While MRI excels at structure and BOLD-signal-based function, positron emission tomography (PET) measures something MRI cannot: the chemistry of the living brain. PET uses small amounts of radioactive tracers that bind to specific molecules — dopamine receptors, serotonin receptors, microglia — and lets researchers see how much of each is present and active. In schizophrenia research, PET has been responsible for some of the most influential findings of the past 30 years.
PET imaging has provided direct biological evidence of altered presynaptic dopamine function in the striatum of people with schizophrenia, and is now extending to other neurotransmitter systems and to neuroinflammation.
How PET works
A PET tracer is a biologically meaningful molecule labelled with a short-lived radioactive isotope (typically carbon-11 or fluorine-18). Once injected, the tracer distributes through the brain and binds preferentially to its target. The radioactive decay produces positrons, which are detected by the scanner. The result is a 3-D map of where the target molecule is and, with appropriate kinetic modelling, how active it is.
Different tracers measure different things:
- Dopamine D2/D3 receptor availability (e.g., [11C]raclopride)
- Presynaptic dopamine synthesis and release (e.g., [18F]DOPA, [11C]raclopride displacement studies)
- Serotonin receptors
- NMDA receptor function (more difficult; tracers are limited)
- Microglial activation (e.g., TSPO tracers)
- Synaptic density (e.g., [11C]UCB-J targeting SV2A)
The dopamine story
The classical dopamine hypothesis of schizophrenia held that excess dopamine activity caused positive symptoms. PET refined this picture substantially. Across many studies and meta-analyses, the most consistent finding has been elevated presynaptic dopamine synthesis capacity in the striatum, particularly in the associative striatum, in patients with schizophrenia compared with healthy controls. This was demonstrated in a series of studies led by Oliver Howes and others, summarised in his influential 2009 paper in the Schizophrenia Bulletin.
Crucially, PET has shown that this dopamine elevation:
- Is detectable in clinical high-risk individuals before the first psychotic episode
- Increases as people transition to full psychosis
- Is more pronounced during acute psychosis
- Is largely a presynaptic phenomenon, not a postsynaptic receptor abundance change
This refinement matters: it suggests that dopamine pathology is upstream of the receptors that antipsychotics block, which may help explain why current dopamine-blocking medications work for many patients but not all, and why some patients are treatment-resistant.
Treatment response and PET
PET studies have shown that patients with treatment-resistant schizophrenia tend not to have the same striatal dopamine elevation seen in treatment-responsive patients. This is consistent with the clinical observation that clozapine — a unique antipsychotic effective in treatment resistance — does not work primarily through high-affinity D2 blockade. The implication is that schizophrenia may include biologically distinct subtypes with different dominant pathologies.
Beyond dopamine
Glutamate
Glutamate PET has been technically challenging. Most evidence on glutamate in schizophrenia comes from MR spectroscopy, not PET. New tracers for the metabotropic glutamate receptors mGluR5 and for NMDA receptor subunits are emerging and may begin to fill that gap.
Serotonin
PET studies of serotonin 5-HT2A receptors — the target shared by many atypical antipsychotics — have produced mixed findings, with some studies showing reduced cortical 5-HT2A binding in schizophrenia. The clinical significance remains under investigation.
Microglia and neuroinflammation
The translocator protein (TSPO) is expressed by activated microglia, the brain's resident immune cells. TSPO PET has been used to test the hypothesis that schizophrenia involves chronic low-grade neuroinflammation. Results have been mixed: some studies, including early work by Doorduin and colleagues, suggested elevated TSPO binding in patients; later well-controlled studies have shown no consistent elevation in chronic patients, and some have shown reduced binding. The field is still working out tracer interpretation, blood-brain barrier issues, and the role of medications. See our piece on neuroinflammation in schizophrenia.
Synaptic density
Newer tracers like [11C]UCB-J bind to synaptic vesicle glycoprotein 2A (SV2A) and are interpreted as a measure of synaptic density. Early PET studies have shown reduced cortical SV2A binding in schizophrenia, consistent with the long-standing model of altered synaptic pruning. This is one of the more exciting recent developments and is being actively replicated and extended.
What PET cannot do
PET has important limitations. The radiation exposure means individuals can only be scanned a limited number of times in their life. Tracers are expensive to produce and require nearby cyclotrons for the shorter-lived isotopes. The temporal resolution is poor compared with EEG. And, like fMRI, PET findings show group differences that are often not strong enough to make individual diagnoses.
Antipsychotic medication occupies dopamine D2 receptors and dramatically alters PET measurements at those receptors. PET dopamine studies have to carefully account for medication exposure, and ideally are done in unmedicated or first-episode patients, which is logistically difficult and ethically delicate.
The radiation dose from a typical research PET scan is comparable to a few months of background radiation exposure. Healthy volunteer participation involves informed consent and ethical oversight.
Clinical use
PET is not used to diagnose schizophrenia in clinical practice. The cost, radiation exposure, and lack of a diagnostically validated tracer all argue against routine use. PET is occasionally used in clinical psychiatry to investigate atypical presentations — for example, FDG-PET to evaluate possible early-onset dementia, or DAT scans in possible Lewy body disease — but these are not schizophrenia-specific.
What PET has changed
The cumulative effect of PET research on the field has been substantial:
- It has anchored the dopamine hypothesis of psychosis in direct biological evidence.
- It has shifted the model from "too many D2 receptors" to "too much presynaptic dopamine release."
- It has supported subtyping schizophrenia by treatment response and underlying biology.
- It is helping connect classical neurotransmitter findings with newer ideas about synaptic density and inflammation.
Where the field is going
Newer tracers, longer-lived isotopes that allow more flexible scanning, combined PET–MRI scanners that capture both modalities at once, and machine-learning analyses of multi-tracer PET datasets are all active areas. The most likely near-term clinical application is treatment selection — using PET signatures to predict response to a specific medication class — rather than diagnosis.
For complementary perspectives, see our articles on the dopamine hypothesis, the glutamate hypothesis, and fMRI research.
This article is for educational purposes only and is not medical advice, diagnosis, or treatment. Always consult a qualified mental health professional. If you or someone you know is in crisis, call or text 988 in the US, or your local emergency number.