Date of Award

Fall 12-2022

Document Type

Thesis (Master's)

Department or Program

Engineering Sciences

First Advisor

Dr. Brian Pogue

Second Advisor

Dr. David Gladstone

Third Advisor

Dr. Petr Bruza

Abstract

Cherenkov imaging in radiation therapy allows a video display of the irradiation beam on the patient’s tissue, for visualization of the treatment. High energy radiation from a linear accelerator (Linac) results in the production of spectrally-continuous broadband light inside tissue due to the Cherenkov effect; this light is then attenuated by tissue features from transport and exits from the delivery site. Progress with the development of color Cherenkov imaging has opened the possibility for some level of spectroscopic imaging of the light-tissue interaction and interpretation of the specific nature of the tissue being irradiated. Generally, there is a linear relationship between Cherenkov emission and dose in a homogenous medium; however human tissue has multiple factors of scatter and absorption that result in the distortion of this linear relationship. This project investigated what color Cherenkov imaging could be used for, in the situation of tissue with different levels of pigmentation present in skin and/or different levels of hemoglobin present inside the tissue. A custom-developed time-gated three-channel intensified camera was used to image the Red Green and Blue (RGB) Cherenkov emission from tissue phantoms that had synthetic epidermal layers and blood. The hypothesis was that RGB color Cherenkov imaging would allow for the detection of signals that varied uniquely in these channels in response to changes in blood content or melanin content, because of their different absorption spectra in the RGB channels. Oxy-hemoglobin in the blood is highly absorbing in the blue & green, but not as much in the red, whereas the melanin is highly absorbing across the channels, falling slightly from blue through green and red. The results showed that these spectral absorption differences did indeed lead to different amounts of exiting light, predominantly in the red wavelength band, where melanin has a higher relative absorption than blood. This observation leads to the provision for future color distortion corrections, and interpretation of more accurate Cherenkov imaging via color-based modeling or correction for dose quantification. Based on this work, it is possible to separate the effects of attenuation from skin color or blood volume based upon the colors seen in the Cherenkov images, as these are emissions that are specific to the patient.

Comments

The time-gated tri-channel intensified camera used in this study is a proprietary device that is unique and one of a kind, designed by Dr. Petr Bruza at Dartmouth College. It was crafted to image Cherenkov radiation emission in three separate channels, specifically red, blue, and green. At the time of writing, there has yet to be any published independent validation of its performance with regard to confirming a differential Cherenkov signal response with regard to changing blood and melanin content. The purpose of this project was to show that color Cherenkov imaging would allow the detection of signals that varied differently in response to changes in blood content or melanin content, and compare and contrast this variable response. Measurements of the mean Cherenkov signal in each channel were done in-house crafted blood solutions and synthetic epidermal layers, with varying concentrations. Analysis was done using CDose and MATLAB.

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