A concussion is a mild traumatic brain injury (mTBI) resulting from a blow to the head or body, causing the brain to move rapidly inside the skull. This triggers a complex physiological process, leading to a temporary alteration in brain function rather than a visible structural injury on standard scans. Because it is largely considered a functional injury, most concussions do not require immediate advanced imaging. The decision to use imaging, such as a CT scan or MRI, depends on the specific symptoms and the timeline following the injury.
Understanding Imaging: CT Scans vs. MRI
Computed Tomography (CT) and Magnetic Resonance Imaging (MRI) serve distinct purposes in the context of head trauma. A CT scan uses X-rays to create cross-sectional images of the brain and skull, making it highly effective at quickly detecting acute structural problems. It is the preferred tool in an emergency setting because it rapidly identifies conditions requiring immediate intervention, such as skull fractures or intracranial hemorrhage (bleeding).
An MRI, by contrast, uses magnetic fields and radio waves to generate detailed images of soft tissues, providing superior visualization of the brain itself. While standard MRI provides a more detailed view than a CT, it is slower to perform and is not the first choice for acute emergencies. The primary limitation is that a conventional CT or MRI scan is often normal even when a person has a clear concussion because the injury is microscopic and functional.
Urgent Imaging: When a Scan Is Needed Immediately
Immediate brain imaging is ordered to rule out dangerous structural injuries, not to diagnose the concussion itself. The rapid acceleration and deceleration of the brain can sometimes cause a bleed, such as an epidural or subdural hematoma, which requires urgent surgical intervention. A CT scan is the standard initial imaging modality in the emergency department due to its speed and ability to detect blood and bone injury.
Healthcare providers look for specific “red flag” symptoms that suggest a severe injury, necessitating an immediate scan.
Red Flag Symptoms
- A severe headache that continuously worsens.
- Repeated or persistent vomiting.
- Any new focal neurological deficit, such as weakness or numbness on one side of the body.
- A seizure after the injury.
- An inability to be easily woken up.
- Loss of consciousness lasting more than 30 seconds.
- Significant change in mental status, such as increasing confusion or slurred speech.
The goal of this immediate imaging is to identify a structural injury that puts pressure on the brain, allowing for prompt treatment. The absence of these findings on the CT scan confirms the injury is likely an uncomplicated concussion, allowing doctors to focus on symptom management and recovery.
Persistent Symptoms: When an MRI Is Considered Weeks Later
If a person’s concussion symptoms persist for weeks or months after the initial injury, a healthcare provider may consider ordering an MRI. This is done in the subacute or chronic phase to investigate potential causes for the prolonged symptoms, often referred to as Post-Concussion Syndrome (PCS). The MRI functions as a diagnostic tool for exclusion, ensuring that the persistent symptoms are not due to subtle structural damage missed earlier or an entirely different underlying condition.
While a standard MRI may not show abnormalities, it can detect subtle lesions or bruising (contusions) that a CT scan might have overlooked. Researchers are also exploring advanced MRI techniques to better understand the functional changes that underpin PCS. These specialized methods, like Diffusion Tensor Imaging (DTI) or functional MRI (fMRI), can measure water movement or track blood flow and activity in different brain networks.
DTI can reveal microstructural damage to the brain’s white matter, which are the long connecting fibers between nerve cells, while fMRI can show disrupted communication between brain regions. Although these advanced techniques are not yet standard clinical practice for diagnosing PCS, they provide valuable insight into the functional injury. When symptoms linger, an MRI helps the medical team create a treatment plan by confirming the absence of significant structural issues while potentially offering clues about the functional disruption.