Imaging of Acoustic Attenuation and Speed of Sound Maps using Photoacoustic Measurements Rene G.H. Willemink a , Srirang Manohar b , Yashasvi Purwar c , Cornelis H. Slump a , Ferdi van der Heijden a and Ton G. van Leeuwen b a Signals and Systems Group, University of Twente, Enschede, The Netherlands b Biophysical Engineering Group, University of Twente, Enschede, The Netherlands c Department of Chemical Engineering, Indian Institute of Technology, Madras, India ABSTRACT Photoacoustic imaging is an upcoming medical imaging modality with the potential of imaging both optical and acoustic properties of objects. We present a measurement system and outline reconstruction methods to image both speed of sound and acoustic attenuation distributions of an object using only pulsed light excitation. These acoustic properties can be used in a subsequent step to improve the image quality of the optical absorption distribution. A passive element, which is a high absorbing material with a small cross-section such as a carbon fiber, is introduced between the light beam and the object. This passive element acts as a photoacoustic source and measurements are obtained by allowing the generated acoustic signal to propagate through the object. From these measurements we can extract measures of line integrals over the acoustic property distribution for both the speed of sound and the acoustic attenuation. Reconstruction of the acoustic property distributions then comes down to the inversion of a linear system relating the obtained projection measurements to the acoustic property distributions. We show the results of applying our approach on phantom objects. Satisfactory results are obtained for both the reconstruction of speed of sound and the acoustic attenuation. Keywords: multi-modality imaging, optoacoustics, ultrasound transmission tomography 1. INTRODUCTION Photoacoustic (PA) imaging is a new imaging technique, suited for medical imaging purposes due to its non- invasive character and its ability to penetrate through soft tissue. The technique involves illuminating the object with pulsed optical or microwave energy; since this is non-ionizing radiation the technique is expected to be relatively safe 1 if the maximum permissible exposures (MPEs) are not exceeded. The imaging process is based on the absorption of the input energy by an object, resulting in a spatial distribution of temperature increases. The resulting temperature rise leads to the generation of outward traveling pressure waves by the thermoelastic effect, 2 which can be measured outside of the object. An image of the optical absorption distribution can be reconstructed by using these measured acoustic signals. 3 The measured acoustic signals are not only dependent on the optical absorption distribution, but also on the acoustic property distribution of the object. An often used assumption in PA imaging is that the acoustic properties of the imaged object are homogeneous. However, this assumption is violated in soft tissue, where each tissue type can have a slightly different speed of sound or acoustic attenuation factor. The result of, for example, assuming a constant value for the speed of sound in an object actually having an inhomogeneous speed of sound will be that the measurement model describing the relation between optical absorption and observed acoustic signals is incorrect. Reconstructing an image of the optical absorption distribution requires inversion of this measurement model and consequently results in an incorrect reconstruction of the optical absorption distribution. When the distribution of the acoustic property of the object is known beforehand it can be incorporated into the measurement model. Using the improved measurement model in the reconstruction phase will result in better agreement of the reconstructed optical absorption distribution with the actual optical absorption distribution. Further author information: (Send correspondence to Rene Willemink) Rene Willemink: E-mail: rene.willemink@utwente.nl Cum Laude Poster Paper Medical Imaging 2008: Ultrasonic Imaging and Signal Processing, edited by Stephen A. McAleavey, Jan D'hooge, Proc. of SPIE Vol. 6920, 692013, (2008) · 1605-7422/08/$18 · doi: 10.1117/12.770061 Proc. of SPIE Vol. 6920 692013-1 2008 SPIE Digital Library -- Subscriber Archive Copy