Technological Updates in Ultrahigh-Field Animal MRI Systems

Academic Background

Animal magnetic resonance imaging (MRI) systems play a crucial role in preclinical research, typically offering superior imaging performance compared to conventional human MRI systems. However, achieving high performance in these systems is challenging due to the multifaceted nature of various system components and the complexity of integration and debugging. In particular, ultrahigh-field animal MRI systems require the generation of extremely high magnetic field strengths and gradient magnetic fields while ensuring field homogeneity and stability. Additionally, system installation, maintenance, and debugging must consider multiple aspects such as magnetic field shielding, mechanical coupling, and thermal management. Although some commercial animal MRI systems are available in the market, detailed reports on the latest technological updates in hardware performance (e.g., superconducting magnets and gradient coils) are still lacking.

This paper, co-authored by Yaohui Wang, Guyue Zhou, Haoran Chen, Pengfei Wu, Wenhui Yang, Feng Liu, and Qiuliang Wang, was published in the 2024 issue of npj Imaging. The research team is affiliated with institutions such as the Institute of Electrical Engineering, Chinese Academy of Sciences, Huazhong University of Science and Technology, and the University of Queensland. The paper provides a detailed description of the design, fabrication, measurement, and integration of a 7 Tesla (T) animal MRI system, highlighting its technological advancements in superconducting magnets and gradient coils.

Research Process and Results

1. Superconducting Magnet Design

The research team designed an actively shielded 7T animal MRI superconducting magnet wound with NbTi superconducting wire. The magnet coil structure includes four parallel solenoid coils, which not only contribute to the main magnetic field strength but also significantly reduce the inhomogeneous harmonic components of the central magnetic field distribution, forming a homogeneous region with a uniformity of 10 ppm (parts per million). Unlike the traditional double-coil shielding pattern, the magnet employs a three-coil shielding pattern, reducing the 5 Gauss line range to ±2.95 meters axially and ±1.85 meters radially, significantly better than the commercial system’s ±3 meters and ±2 meters. This compact shielding design facilitates system installation and maintenance.

2. Gradient Coil Design

The gradient coil design adopts a strategy to minimize Lorentz forces, ensuring that the residual force of the gradient coils is less than 0.1 Newtons under ultrahigh static magnetic field conditions. Through an ultra-shielding optimization strategy, the maximum stray magnetic field intensity of the gradient coils is minimized to 4 Gauss. The gradient coil’s magnetic field strength is 200 mT/m, with a uniformity of ±2.5%. Additionally, the slew rate of the gradient coils is optimized to reach 503.9357 T/m/s (Z-axis), 1070.3454 T/m/s (X-axis), and 929.9312 T/m/s (Y-axis).

3. Magnet Measurement and Shimming

The magnet was measured at a magnetic field strength of 7.02 T, with field locking achieved through a nuclear magnetic resonance (NMR) probe. After more than 7 hours of continuous sampling, the magnetic field decay rate was measured to be 0.0494 ppm/h, indicating sufficient stability for MRI applications. Subsequently, the research team performed shimming using superconducting shim coils and passive shimming iron pieces, achieving a magnetic field homogeneity of 7.66 ppm within a 130 mm spherical volume.

4. System Integration and Debugging

The research team developed proprietary imaging software for debugging the MRI system. The software supports various pulse sequences, including fat suppression, water suppression, dynamic enhanced scanning, motion artifact suppression, diffusion imaging, and angiography. Using this software, the team successfully acquired MRI images of phantoms, fruits, and organisms. For example, spin-echo (SE) sequences were used to image phantoms and oranges, achieving a signal-to-noise ratio (SNR) of 167.6. Additionally, the team conducted experiments on rats, successfully obtaining images of the rat’s head with minimal artifacts and external noise interference.

Conclusions and Significance

This study successfully developed a 7T animal MRI system, showcasing its technological advancements in superconducting magnets and gradient coils. The magnet’s ultra-shielding design significantly reduces the 5 Gauss line range, ensuring system safety and convenience. Through the combination of superconducting shimming and passive shimming, the magnetic field homogeneity reached 7.66 ppm, with a stability of 0.0494 ppm/h, sufficient for standard MRI operations. The gradient coil design minimizes Lorentz forces and stray magnetic fields, ensuring mechanical safety and imaging quality.

The development of this system provides a powerful platform for scientific research on animal models, particularly in the hardware updates of ultrahigh-field MRI technology, with significant scientific value and application prospects. In the future, the research team will further optimize the system’s functionality to promote more scientific outputs.

Research Highlights

1. Ultra-Shielding Design: The magnet’s three-coil shielding pattern significantly reduces the 5 Gauss line range, outperforming commercial systems.
2. Magnetic Field Homogeneity and Stability: Through superconducting and passive shimming, the magnetic field homogeneity reached 7.66 ppm, with a stability of 0.0494 ppm/h.
3. Gradient Coil Optimization: Minimizing Lorentz forces and stray magnetic fields ensures mechanical safety and imaging quality.
4. Proprietary Imaging Software: Supports various pulse sequences, successfully acquiring high-quality images of phantoms, fruits, and organisms.

Other Valuable Information

The research team also detailed the design parameters of the superconducting magnet and gradient coils, including coil dimensions, current density, and magnetic field homogeneity, providing valuable technical references for researchers in related fields. Additionally, the proprietary imaging software and integration platform developed by the team lay the foundation for future functional expansion and optimization.

This study has made significant progress in the hardware design and integration of ultrahigh-field animal MRI systems, providing strong technical support for preclinical research.