Effect of restricted water exchange on cerebral blood flow values calculated with arterial spin tagging: a theoretical investigation.

Arterial spin tagging techniques originally used the one-compartment Kety model to describe the dynamics of tagged water in the brain. The work presented here develops a more realistic model that includes the contribution of tagged water in the capillary bed and accounts for the finite time required for water to diffuse across the blood-brain barrier. The new model was used to evaluate potential errors in cerebral blood flow values calculated using the one-compartment Kety model. The results predict that if the one-compartment Kety model is used to analyze arterial spin tagging data the observed grey matter cerebral blood flow values should be relatively insensitive to restricted diffusion of water across the capillary bed. For instance, the observed grey matter cerebral blood flow should closely approximate the true cerebral blood flow and not the product of the extraction fraction and the cerebral blood flow. This prediction is in agreement with recent experimental arterial spin tagging results.