Combination of non-contact optical fluorescence tomography and micro-CT: A concept and apparatus for small animal imaging

Introduction: In the past decades X-ray-based approaches have been well established in preclinical and clinical applications for fast and high resolution analysis of morphological and anatomical data. Optical fluorescence tomography has proven to be a valuable tool to non-invasively study diseases and pathologies on a molecular level. The goal of this work is to develop and to establish a novel multimodal imaging system by combining anatomical-morphological data from micro-CT and functional-physiological data from optical fluorescence tomography (OFT).

Material and Methods: The imaging system consists of micro-CT and OFT chambers. Both chambers are using a common object tray for a horizontal positioning of the measurement object allowing sequential scans in both modalities. In accordance with the micro-CT scanner, the OFT scanner also provides rotation around the horizontal axis, allowing to perform a non-contact free space imaging. The propagation of light in a strongly scattering phantom is described by a diffusion approximation of the Radiative Transport Equation. Diffuse average intensities of both excitation and emission light are described by a set of two coupled non-homogeneous second order differential equations. The fluorescence emission light originating from a fluorescence object is considered as a point source which radiates isotropically in all directions. The 3D morphological data and surface geometry achieved from micro-CT are used as a priori information in the mathematical model to improve the optical reconstruction algorithm.

Results: Hardware setup: The OFT system was designed and constructed. The setup consists of a concentric 2-ring-system for independent positioning of the excitation light source and a highly sensitive CCD detector in a light-tight chamber. The object tray as well as the light source and the detector are driven by closed-loop stepper motors controlled automatically by the host computer. Image reconstruction: The optical image reconstruction algorithm was implemented. It determines the localization of the fluorescent probes inside a tissue-like phantom by calculating the excitation light intensity distribution in the homogeneous phantom and by using the measured data of the fluorescence light distribution on the surface of the phantom. Emission quantum efficiency is assumed to remain constant for all measurements. For improvement of the resolution a multi-iteration search technique was applied to refine the mesh structure and to reduce the region of interest. In case the fluorescent probes are more condensed and the localizations are already estimated after the first iteration, the regions of interest are reduced in the subsequent iterations.

Discussion: The combination of an x-ray-based approach like micro-CT with optical imaging method such as OFT can provide much deeper insight in the morphology and physiology in a small animal body. The use of a common object tray avoids repositioning the object between both measurements, facilitates an easy and fast imaging procedure and minimizes stress on the animal. With the CCD camera we are able to measure without physical contact to the object. Making multiple projection measurements from different angles reduces the ill-posedness of the reconstruction problem. The multi-iteration reconstruction method may provide higher resolution with the benefit of decreased calculation time. By combining the two modalities a 3D image can be obtained which provides the outer geometry, bone structure, and organs of the animal together with the pathological parts of the organs extracted from OFT.