Imaging description

Diffuse axonal injury (DAI) is caused by shearing forces that occur during rapid acceleration or deceleration of the brain. This results in tearing of the axons. Most lesions are small and multiple. Characteristic locations include the gray–white matter junctions (Figure 14.1), splenium of the corpus callosum, basal ganglia, internal capsules, and dorsolateral brainstem [1, 2].

It has been previously reported that most DAI lesions are non-hemorrhagic. However, both pathologic literature and imaging studies with improved techniques suggest that more lesions are hemorrhagic than previously thought [3]. Lesions may be seen on CT if there is sufficient hemorrhage or edema to produce discernible hyperattenuation or hypoattenuation, respectively. However, CT is very insensitive to the detection of DAI, and this limitation should be realized when imaging a patient with traumatic brain injury.

MRI is more sensitive for the detection of DAI, and will detect many lesions which are not visible on CT [4]. FLAIR and T2-weighted images will depict DAI lesions as foci of increased signal. However, these sequences are also relatively insensitive. Since most blood products are paramagnetic (including deoxyhemoglobin, intracellular methemoglobin, and hemosiderin), they produce susceptibility effect on gradient-recalled echo (GRE) images. GRE images are therefore sensitive to the identification of microhemorrhages and detect more foci of DAI than conventional MRI sequences (Figure 14.2). Susceptibility-weighted imaging (SWI) has been shown to detect even more lesions than GRE images [5].