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13 February 2007 Effects of absorption properties on photoacoustic spectral characteristics: numerical analysis
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In photoacoustics, characteristics of the absorbed optical energy are a major determining factor of the characteristics of the photoacoustic signal. In other words, objects with different absorption properties generate photoacoustic signals with different frequency contents. Therefore, effects of absorption properties on photoacoustic spectral characteristics need to be fully understood in order to devise optimal setup for photoacoustic signal detection. The main purpose of this paper is therefore to numerically investigate such effects using a finite-difference time-domain (FDTD) approach. Photoacoustic signal generation can be described by the thermal conduction equation, the continuity and the Navier-Stokes equations, and the state equations in the system. To reduce computational and storage requirements, an axis-symmetrical cylindrical coordinate system with the z-axis parallel to the laser irradiation direction is adopted in our study. Moreover, the MacCormack scheme, which is fourth-order accurate in space and second-order accurate in time, and the first-order Mur absorbing boundary conditions are introduced to implement the FDTD code. The absorption coefficient range from 1cm-1 to 100 cm-1 and the signals are detected in forward modes. Results show that the peak frequency of the signal with absorbing coefficient 1cm-1, 10cm-1, 20cm-1, 30cm-1, 50cm-1, and 100 cm-1 is 2.4MHz, 2.4MHz, 4.2MHz, 4.8MHz, 6MHz, and 7.8MHz when detected forwardly. Note that although the peak acoustic frequency increases with the absorption coefficient, but the linear relationship between the two parameters does not hold. The results also imply that photoacoustic image contrast can be improved by properly selecting the receiver signal bandwidth. Assuming two objects with absorption coefficient of 10cm-1 and 100cm-1, respectively, it is estimated that a 11.34dB contrast improvement is achievable in the forward mode, while a 8.13dB improvement is possible in the backward mode.
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Yae-lin Sheu, Chen-Wei Wei, and Pai-Chi Li "Effects of absorption properties on photoacoustic spectral characteristics: numerical analysis", Proc. SPIE 6437, Photons Plus Ultrasound: Imaging and Sensing 2007: The Eighth Conference on Biomedical Thermoacoustics, Optoacoustics, and Acousto-optics, 643710 (13 February 2007);

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