Ischemic strokes are not visible on noncontrast CT until at least how many hours after brain infarction?

Ischemic strokes are not visible on noncontrast CT until at least how many hours after brain infarction?

• A. 3 hours
• B. 6 hours
• C. 12 hours
• D. 24 hours

Answer: (B)

Early ischemic changes on a noncontrast CT are often not visible because the initial injury causes only subtle shifts in tissue density that CT doesn’t pick up right away. It takes time for cytotoxic edema to develop and for loss of gray–white differentiation to become apparent. In the hyperacute period, the scan can look normal even though a stroke is present. Generally, signs of ischemia begin to appear around six hours after onset, and become more evident over the next several hours to days. That’s why a noncontrast CT is often normal in the first few hours, but changes are more likely to be seen by about six hours and fuller infarct visibility follows later. This is also why MRI with diffusion is more sensitive for very early ischemia, and why clinical judgment and sometimes additional imaging guide acute management when the CT is normal.

Early ischemic changes on a noncontrast CT are often not visible because the initial injury causes only subtle shifts in tissue density that CT doesn’t pick up right away. It takes time for cytotoxic edema to develop and for loss of gray–white differentiation to become apparent. In the hyperacute period, the scan can look normal even though a stroke is present. Generally, signs of ischemia begin to appear around six hours after onset, and become more evident over the next several hours to days. That’s why a noncontrast CT is often normal in the first few hours, but changes are more likely to be seen by about six hours and fuller infarct visibility follows later. This is also why MRI with diffusion is more sensitive for very early ischemia, and why clinical judgment and sometimes additional imaging guide acute management when the CT is normal.