What More Can MR Imaging Teach Us about Brain Injury?

Head trauma is a major public health problem in the United States. As many as 75% of head-injured patients are classified as having “mild head injury.” Mild head injury is associated with a significant morbidity, which may be associated with deficits in information processing on neuropsychological testing. These tests, as pointed out by McGowan et al in this issue of the AJNR (page 875), may also be sensitive to factors not related directly to the cognitive sequelae of the injury. Therefore, the severity of brain injury should not be evaluated exclusively by the extent of impairment as determined by neuropsychological tests; imaging techniques also should be used to detect anatomic and physiologic abnormalities of tissue in various parts of the brain.

Some investigators recently have proposed that CT should provide the basis for updated classification schemes of head injury. However, Mittl et al (1) showed in their study that MR imaging should play a major role in any classification scheme of injury, especially in mildly head-injured patients. Their results revealed MR imaging changes compatible with nonhemorrhagic and hemorrhagic diffuse axonal injury (DAI) after mild head injury, which were not shown by CT in approximately 30% of cases. It has been accepted for some time that the greater sensitivity of MR imaging makes it a better study than CT for detecting the extent of injury and for predicting patient outcome.

Several different MR sequences have been studied in the evaluation of head trauma. The utility of fluid-attenuated inversion-recovery (FLAIR) MR imaging in head trauma has been studied by several authors. Ashikaga et al (2) examined 56 patients with head injury by using T2-weighted spin-echo and FLAIR sequences, and found the sensitivity of FLAIR images to be equal or superior to spin-echo images in evaluating traumatic lesions. Diffusion-weighted MR imaging findings in traumatic brain injury were studied by Liu et al (3). They studied nine patients with conventional MR imaging as well as echo-planar diffusion-weighted MR imaging. They found that decreased apparent diffusion coefficient values can be demonstrated in patients with DAI in the acute setting and may persist into the subacute period, beyond that described for cytotoxic edema.

The article by McGowan et al in this issue investigates the possible relationships between quantitative magnetization transfer imaging (MTI) and neurocognitive findings in a set of patients who had experienced mild head trauma and had negative conventional MR imaging results. They found that the magnetization transfer ratio (MTR) in the splenium of the corpus callosum was lower in the patient group than in the control group, but no significant reduction in MTR was found in the pons. All of the patients demonstrated impairment of at least three measures of the neuropsychological tests, and in two cases a significant correlation was found between regional MTR values and neuropsychological performance. One of the important aspects of this study is that the authors are trying to find an even more sensitive study than conventional MR imaging, because the set of patients studied had negative conventional MR results. Their hypothesis was that quantitative MTI analysis would offer increased sensitivity over conventional MR imaging for the detection of traumatic brain injury in patients at risk for cognitive deficits secondary to mild traumatic brain injury (TBI).

Preliminary work using MTI has shown success in the detection of DAI in both animal and human studies, even when conventional T2-weighted images do not show the lesion. MTR can be used to detect changes in the structural status of brain parenchyma, which may or may not be visualized on conventional MR images. A clear physiologic explanation for lowered MTR in head trauma, however, has not been established. It is reasonable to suppose, as the authors in this issue have stated, that a lower MTR portends a less favorable outcome.

What is the future for imaging of head trauma? In a comparison of CT with 99-technetium hexamethylpropyleneamine oxime single-photon emission CT (SPECT) of the brain in TBI patients, the effects of brain trauma on regional cerebral blood flow (rCBF) were evaluated. SPECT showed differences in rCBF more often than lesions diagnosed with CT. Does this mean that there also may be a role for perfusion scanning in head trauma? What about MR spectroscopy (MRS)? While MTI provides structural information, MRS permits the detection of in vivo neurochemical alterations. Preliminary work using MRS in animal models and human TBI studies has shown changes indicating neuronal damage. Current animal studies are directed at preventing secondary neuronal damage from mechanisms such as ischemia, apoptosis, and excitatory amino acids. Imaging strategies and algorithms must be directed at the best means of early identification of patients at risk after mild TBI, to determine which patients may benefit from a specific treatment.