In Vivo Optical Imaging of Molecular Function Using Near-Infrared Fluorescent Probes

Abstract

Abstract Tissue observation with light is probably the most common imaging practice in medicine and biomedical research. The original physicians’ visual inspection of their patients has been followed by more elaborate photon technologies applied to biomedical and clinical applications, many of which are described in this book. Light offers unique wavelengthdependent interactions with tissue and has been used to investigate both structural and functional tissue characteristics. A recent development in optical applications is the noninvasive imaging of molecular events in tissues. Underpinning these developments are the discovery of biocompatible, target-specific, and activatable fluorescent imaging probes and the development of highly sensitive imaging technologies forin vivo fluorescent detection. Of particular interest are fluorescent probes that emit in the near infrared, a spectral window in which hemoglobin and water photons to penetrate for several centimeters in tissue. In this chapterwe review technological advances that have allowed molecularimaging in the near infrared. First, we outline design strategies for fluorescent probes developed for in vivo applications and focus on the engineering of molecular beacons, a particular category of near-infrared fluorescent smart probes that maximize imaging capacity. We describe the chemistry of such probes and delineate current and future molecular targets. Second, we describe optical imaging technologies used for noninvasive imaging of the distribution of such probes. We illuminate the advantages and limitations of simple photographic methods and turn our attention to fluorescence-mediated molecular tomography (FMT), a technique that can resolve fluorescence and quenching/activation in deep tissues in three dimensions. We describe theoretical specifics, and we obtain insight on its in vivo capacity and the sensitivity achieved. Last, we discuss its clinical feasibility.