text: Arterial spin labeling utilizes the water molecules circulating with the brain, and using a radiofrequency pulse, tracks the blood water as it circulates throughout the brain. After a period of time in microseconds (enough to allow the blood to circulate through the brain), a label image is captured. A control image is also acquired before the labeling of the blood water. A subtraction technique gives a measurement of perfusion. In order to increase SNR, collections of control and label images can be averaged. There are also other specifications in the MRI that can increase SNR, like the amount of head coils of the MRI, or a stronger field strength (3 T is standard, but 1.5 T is satisfactory). In order to properly scale the perfusion values into cerebral blood flow units (CBF, ml/100g/1 min), a separate proton density map with the same parameters (but longer TR to fully relax the blood spins) is recommended to be acquired as well. Alternatively, the average control image can be used to generate CBF, which is the case for Phillips pCASL readouts. Usually background suppression is also applied to increase the SNR.  Due to the different variations of each implementations, it is recommended that a large multi-scanner study should design a protocol minimizing the variety of readout methods used by each scanner.
  One study has shown that although there are voxel differences when different readout methods are used, average gray matter CBF are still comparable. Differences in SNR are apparent when each voxel compared, but collectively are negligible.