Purpose: To quantify regional gas trapping in the lung by using computed tomographic (CT)-determined specific gas volume and hyperpolarized helium 3 (3He) magnetic resonance (MR) imaging in a porcine model of airway obstruction. Materials and Methods: Four porcine lungs were removed after sacrifice for unrelated cardiac experiments, for which animal studies approval was obtained. Dynamic expiratory thin-section CT and 3He MR imaging were performed during passive deflation from total lung capacity after obstructions were created with inverted one-way endobronchial exit valves in segmental or lobar bronchi to produce identifiable regions of trapped gas. Changes in specific gas volume were assessed from CT data for defined regions of interest within and outside of obstructed segments and for entire lobes. Helium 3 data were analyzed according to the corresponding regional signal reduction during expiration, compared with the total magnetic moment at each time point. Results: In 4.5 seconds of free collapse, volume decreased by 6% ± 2 (standard error) and 53% ± 3, respectively, in trapped-gas lobes and in unobstructed regions (P < .0001). Specific gas volume changed by 6% ± 2 in areas of trapped gas and decreased by 56% ± 3 in unobstructed regions, from 3.4 mL/g ± 0.2 to 1.5 mL/g ± 0.1 (P < .0001). The 3He signal intensity decreased by 25% ± 6 and 71% ± 3, respectively, in trapped-gas and normal regions (P < .0008). In unobstructed regions, the percentage decreases in specific gas volume and 3He signal intensity were not statistically different from one another (P = .89). Conclusion: The results obtained from the model of gas trapping demonstrate that CT-determined specific gas volume and 3He MR imaging can help identify and quantify the extent of regional trapped gas in explanted porcine lungs.