title: Assessment of optical path length in tissue using neodymium and water absorptions for application to near-infrared spectroscopy.
abstract: Quantitative analysis of blood oxygen saturation using near-IR spectroscopy is made difficult by uncertainties in both the absolute value and the wavelength dependence of the optical path length. We introduce a novel means of assessing the wavelength dependence of path length, exploiting the relative intensities of several absorptions exhibited by an exogenous contrast agent (neodymium). Combined with a previously described method that exploits endogenous water absorptions, the described technique estimates the absolute path length at several wavelengths throughout the visible/near-IR range of interest. Isolated rat hearts (n = 11) are perfused separately with Krebs-Henseleit buffer (KHB) and a KHB solution to which neodymium had been added, and visible/near-IR spectra are acquired using an optical probe made up of emission and collection fibers in concentric rings of diameters 1 and 3 mm, respectively. Relative optical path lengths at 520, 580, 679, 740, 800, 870, and 975 nm are 0.41+/-0.13, 0.49+/-0.21, 0.90+/-0.09, 0.94+/-0.01, 1.00, 0.84+/-0.01, and 0.78+/-0.08, respectively. The absolute path length at 975 nm is estimated to be 3.8+/-0.6 mm, based on the intensity of the water absorptions and the known tissue water concentration. These results are strictly valid only for the experimental geometry applied here.
authors.affiliation: Institute for Biodiagnostics, 435 Ellice Ave., Winnipeg, Manitoba R3B 1Y6.
authors.forename: Stephen P
authors.initials: SP
authors.lastname: Nighswander-Rempel
authors.forename: Valery V
authors.initials: VV
authors.lastname: Kupriyanov
authors.forename: R Anthony
authors.initials: RA
authors.lastname: Shaw
mesh_terms: D000042:Absorption; D000818:Animals; D003287:Contrast Media; D005947:Glucose / Q000493:pharmacokinetics; D066298:In Vitro Techniques; D009206:Myocardium / Q000378:metabolism*; D009354:Neodymium / Q000493:pharmacokinetics*; D051381:Rats; D017207:Rats, Sprague-Dawley; D019265:Spectroscopy, Near-Infrared; D014325:Tromethamine / Q000493:pharmacokinetics; D014867:Water / Q000378:metabolism*
chemical_list: D003287:Contrast Media; C074097:Krebs-Henseleit solution; D014325:Tromethamine; D014867:Water; D009354:Neodymium; D005947:Glucose