Accurate modelling of tissue properties in diffuse optical imaging of the human brain

Abstract

Diffuse optical imaging (DOI) is an emerging imaging modality for non-invasive functional medical imaging, using near infrared (NIR) or visible red light. The innovation is to derive functional information about living tissue from measurements of light that has passed through it. Optical imaging can be applied to imaging of tissues as diverse as the central nervous system, female breast, muscle, and joints of fingers. This thesis addresses the application of DOI to studying the human brain. In this thesis, the problems of modelling light propagation in the adult and infant human head, and reconstructing three-dimensional images of functional changes in the brain using optical measurements, are addressed. Difference imaging, where changes from baseline optical parameters rather than absolute parameter values are reconstructed, is considered. The goal was to develop methods for accurate modelling of light propagation and to clarify how specific aspects of the computational modelling affect the reconstruction of functional images from optical measurements of the human brain. Specifically, the significance of anisotropic light propagation in the white matter, and a priori knowledge of the anatomy and the optical properties of the head and brain are studied. Moreover, a generic probabilistic atlas model of the infant head to enhance image reconstruction is developed. Significance of anisotropic light propagation was found to be small in optical imaging of the adult brain. Although anisotropic light propagation may have a larger impact on the measured signal when infants are imaged, results suggest that image reconstruction can be performed without taking anisotropy into consideration. The use of a priori anatomical knowledge was found to significantly improve the accuracy and robustness of image reconstruction in difference imaging. The results suggest that for optimal reconstructions, individual MR imaging based anatomical data should be used when possible. For cases where individual anatomical data is not available, atlas models should be developed. An important consideration is how to obtain the baseline optical parameters of tissue classes in the anatomical model. Literature-derived parameters can be used as a starting point. For optimal results however, methods should be developed for estimating the baseline parameters from measured data.