A concussion, also known as a mild traumatic brain injury (mTBI), is a temporary disruption of normal brain function following a biomechanical force to the head or body. This injury is primarily a physiological disturbance, affecting how brain cells communicate, rather than causing a massive structural defect like a large hemorrhage or skull fracture. Standard clinical imaging tools are designed to detect gross physical damage, meaning they frequently fail to capture the subtle, microscopic changes that define a concussion. This article explores the capabilities and limitations of different MRI techniques in the diagnosis of mTBI.
The Limitations of Standard MRI for Concussion Diagnosis
The standard MRI sequences used in most hospital settings—primarily T1 and T2-weighted imaging—rely on detecting macroscopic changes in brain tissue, such as large areas of swelling, bruising, or bleeding. Concussions typically do not result in these large, visible lesions, which is why the scans of most concussed patients are reported as “normal.” The injury in a concussion is often diffuse axonal injury, involving microscopic shearing and stretching of the brain’s delicate white matter tracts.
Conventional MRI lacks the necessary resolution and sensitivity to visualize this widespread microstructural damage. In the acute setting, standard MRI is typically ordered not to diagnose the concussion itself, but to rule out more severe, life-threatening complications that require immediate intervention, such as a subdural hematoma or a large contusion. A “normal” standard MRI result provides reassurance that a major structural problem is absent, but it does not rule out the presence of a concussion.
Advanced MRI Techniques That Reveal Subtle Injury
While conventional MRI is largely ineffective, specialized, advanced MRI techniques show promise in detecting the subtle pathology of a concussion. These methods move beyond simple structural visualization to measure the physical properties and functional integrity of the brain’s tissue. These techniques are currently used primarily in research settings but are moving toward clinical application.
One such technique is Diffusion Tensor Imaging (DTI), which measures the movement of water molecules within the brain’s white matter. In healthy white matter, water diffusion is highly directed along the nerve fibers, a property called anisotropy. Following a concussion, microscopic damage to the axons disrupts this directionality, causing water movement to become less organized. DTI quantifies this disruption, providing a measurable sign of diffuse axonal injury, a hallmark of concussion pathology.
Another powerful technique is Susceptibility Weighted Imaging (SWI), which is highly sensitive to the magnetic properties of blood products. SWI can detect tiny deposits of iron, which are markers for microhemorrhages, or minute bleeds, that are too small to be seen on a standard MRI or CT scan. The presence of these microhemorrhages can correlate with the severity of the injury and the potential for persistent symptoms.
Clinical Diagnosis and Alternative Imaging
Because standard MRI cannot reliably diagnose a concussion, the condition is diagnosed through a comprehensive clinical evaluation. This process involves a detailed assessment of the injury mechanism, the patient’s symptoms, and a neurological examination. Clinicians rely on the patient’s self-reported symptoms, such as headache, dizziness, nausea, and sensitivity to light, along with observations of balance, coordination, and cognitive changes.
Standardized cognitive testing is also a major component of the diagnostic process, evaluating factors like memory, reaction time, and concentration. These neurocognitive tests provide objective measures of the functional disturbance caused by the concussion. The diagnosis is made when a patient presents with a constellation of symptoms and signs following a traumatic event, regardless of a normal brain scan.
In the acute emergency setting, a Computed Tomography (CT) scan is often the initial imaging modality of choice, not the MRI. The CT scan is faster and better at quickly detecting acute blood, specifically large, life-threatening hemorrhages and skull fractures. However, a CT scan is even less likely than a standard MRI to show any abnormality in a patient with an isolated concussion. Both CT and standard MRI function primarily as screening tools to exclude severe, surgically relevant injuries.