Preliminary tests of a prototype system for optical and radionuclide imaging in small animals

We have assembled a prototype system for multimodal (radionuclide and optical) in vivo planar imaging of small animals (mice) using single photon emission radiotracers (Tc-99m) and a fluorescent marker (hematoporphyrin). Preliminary tests of the separate (optical and radionuclide) prototype imaging systems are presented, aimed at assessing their features and at determining the experimental protocol for in vivo imaging. Tests were performed on anesthetized healthy or tumor bearing mice. The gamma radiation detector is a small area (11 × 11 mm²) hybrid pixel detector based on the Medipix1 ASIC readout technology (64 × 64 square pixels of 170 μm by side), bump-bonded to a 300 μm thick silicon detector. High spatial resolution in radioimaging (in the order of 1 mm) is achieved in vivo with a pinhole tungsten collimator (0.35 mm diameter, 90° acceptance angle, field of view of over 20 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 light source (Nd:YAG 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 low sensitivity CCD camera and a commercial 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 final experimental protocol, only partly implemented here, includes independent and then combined optical/radio imaging of control mice and of a solid tumoral area (human thyroid derived anaplastic carcinoma) after injection of the radiotracer and/or of the fluorophore. In this work, the accumulation of the radionuclide in selected organs and of the fluorophore in the tumor provides the signal contrast in the two 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 tumor in mice will be shown i- n a next paper.