Current Role of MRI in Stroke Imaging

• MRI is the most sensitive imaging modality for detecting cerebral ischemia.
• DWI is best for depicting the infarct core.
• The mismatch between DWI and FLAIR helps to estimate the time from onset ("tissue clock").
• MRI is gold standard for imaging the posterior fossa.
• AI may help avoid misdiagnosis and delayed diagnosis of acute stroke, and ultimately improving patient outcome.

Georgina Gáti from Pécel, Hungary, discussed the role of MRI in stroke imaging and the future potential of AI tools for stroke imaging in MRI.

Time Intervals for Interventions

The time window for intravenous thrombolysis (IVT) is 4.5 to 9 hours after onset.

The time window for mechanical thrombectomy (MT) is 6 to 24 hours after onset.

(AHA/ASA Guidelines. Powers 2019)

Early Time Window with Known Onset

CT is the first choice image modality to exclude hemorrhage and contraindications for IVT which is the evidence-based treatment.

CT plus CT angiography (CTA) can prove the presence of large vessel occlusion (LVO). The presence of LVO makes the patient eligible for MT if the ASPECT score (Alberta Stroke Program Early CT Score) is ≥ 6.

If primary imaging is MRI and the ASPECT score is applied, a score diminished by 1 is considered comparable to CT score.

Most recent randomized controlled studies strongly support the benefit of MT even in cases of low ASPECT score of 3-5 (Sarraj et al. 2023).

Recommended Baseline MRI Protocol for Early Hyperacute Phase

The baseline MRI protocol for the early hyperacute phase includes parenchymal and vascular imaging:

• DWI (diffusion-weighted) for depiction of the infarct core
• DWI/FLAIR mismatch to presume the time from onset ("tissue clock"). FLAIR is usually not positive within the first half hour. FLAIR can also serve for evaluation of occluded vessel.
• SWI (susceptibility-weighted) to visualize the thrombus, exclude hemorrhage. The "brush sign" as a result of increased deoxyhemoglobin indicates the concentration of venous blood and allows for rough lesion localization.

Potential pitfalls for MRI for early hyperacute phase are stent artifacts, the non-visualization of mural or extraluminal abnormalities, and pseudostenosis or pseudoocclusion.

Late Time Window

In the late time window, penumbral imaging by CT perfusion or MR perfusion is mandatory in order to prove the presence of salvageable brain tissue.

In the presence of LVO, routine perfusion imaging is not required for MT in the extended window. This reduces the need for perfusion imaging and expands patients' access for MT.

Alternatively to contrast-enhanced MRI perfusion, arterial spin labeling MRI can be used, providing cerebral blood flow (CBF) map.

Unknown Onset or Wake-up Stroke

In case of unknown onset or wake-up stroke, MRI perfusion imaging can be used to roughly estimate the time since onset. If a DWI/FLAIR mismatch is shown, IVT is considered appropriate.

Patients with more than 4,5 hours from last seen well, but within 9 hours of the midpoint of sleep when they have woken with symptoms should undergo perfusion imaging. A favorable perfusion profile is given when

• mismatch ratio > 1.2
• mismatch volume > 10 ml
• ischemic core volume < 70 ml

Advantages of MRI in Stroke Imaging

In case of unknown onset or wake-up stroke, MRI perfusion imaging can be used to roughly estimate the time since onset. If a DWI/FLAIR mismatch is shown, IVT is considered appropriate.

• Higher spatial resolution, higher sensitivity and specificity are favorable to identify the ischemic lesion.
• Exclusion of stroke mimics like migraine
• DWI is most sensitive to directly visualize infarct core
• DWI plus FLAIR allows to roughly estimate time from onset
• Higher sensitivity for assessing acute stroke involving the posterior fossa and brainstem compared to CT
• Later etiologic workup in cases which were not evident in basic sequences

Disadvantages of MRI in Stroke Imaging

Considering the wide range of advantages, why not use it always, asked Gáti. Main problems with MRI are

• the limited MRI availability at most institutions
• the possible lack of patient’s cooperation during an acute stroke situation
• limited availability of advanced imaging such as MR perfusion

Primary MRI for stroke imaging may cause a significant treatment delay of 20 minutes compared to CT protocols; however, most recently Fladt et al. (Neurology 2024) found no evidence that functional outcome at 90 days was less favorable after MRI/MRA at baseline compared to CT/CTA, despite significant workflow delays.

How AI Can Support Stroke MRI

As part of a clinical decision support system, AI can optimize the workflow by alerting the colleagues via their cell phones. AI can also prioritize patients for the work list.

AI may optimize image quality (reconstruction, denoising, artifact reduction) and speed up acquisition. It can improve spatial resolution and even simulate 7T-like images from 3T systems.

Fast image interpretation is crucial in an acute stroke situation – AI can help in core infarct volume segmentation or by providing automated post-processing of perfusion data. 

AI-based prediction of the time from onset by identifying the DWI/FLAIR mismatch is possible; however, this solution is not yet commercially available.

AI supports outcome prediction by predicting short-term tissue outcome and long-term functional outcome.