A photoacoustic tomography imaging system using a low-coherence interferometer with rapid detection of phase modulation was designed, fabricated, and tested for biologic imaging. A noncontact probing technique was applied to improve the practicability of the system. The technique is experimentally verified by the image of a simulated tissue sample and the blood vessels within a mouse ear flap (pinna) in vivo. The system's axial and lateral resolutions are evaluated at 45 and ~15 μm, respectively. The system's imaging depth is 1mm in a special phantom. The results show that the system has the feasibility to be used as a photoacoustic tomography imaging method, and it may provide a kind of possibility of noncontact real-time PAT.Photoacoustic imaging; Three-dimensional image acquisition; Tomography; Time-resolved imaging.As a routinely used medical imaging modality, photoacoustic imaging (PAI) is becoming more and more useful in investigate the endogenous tissue components that absorb illuminated light [1,2] , because of its advantages in the merits of high tissue contrast of pure optical imaging and high spatial resolution of pure ultrasonic imaging.Generally, PAI has evolved from two categories: photoacoustic microscopy (PAM) and photoacoustic tomography (PAT). PAT is based on the collection of acoustic signals around an object that has been heated by a pulse laser [3] . By taking the advantages from combining optical contrast and high resolution in deep opaque tissues, PAT has broad applications in imaging tissue structure [4] and function [5] and microvasculature [6] in vivo.Although PAT is potentially capable of producing real time, three-dimensional (3D), high resolution images to depths of several centimeters [7] , the more requirements of the ultrasonic detection system were presented. The contact detection mode of PAT is constantly chosen to the ultrasound detection. However, the strong attenuation of ultrasound waves in air, acoustic coupling between the sample and the acoustic transducer, i.e., which limits its applications in biomedical samples. To address this problem, optical ultrasound detection technique is developed to detect ultrasound in a noncontact fashion. Moreover, the detection sensitivity of this type can potentially offer a high imaging resolution for its high detection bandwidth because the probe beam can be focused into a relatively small beam spot size.

Photoacoustic microscopy (PAM), whose image quality largely depends on the optical absorption of samples, provides endogenous information for structural and functional imaging. However, PAM technology in general can not provide edge enhancement imaging for absorbing objects. Therefore, PAM and differential microscopy are integrated for the first time in a single technique to obtain an edge enhancement image. The resolution test target RTA-07 and red blood cells are used as samples to achieve the desired spatial differential photoacoustic imaging. The feasible biomedical application of edge enhancement from the improved differential PAM was demonstrated.