Functional imaging encompasses methods used for visualizing a variety of aspects of cerebral physiology ranging from cerebral blood flow and metabolism to neurotransmitter binding and turnover. Functional imaging has numerous applications in basic and clinical neuroscience, many of which are now in routine clinical use. These techniques may be used to image physiological alterations in the brain that cannot be detected by structural assessment, or to elucidate metabolic changes which underlie structural lesions. The major modalities used for functional imaging of the brain include positron emission tomography (PET), single photon emission tomography (SPECT), and magnetic resonance imaging (MRI). PET and SPECT methods measure the distribution of exogenously administered radioactive tracers, while functional MRI (fMRI) studies primarily utilize endogenous contrast and as such are completely non-invasive. For this reason, over the past 5 years fMRI has begun to replace PET as the technique of choice for mapping regional brain function in response to sensorimotor and cognitive tasks, as well as for imaging alterations in cerebral blood flow and metabolism. However, because of the extreme sensitivity of radioactive tracer techniques, PET and SPECT remain the only means of mapping changes occurring at very low concentrations, such as receptor binding. This chapter will provide an overview of these approaches to physiological imaging of the brain. Applications to neurological diagnosis and management as well as to cognitive neuroscience will also be addressed.
Physiology of regional brain function
A number of cellular and metabolic processes in the brain can be monitored using functional imaging methods, and have relevance to basic and clinical neuroscience. Figure 10.1 illustrates these processes which include neurotransmitter binding and reuptake, glucose utilization (CMRGlu), oxygen metabolism (CMRO2), and hemodynamic parameters of cerebral blood flow (CBF), cerebral blood volume (CBV), and mean transit time (MTT) and time to peak (TTP) for intravascular tracers. Depending on the specific application, some of these parameters may be more relevant than others. For example, studies in cerebrovascular disease have focused on hemodynamic parameters and their effects on oxidative metabolism, as well as on the apparent diffusion coefficient (ADC) in brain, a biophysical parameter available in MRI that reflects early cytotoxic injury.