PET in Psychiatry and Brain Research: A Research Tool

Headlines sometimes suggest that a brain scan can show depression or schizophrenia. The reality is quieter. PET has been one of the most important research tools for studying brain chemistry in psychiatric conditions, yet it is not used as a routine test to diagnose any of them. This page explains what researchers measure with PET, what they have reported in conditions such as depression, schizophrenia and addiction, how research PET differs from the clinical scans described elsewhere on this site, and how the ethics of giving people tiny amounts of radioactive tracers are handled. Anyone worried about their own or a relative's mental health should speak with their doctor or a psychiatrist; nothing here is advice about an individual.

The starting point: mostly research, not routine diagnosis

The American Psychiatric Association published a resource document on neuroimaging, updated in 2017 and approved in 2018, that began as a response to claims that brain imaging was already useful for clinical diagnosis and treatment selection. Its conclusion, despite a number of promising results, was that there are currently no brain imaging biomarkers that are clinically useful for any diagnostic category in psychiatry (American Psychiatric Association). To count as useful, the document says, a test should clearly improve on the current standard of care for establishing a diagnosis.

A 2022 review in Translational Psychiatry gives a similar verdict from inside the research field: most PET research on psychiatric disorders has focused on monoamine systems, such as dopamine and serotonin, and although a series of discoveries has been made, the results have not led to any material changes in clinical practice. It adds that this line of research has not led to new diagnostic tools or treatments for clinical use. The findings describe groups and mechanisms, and do not yet tell a clinician about the individual in front of them.

What researchers measure with PET

Most brain PET research uses a radioligand, a tracer designed to bind to one molecular target, so that the scanner maps how much of that target is present. Targets include receptors that receive chemical signals, transporters that clear them from the gap between nerve cells, the enzymes and proteins involved in signalling, and markers of immune activity. Tracers are introduced in PET radiotracers explained.

The Translational Psychiatry review summarises examples. Studies of dopamine function in psychosis and schizophrenia have collectively shown increased dopamine synthesis and release capacity, indexed by raised F-DOPA uptake and by larger amphetamine-induced drops in D2 receptor binding. In major depressive disorder, findings point to reduced serotonin transporter and serotonin 1A receptor availability, and in anxiety disorders to decreases in serotonin 1A and benzodiazepine receptors. These are group-level findings, and the review also stresses that psychiatric PET studies have often used small samples, which limits what can be concluded.

Synaptic density

A newer approach images synapses, the contact points between nerve cells. Tracers such as UCB-J bind to a protein called SV2A found in nerve terminals, giving a living-brain estimate of synaptic density that was previously available only from post-mortem tissue. A 2024 review in Neuropsychopharmacology describes the authors' findings of lower SV2A synaptic density in people with significant depressive symptoms, and their use of a ketamine challenge to study synapse formation in vivo. This is an active research area and not a clinical test; the review itself frames synaptic density imaging as a way of understanding mechanisms.

Neuroinflammation

Another strand looks at immune activity in the brain. The most established method targets a protein called TSPO, which is expressed in activated microglia (the brain's resident immune cells) and astrocytes. The results are mixed. According to the Translational Psychiatry review, early schizophrenia studies using the first-generation tracer found increased binding, but this was not confirmed with more sensitive tracers, whereas six of seven TSPO studies in major depression showed increased binding compared with controls. The review also cautions that TSPO is not specific to microglia, and that the evidence for what the signal means in different cells is conflicting. Newer, more specific tracers are under investigation.

Addiction research

PET was also central to a body of work on substance use. In an interview reported by Radiology Today (a 2009 trade magazine article), Nora Volkow of the US National Institute on Drug Abuse described a consistent pattern across chronic use of cocaine, heroin, alcohol and amphetamines: reduced availability of dopamine D2 receptors, with lower receptor availability in the striatum linked to lower activity in prefrontal regions involved in motivation and decision-making. The same article records criticism. One psychologist argued that the technology cannot differentiate habitual from addictive use. That caution fits the broader point: such PET findings illuminate biology across groups, and they are not a test for whether any one person has an addiction.

Where PET has changed practice: drug dosing

The Translational Psychiatry review says that the resulting change in dosing regimens remains the most significant contribution of PET research to clinical psychiatry to date. The idea is called target occupancy: PET shows what fraction of a target a medicine is blocking at a given dose. A review in Neuropsychopharmacology Reports gives examples, reporting an adequate dopamine D2 occupancy window for antipsychotic effect of roughly 65 to 70 percent up to 80 percent, and a clinical threshold of 70 to 80 percent serotonin transporter occupancy for antidepressants. This is a use of PET in drug development rather than in diagnosing patients.

How research PET differs from clinical PET

Clinical PET answers a question about one patient, usually with a tracer that is approved and has an established reading method, such as FDG in the brain scans described in PET in dementia and memory evaluation. Research PET asks a question about a mechanism, often in a group of volunteers compared with a control group, frequently with an investigational tracer approved only for a study. The measures are more complex, the groups are small, and the results are averages. The same review points out that the most severely ill patients are often left out of studies because of difficulty with demanding procedures or giving consent, which limits what the findings can say about the full picture of these conditions.

Ethics, radiation and microdosing

Every research scan involves a small radiation exposure with no direct benefit to the volunteer, so it is regulated with care. The Translational Psychiatry review reports that typical effective doses are around 2 millisieverts for carbon-11 tracers and 5 to 10 millisieverts for fluorine-18 tracers, and that the maximum effective dose in clinical research is typically limited to 10 millisieverts depending on the expected impact of the research, which restricts repeat scanning. A review of PET in clinical microdose studies in Advanced Drug Delivery Reviews adds that no formal radiation dose limits are defined across the European Union for radiotracers in clinical research, so risk and benefit are judged case by case by national authorities, and that in the United States a Radioactive Drug Research Committee assesses both the pharmacological and radiation dose. To put such numbers in context, see radiation dose in context.

Microdosing is the other safeguard. The same review describes a microdose as a dose of no more than 100 micrograms (less than one hundredth of the amount expected to have a pharmacological effect), which supports a reduced preclinical safety package. PET tracers are normally given in such tiny amounts that they are meant to trace a process and not to change it. Consent is the third issue. People with severe mental illness may be unable to give informed consent, and the Translational Psychiatry review discusses the tension between protecting them individually and the need to study their conditions at all. The history of how positron imaging reached this point is in a short history of positron imaging.

The short version

PET lets researchers see receptors, transporters, synapses and inflammation in the living brain, and it has informed how psychiatric medicines are dosed. It has not produced a diagnostic test for conditions such as depression, schizophrenia or addiction, and the major psychiatric and research reviews say so plainly. Research scans follow strict ethical and radiation rules.