Fluspidine is a fluorine-18 (¹⁸F) labeled radiotracer used in positron emission tomography (PET) scans to image σ₁ (sigma-1) receptors in the brain and other tissues. Sigma-1 receptors are proteins involved in many neurological and psychiatric processes. They have been linked to pain modulation, psychosis, Alzheimer's disease, and depression.

Fluspidine is a small organic compound with a spirocyclic structure. A 2-fluoroethyl group on the molecule carries the fluorine-18. The compound exists as two mirror image enantiomers, called (R)-fluspidine and (S)-fluspidine. These enantiomers have slightly different properties: (R)-fluspidine has about 4-fold higher affinity for the σ₁ receptor than (S)-fluspidine, but (S)-fluspidine is metabolically more stable in the body.