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We have assembled a prototype system for multimodal (radionuclide and optical) in vivo planar imaging of small animals (mice) using single photon emission radiotracers (Tc99m) and a fluorescent marker (hematoporphyrin). Preliminary tests of the separate (optical and radionuclide) prototype imaging system are presented, aimed at assessing their features and at determining the experimental protocol for in vivo imaging. Tests were performed on anaesthetized healthy tumor-bearing mice. The gamma radiation detector is a small area (11 × 11 mm2) hybrid pixel detector based on the Medipix1 ASIC read-out technology (64×64 square pixels of 170 μm ky side), bump-bonded to a 300 μm thick silicon detector. High spatial resolution in radioimaging is achieved with a pinhole tungsten collimator (0.35 mm diameter, 90 deg acceptance angle, field of view of over 20 mm and FWHM resolution <1 mm at 10 mm source distance). A future setup will use the Medipix2 hybrid detector (256×256 square pixels, 55 μm by side) bump-bonded to a 1 mm thick CdTe pixel detector. The laser-induced in vivo fluorescence imaging system comprises a pulsed laser source (frequency-doubled Nd:VAG laser, λ = 532 nm, energy/pulse = 30 mJ, pulse width = 50 ps, repetition rate = 10 Hz) used to excite the fluorescence emission (600-760 nm) of injected hematoporphyrin compound, a CCD camera and an image analysis system. Images of normal and tumor regions are acquired by using a cut-on filter (λ > 600 nm). Digital image subtraction then enhances the tumor contrast with respect to the background. The experimental protocol includes independent and then combined optical/radio imaging of control mice and of a solid tumoral area (subepidermal fibrosarcoma) in anaesthetized mice, after injection of the radiotracer and/or of the fluorophore. The accumulation of the tracers (in selected organs and) in the tumor provides the signal contrast in both imaging modalities. Fluorescence spectroscopy of excised tissue samples is also performed to help the interpretation of fluorescence images. Results of in vivo combined imaging on mice will be shown in a next paper.