Fast all-optical photoacoustic micro-imaging of vasculature

In an interdisciplinary collaboration of scientists with expertise in Physics, Mathematics and Medicine, photoacoustic imaging with optical ultrasound detection will be advanced towards application in biomedical research. Photoacoustics allows the generation of images with optical absorption contrast and resolution determined by the propagation and detection of ultrasound. Therefore, photoacoustic imaging is very attractive for biomedical research since the distribution of blood vessels and even the oxygen saturation of blood can be determined without contrast agents. The aim of the project is the optimization of a photoacoustic tomograph and the development of a photoacoustic microscope for the application in biomedical research. The common specialty of both devices is the detection of ultrasound waves with an optical phase contrast method and the recording of projection images of the acoustic field with a camera. A tomography system developed in the preceding project needed a rotation of the sample to obtain the projection data for 3D images, limiting its potential applications. The pursued rotatable tomograph will allow performing in-vivo experiments on humans and animals without any movement of the sample. With the optimized tomograph it will be possible to obtain 3D images within an imaging period of one minute and an image resolution of <50 m. The use of optical ultrasound detection with a camera for photoacoustic microscopy is a novel approach. This method uses excitation laser pulses focused to a line onto the sample, oriented orthogonal to the projection direction of the optical phase contrast method. Contrary to common photoacoustic microscopes, only a 1D scan is necessary to obtain 3D images, since the camera image already contains 2D image information. Hence, the data acquisition time is drastically reduced. The recorded C-scan images reveal acoustical resolution in direction parallel to the line and optical resolution in direction perpendicular to the line for superficial structures within the optical transport depth of about 1 mm. 2D images can be recorded in real-time and 3D images in a time determined by the laser repetition rate. For instance, with 10 kHz a C-scan image of size 5x5 mm can be recorded in less than one second. To obtain images from the recorded raw data, efficient image reconstruction algorithms will be implemented, for both tomography and microscopy. The ability of the developed imaging systems for fast, high-resolution 3D imaging will be tested on selected in vivo applications. In these applications also the functional imaging capabilities using multispectral excitation will be explored. The central application will be the monitoring of the wound healing process in mice and rats. Additionally, blood vessels in human skin will be visualized with the newly developed devices.