A decay chart describes the fraction of radioactivity left from a specific isotope after a certain amount of time has passed. The initial point in time (t=0) is called the calibration time. Usually, decay tables also denote the fraction of radioactivity present before the calibration time. Decay charts provide a quick way to calculate the amount of radioactivity in a substance. Which makes it easy to draw the proper dose of a radiopharmaceutical for a patient, no matter long it has been since the calibration of the source.