David Hattery

Quantification of optical properties of a breast tumor using random walk theory

For the first time we use a random walk methodology based on time-dependent contrast functions to quantify the optical properties of breast tumors (invasive ductal carcinoma) of two patients. Previously this theoretical approach was successfully applied for analysis of embedded objects in several phantoms. Data analysis was performed on distributions of times of flight for photons transmitted through the breast which were recorded in vivo using a time-domain scanning mammograph at 670 and 785 nm. The size of the tumors, their optical properties, and those of the surrounding tissue were reconstructed at both wavelengths. The tumors showed increased absorption and scattering. From the absorption coefficients at both wavelengths blood oxygen saturation was estimated for the tumors and the surrounding tissue.

Analytical solutions for time-resolved fluorescence lifetime imaging in a turbid medium such as tissue.

An analytical solution is developed to quantify a site-specific fluorophore lifetime perturbation that occurs, for example, when the local metabolic status is different from that of surrounding tissue. This solution may be used when fluorophores are distributed throughout a highly turbid media and the site of interest is embedded many mean scattering distances from the source and the detector. The perturbation in lifetime is differentiated from photon transit delays by random walk theory. This analytical solution requires a priori knowledge of the tissue-scattering and absorption properties at the excitation and emission wavelengths that may be obtained from concurrent time-resolved reflection measurements. Additionally, the solution has been compared with the exact, numerically solved solution. Thus the presented solution forms the basis for practical lifetime imaging in turbid media such as tissue.

Quantification by random walk of the optical parameters of nonlocalized abnormalities embedded within tissuelike phantoms.

We have extended a random-walk theory that uses time-dependent contrast functions to quantify the cross section and the corrected scattering and absorption coefficients of abnormal nonlocalized targets from time-of-flight (TOF) data obtained in time-resolved transillumination experiments. Experimental TOFs are used to show that this newly developed random-walk method is able to quantify the size and the optical properties of embedded nonlocalized targets in with an error rate of </=25%.

Tissue Characterization by Quantitative Optical Imaging Methods

Optical methods have a long history in the field of medical diagnosis. The biomolecular specificity possible with optical methods has been particularly valuable in microscopy and histopathology while in vivo imaging of deep structures has traditionally been the domain of X-ray and MRI. The use of optical methods in deep tissue has been limited by multiple-scattering which blurs or distorts the optical signal. New stochastic methods which account for multiple scattering have been developed that are extending the usefulness of optical methods deep into tissue. In optical mammography, photons may travel through 10cm of tissue before arriving at the detector. We have developed a method for quantifying parameters of anomalous sites in breast tissue that may be used for functional characterization of tumors. In other work presented here, we are developing fluorescence based methods to detect and monitor tumor status. The immune response to a tumor is a target for fluorescently labeled specific antibodies. We have developed a method to localize the tumor site using CW fluorescence. Additionally, we have developed a method which uses time-resolved data and capitalizes on probe lifetime sensitivity to metabolic parameters such as pH and temperature to obtain functional information from the tumor site.

Analytical calculation of the mean time spent by photons inside an absorptive inclusion embedded in a highly scattering medium.

The mean time spent by photons inside a nonlocalized optically abnormal embedded inclusion has been derived analytically. The accuracy of the results has been tested against Monte Carlo and experimental data. We show that for quantification of the absorption coefficient of absorptive inclusions, a corrective factor that takes into account the size of the inclusion is needed. This finding suggests that perturbation methods derived for very small inclusions which are used in inverse algorithms require a corrective factor to adequately quantify the differential absorption coefficient of nonlocalized targets embedded in optically turbid media.

Fluorescence measurement of localized, deeply embedded physiological processes

Intrinsic and exogenous fluorescent molecules may be used as specific markers of disease processes, or metabolic status. A variety of fluorescent markers have been successfully used for transparent tissue, in-vitro studies, and in cases where the markers are located close to the tissue surface. For example, given fluorescence lifetime measurements of a fluorophore such as bis(carboxylic acid) dye, the known relationship of pH on its lifetime may be used to determine the pH of tissue at the fluorophores location. For fluorophore depths greater than approximately one millimeter in normal tissue, such as might be encountered in in vivo studies, multiple scattering makes it impossible to make direct measurements of characteristics such as fluorophore lifetime. In a multiple scattering environment, the collected intensity depends heavily on the scattering and absorption coefficients of the tissue at both the excitation and emission frequencies. Thus, to obtain values for specific fluorophore characteristics such as the lifetime, a theoretical description of the complex photon paths is required. We have applied Random-walk theory to successfully model photon migration in turbid medias such as tissue. We show how time-resolve intensity measurements may be used to determine fluorophore location and lifetime even when the fluorophore site is located many mean photon scattering lengths from the emitter and detector.

Hyperspectral imaging of Kaposi's Sarcoma for disease assessment and treatment monitoring

Light spectroscopic methods are critical to advances in molecular characterization of disease processes. However, these methods have been limited to in-vitro or cell culture studies. In fact, strong scattering in almost all tissue types causes dispersion of the photons paths which results in poor localization and resolution. Hence, quantitative analysis of spectral data obtained from structures below the tissue surface requires accounting for scattering which affects both the penetration of the photons and the path length over which the photons will be subject to molecularly specific absorption. The goal of much current research is to non-invasively obtain diagnostically useful molecular information from embedded sites. We have designed and built a six-band multi-spectral NIR imaging system which we have used on patients with highly vascularized tumors in the skin called Kaposis Sarcoma. The imaged lesions are undergoing treatment with experimental anti-angiogenesis drugs that are designed to reduce bloodflow and hence growth of the tumors. The NIR data is combined with both 3-5 micron and 8-12 micron infrared images, obtained of the same tumors, which are used to identify thermal signatures of blood volume, as well as three-band visible wavelength data which show the visible extent of the lesions. We have developed a layered model of the skin in which specific analytes exist in specific layers. The spectral signatures of analytes such as oxy- and deoxy-hemoglobin are known. To obtain information on the concentration of those analytes in the tissue, however, the diffuse reflectance NIR images from the patients must be corrected for scattering. The scattering is modeled using analytical solutions developed from a random walk model of photon migration in turbid media. When the hyperspectral patient data is fit to the model, physiologically related parameters, such as to blood volume and oxygenation, are obtained. This provides clinically important data that may be used by the physician for evaluations of drug effectiveness, disease assessment and patient treatment monitoring.

Differential oblique angle spectroscopy of the oral epithelium

Increasing evidence suggests that inflammation may contribute to the process of carcinogenesis. This is the basis of several clinical trials evaluating potential chemopreventive drugs. These trials require quantitative assessments of inflammation, which, for the oral epithelium, are traditionally provided by histopathological evaluation. To reduce patient discomfort and morbidity of tissue biopsy procedures, we develop a noninvasive alternative using diffuse reflectance spectroscopy to measure epithelial thickness as an index of tissue inflammation. Although any optical system has the potential for probing near-surface structures, traditional methods of accounting for scattering of photons are generally invalid for typical epithelial thicknesses. We develop a single-scattering theory that is valid for typical epithelial thicknesses. The theory accurately predicts a distinctive feature that can be used to quantify epithelial thickness given intensity measurements with sources at two different angles relative to the tissue surface. This differential measure approach has acute sensitivity to small, layer-related changes in scattering coefficients. To assess the capability of our method to quantify epithelial thickness, detailed Monte Carlo simulations and measurements on phantom models of a two-layered structure are performed. The results show that the intensity ratio maximum feature can be used to quantify epithelial thickness with an error less than 30% despite fourfold changes in scattering coefficients and 10-fold changes in absorption coefficients. An initial study using a simple two-source, four-detector probe on patients shows that the technique has promise. We believe that this new method will perform well on patients with diverse tissue optical characteristics and therefore be of practical clinical value for quantifying epithelial thickness in vivo.

Noninvasive infrared imaging for quantitative assessment of tumor vasculature and response to therapy

In this study we are investigating three infrared imaging techniques, thermography, multispectral imaging and Laser Doppler imaging (LDI) to assess parameters of vascularity in lesions of Kaposis sarcoma (KS) and response to therapy. Thermography, multispectral imaging and LDI were recorded over the lesion and compare to normal skin either adjacent to the lesion or on the contralateral side. The KS lesions generally had increased temperature, blood volume (as measured by multispectral imaging) and blood flux (as measured by LDI) as compare to normal skin. After the treatment with experimental antiKS drug, temperature, blood volume and blood flow of the lesion were significantly reduced from the baseline. These techniques hold promise to assess physiological parameter in KS lesion and their changes with therapy.

Measuring oral inflammation in vivo with diffuse reflectance spectroscopy

Inflammation of the oral epithelium has been shown to provide a promotional environment for evolving cancer cells. This has led to chemoprevention trials exploring the effectiveness of various anti-inflammatory drugs. These trials require quantitative assessment of epithelial inflammation which are traditionally provided by punch biopsy. To reduce patient discomfort and morbidity, we have developed a non-invasive alternative using diffuse reflectance spectroscopy. Though any optical system has the potential for probing near-surface structures, traditional methods of accounting for scattering of photons are generally invalid for typical epithelial thicknesses. We have developed a theory that is valid in this regime and our Monte Carlo simulations and phantom models of a two layered structure have validated our approach. We use a differential measure with accurate sensitivity to small changes in layer scattering coefficients. Preliminary results from this work are encouraging and further development is planned to enable quantification of epithelial thickness in vivo.