Which technique provides a visual display of brain activity through a radioactive tracer?

The technique that provides a visual display of brain activity through a radioactive tracer is indeed positron emission tomography (PET). This imaging test works by introducing a small amount of a radioactive substance, known as a tracer, into the body, which emits positrons as it decays. When these positrons encounter electrons in the body, they produce gamma rays that can be detected by the PET scanner. The areas of higher radioactive tracer concentration correspond to regions of increased metabolic activity in the brain, allowing researchers and clinicians to visualize how different areas of the brain are functioning during various tasks or in different states, such as rest or illness.

In contrast, other imaging techniques like CT (computed tomography) scans primarily focus on structural information of the brain and do not specifically show brain activity. fMRI (functional magnetic resonance imaging) gives insights into brain activity through changes in blood flow but uses magnetic fields rather than radioactive tracers. EEG (electroencephalogram) measures electrical activity in the brain through electrodes placed on the scalp and does not involve visualizing metabolic activity. Thus, PET stands out as the method specifically linked to the use of a radioactive tracer for observing brain activity.