Quantifying hyperpolarized 129Xe MRI of pulmonary ventilation and gas exchange requires accurate segmentation of the thoracic cavity. This is typically done either manually or semi-automatically using an additional proton scan volume-matched to the gas image. These methods are prone to operator subjectivity, image artifacts, alignment/registration issues, and SNR. Here we demonstrate using a 3D convolutional neural network (CNN) to automatically and directly delineate the thoracic cavity from 129Xe MRI alone. This 3D-CNN uses a combination of Dice-Focal, perceptual loss, and training with template-based data augmentation to demonstrate thoracic cavity segmentation with a Dice score of 0.955 vs. expert readers.