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Description:
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Optical imaging of soft biological tissues is highly desirable since it is nonionizing
and provides sensitive contrast information which enables detection of physiological
functions and abnormalities , including potentially early cancer detection . However ,
due to the diffusion of light , it is dificult to achieve simultaneously both good spatial
resolution and good imaging depth with the pure optical imaging modalities .
This work focuses on the ultrasound -modulated optical tomography - a hybrid
technique which combines advantages of ultrasonic resolution and optical contrast .
In this technique , focused ultrasound and optical radiation of high temporal co -herence are simultaneously applied to soft biological tissue , and the intensity of the
ultrasound -modulated light is measured . This provides information about the optical
properties of the tissue , spatially localized at the interaction region of the ultrasonic
and electromagnetic waves .
In experimental part of this work we present a novel implementation of high -resolution ultrasound -modulated optical tomography that , based on optical contrast ,
can image several millimeters deep into soft biological tissues . A long -cavity confocal
Fabry -Perot interferometer was used to detect the ultrasound -modulated coherent
light that traversed the scattering biological tissue . Using 15 -MHz ultrasound , we
imaged with high contrast light absorbing structures placed 3 mm below the surface
of chicken breast tissue . The resolution along the axial and the lateral directions with respect to the ultrasound propagation direction was better than 70 and 120à ¹m , respectively . This technology is complementary to other imaging technologies ,
such as confocal microscopy and optical -coherence tomography , and has potential for
broad biomedical applications .
In the theoretical part we present various methods to model interaction be -tween the ultrasonic and electromagnetic waves in optically scattering media . We first extend the existing theoretical model based on the diffusing -wave spectroscopy
approach to account for anisotropic optical scattering , Brownian motion , pulsed ul -trasound , and strong correlations between the ultrasound -induced optical phase in -crements . Based on the Bethe -Salpeter equation , we further develop a more general
correlation transfer equation , and subsequently a correlation diffusion equation , for
ultrasound -modulated multiply scattered light . We expect these equations to be
applicable to a wide spectrum of conditions in the ultrasound -modulated optical tomography of soft biological tissues . |