A prototype small animal imaging system was created for coupling fluorescence tomography ͑FT͒ with x-ray microcomputed tomography ͑microCT͒. The FT system has the potential to provide synergistic information content resultant from using microCT images as prior spatial information and then allows overlay of the FT image onto the original microCT image. The FT system was designed to use single photon counting to provide maximal sensitivity measurements in a noncontact geometry. Five parallel detector locations are used, each allowing simultaneous sampling of the fluorescence and transmitted excitation signals through the tissue. The calibration and linearity range performance of the system are outlined in a series of basic performance tests and phantom studies. The ability to image protoporphyrin IX in mouse phantoms was assessed and the system is ready for in vivo use to study biological production of this endogenous marker of tumors. This multimodality imaging system will have a wide range of applications in preclinical cancer research ranging from studies of the tumor microenvironment and treatment efficacy for emerging cancer therapeutics.
Abstract. Tomographic imaging of a glioma tumor with endogenous fluorescence is demonstrated using a noncontact single-photon counting fan-beam acquisition system interfaced with microCT imaging. The fluorescence from protoporphyrin IX ͑PpIX͒ was found to be detectable, and allowed imaging of the tumor from within the cranium, even though the tumor presence was not visible in the microCT image. The combination of single-photon counting detection and normalized fluorescence to transmission detection at each channel allowed robust imaging of the signal. This demonstrated use of endogenous fluorescence stimulation from aminolevulinic acid ͑ALA͒ and provides the first in vivo demonstration of deep tissue tomographic imaging with protoporphyrin IX.
In order to precisely recover fluorescence lifetimes from bulk tissues, one needs to employ complex light propagation models (e.g., the radiative transfer equation or a simpler yet consistent approximation, the diffusion equation) requiring knowledge of the tissue optical properties. This can be computationally expensive and therefore not practical in many applications. We present a novel method to estimate the fluorescence lifetimes of multiple fluorophores embedded in mice. By assuming that the photon diffusion does not significantly change the fluorescence decay slope, the light propagation is simply modeled as a time-delay during lifetime estimation. Applications of this approach are demonstrated by simulation, phantom data, and in vivo experiments.
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