Use of Imaging to Select Patients for Late Window Endovascular Therapy

Gregory W. Albers, MD


Stroke. 2018;49(9):2256-2260. 

In This Article

Evolution of Ischemic Core Estimates Over Time on CTP

One of the most important aspects of CTP to be aware of is that the maps reflect the hemodynamics at the moment that the scan is done. CTP does not provide information about what happened to the patient many hours before the scan. In addition, because ischemic core estimates are based on severe reductions in blood flow, once the blood flow abnormality has improved or resolved, CTP is no longer able to estimate the ischemic core. Figure 1 shows an example of a patient who presents with a left middle cerebral artery occlusion and is scanned at 2 hours after symptom onset. After reperfusion, the ischemic core is no longer visible because the CBF is no longer substantially reduced.

Figure 1.

Patient with a left middle cerebral artery (MCA) occlusion. On the baseline scan, the noncontrast computed tomography (CT) is normal. There is a small, deep region in the brain that has low cerebral blood flow (CBF) and is identified in pink (which denotes that the CBF is <30% of normal). There is a large Tmax >6 s region, shown in green, reflecting the delayed arrival of the contrast agent in the MCA territory. After thrombectomy, there is a substantial increase in the CBF. The green Tmax lesion disappears and so does the pink CBF lesion; although the tissue is irreversibly injured, there is still blood flow in the irreversibly injured region after reperfusion. Therefore, the ischemic core is no longer visible on the CBF map obtained 24 hours after reperfusion.

Under some circumstances, leptomeningeal reperfusion occurs even if recanalization does not. If spontaneous leptomeningeal reperfusion occurs after the tissue has already become irreversibly injured, CTP may not be able to identify the ischemic core in the region where leptomeningeal reperfusion has occurred because the CBF may no longer be severely reduced. Figure 2A shows the CTP mismatch map in a patient who was imaged 24 hours after last known to be well. At the time the stroke occurred, CBF was severely reduced in most of the left middle cerebral artery territory; however, by the time the CTP scan was performed, leptomeningeal collaterals had been recruited. Unfortunately, these collaterals came too late; the tissue was already irreversibly injured. However, because the CBF was no longer low in the frontal region, CTP was unable to identify the core in that area. This reinforces the notion that CTP does not image dead tissue, it can demonstrate low blood flow that is likely to be associated with tissue death.

Figure 2.

This 75-year-old man was last well 24 hours before presentation with a left middle cerebral artery (MCA) occlusion. His computed tomography (CT) perfusion mismatch map (A) demonstrates regions of severe reduction in cerebral blood flow (CBF) in the posterior MCA territory of 34 mL (shown in pink) and significant hypoperfusion of 78 mL (shown in green) resulting in a mismatch of 44 mL. B, shows the noncontrast CT scan coregistered to the perfusion images and demonstrates substantial volume of mild to moderate hypodensity in most of the MCA territory (yellow outline) representing a large subacute infarct. The CBF map demonstrates there has been substantial recruitment of leptomeningeal collaterals into the anterior region of infarct, which explains the underestimation of the ischemic core on the mismatch map. rCBV indicates relative cerebral blood volume.

CTP maps are not sensitive for detecting brain hemorrhage. Therefore, a close evaluation of the noncontrast CT is essential to ensure that subacute or chronic infarcts, as well as acute hemorrhage, are not missed.