Large Field-of-view Photoacoustic Microscopy Based On Transparent Ultrasound Transducer

Photoacoustic imaging（PAI）is a novel non-destructive biomedical imaging technique emerged in recent decades,which combines the advantages of ultrasound imaging and optical imaging,and is a very promising medical imaging method.Photoacoustic microscopy（PAM）has also become one of the current research hotspots as a branch of PAI.However,there is a trade-off between the imaging speed and imaging field of view（FOV）in current PAM.If the mechanical scanning scheme,although the imaging FOV is not limited but the imaging speed is very slow,while if the optical scanning scheme,although the imaging speed is greatly improved but the imaging FOV is very small.The FOV of current PAM systems is 10 mm or less,which is difficult to meet the future requirements for high-resolution imaging of large tissues or organs（extremities,breasts,tumors,and brains）.In addition,due to the use of opaque transducers,conventional PAMs usually require additional approaches and optics to achieve the coaxial alignment of optical and acoustic beams,including using dark-field illumination,using custom optical and acoustic beam combiners,using custom ring-shaped transducers,etc.,which results in complex and costly PAM systems.In this paper,we designed and built a simple and compact optical scanning PAM system with a large centimeter-scale FOV based on a large-size Li Nb O₃transparent transducer.Due to the transparent nature of the transducer,it is feasible to place the transducer directly under the focusing objective to achieve the coaxial alignment of optical and acoustic beams,which greatly simplifies the structure of the PAM system.This scheme addressed the trade-off between scanning speed and imaging FOV in conventional PAM,achieving fast high-resolution imaging over a large area of 20×20 mm².Then,we performed a series of imaging experiments to verify the performance of the PAM system.The lateral resolution of the system was measured by the"edge method"at 18μm,and then we imaged resolution samples such as resolution plates,carbon fibers,and hairs to verify the system resolution.We then imaged a 20×20 mm²area of the leaf vein skeleton,and the imaging results validated the system’s ability to rapidly image a large area sample with high resolution.Finally,we imaged the subcutaneous microvasculature of an ex vivo mouse ear in the range of 10×10 mm²and validated the ability of the PAM system in this paper to resolve the subcutaneous vasculature of biological tissues,and demonstrated a promising application for the imaging diagnosis of large injured or diseased tissues and organs.