Assessment of Opto-mechanical Behavior of Biological Samples by Interferometry

Optoacoustic imaging is a relatively novel biomedical imaging modality that relies on the absorption of light to create pressure transients that can be detected ultrasonically. In most scientific communications, the source of tissue contrast has been described as primarily optical. However, the thermomecahnical properties of tissue, as expressed through the Gruneisen coefficient, also affect the optoacoustic signal. To investigate the effect of thermomechanical tissue properties short pulses (~ 6.5 ns) from an optical parametric oscillator at 750 nm were used to irradiate coagulated and uncoagulated tissue-mimicking albumen phantoms, to emulate normal tissue and tissue that has been heated. The phantoms respond to the laser-induced stress by thermoelastic expansion. This thermomechanical behavior of the samples was assessed using an interferometric system capable of measuring transient displacements with a temporal resolution of less than 10 ns and a spatial resolution of < 10 nm. The experimental measurement allowed determination of the Gruneisen coefficient which is an important thermo-mechanical sample property that can affect generation of optoacoustic signals. An increase in the value of Gruneisen coefficient of 65% was measured when phantoms were coagulated compared to uncoagulated phantoms, consistent with the stiffening of the tissue mimicking material. This suggests that for thermal therapy the changes in the Gruneisen coefficient are also an important source of optoacoustic contrast.