Perfusion Imaging

Cerebral ischemic symptoms occur when blood flow drops below the level required for normal neuronal functioning. Normal cerebral function can be restored either naturally or with medical intervention if hypoperfusion is relatively brief and mild. The effects of arterial hypoperfusion on the brain parenchyma depend not only on the degree of proximal vascular occlusion but also on the presence of collateral vessels. Such vessels are very important in maintaining tissue perfusion during major vessel occlusion and play an important role in preserving tissue viability during cerebral is ischemia.

In contrast to MR angiography, which detects blood flow in large and medium-sized vessels, perfusion imaging measures flow in the distal capillary system. Although distal hypo perfusion can be implied from the proximal vascular occlusion demonstrated on angiography, collateral flow cannot be assessed accurately with this technique. MR perfusion imaging detects flow arising from both the proximal artery and collateral vessels, providing an accurate assessment of tissue oxygen and nutrient supply.

This novel form of MR imaging can define the rate at which a volume of blood is delivered to a unit mass of tissue. A small quantity of blood is first labeled using either intravenous contrast or a magnetic tag. As the labeled blood passes through the brain, repetitive MR sequences are performed every few seconds. The temporal effects of the labeled blood on multiple small volumes of tissue, or voxels, can then be assessed. This technique requires the use of ultra-fast MR sequences, such as echo-planar imaging, to perform rapid and repetitive imaging.


http://www.thebarrow.org/Education_And_Resources/Barrow_Quarterly/205119