Diffuse Axonal (Shear) Injury (DAI)
- Occurs secondary to acceleration and deceleration forces acting at different rates
- Results from traumatic axonal stretch injury
- Can be hemorrhagic or non-hemorrhagic
- Located within
- The gray-white junction, in the subcortical white matter and along the deep gray matter structures (Grade 1),
- Within the corpus callosum (Grade 2) and
- In the brain stem (Grade 3)
- Deeper brain involvement, indicates a greater severity and worse prognosis
Neuroimaging
- CT findings
- Normal appearance in 50-80% cases
- Nonhemorrhagic – appear as small, hypodense foci
- Hemorrhagic – appear as small, hyperdense foci, possibly with surrounding hypodense edema
- Repeat scans may reveal more lesions as the injury evolves
- MRI findings (See Figure 1)
- T1WI
- Usually normal
- If > 1 cm and hemorrhagic, may appear hyperintense for 3-14 days
- T2WI
- Nonhemorrhagic lesions appear as hyperintense foci
- Hemorrhagic lesions appear as hypointense foci
- FLAIR
- Nonhemorrhagic DAI appear as hyperintense foci
- Hemorrhagic DAI appear as hypointense foci
- T2* GRE/SWI
- Most sensitive sequence
- Microbleeds may be visible only on GRE or SWI
- Hypointense foci appear as susceptibility from blood products
- SWI depicts significantly more DAI foci than GRE
- Multifocal hypointense foci may remain for years
- Number of GRE lesions correlates with intracranial hypertension and outcome
- DWI
- May show foci of restricted diffusion
- Damage to WM may be visible on diffusion tensor imaging (DTI) Fractional anisotropy maps
- DTI "tractograms" allow delineation of white matter tract disruption pattern
- DWI may help detect abnormalities when routine imaging, including GRE or SWI, is normal
- MR Spectroscopy
- Decreased N-acetyl aspartate in white matter secondary to neuronal injury
- Increased choline in gray matter is suggestive of inflammation
- Abnormal NAA/Cr and Cho/Cr is predictive of outcome
- T1WI
For more information, please see the corresponding chapter in Radiopaedia.
Contributor: Priya Rajagopalan, MD
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