Hyperspectral and multispectral optical bioluminescence and fluorescence tomography in small animal imaging

For bioluminescence and fluorescence imaging studies in small animals, it is important to be able to accurately estimate the 3D distribution of a light-emitting source within the animal volume in order to draw biological inferences about underlying processes. The spectrum of light produced by an emission source that escapes the subject varies with the depth of the source because of the wavelength dependence of the optical properties of tissue. Consequently, multispectral or hyperspectral data acquisition should help in the 3D localization of deep sources. In this thesis, we develop a framework for fully 3D bioluminescence and fluorescent tomographic image acquisition and reconstruction that exploits spectral information. Singular value analysis is used both for data dimensionality reduction and to illustrate the advantage of using hyperspectral rather than monochromatic or achromatic data. Regularized tomographic reconstruction techniques that use analytic or numerical solutions to the diffusion approximation of photon transport through turbid media were implemented and their properties were studied in phantom and animal studies. A fixed arrangement of mirrors was used in conjunction with a CCD camera of a bioluminescence imaging system (Xenogen Corporation's IVIS200) for simultaneous acquisition of multispectral imaging data over most of the surface of the animal. Phantom studies conducted using this system demonstrate our ability to accurately localize deep point-like sources and show that a resolution of 1.5 to 2.2 mm for depths up to 6 mm can be achieved. Phantom and simulation studies carried out using a fluorescence imaging system (Cambridge Research Instrument's MAESTRO) demonstrate that sources upto 6 mm depth can be reconstructed using only a single view. For a mouse atlas geometry, we present a method for reconstructing multiple fluorescent probes simultaneously. We also show an in-vivo study of a mouse with a brain tumor expressing firefly luciferase. Our results indicate good anatomical localization of the tumor when the 3D bioluminescent image was co-registered with magnetic resonance images. While Optical Bioluminescence Tomography (OBT) and Optical Fluorescence Tomography (OFT) are limited to small animal imaging at this stage, a feasibility study using OFT for breast tumor detection in humans shows the potential use of these methods for clinical in-vivo studies.