In vivo near infra-red spectroscopy instrumentation combining spatially-resolved and intensity modulation techniques

Near infra-red spectroscopy is of great interest for chemical and structural analysis in cells, tissues and organs. The requirements for instrumentation suitable for quantitative measurement are considered here. Problems due to optical scatter make absolute quantitative chemical analysis difficult. In clinical applications semi-quantitative analysis can be achieved by monitoring change within one subject. Approaches to achieve absolute quantitation involve the determination of absorption and scattering parameters of the target sample. Such methods require theoretical models of photon propagation upon which to base a measurement and analysis strategy and these are considered. This includes Transport theory and Diffusion theory. Implementation of the theoretically-based measurement techniques in practice is influenced by the characteristics of light sources and detectors and by digital processing hardware and software. The laboratory use of femto/pico second Ti-Sapphire lasers with streak camera detectors has its place in research, whilst simpler lower cost frequency domain instruments were thought to be more suitable for routine clinical use. However, recent advances in fast laser diodes and fast detectors allow the time-domain method to be re-considered. A system combining spatially-resolved techniques with intensity modulation is described.