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Gamma camera renography with 99Tcm-DTPA: the impact of variations in input plasma curve on estimated GFR

In an attempt to improve the reliability in estimating glomerular filtration rate (GFR) from gamma camera renography without blood sampling, we have investigated the impact of plasma input region of interest (ROI) size and location and the effect of the time delay between the circulation peak in the renal curves and in the plasma curve. An uptake index (UI) was calculated using plasma ROIs of five different sizes (8–56 cm2) and compared with measured 51Cr-EDTA clearance. The spread of the estimated GFR was not affected by the variations in plasma ROI size but different relations were obtained between UI and GFR. The plasma ROI markedly affected the estimated GFR (a deviation of 3.5 cm from the maximal region led to an overestimation of the GFR of 36%). Furthermore, the most reliable GFR estimate (S.D. 10.5 ml min-1) was achieved by including the whole circulation peak in the calculation time interval of the time delay-adjusted renogram, except the first frame. The time delay adjustment also enabled the calculation of a factor a, which represents the ratio between the blood volume within the renal ROI and that within the plasma input ROI. An abnormal α/UI ratio would suggest an unreliable GFR estimation.