A concussion is a mild form of Traumatic Brain Injury (TBI) resulting from a biomechanical force that causes the brain to move quickly within the skull. This sudden movement disrupts normal brain function, leading to physical, cognitive, and emotional symptoms. Patients are often confused when they experience severe symptoms but receive a “normal” result on a brain scan. This discrepancy exists because a concussion is fundamentally a microscopic, functional injury rather than a macroscopic, structural one. Understanding the difference requires looking beyond standard clinical imaging to specialized tests that reveal subtle cellular changes.
Standard Structural Imaging
When a patient arrives at the emergency department following a head injury, the primary goal of initial imaging is to rule out a severe, life-threatening injury. Physicians typically rely on Computed Tomography (CT) scans or conventional Magnetic Resonance Imaging (MRI). The CT scan uses X-rays to generate cross-sectional images and is fast and effective at detecting acute issues like skull fractures, large blood clots, or swelling (edema). These structural problems may require immediate surgical intervention.
Conventional MRI provides more detailed images of soft tissues but remains primarily a structural tool. It is excellent for identifying larger lesions, contusions, or hemorrhages that a CT scan might miss. However, in the vast majority of concussions, both CT and conventional MRI scans appear entirely unremarkable. These technologies are designed to visualize large-scale anatomical damage, such as a tear or a bleed, which are not characteristic of a concussion. A concussed brain looks “normal” because the injury occurs at a level too small for macro-imaging techniques to resolve.
The Functional Nature of Concussion
Standard structural scans do not detect a concussion because the injury is rooted in physiological mechanisms. A concussion is not typically a physical tear of tissue, but rather a disruption of how brain cells communicate and manage energy. The biomechanical forces—the shearing and stretching of tissue—trigger a complex sequence known as the neurometabolic cascade. This cascade begins with an indiscriminate release of neurotransmitters and a massive efflux of potassium ions from the neurons.
The brain attempts to restore this ionic imbalance by activating cellular pumps, which requires an enormous amount of energy. This demand leads to a temporary, rapid increase in glucose utilization, followed by a period of reduced metabolism. The concussed brain enters a state of energy crisis, struggling to meet its basic metabolic needs for days or weeks. This functional impairment, involving transient changes in ion balance, glucose use, and blood flow regulation, leaves no visible mark on a standard anatomical scan.
Specialized Scans Revealing Subtle Changes
While conventional imaging shows a normal brain, advanced neuroimaging techniques can visualize subtle differences in a concussed brain by focusing on microstructure and function. Diffusion Tensor Imaging (DTI) is one technique that maps the brain’s white matter tracts, the communication cables. DTI measures the directionality of water molecule movement, a parameter quantified as fractional anisotropy (FA).
In a healthy brain, water flows directionally along the organized axons, resulting in high FA. After a concussion, shearing forces can cause microstructural damage to these axons, disrupting the organized flow of water. This disruption is visualized as areas of decreased FA compared to a normal brain, indicating damage to white matter integrity. These abnormalities are frequently observed in regions like the corpus callosum, the large bundle of fibers connecting the two hemispheres.
Another advanced method is functional MRI (fMRI), which detects changes in brain activity by measuring the Blood Oxygen Level-Dependent (BOLD) signal. The BOLD signal reflects localized changes in blood flow and oxygenation when a specific area of the brain is active. When a concussed individual performs a cognitive task, fMRI often reveals altered activation patterns compared to a healthy individual. This alteration can manifest as either increased activity, suggesting the brain recruits more resources for a simple task, or decreased activity in responsible areas.
Other specialized techniques examine the metabolic disturbances resulting from the injury. Positron Emission Tomography (PET) and Single-Photon Emission Computed Tomography (SPECT) scans use radioactive tracers to visualize metabolic activity and cerebral blood flow (CBF). These scans capture the concussed brain’s characteristic hypometabolic state, showing areas of reduced glucose uptake or diminished CBF compared to a normal metabolic map. These advanced scans, though often used only in research settings, provide objective visual evidence of functional and microstructural differences.
Diagnosis Without Visual Evidence
Since standard imaging often fails to detect the injury, the diagnosis of a concussion relies heavily on clinical judgment and a thorough evaluation of symptoms and functional deficits. The primary diagnostic tools are non-visual, focusing on the patient’s subjective experience and observable neurological function. A medical professional uses a standardized symptom checklist to document complaints, which commonly include headache, dizziness, and difficulty concentrating.
The assessment also includes objective measures of neurological function, such as cognitive testing and balance evaluation. Tools like the Sport Concussion Assessment Tool (SCAT5) incorporate a full symptom review alongside tests of memory, orientation, and balance, such as the Modified Balance Error Scoring System (mBESS). Ultimately, the diagnosis confirms the presence of a brain injury based on the patient’s history and functional deficits, rather than requiring visual confirmation on a standard structural scan.