Pulmonary clearance of 99mTc-DTPA: influence of background activity

We examined the effect of intravascular and tissue accumulation of tracer when measuring pulmonary clearance of sodium pertechnetate-labeled diethylenetriamine pentaacetate (99mTc-DTPA). Pigs were intubated with endobronchial tubes, permitting deposition of an aerosol of 99mTc-DTPA only into the left lung. Scintillation detectors recorded radioactivity separately from one thigh and from the lung and chest wall on the left and right side. 99mTc-DTPA was given intravenously after 30 min, so that the chest counts from the left lung could be corrected for background activity in either the right lung or the thigh. The uncorrected clearance half time (t1/2) mean +/- SE from the left chest was 118.5 +/- 14.4 min. When corrected for background activity in the right chest, the t1/2 was 82.1 +/- 10.5 min, and when corrected for background activity in the thigh, the t1/2 was 80.9 +/- 10.6 min. There was no significant difference between t1/2 corrected by the measurements from the right chest or the thigh, and in four of five animals the corrected t1/2 by either method was significantly different from the uncorrected t1/2 (P less than 0.05). There was no correlation between the uncorrected t1/2 and the magnitude of the required correction. We conclude that correction for intravascular and tissue accumulation of tracer is an important refinement of the technique and can easily be accomplished by measuring accumulation of tracer in the thigh.