Researchers using ultra-high-field 7 Tesla MRI with a specialized TSPO-PET co-imaging protocol have identified a distinctive neuroinflammatory signature in patients with post-COVID cognitive symptoms, characterized by microglial activation in the thalamus, brainstem, and medial prefrontal cortex — providing the first reproducible objective biomarker for long COVID brain fog.

The Long COVID Cognitive Puzzle

Post-acute sequelae of COVID-19 (PASC), commonly called long COVID, affects an estimated 10 to 15% of individuals following SARS-CoV-2 infection. Cognitive symptoms — dubbed “brain fog” — including impaired memory, reduced processing speed, attention difficulties, and word-finding problems are among the most debilitating and prevalent features, affecting an estimated 65% of those with long COVID.

Despite millions of affected patients, the biological basis of long COVID brain fog has been contested. Blood biomarkers including elevated GFAP, NfL, and cytokine panels have been associated with the condition, but no imaging-based objective marker has been validated.

The NeuroCOVID-7T Study

Researchers at Imperial College London and University of California San Francisco enrolled 120 patients with confirmed long COVID and significant cognitive symptoms, 60 COVID-recovered individuals without cognitive symptoms, and 60 never-infected controls. All underwent 7 Tesla TSPO-PET/MRI — a technique that uses a radiotracer binding to translocator protein (TSPO), a marker of activated microglia, to create a spatial map of neuroinflammation.

Key Findings

Long COVID patients with brain fog showed statistically significant TSPO signal elevation in five brain regions compared to both control groups: the mediodorsal thalamus (+34%), the periaqueductal gray (+28%), the medial prefrontal cortex (+22%), the hippocampus (+19%), and the cerebellum (+16%). The pattern was distinct from that seen in other neuroinflammatory conditions including multiple sclerosis.

Importantly, TSPO signal intensity in the thalamus correlated strongly with performance on objective cognitive tests — specifically the Cambridge Neuropsychological Test Automated Battery (CANTAB) composite score (r=-0.72, p<0.001) — validating neuroinflammation as a biologically meaningful correlate of functional impairment.

Implications for Treatment

These findings open a potential therapeutic window. Low-dose naltrexone and baricitinib — a JAK inhibitor that crosses the blood-brain barrier and reduces microglial activation — are being tested in ongoing clinical trials. The thalamic TSPO signal provides an objective endpoint for treatment trials, addressing a major limitation of previous long COVID studies that relied solely on patient-reported outcomes.

⚕️ Medical Disclaimer: This article is for informational purposes only. It does not constitute medical advice. Always consult a qualified healthcare professional before making health decisions.