Extrapulmonary radioactivity in lung permeability measurements.

The pulmonary clearance rate of aerosolized and deposited [99mTc]DTPA is used to assess pulmonary epithelial permeability to solutes. To evaluate whether it is necessary to correct these measurements for radioactivity in the chest wall and pulmonary vasculature five patients with no ventilation to one hemithorax were studied. After inhaling a submicronic aerosol of [99mTc]DTPA a gamma camera measured count rates over both hemithoraces for 20 min. The observed T1/2 for clearance from the normal hemithorax was 56 min (range 18-115 min), and when this was corrected for chest wall contribution (derived from the abnormal hemithorax) the average T1/2 was 52 min (range 17-107 min). To simulate infinitely permeable lungs we measured thoracic radioactivity in two adults and six children who were injected i.v. with a known amount of [99mTc]DTPA. The count rate over the thorax was only 8.1% (range 2.7%-11%) of that obtained when a similar amount of aerosolized [99mTc]DTPA was deposited in the lungs during a subsequent study. We conclude that it is not necessary to correct for nonpulmonary epithelial radioactivity during the measurement of [99mTc]DTPA clearance rate from the lungs.