James D. Boyer

Predictions and measurements of scattering and absorption over broad wavelength ranges in tissue phantoms

Predictions from Mie theory regarding the wavelength dependence of scattering in tissue from the near UV to the near IR are discussed and compared with experiments on tissue phantoms. For large fiber separations it is shown that rapid, simultaneous measurements of the elastic scatter signal for several fiber separations can yield the absorption coefficient and reduced scattering coefficient. With this information, the size of the scattering particles can be estimated, and this is done for Intralipid. Measurements made at smaller source detector separations support Mie theory calculations, demonstrating that the sensitivity of elastic scatter measurements to morphological features, such as scatterer size, is enhanced when the distance between the source and detector fibers is small.

Influence of the scattering phase function on light transport measurements in turbid media performed with small source–detector separations

Many methods of optical tissue diagnosis require that measurements be performed with small source-detector separations in a backscatter geometry. Monte Carlo simulations are used to demonstrate that for these situations light transport depends on the exact form of the angular scattering probability distribution, P(theta). Simulations performed with different forms of P(theta) with the same value of ?cos theta? result in the collection of significantly different fractions of the incident photons, particularly when small-numerical-aperture delivery and collection fibers are employed. More photons are collected for the distribution that has a higher probability of scattering events with theta > 125 degrees . For the clinically relevant optical parameters employed here, the differences in light collection are >60%.

Elastic scattering spectroscopy as a diagnostic tool for differentiating pathologies in the gastrointestinal tract: preliminary testing

We report preliminary clinical testing of elastic-scattering spectroscopy for the detection of pathologies of the gastrointestinal tract. Tissue pathologies are detected and diagnosed using spectral measurements of elastically scattered light in an optical geometry that results in sensitivity to both the absorption and scattering properties of the tissue, over a wide range of wavelengths (300 to 750 nm). The system employs a small fiber optic probe, which is amenable to use with most endoscopes or catheters, or to direct surface examination, as well as interstitial needles. In this paper we report the results of preliminary clinical measurements on various organ sites of the gastrointestinal tract. In several instances the data indicate promise for this diagnostic method to distinguish malignant and dysplastic conditions from normal or other diagnoses.

Detection of gastrointestinal cancer by elastic scattering and absorption spectroscopies with the Los Alamos Optical Biopsy System

The Los Alamos National Laboratory has continued the development of the Optical Biopsy System (OBS) for noninvasive, real-time in situ diagnosis of tissue pathologies. In proceedings of earlier SPIE conferences we reported on clinical measurements in the bladder, and we report here on recent results of clinical tests in the gastrointestinal tract. With the OBS, tissue pathologies are detected/diagnosed using spectral measurement of the elastic optical transport properties (scattering and absorption) of the tissue over a wide range of wavelengths. The use of elastic scattering as the key to optical tissue diagnostics in the OBS is based on the fact that many tissue pathologies, including a majority of cancer forms, exhibit significant architectural changes at the cellular and subcellular level. Since the cellular components that cause elastic scattering have dimensions typically on the order of visible to near-IR wavelengths, the elastic (Mie) scattering properties will be wavelength dependent. Thus, morphology and size changes can be expected to cause significant changes in an optical signature that is derived from the wavelength-dependence of elastic scattering. Additionally, the optical geometry of the OBS beneficially enhances its sensitivity for measuring absorption bands. The OBS employs a small fiber optic probe that is amenable to use with any endoscope or catheter, or to direct surface examination, as well as interstitial needle insertion. Data acquisition/display time is < 1 second.

Noninvasive identification of bladder cancer with subsurface backscattered light

We have developed and are testing early prototypes of an optical biopsy system (OBS) for detection of cancer and other tissue pathologies. The OBS invokes a unique approach to optical diagnosis of tissue pathologies based on the elastic scattering properties, over a wide range of wavelengths, of the microscopic structure of the tissue. Absorption bands in the tissue also add useful complexity to the spectral data collected. The data acquisition and storage/display time with the OBS instrument is approximately 1 second. Thus, in addition to the reduced invasiveness of this technique compared with current state-of- the-art methods, the OBS offers the possibility of impressively faster diagnostic assessment. The OBS employs a small fiber-optic probe that is amendable to use with any endoscope, catheter or hypodermic, or to direct surface examination (e.g., as in skin cancer or cervical cancer). We report here specifically on its potential application in the detection of bladder cancer.

Monte Carlo investigations of elastic scattering spectroscopy applied to latex spheres used as tissue phantoms

An optical-fiber-coupled, elastic-scatter spectrometer has proven effective in discriminating between malignant and nonmalignant tissue in the human bladder and gastrointestinal tract. The system injects broadband light into the tissue with an optical fiber and spectrally analyzes the returning light collected by an adjacent fiber. The collected photons have experienced multiple scattering events and therefore arrive at the analysis fiber after traveling varied paths. The diameter of the source fiber is comparable to its separation from the collection fiber. The diffusion model is inappropriate for this geometry; therefore, Monte Carlo simulations are used. In addition, the size of the scattering sites in tissue are expected to be of the same order as the excitation wavelengths, and Mie theory is expected to provide the best description of the scattering and extinction. We will present and compare the results of simulations and measurements of the elastic scatter signal for suspensions of latex spheres in hemoglobin solutions of varying concentrations.

Optical diagnostics based on elastic scattering: An update of clinical demonstrations with the Optical Biopsy System

The Los Alamos National Laboratory has continued the development of the Optical Biopsy System (OBS) for noninvasive, real-time in situ diagnosis of tissue pathologies. Our clinical studies have expanded since the last Biomedical Optics Europe conference (Budapest, September 1993), and we report here on the latest results of clinical tests in gastrointestinal track. The OBS invokes a unique approach to optical diagnosis of tissue pathologies based on the elastic scattering properties, over a wide range of wavelengths, of the tissue. The OBS employs a small fiber-optic probe that is amenable to use with any endoscope or catheter, or to direct surface examination. The probe is designed to be used in optical contact with tissue under examination and has separate illuminating and collecting fibers. Thus, the light that is collected and transmitted to the analyzing spectrometer must first scatter through a small volume of the tissue before entering the collection fiber(s). Consequently, the system is also sensitive to the optical absorption spectrum of the tissue, over an effective operating range of <300 to 950 nm, and such absorption adds valuable complexity to the scattering spectral signature. More detailed discussions of the technology have appeared in earlier publications.

Coating development for high-energy KrF excimer lasers

Damage resistant optical coatings are needed for cost effective beam delivery systems for high energy laser applications. At excimer wavelengths the photon energy either exceeds or is comparable to bandgap energies of most materials and limits the materials suitable for optical coatings. Other constraints are imposed by limits on near specular scatter and fluorine resistance for selected components.

Angular Dependence of Thin-Film Dielectric Coating Damage Thresholds Revisited

Newnam et al. [1] reported experiments showing that the angular dependence of 351-nm laser damage thresholds in HfO 2 /SiO 2 multilayer dielectric reflectors was much weaker than even the 1/cosθ expected from simple geometric fluence dilution. Several plausible explanations were suggested, but none were convincing. We propose a simple geometric model based on a cylindrical form for the coating defect responsible for damage initiation. We have measured 248-nm damage thresholds for bare fused silica, evaporated aluminium films, and HfO 2 /SiO 2 and Al 2 O 3 /SiO 2 dielectric reflectors at angles out to 85°. The measured data agree well with our simple model.