Photoacoustic Tomography Research Based On The Focused Fiber-laser Ultrasonic Sensor

As a non-invasive hybrid imaging modality,photoacoustic imaging works based on optical excitation and acoustic detection,combining the advantages of high contrast of optical imaging and large penetration depth of ultrasonic imaging.In recent years,photoacoustic imaging has been widely exploited for early diagnosis of cardiovascular diseases and cancers and shown great potential for practical applications.However,most of the existing photoacoustic imaging systems use piezoelectric ceramics as the ultrasonic sensor.The probes are large in size,optically opaque,and susceptible to electromagnetic interference,which increases the complexity and cost of the system,and also limits the image quality.To address the above problems,this thesis proposed a dual-frequency fiber laser as the sensitive element for acoustic wave detection.The optical fiber was bent to form ultrasonic focusing which improves the detection sensitivity.The focused fiber-laser ultrasound sensor was employed for deep-tissue imaging.Furthermore,a virtual point concept was combined with the synthetic aperture focusing technology to overcome the problem of degraded image resolution and contrast in the out-of-focus region.In addition,for objects with strong or weak absorption,photoacoustic imaging is difficult to obtain high-quality images.Therefore,a fiber-optic ultrasonic emitter based on the thermal-cavitation effect was proposed for photoacoustic and ultrasonic dual-modality imaging system to obtain more comprehensive image information.The specific research contents are as follows:1.Photoacoustic tomography based on the focused fiber-laser ultrasonic sensor:Wavefront phase matching of the spherical ultrasound wave emitted from a point source was achieved by bending the optical fiber,realizing an improving detection sensitivity.By using the optical fiber sensor with a bending curvature radius of 25 mm,three-dimensional high spatial resolution of 150μm×85μm×300μm was obtained,and the sensor was successfully applied for imaging abdominal blood vessels in rats.2.Optimization of the imaging resolution based on virtual point synthetic aperture technology:The focus of the fiber-laser ultrasonic sensor was used as the virtual detection point,which is combined with the synthetic aperture focusing technology for improving the acceptance angle of ultrasound.With an equivalent numerical aperture of 0.36,the reconstructed image have a resolution improved by a factor of nearly 10.3.Optical fiber ultrasonic emission technology based on thermal-cavitation effect:980 nm continuous laser was used to heat the Cu（NO₃）₂solution at the end face of the fiber to generate thermal cavitation,accompanied by strong emission of ultrasonic pulse during the instant collapse of the microbubble.Under the heating power of 80 m W,the emitted ultrasonic pulse had a pressure of 750 k Pa and a frequency bandwidth of 15 MHz.This fiber-optic emitter meets the needs of ultrasonic imaging and can be used to build an all-fiber ultrasonic-photoacoustic dual-modality imaging system in the future.