Imaging, Intervention, and Workflow in Acute Ischemic Stroke: The Calgary Approach

A, Axial multiphase CT angiography (3 phases) maximum-intensity-projection images (arterial, phase 1 [P1]; midvenous, phase 2 [P2]; and late venous, phase 3 [P3] from top to bottom) show left middle cerebral artery segment 1 occlusion. In addition, during phase 1, there is a delay in contrast opacification of the left middle cerebral artery branches compared with the contralateral normal side; however, during phase 2, the contrast opacification appears to be symmetric. This is labeled as “one phase delay.” There is also symmetric “extent” of collaterals (red oval) compared with the contralateral normal side (yellow oval). Overall, this gives an impression of fairly good collaterals. B, Magnified axial MIP image in the second delayed phase (P3) shows the possible clot length (arrow) calculated as the distance between the site of abrupt vessel cutoff (proximal end) and the site of distal vessel opacification, either through slow anterograde or retrograde collateral filling (distal end). This can help in preplanning the length of the stent retriever to be used.