In EPI the whole picture is collected in one go. This speeds up scanning immensely, allowing dynamic processes to be studied, and overcoming many problems of subject motion. This method is based on magnetically 'labelling' the blood outside the images, and then watching it flow in. This is a technique that shows activation of various parts of the brain. This is a contrast agent.
Contrast agents are chemicals used to highlight variations in certain measurable quantities.
In this case, it is an intra-vascular contrast agent, indicating that it is injected into the blood stream where it alters a quantity of the blood, making it stand out from other body tissues in MR pictures. HDR Hemodynamic Response. This term is gernally used to describe the time course of the signal change that occurs in the MR scans of the brain in response to brain activity. More specifically it is the terms for the time course of the changes in blood flow, blood volume and blood oxygenation that occur in the brain in response to brain activity. In Vivo Viscometry Measuring viscosity in the body.