Autism Spectrum Disorder (ASD) is a heterogeneous developmental condition characterized by difficulties in social communication and the presence of repetitive, restricted patterns of behavior.1,2 It is estimated that approximately 1 in 100 children is affected by autism worldwide.3 This figure reflects a general average, but prevalence rates differ significantly between studies. Scientific evidence indicates that both genetic and environmental factors likely contribute to autism.4, 5, 6 In a large-scale analysis using population-based data from three Nordic countries, which included 22,156 individuals diagnosed with ASD, researchers estimated that 81% of the variation in autism-related traits could be explained by genetic factors (95% confidence interval: 74%–85%).7 The same study found that environmental influences accounted for approximately 14% to 22% of the risk associated with ASD. A range of genetic and environmental factors have been linked to ASD, though none are uniquely responsible for its development. The symptoms of ASD can vary based on a patient's sex, age, cognitive abilities, and language proficiency. During the first two years of life, typical signs include delays or regression in language development and the use of communicative gestures. Children may also show limited social responsiveness and engage in repetitive behaviors, such as not responding to their name, flapping their hands, or arranging toys in a specific pattern. One commonly used tool, the Modified Checklist for Autism in Toddlers, Revised (M-CHAT-R),8 helps identify toddlers at risk for ASD, but any positive screen should be followed by a comprehensive evaluation and early intervention. The criteria for diagnosing ASD is a multidisciplinary clinical consensus based on developmental history and behavioral observation using standardized tools. Common assessments include the Autism Diagnostic Interview–Revised (ADI-R) and the Autism Diagnostic Observation Schedule, Second Edition (ADOS-2), with respective sensitivities of 80% and 91%.9 The primary aim of therapy for ASD is to enhance functioning and overall well-being through evidence-based interventions, although no medications have proven effective for the core symptoms.10,11 However, medications such as aripiprazole and risperidone may help manage associated behavioral challenges.12

Although several compelling theories have been proposed to explain the etiology and pathogenesis of the disorder since its first attempt, the causes and developmental mechanisms of ASD remain largely unknown.13 Currently, the most prominent theories regarding the pathogenesis of ASD include disruptions in neural connectivity, abnormalities in neuronal migration, imbalances between excitatory and inhibitory neural activity, impaired synaptogenesis and dendritic morphogenesis, disturbances in neuroimmune signaling—with particular emphasis on glial cell dysfunction—and the broken mirror neuron hypothesis.14, 15, 16, 17, 18

Traditional neurodevelopmental research methods—such as neuropsychological testing, behavioral assessments, and electrophysiological studies—have provided essential insights into the cognitive and functional features of ASD. However, to investigate the molecular and dynamic neural mechanisms proposed by emerging pathophysiological models, there is a growing need for complementary tools such as advanced neuroimaging. Molecular imaging techniques such as positron emission tomography (PET), have demonstrated significant potential in advancing research on ASD by enabling the in vivo characterization of functional and neurochemical brain abnormalities.19 Although PET is not currently employed in routine clinical diagnosis for ASD, its ability to map alterations in glucose metabolism, cerebral blood flow, neurotransmitter systems, neuroinflammation and synaptic density (Table 1 and Fig. 1) provides valuable insights into the underlying neurobiology of ASD and may inform the development of targeted therapeutic strategies in the future. This review explores the contributions of PET to ASD research and clinical translation.

The aim of this review is to provide a deeper understanding of the pathophysiology of ASD by highlighting the contributions of molecular pathology and molecular imaging, particularly advances in PET. Special emphasis is placed on recent innovations in PET technology—including high-resolution brain imaging systems, total-body PET, and novel radiotracers—that have expanded our ability to investigate neurochemical, synaptic, and neuroimmune mechanisms in ASD.

[18F]Fluorodeoxyglucose (FDG) PET is a widely used molecular imaging technique that measures regional cerebral glucose metabolism, serving as a proxy for neuronal activity, which is markedly higher in gray matter including the cerebral cortex, cerebellar cortex, thalamus and basal ganglia compared to white matter (Fig. 1).20,21 In the healthy brain, [18F] FDG uptake patterns vary with age, reflecting the dynamic nature of neurodevelopment and neurodegeneration. In early life, glucose metabolism is highest in sensorimotor, visual, and subcortical regions, gradually shifting toward the prefrontal cortex and associative areas as cognitive functions mature.22 With aging, a gradual decline in metabolic activity is typically observed in the frontal cortex and posterior cingulate gyrus correlating with normal cognitive aging processes.23 Therefore, to avoid misinterpreting normal development and aging as pathology, an age-matched FDG PET controls or reference database is essential for both research and clinical diagnostics.

The majority of [18F]FDG PET studies have explored altered regional glucose metabolism in individuals with ASD, revealing diverse yet converging patterns of metabolic alteration due to differences in experimental subjects and designs. Chugani et al. identified decreased glucose metabolism in the bilateral temporal cortex in children with infantile spasms showed poor long term outcome and the majority are autistic.24 They also reported that decreased metabolic activity in the anterior cingulate gyrus, frontal cortex, right temporal cortex, bilateral medial temporal regions and cerebellum in four children with presumable Sturge-Weber syndrome who were also autistic.25 Kadwa et al. reported sensory processing abnormalities were present in all children with severe autism, compared to 40% in those with mild to moderate autism. [18F]FDG PET was abnormal in 17% of severe cases, showing diffuse cerebral or temporal hypometabolism, increased in bilateral frontal lobes, and moderate decrease in parietal lobe (left > right).26 And several studies have reported that individuals with ASD exhibit both hypo- and hypermetabolism across different brain regions.27,28 Thus, there is no clear consensus regarding specific patterns of metabolic alteration in the ASD brain. However, the most consistently reported finding is reduced glucose metabolism in the temporal lobe among individuals with ASD.

In recent decades, analyzing metabolic connectivity patterns has gained considerable interest as a means to investigate the mechanisms behind human behavior and the neural basis of neurological brain disorders including ASD.29 Reduced salience network and increased cortico-cerebellar activity correlated with social deficits and may mark autistic-like traits in valproic acid rat models, more pronounced in male.30 Individuals with ASD show reduced brain glucose metabolism compared to controls. They also exhibit fewer regions with leftward laterality, suggesting that impaired left hemisphere dominance may contribute to ASD.31

[18F]FDG PET studies in individuals with ASD and comorbid epilepsy (Fig. 2), particularly those with Lennox Gastaut syndrome (Fig. 3), have revealed hypometabolism in prefrontal and premotor cortex, anterior and posterior cingulate, inferior parietal lobule, and precuneus on [18F]FDG PET.32 These metabolic disturbances likely indicate combined disruptions in brain networks caused by both seizure activity and neurodevelopmental changes, supporting the idea that epilepsy may amplify the underlying brain pathology of ASD and lead to more pronounced clinical symptoms. These findings highlight the utility of [18F]FDG PET in characterizing the neurobiological impact of ASD-epilepsy comorbidity and in informing treatment strategies.

[18F]FDG PET is also used for evaluating treatment effects in individuals with ASD enabling to observe both baseline abnormalities and post-treatment changes, providing insight into the neurobiological impact of therapeutic interventions. Six patients with ASD and one patient with Asperger's syndrome enrolled fluoxetine trial and underwent [18F]FDG PET at baseline and after treatment. Following fluoxetine treatment, relative metabolic rates increased notably in the right frontal lobe, particularly in the anterior cingulate gyrus and orbitofrontal cortex. Patients who initially exhibited higher metabolism in the medial frontal and anterior cingulate without medication were more likely to show a positive response to fluoxetine.33 These findings suggest that treatment response can be predicted based on baseline regional metabolic activity on [18F]FDG PET in individuals with ASD.

The human brain consumes approximately 15% of the body's cardiac output, making it the most blood flow–dependent organ. This underscores the crucial role of cerebral perfusion in maintaining neural activity and its significance in various neurological and cerebrovascular disorders including ASD.34,35 Cerebral blood flow (CBF) PET imaging, using tracers including [15O]H₂O and [11C]butanol, is a valuable tool for assessing regional perfusion and understanding functional brain abnormalities in individuals with ASD. These tracers cross the blood–brain barrier and accumulate in brain tissue in proportion to regional CBF (rCBF). Since rCBF is closely linked to metabolic demands and neural activity, perfusion imaging provides an indirect yet informative measure of localized brain function.36

Zilbovicius et al. reported a localized dysfunction of the temporal lobes in school-aged children with idiopathic autism using [15O]H₂O PET.37 The decreased blood flow was observed in both temporal lobes, with three focal areas in the right lobe and one in the left, primarily located in Brodmann areas 22 and 42, encompassing the left and right superior temporal gyrus and the right superior temporal sulcus. Meresse et al. invested a relationship between clinical profile and rCBF of 45 autistic children.38 Significant negative correlation between ADI-R scores and rCBF in the left superior temporal gyrus, suggesting these findings are related to autistic behavior severity. Boddaert et al. performed activation study in autistic children and found decreased rCBF in the left middle temporal and precentral frontal gyrus – left speech-related areas- when subjects with autism were listening to speech-like sounds.39 In individuals with autism, activity in the right dentate nucleus and left frontal Brodmann area 46 was lower during tasks involving verbal auditory processing and expressive language, but showed increased activation during motor speech functions, compared to controls.40 These findings suggest abnormal functional specialization within the dentato-thalamo-cortical pathway and support a model proposing region-specific biochemical disruptions in the developing brain of individuals with autism.41 Castelli et al. observed decreased perfusion in the superior temporal sulcus, temporal poles, and medial prefrontal cortex when participants viewed animations designed to evoke mentalizing processes.42 Hall et al. observed increased rCBF in the right anterior temporal pole, anterior commissure, and thalamus during tasks involving emotional recognition. Conversely, they noted reduced rCBF in the inferior fusiform gyrus and frontal brain regions.43 Pagani et al. investigated in the anatomo-functional connectivity of the “social” brain of limbic-striatal area using [11C]butanol PET.35 When compared to controls, individuals with ASD showed increased rCBF in the putamen, caudate, substantia nigra, right parahippocampal, posterior cingulate, primary visual and temporal cortex.

The cerebral cortex of the human brain is widely innervated by serotonin (5-hydroxytryptamine, or 5-HT)-containing axons originating from raphe nuclei.44 During early brain development, the serotonergic system in the cortex plays a crucial role in guiding cortical maturation and plasticity, highlighting serotonin’s regulatory function in promoting neuronal proliferation, guiding cell migration, and preventing programmed cell death.45,46

[11C]AMT, a tracer that binds to tryptophan hydroxylase—the key enzyme in 5-HT synthesis—has been widely used to measure brain serotonin production.47 Chugani et al. found asymmetrical 5-HT synthesis in the dentato-thalamo-cortical pathway in boys with autism, particularly showing reduced synthesis in the frontal cortex and thalamus on one side and elevated synthesis in the contralateral cerebellar dentate nucleus using [11C]AMT PET.48 Later studies with [11C]AMT PET revealed that nonautistic children have serotonin synthesis levels over twice adult values before age 5, then decrease with age—earlier in girls than boys. In contrast, autistic children showed a gradual increase in serotonin synthesis from ages 2 to 15, reaching 1.5 times adult levels, with no sex differences.49 These findings indicate that typical developmental serotonin regulation is altered in autism, suggesting disrupted maturation of the serotonergic system. Additionally, Chandana et al. reported children with autism who have reduced [11C]AMT binding in the left cortex showed more severe language impairment, while there was a higher autism prevalence with reduced [11C]AMT binding in the right cortex among those that were left-handed and ambidextrous. Abnormal asymmetrical development of the serotonin system both globally and locally may result in faulty neural circuitry that specifies hemispheric specialization.50

The serotonin transporter (5-HTT) plays a crucial role in regulating serotonin signaling by reabsorbing serotonin from the synaptic cleft back into presynaptic neurons. This process controls the duration and intensity of serotonergic transmission, making SERT essential for maintaining serotonin homeostasis. Abnormalities in SERT functions such as altered expression or genetic polymorphisms like the upstream regulatory region (5-HTTLPR) have been associated with increased risk for neurodevelopmental disorders, including ASD.51 These alterations may disrupt early brain development by affecting serotonin availability during critical periods of neuronal growth, synaptogenesis, and circuit formation. As such, SERT is a key component in the serotonergic system's influence on brain maturation and behavioral outcomes. 5-HTT levels throughout the brain of men with autism were significantly lower than in age- and IQ-matched control using [11C]McN5652 PET.52 [11C]DASB, which showed higher signal to noise ratio than [11C]McN5652, is more frequently used, however, no statistical difference was observed between individuals with Asperger’s disorder and control subjects.52,53 Compared to controls, individuals with ASD showed significantly reduced 5-HTT availability in total gray matter, the brainstem, and several brain subregions using [11C]MADAM PET.54 These reductions were also linked to impairments in social cognition. The findings support the longstanding hypothesis that serotonin plays a key role in ASD neurodevelopment and highlight the importance of further research into the serotonin system in ASD.

5-HT influences a wide range of physiological functions across the central nervous, cardiovascular, immune, and gastrointestinal systems through the activation of diverse 5-HT receptors (5-HTR).55 Currently, 14 subtypes of 5-HTR have been identified.56 To date, only the 5-HT₂AR, which is primarily involved in excitatory neurotransmission, sensory perception, and cognition, and the 5-HT₁AR, known for its inhibitory role in regulating mood, anxiety, and social behavior, have been investigated in ASD using PET imaging.57,58 Probes targeting the 5-HT2AR include [11C]MDL100907, while probes targeting the 5-HT1AR primarily consist of [18F]MPPF. However, studies utilizing these two tracers have not demonstrated any statistically significant differences between individuals with ASD and controls.

The dopamine system comprises a network of neurons that synthesize and release dopamine, a crucial neurotransmitter involved in a wide array of brain functions, including movement, motivation, reward processing, and emotional regulation.59 It plays a key role in ASD pathogenesis, with genes involved in dopamine signaling—such as receptors, transporters, and metabolic enzymes—being linked to the disorder.60 Multiple genetic studies and investigations involving dopamine-modulating medications suggest that disruptions in the dopamine system are present in individuals with ASD.61, 62, 63 Furthermore, the clinical effectiveness of psychotropic medications that act on the dopamine system in alleviating symptoms in individuals with ASD supports the involvement of dopamine signaling in regulating behavior in this population.64

Ernst et al. reported FDOPA ratio was decreased in the anterior medial prefrontal cortex in individuals with autism compared to the controls.65 However, Schalbroeck et al. found no statistical difference in striatal dopamine synthesis capacity between individuals with ASD and controls and was not associated with loneliness scale.66 They suggest unmedicated, nonpsychotic ASD do not exhibit elevated striatal dopamine synthesis capacity, nor are it linked to experiences of social defeat. The variability in findings across these studies is likely due to differences in the characteristics of the study populations.

Nakamura et al. investigated relationship between dopamine and serotonin systems and found individuals with ASD had significant higher dopamine transporter (DAT) binding in the orbitofrontal cortex measured using [11C]WIN-35,428. And DAT binding in the orbitofrontal cortex was inversely correlated with 5-HTT binding measured by [11C]McN-5652 suggesting a autistic individuals exhibit altered binding of both SERT and DAT in the brain.52

Kubota et al. studied dopamine D1 receptor (D1R) and noradrenaline transporter (NAT) using [11C]SCH23390 and (S,S)-[18F]FMeNER-D2 in adults with ASD and controls.67 There were no significant group differences in D1R binding in the striatum, anterior cingulate cortex and temporal cortex or NAT binding in the thalamus and pons. However, D1R binding was showed significant negative correlation with attention to detail subscale score and a positive correlation with emotion perception ability. Zürcher et al. recently showed D2R/D3R binding in the putamen and caudate nucleus in adults with ASD is decreased with controls using [11C]raclopride.68

Gamma-aminobutyric acid (GABA) is a key inhibitory neurotransmitter in the adult mammalian brain and is considered to have versatile roles in the central and peripheral nervous systems, as well as in non-neuronal tissues. It is mainly produced from glutamate by the enzyme glutamate decarboxylase.69 Direct evidence of GABA dysfunction in autism comes from studies on mice with a mutated subunit of GABA receptors, which showed impairments in sociability and repetitive behaviors, key characteristics of ASD.70 A dysfunction of the GABAergic signaling early in development can cause an imbalance between excitation and inhibition in the neural circuits, a phenomenon linked to ASD.71,72

On [11C]flumazenil PET scans (Fig. 1), the uptake and distribution in the brain are mainly influenced by the density of GABAA receptors, which play a key role in inhibitory neurotransmission.73 The distribution of flumazenil typically reflects the density of these receptors. In areas like the cortex, hippocampus, and cerebellum, where GABAA receptors are abundant, flumazenil uptake is more noticeable. In contrast, regions with fewer receptors, such as the brainstem, exhibit lower uptake.

While substantial evidence indicates that GABA dysfunction might be associated with ASD, there are only a limited number of PET studies on GABA. Mendez et al. demonstrated decreased binding of [11C]RO15-4513 in the bilateral amygdala in three high-functioning adults with autism.74 However, other studies with [11C]flumazenil showed no significant difference between adults with ASD and controls.75,76

While the exact etiology of ASD remains unknown, a growing body of research suggests that prenatal and gestational immune system dysfunction may contribute to its pathophysiology.77 Studies have suggested that immune-mediated mechanisms, such as neuroinflammation including microglial activation, may lead to changes in immune responses that could impact brain development and function, potentially contributing to the onset of ASD.78

Neuroinflammation PET studies have long been focused on targeting the translocator protein (TSPO),79 which is a mitochondrial protein that is expressed mostly by microglial cells upon activation, cells that play a crucial role in immunity within the central nervous system.80 Although this supports the potential of TSPO PET as a biomarker for neuroinflammation, the clinical application of current TSPO PET radioligands has been limited by their lower affinity for a common polymorphic form of TSPO (A147T) compared to the wild-type TSPO.81

Suzuki et al. found increased [11C]PK11195 binding in the fusiform gyrus, prefrontal cortex, cingulate cortex, midbrain, and cerebellum of individuals with ASD and controls.82 By comparing TSPO in 15 young adult males with ASD with 18 age- and sex-matched controls, Zürcher et al., showed that individuals with ASD exhibited lower regional TSPO expression in several brain regions measured by [11C]PBR28 PET, including the bilateral insular cortex, bilateral precuneus/posterior cingulate cortex, and bilateral temporal, angular, and supramarginal gyri, which have previously been implicated in autism in functional magnetic resonance imaging studies. No brain region exhibited higher regional TSPO expression in the ASD group compared with the control group.83 The inconsistency between the results of these two studies may be due to the heterogeneity of ASD patients, including factors such as age, gender, IQ, subtype, and the potential impact of TSPO gene polymorphisms. Additionally, temporal variations in the level of inflammation at the time of the PET scan could also account for these discrepancies.81

Synapses are essential for neurotransmission.84 In a healthy human brain, there are hundreds of billions of neurons interconnected by synapses. Each neuron can establish tens of thousands of synaptic connections with other neurons. Consequently, the brain contains hundreds of trillions of synapses, with roughly 150 to 164 trillion located in the cerebral neocortex.85 Synaptic dysfunction is linked to a range of neuropsychiatric disorders including ASD.86

Regional synaptic density has traditionally been assessed using techniques such as stereology, immunohistochemistry, and electron microscopy.87 However, PET imaging of synaptic vesicle glycoprotein 2A (SV2A), which is expressed GABAergic and glutamatergic neurons of the central nervous system, was developed as the first noninvasive method to measure synaptic density in vivo.88 Since SV2A is widely expressed in nearly all presynaptic vesicles, it serves as an appropriate biomarker for synaptic density,89 alongside other proteins found in the presynaptic proteome and synaptic vesicles.90 Matuskey et al. reported all brain regions in individuals with autism showed lower binding potential measured by [11C]UCB-J PET which targeted SV2A.91 Significant differences were observed across multiple individual regions, including the prefrontal cortex. Synaptic density was significantly associated with clinical measures across the whole cortex. These findings suggest that the overall synaptic density in the brain could serve as an unidentified molecular foundation for the clinical features of autism, as well as the widespread changes across various neural processes associated with the disorder.

Recent advancements in PET technology have significantly enhanced its clinical and research applications. The development of digital PET scanners has led to improved spatial resolution, higher sensitivity, and faster acquisition times, which are particularly beneficial when imaging pediatric or neurologically diverse populations.92,93 Moreover, total-body PET systems allow for simultaneous imaging of the entire body with ultra-high sensitivity, enabling researchers to study brain-body interactions, systemic inflammation, and peripheral biomarkers potentially relevant to ASD.94 This is particularly beneficial for pediatric populations, as it enables comprehensive whole-body imaging in a single, shorter session—reducing motion-related artifacts and improving scan tolerance in children with ASD—while also allowing for significant radiation dose reduction due to the system’s enhanced sensitivity.95,96 Complementing this, a dedicated human brain PET scanner with high spatial resolution provide unparalleled detail in brain-specific molecular imaging, supporting precise location and quantification and can offer insights into physiologic and pathologic processes.97,98

In parallel, the introduction of the novel PET radiotracers has significantly enhanced the ability to investigate synaptic and neurotransmitter-related abnormalities in ASD. For instance, [¹¹C]UCB-J, which targets synaptic vesicle glycoprotein 2A (SV2A), enables in vivo quantification of synaptic density, offering a direct measure of synaptic integrity—a process thought to be disrupted in ASD.99 Similarly, [¹¹C]K-2, a tracer developed for visualizing AMPA-type glutamate receptor distribution, allows researchers to examine excitatory neurotransmission dynamics with high specificity.100 These molecular tools facilitate more targeted investigations into the synaptic and receptor-level alterations hypothesized in ASD pathophysiology, thereby bridging the gap between theoretical models and measurable in vivo biomarkers.

These innovations collectively position PET as a powerful tool for advancing our understanding of the neurobiological mechanisms underlying ASD and for identifying potential biomarkers to guide diagnosis and treatment.