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Abdominal ultrasonography, where gallstones create acoustic shadowing of the ultrasound, seen at bottom. A short-distance acoustic shadow occurs behind a building or a sound barrier. The sound from a source is shielded by the obstruction. Due to diffraction around the object, it will not be completely silent in the sound shadow. The amplitude ...
The acousto-optic effect couples the optical refractive index of the medium with its density and pressure. Thus, spatial and temporal variations in pressure (e.g., due to ultrasound radiation) induces corresponding variations in refractive index. Optical wavelength and wavenumber in medium depend on refractive index.
The generated refractive index, (2), gives a diffraction grating moving with the velocity given by the speed of the sound wave in the medium. Light which then passes through the transparent material, is diffracted due to this generated refraction index, forming a prominent diffraction pattern .
A head shadow (or acoustic shadow) is a region of reduced amplitude of a sound because it is obstructed by the head. It is an example of diffraction. [1] [2]Sound may have to travel through and around the head in order to reach an ear.
A shadowgram is not a focused image, rather it is a mere shadow. In the shadowgram, the differences in light intensity are proportional to the second spatial derivative of the refractive index field in the transparent medium under study. Once the distance from the transparent disturbance to the cast shadow becomes too large, then the shadow no ...
The light that is deflected toward or away from the knife edge produces a shadow pattern depending upon whether it was previously blocked or unblocked. This shadow pattern is a light-intensity representation of the expansions (low density regions) and compressions (high density regions) which characterize the flow.
Applying these ultrasound waves, or an ultrasound field, to a region of tissue will change the optical properties of the tissue in time and space. This region of ultrasound-modulated tissue is the region of interest (ROI) which will be analyzed. Photons are then sent into the tissue from some source, such as a laser.
The ultrasound waves create changes in the air's index of refraction. When the laser encounters these changes, the beam slightly deflects and displaces to a new course. This change is detected and converted to an electric signal by a custom-built photodetector.