New results for the development of infrared absorption imaging

Near-infrared light may be detected in transillumination through several centimetres of tissue. Spectral changes in this light are routinely used for globally monitoring blood volume and oxygenation in the brain of newborn infants and observing the enhanced vascularity surrounding tumours in the breast. The imaging problem may be identified as the inversion of strongly multiply scattered light. The reconstruction method previously proposed is an iterative one requiring a sophisticated forward model and an analysis of the ill-posedness at each stage. The experimental arrangement for studies on a cylindrical phantom takes measurements at 32 equally angularly spaced locations for each of 32 similarly arranged input locations. The phantom that has been developed allows the values of the scattering coefficient (μs), absorption coefficient (μa), and angular scattering probability function f(s,s') [direction s' -> s] to be independently controlled. Absorbing objects are placed in the phantom to produce inhomogeneous data. Experimental results are compared with both Monte-Carlo and Finite Element simulations. The experimental and simulated data may be collected in both continuous and time-resolved form. The former shows a definite spatial variation with phantom inhomogeneity. The latter may afford additional information, but is limited at present by the poor signal-to-noise ratio available from the insirumentation.