abstract
| - Visible and near infrared (NIR) light interact with biological tissue predominantly by absorption and elastic scattering. There are several physiologically interesting molecules which have characteristic absorption spectra at these wavelengths. In particular, the spectra of oxy-haemoglobin (HbO) and deoxy-haemoglobin (HHb) differ markedly, as do the oxygenation-dependent spectra of cytochrome oxidase. Haemoglobin provides an indicator of blood volume and oxygenation, whereas the cytochrome enzymes indicate tissue oxygenation. Unfortunately, while most of the physiological information is contained in the absorption coefficient (the number of absorption events per unit length, µa), the scatter coefficient (the number of scattering events per unit length, µs) in tissue is generally considerably larger, so that signals measured over distances of a few millimetres or larger are dominated by diffuse light. The physics of light transport in tissue has been explained in a number of recent review articles, for example Boas et al. (2001a), Schweiger et al. (2003) and Dunsby and French (2003), and will not be covered in detail here. The different absorption spectra of HbO and HHb are routinely exploited in physiological monitoring techniques such as pulse oximetry and near infrared spectroscopy (NIRS). Diffuse optical imaging techniques aim to process this information further and produce spatially resolved images. These images may display the specific absorption and scattering properties of the tissue, or physiological parameters such as blood volume and oxygenation. Diffuse optical imaging generally falls into one of two categories: topography or tomography. The distinction between these two techniques has become somewhat blurred, but we use the term optical topography when referring to methods which produce two-dimensional (2D) images of a plane parallel to the sources and detectors with limited depth information, and use optical tomography to describe techniques which generate full three-dimensional (3D) images from measurements taken from sources and detectors widely spaced over the surface of the object.
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