Arterial Spin Labeling Perfusion Imaging

Instead of administering exogenous contrast to label flowing blood, arterial spin labeling labels blood protons with a magnetic tag. This technique has the advantages of being both completely noninvasive and able to obtain quantitative measures of CBF directly.

Technique. This technique applies a short-lasting magnetic tag to flowing blood just before it enters the brain.[33] An inversion recovery pulse causes protons to be flipped 180º out of phase with protons within the brain par enchyma. As tagged blood flows into the brain, its change in orientation allows it to be identified from protons within the brain parenchyma. Multiple repetitive images of the brain both with and without application of the magnetic tag are obtained to allow signal from the background tissue to be separated from flowing blood. From these data, it is possible to quantify CBF accurately in normal subjects with better spatial and temporal resolution than other perfusion techniques such as PET or Xenon-CT.[34]

Imaging of Cerebral Ischemia. Arterial spin labeling is less successful when applied to subjects with cerebral ischemia. This situation is largely due to the short half-life of the applied magnetic tag, which reflects the T1 value of blood. In cerebral ischemia, where perfusion is slow, the magnetic tag may wear off before it reaches the area of interest. Under these circumstances, signal will be lost and CBF will be underestimated. Further experimental studies are required before this technique can be used routinely in clinical practice.



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