Combined optical and single photon emission imaging: preliminary results.

In vivo optical imaging instruments are generally devoted to the acquisitionof light coming from fluorescence or bioluminescence processes. Recently,an instrument was conceived with radioisotopic detection capabilities (Kodakin Vivo Multispectral System F) based on the conversion of x-rays from thephosphorus screen. The goal of this work is to demonstrate that an opticalimager (IVIS 200, Xenogen Corp., Alameda, USA), designed for in vivoacquisitions of small animals in bioluminescent and fluorescent modalities,can even be employed to detect signals due to radioactive tracers. Our systemis based on scintillator crystals for the conversion of high-energy rays and acollimator. No hardware modifications are required. Crystals alone permit theacquisition of photons coming from an in vivo 20 g nude mouse injected witha solution of methyl diphosphonate technetium 99 metastable (Tc99m-MDP).With scintillator crystals and collimators, a set of measurements aimed to fullycharacterize the system resolutionwas carried out. More precisely, system pointspread function and modulation transfer function were measured at differentsource depths. Results show that system resolution is always better than1.3 mm when the source depth is less than 10 mm. The resolution of theimages obtained with radioactive tracers is comparable with the resolutionachievable with dedicated techniques. Moreover, it is possible to detect bothoptical and nuclear tracers or bi-modal tracers with only one instrument.