ABSTRACT
I asked one of my MS students to study the impact of refractive index variation on the reconstruction of absorption and scattering coefcients using the new diffusion equation. Even though the study was interesting, it did not lead to signicant improvement of the image quality. On a Saturday in 2000 when I was sitting in the study room in the quiet basement of our house and reading a news report on the use of the refractive index difference between cancerous and normal cells for cellular optical sensing, I suddenly realized that the refractive index of cancerous tissue may differ from
that of normal tissue, and that the new diffusion equation I derived would allow us to reconstruct the spatial distribution of refractive index or phase contrast, making phase-contrast DOT possible. I was excited and was able to spend a couple of weeks implementing the reconstruction code myself. Yong Xu, then a postdoctoral fellow in my laboratory, tested the code using both phantom and clinical data. Surprisingly, we found that this new contrast mechanism allowed us to differentiate malignant from benign breast tumors while it was impossible to do so using the absorption and scattering images (see Section 8.2). Section 7.2 details the derivation of the new diffusion equation and phase-contrast reconstruction algorithm based on this diffusion equation. A two-step strategy that can improve the quantitative accuracy for phase-contrast DOT is presented in Section 7.3.
