Characterization and Comparison of Two Chromophore Determination Techniques: The Modified Beer-Lambert Law (MBLL) and Spatial-Frequency Domain Imaging (SFDI)

Optical imaging meets a growing need to non-invasively quantify tissue health in a number of different clinical situations. As optical imaging technology has improved, the ability to quantify biologically-relevant chromophores has grown more accurate but at the cost of increased data acquisition and computation time. For applications such as measurements of functional cerebral activation, in which vascular response occurs on the order of milliseconds to seconds, there is a need for imaging modalities that acquire data quickly and accurately. Spatial-Frequency Domain Imaging (SFDI) is a technique that projects different wavelengths of light in spatially-modulated frequency patterns to quantify absolute optical properties and chromophore concentrations. Since SFDI projects light at different wavelengths and spatially-modulate frequency patterns, it requires several seconds to acquire data. In addition, SFDI requires mathematically rigorous processing techniques to calculate optical properties and chromophore concentrations, which can take several seconds to several hours to complete. A technique called Multi-Spectral Reflectance Imaging (MSRI) relies only on planar reflectance images to quantify changes in chromophore concentration using the Modified Beer-Lambert Law (MBLL), resulting in acquisition and computation times on the order of milliseconds. However, MBLL relies on the assumption of baseline chromophore concentrations in order to deduce absolute chromophore concentrations.;In this thesis I will be investigating a new technique that combines the use of MBLL with SFDI, with the goal of quickly and accurately quantifying chromophore concentrations. I validated the combination technique by first using SFDI to image tissue-simulating phantoms with varying concentrations of absorber, Naphthol Green-B, and constant reduced scattering. Both SFDI and the combination technique were able to calculate the concentration of Naphthol Green-B within 10 percent of the actual concentration after a 100 percent change in Naphthol Green-B concentration. The combination technique took approximately two to three milliseconds on average to calculate the concentration of Naphthol Green-B per iteration of acquisition while SFDI took approximately three to five minutes on average to calculate the concentration of Naphthol Green-B per iteration of acquisition (using a White Monte Carlo processing method). I then analyzed data from an experiment where SFDI was used to measure vascular response in a mouse brain after cortical activation. The combination technique differed from SFDI-calculated chromophore concentrations by 50 percent on average. However, the combination technique took approximately two to three milliseconds on average to calculate chromophore concentrations per iteration of acquisition while SFDI took approximately three to five seconds on average to calculate chromophore concentrations per iteration of acquisition (using the look-up table method). Despite the assumptions made while using the combination technique, the results presented in my thesis show that the combined use of MBLL and SFDI provides a way of quickly and accurately calculating chromophore concentrations for a number of imaging situations.