New AAN Guideline Advocates MRI Over CT for Stroke Diagnosis

Pauline Anderson

July 15, 2010

July 15, 2010 — After reviewing the relevant literature, a panel of neurologists, neuroradiologists, and radiologists has concluded that diffusion-weighted imaging (DWI) MRI is superior to noncontrast computed tomography (CT) scans, which are the current imaging standard, for diagnosing acute ischemic stroke within 12 hours of symptom onset.

However, although the research shows that DWI is better than CT, the decision about which imaging test to use in clinical practice will depend on issues such as availability and cost, the panel concludes in the new guideline from the American Academy of Neurology.

"The doctors taking care of acute stroke patients, as well as the patients themselves, need to be aware that MRI-DWI is a superior diagnostic tool in acute stroke less than 12 hours," lead author Peter Schellinger, MD, PhD, from Johannes Wesling Clinical Center and the University of Erlangen, Germany, told Medscape Medical News.

"Whether this translates into a change in practice remains to be seen," Dr. Schellinger added. "Logistical, financial, and personnel requirements need to be weighed against better diagnosis which — and this was not within the scope of our assessment — may influence management and ultimately the outcome of the stroke."

The panel's recommendations appear in the July 13 issue of Neurology.

For their review, the panel members addressed 2 questions:

  1. Are DWI and PWI (perfusion-weighted imaging) sensitive and specific in the diagnosis of acute ischemic stroke compared with concurrent imaging with other techniques, established by follow-up imaging, clinical follow-up, and final discharge diagnosis?

  2. Does the volume of the DWI or PWI abnormality predict initial clinical severity, final infarct size, and late clinical outcome?

The panel performed a literature search of Medline, Embase, and Biosis up to January 2008. For the first question, the search was restricted to studies with a time window of 12 hours after symptom onset. For the second, the search was restricted to studies related to acute ischemic stroke less than 24 hours after symptom onset.

The panel classified evidence using a 4-tier classification scheme for diagnostic (question 1) and prognostic (question 2) articles. Recommendations were based on these levels of evidence.

For question 1, the panel identified 62 articles that fulfilled the inclusion criteria. For DWI, there was 1 class I and 3 class II studies. All PWI studies were class IV.

The class I study assessed the accuracy of MRI (DWI and gradient echo scans) vs CT in 356 consecutive patients presenting to a hospital emergency department over 18 months with a possible diagnosis of acute stroke. In the subset of 221 patients scanned within 12 hours of symptom onset, the majority of blinded readers correctly diagnosed acute ischemic stroke by MRI more often than by CT (94 vs 22; P < .0001).

Single Study Justifies Level A Recommendation

"The odds ratio and its 95% confidence interval (CI) of the difference in the proportions was 25 (8-79), indicating an effect size sufficiently large for this single study to justify a Level A recommendation," the authors write. "A similar direction and magnitude of difference were also seen in the subset of 90 patients scanned within 3 hours of onset."

They added that the sensitivity, specificity, and accuracy of DWI in this study were 77%, 96%, and 86% respectively, compared with 16%, 97% and 55% for CT.

One of the class II studies prospectively evaluated 50 patients with ischemic stroke and 4 with transient ischemic attacks. Patients were randomly assigned to receive MRI or CT within 6 hours of stroke onset. The sensitivity of infarct detection by experts blinded to the patients' symptoms but aware that it was an ischemic stroke population was significantly better with DWI (91%) than CT (61%), as was the accuracy (DWI, 91%; CT, 61%).

The sensitivity of DWI for the diagnosis of ischemic stroke in patients with possible stroke is not perfect; the panel found that its "true sensitivity" is probably 80% to 90%.

The panel concluded from this and the other evidence that DWI is superior to CT for the diagnosis of acute ischemic stroke within 12 hours of symptom onset.

For question 2, the panel identified 8 studies fulfilling the inclusion criteria — 4 class IV studies that assessed only the correlation of baseline DWI and PWI lesion volume with chronic lesion volume, 1 class II study that assessed only the correlation of baseline DWI and PWI lesion volume with clinical outcome, and 2 class II studies and 1 class III study that assessed both clinical and morphologic outcomes. None of the studies compared CT with MRI in predicting these outcomes.

From this research, the panel concluded that baseline DWI volume probably predicts baseline clinical stroke severity and final lesion volume in anterior-circulation stroke syndromes and is possibly accurate in predicting clinical outcome. In addition, the evidence showed that baseline DWI volume possibly does not predict the baseline National Institute of Health Stroke Scale score in posterior circulation stroke syndromes.

In addition to its recommendation that DWI should be considered superior to CT for the diagnosis of ischemic stroke in patients presenting within 12 hours of symptom onset, the panel recommended that baseline DWI volume should be considered useful in predicting baseline clinical stroke severity and final lesion volume in anterior-circulation stroke syndromes, but not in predicting baseline National Institute of Health Stroke Scale score in posterior-circulation stroke syndromes.

The panel found there was insufficient evidence to support or refute the value of PWI in diagnosing acute ischemic stroke but said baseline PWI volume may be considered useful in predicting baseline clinical stroke severity. Prospective, well-designed studies are needed to investigate the diagnostic utility of PWI in acute stroke, according to the panel.

When CT Is the Best Diagnostic Tool

Still, there are circumstances under which CT should remain the primary diagnostic tool, said Dr. Schellinger. "Most typically, this would be in comatose patients, patients with contraindications for MRI such as implanted cardiac pacemakers, and in patients who are candidates for intravenous thrombolytic therapy with rt-PA [tissue plasminogen activator]. Here, a CT-based exclusion of intracranial hemorrhage is enough to make a treatment decision."

A plain CT scan is usually performed faster than a multisequence MRI scan, noted Dr. Schellinger. "Loss of time from arrival at the hospital to initiation of thrombolytic therapy is associated with a loss of efficacy and reduction of chance for a good outcome, and potentially also with a higher bleeding risk, and therefore should be avoided by all means."

In situations in which CT was performed first — for example, in a candidate for thrombolysis — and diagnostic uncertainty remains, MRI may be performed in addition to CT after initiation of thrombolytic therapy to optimize diagnostic assessment, added Dr. Schellinger.

A disadvantage of MRI imaging in acute stroke is its relatively high cost. According to Dr. Schellinger, superior technologists often cost more, although this study did not address cost implications of using MRI to diagnose stroke. "Our objective was an assessment; how and whether this is taken as a means to change [emergency department] practice and stroke care practice remains to be seen."

Lack of Availability

Another perceived disadvantage of MRI is its lack of ready availability. MRIs should be as accessible as CT scans in typical hospital settings, said Dr. Schellinger, adding that the technology has been available at all hours in every center he himself has worked in. "Many of the major stroke services in the United States have implemented MRI as an emergency imaging tool. It is a question of dedication and also a question of whether stroke patients should be treated in stroke centers or not."

Until now, noncontrast CT has been the diagnostic standard for acute stroke. "There was nothing else available, and it was clear pretty early that the most important differential diagnosis of acute ischemic stroke — for example, intracranial hemorrhage — can be detected by CT with a close to 100% sensitivity," explained Dr. Schellinger. "By deduction, it is assumed that a clear stroke syndrome that is not caused by hemorrhage likely is caused by ischemic stroke, even if the CT does not show it."

Best Identification of Early Strokes

Approached for a comment, Gary Abrams, MD, professor of neurology at the University of California–San Francisco, said the new guideline is "very timely and important," as therapeutic interventions for acute ischemic stroke continue to advance.

The article validates what most clinicians already know — that DWI is the most effective and sensitive way to diagnose an acute stroke and is superior to CT — said Dr. Abrams. "As we move forward in the future, this may the way that we need to go in terms of best identification of early strokes and identifying the group that's most amenable to treatment."

As well, the guideline begins to address the issue of PWI, added Dr. Abrams. "This is much less widespread in terms of availability and clinical impact, but it's important, and as the authors suggest, the combination of DWI and PWI may turn out to be the most effective way to understand what's going on in terms of diagnosis and prognosis in acute stroke."

Dr. Schellinger has served or serves on scientific advisory boards for Boehringer Ingelheim, ImaRx Therapeutics, Photothera, Cerevast, and CoAxia Inc; has served or serves on speakers' bureaus for and received funding for travel and speaker honoraria from Boehringer Ingelheim, Sanofi, ImaRx Therapeutics, Photothera, Cerevast, CoAxia Inc, Solvay Pharmaceuticals Inc, and GlaxoSmithKline; serves on editorial boards of Stroke and European Neurology; receives royalties from the publication of NeuroIntensiv (Springer, 2008) and received royalties from the publication of Stroke MRI (Steinkopff, 2004); has served as a consultant for CoAxia Inc, Photothera, Cerevast, ImaRx, and Boehringer Ingelheim; and has provided expert testimony, affidavits, and acted as a witness or consultant in legal proceedings. For disclosure information on the other authors, please see the original article.

Neurology. 2010;75:177-185. Abstract