Optoacoustic imaging with improved synthetic focusing

Optoacoustic imaging takes advantage of high optical contrast and low acoustical scattering, and has found several biomedical applications. Optoacoustic signals are produced by irradiating laser pulse to a sample, which absorbs light energy and generates ultrasound waves. In the common backward mode optoacoustic imaging, a laser beam illuminates the image object and an acoustic transducer located on the same side as the laser beam. A cross-sectional image is formed by laterally scanning the transducer. Although the laser beam width is generally narrow, strong optical scattering in tissue broadens the optical illumination energy and thus degrades the lateral resolution in optoacoustic image. Therefore, a time-domain delayed and summed technique has been proposed to locate the optoacoustic sources in the tissue. In this study, a combination of synthetic aperture focusing technique and coherence weighting is proposed. Specifically, the focusing quality of the synthetic aperture technique is further improved by using the signal coherence as an image quality index. In this article, we demonstrate the efficacy of the proposed method using numerical simulations and phantom experiments with a phantom comprising hair threads in a 1% milk solution. The results show that the proposed technique improved lateral resolution by 4-8 times and the signal-to-noise ratio by 7-23 dB over the conventional techniques.