Photoacoustic imaging with focused ultrasonic probe beam

This paper presents a novel model of photoacoustic tomography with an ultrasonic probe beam. A strict theory describes the nonlinear interaction between pholoacoustic wave and ultrasonic probe beam. When a pulse laser irradiates on biological tissues, die laser energy will be absorbed by the identity molecule of the tissues, and be converted into heat. Subsequently, the thermal expansion of the instantaneously heated tissues induces photoacoustic waves. So the density, temperature and compressibility of the absorption area will be changed. A focus ultrasonic probe beam with a single frequency passes through the irradiated area, the changes of the properties of the irradiated medium will be coupled with the ultrasonic probe beam, which will propagate through the medium with minimal distortion and can be detected at the surface of the medium. The PA signal in situ can be obtained by demodulating the detected ultrasonic beam. It will take out much more information from the interaction area. which can reflect not only the intrinsic optical properties but also the mechanical and acoustical properties of the tissue. In our experimentation, a Q-switched Nd: YAG pulse laser operated at 1064nm was employed to generate photoacoustic signal., the frequency of ultrasonic probe beam is 10M. By demodulating the detected ultrasonic beam. we obtained very high quality tomography images. So it will provide a new promising method for tumour detection and noninvasion function imaging.