Refining seizure foci localization: the potential of TSPO-PET.

To understand the role of neuroinflammation in Alzheimer's disease (AD), we characterized immune-related proteins in central and peripheral biofluids. Selection of participants from the Translational Biomarker of Aging and Dementia (TRIAD) cohort with available translocator protein (TSPO) positron emission tomography (PET), cerebrospinal fluid (CSF) (n=97), and plasma (n=165). Biofluid samples analyzed with Olink technology (368 inflammation proteins). Elevated proteins levels in CSF of TSPO-positive individuals were identified. Functional enrichment analysis of CSF proteins revealed processes implicated in AD (MAPK, ERK cascades, cytokine, and leukocyte signaling). Selected candidates (CXCL1 and TNFRSF11B) showed high correlation with each other in CSF and with TSPO PET signal, but weaker associations with amyloid and tau PET. No significantly changed proteins in plasma between TSPO groups were found. This explorative study identified two potential targets in CSF showing correlations with TSPO, amyloid and tau PET, suggesting a direct link between neuroinflammation, expression of these proteins and their potential implication in AD. Several proteins are elevated in CSF of TSPO PET-positive individuals, linking them to neuroinflammation. Elevated CSF proteins were enriched in pathways such as MAPK, ERK, and cytokine signaling, linking them to the AD pathophysiology. Candidate proteins (CXCL1 and TNFRSF11B) correlated strongly with TSPO PET, particularly in brain regions known to be affected in AD. Although none of the plasma proteins remained significant after multiple comparisons correction when comparing their expression between TSPO groups, as done for CSF, candidate CSF proteins were found to correlate with plasmatic proteins, highlighting the complexity of the immune system.

Microglia measured by TSPO PET are associated with Alzheimer's disease pathology and mediate key steps in a disease progression model

Epilepsy is one of the most common neurological disorders and affects both the young and adult populations. The question we asked for this review was how positron emission tomography (PET) imaging with translocator protein (TSPO) radioligands can help inform the epilepsy clinic and the development of future treatments targeting neuroinflammatory processes.Even though the first TSPO PET scans in epilepsy patients were performed over 20years ago, this imaging modality has not seen wide adoption in the clinic. There is vast scientific evidence from preclinical studies in rodent models of temporal lobe epilepsy which have shown increased levels of TSPO corresponding to neuroinflammatory processes in the brain. These increases peaked sub-acutely (1-2weeks) after the epileptogenic insult (e.g. status epilepticus) and remained chronically increased, albeit at lower levels. In addition, these studies have shown a correlation between TSPO levels and seizure outcome, pharmacoresistance and behavioural morbidities. Histological assessment points to a complex interplay between different cellular components such as microglial activation, astrogliosis and cell death changing dynamically over time.In epilepsy patients, a highly sensitive biomarker of neuroinflammation would provide value for the optimization of surgical assessment (particularly for extratemporal lobe epilepsy) and support the clinical development path of anti-inflammatory treatments. Clinical studies have shown a systematic increase in asymmetry indices of TSPO PET binding. However, region-based analysis typically does not yield statistical differences and changes are often not restricted to the epileptogenic zone, limiting the ability of this imaging modality to localise pathology for surgery. In this manuscript, we discuss the biological underpinnings of these findings and review for which applications in epilepsy TSPO PET could bring added value.

Emerging evidence from autopsy studies indicates that interface astroglial scarring (IAS) at the gray–white matter junction is a pathological signature of repeated blast brain injury in military personnel. However, there is currently no in vivo neuroimaging test that detects IAS, which is a major barrier to diagnosis, prevention, and treatment. In 27 active-duty U.S. Special Operations Forces personnel with high levels of cumulative blast exposure, we performed translocator protein (TSPO) positron emission tomography (PET) using [11C]PBR28 to detect neuroinflammation at the cortical gray–white matter interface, a neuroanatomic location where IAS has been reported in autopsy studies. TSPO signal in individual Operators was compared with the mean TSPO signal in a control group of nine healthy civilian volunteers. We identified five Operators (18.5%) with TSPO signal at the cortical gray–white matter interface that was more than 2 standard deviations above the control mean. Cumulative blast exposure, as measured by the generalized blast exposure value, did not differ between the five Operators with elevated TSPO signal and the 22 Operators without elevated TSPO signal. While the pathophysiologic link between neuroinflammation and IAS remains uncertain, these preliminary observations provide the basis for further investigation into TSPO PET as a potential biomarker of repeated blast brain injury.

Microglia, the resident immune cells of the central nervous system (CNS) play a key role in regulating and maintaining homeostasis in the brain. However, the CNS is also vulnerable to infections and inflammatory processes. In response to CNS perturbations, microglia become reactive, notably with expression of the translocator protein (TSPO), primarily on their outer mitochondrial membrane. Despite TSPO being commonly used as a marker for microglia, it is also present in other cell types such as astrocytes. Positron emission tomography (PET) ligands that target the TSPO enable the noninvasive detection and quantification of glial reactivity. While some limitations were raised, TSPO PET remains an attractive biomarker of CNS infection and inflammation. This book chapter delves into the development and application of microglial PET imaging with a focus on the TSPO PET. First, we provide an overview of the evolution of TSPO PET radioligands from first-generation to second-generation ligands and their applications in studying neuroinflammation (or CNS inflammation). Subsequently, we discuss the limitations and challenges associated with TSPO PET. Then we go on to explore non-TSPO targets for microglial PET imaging. Finally, we conclude with future directions for research and clinical practice in this field.

TSPO PET in multiple sclerosis: Emerging insights into pathophysiology, prognosis, and treatment monitoring.

The aim was to study brain innate immune cell activation in teriflunomide-treated patients with relapsing-remitting multiple sclerosis. Imaging with 18-kDa translocator protein positron emission tomography (TSPO-PET) using the [11 C]PK11195 radioligand was employed to assess microglial activity in the white matter, thalamus and areas surrounding chronic white matter lesions in 12 patients with relapsing-remitting multiple sclerosis who had been treated with teriflunomide for at least 6 months before inclusion. Magnetic resonance imaging (MRI) was used to measure lesion load and brain volume, and quantitative susceptibility mapping (QSM) was used to detect iron rim lesions. These evaluations were repeated after 1 year of inclusion. Twelve age- and gender-matched healthy control subjects were imaged for comparison. Half of the patients had iron rim lesions. In TSPO-PET, the proportion of active voxels indicating innate immune cell activation was slightly greater amongst patients compared with healthy individuals (7.7% vs. 5.4%, p = 0.033). The mean distribution volume ratio of [11 C]PK11195 was not significantly different in the normal-appearing white matter or thalamus amongst patients versus controls. Amongst the treated patients, no significant alteration was observed in positron emission tomography distribution volume ratio, the proportion of active voxels, the number of iron-rim-positive lesions, lesion load or brain volume during follow-up. Compared to controls, treated patients exhibited modest signs of diffuse innate immune cell activity, which was unaltered during follow-up. Lesion-associated smoldering inflammation was negligible at both timepoints. To our knowledge, this is the first study applying both TSPO-PET and QSM-MRI to longitudinally evaluate smoldering inflammation.

Neuroinflammation is an inflammatory response in the brain and spinal cord, which can involve the activation of microglia and astrocytes. It is a common feature of many central nervous system disorders, including a range of neurodegenerative disorders. An overlap between activated microglia, pro-inflammatory cytokines and translocator protein (TSPO) ligand binding was shown in early animal studies of neurodegeneration. These findings have been translated in clinical studies, where increases in TSPO positron emission tomography (PET) signal occur in disease-relevant areas across a broad spectrum of neurodegenerative diseases. While this supports the use of TSPO PET as a biomarker to monitor response in clinical trials of novel neurodegenerative therapeutics, the clinical utility of current TSPO PET radioligands has been hampered by the lack of high affinity binding to a prevalent form of polymorphic TSPO (A147T) compared to wild type TSPO. This review details recent developments in exploration of ligand-sensitivity to A147T TSPO that have yielded ligands with improved clinical utility. In addition to developing a non-discriminating TSPO ligand, the final frontier of TSPO biomarker research requires developing an understanding of the cellular and functional interpretation of the TSPO PET signal. Recent insights resulting from single cell analysis of microglial phenotypes are reviewed.

Recent advancements in neuroinflammation research have significantly enhanced our understanding of its pivotal role in the pathogenesis and pathophysiology of psychiatric disorders. Positron emission tomography (PET) imaging, leveraging its unique ability to quantify biological processes in vivo non-invasively, has emerged as a transformative tool for investigating neuroimmune mechanisms. The 18 kDa translocator protein (TSPO), a biomarker of activated microglia and astrocytes during neuroinflammation, enables PET-based visualization of neuroinflammatory activity, offering novel insights into the neurobiological underpinnings of psychiatric conditions. This review synthesizes recent TSPO PET imaging findings across major psychiatric disorders, including major depressive disorder (MDD), obsessive–compulsive disorder (OCD), posttraumatic stress disorder (PTSD), schizophrenia, and psychosis. We critically evaluate how TSPO PET elucidates neuroinflammatory signatures linked to disease progression, treatment responses, and therapeutic stratification. Furthermore, we explore the translational potential of anti-inflammatory agents (e.g., celecoxib and minocycline) and TSPO-targeted ligands (e.g., etifoxine and XBD173) in modulating neurosteroid synthesis and neuroimmune interactions. By bridging methodological innovations with clinical applications, this review underscores the promise of TSPO PET in advancing diagnostic precision, personalized treatment strategies, and mechanistic insights into neuroinflammation-driven psychiatric pathologies. Challenges such as genetic polymorphisms (e.g., rs6971), partial volume effects (PVEs), and quantification biases are discussed to guide future research directions.Graphical Schematic overview of TSPO PET imaging in psychiatric disorders. (Created with BioRender.com). black diamond, consistent TSPO elevation region; black circle, conflicting findings. Abbreviations: ACC, anterior cingulate cortex; PFC, prefrontal cortex; OFC, orbitofrontal cortex; TSPO, translocator protein; PET, positron emission tomography; MDD, major depressive disorder; OCD, obsessive–compulsive disorder; PTSD, posttraumatic stress disorder; PVE, partial volume effect; HAB, high-affinity binder; MAB, mixed-affinity binder; LAB, low-affinity binderSupplementary InformationThe online version contains supplementary material available at 10.1007/s12035-025-05177-w.

Positron Emission Tomography (PET) of 18 kDa translocator protein (TSPO) has been investigated as putative marker of neuroinflammation but faces substantial methodological challenges. These include issues with arterial blood sampling for kinetic modeling, the absence of suitable reference regions, genetic polymorphisms affecting tracer affinity, altered blood-to-brain tracer delivery in inflammatory conditions, and high signal variability. This study presents a novel blood-free reference-free method for TSPO PET quantification, leveraging a logistic regression model to estimate the probability of TSPO overexpression across brain regions. Validation was performed on 323 human brain scans from five datasets and three radiotracers. The quantified TSPO topology in healthy controls showed strong concordance with constitutive TSPO gene expression for all tracers. When using [11C]PBR28 PET data, the method replicated previous findings in schizophrenia, Alzheimer's disease, chronic pain, and XBD173 blocking. However, model extension to [18F]DPA-714 and [11C]-(R)-PK11195 revealed small effect sizes and high variability, suggesting the need for tracer-specific model optimization. Finally, validation in a rat model of lipopolysaccharide-induced neuroinflammation confirmed previous evidence of increased brain TSPO uptake after systemic challenge. This novel non-invasive method provides individualized TSPO PET quantification, demonstrating broad applicability across TSPO PET tracers and imaging sites, assuming sufficient training data for model development.

Positron Emission Tomography (PET) of the 18 kDa translocator protein (TSPO) is critical for neuroinflammation studies but faces substantial methodological challenges. These include issues with arterial blood sampling for kinetic modeling, the absence of suitable reference regions, genetic polymorphisms affecting tracer affinity, altered blood-to-brain tracer delivery in inflammatory conditions, and high signal variability. This study presents a novel blood-free reference-free method for TSPO PET quantification, leveraging a logistic regression model to estimate the probability of TSPO overexpression across brain regions. Validation was performed on 323 human brain scans from five datasets and three radiotracers. The quantified TSPO topology in healthy controls showed strong concordance with the constitutive TSPO gene expression for all tracers. When using [11C]PBR28 PET data, the method replicated previous findings in schizophrenia, Alzheimer's disease, chronic pain, and XBD173 blocking. However, model extension to [18F]DPA-714 and [11C]-(R)-PK11195 revealed small effect sizes and high variability, suggesting the need for tracer-specific model optimization. Finally, validation in a rat model of lipopolysaccharide-induced neuroinflammation confirmed previous evidence of increased brain TSPO uptake after a systemic challenge. This novel non-invasive method provides individualized TSPO PET quantification, demonstrating broad applicability across TSPO PET tracers and imaging sites, assuming sufficient training data for model development.

The increased expression of 18 kDa Translocator protein (TSPO) is one of the few available biomarkers of neuroinflammation that can be assessed in humans in vivo by positron emission tomography (PET). TSPO PET imaging of the central nervous system (CNS) has been widely undertaken, but to date no clear consensus has been reached about its utility in brain disorders. One reason for this could be because the interpretation of TSPO PET signal remains challenging, given the cellular heterogeneity and ubiquity of TSPO in the brain.The aim of the current study was to ascertain if TSPO PET imaging can be used to detect neuroinflammation induced by a peripheral treatment with a low dose of the endotoxin, lipopolysaccharide (LPS), in a rat model (ip LPS), and investigate the origin of TSPO signal changes in terms of their cellular sources and regional distribution. An initial pilot study utilising both [18F]DPA-714 and [11C]PK11195 TSPO radiotracers demonstrated [18F]DPA-714 to exhibit a significantly higher lesion-related signal in the intracerebral LPS rat model (ic LPS) than [11C]PK11195. Subsequently, [18F]DPA-714 was selected for use in the ip LPS study.Twenty-four hours after ip LPS, there was an increased uptake of [18F]DPA-714 across the whole brain. Further analyses of regions of interest, using immunohistochemistry and RNAscope Multiplex fluorescence V2 in situ hybridization technology, showed TSPO expression in microglia, monocyte derived-macrophages, astrocytes, neurons and endothelial cells. The expression of TSPO was significantly increased after ip LPS in a region-dependent manner: with increased microglia, monocyte-derived macrophages and astrocytes in the substantia nigra, in contrast to the hippocampus where TSPO was mostly confined to microglia and astrocytes. In summary, our data demonstrate the robust detection of peripherally-induced neuroinflammation in the CNS utilising the TSPO PET radiotracer, [18F]DPA-714, and importantly, confirm that the resultant increase in TSPO signal increase arises mostly from a combination of microglia, astrocytes and monocyte-derived macrophages.

Translocator protein 18 kDa (TSPO) PET imaging is used to monitor glial activation. Recent studies have proposed TSPO PET as a marker of the epileptogenic zone (EZ) in drug-resistant focal epilepsy (DRFE). This study aims to assess the contributions of TSPO imaging using [18F]DPA-714 PET and [18F]FDG PET for localizing the EZ during presurgical assessment of DRFE, when phase 1 presurgical assessment does not provide enough information. We compared [18F]FDG and [18F]DPA-714 PET images of 23 patients who had undergone a phase 1 presurgical assessment, using qualitative visual analysis and quantitative analysis, at both the voxel and the regional levels. PET abnormalities (increase in binding for [18F]DPA-714 vs decrease in binding for [18F]FDG) were compared with clinical hypotheses concerning the localization of the EZ based on phase 1 presurgical assessment. The additional value of [18F]DPA-714 PET imaging to [18F]FDG for refining the localization of the EZ was assessed. To strengthen the visual analysis, [18F]DPA-714 PET imaging was also reviewed by 2 experienced clinicians blind to the EZ location. The study included 23 patients. Visual analysis of [18F]DPA-714 PET was significantly more accurate than [18F]FDG PET to both, show anomalies (95.7% vs 56.5%, p = 0.022), and provide additional information to refine the EZ localization (65.2% vs 17.4%, p = 0.019). All 10 patients with normal [18F]FDG PET had anomalies when using [18F]DPA-714 PET. The additional value of [18F]DPA-714 PET seemed to be greater in patients with normal brain MRI or with neocortical EZ (especially if insula is involved). Regional analysis of [18F]DPA-714 and [18F]FDG PET provided similar results. However, using voxel-wise analysis, [18F]DPA-714 was more effective than [18F]FDG for unveiling clusters whose localization was more often consistent with the EZ hypothesis (87.0% vs 39.1%, p = 0.019). Nonrelevant bindings were seen in 14 of 23 patients in visual analysis and 9 patients of 23 patients in voxel-wise analysis. [18F]DPA-714 PET imaging provides valuable information for presurgical assessments of patients with DRFE. TSPO PET could become an additional tool to help to the localization of the EZ, especially in patients with negative [18F]FDG PET. Eudract 2017-003381-27. Inclusion of the first patient: September 24, 2018. This study provides Class IV evidence on the utility of [18F]DPA-714 PET compared with [18F]FDG PET in identifying the epileptic zone in patients undergoing phase 1 presurgical evaluation for intractable epilepsy.

To describe five patients with ictal aphasia and one patient with ictal amnesia, who had focal positron emission tomography (PET) hypermetabolism but no clear ictal activity on electroencephalography (EEG). (18)F-Fluorodeoxyglucose (FDG)-PET scans with concomitant EEG were obtained in five patients with suspected ictal aphasia or ictal amnesia without ictal activity on EEG. We reviewed medical history, EEG, imaging data, and treatment outcome. Brain magnetic resonance imaging (MRI) showed no structural abnormalities in any of the patients. EEG showed left temporal irregular delta activity in three patients, with aphasia and nonspecific abnormalities in two other patients, all without clear ictal pattern. All patients demonstrated focal hypermetabolism on PET scan. The hypermetabolism was in the left frontotemporal region in patients with ictal aphasia and in the bilateral hippocampal region in the patient with amnesia. Three patients who received intravenous benzodiazepines during their episodes had transient clinical improvement. With antiepileptic drug (AED) treatment, symptoms gradually resolved in all patients. Concomitant resolution of PET hypermetabolism was documented in three patients who had follow up scans. One patient with ictal aphasia later developed recurrent episodes, each with recurrent PET hypermetabolism. This patient and one other patient required immune-modulating therapy in addition to AEDs. FDG-PET imaging should be considered as a diagnostic tool in patients with suspected ictal aphasia or amnesia, who fail to show clear evidence of ictal activity on EEG.

Recent developments on PET radiotracers for TSPO and their applications in neuroimaging