Proceedings Article | 13 February 2007
KEYWORDS: Photoacoustic spectroscopy, Absorption, Signal detection, Finite-difference time-domain method, Acoustics, Pulsed laser operation, Wave propagation, Numerical analysis, Biomedical optics, Signal generators
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.
