Abstract
Multi-spectral imaging enables non-invasive sensing of chemical concentrations in biological tissue based on measurement of optical absorption, but invariably in the presence of high levels of scatter. Absorption is normally inferred from measurement of contrast of biological features, such as the vasculature, and so accuracy is degraded by the poorly characterized modulation-transfer function of the imaging optics and overlying tissue. We report how experimental characterization of the spectral variation of the tissue point-spread function and associated objective speckle pattern can be used to characterize the absorption spectrum and chromophore concentration, with a particular emphasis on determination of the ratio of oxygenated to deoxygenated hemoglobin within blood. Absorption measurements are determined purely by the geometry of the experiment, without degradation due to optical aberrations and associated light scatter. The technique offers enhanced and low-cost determination of in vitro or in vivo chromophore characterizations, including blood-gas analysis.
Highlights
The accurate optical measurement of chemical concentrations in scattering media is important for in vitro, in vivo and ex vivo sensing within biological tissue and fluids and for industrial monitoring
The standard form of the Beer-Lambert law states that the ratio of the transmitted to incident intensities of a collimated beam of light transmitted a distance d to a nominal ‘detection point’ within a turbid medium is given by
We describe here a non-imaging, lensless technique for determination of c, which is based on the spatial-frequency spectrum of the objective speckle recorded when laser light is focused onto a turbid medium
Summary
The accurate optical measurement of chemical concentrations in scattering media is important for in vitro, in vivo and ex vivo sensing within biological tissue and fluids and for industrial monitoring. In contrast to existing methods, the technique does not require features in the scene, such as blood vessels, to provide contrast and since no image is formed there is no degradation due to optics-induced contrast reduction It offers the possibility of simple and low-cost accurate measurement of optical absorption in scattering media, such as underpins important requirements for in vitro and in vivo blood-gas analysis [17]. This new measurement technique offers particular advantages for oximetry within the microvascular tissue where capillaries are not imaged due to their low contrast and high transparency.
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