Clearance of Tc-99m DTPA aerosol in mismatched and matched pulmonary perfusion defects

PURPOSE: To evaluate clearance changes of Tc-99m DTPA aerosol in mismatched and matched pulmonary perfusion defects. MATERIALS AND METHODS: Twenty-one patients (14 women, 7 men; mean age, 51 +/- 14 years) with possible pulmonary embolism were included in the study. On the day after perfusion (Q) scintigraphy with 5 mCi Tc-99m MAA, radioaerosol inhalation scintigraphy was performed using 45 mCi Tc-99m DTPA. Immediately and 45 minutes after the inhalation, early and delayed inhalation images (EI and DI, respectively) were obtained. Group 1 included 11 patients with mismatched defects who had a high probability of pulmonary embolism according to the Q/EI scan results. Group 2 included 10 patients with matched defects who had a low probability of PE. Contralateral normal lungs of 7 patients in group 2 served as controls (group 3). In groups 1 and 2, regions of interest were drawn over the mismatched and matched perfusion defects where they were best visualized, and this region of interest was mirrored to the same region on EI and DI images. For the control group, this was done in the contralateral normal lung. Mean counts in each region of interest were used for quantitative analysis, and the percentage clearance ratio was calculated using the following formula: early counts - late counts/early counts x 100. RESULTS: The average percentage clearances for the three groups were as follows: group 1, 37% +/- 10%; group 2, 21% +/- 4%; group 3, 24% +/- 7%. Differences between groups 1 and 3 were significant, as were those between groups 1 and 2 (P < 0.05). Patients with mismatched perfusion defects had increased DTPA clearance compared with the control group and those with matched defects. CONCLUSIONS: Vascular occlusion may lead to impairment of the alveolar-capillary barrier and consequently an increase in the clearance of Tc-99m DTPA aerosol in embolized regions. Immediately after inhalation, Tc-99m DTPA imaging should be started in the projection where perfusion defects are best seen to avoid potential misinterpretation of pulmonary embolism.