AAN Summary of Evidence-based Guideline for CLINICIANS
The Role of Diffusion and
Perfusion MRI for the Diagnosis
of Acute Ischemic Stroke
This is a summary of the American Academy of Neurology (AAN) guideline (Neurology ® 2010;75:177–185) regarding the role of diffusion and
perfusion MRI for the diagnosis of acute ischemic stroke.
Please refer to the full guideline at www.aan.com for more information.
Are diffusion-weighted imaging (DWI) and perfusion-weighted imaging (PWI) sensitive and specific in the diagnosis of acute
ischemic stroke (compared to concurrent imaging with other techniques, established by follow-up imaging, clinical follow-up,
and final discharge diagnosis)?
Strong evidence
DWI should be considered superior to noncontrast CT scan for the diagnosis of acute ischemic stroke in patients
presenting within 12 hours of symptom onset (Level A†).
Insufficient evidence
There is insufficient evidence to support or refute the value of PWI in diagnosing acute ischemic stroke (Level U).
Does the volume of the DWI or PWI abnormality predict initial clinical severity, final infarct size, and late clinical outcome?
Good evidence
Baseline DWI volume should be considered useful in predicting baseline clinical stroke severity and final
lesion volume in anterior-circulation stroke syndromes (Level B).
Baseline DWI volume may be considered not useful in predicting baseline National Institute of Health Stroke
Scale (NIHSS) score in posterior-circulation stroke syndromes (Level C).
Weak evidence
Baseline DWI volume may be considered useful in predicting clinical outcome as measured by the NIHSS and
Barthel Index (Level C).
Baseline PWI volume may be considered useful in predicting baseline clinical stroke severity (Level C).
CLINICAL CONTEXT
In patients presenting with acute neurologic impairment, noncontrast CT imaging is used to evaluate for infarct and to exclude
hemorrhage and other structural lesions that may mimic stroke. The evidence from our report demonstrates that DWI is accurate and
superior to CT for the diagnosis of acute ischemic stroke relative to clinical and imaging outcomes. However, in clinical practice, the
availability and cost of imaging modalities and the requirements of medical management enter into the decision about which test to
perform in the acute period.
The true sensitivity of DWI for the diagnosis of ischemic stroke is not 100% and is probably closer to 80%–90% in a general sample
of patients presenting for emergency evaluation of possible stroke. Many of the initial case series and small CT comparative studies
reported a near-100% sensitivity for DWI in the hyperacute stage of stroke in highly selected subsets of patients. Increasingly,
however, cases of DWI-negative stroke were reported. False-negative DWI in ischemic stroke may be attributable to mild (small)
strokes, brainstem location, and the earliest times from onset, and may become less frequent as imaging technology continues
to improve.
DWI-positive scans in TIA are common. According to the literature, acute ischemic DWI lesions are present in 40.1% of patients
with the clinical diagnosis of a TIA (10 studies, 234/584 patients), a finding that correlates with symptom duration. Only one of the
studies involved DWI performed within 24 hours of symptom onset. A recent study estimated the epidemiologic impact of DWI-based
diagnosis would result in reduced annual TIA incidence (33%) and increased stroke incidence (7%) in the United States.
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†Classification of Recommendations: A = Established as effective, ineffective or harmful (or established as useful/predictive or not useful/predictive) for the given condition
in the specified population. (Level A rating requires at least two consistent Class I studies.)* B = Probably effective, ineffective or harmful (or probably useful/predictive or not
useful/predictive) for the given condition in the specified population. (Level B rating requires at least one Class I study or two consistent Class II studies.) C = Possibly effective,
ineffective or harmful (or possibly useful/predictive or not useful/predictive) for the given condition in the specified population. (Level C rating requires at least one Class II study
or two consistent Class III studies.) U = Data inadequate or conflicting; given current knowledge, treatment (test, predictor) is unproven.
*In exceptional cases, one convincing Class I study may suffice for an “A” recommendation if 1) all criteria are met, 2) the magnitude of effect is large (relative rate improved outcome >5 and the lower limit
of the confidence interval is >2).
Classification of Evidence for Studies of Diagnostic Accuracy: Class I = A cohort study with prospective data collection of a broad spectrum of persons with the suspected
condition, using an acceptable reference standard for case definition. The diagnostic test is objective or performed and interpreted without knowledge of the patient’s clinical
status. Study results allow calculation of measures of diagnostic accuracy. Class II = A case control study of a broad spectrum of persons with the condition established by an
acceptable reference standard compared to a broad spectrum of controls or a cohort study where a broad spectrum of persons with the suspected condition where the data
was collected retrospectively. The diagnostic test is objective or performed and interpreted without knowledge of disease status. Study results allow calculation of measures of
diagnostic accuracy. Class III = A case control study or cohort study where either persons with the condition or controls are of a narrow spectrum. The condition is established
by an acceptable reference standard. The reference standard and diagnostic test are objective or performed and interpreted by different observers. Study results allow
calculation of measures of diagnostic accuracy. Class IV = Studies not meeting Class I, II, or III criteria including consensus, expert opinion, or a case report.
Classification of Evidence for Prognostic Articles: Class I = A cohort study of a broad spectrum of persons at risk for developing the outcome (e.g., target disease, work status).
The outcome is defined by an acceptable reference standard for case definition. The outcome is objective or measured by an observer who is masked to the presence of the
risk factor. Study results allow calculation of measures of prognostic accuracy. Class II = A case control study of a broad spectrum of persons with the condition compared
to a broad spectrum of controls or a cohort study of a broad spectrum of persons at risk for the outcome (e.g., target disease, work status) where the data was collected
retrospectively. The outcome is defined by an acceptable reference standard for case definition. The outcome is objective or measured by an observer who is masked to the
presence of the risk factor. Study results allow calculation of measures of prognostic accuracy. Class III = A case control study or a cohort study where either the persons
with the condition or the controls are of a narrow spectrum where the data was collected retrospectively. The outcome is defined by an acceptable reference standard for
case definition. The outcome is objective or measured by an observer who did not determine the presence of the risk factor. Study results allow calculation of measures of a
prognostic accuracy. Class IV = Studies not meeting Class I, II, or III criteria including consensus, expert opinion, or a case report.
This is an educational service of the American Academy of Neurology. It is designed to provide members with evidence-based guideline recommendations to assist
the decision making in patient care. It is based on an assessment of current scientific and clinical information and is not intended to exclude any reasonable alternative
methodologies. The AAN recognizes that specific patient care decisions are the prerogative of the patient and the physician caring for the patient, and are based on the
circumstances involved. Physicians are encouraged to carefully review the full AAN guidelines so they understand all recommendations associated with care of these patients.
©2010 American Academy of Neurology. All rights reserved.
Copies of this summary and additional companion
tools are available at www.aan.com or through
AAN Member Services at (800) 879-1960.
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