Radiography
Findings
No specific findings related to DAI can be made using conventional radiography; however, other signs of head trauma can be appreciated, such as facial bone fractures or fluid levels within the paranasal sinuses.
Degree of Confidence
The degree of confidence is low, since conventional radiography cannot demonstrate subtle soft-tissue changes. While radiographs can clearly demonstrate skull fracture, this is not helpful in DAI, since DAI is rarely associated with skull fracture.
False Positives/Negatives
Many false negatives are possible, since a negative skull radiograph in no way excludes a parenchymal brain injury.
Computed Tomography
Findings
Among patients eventually proven to have DAI, 50-80% demonstrate a normal CT scan upon presentation. Delayed CT scanning may be helpful in demonstrating edema or atrophy, which are later findings. Small petechial hemorrhages, located at the gray-white matter junction, as well as in the corpus callosum and brainstem, are characteristic of CT-scan findings in the acute setting.
The following CT-scan criteria have been suggested by Wang and colleagues2 :
- One or more small intraparenchymal hemorrhages less than 2 cm in diameter, located in the cerebral hemispheres
- Intraventricular hemorrhage
- Hemorrhage in the corpus callosum
- Small focal areas of hemorrhage less than 2 cm in diameter, adjacent to the third ventricle
- Brainstem hemorrhage
One may also observe small focal areas of low density on CT scans; these correspond to areas of edema occurring where shearing injury took place.
MRI is more sensitive in the detection of subtle soft-tissue abnormalities; however, CT scanning is more available and practical in the current medical environment and is therefore, according to Teasdale, the "mainstay of acute investigation of head injury."3
Degree of Confidence
The degree of confidence in CT scanning is moderate, since the only finding may be petechial hemorrhage, and fewer than 20% of patients with DAI demonstrate this finding on CT scanning alone. When petechial hemorrhages are observed with the appropriate clinical findings, the sensitivity of CT scanning in the detection of DAI is high.
False Positives/Negatives
As with conventional radiographs, frequent false negatives are possible, since normal CT-scan findings are common in patients with DAI.
Magnetic Resonance Imaging
Findings
Recommended sequences include T1-weighted, T2-weighted, T2 – gradient-echo, proton density – weighted, and diffusion-weighted images.
- T1-weighted images are helpful for anatomic localization; however, nonhemorrhagic lesions may be isointense to surrounding tissue. Hemorrhagic lesions appear hyperintense on T1-weighted images. Nonhemorrhagic lesions appear hyperintense on T2-weighted sequences. Diffusion-weighted sequences can reveal hyperintensities in areas of axonal injury.
- Gradient-echo sequences are particularly useful in demonstrating the paramagnetic effects of petechial hemorrhages. Gradient-echo imaging can often demonstrate signal abnormality in areas that appear normal in T1- and T2-weighted spin-echo sequences. For this reason, gradient-echo imaging has become a mainstay of MRI exams for patients with suggested shearing-type injuries. The abnormal signal on gradient-echo images can persist for many years after the injury.
- The most common MRI finding is the presence of multifocal areas of abnormal signal (bright on T2-weighted images) at the white matter in the temporal or parietal corticomedullary junction or in the splenium of the corpus callosum.
- Other areas that frequently are abnormal include the dorsolateral rostral midbrain and the corona radiata (see DAI stages in Anatomy).
- Eventually, nonspecific atrophic changes are observed.
One area of research has been magnetization transfer imaging. Studies have reported that the magnetic transfer ratio has shown promise in identifying areas of injury not visible on the above MRI pulse sequences. This may allow the radiologist to appreciate a truer representation of the degree of microscopic injury. Studies have indicated that MRI can play a role in predicting the length of coma in DAI patients. The volume of white-matter lesions has been correlated to the degree of injury, as measured by MRI. MRI has also been used to quantify cerebral blood flow in damaged areas of the brain, thus predicting injury severity.
Degree of Confidence
The degree of confidence is high, since abnormal signal in the characteristic locations, discovered in the clinical setting of recent trauma, leaves little doubt about the diagnosis of DAI.
Multiple sclerosis (MS) is a progressive neurologic disorder that can involve multiple foci of white-matter signal abnormality on MRI; however, MS lesions typically are oval or oblong and are oriented in a direction perpendicular to the border of the lateral ventricles (Dawson fingers). In addition, MS lesions may involve the spinal cord, a finding not associated with DAI, and the clinical course of MS is dramatically different from that of DAI.
Nuclear Imaging
Findings
Nuclear medicine currently has no role in the routine diagnostic workup of patients with possible DAI; however, studies have suggested that iodine-123 single-photon emission CT (SPECT) imaging demonstrates areas of hypoperfusion in areas of known injury and reveals additional areas of injury not visualized with MRI.
More on Diffuse Axonal Injury |
| Overview: Diffuse Axonal Injury |
 Imaging: Diffuse Axonal Injury |
| Follow-up: Diffuse Axonal Injury |
| Multimedia: Diffuse Axonal Injury |
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References
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Further Reading
Keywords
DAI, axonal shear injury, axonal shear-strain injury, traumatic brain injuries
