A concussion is a mild traumatic brain injury (mTBI) that results from a blow or jolt to the head or body, causing the brain to move rapidly inside the skull. The effects are primarily functional, involving a temporary disruption of normal brain activity rather than a massive physical trauma. Because of this functional nature, a standard Magnetic Resonance Imaging (MRI) scan generally cannot detect a concussion. The injury involves a complex biological event known as a neurometabolic cascade, which is invisible to conventional imaging designed to capture large-scale damage.
Why Standard MRI Is Insufficient
A concussion is fundamentally a temporary energy crisis in the brain cells, not a structural demolition easily visible on a scan. The rapid movement of the brain causes the stretching of neurons, leading to an unregulated flow of ions across cell membranes. This widespread firing of neurons triggers an “excitatory” phase that demands vast amounts of energy, which the brain struggles to produce. This mismatch between high energy demand and low energy supply is the core physiological effect of the concussion.
Conventional MRI, which includes T1-weighted and T2-weighted sequences, is designed to detect macroscopic structural changes. T1-weighted images visualize normal brain anatomy, while T2-weighted images identify pathology like inflammation or large lesions. These scans primarily look for physical abnormalities such as major swelling, tumors, or hemorrhages. Since a concussion involves cellular dysfunction and subtle chemical imbalances, a standard MRI typically appears completely normal.
Current Methods for Concussion Diagnosis
Because standard imaging cannot capture the injury, a concussion remains a clinical diagnosis based on a patient’s symptoms and cognitive function. Clinicians rely heavily on a detailed assessment of reported symptoms, such as headache, dizziness, nausea, sensitivity to light, and cognitive fog. Standardized tools like the Sport Concussion Assessment Tool (SCAT5) are used to systematically evaluate memory, balance, and symptom severity in patients aged 13 and older.
When a patient receives a CT scan or standard MRI in an emergency room, the primary goal is not to diagnose the concussion itself. The purpose of this structural imaging is to immediately rule out more serious, life-threatening injuries requiring urgent medical intervention. These injuries include skull fractures or intracranial hematomas (large bleeds on the brain). A negative result only confirms the absence of a large structural injury; the diagnosis still depends on the patient’s clinical presentation.
Advanced MRI Techniques for Subtle Brain Injury
While conventional MRI is insufficient, advanced neuroimaging techniques are being used in research settings to detect the subtle microscopic changes associated with concussion. These specialized methods look beyond gross anatomy to examine the brain’s microstructure and function. They are designed to capture damage to the brain’s “wiring” and connectivity missed by routine clinical scans.
Diffusion Tensor Imaging (DTI)
One promising technique is Diffusion Tensor Imaging (DTI), which measures the direction and speed of water molecule movement along the brain’s white matter tracts. These tracts are the bundles of axons that connect different brain regions. Damage to these tracts, known as diffuse axonal injury (DAI), changes how water molecules move. DTI can detect these alterations even when the structural tissue appears normal, providing an objective measure of microstructural damage.
Functional MRI (fMRI)
Functional MRI (fMRI) detects brain activity by measuring changes in blood flow, known as the Blood-Oxygen-Level-Dependent (BOLD) signal. Following a concussion, the brain’s functional connectivity and activation patterns can be disrupted. Resting-state or task-based fMRI captures these abnormal patterns, showing areas of the brain that are either under- or over-activated compared to a healthy brain. This technique offers insight into the functional consequences of the injury on neural networks.
Susceptibility-Weighted Imaging (SWI)
Susceptibility-Weighted Imaging (SWI) is highly sensitive to the presence of blood products. This technique detects tiny, otherwise invisible hemorrhages, or microbleeds, that can occur following a traumatic event. These microbleeds are too small to be seen on standard T2-weighted images. SWI is a valuable research tool for identifying minimal bleeding that accompanies some mild traumatic brain injuries.
While these advanced techniques show significant potential to serve as objective biomarkers for concussion, they are not yet widely available or standardized for routine clinical diagnosis.