Development of a fast-scanning combined ultrasound-photoacoustic biomicroscope Roger J. Zemp*, Huihong Lu, Kory Mathewson, Janaka Ranasinghesagara, Yan Jiang, Andrew Walsh, Xuhui Chen Department of Electrical and Computer Engineering, University of Alberta, Edmonton, Alberta, Canada, T6G 2V4 *Email: zemp@ece.ualberta.ca ABSTRACT Recently a realtime photoacoustic microscopy system has been demonstrated. Unfortunately, however, displayed B-scan images were sometimes difficult to interpret as there was little structural context. In this work, we provide structural context for photoacoustic microscopy images by adding ultrasound biomicroscopy as a complementary and synergistic modality. Our system uses a voice-coil translation stage capable of 1” lateral translation, and can operate in excess of 15 Hz for 1-cm translations, providing up to 30 ultrasound frames per second. The frame-rate of the photoacoustic acquisitions is limited by the 20-Hz pulse-repetition rate of the laser, but can be increased with a faster-repetition-rate laser. Data from the system is streamed in real time from a 2GS/s PCI data acquisition card to the host PC at rates as high as 200 MB/s. The system should prove useful for various in vivo studies, including combined ultrasound Doppler and photoacoustic imaging. Keywords: photoacoustic imaging, high-frequency ultrasound, Q-switched lasers, biophotonics, cardiovascular research 1. INTRODUCTION Recently photoacoustic microscopy has emerged as a promising technology for visualizing optically-absorbing structures in vivo with ultrasonic spatial resolution. Short laser pulses are fired into tissue producing rapid but minimal heating in optically-absorbing pigments, and causing temperature-induced expansion. This transient mechanical perturbation serves as an internal source of ultrasonic waves which are detected by an ultrasound transducer in the imaging system. The received photoacoustic signals are subsequently reconstructed to form images of subcutaneous optical absorption. Spatial resolution in this case is determined primarily by ultrasonic focusing and receiver frequency response, and multiply-scattered light is well-tolerated, enabling optical-contrast imaging with high-resolution at new depths. Maslov et al [1] and Zhang et al [2] have demonstrated images of subcutaneous microvasculature using dark- field confocal photoacoustic microscopy (PAM), a raster-scanning-based reflection geometry imaging system. Leveraging known oxy-hemoglobin and deoxy-hemoglobin absorption spectra, photoacoustic methods possess the ability to image blood oxygen saturation as well as relative concentration of total hemoglobin [2-4]. Emerging photoacoustic methods are enabling molecular imaging in small animals, including imaging of gene expression [5] and distribution of cell receptors [6]. C-scan and accompanying 3-dimensional imaging methods currently require fairly long scan times. Recently, a 50- frame-per-second B-scan system has been reported using a high-frequency array transducer, a high-repetition-rate laser, and a multicore computer for realtime beamforming and display [7-9]. This system has been used to image the beating hearts of small animals in realtime. Unfortunately, these images were sometimes difficult to interpret, and were without structural context. While optical imaging technology, and in particular, photoacoustic methods, have made significant inroads for imaging small animals, high-frequency ultrasound [10, 11] has seen an explosion of growth in the pre-clinical imaging sector, partially due to recent availability of commercial pre-clinical high-frequency ultrasound systems. This adoption of high- frequency ultrasound by the biomedical research community is timely considering the vast array of animal models used in preclinical research. Photons Plus Ultrasound: Imaging and Sensing 2009, edited by Alexander A. Oraevsky, Lihong V. Wang, Proc. of SPIE Vol. 7177, 717711 · © 2009 SPIE · CCC code: 1605-7422/09/$18 · doi: 10.1117/12.809296 Proc. of SPIE Vol. 7177 717711-1