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Proceedings Paper

Image reconstruction in photoacoustic tomography using integrating detectors accounting for frequency-dependent attenuation
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Paper Abstract

Photoacoustic Imaging (also known as thermoacoustic or optoacoustic imaging) is a novel imaging method which combines the advantages of Diffuse Optical Imaging (high contrast) and Ultrasonic Imaging (high spatial resolution). A short laser pulse excites the sample. The absorbed energy causes a thermoelastic expansion and thereby launches a broadband ultrasonic wave (photoacoustic signal). This way one can measure the optical contrast of a sample with ultrasonic resolution. For collecting photoacoustic signals our group introduced so called integrating detectors a few years ago. Such integrating detectors integrate the pressure in one or two dimensions -a line or a plane detector, respectively. Thereby the three dimensional imaging problem is reduced to a two or a one dimensional problem for the projections and a two or three dimensional inverse radon transform as a second step to get the three dimensional initial pressure distribution. The integrating detectors are mainly optical detectors and thus can provide a high bandwidth up to several 100 MHz. Using these detectors the resolution is often limited by the acoustic attenuation in the sample itself, because attenuation increases with higher frequencies. Stoke's equation describes the attenuation of photoacoustic generated waves in liquids very well, which results in an increase of the acoustic attenuation with the square of the frequency. For fat tissue an absorption coefficient which is approximately linear proportional to frequency is reported. Presented measurements give an exponential power law dependency with an exponent between 1.31 and 1.36 in fat tissue near the skin of a pig. These equations describing frequency dependent acoustic attenuation have been solved in the past by decomposing the pressure wave into plane waves damped in space, described by the complex part of the wave number equal to the attenuation coefficient. One main result of this paper is that for Photoacoustic Tomography another description seems to be very useful: like for a standing wave in a resonator the wave number is real but the frequency is complex. The complex part of the frequency is the damping in time. Both descriptions are equivalent, but with the complex frequency description acoustic attenuation can be included in all "k-space" methods well known in Photoacoustic Tomography just by introducing a factor describing the exponential decay in time.

Paper Details

Date Published: 24 February 2010
PDF: 8 pages
Proc. SPIE 7564, Photons Plus Ultrasound: Imaging and Sensing 2010, 75640O (24 February 2010); doi: 10.1117/12.843220
Show Author Affiliations
P. Burgholzer, RECENDT GmbH (Austria)
H. Roitner, RECENDT GmbH (Austria)
J. Bauer-Marschallinger, RECENDT GmbH (Austria)
G. Paltauf, Karl-Franzens-Univ. Graz (Austria)

Published in SPIE Proceedings Vol. 7564:
Photons Plus Ultrasound: Imaging and Sensing 2010
Alexander A. Oraevsky; Lihong V. Wang, Editor(s)

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