OPTICAL IMAGING IN MEDICINE .2. MODELING AND RECONSTRUCTION

The desire for a diagnostic optical imaging modality has motivated the development of image reconstruction procedures involving solution of the inverse problem. This approach is based on the assumption that, gi ven a set of measurements of transmitted light between pairs of points on the surface of an object, there exists a unique three-dimensional distribution of internal scatterers and absorbers which would yield th at set. Thus imaging becomes a task of solving an inverse problem usin g an appropriate model of photon transport. In this paper we examine t he models that have been developed for this task, and review current a pproaches to image reconstruction. Specifically, we consider models ba sed on radiative transfer theory and its derivatives, which are either stochastic in nature (random walk, Monte Carlo, and Markov processes) or deterministic (partial differential equation models and their solu tions). Image reconstruction algorithms are discussed which are based on either direct backprojection, perturbation methods, nonlinear optim ization, or Jacobian calculation. Finally we discuss some of the funda mental problems that must be addressed before optical tomography can b e considered to be an understood problem, and before its full potentia l can be realized.