancing system performance.
5. Application Value: Successful applications in mouse brain imaging, defect detection in plastic gears, and petroleum core analysis demonstrate the system’s broad potential in scientific research and industrial fields.

Other Valuable Information

The research team also noted that the cryogen-free MRI system has been continuously operating since August 2023, exhibiting good stability and reliability. Due to its compact size and lack of need for liquid helium, the system is particularly suitable for installation in research institutions with limited space or in high-rise buildings, further reducing the spatial and installation costs of MRI systems. In the future, the research team plans to further optimize the system’s scanning efficiency, especially in terms of the acceleration factor in compressed sensing techniques, to further enhance imaging speed and practicality.

Through this study, cryogen-free MRI technology has taken an important step forward in high-field applications, providing new solutions for the popularization and promotion of MRI technology.