Tissue viscosity is correlated with tissue pathological changes and provides information for tissue characterization. In this study, we report an optical method to track continuous shear-wave propagation at centimeter depths in an optically turbid medium. Shear-wave attenuation coefficients were measured at multiple frequencies using shear-wave laser speckle contrast analysis (SW-LASCA) to quantitatively estimate tissue viscosity using the Voigt model. Shear waves were generated within tissue-mimicking phantoms by an amplitude-modulated ultrasound (modulation frequency: 100 to 600 Hz) and tracked by time-resolved laser speckle contrast difference received on a charged-coupled device camera. Averaged contrast difference over a selected time window was related to shear-wave amplitude and used to calculate the shear-wave attenuation coefficient. Phantoms of varying viscosities (0.1 and 0.3 Pa s) were studied. Attenuation coefficients for different shear-wave frequencies (100 to 600 Hz) were calculated. Derived viscosity values had a maximum standard deviation of 9%, and these values were consistent with the independent measurements reported in a previous study using nonoptical methods.
In this paper we present a method to visualize the pressure field of an ultrasound beam in a single shot of the CCD and
to image the shear wave propagation based on acousto-optic laser speckle contrast analysis. The contrast images show
features in the near field, far field and central region of the ultrasound beam and the pressure profile fits with that
measured with a hydrophone. The shear wave propagation was acquired by changing the imaging delay time after the
ultrasound burst. This method can be used to study the shear wave properties of common tissue phantoms to guide
experiments on tissue.
Ultrasound modulated optical tomography (UOT) is a hybrid technique which combines optical contrast with ultrasound
resolution and has shown some potential for early cancer detection, functional and molecular imaging. However, one
current problem with this technique is the weak optical modulation signal strength and consequently low Signal-to-Noise
Ratio (SNR). In this study, the effect of increasing the amplitude of the ultrasound induced particle displacement on the
UOT signal is investigated using a Monte Carlo simulation tool. The simulation software was validated against those
reported in the literature and good agreement was achieved. The simulated amplitude of particle displacement was varied
from 0.1 to 500nm . The results showed a significant increase in UOT signal with particle displacement for low
displacements, followed by saturation when displacement increased beyond a certain level. Further simulations were
performed to investigate the saturation by changing the optical wavelength from 400nm to 600nm . The results show
that the UOT signal saturates at lower particle displacements for smaller wavelengths. This suggests that the phase
variations along a photon path can be large enough to cause cancellation as particle displacement increases. This study is
part of ongoing efforts to improve the SNR of UOT through using the large particle displacements created by high
amplitude ultrasound and radiation forces.