Principle of Diffuse Optical Tomography

Typical values of p s are around 10 cm Consequently, there are practieally no unscattered or ballistic photons for tissue thicker than 1 cm [149]. Instead, the photons must be considered to diffuse through the tissue. Consequently, the spatial resolution of DOT images is extremely poor and cannot compete with positron emission, X-ray and MRI techniques. 

The absorption in tissue is dominated by oxy-haemoglobin, deoxy-haemo-globin, lipids, and water [121]. The extinction coefficients of the tissue constituents are shown in Fig. 5.41, left. Absorption spectra of tissue measured in vivo are shown right. There is an absorption window from approximately 650 to 900 nm. Therefore, NIR light can be transmitted and detected through tissue layers as thick as 10 cm. Absorption coefficients for various types of tissue are given in [367]. 

In spite of the poor spatial resolution, DOT in the NIR has the benefit that the measured absorption eoefficients are related to the bioehemical constitution of the tissue, sueh as haemoglobin concentration and blood oxygenation [121, 346]. If exogenous markers are used, the absorption or fluoreseence delivers additional information about blood flow, blood leakage, ion eoneentrations, or protein binding state [135, 369, 460]. 

Increases in both scattering and absorption decrease the output intensity. However, increased scattering increases the pulse width while increased absorption tends to decrease it [512]. Therefore, the shape of the time-of-flight distribution of the photons can be used to distinguish between scattering and absorption. Qualitatively, early photons are mainly influenced by scattering, whereas later photons are increasingly influenced by absorption as well. 

In diffuse reflection experiments, the depth of scattering and absorption changes in the tissue can be derived from time-resolved data [481]. The first and second moments of the time-of-flight distributions are especially sensitive to changes in deep tissue layers [325, 328]. 

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