Potential recuperation ratio (PRR)

Potential recuperation ratio (or Lausanne Stroke Index) is a simple quantitative measure of the relative size of ischemic penumbra compared to the non-salvageable ischemic core region in acute stroke. To calculate the PRR, a perfusion CT with infarct and penumbra maps is required. To do this, first, maps of cerebral blood flow (CBF) and volume (CBV) have to be obtained, based upon which the infarct and penumbra map can be created: low CBF and CBV = infarct core; low CBF and increased CBV = penumbra ¹.

PRR can be calculated with the following formula, though this is usually performed automatically by the software:

PRR = penumbra size / (penumbra size + infarct size) ¹

Generally, a higher index indicates a greater relative size of penumbra and therefore better prognosis. 

While PRR has been originally proposed as a prognostic indicator of therapeutic efficacy, later on, concerns had been raised about its high variability, depending on scanner and software equipment ². Some studies found that while in general, a greater calculated infarct core predicts a worse functional outcome, perfusion CT-derived parameters are unable to reliably identify patients that could benefit from endovascular therapy ³. However recently other clinical trials have found high relative penumbra size to be not only a good predictor of functional outcome but also to be suitable for selecting patients for endovascular therapy ⁴.