High-speed Cross-scale Photoacoustic Microscopy Imaging System Based On Polygonal Mirror Scanning

Photoacoustic imaging(PAI)is a kind of in vivo imaging technique with high optical contrast and large ultrasonic detection depth.it has the advantages of high resolution,label-free,safe to biological tissue,so it is widely used in a variety of biomedical research.Photoacoustic microscopy(PAM),as an important branch of photoacoustic imaging,is also divided into optical resolution photoacoustic microscopy(OR-PAM)and acoustic resolution photoacoustic microscopy(AR-PAM).They have different resolutions and imaging depths.OR-PAM can provide superficial vascular information with high optical resolution,while AR-PAM can provide deep tissue information with acoustic resolution.In this thesis,an imaging system integrating OR-PAM and AR-PAM is developed,which makes the system have multi-scale imaging ability.However,the traditional mechanical scanning method limits its imaging speed and imaging field of view,and hinders its application.In order to improve the scanning speed of the system and enable it to obtain the dynamic information of the sample,this thesis introduces polygon-scanning as the fast axis scanning of the system,and cooperates with the slow axis motor linear translation stages to carry out two-dimensional scanning to achieve multi-scale three-dimensional imaging with high-speed and wide field of view.Compared to the traditional mechanical scanning of PAM,this thesis proposes a novel polygon-scanning method.First the principle and characteristics of polygon-scanning were introduced,then the causes of image dislocation in polygon-scanning were enumerated,and puts forward three effective methods to reduce the dislocation:(1)ultrasonic assisted image correction;(2)The output pulse signal of the rotary motor encoder is used to trigger the laser synchronously,so that the laser pulses emit with the equal angular interval of the polygonscanning to reduce the image dislocation;(3)Design the transmission structure of shaft and bearing with high fit tolerance to reduce the image dislocation caused by the vibration of the polygon-scanning.This thesis also introduces detailed design and build of a high-speed multi-scale PAM system based on polygon-scanning.First of all,the design of the optical and acoustic part of the system is introduced.The system uses 532 nm and 1064 nm wavelength lasers,and dual light sources provide OR and AR illumination respectively.According to the designed optical/acoustic system,the performance parameters of the imaging system are determined.Then the specific design steps of the important structural components of the system(photoacoustic coupler and polygon-scanning)are introduced,and the whole system is modeled by Solid Works,and the imaging system is built after confirming that the design is correct.In addition,the working time sequence and control system of the system are introduced,the time sequence signal of each module is controlled by microcontrollers,and the master computer control program is written by Labview,so as to realize the automatic control of motor motion,data acquisition and image display.Through phantom experiments,the lateral resolution of OR-PAM is 7.1 μm,the lateral resolution of AR-PAM is 112 μm,the axial resolution of the system is 44.275 μm,the imaging depth of OR-PAM is 1mm,the imaging depth of AR-PAM is 2.7mm,and the effective scanning range of polygon-scanning is 10mm.It takes an average of 14 s for the system to scan and image the 2 × 10mm range with the laser repetition rate of 40 KHz,which is about 180 times faster than that of the traditional machine.At present,the imaging speed of the system is still limited by the repetition rate of the laser,and the scanning speed can be faster by using a laser with higher frequency.Finally,through the multi-scale imaging of the melanin tumor in the leg of nude mice,the superficial microvascular structure information and tumor boundary information of melanin tumor were obtained.At the same time,several scanning imaging experiments were carried out on the ears of nude mice.The images of the blood perfusion process of ear vessels in nude mice were observed,and the in vivo high-speed multi-scale imaging ability of the system was verified.