Acute Ischemic Stroke Imaging Guidelines

Initial Imaging and Diagnosis

• The American College of Radiology recommends that for patients within the 4.5-hour window who are IV tPA candidates, either non-contrast CT (NCCT) or MRI is used to exclude intracranial hemorrhage before treatment administration, with the primary goal of distinguishing hemorrhagic from ischemic stroke rapidly 1, 2.
• The American Heart Association suggests that MRI with diffusion-weighted imaging (DWI) is more sensitive for early ischemic changes, but the key principle is that imaging should not delay treatment, and whichever modality is immediately available should be used 1, 3.

Sensitivity of DWI for Acute Ischemia

• The American College of Radiology states that DWI has a sensitivity of 91% and specificity of 95% for detecting acute ischemia within 6 hours, compared to CT's sensitivity of only 61% and specificity of 65% 3.

Vascular Imaging in Acute Stroke

• The American Heart Association recommends that for acute stroke patients who are candidates for endovascular therapy, vascular imaging (CTA, MRA, or conventional angiography) is strongly recommended during the initial imaging evaluation, even within the 3-hour window, if it does not unduly delay IV tPA administration 2, 4, 5, 6.
• The presence of large vessel occlusion significantly influences treatment decisions and prognosis, and CTA, DSA, and MRA far surpass the sensitivity of non-vascular studies like NCCT, FLAIR, or gradient-echo MRI for detecting intravascular thrombus 5, 6.

Gradient-Echo (GRE) Sequence

• The American College of Radiology suggests that GRE sequence detects acute, subacute, and chronic hemorrhage with superior sensitivity compared to CT, and can detect microhemorrhages, indicating the presence of amyloid angiopathy, hypertension, small vascular malformations, and other vascular diseases 1, 3.

Imaging Beyond 6 Hours

• The American Heart Association recommends that for patients beyond 6 hours from symptom onset, multimodal imaging with perfusion assessment is essential for treatment selection, and a vascular study is strongly recommended during initial imaging evaluation, especially if intra-arterial thrombolysis or mechanical thrombectomy is contemplated 4, 5, 6, 7, 8.
• Perfusion imaging identifies the ischemic penumbra (salvageable tissue) and helps distinguish it from the irreversibly infarcted core, and CT perfusion differentiates between infarcted core and salvageable penumbra, essential for selecting patients beyond 6 hours from symptom onset 4, 7, 8.

Common Pitfall in Acute Stroke Imaging

• The American College of Radiology states that focusing solely on NCCT without vascular imaging may miss large vessel occlusions requiring endovascular therapy, representing a critical missed opportunity for potentially life-saving intervention, and vascular imaging is strongly recommended during initial evaluation for patients who are endovascular therapy candidates 2, 4.

Thrombectomy in Extended Time Windows

Patient Selection and Outcomes

• The American Heart Association recommends thrombectomy in extended time windows (6-24 hours) using perfusion imaging for patient selection, which produces excellent functional outcomes, with 45-49% of patients achieving functional independence compared to 13-17% with medical therapy alone, representing a number needed to treat of less than 2 9.
• A core-penumbra mismatch ratio of ischemic tissue volume to infarct volume ≥1.8 is a key selection criterion for thrombectomy in extended windows 9.

Imaging Requirements and Techniques

• Either CT perfusion or MR perfusion can effectively identify salvageable tissue, though the specific technique matters less than proper patient selection 10, 11.
• MR diffusion/perfusion mismatch is a qualitative marker for potential infarct expansion, used successfully in trials up to 9 hours 10, 11, 12.

Safety Profile and Efficacy

• The American College of Cardiology and American Heart Association guidelines support the use of thrombectomy in extended windows with perfusion selection, which maintains excellent safety and efficacy, with a symptomatic intracranial hemorrhage rate of 7% and 90-day mortality rate of 14% 9, 13.

Mismatch Ratio Thresholds in Acute Ischemic Stroke Imaging

1. Evidence from Randomized Trials

• In the SWIFT‑PRIME trial, a perfusion‑CT mismatch ratio > 1.8 was required only for the first 71 enrolled patients; the protocol was then changed to use an ASPECTS ≥ 6 threshold for centers without perfusion‑CT capability (American Heart Association/ACC)【14】【15】.
• The EXTEND‑IA trial selected patients with a mismatch ratio > 1.2, an absolute mismatch volume > 10 mL, and an infarct core < 70 mL (American Heart Association/ACC)【14】【15】.

2. Recommended Imaging Thresholds for the Extended Time Window (6–24 h)

• The American Heart Association recommends a mismatch ratio ≥ 1.8 together with an infarct‑core volume ≤ 50 mL as key selection criteria for thrombectomy in the 6–24 h window【14】.
• For patients 6–24 h after onset, the guideline also specifies a core volume < 70 mL, mismatch ratio ≥ 1.8, and an absolute mismatch volume > 10–15 mL【14】.

3. Role of Perfusion Imaging by Time Window

• Within the first 3–6 h after stroke onset, > 80 % of patients have salvageable tissue, making strict perfusion‑CT criteria less critical (observational data)【16】【17】.
• Beyond 6 h, multimodal imaging that includes perfusion assessment becomes essential for patient selection (observational data)【16】.

4. Limitations and Validation Gaps

• The American Heart Association notes that none of the five major stent‑retriever trials were specifically designed to validate extended‑window perfusion criteria (level B evidence)【14】.
• The MR RESCUE trial failed to show overall benefit of endovascular therapy even in the subgroup with a favorable penumbra pattern (level A evidence)【14】.

5. Practical Decision Algorithm

• ≤ 6 h window: An initial assessment using ASPECTS ≥ 6 is usually sufficient; adding perfusion‑CT may provide extra information but should not delay treatment (American Heart Association)【14】.
• 6–24 h window: Mandatory perfusion imaging; apply the combined criteria of core < 70 mL, mismatch ratio ≥ 1.8, and absolute mismatch > 10–15 mL to identify patients who achieve functional independence in ~45–49 % of cases versus ~13–17 % with medical therapy alone (derived from trial data)【14】.

6. Caution Against Over‑reliance on a Single Threshold

• Solely using a mismatch ratio ≥ 1.8 may exclude patients who could still benefit from reperfusion therapy; selection should also consider time since onset, core volume, occlusion location, collateral status, and overall clinical condition (American Heart Association)【14】.

CT Perfusion Indications in Acute Ischemic Stroke

Early Presentation (≤ 6 Hours)

• For patients who are candidates for intravenous tPA within 4.5 hours, a non‑contrast CT alone is sufficient to exclude hemorrhage; adding CT perfusion should not be performed because it can delay treatment. (AHA/ASA guideline) 18
• For patients being considered for endovascular therapy within 6 hours, vascular imaging with CTA is strongly recommended to identify large‑vessel occlusions, while CT perfusion remains optional because > 80 % of patients in this time window already have salvageable tissue. (AHA/ASA guideline) 19
• CT perfusion has not been shown to improve acute treatment decisions within the first 6 hours and must never postpone IV tPA administration. (AHA/ASA guideline) 20, 21

Late Presentation (6 – 24 Hours)

• Multimodal imaging that includes CT perfusion becomes essential for selecting patients for reperfusion therapy when they present 6–24 hours after symptom onset. (AHA/ASA guideline) 19
• CT perfusion identifies the ischemic penumbra (salvageable tissue) and distinguishes it from the irreversibly infarcted core, which is critical for selecting patients who will benefit from late‑window mechanical thrombectomy. (AHA/ASA guideline) 19
• In patients with a large‑vessel occlusion presenting 6–24 hours after onset, non‑contrast CT + CTA + CT perfusion are all mandatory; patients must meet strict perfusion thresholds (core volume < 70 mL and mismatch ratio ≥ 1.8) to proceed with thrombectomy. (AHA/ASA guideline) 19

Specific Clinical Scenarios

• When the exact time of stroke onset is unknown (e.g., wake‑up stroke), CT perfusion helps identify patients with salvageable tissue, enabling biologically‑based treatment decisions rather than time‑based ones. (AHA/ASA guideline) 19
• In patients who are ineligible for IV tPA and are being considered for intra‑arterial thrombolysis or primary mechanical thrombectomy, CT perfusion assists in delineating at‑risk but potentially salvageable brain regions. (AHA/ASA guideline) 19

Technical Performance and Limitations

• Dynamic CT perfusion provides absolute quantitative maps of cerebral blood flow (CBF), mean transit time (MTT), and cerebral blood volume (CBV), although current scanners cover only 2–4 brain slices, which may miss some vascular territories. (AHA/ASA guideline) 20, 22
• CT perfusion demonstrates high sensitivity and specificity for detecting cerebral ischemia and can differentiate reversible from irreversible ischemic thresholds. (AHA/ASA guideline) 20
• The technique entails exposure to iodinated contrast, additional radiation dose, and requires rapid data acquisition on conventional CT equipment. (AHA/ASA guideline) 20, 21

Implementation Pitfalls

• Never delay IV tPA administration to obtain CT perfusion in patients presenting within 4.5 hours; additional imaging must not postpone treatment. (AHA/ASA guideline) 18

Practical Imaging Algorithm (Guideline Summary)