Arterial Spin Labeling (ASL) is a powerful technique to evaluate cerebral blood flow. To analyze hemodynamics effects with ASL, multiples acquisitions over the time are realized, which is called multiphase ASL. In conventional multiphase ASL methods, the later phases has low contrast to noise ratio, so it becomes difficult to analyze it. This study purposes a solution to this problem, through a modulation in the acquisition flip angle. With this technique, the flip angle of all phases follows a modulation equation, so that the ASL signal over the phases becomes nearly constant.