André Kemmling

Recent randomized trials have shown the efficacy of imaging-based patient selection in predicting benefit from thrombectomy after acute proximal occlusion in the anterior circulation. The underlying concept of imaging-based criteria rests on the assumption that a small irreversible infarct (core lesion) in relation to a large area of potentially treatable hypoperfused critical ischemia identifies a patient with the optimal constellation for therapy. The trials have outlined imaging criteria that are favorable for endovascular intervention as follows:

• ESCAPE: Core lesion size limited by CT (ASPECTS > 5) in relation to extent of good collaterals (CTA collateral score with intermediate to good vasculature contrast > 50% of MCA territory)¹
• EXTEND-IA: Core lesion size limited by MR-DWI or CTP-CBF (< 70 mL; CBF reduction < 30% relative to normal) in relation to size of critical perfusion Tmax > 6 s (at least 10 mL and 20% larger than core lesion)²
• SWIFT Prime*: Core lesion size limited by MR-DWI or CT (< 50 mL) in relation to size of critical perfusion Tmax > 6 s (at least 15 mL and 80% larger than core lesion; in addition, size of severely critical perfusion Tmax > 10 s no larger than 100 mL).³

The pragmatic approach of the ESCAPE criteria may seem more appealing for fast and robust clinical decision-making in the acute stroke setting than the more complex, operational definition of a target mismatch in the perfusion image by EXTEND-IA or SWIFT Prime (including a “black box” analysis with purported automated RAPID software).

In CT or MR angiography, the term “collaterals” has been associated with the imaging feature of abundance of visible vasculature distal to the proximal brain artery occlusion, and collateral scoring has been based on grading the apparent asymmetry of vascularity between the ischemic and contralateral nonischemic hemisphere. How is this different from perfusion imaging, if there is any difference pragmatically? Primarily, collateral grading emphasizes the abundance of macrovascular vessel contrast, which, in the end, will probably have an equivalent relevance for patient selection as the additional tissue contrast seen in perfusion imaging.

*Initial criteria, later revised and simplified to “core lesion ASPECTS > 5” to include for endovascular intervention

Poorly or not visible collaterals highly correlate with the size of CBV lesions; both essentially show the area where no contrast medium arrives, an area of high infarct probability (i.e., core lesion) regardless of successful recanalization.

However, conventional CT or MR angiographic collateral grading does not include the time component of bolus speed and arrival, which is encoded in perfusion parameters such as CBF, MTT, or Tmax and also in conventional DSA. In other words, collateral grading in conventional CT or MR angiography will differentiate the abundance of vasculature, but not the velocity, and this could be relevant in acute stroke triage: plentiful collaterals with fast arrival may designate a patient with better outcome from thrombectomy in contrast to a patient with plentiful but slow collaterals and late arrival. Also, merely scoring the "abundance of collaterals" is prone to error because late collaterals may be missed in fast multidetector CT scanners that only acquire a snapshot of the contrast bolus.⁴ The ESCAPE imaging criteria address this problem by using a multiphase-CTA protocol.

Our goal was to investigate collateral grading in MRA for tissue outcome prediction in proximal anterior circulation occlusive stroke.⁵ We quantified contrast of vasculature distal to the proximal occlusion in contrast-enhanced MRA and TOF-MRA, concurrently. The hypothesis was that the flow-weighted signal in TOF-MRA contains additional information with regard to severity of ischemia compared with the blood volume–weighted signal in contrast-enhanced MRA alone. By quantitative signal analysis, we found that analogous to "perfused tissue-at-risk," a brain area that shows missing flow-weighted collateral signal in TOF imaging concurrent with present collateral signal in contrast-enhanced MRA could be an indicator of "collateralized tissue-at-risk." Pragmatically, if MR imaging is used for acute stroke triage, contrast-enhanced MRA should be the primary protocol to assess abundance of collaterals; TOF-MRA should not be used alone because of poor signal in slow-flow collaterals.

References

1. Goyal M, Demchuk AM, Menon BK, et al; ESCAPE Trial Investigators. Randomized assessment of rapid endovascular treatment of ischemic stroke. N Engl J Med 2015;372:1019–30. doi: 10.1056/NEJMoa1414905
2. Campbell BC, Mitchell PJ, Kleinig TJ, et al. Endovascular therapy for ischemic stroke with perfusion-imaging selection. N Engl J Med 2015;372:1009–18. doi: 10.1056/NEJMoa1414792
3. Saver JL, Goyal M, Bonafe A, et al. Stent-retriever thrombectomy after intravenous t-PA vs. t-PA alone in stroke. N Engl J Med 2015;372:2285–95. doi: 10.1056/NEJMoa1415061
4. Frolich AM, Wolff SL, Psychogios MN, et al. Time-resolved assessment of collateral flow using 4D CT angiography in large-vessel occlusion stroke. Eur Radiol 2014;24:390–96. doi: 10.1007/s00330-013-3024-6
5. Ernst M, Forkert ND, Brehmer L, et al. Prediction of Infarction and Reperfusion in Stroke by Flow- and Volume-Weighted Collateral Signal in MR Angiography. AJNR Am J Neuroradiol 2015;36:275–82. doi: 10.3174/ajnr.A4145

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