Integrative Fast Photoacoustic Imaging System Based On Phase Controlled Technique

Photoacoustic (PA) tomography is a novel technique for medical imaging. When a laser pulse is introduced into biological tissues, the resulting heat-related expansion of the tissues produces PA waves. By processing the acquired PA signals from a target tissue, we can reconstruct its optical absorption distribution. The large differences in optical absorption of the various tissues, which are associated with their physiological and pathological status, provide significant contrasts in imaging. In this paper, the following work is studied for PA imaging:First, the paper introduces the progress of the PA tomography for the tissue in past years.Second, We have explored the frequency spectrum of absorbers with different sizes and the influence of photoacoustic signals with different spectral components on photoacoustic imaging. The main frequency ranges of photoacoustic pressures of absorbers with diameters of ~cm, ~mm and hundreds of urn are about 20kHz~ 300kHz, 70kHz~ 2.5MHz and 400kHz ~20MHz, respectively. The low spectral components of photoacoustic signals contributed to the non-boundary region of absorbers, and the high spectral components contributed to the small structure, especially, to the boundary. The simulations and experiments demonstrated that the ultrasonic transducers used to detect photoacoustic pressures should be designed and selected according to the frequency ranges of absorbers. When the frequency response range of transducers accords to that of absorbers, almost whole frequency components of photoacoustic pressure can be detected. That results in good reconstructed images.Third, the influences of attenuation coefficients of photoacoustic signals, which depend to frequencies, to the quality of reconstructed photoacoustic images has been explored. It reveals that the attenuation coefficients of low frequent components are less than that of high frequencies, and the latter is more important for photoacoustic imaging with high resolution. Based on the ultrasonic attenuation theory, the method of photoacoustic imaging with rectification of the attenuation of different frequentcomponent was performed. The experiments results show that this method improves the resolution of reconstructed images, and the reconstructed images resolution improved from 0.3mm to 0.2mm.Fourth, We have explored photoacoustic imaging with different filters, such as RL, SL, Modi-SL and Kwoh-Reed, which take important roles on reconstructed images. The results of simulations and experiments show that the filter of Kwoh-Reed can restrain noise effectively and improve the contrast of images compares with the filters of RL, SL, and Modi-SL in the presence of strong noise.Fifth, we have explored the velocity of some kinds of coupling mediums. The ultrasonic velocity of coupling medium has the same that of tissue by adjusting solubility, temperature or proportion of coupling mediums. When the ultrasonic velocity of coupling medium is the same as that of a'tissue, the refraction of ultrasonic can be avoided, dispersion of ultrasonic can be decreased and the ultrasonic path difference can be rectified. The experiments results show that this method improves the resolution and contrast of reconstructed images.Sixth, we have constructed an integrative fast photoacoustic (PA) imaging system, which includes a fiber, ultrasonic coupling medium, and a multi-element linear transducer array. The PA signals were received by the multi-element linear transducer array in a reflection mode and collected by a computer, reconstructed by phase-controlled focus algorithm. The PA images of different depth of phantom lard and animal blood vessels of different diameters were obtained. The resolution of the system was 0.5mm, and the PA imaging time was less than 8s. It would provide a new approach to tissue functional images in vivo, and may have potentials in developing into an appliance for clinic diagnosis of disease.