Oximetry considerations in the small source detector separation limit

Oximetry is a common blood monitoring technique, useful for the assessment of blood flow and blood oxygen saturation information. Commercial oximeters generally utilize an optical transmission measurement scenario which necessitates the use of wavelengths residing in the optical absorption window (650-1100 nm) which are capable of traveling long distances before absorption. When the source and detector fibers are brought close together (les1 mm) such wavelengths undergo too few scattering events for meaningful data abstraction. Small source-detector separation scenarios are becoming more common as tissue spectroscopy is conducted using catheter and small surface probes. This paper will demonstrate the insufficiencies of present oximetry techniques and the need to use visible wavelengths when conducting oximetry at small source-detector separations. It will begin with a theoretical derivation of the problems with NIR wavelengths in the small source detector separation limit. The theory will be compared to Monte Carlo derived data and in vivo data collected with a surface probe with les1 mm of separation between the source and detector fibers. The study demonstrates that visible wavelengths are more than two orders of magnitude more sensitive to changes in hemoglobin saturation when small source detector fiber separations are used for oximetry measurements