Page 20 - Summer 2007

ures the magnitude and phase of the diffuse optical field emitted. The modulated laser light propagates through dif- fuse media with a coherent wave front (referred to as photon density waves) and the dispersion of such waves is highly dependent on the absorption and scattering properties of the media. The media can be illuminated by modulated sources at multiple wavelengths, and a tomographic reconstruction algorithm used to spatially resolve tissue optical properties, from which tissue constituents can be inferred. The main drawback of diffuse optical tomography is its reliance on dif- fuse light, which propagates along tortuous paths, to create an image of subsurface features. This inherently limits the resolution that can be achieved using the technique to ~1 cm.
Acousto-optic (AO) sensing is a new hybrid technique that combines ultrasound with diffuse light to achieve deep- tissue imaging of optical properties with the spatial resolu- tion of ultrasound. In this technique, the sample is illuminat- ed by a continuous wave (CW) laser source to produce a dif- fuse optical field. To provide spatially resolved optical infor- mation, the sample is simultaneously ensonified by an ultra- sound beam. In the acousto-optic sensing volume, defined as the region where the light and sound overlap, the ultrasound modulates the local optical field. The modulated or “tagged” light propagates to the sample boundaries where it is processed and detected. The acousto-optic signal, or intensi- ty of the tagged light, gives an indication of the strength of the acousto-optic interaction. Although the acousto-optic signal in itself is not a direct measure of the optical properties of the sample, it does scale with the optical fluence through the sensing volume and is thus indicative of the local optical properties of the tissue—in particular the local absorption coefficient. By pulsing the ultrasound and scanning the beam across a plane in a manner similar to conventional B-mode ultrasound imaging, a 2-D acousto-optic image can be con- structed. One of the key advantages of acousto-optic imaging over diffuse optical tomography, for example, is that the acousto-optic signal is localized by the ultrasound. It origi- nates only from the acousto-optic sensing volume which has a spatial extent given by the ultrasound-pulse used. The net result is a subsurface image of the acousto-optic interaction strength at depth and with a spatial resolution dictated by the ultrasound field.
Background
The mechanism for the interaction of ultrasound with
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Propagation of an acoustic wave through a homogeneous
media will produce a periodic variation in the index of
refraction, due to periodic variations in pressure along the
acoustic wave path via the piezo-optic effect. Ballistic light
incident on this media will scatter or diffract on the index
grating. In the case of a moving grating, the diffracted beam
will also be frequency-shifted. This phenomenon is used in a
wide range of acousto-optic devices including optical fre-
quency and amplitude modulators, switches, and signal
processors. The interaction of ultrasound with diffuse light,
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ballistic light has been well understood for quite some time.
on the other hand, has received considerably less attention. The treatment of diffuse wave interaction with ultrasound typically involves decomposing the optical field into partial
Fig. 2. The primary mechanisms for the ultrasound modulation of light in optical- ly scattering media.
waves, each of which follows a different meandering path through the media (see Fig. 2). Along each path, the light experiences a sequence of scattering events, with the average distance between these events given by the transport mean free path. In the absence of ultrasound, the scattering sites remain stationary less any Brownian motion or physiological motion. In the presence of ultrasound, the scattering sites move with the periodic displacement of the media. The coherent motion of all scatterers along a given partial wave path produces an accumulated phase shift of the light propa- gating along that path. A second important modulation mechanism results from the periodic variation in the refrac- tive index associated with the ultrasound wave propagation. Again, a change in optical path length is accumulated over each partial wave path leading to a net phase shift. The total phase shift is then given by a combination of both modula- tion mechanisms, and the net optical field observed external to the sample is given by the sum of partial waves propagat- ing over all paths. In both modulation effects, the optical field is modulated at the ultrasound frequency, and their relative contributions depend on the ratio between the transport mean free path and the acoustic wavelength. If the mean free path is large compared to the wavelength, variations in the index of refraction dominate the response.
In an acousto-optic imaging experiment, the sample is illuminated by a laser source and the scattered light is detect- ed and processed to deduce information regarding the prop- erties of the sample in the region where the optical and ultra- sonic fields interact. As discussed above, the optical field at any point in space external to the object is composed of many partial waves, each originating from a different point in the object. The interference of these waves will cause the result- ing intensity to be anywhere from fully bright to fully dark, depending on the relative de-phasing between paths. This effect leads to the formation of a speckle pattern. When an ultrasonic beam is introduced, the phase of the light passing through the ultrasound is shifted with respect to the light passing through other regions of the sample. Interference between modulated and un-modulated light leads to the for- mation of a time varying speckle pattern. However, modulat- ed light emanating from a highly diffuse media has a spatial- ly random phase shift. Although the intensity of each indi- vidual speckle is modulated at the ultrasound frequency, the phase of this time varying signal is random from speckle to
18 Acoustics Today, July 2007