Polarized photoacoustic microscopy based on high-order harmonics for anisotropy detection

BACKGROUND: Anisotropy which encodes rich structure and function information is one of the key and unique characteristics of tissues. Polarized photoacoustic imaging shows tremendous potential for the detection and quantification of the anisotropy of tissues. The existing polarized photoacoustic imaging methods cannot quantify anisotropy and detect the orientation of the optical axis in 3D imaging. METHODS: We proposed a versatile polarized photoacoustic imaging method based on the detection of high-order harmonics of the photoacoustic signal, which can be used for both 2D and 3D polarized photoacoustic imaging, This method can detect and quantify the anisotropy and the orientation of the optical axis of the anisotropic objects by the amplitude and initial phase of the high-order harmonics. A double-focusing polarized photoacoustic microscopy was developed to validate the proposed method. Experiments were conducted on 2D and 3D anisotropic phantoms. RESULTS: The results showed that the anisotropy and the orientation of the optical axis of the anisotropic object can be detected and quantified accurately by the amplitude and initial phase of the high-order harmonics, even at a depth of triple transport mean free path. The imaging depth of the polarized photoacoustic microscopy is mainly limited by laser energy attenuation rather than depolarization. CONCLUSIONS: Polarized photoacoustic microscopy based on high-order harmonics has tremendous potential for imaging the anisotropy of deep biological tissues in vivo. It also extends the capability of photoacoustic microscopy to image the anisotropy of tissues.