Developments in imaging technologies have led to groundbreaking advances in our understanding of neural and physiologic functions in normal and diseased humans and animals by offering a view of the living brain at work (Hoehn et al., 2001). The technologies are generally less invasive than other investigative scientific methods and offer an opportunity to address questions of structure and function without significant consequences for the research subjects (Balaban and Hampshire, 2001).
Several imaging techniques are used in animals. They include positron-emission tomography (PET), single-photon emission computed tomography (SPECT), magnetic resonance imaging (MRI) and functional MRI (fMRI), nuclear magnetic resonance imaging or spectroscopy (NMR), near-infrared spectroscopy, ultrasonography, computed tomography (CT) and optical imaging (Balaban and Hampshire, 2001; Hoehn et al., 2001; Rolfe, 2000). Some of the techniques, such as PET and SPECT, enable measurement of blood flow, oxygen and glucose metabolism, receptor density, or drug concentrations in regions of the living brain (Mathias, 1996). Others, such as MRI and NMR, provide imaging of superficial and deep brain structures with a high degree of anatomic detail. High-field MRI, SPECT, and PET techniques can also be used to provide in vivo longitudinal evaluation of receptor binding and gene expression following gene therapy (Auricchio et al., 2003; Kasper et al., 2002).
Each of those techniques allows researchers to test hypotheses about the functions of different regions of the brain on the basis of functional composition or physiologic activity. The hypotheses can often be explored further with human subjects performing specific tasks during PET, SPECT, or fMRI. However, many of the technologies provide even better resolution when used in small mammals, providing more information about physiologic function than can be obtained with human subjects (Balaban and Hampshire, 2001). Animal models enable variables associated with specific diseases to be manipulated and controlled to a degree that is not possible with human patients. Furthermore, individual animals can be evaluated repeatedly during the course of a disease or can serve as their own control instead of sacrificing large groups of animals at different time points, and thereby reducing the number of animals used (Hoehn et al., 2001).
Imaging generally requires anesthesia so that the animal remains motionless throughout the duration of image collection. The exception is ultrasonographic images, which can be collected from a restrained nonanesthetized animal, pro-