The interferometric measurement of laser induced thermoelastic expansion of tissue samples can be used to estimate
their optical, thermal and mechanical properties. This method was used to assess the Gruneisen coefficient and optical
attenuation depth for native and coagulated ex-vivo bovine liver and porcine kidney samples. The results demonstrate
decreases of 54% and 60% in the optical attenuation depth in bovine liver and porcine kidney after coagulation,
respectively. The Gruneisen coefficient of native porcine kidney was determined to be 58% smaller (p < 0.05) that native
bovine liver. The measured Gruneisen coefficients for native and coagulated ex-vivo porcine kidney were 0.07 ± 0.03
and 0.105 ± 0.02, respectively, whereas the Gruneisen coefficients for native and coagulated liver were 0.126 ± 0.036
and 0.127 ± 0.04, respectively. Our measurements indicate significant inter sample variability due likely to inherent
variations in tissue optical absorption and surface preparation.
Several studies have explored the potential of optoacoustic imaging for monitoring thermal therapies, yet the origin of the contrast in the images is not well understood. A technique is required to measure the changes in the optical and thermomechanical properties of tissues upon coagulation to better understand this contrast. An interferometric method is presented for measuring simultaneously the optical and thermomechanical properties of native and coagulated ex-vivo bovine tissue samples based on analysis of the surface displacement of irradiated samples. Surface displacements are measured after irradiation by short laser pulses at 750 nm. A 51% decrease in the optical attenuation depth is observed for coagulated liver samples compared to native samples. No significant differences in the Grüneisen coefficient are measured in the native and coagulated tissue samples. A mean value of 0.12 for the Grüneisen coefficient is measured for both native and coagulated liver tissues. The displacement profiles exhibit consistent differences between the two tissue types. To assess the changes in the sample mechanical properties, the experimental data also are compared to numerical solutions of the equation for thermoelastic deformation. The results demonstrate that differences in the tissue expansion dynamics arise from higher values of elastic modulus for coagulated liver samples compared to native ones.
Dynamics of the thermoelastic expansion of native and coagulated
ex-vivo bovine liver tissues after their irradiation
by short laser pulses were studied. The differences in optical and thermo-mechanical properties of the native and
coagulated samples such as their Gruneisen coefficient and optical attenuation depth were quantitatively determined. It
was found that for coagulated ex-vivo bovine liver samples, the optical attenuation depth decrease by an average of 47%.
Also significant differences were observed in the dynamics of thermoelastic expansion of the tissue surface. These
differences can be potentially linked to differences in
thermo-mechanical properties between native and coagulated
samples. The changes in these properties may be
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.
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