Figure 7a.  (a) Graph shows the time-activity curve for the case of constant activity in the source organ. Ã is then just the product of fA₀ and the time of interest t. (b, c) Graphs show the activity in a source organ in which the activity is being removed exponentially over time, with effective half-time T_1/2, by a combination of physical decay and biologic washout. In b, the cumulated activity over the limited time of interest, from 0 to t, is the area under the exponential curve. This area is given by the indicated equation. In c, the time of interest has been expanded to t , and the area under the whole exponential curve is given by 1.44fA₀T_1/2.