Positron Emission Tomography (PET) is one of the most important techniques for measuring the blood flow and energy consumption in the brain. This method of measuring brain function is based upon the detection of radioactivity emitted when positron, positive charged particles, undergo radioactive decay in the brain. Small amounts of radiation are introduced in the blood, which is then taken up in different areas of the brain in proportion to how hard the neurons are working. The amount of radiation emitted from these different areas is subsequently used by computers to construct three-dimensional images of the brain.

PET studies have helped scientists understand more about how drugs affect the brain, what happens during learning, what happens while using language and how certain brain disorders work, such as stroke, depression and Parkinson’s disease. Within the next few years, it seems likely that PET studies will enable researchers to identify the biochemical nature of neurological and mental disorders and determine how well certain therapies are working in patients. It has already revealed marked changes in the depressed brain. Locating these changes could help to understand the causes of depression and monitor the effectiveness of specific treatments.

MEG

Magnetoencephalography (MEG) is a recently developed technique that reveals the source of weak magnetic fields emitted by neurons. Several cylinder-shaped sensors monitor the magnetic field pattern near the patient’s head to determine the position and strength of activity in various regions of the brain. In contrast with other techniques, it can characterize rapidly changing patterns of neural activity (resolution of milliseconds) and can provide a quantitative measure of strength of this activity.

An exciting development in neural imaging is the combination of fMRI and MEG. The former provides information about the areas of brain activity in specific tasks, while the latter tells researchers when certain areas become active. When combined, this information leads to a detailed understanding of how the brain works in health and disease.

Optical Imaging

This group of techniques relies on shining weak lasers through the skull to visualize brain activity. These techniques are portable and relatively inexpensive. Furthermore, they are also silent and safe. Since only very weak lasers are used, they can even be implemented on infants. Two examples of such techniques are:

Near Infrared Spectroscopy (NIRS): Lasers at near infrared frequencies are used. Blood with a high oxygen content absorbs different frequencies than blood with a low oxygen content, allowing observations of the blood flow in the brain.
Event-related Optical Signal: This technique records how light is scattered in response to cellular changes that arise when neurons fire. It can quickly assess changes in neural activity.