Final answer:
A concussion is a brain injury that is initially microscopic and often not visible on a CT scan. It usually heals on its own and is common in contact sports.
Step-by-step explanation:
The brain injury that will be initially at a microscopic level and not detectable by a computerized tomography (CT) scan is a concussion. A concussion is often considered the mildest and most common type of brain injury. It is essentially a bruise on the surface of the brain that may cause temporary issues such as headache, drowsiness, and confusion. Unlike a hemorrhagic stroke or hematoma, where there is an accumulation of blood visible on CT scans, or edema, where swelling in brain tissue would be evident, concussions may not show any visible signs of injury on a CT scan. These injuries typically heal on their own within a few days.