"post-surgical"
"post-percutaneous intervention, e.g. pleural drain insertion"
"gas-producing infection(s), e.g necrotising fasciitis"
"There are often striated lucencies in the soft tissues that may outline muscle fibres. If affecting the anterior chest wall, subcutaneous emphysema can outline the pectoralis major muscle, giving rise to the ginkgo leaf sign 2. Often there are displaced rib fractures indicating a cause of the gas."