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images must be acquired within the speckle decorrelation time in order to achieve high sensitivity detection. In the case of photorefractive crystal-based detection, the crystal response must be fast enough to follow the speckle decorre- lation, i.e., the crystal must be capable of rewriting the index grating on the order of 1.0 ms, such that the diffracted refer- ence beam is held in a fixed phase relationship with the trans- mitted signal beam. Fortunately, semiconductor photorefrac- tives, operating in the near-infrared wavelength band, pos- sess response times in the desired range.
In summary, acousto-optic imaging is a promising new modality that could fuel improvements in the detection and characterization of any tissue abnormalities that exhibit con- comitant changes in optical properties. One target application is in the diagnosis of breast cancer, where a dual mode ultra- sound/ acousto-optic imaging system may provide improve- ments in the ability to differentiate benign from malignant lesions. Technical challenges in the transition from a laborato- ry to a clinical setting are primarily associated with the sensi- tivity limitations imposed by the low photon flux and over- coming the effects of physiological motion on the diffuse opti- cal field. Recent advances in the detection of acousto-optic sig- nals, however, offer hope that dual-mode ultrasound/ acousto- optic imaging in vivo may be just over the horizon.
Acknowledgments
This work was supported by the Bernard M. Gordon Center for Subsurface Sensing and Imaging Systems via a National Science Foundation Engineering Research Center award number EEC-9986821.AT
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