Passive Beamforming Metasurfaces for Microwave-Induced Thermoacoustic Imaging

Electrical Engineering Electronic Information Engineering Information Science Medical Imaging Life Sciences Bioengineering Medicine
Microwave-Induced Thermoacoustic Imaging Metasurfaces Brain Imaging Signal-to-Noise Ratio Tissue Imaging

Passive Beamforming Metasurfaces for Microwave-Induced Thermoacoustic Imaging

Academic Background

Microwave-induced thermoacoustic imaging (MTAI) is an emerging medical imaging technology that combines the advantages of microwave and ultrasound imaging. It generates ultrasonic waves (i.e., thermoacoustic signals) by irradiating biological tissues with microwave pulses, which are then absorbed by the tissue, causing thermal expansion. These signals carry morphological and functional information about the internal structure of the tissue. MTAI offers non-invasive, high-resolution, deep-penetration, and high-contrast imaging, making it widely applicable in areas such as breast cancer screening, brain imaging, and joint imaging. However, as imaging depth increases, the attenuation of microwave energy leads to a significant reduction in the signal-to-noise ratio (SNR) and contrast of thermoacoustic signals, limiting its application in deep tissue imaging.

To address this issue, researchers have proposed various methods, such as using high-power microwave sources and multi-antenna coupling. However, these approaches present challenges such as biosafety concerns, complex circuit designs, and high costs. Therefore, developing a safe, low-power, and easily integrable technology to enhance the microwave response of biological tissues has become a key focus of current research.

Paper Source

This paper was co-authored by Shuangfeng Tang, Yichao Fu, Yu Wang, Xiaoyu Tang, Lizhang Zeng, and Huan Qin, who are affiliated with the MOE Key Laboratory of Laser Life Science and the Key Laboratory of Brain, Cognition, and Education Sciences at South China Normal University. The paper was published in 2025 in the journal IEEE Transactions on Biomedical Engineering, titled “Passive Beamforming Metasurfaces for Microwave-Induced Thermoacoustic Imaging.”

Research Workflow

1. Design and Principle of Passive Beamforming Metasurface (PB-MS)

The researchers proposed a passive beamforming metasurface (PB-MS) that enhances the SNR and contrast of deep-tissue imaging by focusing microwave energy on target regions through phase control principles. The PB-MS consists of 27 superstructure units, each composed of multiple layers of metal (copper) and dielectric material (F4B-M2). When excited by microwave fields, these units generate surface plasmons, and by arranging them, the microwave field is reshaped to focus and distribute evenly in the target area.

The design of the PB-MS is based on phase control principles, where the geometric dimensions and arrangement directions of the units are adjusted to alter the phase of the microwaves passing through the units, thereby achieving focusing of the microwave field. The researchers used electromagnetic simulation software CST (CST Studio Suite 2022) to model and simulate the PB-MS, ultimately designing a metasurface array measuring 270 mm × 270 mm × 5 mm.

2. Integration of PB-MS into the MTAI System

The researchers integrated the PB-MS into the MTAI system, which includes a microwave source, antenna, ultrasonic transducer, and data acquisition system. The microwave source generates 6.7 GHz pulsed microwaves, which pass through the PB-MS before irradiating the imaging object. The resulting thermoacoustic signals are received by a 128-channel semi-ring focused ultrasonic transducer and processed by the data acquisition system. Data reconstruction employs the delay-and-sum (DAS) algorithm, yielding the final microwave absorption distribution image of the tissue.

3. Simulation and Experimental Validation

To validate the feasibility of the PB-MS, the researchers conducted simulations and experiments. Simulation results showed that the PB-MS significantly enhances the energy density of the microwave field while maintaining uniformity along the propagation direction. The experimental section included muscle phantom imaging, sensitivity experiments with varying conductivity phantoms, and mouse brain tissue imaging.

  • Muscle Phantom Imaging Experiment: At different depths (0.5 cm to 7.5 cm), the MTAI system with PB-MS maintained a high SNR of 22.2 dB at a depth of 7.5 cm, with thermoacoustic signal amplitude and SNR increasing by 3.9 times and 4.6 times, respectively.
  • Conductivity Sensitivity Experiment: Through simulations and experiments, the researchers demonstrated that the PB-MS could detect conductivity changes as small as 0.095 S/m, indicating its high sensitivity to minor changes in electromagnetic parameters.
  • Mouse Brain Tissue Imaging Experiment: The PB-MS significantly improved the contrast of brain tissue imaging, enabling clearer visualization of internal structures. In a mouse brain hemorrhage model, the PB-MS detected micro-liter level bleeding, enhancing contrast by 2.78 times.

Research Results

The study showed that the PB-MS significantly improves the imaging depth, SNR, and contrast of the MTAI system. In muscle phantoms, the PB-MS allowed the MTAI system to maintain a high SNR at a depth of 7.5 cm. In conductivity sensitivity experiments, the PB-MS detected conductivity changes as small as 0.095 S/m. In mouse brain tissue imaging, the PB-MS significantly enhanced imaging contrast and detected micro-liter level bleeding.

Conclusions and Significance

The conclusion of this study is that the PB-MS achieves efficient focusing of microwave energy through phase control principles, significantly improving the imaging depth, SNR, and contrast of the MTAI system. The introduction of PB-MS provides new possibilities for the clinical application of MTAI, particularly in deep tissue imaging and early disease detection. Additionally, the ease of integration and broad applicability of the PB-MS make it highly valuable for future clinical applications.

Research Highlights

  1. Innovative Design: The PB-MS is the first passive beamforming metasurface applied to MTAI, achieving efficient focusing of microwave energy through phase control principles.
  2. Deep Imaging Capability: The PB-MS enables the MTAI system to maintain a high SNR at a depth of 7.5 cm, significantly enhancing deep tissue imaging capabilities.
  3. High Sensitivity: The PB-MS can detect conductivity changes as small as 0.095 S/m, demonstrating its high sensitivity to minor changes in electromagnetic parameters.
  4. Clinical Potential: The PB-MS detected micro-liter level bleeding in a mouse brain hemorrhage model, showcasing its potential value in clinical diagnostics.

Other Valuable Information

The researchers noted that future work will further optimize the design of the PB-MS, expand its operating frequency range, and introduce FPGA-controlled metasurfaces to achieve real-time control of the microwave field’s focal point, focal depth, and polarization direction, thereby meeting more diverse clinical needs.