Implementation of Full-Field Laser Ultrasound Imaging System for 3D Image Reconstruction

Abstract

When light of a wavelength that can be absorbed well is irradiated to the sample for a very short time, the sample instantaneously expands and contracts to generate ultrasound wave. This phenomenon is called the photoacoustic effect. Laser ultrasound imaging is a technology of generating ultrasound wave by generating a photoacoustic effect on the surface of the sample, measuring the reflected ultrasound wave while propagating into the inside of the sample, and imaging the structure of the target inside the sample. Conventional laser ultrasound technology uses an ultrasound transducer to measure ultrasound wave, which requires contact with the sample surface. Therefore, infection or damage due to physical contact may occur. To overcome this, a technology for measuring ultrasound wave using a laser has been developed, which can only measure one point at a time. Therefore, scanning is essential to image the structure of the target inside the sample. In this study, by overcoming the disadvantages of the existing laser ultrasound imaging system, we implemented a system that can measure ultrasound wave without scanning and reconstruct it to obtain a 3D structure of the target. The system consists of an excitation laser to generate ultrasound wave and a Michelson interferometer to detect ultrasound wave. In addition, an sCMOS camera was used to acquire ultrasound wave from a 2D sample surface at a time. The off-axis holography scheme was applied to acquire only the phase information of the sample surface because the surface displacement by ultrasound wave is very small. And since the speed of ultrasound wave is faster than the frame rate of the camera, the probe laser was irradiated with pulse to acquire the surface displacement at a specific moment. Only the surface displacement by ultrasound wave was obtained from the difference of the phase information of the surface when the excitation laser was not irradiated and when it was irradiated. A system constructed using a sample that was put into a transparent PDMS with a pencil lead as a target, mixed with PDMS and carbon black powder and applied thinly on the transparent PDMS was applied. The surface displacement by the ultrasound wave that was reflected from the target was acquired from the sample surface at different times. By applying this to the image reconstruction algorithm, the 3D structure of the target inside the sample was obtained.