From the Guidelines
For a patient with suspected stroke, a non-contrast computed tomography (CT) scan of the brain should be ordered immediately as the first-line imaging study. This recommendation is based on the most recent and highest quality evidence from the Canadian Stroke Best Practice Recommendations for Acute Stroke Management, 6th edition, update 2018 1. The CT scan can quickly rule out hemorrhagic stroke and identify early signs of ischemic stroke, taking only minutes to complete.
Some key points to consider when ordering brain imaging for suspected stroke include:
- All patients with suspected acute stroke should undergo brain imaging with non-contrast CT (NCCT) or MRI, with NCCT being the preferred initial test due to its speed and accessibility 1.
- For patients with suspected acute ischemic stroke who arrive within 4.5 hours and are potentially eligible for intravenous thrombolysis, immediate brain imaging with non-contrast CT should be performed without delay to determine eligibility for thrombolysis 1.
- CT angiography (CTA) may also be appropriate to evaluate for large vessel occlusions if the patient is a potential candidate for mechanical thrombectomy, and should be performed without delay in patients who arrive within 6 hours of symptom onset 1.
- Advanced CT imaging such as CT perfusion (CTP) or multiphase or dynamic CTA may be considered as part of initial imaging to aid patient selection, but should not substantially delay decision and treatment with intravenous thrombolysis or endovascular therapy 1.
It's also important to note that the choice between CT and MRI may depend on local availability, with CT generally being more accessible and faster, making it the preferred initial test in most emergency settings. Additionally, the American Heart Association/American Stroke Association guidelines recommend that systems be established to perform brain imaging studies within 20 minutes of arrival in the emergency department for at least 50% of patients who may be candidates for IV alteplase and/or mechanical thrombectomy 1.
Overall, the key principle is to obtain imaging urgently, as "time is brain" - every minute delay results in the loss of approximately 1.9 million neurons.
From the Research
Brain Imaging for Suspected Stroke
The following brain imaging options are available for a patient with suspected stroke:
- Non-contrast computed tomography (CT) brain: helps differentiate ischemic stroke and intracerebral hemorrhage (ICH) 2
- Magnetic resonance imaging (MRI): can help differentiate ischemic stroke and ICH, and can also assist in making a diagnosis of ischemic stroke versus mimics such as migraine, Todd's paresis, or functional disorders 2
- Vascular and perfusion imaging: can help identify medium-large vessel occlusions and salvageable brain tissue in patients who are likely to benefit from reperfusion therapies 2
- CT or MR angiography: can help identify medium-large vessel occlusions and trigger consideration of endovascular thrombectomy (EVT) 2
- Perfusion imaging: can help identify salvageable brain tissue in patients who are likely to benefit from reperfusion therapies, particularly in the ≥6 h window 2
Factors Influencing the Choice of Brain Imaging
The choice of initial brain imaging in patients with suspected stroke may be influenced by several factors, including:
- Patient characteristics: older age, preexisting disability, use of anticoagulants 3
- Stroke characteristics: stroke of unknown onset 3
- Management factors: onset-to-door time, initial admission to ED or intensive care unit, personal transport, and admission during working hours 3
- Availability of CT or MRI 3
Comparison of CT and MRI
Studies have compared the use of CT and MRI in patients with suspected stroke, including:
- A study that found that CT perfusion and MRI perfusion-diffusion mismatch can be used to select patients most likely to respond to thrombolysis 4
- A study that found that multiphasic perfusion CT is useful and of comparable utility to diffusion and perfusion MRI for predicting final infarct volume, infarct growth, and clinical severity in acute ischemic stroke 5
- A study that highlighted the importance of CT in stroke diagnosis, assessment, and treatment, due to its affordability, wide availability, and speed of acquiring images 6