When people experience a head injury, they often expect diagnostic imaging to provide a clear answer about their condition. The question of whether a concussion, a form of mild traumatic brain injury (mTBI), can be seen on a scan is a source of frequent confusion. The nature of a concussion means its detection method is fundamentally different from identifying a broken bone or a large internal bleed. Understanding the distinction between a structural injury and a functional injury explains why traditional scans may not capture the damage caused by a concussion.
What Defines a Concussion
A concussion is medically classified as a mild traumatic brain injury resulting from a direct blow to the head or an indirect force, such as a whiplash injury, that causes the brain to move rapidly within the skull. This sudden, forceful movement triggers a temporary alteration in brain function, which is the defining characteristic of the injury. The resulting symptoms stem from a complex cascade of neurochemical and metabolic changes rather than a visible tear in the tissue. Common immediate effects include a transient loss of consciousness, confusion, headache, dizziness, nausea, and difficulty with balance or coordination.
The injury is primarily considered a functional disturbance because the brain’s ability to process information and maintain normal energy levels is temporarily impaired. This disruption involves the stretching and shearing of axons, the long connecting fibers of nerve cells, which leads to an energy deficit in the brain. The symptoms a person experiences are a direct manifestation of this physiological crisis, even if the brain’s overall structure remains intact.
The Purpose of CT Scans in Head Trauma
A Computed Tomography (CT) scan is a form of structural imaging that uses X-rays to create detailed cross-sectional images of the brain and skull. When a patient arrives with a head injury, a CT scan is often ordered immediately to rule out acute, life-threatening conditions. The primary purpose of this quick, non-invasive scan is to identify macroscopic structural damage that may require immediate neurosurgical intervention.
Specifically, the scan is exceptionally good at detecting dense structures like bone and fresh blood. Clinicians use it to identify skull fractures, epidural or subdural hematomas (bleeding between the brain and the skull), and significant cerebral edema, which is dangerous brain swelling. The decision to perform a CT scan in the acute setting is often guided by standardized clinical decision tools, such as the Canadian CT Head Rule or the New Orleans Criteria. These rules help physicians determine if a patient’s symptoms or injury mechanism place them at risk for a serious intracranial injury that the CT scan is designed to detect.
Why CT Scans Miss Concussions
The CT scan’s ability to detect life-threatening issues is precisely why it is ineffective at diagnosing a concussion. The technology relies on visualizing structural changes large enough to be seen with X-ray attenuation, such as a mass of accumulated blood or a fractured bone. A concussion, however, does not involve a large, visible structural lesion; instead, it is a microscopic injury affecting the function of the brain cells.
The physiological changes that cause a concussion—the stretching of neuronal axons and the resulting temporary metabolic dysfunction—are invisible at the resolution of a conventional CT scan. Even a person experiencing severe concussive symptoms like disorientation and persistent nausea will typically have a completely “normal” CT scan. The brain tissue itself appears structurally sound, meaning the imaging tool cannot capture the underlying disturbance in the brain’s chemical and energy balance.
How Concussions Are Clinically Diagnosed
Since imaging cannot confirm the presence of a concussion, the diagnosis remains a process of clinical judgment based on a thorough medical evaluation. The assessment begins with a detailed patient history, focusing on the mechanism of injury and the subsequent onset and evolution of symptoms. Subjective reporting of symptoms, such as headache severity, memory issues, or sensitivity to light, is a major component of the diagnostic process.
Healthcare professionals use standardized tools like the Sports Concussion Assessment Tool 5 (SCAT5) to systematically evaluate the patient’s condition. This tool incorporates a comprehensive symptom checklist, a cognitive screening test, and a neurological examination that assesses balance and coordination. Ultimately, a concussion is diagnosed when a patient exhibits a pattern of physical, cognitive, emotional, and sleep-related symptoms consistent with a functional brain injury following a traumatic event.