Schematic diagram illustrates the continuous arterial spin labeling (CASL) method. This technique is very similar to positron emission tomography (PET) scanning, but is entirely non-invasive and does not require injections or radioactivity. In this elegant technique, water molecules in the patient's own blood are 'tagged' by the functional magnetic resonance imaging (fMRI) magnet and used as the natural contrast agent in order to measure cerebral blood-flow.

Image Courtesy: Jiongjiong Wang, PhD, and John A. Detre, MD, University of Pennsylvania School of Medicine