2-D Optical Tomography for Medical Applications

Abstract

Optical spectroscopy techniques are frequently used to non-invasively monitor various physiological functions such as tissue and blood oxygenation. It is desirable to develop methods to localise these measurements and quantify the parameters of interest, e.g. oxygen saturation. There is also a need for the localisation of abnormal tissues using non-harmful radiation, for example localisation and characterisation of breast tumours. This is possible in principle, provided the optical properties of abnormal tissues are sufficiently different. To establish a baseline reference, against which more advanced optical imaging systems and methods can be compared, a rotate-translate optical imaging system was developed. Image reconstruction was performed using a conventional filtered back projection scheme, originally developed for CT. With this system the limits of spatial and contrast resolution as a function of the absorption and reduced scattering coefficient were investigated. The limiting factor was found to be contrast resolution. Given the often small differences in the absorption and scattering properties of different tissues this makes such a system of limited use. A second generation imaging system, using a rotate-rotate type scanning system, has been constructed. Data obtained with this system utilises all the light emerging in the plane of measurement, and does not assume rectilinear propagation. The image reconstruction algorithm developed for this uses diffusion theory to model the light transport, and an iterative reconstruction algorithm. Results obtained with this model are presented. A further method to improve image quality is the use of a phase resolved technique", which uses amplitude modulation of the light source. An outline is given of the principle of this technique, and the system designed for the measurement is discussed.