Diagnosis of stroke: CT and MRI

1. Why is a CT without contrast ordered?
2. How does the appearance of an infarct evolve on CT scan?
3. How does SAH appear on CT?
4. What is the benefit of using MRI over CT?
5. What is seen on the MRI for a cerebral infaction?
6. What is seen on the MRI for intracerebral hemorrhage?
7. What is seen on the MRI for SAH or IVH?

Answers:
1. If a bleed is suspected, contrast may be confused with bleeding.
2. Initially normal for first few hours, ill-defined hypodensity (black) after 24-48 hrs, edema (hypodense) better defined after 3-4 days. Hemorrhage in infarcted area appears as hyperdense mass within hypodense edema.
3. 90% of CTs visualize hyperdense area in basal cisterns or around brainstem within 4-5 days.
4. MRI is more sensitive than CT for small infarcts and posterior fossa infarcts. Ischemic edema detected earlier with MRI (within hours).
5. Early there is increased white signal on T2 images, more pronounced at 24hrs to 7 days. Chronically (>21 days) there is decreased T1 and T2 signals.
6. With acute hemorrhage, there is decreased (black) signal on T1 and T2 image. As hemorrhage ages, it develops increased signal on T1 and T2.
7. Acute, there is low signal (black) on T1 and T2.