IMAGE-RECONSTRUCTION IN OPTICAL TOMOGRAPHY

Optical tomography is a new medical imaging modality that is at the th reshold of realization.,A large amount of clinical work has shown the very real benefits that such a method could provide. At the same time a considerable effort has been put into theoretical studies of its pro bable success. At present there exist gaps between these two realms. I n this paper we review some general approaches to inverse problems to set the context for optical tomography, defining both the terms forwar d problem and inverse problem. An essential requirement is to treat th e problem in a nonlinear fashion, by using an iterative method. This i n turn requires a convenient method of evaluating the forward problem, and its derivatives and variance. Photon transport models are describ ed and methods for obtaining analytical and numerical solutions for th e most commonly used ones are reviewed. The inverse problem is approac hed by classical gradient based solution methods. In order to develop practical implementations of these methods, we discuss the important t opic of photon measurement density functions, which represent the deri vative of the forward problem. We show some results that represent the most complex and realistic simulations of optical tomography yet deve loped. We suggest, in particular, that both time-resolved, and intensi ty-modulated systems can reconstruct variations in both optical absorp tion and scattering, but that unmodulated, non-time-resolved systems a re prone to severe artefact. We believe that optical tomography recons truction methods can now be reliably applied to a wide variety of real clinical data. The expected resolution of the method is poor, meaning that it is unlikely that the type of high-resolution images seen in c omputed tomography or medical resonance imaging can ever be obtained. Nevertheless we strongly expect the functional nature of these images to have a high degree of clinical significance.