Passive Beamforming Metasurfaces for Microwave-Induced Thermoacoustic Imaging

Objective: Microwave-induced thermoacoustic imaging (MTAI) responds to the electromagnetic properties of biological tissues, providing non-ionizing, high-resolution, and deep-penetration imaging, with significant potential for clinical diagnostics and treatment. However, the current MTAI faces issues of reduced signal-to-noise ratio (SNR) and contrast when imaging deep tissues. Methods: In this study, we propose a passive beamforming metasurface (PB-MS) (270 mm × 270 mm × 5 mm), designed to focus microwave energy on deeper regions using phase control, enabling more sensitive MTAI of deep tissues. The PB-MS is composed of 27 superstructure units, which generate surface plasmons when excited by microwave fields. By arranging these units, the microwave field is reshaped to focus and distribute evenly, increasing the energy density in target areas. This enhances thermoacoustic signals, improving the imaging SNR and contrast. Results: Both simulations and experiments were conducted to evaluate the practical feasibility of MTAI with PB-MS. The results showed that with PB-MS, the SNR remained as high as 22.2 dB in muscle phantoms at a depth of 7.5 cm. The MTAI system, equipped with PB-MS, is capable of detecting a minimum conductivity change of 0.095 S/m and identifying micro-liter level hemorrhages in a mouse model of hemorrhagic stroke. Conclusion: These results demonstrate that PB-MS optimizes energy delivery in MTAI, enabling deeper and more sensitive imaging. Significance: PB-MS effectively enhances MTAI imaging quality, representing a critical step toward its clinical application.