Abstract
The objective of this work was to compare the accuracy of analyte concentration estimation when using transmission versus diffuse reflectance spectroscopy of a scattering medium. Monte Carlo ray tracing of light through the medium was used in conjunction with pure component absorption spectra and Beer-Lambert absorption along each ray's pathlength to generate matched sets of pseudoabsorbance spectra, containing water and six analytes present in skin. PLS regression models revealed an improvement in accuracy when using transmission compared to reflectance for a range of medium thicknesses and instrument noise levels. An analytical expression revealed the source of the accuracy degradation with reflectance was due both to the reduced collection efficiency for a fixed instrument etendue and to the broad pathlength distribution that detected light travels in the medium before exiting from the incident side.
Highlights
For some biomedical optical spectroscopy applications, it is possible to measure the tissue of interest in either transmission mode or diffuse reflectance mode
Pulse oximetry is most commonly performed in transmission mode on the finger or earlobe, but devices have been developed to use reflectance mode on the forehead.[1]
Diffuse reflectance spectroscopy from a scattering medium can lead to severely degraded quantitative accuracy compared to transmission spectroscopy through a thin slab of the same medium, over a broad range of analyte signal size and instrument noise conditions
Summary
For some biomedical optical spectroscopy applications, it is possible to measure the tissue of interest in either transmission mode or diffuse reflectance mode. Regional oximetry cannot be made in transmission mode, due to the extreme attenuation of the optical signal along the long transmission path through the head or arm Another application, noninvasive glucose monitoring using near-infrared (NIR) spectroscopy, has been attempted by many researchers at a variety of sites using either mode, including the finger in transmission,[10] inner lip in reflectance,[11] finger in reflectance,[12] forearm in reflectance,[13,14] and tongue in Intuitively, it makes sense that a transmission measurement through a tissue that is relatively thin compared to its inverse scattering coefficient would yield better accuracy than a measurement made in diffuse reflectance mode from a thick tissue. Partial least-squares regression of spectral changes to analyte concentration changes were made to compare the quantitative accuracy of reflectance and transmission
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