WCN 2021 | Imaging approaches for acute ischemic stroke

Basically when we think of imaging for acute ischemic stroke patients, there are two choices: CT or MRI. Most places do CT because MRI is not readily available. At my hospital, that’s what we use. I know in the United States that most other hospitals are doing the same. We have a stroke imaging protocol that the Radiology Department has agreed with the Stroke Service on. We quickly obtain a plain head CT to rule out hemorrhage and to get some approximation of the extent of the ischemic injury. Then we also do, in pretty much all cases, a CT angiogram of the head and neck vessels, which allows us to identify whether there’s a large vessel occlusion, which is amenable to having thrombectomy. Now in many cases, we also add on CT perfusion, which gives an approximation of the extent of the ischemic injury and what percentage of that is actually already irreversibly damaged or what we call the ischemic core.

I would say in the majority of our acute stroke patients, we’re doing all three: a head CT, CT angiogram, and CT perfusion. There are many centers that don’t do the CT perfusion as readily as we do at our center, but I think that it does have some potential value especially in a later time window for identifying thrombectomy candidates. Before I get into specifics about identifying candidates, let’s also talk briefly about MR. With MR, we have a brain MRI, which has multiple sequences. For acute stroke, the most important sequence is diffusion imaging. Because with diffusion imaging, you can readily see the region of ischemia and it actually is much better than CT for identifying acute ischemic injury.

We also have MR angiography, which is pretty similar to CT angiography for identifying large vessel occlusion. On MR, we also have another modality called MR perfusion imaging, which focuses on blood flow. That can be used to identify the regions of the brain that are not being perfused well. If we overlap the diffusion lesion with the perfusion lesion, the region where there’s a mismatch, in other words, there’s a perfusion abnormality, but no diffusion abnormality would approximate, would approximate what’s called the ischemic penumbra or tissue at risk of becoming infarcted. Now the diffusion lesion is considered by many to be the ischemic core, but that’s not exactly true. Because early after stoke onset with rapid reperfusion, you can actually salvage some of that tissue that’s abnormal in diffusion.