sentences:  The oxygenation level dependent signal, known now as BOLD, has become the most popular because of its greater overall contrast/noise, but Kwong showed also that MRI could be used to detect a blood "flow" signal through the apparent change in T1 relaxation rates associated with the replenishment of blood in brain tissue, and demonstrated how the measured signal changes could be used to directly infer a quantitative measurement of the change in brain perfusion.This forms the basis of a second set of modern methods known now as arterial spin labeling, increasingly used when quantification of baseline and changing physiology is required.Kwong's was clearly the first work in this field to apply these methods to human brain mapping.