Introductory Remarks

Photoacoustic (PA) imaging is a vital technology in biomedical imaging because it provides better contrast, resolution, and penetration depth than typical optical or ultrasonic imaging systems [1]. Acoustic-Resolution Photoacoustic Microscopy (AR-PAM) is a type of PA imaging that makes use of a laser and ultrasonic transducers to improve the image quality. However, acoustic diffraction can restrict AR-PAM by reducing lateral resolution and signal-to-noise ratio [2]. The synthetic aperture focusing technique (SAFT) employing virtual detectors (VD) can recover lateral resolution in such areas, resulting in a depth-invariant resolution system, however, SAFT cannot enhance lateral resolution in the focused region due to the acoustic diffraction limit [3-5].

Structured illumination has been employed for overcoming this barrier, allowing the extraction of high-frequency components in the imaging system's pass band. Through point-by-point raster scanning and pattern illumination, the system gathers a wide variety of spatial frequency components to increase image quality.

The proposed structure uses 13 illumination patterns generated by a high-speed digital micromirror device (DMD) to achieve near-isotropic resolution in four orientations and with three perpendicular phase shifts. The three-phase shifting method helps to improve the lateral resolutions more than two-fold and noise level reduction compared to conventional photoacoustic microscopy (PAM) systems without deteriorating the depth of field. The idea validated the approach through in vivo and ex vivo experiments using mouse ear, zebrafish body, and mouse liver as imaging targets. According to the experimental findings, the focal regions of the AR-PAM system have an enhanced lateral resolution of 25 μm compared to 55 μm that a conventional AR-PAM system provides. Moreover, the resolution of the implemented system remains steady at 29 μm for depths over 1.2 mm, whereas in a typical system, it degrades to 287 μm.

By modifying the phase compounding method, we can obtain a system with wider bandwidth that results in finer lateral resolution in foci and out of focal regions by using 17 illuminations. The modified phased compounding method helps to extract the second harmonic of the illuminated pattern by the DMD and down-converting the higher spatial frequencies of the imaging target two times higher than the three-phase shifting method. Therefore, the resolution of the system will be enhanced five times compared to the conventional PAM systems, theoretically. The experimental results demonstrate lateral resolution improvement from 44.6 μm in the conventional systems to 19.8 μm in the modified phase compounding structured illumination. Moreover, the signal-to-noise ratio (SNR) of the reconstructed images shows 22 dB improvement.

Implementation of the reconstruction algorithm in the Fourier domain and high-speed DMD enables reaching a high-speed three-dimensional high-resolution imaging system for biomedical applications. The superiority of the system was proved through in vivo and ex vivo experiments.

References:

[1] Wang, X., Pang, Y., Ku, G., Xie, X., Stoica, G., & Wang, L. V. (2003). Noninvasive laser-induced photoacoustic tomography for structural and functional in vivo imaging of the brain. Nature biotechnology, 21(7), 803-806.

[2] Park, S., Lee, C., Kim, J., & Kim, C. (2014). Acoustic resolution photoacoustic microscopy. Biomedical Engineering Letters, 4, 213-222.

[3] Li, M. L., Zhang, H. F., Maslov, K., Stoica, G., & Wang, L. V. (2006). Improved in vivo photoacoustic microscopy based on a virtual-detector concept. Optics letters, 31(4), 474-476.

[4] Amjadian, M., Mostafavi, S. M., Chen, J., Kavehvash, Z., Zhu, J., & Wang, L. (2021). Super-resolution photoacoustic microscopy using structured-illumination. IEEE Transactions on Medical Imaging, 40(9), 2197-2207.

[5] Amjadian, M., Mostafavi, S. M., Chen, J., Wang, L., & Luo, Z. (2022). Super-resolution photoacoustic microscopy via modified phase compounding. IEEE Transactions on Medical Imaging, 41(11), 3411-3420.