sentences:  At the same time, methods to directly measure brain perfusion using spin inverted water (arterial spin labeling) were pioneered in animal models by John Detre and Alan Koretsky.All of this was possible without the introduction of blood borne contrast agents.With this background, Kwong reasoned that the concepts of functional mapping by brain perfusion, and the assessment of oxygenation from purely endogenous signals could be combined into an entirely new method of studying human brain activity.