Photoacoustic imaging of oxygen saturation (sO2) in deep tissue has broad preclinical and clinical applications. Because the magnitude of photoacoustic signal is proportional to the product of optical absorption coefficient and local fluence, quantitative imaging of oxygen saturation usually requires knowledge of the local optical fluence. Especially in deep biological tissue, wavelength dependent optical attenuation of biological tissue presents a challenge to measure the absolute oxygen saturation. Here, we present a new method to measure the sO2 without knowing the local fluence. We measure photoacoustic signals at different wavelengths and different sO2 values. Because the optical fluence at each optical wavelength does not change with a certain sO2, the unknown optical fluence at one wavelength can be cancelled via taking the ratio between two photoacoustic amplitudes at the same optical wavelength but different sO2 values. Three wavelengths, i.e. 760,798,820nm, have been utilized to quantify the absolute sO2. Compared with conventional two-wavelength method, the proposed three wavelength dynamic sO2 method has a better performance on the estimation of absolute sO2. Preliminary phantom experiments have validated the feasibility of this method. This new method enables calibration-free quantitative imaging of absolute sO2 in deep biological tissue.
Traditional back-projection(BP) method suffers from low resolution and high peak sidelobe level (PSL). A new method named eigenspace based adaptive beamforming (EAB) is proposed for photoacoustic computed tomography (PACT). Adaptive beamforming (AB) suppresses the interference and noise signal to improve the lateral resolution, eigenspace can further restrain the off-axis signal to eliminate the artifacts. The simulated experiment shows that the lateral resolution after BP, AB and EAB are 0.99, 0.31, and 0.31mm, respectively. The actual point targets experimental results show that the new method improves the lateral resolution by 67.9% compared with BP, which is agreed with simulated results. New method also improves the PSL by 39.6dB and 42.6dB compared with BP and AB, eliminates the artifacts and improvs imaging quality of the PACT.
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