Brain Infarction (Stroke)
Arterial occlusion (stroke) results in infarction of brain in the distribution of that vessel. Brain infarct can be plain (80%) or hemorrhagic (20%).
Describe the evolution of changes in CT following CVA.
- An infarcted brain is pale initially.
- Within hours to days, the gray matter becomes congested with engorged, dilated blood vessels and minute petechial hemorrhages.
- The development of edema within the brain produces subtle parenchymal hypo density changes and mass effect.
- These initial edematous changes are characterized as a loss of distinction between gray and white matter (blurring of the gray-white interface).
- As the ischemic insult continues, there is mounting cellular death and a combination of cytotoxic and vasogenic edema occurs.
- This produces areas of parenchymal hypo attenuation in the region of the affected artery.
- Finally, as the infarct progresses, mass effect develops and progresses to sulcal effacement and ventricular compression.
- When an embolus blocking a major vessel migrates, lyses, or disperses within minutes to days, re circulation into the infarcted area can cause a hemorrhagic infarction and may aggravate edema formation due to disruption of the blood-brain barrier.
- After 24 hours, a cerebral or cerebellar infarct is usually a hypo dense area involving both the gray and white matter in a typical vascular distribution.
- The mass effect initially increases due to edema and is at its maximum 3-5 days post infarct.
- The mass effect usually resolves within 7-10 days.
- This hypo attenuation may regress completely and is related to the breakdown of infarcted tissue, cell wall lysis, leaking of intracellular proteins, and reduction of the edema.
- Chronic ischemic infarcts are characterized by hypo density and sharply demarcated margins.
- As the tissue continues to break down and phagocytosis occurs, the parenchyma is replaced with a cystic CSF-filled space.
- A primary intra cerebral hemorrhage damages the brain directly at the site of the hemorrhage by compressing the surrounding tissue.
- Clot formation occurs within 1-2 hours in this area.
- The brain is unable to repair itself, forming only fibrogliotic scar tissue at the site of infarction or hemorrhage.
What imaging procedures are useful to evaluate storke?
CT and MRI are useful modalities in the evaluation of stroke.
- The first diagnostic study which should be performed in patients with suspected stroke is a non contrast CT.
- In the acute setting, CT is used to differentiate ischemia from hemorrhage.
- Evidence of hemorrhage will be a contraindication to the use of thrombolytic or anticoagulant agents.
- Advantages of CT are widespread access, noninvasiveness, and speed of acquisition.
- MRI is more sensitive than CT for the early diagnosis of brain infarction.
Imaging Findings of Stroke
Acute Stroke (up to 7 days):
- MR imaging of the brain is far more sensitive than CT imaging to recognize acute infarction.
- Diffusion wtd. pulse sequence (DW imaging) is the most sensitive MR sequence to demonstrate stroke. This sequence is sensitive to restricted diffusion within the cell from stroke-induced cytotoxic edema and the region of acute stroke is seen as an area of bright signal on DWI . Cytotoxic edema can occur immediately after the initial insult, thus DWI images can reveal the area of acute infarct immediately after the insult.
- Intravascular contrast enhancement, another sign of early stroke .
- Sulcal effacement, gyral edema , loss of gray-white matter interface can occur within 12 hours of stroke.
- Parenchymal contrast enhancement , mass effect and hemorrhage can occur within 1-7 days of insult.
Subacute Infarct (1 to 8 weeks):
- Contrast enhancement slowly decreases in time but can persist for 8 weeks, with decreasing mass effect and abnormal signal intensity.
- Focal area of encephalomalacia.
- Porencephalic dilatation of adjacent ventricle.
- Residual old blood products may be present.
- Accumulation of intracellular fluid
- Focal swelling of brain causes mass effect and shifts structures
- Infarcts undergo evolution with time and has varying appearance.
- Image findings
- Accumulation of intracellular fluid causes swelling of the cortical gyri resulting in sulcal effacement.
- Parenchymal hypodensity can be detected in 45-85% of cases
- Early focal brain swelling occurs in 40% of patients.
- Blurring of the clarity of the internal capsule
- Loss of distinctness of the insular ribbon cortex
- Loss of differentiation between the cortical gray and subjacent white matter.
- Acute intraparenchymal bleeding is readily seen as a white density in CT located within the brain tissue.
- Occasionally, intraparenchymal hemorrhage will extend into the subarachnoid space and ventricles.
- Intraparenchymal bleeding (especially if supratentorial) will cause a shifting of the adjacent structures away from the area of hemorrhage.