A theory is provided for the detection efficiency of diffuse light whose frequency is modulated by an acoustical wave. We derive expressions for the speckle pattern of the modulated light, as well as an expression for the signal-to-noise ratio for the detector. The aim is to develop a new imaging technology for detection of tumors in humans. The acoustic wave is focused into a small geometrical volume, which provides the spatial resolution for the imaging. The wavelength of the light wave can be selected to provide information regarding the kind of tumor.There is a need to develop new imaging technology to detect cancers and tumors in humans. The present technology needs to be improved, and many different techniques have been suggested (1-6). For example, x-rays and ultrasound show tumors and provide accurate spatial information, but they do not provide information on the tumors' nature. Infrared light diffuses in humans and cannot provide information on spatial positions unless the tumor is very near to the surface. Light can, however, provide information on the nature of the tumor, because different tumors have different absorption bands.Recently, a new imaging technology that combines the benefits of ultrasound and diffuse light was proposed (7). Since then it has been under continuous development (8-13). By having the ultrasound and light present simultaneously, and by detecting the light intensity at the frequency modulated by the sound wave, one detects photons that have interacted with both sound and light. We use the phrase "tagging" to denote the process of modulating the light wave by the ultrasound frequency. UTL is ultrasound tagging of light. By focussing the sound wave into a small geometrical volume, one can provide spatial information regarding where the tagging occurred. The spatial resolution is about the wavelength of the ultrasound, which is about 1 mm, depending on the sound frequency. High frequency sound is used because of its shorter wavelength. Pulsing the laser light allows multiple images as the slow sound pulse travels through the medium. Phased arrays can be used to send ultrasound in different directions, which allows for sweeping over the complete volume. The tagged signal should be different in the volume occupied by a tumor. Varying the wavelength of the light can then provide information about the nature of the tumor, and the ultrasound can provide information regarding the tumor's location. When finally developed, this UTL technology could provide a new method of imaging.Although this idea is attractive, the UTL technology is still in its embryonic stage. Here we develop a mathematical model of the imaging process. We consider what kind of signal will be detected in this measurement. Several related issues bear upon this answer. First, what is the nature of the speckle pattern for a modulated signal? Diffuse light from a coherent source is known to be emitted with a interference patternThe publication costs of this article were defrayed in part by page charge payment. T...

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