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The medical use of light to probe deep tissues dates back to the 19th century, beginning with Bright’s 1831 report on hydrocephalus. Bright noted that sunlight or light from a candle was able to shine through the head of a patient who suffered from hydrocephalus, a disorder characterized by excess cerebrospinal fluid in and around the brain. The ability of light to transilluminate tissues was later noted by Curling in 1843 in conjunction with problems of the scrotum, and by Cutler in 1929 in relation to breast lesions. The year 1977 represented another pivotal point for the field of optical biomedical diagnostics. It was then that Frans Jöbsis demonstrated the deep transillumination of mammalian tissues using near-infrared (NIR) light, including the chest of a dog and an adult human head from temple to temple. Furthermore, by combining the penetrating properties of NIR light with spectroscopy, he demonstrated the diagnostic value of NIR light for the assessment of hemoglobin oxygen saturation and the cytochrome a-a3 redox state. Given the cellular metabolic information afforded by NIR spectroscopy, coupled with its deeply penetrating and noninvasive properties and the intrinsic low cost of optical technology, it is apparent that this new biomedical tool holds great promise for diagnosing a variety of pathologies. One fundamental limitation of the use of NIRS for diagnostic purposes is the problem of localizing the origin of the signals. One can easily demonstrate the extent of this limitation with simple experiments, which most of us have done at one time or another. For example, in a darkened room, take a bright penlight or a red laser pointer and place it under your thumb. As you activate the light source, you will readily observe your fingernail and the surrounding tissue glowing with deep red light, confirming the ability of red/NIR radiation to transilluminate thick mammalian tissue. A typical pattern of light emerging from the finger is shown in Fig. 6.1. What is notably missing from this light pattern is imaging information about the internal structure of the thumb. There is no evidence, for example, that the bone in the finger is casting a shadow. To the unaided eye, the inside of the thumb appears to be filled with a homogeneous medium.
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