Lung Clearance of Inhaled 99mTc-DTPA in the Dog

We studied the reproducibility of measuring an index of permeability of respiratory epithelium in dogs using aerosolized 99mTc-DTPA (diethylenetriaminepentaacetate). The method uses a gamma camera to measure the rate of clearance of soluble radioactive aerosol deposited in the lung. A solution of 99mTc-DTPA in normal saline was aerosolized by an ultrasonic nebulizer. Eleven anesthetized dogs breathing spontaneously inhaled the resulting droplets for 2 min. Mass median aerodynamic diameter of the droplets was 4.4 micron with a geometric standard deviation of 2.1. Clearance from the lung was monitored by quantitative gamma camera imaging for up to 2 h. For a 60-min observation period, the biological half-life for clearance of 99mTc-DTPA from both lungs was 66 +/- 11 (SD) min. Apical regions cleared significantly slower than basal regions, probably because of a larger portion of bronchial tissue in the apical region of the dog's lung. The best reproducibility of absorption of 99mTc-DTPA in the dog was obtained from basal regions and peripheral zones of the lung within 30 min after inhalation of the radioaerosol.