text: Perfusion-weighted imaging (PWI) is performed by 3 main techniques:
  * Dynamic susceptibility contrast (DSC): Gadolinium contrast is injected, and rapid repeated imaging (generally gradient-echo echo-planar T2 weighted) quantifies susceptibility-induced signal loss.
  * Dynamic contrast enhanced (DCE): Measuring shortening of the spin–lattice relaxation (T1) induced by a gadolinium contrast bolus.
  * Arterial spin labelling (ASL): Magnetic labeling of arterial blood below the imaging slab, without the need of gadolinium contrast.
  The acquired data is then postprocessed to obtain perfusion maps with different parameters, such as BV (blood volume), BF (blood flow), MTT (mean transit time) and TTP (time to peak).
  In cerebral infarction, the penumbra has decreased perfusion. Another MRI sequence, diffusion-weighted MRI, estimates the amount of tissue that is already necrotic, and the combination of those sequences can therefore be used to estimate the amount of brain tissue that is salvageable by thrombolysis and/or thrombectomy.