A concussion, medically defined as a mild traumatic brain injury (mTBI), is a temporary functional disturbance of the brain caused by a biomechanical force. When a person sustains a concussion, the immediate concern is the potential for a more serious, life-threatening injury. Brain imaging plays a dual role: its primary function is to rule out dangerous structural damage, while its secondary role involves seeking insights into the brain’s functional changes. Whether a scan can detect a concussion depends entirely on the type of scan performed and what it is designed to visualize.
Standard Imaging: Ruling Out Structural Injury
The initial assessment of a head injury in an emergency setting relies heavily on standard structural imaging, primarily the Computerized Tomography (CT) scan. A CT scan uses X-rays to create cross-sectional images of the brain, making it excellent at rapidly detecting acute issues like bleeding and fractures. It is the preferred first-line tool because it is fast, widely available, and highly effective at identifying conditions that require immediate neurosurgical intervention, such as hematomas or larger brain contusions.
Standard Magnetic Resonance Imaging (MRI) is also a structural scan but is generally used less often immediately following an injury. MRI provides a more detailed picture of soft tissues compared to CT, sometimes revealing small areas of bruising or minute bleeding that a CT scan might miss. However, the time required and the need for patients to remain perfectly still make it less practical for unstable patients requiring rapid triage. Both CT and conventional MRI focus on visualizing the physical integrity of the brain structure, looking for visible tears, swelling, or hemorrhages.
Why Standard Scans Often Appear Normal
The vast majority of standard CT and MRI scans appear normal following a concussion, which can be confusing for patients experiencing significant symptoms. This typical finding results from how a concussion affects the brain at a cellular level, causing a functional disturbance rather than a gross structural injury. Concussions involve chemical and metabolic changes known as the neurometabolic cascade.
This cascade begins when mechanical force causes the stretching and temporary disruption of neurons and connecting fibers, leading to an abrupt release of excitatory neurotransmitters. The brain attempts to restore chemical balance, activating energy-requiring membrane pumps that trigger excessive glucose use. This sudden, high-energy demand, often coupled with reduced cerebral blood flow, creates a temporary energy crisis in the brain cells.
These microscopic changes, including ionic shifts and impaired axonal function, do not create a large enough lesion or mass effect to be resolved by conventional imaging technology. Therefore, a “normal” scan simply confirms the absence of a life-threatening bleed or major structural damage. The injury itself is physiological and functional, residing in the brain’s circuitry and chemistry.
Advanced Scans: Detecting Functional Changes
To visualize the subtle, non-structural effects of a concussion, researchers use advanced neuroimaging techniques focusing on brain function and microstructural integrity. Diffusion Tensor Imaging (DTI) is an advanced MRI technique that maps the movement of water molecules within the brain’s white matter tracts. DTI can reveal microscopic damage to these nerve fibers, not visible on conventional MRI, by detecting changes in water diffusion patterns that suggest impaired axonal integrity.
Functional MRI (fMRI) is another specialized tool that detects changes in blood oxygenation levels in the brain, known as the Blood Oxygen Level Dependent (BOLD) signal. This technique maps neural activity, showing how different brain regions communicate and activate during specific cognitive tasks or at rest. Following a concussion, fMRI studies have shown abnormal activation patterns, suggesting a functional disconnection or reorganization of brain networks.
Other techniques, such as Positron Emission Tomography (PET) scans measuring glucose metabolism, or Magnetoencephalography (MEG) mapping brain electrical activity, are also used in research settings. While these advanced methods are promising for understanding the long-term effects and recovery trajectories of concussion, they are not standardized for routine clinical diagnosis in the acute setting. They are primarily used to guide research or manage patients with persistent symptoms.
Clinical Guidelines: Determining the Need for a Scan
Because a concussion is a clinical diagnosis based on symptoms and physical examination, standard imaging is not necessary for every patient with a head injury. Physicians rely on specific clinical decision rules to determine when a scan is warranted, minimizing unnecessary radiation exposure and cost. These guidelines focus on identifying “red flags” that indicate a higher risk of intracranial injury.
Immediate imaging, typically a CT scan, is indicated if a patient presents with warning signs. These include:
- A severe, worsening headache.
- More than one episode of vomiting.
- Signs of a basal skull fracture.
- Post-traumatic seizures, a newly developed focal neurological deficit, or a persistently low or decreasing score on the Glasgow Coma Scale.
Age over 60, a history of bleeding or clotting disorders, or a high-risk injury mechanism also heighten the suspicion for serious injury.
Clinical decision tools, such as the Canadian CT Head Rule or the New Orleans Criteria, provide doctors with a structured way to assess these risk factors. These rules are designed with high sensitivity to ensure that patients who might have a serious brain injury are scanned. Ultimately, the decision to scan is a safety precaution to rule out the small chance of a life-threatening structural problem, not a diagnostic step for the concussion itself.