Which imaging technique utilizes a radioactive substance to observe brain activity?

The choice of a positron emission tomography (PET) scan is appropriate because it specifically utilizes a radioactive substance, known as a radiotracer, to observe brain activity. When this substance is injected into the body, it emits positrons, which can be detected by the PET scanner. This process allows researchers to visualize metabolic processes in the brain, thus enabling the observation of areas with higher activity levels based on glucose consumption or blood flow.

In contrast, the other imaging techniques do not employ radioactive materials for brain activity observation. A CT scan uses X-rays to create detailed images of brain structures but does not provide functional data regarding brain activity. Functional magnetic resonance imaging (fMRI) measures brain activity indirectly by detecting changes in blood flow and oxygen levels but does so without any radioactive materials. Similarly, MRI focuses on imaging the brain's structure using magnetic fields and does not involve radioactive substances. Therefore, the unique feature of PET in utilizing a radioactive substance for monitoring brain activity distinguishes it from the other imaging modalities.