Neurorads pinpoint extent of traumatic brain injuries

October 1, 2006

Specialists apply complex analyses, innovative technologies to diagnose life-threatening injuries caused by blows to the head

Specialists apply complex analyses, innovative technologies to diagnose life-threatening injuries caused by blows to the head

Traumatic brain injury keeps the neuroradiologists busy at San Francisco General Hospital, the city's only Level 1 trauma center. Unconscious and bloodied from blows to the head, patients are immobilized and stabilized in the emergency room before moving on to CT imaging and evaluation by a neuroradiologist who tries to identify the full extent of injury and its implications for treatment and recovery.

Time is brain for TBI patients. Every minute treatment is delayed can translate into the death of more functioning brain tissue. Resulting neurological deficits can lead to death or physical handicaps that last a lifetime, and clinical outcomes can depend on the neuroradiologist's decisions.

This intense event in radiological practice occurs more than 2000 times a day across the country. About 500,000 U.S. residents are hospitalized annually with brain injuries from vehicular crashes, falls, and physical assaults. TBI kills 50,000 of them, according to the Centers for Disease Control and Prevention. It is the leading cause of death and disability among children and young adults, at an estimated annual cost of $50 billion, according to the National Institute of Neurological Disorders and Stroke.

The American Society of Neuroradiology devoted a plenary session to TBI imaging at its annual meeting in April. Dr. Alisa D. Gean, a professor of radiology, neurology, and neurosurgery at the University of California, San Francisco, described how TBI is addressed at San Francisco General. Dr. Lawrence F. Marshall, chief of neurosurgery at the University of California, San Diego Medical Center, stressed the importance of multimodal diagnostic approaches and expressed concern about overreliance on CT for initial evaluation. Dr. Robert I. Grossman, radiology chair at New York University Medical Center, considered whether thalamic injury can explain troubling residual symptoms that sometimes linger after mild traumatic head injury.

Gean led physicians through a step-by-step process for characterizing the nature and extent of TBI. Multislice CT is by far the imaging instrument of choice for workup. Speed, convenience, accessibility, and excellent depiction of bone and blood are keys to CT's popularity. Although more cumbersome than CT, MR imaging is preferred for examining white matter injuries as well as for determining the true extent of cortical contusion and ischemia. CT and MR angiography are used as complementary approaches for the identification of vascular injuries.

NEUROSURGEON'S NEEDS

The neuroradiologist supplies the neurologist and neurosurgeon with diagnostic information for emergent treatment planning and surgical intervention. The assessment begins with a clear statement about the volume status of the brain, including the appearance of the sulci, Marshall said. The neuroradiologist should report the volume of intracranial contusions and their rate of expansion. Brain swelling is a concern, especially in younger patients. The report must cover the status of the ambient cistern, the spaces around the brain stem, as compression here can quickly translate into neuronal damage leading to death or catastrophic disability.

The neurosurgeon needs to know the presence, location, and size of blood clots and the presence or absence of subarachnoid hemorrhage. Midline displacement also affects prognosis and treatment.

Gean has learned from more than 20 years of neurotrauma experience to look for specific indications. She is alert for danger signs of an acute subdural hematoma (SDH) on CT (Figure 1). Heterogeneous appearance of blood collection suggests active bleeding. A convex shape is more ominous than a concave appearance. An SDH tends to be more benign when it appears as a crescent-shaped collection layered over the hemisphere. The simultaneous presentation of an SDH and subarachnoid hemorrhage suggests a poorer prognosis than does an SDH alone, Gean said.

The likelihood for recovery lessens when CT discloses disproportionate mass effect for the volume of the subdural collection. Prognosis is also poor when the degree of midline shift is greater than would be expected from the volume of subdural blood collection. This finding strongly suggests the presence of intraparenchymal injury that has not yet declared itself hemorrhagically or ischemically.

Presence or absence of the basal cistern also affects the patient's prognosis (Figure 2). CT and MR reveal the compression of the basal cisterns by the effacement of the cerebrospinal fluid spaces surrounding the upper brain stem. Various scoring systems, including the Marshall and Rotterdam systems, have found that compression of the basal cisterns has a statistically significant effect on patient mortality. Gean always reports whether effacement or preservation of the basal cistern is observed in head trauma cases.

SKULL FRACTURES

The same focused approach applies to skull fractures. Gean looks for specific worrisome signs that affect prognosis and the proper course of treatment, including fractures that:

- overlie the dural sinus, carrying the risk of venous thrombosis or subsequent intracranial hypertension;

- overlie the meningeal artery, suggesting the possibility of epidural bleeding;

- overlie the eloquent cortex, especially troublesome when the fracture is depressed more than a table width (the thickness of the calvarium);

- interrupt the cribiform plate or mastoid air cells, raising concern about a CSF leak and susceptibility to meningitis; and

- traverse vascular canals of the base of the skull, possibly resulting in a dissection, pseudoaneurysm, or arteriovenous fistula (Figure 3).

Gean pays particular attention to temporal bone injury and how it may affect hearing and balance. She looks for disruption of the ossicles and inner ear structures. The presence of air within the inner ear (pneumolabyrinth) may explain a patient's dizziness. The extension of a fracture across the canal of the internal carotid artery raises suspicion of a carotid cavernous sinus fistula or vascular dissection (Figure 4). An arteriovenous fistula creates a passage for arterialized blood to drain from various tributaries of the cavernous sinus. Venous hypertension and possible venous infarction result from the pressure of arterial blood flooding the venous system.

Gean never completes a TBI study without examining the corpus callosum, specifically the splenium, for signs of shearing injury. She looks for a triad of injuries. First, a lobal white matter shearing injury is the most common location for diffuse axonal injury (DAI). This finding leads Gean to look for the second point in the triad: shearing injuries in the corpus callosum. She then looks for trauma in the brain stem, the third point in the triad.

"If you see callosal involvement, your eyes should next shift to the brain stem, specifically to the dorsolateral midbrain," she said. "When you have one shearing injury, you should look for two. When you have two, you should look for three."

The patient's prognosis worsens as DAI lesions are found deeper and deeper in the brain.

LIMITATIONS OF CT

Although CT can visualize the corpus callosum, it does not match the sensitivity of MRI for identifying white matter injury, Gean said (Figure 5). She recommends MRI whenever the CT does not explain problems with the patient's motor, language, or cognitive skills. It is also routinely performed when an accompanying spinal cord injury or child abuse is suspected. In cases of suspected child abuse, MRI is performed three to five days after the incident to coincide with the time of maximum declaration of cellular injury, she said.

CT may also miss vascular dissections easily detected with MR angiography, Marshall said. He is suspicious of vascular dissection when CT fails to explain the source of persistent facial pain following minor or moderate TBI. Combined MRI/MRA is prescribed to rule out the possibility of dissection. Three of Marshall's patients in the first three months of 2006 were diagnosed with vascular dissection on the basis of MRA findings. Without MRA, they would have not received urgent anticoagulation therapy needed to treat their injuries.

Gean saw the diagnostic power of MRI at work last year when a physician friend arrived at San Francisco General with a head injury from a bicycle accident. The CT study was unremarkable for a neurosurgical lesion, but the patient's left-side paralysis led to an MR examination that showed the presence of a brain stem lesion with damage to the right cerebral peduncle, explaining the paralysis.

CT has become so fast and convenient that it may discourage the use of other potentially useful imaging techniques, Marshall said. The danger of using CT as a one-stop shop for evaluating trauma has been apparent since CT replaced x-ray angiography as the front-line modality of choice for TBI in the 1970, he said.

One of x-ray angiography's strengths was its ability to detect vasospasm, a symptom of blood vessel narrowing causing cerebral ischemia or infarction. About 20% of patients were diagnosed with vasospasm when x-ray angiography was routinely used to examine TBI. Calcium channel blockers may be effective in treating traumatic vasospasms.

"Vasospasm did not go away, but our ability to image it diminished when CT became the modality of choice," Marshall said.

The temptation to overlook the potential contributions of MRI grows stronger as the speed of MSCT continues to increase, he said. Although CT identifies brain swelling, diffusion-weighted MRI goes further by identifying its causes.

"Is it blood-brain barrier breakdown? Is it cytotoxic edema? What is going on in the tissues of the brain? Diffusion MRI helps you decide what to do in terms of therapeutic interventions," Marshall said.

Recent research has established, for example, that diffuse axonal injury is not irreversible, as was once thought, Marshall said. The human brain, especially in younger patients, can sprout new axons, reflecting a level of plasticity considered impossible a few years ago. If new MRI technologies such as diffusion tensor imaging are applied to identify the early signs of such changes, the neurosurgeon could have a much better idea about which patients to treat and what treatments to apply.

Although still a work-in-progress, DTI appears well suited for characterizing the extent of DAI and its responses to therapy. Shearing injuries are more conspicuous with 3T MR, and MR spectroscopy. PET imaging promises to reduce the guesswork involved in prescribing TBI drug treatment.

"More information will help us restore the brain to a state that is optimal for recovery," Marshall said.

THALAMIC INVOLVEMENT

Grossman's research suggests that MRI may help identify thalamic injuries that he has linked to lingering physical symptoms following mild traumatic brain injury. Problems with attention and memory arise in 15% to 29% of mild brain trauma cases and can persist for years, Grossman said. MTBI affects more than one million people in the U.S. annually and costs about $17 billion.

Despite debilitating symptoms, MTBI often fails to show up on brain imaging. Anatomic CT and MR scans usually appear normal. Some changes can be detected with diffusion MR or diffusion tensor imaging, but most scans are unremarkable.

"So are we missing something?" Grossman asked. "You bet we are missing something, because these patients are symptomatic!"

That something missing could be a connection between MTBI and thalamic trauma that Grossman identified with 3T MR. MP-RAGE imaging found a trend toward lower thalamic volume in 29 MTBI patients compared with normal 13 subjects. He also observed a trend toward atrophy among the MTBI patients over time.

Diffusion kurtosis was significantly lower in the thalami of MTBI patients, indicating elevated iron content. Elevated iron concentrations can induce oxidative stress, resulting in neuronal degeneration and death, Grossman said.

In the same study, 3D MR spectroscopy also found significantly less N-acetylaspartate metabolite in the thalami of MTBI patients.

Similarities in the symptoms of MTBI and postconcussive thalamic injury suggest a connection between two conditions. Both can involve headache, insomnia, chronic fatigue, and disrupted cognition.

"Obviously, this needs significant future confirmation, but the thalamus does pose very important diagnostic and treatment implications," Grossman said.

Mr. Brice is senior editor of Diagnostic Imaging.