A concussion is one of the most common injuries a UK adult will face, and one of the most misunderstood. Patients arrive at clinics expecting an MRI to confirm the diagnosis, quantify the damage and predict recovery. Most of the time it can do none of those things. The scan a patient really needs after a head injury depends less on the mechanism of injury and more on how the days and weeks after it unfold.
This piece is what MRI actually can and cannot show after a concussion, when it is worth doing, what protocol to ask for, and how to think about the results when they come back "normal" but the symptoms are anything but.
The one-line answer
Most concussions do not need any MRI. Standard MRI is offered when symptoms persist beyond four weeks, when they worsen, or when new neurological signs appear. Even then, the scan is often normal - because concussion is a clinical diagnosis, not an imaging one.
Concussion is a clinical diagnosis, not an imaging one
Concussion, also called mild traumatic brain injury (mTBI), is diagnosed on the story and the examination, not on a scanner. A blow, a rapid deceleration, a fall - followed by any of confusion, brief loss of consciousness, headache, nausea, sensitivity to light and sound, balance problems, brain fog, or emotional changes. That is a concussion. The scan does not need to prove it.
Standard clinical MRI, even done at 3 Tesla with a good post-concussion protocol, is often entirely normal in a patient with textbook concussion symptoms. That is not a scanner failure. It is that the injury of concussion is largely microstructural and neurometabolic. Axons are stretched but not torn, neurotransmitter release is disordered, glucose metabolism dips, but the pictures MRI generates in clinical practice do not resolve any of that. The report says "no acute intracranial abnormality" and the symptoms carry on regardless.
That mismatch is one of the hardest things for patients to accept, and one of the most important things a good clinician explains up front, before ever ordering the scan.
When to escalate to MRI
UK and international guidance is fairly consistent on when imaging shifts from unnecessary to indicated. The trigger points are:
- Symptoms persisting beyond four weeks. Most concussions resolve inside two to four weeks. Once symptoms cross the four-week mark - persistent headache, dizziness, cognitive fog, mood change - imaging becomes reasonable to rule out a structural cause that has been mistaken for concussion.
- Worsening symptoms after an initial plateau. Concussion recovery is not always linear, but a genuine second decline days or weeks in should prompt imaging to exclude a delayed subdural haematoma, particularly in older patients or those on anticoagulants.
- New neurological signs. Focal weakness, new seizures, progressive cognitive change, visual field loss, or new speech disturbance. These are not concussion symptoms. They demand imaging.
- Suspected chronic changes. Repeated concussions in contact-sport athletes, ex-boxers, ex-military, or a history of significant childhood head injury may warrant MRI as part of a wider workup, even without acute symptoms.
If none of those apply, the honest answer to "do I need an MRI" is often "not yet, and possibly not at all". A good private consultant will say that even when they are the one being paid to arrange the scan.
What MRI can show after concussion
Where MRI does earn its place after a head injury, it is usually one of four findings that a good post-concussion protocol is looking for:
- Microhaemorrhages on SWI. Susceptibility-weighted imaging is exquisitely sensitive to tiny bleeds - blood breakdown products a few millimetres across, invisible on standard T1 or T2 sequences. In moderate to severe TBI these correlate with diffuse axonal injury. In mild concussion they are less common but occasionally present.
- Contusions. Small parenchymal bruises, typically at the frontal and temporal poles where the brain strikes bone on rapid deceleration. Usually visible on FLAIR and T2.
- Subacute subdural or extra-axial collections. Small collections of blood between the brain and skull that were missed or too early to see on initial CT, and have since become visible on MRI. Particularly important in older adults or anticoagulated patients.
- Signs of diffuse axonal injury on DTI. Diffusion tensor imaging, available on advanced protocols at some tertiary centres, can suggest disrupted white matter tracts in patients with moderate to severe injury. It is not routinely used or reported in mild concussion.
A well-designed post-concussion brain MRI protocol includes T1, T2, FLAIR, DWI, SWI and often a 3D volumetric sequence, and takes about 25 to 35 minutes on a 3T scanner. If a clinic is offering a "concussion MRI" and it does not include SWI, ask why.
What MRI cannot show
The list of what standard clinical MRI cannot show after concussion is longer than the list of what it can.
It cannot show the neurometabolic injury that underlies most concussion symptoms - the disordered ion flux, glutamate release and glucose metabolism dip that follows a mild TBI. It cannot show most microstructural axonal stretch injury on a routine protocol. It cannot quantify functional deficits like attention lag, working memory disruption or slowed processing - those are found on neuropsychological testing, not on a scanner. It cannot predict recovery time. And critically, it cannot diagnose chronic traumatic encephalopathy (CTE) in a living patient. CTE is currently a post-mortem neuropathological diagnosis, and any clinic implying otherwise is overreaching.
This is why a normal MRI after a concussion is not reassurance that "nothing is wrong". It is a rule-out of the structural things MRI is good at finding. The symptoms are real. The scan simply cannot see them.
