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Figure 1. Schematic of progression of enhancement as defined with time-signal intensity curves. Curve A shows arterial enhancement, with rapid progression of signal intensity that reaches a maximum value very shortly after arrival of the bolus of contrast material and declines immediately thereafter (washout). Curve 1 is exponential, parallel to the first phase of arterial enhancement, followed by a maximum plateau or a slight increase in enhancement. Curve 2 represents gradual progression of enhancement over time, between the level of early arterial enhancement and the prolonged reference tissue enhancement. No part of this curve parallels the early arterial enhancement. Lower absolute enhancement was obtained compared with that shown by the exponential curve. Curve 3 shows no or slightly progressive enhancement equal to that of the reference tissue. The signal intensity of this curve depends on the type of reference tissue used.