Use of Imaging to Select Patients for Late Window Endovascular Therapy

The substantial clinical benefits of late window thrombectomy that were recently documented in the DAWN (Triage of Wake-up and Late Presenting Strokes Undergoing Neurointervention With Trevo) and DEFUSE 3 (Endovascular Therapy Following Imaging Evaluation for Ischemic Stroke 3) studies led to expansion of the treatment window for thrombectomy from 6 to 24 hours in the 2018 American Heart Association stroke guidelines. The new clinical trial data and guidelines have led a large number of stroke centers to begin using advanced imaging with computed tomography perfusion (CTP) or magnetic resonance imaging (MRI) to evaluate patients who present with a possible large vessel occlusion in an extended time window. These techniques can provide quantitative estimates of ischemic core and penumbra without user input and have excellent interobserver agreement. However, these techniques also have limitations, and therefore it is important to review all available imaging data before making a decision to proceed with thrombectomy. The purpose of this article is to discuss the recommended imaging options for selecting patients for late window thrombectomy and to review how to interpret CTP maps and MRI images both before and after reperfusion has occurred. The new AHA guidelines recommend that DAWN or DEFUSE 3 eligibility should be strictly adhered to in clinical practice; therefore, it is important to understand how these trials selected eligible patients. DEFUSE 3 enrolled patients who could be treated between 6 and 16 hours after last known well, and DAWN enrolled patients who could be treated between 6 and 24 hours. Patients in these trials rarely received tPA (tissuetype plasminogen activator; <10%) because they typically presented beyond the tPA time window. Both trials used the Rapid Processing of Perfusion and Diffusion (RAPID) automated software platform (iSchemaView, Menlo Park, CA) to determine imaging eligibility for all patients. Imaging selection for patients in both DEFUSE 3 and DAWN required either CTP or MRI, with the majority being selected by CTP. Ischemic core volumes were based on a RAPID relative cerebral blood flow (CBF) lesion volume using a <30% threshold or a RAPID diffusionweighted lesion (DWI) lesion volume with an apparent diffusion coefficient (ADC) threshold of <620×10−3 mm/s. The volume of salvageable tissue in DEFUSE 3 was estimated using the T max perfusion parameter with a >6 seconds (T max >6 seconds) threshold on both CTP and MR perfusion imaging. Subtracting the ischemic core volume from the T max >6 seconds volume provides the mismatch volume and dividing the T max >6 seconds volume by the core volume provides the mismatch ratio. Table 1 summarizes the key imaging selection criteria for both studies.