PET and SPECT as platforms for molecular imaging

With the development of new molecular imaging probes, sophisticated animal models, and small-animal imaging devices as well as advances in molecular biology techniques, molecular imaging has become increasingly important for understanding basic biological processes in living subjects. Molecular imaging can be defined as the characterization of biological processes at the cellular and molecular levels in living animals or humans using remote imaging detectors (1). This can differ from “classical” diagnostic imaging that focuses on imaging the end result of these biological processes (2). Molecular imaging modalities include nuclear imaging, optical imaging, magnetic resonance imaging (MRI), computed tomography (CT), and ultrasound. All of these modalities offer different advantages and limitations for molecular imaging. Most of these advantages and limitations are centered on issues of spatial and temporal resolution, sensitivity, molecular probe availability, image acquisition and analysis time, and cost. The focus of this chapter will be on nuclear imaging, in particular single photon emission computed tomography (SPECT) and positron emission tomography (PET). These techniques have the advantages of having a high sensitivity, being extremely quantitative, and having a number of probes readily available. They are, however, relatively expensive, expose the subject to radiation, and have lower spatial resolution than some of the other imaging modalities. This chapter will describe the basics of SPECT and PET imaging, describe the advantages and disadvantages of each modality, and give examples of how each platform is used for molecular imaging applications.