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. O n the day after perfusion (Q) scintigraphy with 5 mCi Tc-99m MAA, radioaero sol inhalation scintigraphy was performed using 45 mCi Tc-99m DTPA. Immedia tely and 45 minutes after the inhalation, early and delayed inhalation imag es (El and DI, respectively) were obtained. Group 1 included 11 patients wi th mismatched defects who had a high probability of pulmonary embolism acco rding to the Q/EI scan results. Group 2 included 10 patients with matched d efects who had a low probability of PE. Contralateral normal lungs of 7 pat ients in group 2 served as controls (group 3). In groups 1 and 2, regions o f interest were drawn over the mismatched and matched perfusion defects whe re they were best visualized, and this region of interest was mirrored to t he same region on El 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 cal culated using the following formula: early counts - late counts / early cou nts x 100. Results: The average percentage clearances for the three groups were as fol lows: group 1, 37% +/- 10%; group 2, 21% +/- 4%; group 3, 24% +/- 7%. Diffe rences between groups 1 and 3 were significant, as were those between group s 1 and 2 (P < 0.05). Patients with mismatched perfusion defects had increa sed DTPA clearance compared with the control group and those with matched d efects. Conclusions: Vascular occlusion may lead to impairment of the alveolar-capi llary barrier and consequently an increase in the clearance of Tc-99m DTPA aerosol in embolized regions. Immediately after inhalation, Tc-99m DTPA ima ging should be started in the projection where perfusion defects are best s een to avoid potential misinterpretation of pulmonary embolism.