What is the difference between Perfusion MRI and Contrast MRI?

Perfusion MRI is a type of contrast MRI but it is more sensitive for detecting blood flow. For both MRIs, a dye is injected into blood circulation before taking the MRI pictures. With the perfusion MRI, the MRI images are processed by computer to highlight where the blood flow is present or absent in the body. In a child with Perthes disease who has a whole femoral head necrosis, most of the femoral head will show an absence of blood flow (dye) in the femoral head in the early stage of the disease. Perfusion MRI can detect how much of the femoral head is involved earlier than regular, non-contrast MRI and x-ray.

Additional read on the subject is available at:
http://www.ncbi.nlm.nih.gov/pubmed/25031369,24590346,23872796,23478799