We all know someone who's had a head injury that was 'only' a concussion, but even mild traumatic brain injuries can cause lasting damage, and the vast majority are never diagnosed. A team created a way to diagnose mild TBIs (mTBIs) even when MRI scans showed nothing, using the body's own immune cells called macrophages. They attached microparticle 'backpacks' containing a common MRI contrast agent to the cells, which migrated to the brain and allowed them to see evidence of inflammation in pigs with a mock mTBI.