(CASL) In order to take spins in a different magnetic state to that of the static tissue a train of RF pulses repeatedly saturates blood water spins flowing through the neck. The saturated (or 'labeled') spins flow into the brain and, assuming water is a freely diffusible tracer, exchange completely with brain tissue water, thus reducing the overall tissue magnetization. A steady state progresses where the regional magnetization in the brain is directly related to cerebral blood flow.