Basic Principles

How is light propagated in biologic media?

Absorption and scattering

Unlike X-rays, light radiation interacts with biological tissue not only through absorption events, but also through multiple scattering events, from which the turbid nature of the medium comes. This provides the possibility of obtaining tissue properties different from those observable by other techniques, constituting important complementary information for diagnosis.

In the study of the turbid media optics there are three fundamental parameters that characterize the medium: the scattering coefficient, the absorption coefficient and the anisotropy coefficient. The scattering coefficient and the absorption coefficient represent the inverse of the mean path between collisions and absorption events respectively and, therefore, have units of reciprocal length. The coefficient of anisotropy is a dimensionless parameter that is related to the average value of the cosine of the angle between the direction of incidence and the direction of exit after a scattering event.

Different medical studies show that optical properties differ between malignant and benign neoplasms , so techniques based on turbid media optics can discriminate the malignancy of tumors located in optically accessible areas.

Optical Window

The absorption of the different components that make up a typical biological tissue depends on the wavelength (color) of the light that crosses it. In particular, the absorption is relatively low between 750 nm and 950 nm. Below 600 nm hemoglobin absorbs a lot and above 1100 nm the water starts to stop being transparent.

EIt is in this zone of wavelengths, of red and near infrared, called NIR (near infrarred), where the development of methods of formation of images with light radiation becomes viable since the radiation can penetrate several centimeters.

Transmittance and reflectance

It is possible to obtain optical information from a sample using different geometric detection arrangements. The sources and detectors can be inside the tissue; they can be on the same side; or they can be on opposite sides.

When the sources and detectors are on opposite sides, the light diffusely transmitted by the tissue is used to infer its composition. This is how X-ray systems work, for example. In biomedical optics this is called diffuse transmittance .

If, on the other hand, the sources and the detectors are on the same side, diffusely reflected light is used to obtain information on the tissue. This is called diffuse reflectance .

The exact region explored will depend on the position of the sources and detectors and the path that the light makes between them.