GFR Determination by a Modification of the Gates Method: The Conventional Renal Examination with a Semi-Automated GFR Measurement

The conventional radionuclide renal examination using a 1020-mCi dose of technetium-99m t mTc) DTPA (Pentetate), conducted over a 30-60-min duration, does not accommodate the Gates method for GFR determination. Simple modifications to the Gates technique will, however, permit a GFR determination to be made without sacrificing the components of a complete conventional renal examination, and will not significantly alter the technologist's examination routine or time commitment. These modifications allow the Gates method to be used on virtually any gamma camera/collimator combination with an accompanying computer system, dose calibrator, and a lead slab syringe attenuator. With the modifications, GFR determinations can be obtained using doses of tracer as high as 10-20 mCi, while eliminating the direct gamma camera counting of the residual activity in the postinjection syringe. The net injected syringe counts are calculated using the gamma camera counts from the preinjection syringe attenuated through lead and the dose calibrator's preand postinjection measurements of activity to determine the fraction of injected counts from the measured syringe dose. Excellent results are obtained when the gamma camera counts of the syringe activity using the original Gates method are compared to the counts derived from the modified technique using the lead slab attenuator and the dose calibrator (r > 0.99). We have found that this particular modification of the Gates method allows for a full examination by removing the dose and time restrictions imposed by the original Gates technique.