For dose calibration in functional imaging, different precision sources for a same long-lived isotope are used to calibrate, avoiding having to ship one source from one location to another location. A ratio of sensitivities of a gas ion chamber-based dose calibrator at a reference laboratory to the precision source of the long-lived isotope to a source with an isotope to be used for imaging is found. At the clinical site, a measure of the sensitivity of a local gas ion chamber-based dose calibrator to the other source with the long lived isotope and the ratio from the remote gas ion chamber-based dose calibrator are used to determine sensitivity of the local gas ion chamber based dose calibrator to the isotope of the radiopharmaceutical. The bias and corresponding dose for the radiopharmaceutical to be used for imaging a patient are based on activity for the radiopharmaceutical as calibrated.
