Estimating the plasma time-activity curve during radionuclide renography.

Quantitative analysis of the radionuclide renogram requires an estimate of the plasma time-activity curve. This estimate is usually obtained from an externally detected blood-pool curve that is calibrated with a single plasma sample. However, external probes detect activity in the extravascular as well as the intravascular space. This may lead to significant errors in estimating the plasma time-activity curve. A method for overcoming this problem by using a continuously varying calibration factor based upon multiple plasma samples was therefore evaluated. Externally detected blood-pool curves were used to estimate the plasma time-activity curves obtained from rabbits during radionuclide renography. Estimates obtained using the externally detected curves calibrated with a constant calibration factor were found to be significantly biased, while estimates obtained using the externally detected curves calibrated with the continuously varying calibration factor were not.