We examined the effect of chest extracellular 99mTc-diethylenetriamine pentaacetate (DTPA) as a background in the measurement of pulmonary 99mTc- 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 99mTc-DTPA aerosol by using a gamma camera placed over the anterior chest. The rate constant of pulmonary 99mTc-DTPA clearance (k; %/min) was assessed by calculating the slope of the decrease in the gamma counts. The chest background, estimated by 99mTc-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 [2.19 ± 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 99mTc-DTPA clearance.
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