Influence of chest background on pulmonary ^99mTc-DTPA clearance in interstitial lung disease

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.