Figure 4. Perfusion scaled impulse response function (IRF). The perfusion (flow over volume) scaled IRF curve initially rises instantaneously to a plateau while all of the contrast material is in the tissue. The length of the plateau is the minimum transit time. The curve will fall to zero as the contrast material is washed out. Perfusion is given by the height of the plateau, the blood volume by the area under the curve. Mean transit time is blood volume divided by the perfusion.