ThC4 (I nvited) 9:30 am - 10:00 am HIGH-RESOLUTION PHOTOACOUSTIC TOMOGRAPHY Lihong V. Wang*, Xueding Wang, Geng Ku, Xueyi Xie, George Stoica Optical Imaging Laboratory, Department of Biomedical Engineering Texas A&M University, 3120 TAMU College Station, Texas 77843-3120 URL: http://oilab.tamu.edu; Email: LWang@tamu.edu * Corresponding author. Abstract: Optical contrast is sensitive to physiological parameters, such as the oxygen saturation and total concentration of hemoglobin, in biological tissues. Photoacoustic tomography is based on the high optical contrast yet utilizing the high ultrasonic resolution. Our work in this emerging area of research will be summarized in this invited talk. In this technology, a difraction-based inverse-source problem is solved in the image reconstruction, for which we developed the rigorous reconstruction theory. We implemented a prototype and accomplished non-invasive transdermal and transcranial functional imaging of small-animal brains in vivo. Changes in the cerebral blood oxygenation and blood volume of a rat, as a result of the alternation from hyperoxia to hypoxia, were imaged successfully. Keywords Photoacoustic tomography; optical contrast; ultrasonic resolution; inverse-source reconstruction; oxygen saturation of hemoglobin I. INTRODUCTION Photoacoustic tomography (PAT, also referred to as optoacoustic or thermo acoustic tomography) involves both photons and ultrasound. A short-pulsed laser source is used to irradiate the biological tissue samples under investigation. A temperature rise of the order of m will be produced in a short time frame. Consequently, thermoelastic expansion will cause emission of acoustic waves, referred to as photoacoustic waves. The photoacoustic waves are measured by wideband ultasonic transducers around the sample, and the acquired photoacoustic waves are used to reconstruct the optical absorption distributions. Because the laser pulse is short, proportionately high frequency ultasonic waves will be produced, which can provide diffraction-limited spatial resolution. In summary, the contrast in PAT is determined primarily by the optical properties of the biological tissues, and the spatial resolution in PAT is determined primarily by the photoacoustic waves originating from within the biological tissues. II. FUNDAMENTALS OF PHOTOACOUSTICS: RECONSTRUCTION ALGORITHM If the electromagnetic pumping pulse duration is much shorter than the thermal diffsion time through a voxel, thermal diffsion can be neglected; this is known as the 0-7803-8557-8/04/$20.002004 IEEE 767 thermal confinement. In this case, the acoustic wave p(rJ) is related to electromagnetic absorption, H (rJ) , by the following wave equation: a 2 p(rJ) , 2 ( - ) _ jvs aH (rJ) �:" 2� v p r,t - , a1 C at (1) where 1 = ts; v s is the acoustic speed, assumed to be constant; C is the specific heat; and j is the coeficient of volume thermal expansion. H(rJ) can further be written as the product of a purely spatial component q(r) that describes the electomagnetic absortion properties of the medium at r and a purely temporal component '(1) that describes the shape of the irradiating pulse. Our group derived the exact inverse solutions in planar, spherical, and cylindrical geometries [1-3]. When the distance between the photoacoustic sources and the detector is much longer than the wavelengths of the high frequency photoacoustic waves that are usefl for imaging, the following approximate inverse solution holds: q (r) = C f dSo cos( B d ) 1 ap(ro,t) 1 ' (2) s t at t=l� -rllv o 0 .1 where C is a constant; So is the surface of detection; and Bd is the angle between the normal of dSo and r - r o . III. EXPERIMENTS The experimental system for PAT of a rat brain in vivo has been described before [see Ref. (4)]. In this work, through the modulation of the inhaled oxygen concentration, we changed the systemic physiological status of the rat. Firstly, provided with pure oxygen, the rat was under the hyperoxia status. Two photoacoustic images of optical absortion in the cerebral cortex of the rat brain were acquired with laser light at the 584-n and the 600-n wavelengths, respectively [see Figs. leA) and (B)]. Then the breathing gas was changed slowly to a mixed gas with a low concentration of oxygen (�8% O2, �5% CO2 and �87% N2). Then, under the hypoxia status, another two images of the cerebral cortex corresponding to the same two wavelengths were acquired (not shown). With the high optical contrast between the blood and background brain tissues, each brain image presents the vascular structure in the cerebral cortex clearly and matches well with the open-skll anatomical photograph obtained after the photoacoustic imaging experiment [see