Date

July 2007

Document Type

Dissertation

Degree Name

Ph.D.

Department

Dept. of Biomedical Engineering

Institution

Oregon Health & Science University

Abstract

Fiber optics can facilitate a non-invasive means of tissue biopsy through optical inspection in small volumes to differentiate between normal and diseased tissue. Optical fibers provide a non-complex means to deliver and collect reflected light allowing for the determination of the absorption and scattering properties. These optical properties correlate with the chemical and structural state of the tissue. The arrangement of the emission and collection fibers dictates the volume of tissue that is optically sampled. Fiber-optic probes that sample the smallest possible volume by emitting and collecting their own backscattered light are investigated. Two complimentary studies are presented that are necessary to provide the tools to evaluate the behavior of fiber-optic probes. First, a method to fabricate optically stable phantoms is investigated. The optical properties of the phantoms are defined at two wavelengths. Linear relations are given for the concentration of dyes and TiO[subscript]2 scattering agent that predict the absorption and scattering properties of the finished phantoms with less than 4% error. The phantoms are demonstrated to be stable over a period exceeding one year. Second, a combined inverse adding-doubling and Monte Carlo model is presented to evaluate the optical properties of the phantoms using integrating sphere measurements. The combined IAD/MC model is demonstrated to accurately determine optical properties of homogenous optically turbid samples with a reasonable precision using multiple sample thicknesses and sample port sizes for both single and double sphere experiments. The scattering is predicted with 1% error and absorption error is 2-4%. The nomenclature for integrating sphere measurements is simplified and rationalized using the concept of sphere gain to express the results. Explicit directions for determining sphere parameters were shown. Formulas were given that work for diffuse incidence or collimated incidence or any combination thereof. A new kind of fiber optic probe, a sized-fiber reflectometry device is presented and investigated. Experimental studies are performed using phantoms with known absorption and scattering properties. A Monte Carlo model is developed to simulate the device behavior to evaluate effects due to absorption scattering, scattering anisotropy, and optical sampling volume. The model is validated by comparison to experimental results. Both experiments and Monte Carlo simulations of the sized-fiber device indicate that 50% of the signal arises from roughly 1.2 and 1.9 reduced mean free paths for the 200 and 600 µm fibers respectively and that in general larger fibers sample deeper optically. Specular reflectance is shown to act as a noise source comparable in magnitude to the diffuse reflection signal for perpendicularly polished fibers and can be rejected with bevel-tipped fibers. The measurement variability decreases 6 fold to 4.5% on in vivo skin with a 200 micron fiber with the beveled-tip fiber and to 2.2% for a 1000micron fiber with the beveled-tip fiber. The absorption and scattering sensitivity is presented for a bevel- tipped sized fiber device using a Monte Carlo generated grid to invert optical properties from the measured diffuse reflectance. The scattering coefficient could be predicted with an error of 1.5 ± 0.2% over the entire range of absorption and scattering properties. A second two-fiber probe design is investigated that uses two identical diameter fibers with only a single source fiber and both fibers collecting reflected light. The inversion of absorption and the reduced scattering is investigated using a heuristically determined closed form relationship from device simulations. The scattering coefficient could be predicted with a mean error of ±4.3%. Typical error for absorption coefficient determination is shown to be between 50-100% for absorption less than 2 cm[superscript]-1. The poor resolution of absorption is related to the mean optical path for collected light which typically is less than 2mm for tissue. A clinical study is presented using a dual 400micron fiber probe to distinguish oral pigmented lesions caused by either melanin or dental amalgam. Two methods of discrimination were investigated. The first method used the spectral features of melanin in the 640-720nm wavelength band. The pigmented lesions containing melanin exhibited a higher change in reflectance with respect to wavelength over this band in comparison to amalgam tattoos or non-pigmented sites. The sensitivity and specificity for identifying melanotic lesions from amalgam tattoo was 98% and 92% respectively. The second method of discriminating amalgam tattoo, melanin pigment and non-pigmented sites uses discriminant function analysis on uniformly spaced wavelength bands of reflectance to simulate spectrally filtered reflected light. The sensitivity and specificity was 94% and 100% respectively for classifying melanin pigmented sites.

Identifier

doi:10.6083/M4Q23X5K

School

OGI School of Science and Engineering

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