Prof. Robert S. Parker Profile

Prof. Robert S. Parker

at Univ of Pittsburgh

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Proceedings Article | 12 February 2008 Paper

In silico analysis of quantifying tumor hemoglobin saturation using the optical pharmacokinetic system (OPS)

Stephen Chad Kanick, Robert Parker

Proceedings Volume 6845, 684512 (2008) https://doi.org/10.1117/12.763486

KEYWORDS: Tissues, Oxygen, Tumors, Tissue optics, Photodynamic therapy, Monte Carlo methods, Absorption, Hypoxia, Geometrical optics, Laser tissue interaction

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The Optical Pharmacokinetic System (OPS) is an elastic-scattering spectroscopic device that is capable of detecting changes in hemoglobin saturation in tissue in vivo. This measurement is important to the field of photodynamic therapy (PDT) because it may be possible to use knowledge of tissue oxygen concentration to improve treatment effect. The OPS measures a 'bulk' signal that is representative of the total tissue volume sampled optically. This measurement may not be sensitive to the presence or development of small, non-uniform hypoxic regions within tissue, and therefore, the clinical relevance of such a measurement is not well understood. This study utilizes mathematical models to investigate the sensitivity of the OPS to chronic hypoxic regions that exist in tumor tissue at steady state and acute hypoxic regions caused by PDT-induced damage. A Monte Carlo model of light propagation is used to emulate the measurement of tissue by the OPS. Tissue geometry is constructed to mimic the tumor microvascular environment, with discrete blood vessels interspersed throughout. A finite element-based transport model is used to describe spatial distributions of oxygen, reactive oxygen species, and hemoglobin saturation throughout the tissue at steady state and following the PDT reaction. PDT-induced damage is estimated and used to approximate the effect of vascular damage on tissue oxygen concentration, thereby simulating acute hypoxia. The volume-averaged hemoglobin saturation measured by the OPS shows potential to identify the presence hypoxic vessels in the chronic case. However, results suggest that the clinical utility of the OPS to detect PDT-induced hypoxia may be limited.

Proceedings Article | 28 February 2007 Paper

Monte Carlo simulation of elastic-scattering spectroscopic measurement using the optical pharmacokinetic system (OPS): analysis of sensitivity to heterogeneous chromophore distribution

Stephen Chad Kanick, Robert Parker

Proceedings Volume 6427, 642710 (2007) https://doi.org/10.1117/12.699137

KEYWORDS: Tissue optics, Absorption, Tissues, Signal detection, Chromophores, Monte Carlo methods, Blood vessels, Oxygen, Photodynamic therapy, Optical properties

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Effective photodynamic therapy depends on an adequate supply of photosensitizer, oxygen, and light fluence. During light exposure, the rapid depletion of molecular oxygen within tissue can lead to the development of hypoxic regions, which decreases the generation of reactive oxygen species and can result in non-uniform tissue necrosis. The Optical Pharmacokinetic System (OPS) is a fiber-optic based spectroscopy device that may be able to monitor local tissue oxygen concentrations during treatment and identify problematic hypoxic regions in vivo. However, the 'bulk' signal detected by the OPS is potentially limited in its ability to discern the development of small hypoxic regions within tissue. This study employs a Monte Carlo simulation of the elastically-scattered light as measured by the OPS to investigate the effect of heterogeneous chromophore distributions on the detected signal. The model tissue geometry is constructed to mimic tissue in vivo, with discrete capillaries interspersed throughout. Tissue optical properties are specified spatially, allowing investigation of heterogeneous chromophore distribution. Simulations investigate the effect that discrete, highly absorbing regions within a measured sample have on the light collected by the OPS. Simulations also consider OPS measurement of a sample with a depth-dependent chromophore concentration gradient and quantitate the ability of the OPS to detect the presence of a sub-population of hypoxic vessels within a network of oxygenated vessels.

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