In vivo hematoporphyrin mediated fluorescence reflectance imaging: early tumor detection on small animals

8 Introduction The present work is included in the research field of the optical biomedical imaging on small animals that, in the last years, has reached a prodigious growth due to various factors of different kind. We will remember, on the one hand, the quick and easy implementation of the optical techniques supported by the technological development of the instrumentation, with the availability of highly sensitive PC controlled sensors (typically CCD camera), in parallel with the development of sophisticated software for the images acquisition and analysis. On the other hand, the genetic and biomolecular revolution has allowed approaching the biological phenomena by a completely new point of view: let think to the sequencing of the human genoma, and then of the mouse genoma, in combination with animal models of human diseases, as a link between in vitro and clinical studies. Finally, the birth of the combinatorial chemistry has produced a large amount of molecules that can interact with a particular biological target of interest. The integration of these factors has been fostered by the erosion of traditional boundaries that separated the different scientific fields and by the consequent establishment of interdisciplinary teams and programmes, where are involved physicians, pathologists, histologists, physicists, chemists, biologists, engineers, mathematicians, etc, each bearing its own competences, but working in synergy (1). The imaging process allows a spatial and/or temporal distribution of a physical quantity be registered in an image. In the optical imaging the image acquired by a detector visualizes the spatial distribution of the radiation intensity emitted by an (eventually irradiated) object. Particular success has obtained in the last years the application of the fluorescence reflectance imaging (FRI) technique in biomedical in vivo imaging on small animals. It is a very simple technique where a CCD camera, with entrance filtered to the fluorescence emission wavelength, collects the spatial distribution of the fluorescence radiation emitted by the object irradiated by light at the fluorescence excitation wavelength (2). What is the object? As it was specified above, it is a small animal, typically a mouse of 20 g body weight, where the biomolecular or genetic target of interest has been labeled with a fluorescent marker. Depending on the biomedical target of interest, the more suitable fluorescence targeting strategy will be selected by using a non specific, or specific, or targeted, or “smart” fluorescent contrast agent, or fluorescent or bioluminescent protein (3). The scope of imaging studies can be of various kind: the understanding of the biomolecular mechanisms that control the functions of the healthy and diseased cells and tissues; or the (possibly) early, diagnosis of oncologic (and some non tumoral) diseases, the subsequent analysis of the disease evolution up to the identification of metastases formation and dissemination; or efficacy tests of new anticancer drug, by monitoring the possible disease regression and/or drug side effects (4). Different reasons justify the choice of the optical modality of imaging: optics is a branch of the physics that developed technicologically advanced imaging instrumentation and techniques, that are much safer than other imaging modalities, as, for example, radionuclide imaging. Indeed, the optical markers are generally non toxic and the optical radiation, being non-ionizing, doesn’t damage the biological tissues, so that the optical methods are intrinsically non invasive. Finally imaging optical systems can be designed having: i) high sensitivity, so allowing for the use of very low dose of fluorescent marker; ii) high spatial resolution, so they are suitable for the human disease model studies on