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Spectral imaging techniques, using visible and near-infrared radiation, are being investigated for use in detecting abnormal regions deeply embedded within normal tissue. These involve time-resolved imaging techniques that enhance spatial resolution by detecting the time-of-flight of photons within the tissue. To evaluate the performance of such time-resolved transillumination techniques, we used random walk theory (RWT). Our evaluation showed that the strong scattering properties of tissues prevent direct imaging of abnormalities. We thus derived theoretical constructs to separate the effects of scattering from absorption, thus allowing us to map the optical coefficients as spectroscopic signatures of an abnormal target. By utilizing our method for different wavelengths, one can obtain diagnostic information (for example, estimates of blood oxygenation of the tumor) from corresponding absorption coefficients that no other imaging modality can directly provide.
We have verified our methodology by applying it to two transillumination and fan geometry data sets provided by collaborators at University College, London. The data were obtained from an experimental phantom that had the optical properties, thickness, and characteristics of the embedded, relatively small, abnormal target that were similar to those measured in human breast. To improve performance of our algorithm, we have extended the analysis to non-localized abnormalities. We successfully applied the RWT approach to quantification of the optical characteristics and dimensions of larger inclusions (increased scattering and/or absorption) that were realized in the tissue-like phantoms of our collaborators at the Politecnico di Milano. We also have begun an analysis of real clinical data, involving tumors embedded inside normal tissues. This work is a joint project with researchers at Physikalisch-Technische Bundesanstalt of Berlin (PTB), who have designed a clinically practical optical imaging system capable to implementing time-resolved in vivo measurements on human breast. Our first goal is to quantify the optical parameters at several wavelengths and thereby estimate blood oxygen saturation of the tumor and surrounding tissue. After data processing that includes filtering and deconvolution of the raw time-resolved data, we create linear contrast scans passing through the tumor center and analyze these scans using our algorithms. Preliminary results of data obtained from a patient with invasive ductal carcinoma suggest that the tumor tissue is in a slightly deoxygenated state, with higher blood volume than surrounding tissue. These results have encouraged us to extend our collaboration with the Berlin group and to start a study with the Oncology Radiology Program at the NCI, using a new imaging device that will be installed at the NIH campus.
Chernomordik, V., A. Gandjbakhche, M. Lepore, R. Esposito and I. Delfino. Depth dependence of the analytical expression for the width of the point spread function (spatial resolution) in time resolved transillumination. Journal of Biomedical Optics 6(4), 441-445 (Oct 2001).
Chernomordik, V., R. Nossal and A. Gandjbakhche. Point spread functions of photons in time-resolved transillumination experiments using simple scaling arguments. Med. Phys. 23(11), 1857-1861 (Nov 1996).
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