RecruitingDevice studyThe Effects of the Female Hormones on Cerebral PerfusionMeasuring brain perfusion is biased by a inter- and intrasubject variability, caused by physiological and lifestyle factors. In this study, the investigators want to investigate the variations in cerebral perfusion and other brain parameters (grey matter, resting-state brain activity, brain connectivity and white matter diffusion) caused by the female sex hormones and hormonal contraception.