A CT scan is the standard imaging test used after a suspected concussion. It’s fast, widely available, and effective at spotting bleeding or fractures inside the skull. But here’s the important nuance: most concussions don’t actually show up on any scan. The injury is functional, not structural, so the primary goal of imaging is to rule out something more dangerous rather than to confirm the concussion itself.
Why CT Scans Come First
In an emergency department, a CT scan of the head is the go-to choice because it takes only minutes and excels at detecting the two things doctors are most worried about after a head injury: bleeding inside the skull and bone fractures. These are rare in mild traumatic brain injury, but they’re life-threatening when they do occur, so speed matters. An MRI can detect some of these same problems with equal or better sensitivity, but it takes significantly longer and isn’t always immediately available, making it impractical as a first-line tool in urgent situations.
Not Everyone Needs a Scan
Doctors don’t order a CT scan for every bump on the head. They use clinical decision rules, essentially checklists of risk factors, to determine whether imaging is necessary. For adults, these rules weigh things like whether you lost consciousness, how alert and oriented you are, whether you’re vomiting, whether you’re on blood thinners, and how the injury happened. A fall from a significant height or a high-speed car accident, for instance, raises the risk level compared to a minor sports collision.
For children, emergency physicians follow a separate set of criteria known as the PECARN algorithm. Kids under two are evaluated based on their mental status, behavior changes reported by a caregiver, whether they lost consciousness, and whether there’s a scalp hematoma (a swollen lump) in a location other than the forehead. Children ages two through fifteen are assessed on similar factors plus vomiting and severe headache. If a child meets all the low-risk criteria, the chance of a clinically significant brain injury is very small, and doctors will typically observe rather than scan, avoiding unnecessary radiation exposure.
What a Concussion Actually Does to the Brain
The reason scans look normal after most concussions is that the damage happens at a microscopic level. A concussion disrupts how brain cells communicate rather than tearing tissue or causing visible bleeding. The impact triggers a cascade of chemical changes: neurons fire chaotically, releasing a flood of signaling chemicals, while blood flow and energy metabolism in the brain become temporarily altered. The nerve fibers connecting different brain regions (called axons) can stretch and swell, disrupting the signals they carry.
None of this is visible on a standard CT or even a conventional MRI. In fact, the absence of findings on standard imaging is part of the formal diagnostic criteria for concussion. If a scan does show bleeding or a significant bruise on the brain, the injury is typically classified as something more severe than a simple concussion.
When MRI Gets Involved
If your symptoms persist beyond the typical recovery window, which is two to four weeks for adults and about four weeks for children and teens, your doctor may order an MRI. This isn’t to diagnose the concussion itself but to rule out other problems that might explain lingering symptoms, such as a small bleed that was missed, structural abnormalities, or an unrelated condition. MRI is better than CT at detecting subtle findings like minor bruising on the brain surface, small areas of swelling, and old blood deposits. It also avoids radiation, which makes it preferable for follow-up imaging, especially in younger patients.
That said, even MRI results are often normal in people with persistent post-concussion symptoms like headaches, dizziness, vision problems, and difficulty concentrating. No single brain scan can definitively confirm or rule out persistent post-concussion symptoms. Treatment in those cases is guided by your specific symptoms rather than imaging findings.
Advanced Imaging and Blood Tests
Researchers have developed more sensitive imaging techniques that can detect concussion-related changes invisible to standard scans. One of the most studied is diffusion tensor imaging, a specialized type of MRI that maps the brain’s white matter pathways. It can reveal subtle damage to nerve fiber tracts that conventional MRI misses entirely. Functional MRI, which measures brain activity patterns rather than structure, has shown that some concussed athletes display abnormal activation in areas involved in working memory and attention, even after their symptoms have resolved. These tools are valuable for research but aren’t yet part of routine clinical care for most patients.
On the blood test front, the FDA cleared a point-of-care blood test in 2021 that measures two proteins released by brain cells after injury. The test is designed to help emergency physicians decide whether a CT scan is necessary. If the protein levels are below a certain threshold, the likelihood of a finding on CT is very low, potentially sparing patients from unnecessary radiation and reducing emergency department wait times. It’s an early step toward diagnosing brain injuries without imaging, though it identifies who can safely skip a CT rather than confirming a concussion diagnosis on its own.
What to Expect in Practice
If you go to an emergency department after hitting your head, the doctor will first assess you clinically: asking questions, checking your pupils, testing your balance and memory, and evaluating your level of alertness. If you have risk factors like loss of consciousness, repeated vomiting, worsening confusion, or a dangerous mechanism of injury, you’ll likely get a CT scan. The scan itself takes about five to ten minutes, and results are usually available quickly.
If the CT is normal, that’s good news. It means there’s no bleeding or fracture. But it doesn’t mean you’re fine. Your concussion symptoms, things like headache, fogginess, sensitivity to light, and fatigue, are real and expected. They just stem from the kind of cellular disruption that no standard scan can photograph. Recovery is guided by how you feel, not by what a scan shows. If symptoms aren’t improving after two weeks, or they’re getting worse at any point, that’s the signal to follow up with a healthcare provider who can assess whether additional imaging or specialized treatment is warranted.