Glossary
Positron emission tomography
What is Positron emission tomography?
Positron emission tomography (PET) uses positron-emitting tracers to estimate the distribution or behavior of selected molecules in living tissue. In brain research, the process studied depends on the tracer and analysis.
How it works
A tracer emits positrons; their interaction with electrons produces paired photons detected by the scanner. Different tracers support different measurements. FDG provides information about glucose utilization, while other tracers target selected molecular processes. Radiation exposure, resolution, timing and interpretation depend on the scan and tracer.
PET research example: addiction
In a study by Martinez and colleagues, 15 cocaine-dependent volunteers and 15 matched comparison participants underwent PET with [11C]raclopride, a tracer used to estimate dopamine D2/D3 receptor availability. Before an experimental dopamine-depletion procedure, the cocaine-dependent group had lower striatal binding potential. This is a tracer-binding estimate, not a direct count of every receptor.
Availability depends in part on whether the body’s own dopamine already occupies receptors. The researchers therefore repeated scans after dopamine depletion; the results did not support excess baseline dopamine as the explanation for the lower binding in the cocaine-dependent group. This illustrates how a tracer and an experimental manipulation can refine a molecular interpretation. It still does not establish whether cocaine use caused the group difference or whether it existed before use.
Why it matters
PET adds molecular information to imaging. Its estimates are tracer-dependent and should not be treated as direct measurements of every neurotransmitter or mental process.
Source: NIBIB explains the tracer-specific basis of PET measurements.