基于单纵模纳秒脉冲激光的光学分辨率光声显微成像

Optical Resolution Photoacoustic Microscopy Based on Single-longitudinal-mode Nanosecond Laser

本文设计并搭建了一套激励光和超声探测器对向共轴共焦的具备亚细胞尺度成像分辨率和毫米级成像深度的光学分辨率光声显微成像(OR-PAM:Optical-resolution photoacoustic microscopy)系统。与基于多纵模脉冲激光激励的光声成像系统相比,使用自制的单纵模纳秒532 nm脉冲激励光源,将光声探测的信号强度提高了1.35倍,测得成像系统的轴向分辨率为18μm,横向分辨率为8μm,成像深度为1.54 mm,成像深度-横向空间分辨率比达192.5。利用研制的OR-PAM系统对嵌有碳纤维丝的仿体样品以及活体小鼠的耳朵进行成像,获得了高分辨率的图像。

Optical resolution photoacoustic microscopy(OR-PAM), benefiting from rich optical contrast, scalable acoustic resolution, and deep penetration depth, is a rapidly developing biomedical imaging technology in recent years. To meet the need of shallow cells imaging in large-scale living organisms, an OR-PAM system with subcellular level imaging resolution and more than one millimeter imaging depth was designed and constructed in this paper. In the OR-PAM system, a homemade single longitudinal mode(SLM) nanosecond(ns) 532 nm pulse laser was adopted as excitation source, and an ultrasonic sensor with a center frequency of 50 MHz was adopted as the acoustic detector, noting that the excitation laser and the ultrasonic detector were coaxially mounted with a common focus. Two-dimensional(2D) and three-dimensional(3D) images was obtained based on a homemade program-controlled 2D electric translation table, as well as an automatic signal acquisition and image reconstruction program. In a single point photoacoustic detection experiment using a agar block embedded with black tape as sample, the signal-to-noise ratio(SNR) of our OR-PAM was 1.35 times higher than that of a conventional OR-PAM system excited by a multi-longitudinal mode pulsed laser, owing to the low amplitude noise, less clutter and good beam quality of the self-made SLM 532 nm excitation laser.On this basis, the key performance index of the OR-PAM system was measured. The imaging depth of the system was determined by a series of single point photoacoustic detection with different excitation laser focus position, while the absorbers were 10 hairs placed at different depths in an imitation sample. Since the measured imaging depth was 1.54 mm, the axial and lateral resolutions of the OR-PAM system was determined using two samples embedded with a human hair and a sharp knife edge at the depth of1.54 mm, respectively. The measured photoacoustic signal of the human hair was copied and translated in time domain, and when the translated signal and the original signal was added, there existed two maximum peaks in the added signal. Then the axial resolution of the system can be determined using the minimum moving distance corresponding to the translation time that made the two peaks distinguishable, which was 18 μm in our experiment. The lateral resolution of the system was tested by scanning along the direction perpendicular to the edge of the sharp knife, which was 8 μm in our experiment. It was concluded that the ratio of the imaging depth to the lateral spatial resolution of our OR-PAM system was as high as 192.5. The developed OR-PAM system was also used to reconstruct the 2D images of an imitation samples embedded with carbon fiber filament and the ear of a living mouse,respectively. High-resolution images were obtained verifying the usability and performance of the OR-PAM system.