Anthony J. Durkin

Modulated imaging: quantitative analysis and tomography of turbid media in the spatial-frequency domain

Experiments performed on turbid phantoms demonstrate that spatially modulated illumination facilitates quantitative wide-field optical property mapping and tomographic imaging in turbid media.

Fluorescence Excitation Emission Matrices of Human Tissue: A System for in Vivo Measurement and Method of Data Analysis

We describe a system capable of measuring spatially resolved reflectance spectra from 380 to 950 nm and fluorescence excitation emission matrices from 330 to 500 nm excitation and 380 to 700 nm emission in vivo. System performance was compared to that of a standard scanning spectrofluorimeter. This “FastEEM” system was used to interrogate human normal and neoplastic oral cavity mucosa in vivo. Measurements were made through a fiber-optic probe and require 4 min total measurement time. We present a method based on autocorrelation vectors to identify excitation and emission wavelengths where the spectra of normal and pathologic tissues differ most. The FastEEM system provides a tool with which to study the relative diagnostic ability of changes in absorption, scattering, and fluorescence properties of tissue.

In vivo quantification of optical contrast agent dynamics in rat tumors by use of diffuse optical spectroscopy with magnetic resonance imaging coregistration

We present a study of the dynamics of optical contrast agents indocyanine green (ICG) and methylene blue (MB) in an adenocarcinoma rat tumor model. Measurements are conducted with a combined frequency-domain and steady-state optical technique that facilitates rapid measurement of tissue absorption in the 650-1000-nm spectral region. Tumors were also imaged by use of contrast-enhanced magnetic resonance imaging (MRI) and coregistered with the location of the optical probe. The absolute concentrations of contrast agent, oxyhemoglobin, deoxyhemoglobin, and water are measured simultaneously each second for approximately 10 min. The differing tissue uptake kinetics of ICG and MB in these late-stage tumors arise from differences in their effective molecular weights. ICG, because of its binding to plasma proteins, behaves as a macromolecular contrast agent with a low vascular permeability. A compartmental model describing ICG dynamics is used to quantify physiologic parameters related to capillary permeability. In contrast, MB behaves as a small-molecular-weight contrast agent that leaks rapidly from the vasculature into the extravascular, extracellular space, and is sensitive to blood flow and the arterial input function.

Quantitative optical tomography of sub-surface heterogeneities using spatially modulated structured light

We present a wide-field method for obtaining three-dimensional images of turbid media. By projecting patterns of light of varying spatial frequencies on a sample, we reconstruct quantitative, depth resolved images of absorption contrast. Images are reconstructed using a fast analytic inversion formula and a novel correction to the diffusion approximation for increased accuracy near boundaries. The method provides more accurate quantification of optical absorption and higher resolution than standard diffuse reflectance measurements.

Three-dimensional surface profile intensity correction for spatially modulated imaging

We describe a noncontact profile correction technique for quantitative, wide-field optical measurement of tissue absorption (microa) and reduced scattering (micros) coefficients, based on geometric correction of the samples Lambertian (diffuse) reflectance intensity. Because the projection of structured light onto an object is the basis for both phase-shifting profilometry and modulated imaging, we were able to develop a single instrument capable of performing both techniques. In so doing, the surface of the three-dimensional object could be acquired and used to extract the objects optical properties. The optical properties of flat polydimethylsiloxane (silicone) phantoms with homogenous tissue-like optical properties were extracted, with and without profilometry correction, after vertical translation and tilting of the phantoms at various angles. Objects having a complex shape, including a hemispheric silicone phantom and human fingers, were acquired and similarly processed, with vascular constriction of a finger being readily detectable through changes in its optical properties. Using profilometry correction, the accuracy of extracted absorption and reduced scattering coefficients improved from two- to ten-fold for surfaces having height variations as much as 3 cm and tilt angles as high as 40 deg. These data lay the foundation for employing structured light for quantitative imaging during surgery.

Noninvasive assessment of burn wound severity using optical technology: A review of current and future modalities

Clinical examination alone is not always sufficient to determine which burn wounds will heal spontaneously and which will require surgical intervention for optimal outcome. We present a review of optical modalities currently in clinical use and under development to assist burn surgeons in assessing burn wound severity, including conventional histology/light microscopy, laser Doppler imaging, indocyanine green videoangiography, near-infrared spectroscopy and spectral imaging, in vivo capillary microscopy, orthogonal polarization spectral imaging, reflectance-mode confocal microscopy, laser speckle imaging, spatial frequency domain imaging, photoacoustic microscopy, and polarization-sensitive optical coherence tomography.

Relation between fluorescence spectra of dilute and turbid samples

We present a method to extend rank-annihilation-factor analysis (RAFA) for the analysis of fluorescence from homogeneous turbid samples. The method is based on a fundamental relationship between the fluorescence of a dilute solution and that of a turbid solution. We have derived this relationship, known as the transfer function, for turbid materials using the two-flux Kubelka-Munk theory. The method is tested with spectroscopic data from optically thin and turbid samples of the media of a human aorta. At 450-nm excitation, agreement between the measured and predicted dilute-solution fluorescence spectra is within 5% at all emission wavelengths; at 340-nm excitation, agreement is within 20% at all wavelengths, with some residual Soret-band absorption. The simulations presented indicate that the transfer function is markedly more sensitive to absorption than to scattering properties.

In vivo determination of skin near-infrared optical properties using diffuse optical spectroscopy.

We develop a superficial diffusing probe with a 3 mm source-detector separation that can be used in combination with diffuse optical spectroscopic (DOS) methods to noninvasively determine full-spectrum optical properties of superficial in vivo skin in the wavelength range from 650 to 1000 nm. This new probe uses a highly scattering layer to diffuse photons emitted from a collimated light source and relies on a two-layer diffusion model to determine tissue absorption coefficient mu a and reduced scattering coefficient mus. By employing the probe to measure two-layer phantoms that mimic the optical properties of skin, we demonstrate that the probe has an interrogation depth of 1 to 2 mm. We carry out SSFDPM (steady state frequency-domain photon migration) measurements using this new probe on the volar forearm and palm of 15 subjects, including five subjects of African descent, five Asians, and five Caucasians. The optical properties of in vivo skin determined using the superficial diffusing probe show considerable similarity to published optical properties of carefully prepared ex vivo epidermis+dermis.

First-in-human pilot study of a spatial frequency domain oxygenation imaging system.

Oxygenation measurements are widely used in patient care. However, most clinically available instruments currently consist of contact probes that only provide global monitoring of the patient (e.g., pulse oximetry probes) or local monitoring of small areas (e.g., spectroscopy-based probes). Visualization of oxygenation over large areas of tissue, without a priori knowledge of the location of defects, has the potential to improve patient management in many surgical and critical care applications. In this study, we present a clinically compatible multispectral spatial frequency domain imaging (SFDI) system optimized for surgical oxygenation imaging. This system was used to image tissue oxygenation over a large area (16×12 cm) and was validated during preclinical studies by comparing results obtained with an FDA-approved clinical oxygenation probe. Skin flap, bowel, and liver vascular occlusion experiments were performed on Yorkshire pigs and demonstrated that over the course of the experiment, relative changes in oxygen saturation measured using SFDI had an accuracy within 10% of those made using the FDA-approved device. Finally, the new SFDI system was translated to the clinic in a first-in-human pilot study that imaged skin flap oxygenation during reconstructive breast surgery. Overall, this study lays the foundation for clinical translation of endogenous contrast imaging using SFDI.

Reflectance-based determination of optical properties in highly attenuating tissue

Accurate data on in vivo tissue optical properties in the ultraviolet A (UVA) to visible (VIS) range are needed to elucidate light propagation effects and to aid in identifying safe exposure limits for biomedical optical spectroscopy. We have performed a preliminary study toward the development of a diffuse reflectance system with maximum fiber separation distance of less than 2.5 mm. The ultimate objective is to perform endoscopic measurement of optical properties in the UVA to VIS. Optical property sets with uniformly and randomly distributed values were developed within the range of interest: absorption coefficients from 1 to 25 cm(-1) and reduced scattering coefficients from 5 to 25 cm(-1). Reflectance datasets were generated by direct measurement of Intralipid-dye tissue phantoms at lambda=675 nm and Monte Carlo simulation of light propagation. Multivariate calibration models were generated using feed-forward artificial neural network or partial least squares algorithms. Models were calibrated and evaluated using simulated or measured reflectance datasets. The most accurate models developed-those based on a neural network and uniform optical property intervals-were able to determine absorption and reduced scattering coefficients with root mean square errors of +/-2 and +/-3 cm(-1), respectively. Measurements of ex vivo bovine liver at 543 and 633 nm were within 5 to 30% of values reported in the literature. While our technique for determination of optical properties appears feasible and moderately accurate, enhanced accuracy may be achieved through modification of the experimental system and processing algorithms.