Pulsed optoacoustic characterization of layered media

Abstract

Thermoelastic waves generated by absorption of short laser pulses are used to characterize the layer structure of materials. The method is based on the analysis of the distribution of absorbed laser energy from temporal profiles of recorded acoustic signals. Particularly in view of noninvasive medical applications, optoacoustic front surface transducers are investigated in this study, where irradiation of the surface and detection of the acoustic wave take place on the same side of the sample. Front surface detection of optoacoustic waves is studied theoretically and experimentally, with special emphasis on acoustic diffraction and the differences between measurements in the acoustic near and far field. In the experiments, samples with stepwise and continuously varying depth profiles of absorption coefficient were irradiated with laser pulses of 6–8 ns duration. For the detection of the acoustic waves either an optical ultrasound sensor or an annular piezoelectric film was used. Generating the optoacoustic waves with a flat top laser beam profile and detecting the signals in the acoustic near field yields optimal conditions for direct measurements of the distribution of absorbed energy and the absorption coefficient in the medium. Far field measurements are advantageous for detecting and imaging layer boundaries at large depths in the sample.

Photoacoustic imaging in biomedicine

“…However, except for phantom samples, 44,45,47 the relative measurement error in these experiments 43,46 is around 10% because of the property variations in tissue samples. Besides, the deduction of optical properties from PA signals in the range of the acoustic frequencies, where diffraction and attenuation are not negligible, can be complicated.…”

“…In such a case, the tissue properties ͑e.g., the absorption coefficient in a nonscattering medium͒ and structure can be characterized by analyzing the temporal PA signal. [42][43][44][45][46][47][48] Other reviews 22,23 have discussed 1D or depth-resolved optoacoustic profiling. Here, we only give an example to illustrate the principle of depth profiling.…”

“…44,45 In the fitting process, the temporal profile of the laser pulse can be taken into account although the pulse width is typically quite small. In general, if the absorption or attenuation coefficient is a continuous function of depth in the sample, a reconstruction algorithm is required to extract the absorption information from the detected PA temporal signals.…”

Quantitative spectroscopic photoacoustic imaging: a review

“…As a final comment on 1-D methods, it is worth recalling that diffraction cannot occur in 1-D, hence divergences from 1-D are sometimes described as "problems of diffraction." 57,58,68,69 Indeed, the lack of diffraction and the positivity of the initial pressure distribution give a simple way to check experimentally that the 1-D assumption holds: a truly planar 1-D PA signal detected by a sufficiently broadband detector will not contain negative components. 70 …”

Imaging of small vessels using photoacoustics: An in vivo study

“…Layered tissue structures have been studied by Paltauf et al [15,16]. Oraevsky et al [17] have developed a laser optoacoustic imaging system (LOIS) for mammography.…”