INFLUENCE OF CHEST BACKGROUND ON PULMONARY TC-99M-DTPA CLEARANCE IN INTERSTITIAL LUNG-DISEASE

We examined the effect of chest extracellular Tc-99m-diethylenetriamine pentaacetate (DTPA) as a background in the measurement of pulmonary Tc-99m-DTPA clearance in patients with interstitial lung disease (ILD). Eight healthy nonsmokers (HN) and eight patients with ILD were studied. We monitored changes in gamma counts after the inhalation of Tc-99m-DTPA aerosol by using a gamma camera placed over the anterior chest. The rate constant of pulmonary Tc-99m-DTPA clearance (k; %/min) was assessed by calculating the slope of the decrease in the gamma counts. The chest background, estimated by Tc-99m-DTPA intravenous injection, was subtracted from the original data to obtain the corrected DTPA clearance (k(c); % /min). In patients with ILD, k was significantly greater 12.19 +/- 1.03 (SD) %/min; n = 8] compared with HN (0.86 +/- 0.17%/min; n = 8; P < 0.01). In patients with ILD, k(c) was also greater (2.80 +/- 1.15%/min; n = 8; P < 0.01) compared with HN (1.20 +/- 0.12%/min; n = 8). There was no difference in percent underestimation of k between the two groups (29.1 +/- 8.8% for HN, 22.5 +/- 7.9% for patients with ILD). There was a significant correlation between k and k(c) among all subjects (r = 0.987, P < 0.01). We conclude that background causes significant underestimation of pulmonary Tc-99m-DTPA clearance.