Hypoperfusion Intensity Ratio (HIR)

Hypoperfusion Intensity Ratio (HIR) is a cerebral perfusion imaging biomarker used in acute ischemic stroke to quantify the severity of hypoperfusion and assess collateral blood flow.

HIR = Volume of tissue with Tmax >10 seconds / Volume of tissue with Tmax >6 seconds

• Tmax >6 sec: represents all hypoperfused tissue (potentially salvageable penumbra).
• Tmax >10 sec: represents severely hypoperfused tissue (more likely to become infarct core).

• High HIR (~1):
  • A large portion of hypoperfused tissue is severely delayed.
  • Indicates poor collateral flow.
  • Associated with greater infarct growth and worse clinical outcome.

• Low HIR (~0):
  • Most of the hypoperfused tissue has only moderate delay.
  • Suggests good collateral circulation.
  • Associated with better response to reperfusion therapies.

HIR is used alongside other imaging parameters (CBF, CBV, core/penumbra mismatch) to:

• Predict infarct growth and clinical outcomes.
• Select patients for endovascular therapy in extended time windows (e.g., DEFUSE 3, DAWN).
• Guide treatment decisions when standard time-based criteria are insufficient.