A sequence of magnetic resonance images taken at 1-second intervals after a woman received an intravenous blood marker progresses from light to dark to light as her cerebral blood flow carries the marker into her brain and washes it back out again. The total marker present in all the images, measured as their intensity (darkness), reflects total cerebral blood volume. To study the hemodynamic effects of cocaine, investigators compared two such sequences, obtained before and after administration of the drug, during each menstrual period.