CT in acute ischemic stroke
Hyperacute phase: 0-24 hours
- Early hyperacute phase: 0-6 hours. Late hyperacute phase: 6-24 hours
- A hyperdense vessel may be the only finding on unenhanced brain CT in the early hyperacute phase
- Loss of grey-white matter differentiation
- Hypoattenuation and “disappearance” of deep nuclei
- Hypoattenuation of cortex and parenchymal swelling results in gyral effacement (eg “insular ribbon” sign)
- CTA may reveal vessel occlusion, dissection or thrombosis
- Perfusion CT evaluates the cerebral blood flow CBF, the cerebral blood volumeCBV and the mean transit time MTT. These three parameters are related as follows:
- All ischemic tissue will show reduced MTT
- The irreversibly injured infarct core will show reduction in both CBF and CBV
- The potentialy salvable ischemic penumbra will show mismatch between reduced CBF but normal or even transiently increased CBV
- 15-20% of large MCA infarcts will show reduced CBF in the contralateral cerebellum (crossed cerebellar diaschisis)
- The CT hyperdense MCA sign should not be used as a criterion to withhold IV alteplase from patients who otherwise qualify.
- Multimodal CT and MRI, including perfusion imaging, should not delay administration of IV alteplase
- It may be reasonable to incorporate collateral flow status into clinical decision making in some candidates to determine eligibility for mechanical thrombectomy.
Acute phase: 24 hours - 1 week
- Hypoattenuation and development of cytotoxic edema become more marked
- Mass effect from the edema may result in parenchymal herniation or vessel compromise and further infarct development
- Ischemic vascular damage may result in hemorrhagic transformation in 20-25% of cases
- When CTA has been performed, contrast extravasation caused by a disrupted blood brain barrier may mimic hemorrhagic transformation. Dual-energy CT scanning helps in differentiating the two.
- Patchy enhancement may be seen on contrast-enhanced CT. It appears ~2 days after onset, peaks at 2 weeks and disappears by 2 months (“2-2-2 rule”)
Subacute phase: 1-3 weeks
- The edema starts to subside 7-10 days after stroke onset
- Cortical or deep grey substance small petechial hemorrhages (not to be confused with hemorrhagic transformation) may develop which increase the attenuation and contribute to the ”fogging phenomenon”
- Fogging phenomenon is the appearance of infarcted grey matter as normal or near normal on non-enhanced CT usually performed in the subacute phase of the infarct. Factors that may contribute to this phenomenon include:
- Migration of lipid-laden macrophages into the infarcted tissue
- Decrease in edema
- Development of small petechial hemorrhages
- Capillary proliferation
If this presents a diagnostic problem, an MRI scan or contrast-enhanced CT will promptly demarcate the infarcted region.
Chronic phase: >3 weeks
- Sharp demarcation of infarcted area
- Dystrophic calcification may occur
- Secondary atrophy in the affected hemisphere or contralateral cerebellum
- Wallerian degeneration may result in volume loss in the ipsilateral cerebral peduncle.
- Infarcts older than 2 weeks should not enhance on contrast-enhanced CT scans