It was recognised early in the history of optical imaging that photons which have been scattered a small number of times carry more spatial information than diffusive photons. Furthermore, if measurements could be made of minimally scattered photons, images could be reconstructed using the Radon transform as in x-ray CT, avoiding most of the difficulties associated with diffuse optical image reconstruction. Methods which can isolate minimally scattered photons from the diffusely scattered background, such as collimated detection, coherent techniques, and time-gating were reviewed in detail by Hebden et al. (1997). However, the fraction of minimally scattered photons transmitted across large (> several cm) thicknesses of tissue is immeasurably small, making this approach unsuitable for medical imaging. The length scale over which a collimated beam effectively becomes diffuse is known as the transport scattering length, which is about 1 mm in most biological tissues at NIR wavelengths. The transport scattering length is equal to the reciprocal of the transport scatter coefficient µ's, which is defined as, where µs is the scatter coefficient and g is the mean cosine of the scattering phase function.
Diffuse optical tomography can be contrasted with optical coherence tomography (OCT), which is a rapidly developing medical imaging modality, particularly in the area of ophthalmology. It is a range-gating technique which exploits the coherent properties of back-reflected light to generate very high spatial and temporal resolution images of tissues with a penetration depth of a few millimetres (Boppart 2003, Fercher et al. 2003, Fujimoto 2003).
Recently, the emphasis of research in medical imaging with diffuse light has moved away from the pursuit of high (~ mm) spatial resolution and towards functional imaging. It is widely appreciated that diffuse optical imaging can never compete in terms of spatial resolution with anatomical imaging techniques (e.g. x-radiography, ultrasound, and magnetic resonance imaging (MRI)), but offers several distinct advantages in terms of sensitivity to functional changes, safety, cost, and use at the bedside. In the following sections we review the alternative technological approaches currently being pursued, and discuss their relative merits and disadvantages.
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