Residency · Residency · Diagnostic Radiology
Traumatic Brain Injury: Imaging Classification and Prognosis
Overview
Classification by Severity
TBI is classified by severity using the Glasgow Coma Scale. Mild TBI (GCS 13-15) represents concussion; CT is often normal, and this is the most common form. Moderate TBI (GCS 9-12) involves focal neurologic deficits, and CT may show contusions or hemorrhage. Severe TBI (GCS 3-8) involves significant structural injury with high mortality and morbidity.
Imaging Approach
NCCT is the first-line modality for acute TBI evaluation because it is rapid, widely available, and detects surgical emergencies. MRI is more sensitive than CT for diffuse axonal injury, brainstem injury, and non-hemorrhagic contusions and is used for prognostication. The Canadian CT Head Rule and New Orleans Criteria guide imaging decisions in mild TBI.
Primary Injuries
Scalp Injuries
A subgaleal hematoma lies between the galea aponeurotica and the periosteum and can cross suture lines. A cephalohematoma in neonates is subperiosteal and limited by the sutures.
Skull Fractures
A linear fracture is the most common type and appears as a nondisplaced lucent line crossing the calvarium. A depressed fracture involves inward displacement of a bone fragment and is surgical if depressed more than one table thickness or more than 1 cm. Basilar skull fractures involve the petrous bone, sphenoid bone, or orbital plates, with signs including fluid in the mastoid air cells or sphenoid sinus, pneumocephalus, and soft tissue swelling overlying the fracture. Temporal bone fractures are classified as longitudinal (more common, causing conductive hearing loss) or transverse (causing sensorineural hearing loss and facial nerve palsy). A diastatic fracture involves widening of a suture line and is more common in children.
Epidural and Subdural Hematomas
EDH is biconvex, usually arterial, and a surgical emergency when large. SDH is crescent-shaped, venous (from bridging veins), and common in elderly and anticoagulated patients. These are discussed in detail in Topic 32.
Cerebral Contusions
Cerebral contusions are bruises of the brain parenchyma. Coup contusions occur at the site of impact, while contrecoup contusions occur opposite the site of impact and are more common and more severe. Common locations include the frontal poles, temporal poles, and inferior surfaces (orbital frontal and temporal lobe), resulting from impact against the irregular skull base. On CT they appear as petechial hemorrhage (salt-and-pepper pattern) with edema and may "blossom" (enlarge) in the first 24-48 hours. On MRI they are T2/FLAIR hyperintense with hemorrhagic foci visible on GRE/SWI.
Diffuse Axonal Injury (DAI)
DAI results from shearing injury caused by rotational acceleration-deceleration forces. CT is often normal or shows only subtle petechial hemorrhages, making MRI essential. SWI/GRE is the most sensitive conventional sequence for hemorrhagic DAI.
| Adams DAI Grade | Location | Prognosis |
|---|---|---|
| 1 | Gray-white matter junction (frontal/temporal) | Best (most common) |
| 2 | Corpus callosum (especially splenium) | Intermediate |
| 3 | Dorsolateral brainstem (midbrain/pons) | Worst |
The Adams classification grades DAI by location: Grade 1 involves the gray-white matter junction (frontal and temporal lobes) and is the most common; Grade 2 involves the corpus callosum (especially the splenium); Grade 3 involves the dorsolateral brainstem (midbrain and pons) and carries the worst prognosis. DWI shows restricted diffusion in acute non-hemorrhagic DAI lesions, and DTI demonstrates white matter tract disruption with an emerging role in prognosis. DAI is often clinically severe (coma) with disproportionately normal-appearing CT.
Intraventricular Hemorrhage
Traumatic IVH may result from shearing of subependymal veins or extension from adjacent contusion or hemorrhage. It is associated with poor prognosis and carries a risk of hydrocephalus.
Traumatic Subarachnoid Hemorrhage
Traumatic SAH is the most common type of traumatic intracranial hemorrhage. It characteristically involves the convexity sulci adjacent to contusions and the interpeduncular fossa. Unlike aneurysmal SAH, it is typically located along the convexity rather than in the basal cisterns.
Secondary Injuries
Brain Herniation
Subfalcine (cingulate) herniation occurs when the cingulate gyrus herniates under the falx, compressing the ACA and potentially causing ACA territory infarction. Transtentorial (uncal) herniation occurs when the medial temporal lobe (uncus) herniates through the tentorial notch, compressing CN III (causing ipsilateral pupil dilation), the PCA (causing occipital infarction), and the cerebral peduncle. The Kernohan notch phenomenon involves contralateral cerebral peduncle compression against the tentorial edge, causing false-localizing ipsilateral hemiparesis. Tonsillar herniation involves the cerebellar tonsils herniating through the foramen magnum, compressing the medulla, and is fatal if untreated. Ascending transtentorial herniation occurs when a posterior fossa mass pushes the cerebellum upward. External (transcalvarial) herniation involves brain tissue herniating through a skull defect.
Diffuse Cerebral Edema
Diffuse cerebral edema produces global brain swelling with loss of gray-white differentiation and effacement of sulci, cisterns, and ventricles. The "white cerebellum sign" (seen in children) describes the cerebellum appearing relatively white compared to diffusely edematous hypodense cerebral hemispheres and indicates severe injury.
Post-Traumatic Hydrocephalus
Communicating hydrocephalus results from impaired CSF absorption due to blood products in the subarachnoid space. Non-communicating hydrocephalus results from obstruction by a blood clot or edema. It may develop days to weeks after injury.
Post-Traumatic Infarction
Post-traumatic infarction may result from vascular compression due to herniation (ACA or PCA), dissection of the carotid or vertebral arteries, or vasospasm from traumatic SAH.
Pediatric-Specific Considerations
Abusive Head Trauma (Non-Accidental Trauma)
Findings suggestive of abusive head trauma include subdural hematomas of different ages (mixed-density or bilateral), interhemispheric SDH (particularly in infants), diffuse cerebral edema out of proportion to the history, retinal hemorrhages, and associated skeletal injuries (rib fractures and classic metaphyseal lesions). MRI with SWI is essential for dating hemorrhages and identifying shearing injuries.
Birth Trauma
A cephalohematoma is subperiosteal and limited by sutures. A subgaleal hemorrhage can be large and life-threatening and crosses sutures. Germinal matrix hemorrhage in premature infants is graded I through IV on cranial ultrasound.
Imaging for Prognosis
CT Predictors of Poor Outcome
CT findings predicting poor outcome include compressed or absent basal cisterns, midline shift exceeding 5 mm, diffuse brain swelling, and large hematoma volume exceeding 25 mL.
MRI Predictors
MRI predictors include DAI involving the corpus callosum and brainstem (Grade 2-3), the number and volume of DAI lesions on SWI, the extent of restricted diffusion in the white matter on DWI, and fractional anisotropy reduction in major white matter tracts on DTI.
<image>A NCCT panel showing the spectrum of traumatic brain injury. (1) Axial CT showing bifrontal contusions (contrecoup) with petechial hemorrhage in the frontal poles and surrounding edema. (2) Axial CT showing a large left-sided acute subdural hematoma with midline shift and subfalcine herniation. (3) Axial CT showing diffuse cerebral edema with loss of gray-white differentiation, compressed basal cisterns, and slit-like ventricles. (4) Axial CT showing intraventricular hemorrhage with blood layering in the occipital horns. Each image is labeled with arrows pointing to key findings.</image>
<image>An MRI panel demonstrating diffuse axonal injury (DAI) graded by severity. (1) Axial SWI image showing multiple punctate foci of susceptibility (microhemorrhages) at the gray-white matter junction of the frontal lobes (Grade 1). (2) Axial SWI image showing hemorrhagic foci in the splenium of the corpus callosum (Grade 2). (3) Axial SWI image showing hemorrhagic foci in the dorsolateral brainstem/midbrain (Grade 3). A DWI image inset shows restricted diffusion in a non-hemorrhagic DAI lesion at the gray-white junction. The Adams grading system is annotated alongside with corresponding prognosis.</image>
<image>An illustration of brain herniation types on a coronal section of the brain. Five herniation types are labeled with arrows and direction of herniation: (1) Subfalcine herniation with the cingulate gyrus displaced under the falx and compression of the ipsilateral lateral ventricle. (2) Uncal (transtentorial) herniation with the medial temporal lobe herniating through the tentorial notch, compressing CN III and the PCA. (3) Tonsillar herniation with cerebellar tonsils displaced through the foramen magnum. (4) Ascending transtentorial herniation with upward displacement of the cerebellum. (5) External herniation through a skull defect. Associated vascular compressions and clinical consequences are labeled for each type.</image>
Clinical Pearls
CT is often normal in diffuse axonal injury; if the clinical severity (coma, GCS) is disproportionate to the CT findings, MRI with SWI is essential to detect DAI. Contusions commonly "blossom" in the first 24-48 hours, and follow-up CT should be performed if there is clinical deterioration. The Adams classification of DAI correlates with prognosis, with Grade 3 (brainstem involvement) carrying the worst outcome. In pediatric patients, subdural hematomas of different ages, interhemispheric SDH, and diffuse cerebral edema out of proportion to the stated mechanism should raise suspicion for abusive head trauma. Subfalcine herniation can cause ACA territory infarction, and uncal herniation can cause PCA territory infarction -- these are important secondary injuries to identify on follow-up imaging. SWI is approximately 3-6 times more sensitive than conventional GRE for detecting hemorrhagic DAI lesions and should be included in all MRI protocols for TBI evaluation.
References
- Adams JH, et al. "Diffuse Axonal Injury in Head Injury: Definition, Diagnosis, and Grading." Histopathology, 1989
- Stiell IG, et al. "The Canadian CT Head Rule for Patients with Minor Head Injury." Lancet, 2001
- ACR Appropriateness Criteria: Head Trauma, 2020
- Tong KA, et al. "Hemorrhagic Shearing Lesions in Children and Adolescents with Post-Traumatic Diffuse Axonal Injury: Improved Detection and Initial Results." Radiology, 2004
