Dr. Aysenil Belger is a professor of psychiatry at UNC-Chapel Hill and an adjunct professor at the Duke-UNC Brain Imaging and Analysis Center. Here she explains the mechanics and applications of neuroimaging. Questions and answers have been edited.
PET and MRI scans can provide images of brain anatomy, structure and function. When a person perceives stimuli or thinks certain thoughts, neurons (nerve cells) in specific regions of the brain get “activated.” As these neural circuits or brain regions are activated, their metabolic needs increase and they consume more oxygen, which is supplied by increased blood flow. PET scans can measure blood flow, velocity and volume, as well as oxygen and glucose consumption.
They can also be used to measure the location and density of specific “receptors” or chemical channels, such as dopamine receptors or other neurotransmitters, through radioactive tags. PET is often used to discover the mechanisms of disease or develop new drugs.
MRI can be used for structural imaging of brain anatomy (white matter, gray matter, sizes and shapes of brain structures) but also functional imaging of brain physiology. Specifically, MRI scanners can be “tuned” to visualize the regional changes in brain activity related to particular sensory or cognitive events. Using this method, we can visualize which regions of the brain are engaged when an individual makes a decision, experiences particular emotions, or remembers specific events.
Because the brain is a complex network with highly interconnected regions, our ability to gather a person’s thoughts from a static brain image is limited. However, if we detect activation of specific regions known to be associated with the perception of emotions such as fear or sadness or anger, we can infer that the individual whose brain is being scanned may be processing those stimuli. But their “thoughts” about the stimuli remain unknown.
Imaging studies from my lab have demonstrated abnormalities in activation of regions of the brain associated with the processing of emotion in patients with schizophrenia. We have also demonstrated abnormal brain activations during processing of faces or other social and emotional events in individuals with autism. These functional changes were also accompanied by mild brain anatomical changes, suggesting structural alterations in many brain regions in these disorders.