====== 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.

===== Definition =====
**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).

===== Interpretation =====
  * **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.

===== Clinical Relevance =====
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.