For dose calibration in functional imaging, an amount of bias in a dose calibrator measurement of activity is determined using a spectroscopic detector. The bias may then be used to correct dose values for the same isotope used to determine a factory-based sensitivity of the functional imaging system. When local functional imaging systems are calibrated, any difference in sensitivity from the factory measured sensitivity may be due to local dose calibrator bias, so the difference in sensitivity is used to determine a local correction.