Miniaturized Photoacoustic Microscope Based On Diode Excitation

The optical-resolution photoacoustic microscopy(OR-PAM)is a hybrid imaging technique that acoustically detects optical contrast via the photoacoustic effect,and it can provide images of biological subcutaneous micro-vessels and other structures with micron-scale lateral resolution.It has successfully found applications in many biomedical imaging fields such as dermatology,tumor neovascularization,brain structure and function.Among these imaging techniques,compared with the solid-state laser OR-PAM system,the laser diode OR-PAM system has the advantages of inexpensive,compact and high repetition frequency,leading to possibility of the integration and incorporation of the photoacoustic microscopic imaging system and envisaging broad application prospect.However,with the low peak power(generally several hundred microjoules),the pulsed laser diode needs to employ a high numerical aperture(NA)lens which is used to enable the microscopy and focus of the laser beam to generate photoacoustic signals,resulting in that the working distance of the OR-PAM system is very short(only 2-3 mm)and that the photoacoustic excitation source and the ultrasonic sensor cannot be arranged on the same side and in a coaxial confocal structure.Therefore,the arrangement methods of light source of the existing laser diode photoacoustic microscopy imaging system as well as the sensor is either oblique incidence structure or transmittance structure,making it difficult to meet clinical needs in reality.In this work,a long-pulse visible light laser diode carrying a low-NA reflective microscopic objective as well as an OR-PAM imaging system with a 22-mm long working distance was designed and built,including pulsed laser diode,reflective microscopic objective,ultrasonic sensor,and 3D electronic control translation stage,oscilloscope data acquisition module,etc.Furthermore,LabVIEW software was employed to achieve system hardware control and photoacoustic data acquisition.In the system,the 450 nm pulsed laser diode whose repetition rate,pulse width and energy were respectively 1 KHz,about 650 ns and 1 ?J,was used.The coaxial confocal structure optimized photoacoustic excitation and sensing efficiency with a lateral resolution of approximately 10 ?m,enabling a larger scanning range in the depth direction than existing laser diode photoacoustic microscopes.Furthermore,by combining 3D printing of small-sized low-frequency ultrasound sensor and multi-wavelength laser diode,the system presented here is expected to develop into alabel-free,low-cost,miniaturized reflective photoacoustic functional microscope technology,which provides a new detection method for real-time monitoring and imaging of subcutaneous microcirculation structure and biometric identification,thereby,expanding application range of high-resolution photoacoustic microscopy imaging.