APPROPRIATE BACKGROUND CORRECTION FOR DTPA AEROSOL CLEARANCE

Measurement of the clearance rate of inhaled aerosols of Tc-99m-diethy lenetriamine pentaacetic acid (DTPA) from distal airway to pulmonary c apillary is a sensitive technique for the detection of lung injury. As the solute diffuses across the blood-gas barrier, the concentration i n circulating blood increases, giving rise to a background signal supe rimposed on the signal from residual DTPA in the airway. Background su btraction is conventionally based on the thigh, but this tissue has th e disadvantage in that its composition, in terms of the relative volum es of its extracellular extravascular and intravascular compartments ( a ratio of similar to 4:1), is quite different from that of the lung ( <1:6). With comparison to the thigh, we examined alternative regions f or background, liver, and cranium, which have extravascular-to-intrava scular compartment ratios much closer to these for the lung, to determ ine the most appropriate background for correction of the pulmonary si gnal. From 1 min after intravenous injection of Tc-99m-DTPA, the time- activity curves recorded by a gamma camera over the liver and lung in a group of otherwise normal cigarette smokers decreased up to 30 min a fter injection, with time courses that could essentially be superimpos ed on each other; the curve recorded over the thigh with a separate sc intillation probe continued to increase. The curve recorded over the c ranium had a time course similar to that for the liver and lung. Follo wing aerosol inhalation, the lung clearance rates over the initial 7 m in when background subtraction was used, based on the liver, cranium, and thigh were, respectively, 4.9 +/- 2.9, 4.7 +/- 2.6, and 5.4 +/- 3. 4 (SD) %/min, compared with 4.1 +/- 2.2%/min without subtraction. The corresponding values based on 30 min of data were 3.3 +/- 1.4, 3.4 +/- 1.4, 4.2 +/- 2.3, and 2.8 +/- 1.0%/min. When the liver was used for b ackground, the lung clearance curves were clearly multiexponential, wh ereas thigh correction tended to give curves that were monoexponential or even convex upward on semilogarithmic axes. With an appropriate re gion for background, the true shape of a lung curve can be identified, which permits the study of an intervention on the clearance while it is in progress. The intravenous DTPA, required for calibrating the bac kground regions, can be given before inhalation of the tracer.