Perfusion MRI or perfusion-weighted imaging (PWI) is perfusion scanning by the use of a particular MRI sequence. The acquired data are 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.
There are 3 main techniques for perfusion MRI:
Dynamic contrast-enhanced (DCE) imaging gives information about physiological tissue characteristics. For example, it enables analysis of blood vessels generated by a brain tumor. The contrast agent is blocked by the regular blood-brain barrier but not in the blood vessels generated by the tumor. The concentration of the contrast agent is measured as it passes from the blood vessels to the extracellular space of the tissue (it does not pass the membranes of cells) and as it goes back to the blood vessels.
The contrast agents used for DCE-MRI are often gadolinium-based. Gadolinium injection causes the relaxation time to decrease, and therefore images done after gadolinium injection have higher signal. First a regular T1-weighted MRI scan is done (with no gadolinium), then gadolinium is injected (usually at a dose of 0.05-0.1 mmol/kg) and another T1-weighted scan is done. By comparing the values of T1 in both scans, for each voxel, it is possible to identify permeable blood vessels and tumor tissue. In tissues with healthy cells or a high cell density, gadolinium re-enters the vessels faster since it cannot pass the cell membranes. In damaged tissues or tissues with a lower cell density, the gadolinium stays in the extracellular space longer.