<p>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 led by Samir Mitragotri at the Wyss Institute, Harvard SEAS, and Boston Children's Hospital 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.</p>