MRI in Acute Ischemic Infarction Diagnosis

MRI in Acute Ischemic Infarction

• MRI has higher sensitivity and specificity than CT in diagnosing acute ischemic infarction in the first few hours after onset.

Early Hyperacute Stage

• Within minutes of arterial occlusion, diffusion-weighted imaging (DWI) demonstrates increased signal and reduced ADC values.
• Affected parenchyma appears normal on other sequences, but changes in flow will be detected (occlusion on MRA) and the thromboembolism may be detected.
• Cortical contrast enhancement may be seen as early as 2-4 hours if infarction is incomplete.

Late Hyperacute Stage

• After 6 hours, high T2 signal will be detected, initially more easily seen on FLAIR.
• T1 hypointensity is only seen after 16 hours and persists.

Acute Stage

• During the first week, infarcted parenchyma continues to demonstrate high DWI signal and low ADC signal.
• Infarct remains hyperintense on T2 and FLAIR, with T2 signal progressively increasing during the first 4 days.
• T1 signal remains low, but some cortical intrinsic high T1 signal may be seen as early as 3 days after infarction.
• After day 5, the cortex usually demonstrates contrast enhancement.
• Less common patterns of enhancement include arterial enhancement and meningeal enhancement.

Hemorrhage

• Hemorrhage is most easily seen on susceptibility-weighted imaging (SWI) and is usually seen after 12 hours and within the first few days.

Subacute Stage

• ADC demonstrates pseudonormalization typically occurring between 10-15 days.
• DWI remains elevated due to persistent high 2/FLAIR signal (T2 shine through) unless hemorrhage or cystic encephalomalacia.
• Cortical enhancement is usually present throughout the subacute period.
• T1 weighted sequences continue to show hypointensity throughout the area of infarct with cortical intrinsic high T1 signal.

Chronic Stage

• T1 signal remains low with intrinsic high T1 in the cortex if cortical necrosis is present.
• T2 signal is high.
• Cortical contrast enhancement usually persists for 2 to 4 months.
• ADC values are high.
• DWI signal is variable, but as time goes on, signal progressively decreases.

Transcranial Doppler Ultrasound

• Transcranial Doppler ultrasound can be used for the diagnosis of intracranial vessel occlusion and differentiation between ischemic and hemorrhagic stroke.
• Diagnostic criteria for occlusion of an isolated vessel include:
  • Complete absence of color flow Doppler signals.
  • Absence of pulsed-wave Doppler signals.
  • Concurrent adequate visualization of surrounding parenchyma and vessels.

Treatment and Prognosis

• Acute treatment focuses on prompt application of reperfusion therapies, including intravenous or intra-arterial thrombolysis.