Stroke Imaging: Whats Best vs. Whats Practical

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CT images in a patient with left hemiparesis. Admission CT perfusion images show A) cerebral blood flow (CBF), B) cerebral blood volume (CBV), and C) mean transit time (MTT). The presence of a matched CBF/CBV perfusion deficit suggests irreversibly ischemic infarct “core,” likely to correlate with MRI diffusion-weighted imaging findings, and not a target for reperfusion therapies. Image courtesy of Michael H. Lev, MD, and Angelos Konstas, MD, Massachusetts General Hospital, Boston.

The solid evidence points to MRI as the first-line test for acute stroke assessment. Limited scanner availability, however, has prompted many to rely on CT, despite the lack of rigorous clinical studies to support its use.

MRI is better, but…

Recently, the American Academy of Neurology (AAN) reviewed the evidence for MR diffusion-weighted imaging (DWI) in the assessment of acute ischemic stroke and concluded that it should be the first-line test (Neurology 2010;75;177-185). While many neuroradiologists agree with this conclusion, many emergency departments (EDs) do not have easy access to an MRI scanner. Consequently, the first-line test for suspected stroke is a non-contrast brain CT to rule out hemorrhage.  

“No radiology or neurology specialists would question whether MRI is more sensitive to the presence of acute stroke, although CT imaging properly performed and read also serves as a very good test,” says Lawrence N. Tanenbaum, MD, director of MRI, CT and outpatient/advanced development at Mount Sinai School of Medicine in New York City. However, there are practical considerations with MRI as a first-line test, including scanner availability, substantial delays between ordering and obtaining an MR exam and imaging times up to 40 minutes—“a long time in the life of a stroke.”

“It is possible to design an efficient MR exam that takes less than 10 minutes and includes MR angiography, diffusion-weighted imaging and perfusion-weighted imaging,” Tanenbaum says. “Performing the exam quickly is more practical than it was 10 years ago, but MR scanners are still less accessible than CT scanners—and that is the crux of the debate.”

Steven Warach, MD, PhD, senior author of the AAN guidelines, would like to see stronger lobbying from ED physicians and neurologists to make MRI scanners more readily available for the assessment of suspected stroke.

“Emergency physicians, as the main gatekeepers, are making decisions about which test has to be ordered. Hopefully, these guidelines will encourage them to take a more active role in making sure these patients get the best test,” says Warach, chief of stroke diagnostics and therapeutics at the National Institutes of Neurological Disorders and Stroke, part of the National Institutes of Health, in Bethesda, Md. He notes that the guidelines do not advocate for what hospitals should do regarding the availability of MRI scanners.

In a review of the literature up to 2008, Warach and colleagues concluded that DWI is the most sensitive and specific technique to determine at admission how much brain is likely to be irreversibly infarcted (Level A). The next conclusion is that the amount of brain infarcted at admission does have a reasonably good correlation with not only current clinical status but with a patient’s outcome (Level B). This evidence holds true for anterior territory strokes, while evidence for DWI in vertebral basilar territory stroke is not as strong (Level C). They found insufficient evidence to offer a conclusion regarding MR perfusion’s role in assessing acute ischemic stroke.

Bleed or no bleed

In the seminal study showing the utility of IV tissue plasminogen activator (tPA) to thrombolize ischemic stroke within three hours of onset, one of the eligibility criteria was “a baseline CT scan of the brain that showed no evidence of intracranial hemorrhage” (N Engl J Med 1995; 333:1581-1588). When the FDA subsequently approved tPA, the indications included a non-contrast head CT to rule out hemorrhage.

“That is still the standard today,” says Michael H. Lev, MD, director of emergency neuroradiology and the neurovascular lab at Massachusetts General Hospital (MGH) in Boston. “That is evidence-based medicine for IV-tPA administration that no one will dispute. What the AAN guidelines are saying is that if you want to diagnose the presence of acute ischemic stroke, or if you are trying to determine the extent of the stroke at admission, there is no better way to do that in a typical ED setting