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Background
A variety of abnormalities are associated with the release of free air escaping from the lungs, airways, abdomen, and the spine. Once released, free air dissects through tissue planes forming abnormal collections, or it can migrate in other anatomic regions causing symptoms. Medical procedures, blunt trauma, unaccustomed positive pressure activities, and infection are among the conditions associated with pneumatosis in the craniocervical and thoracic area, and the spine. As such, there are cases in the literature reporting the sporadic collection of free air or other gas in the cervicothoracic region and the spinal epidural space. Included among the conditions predisposing to pneumatosis are surgery, fractures, underwater diving, strenuous cough, bronchial asthma, inflammatory bowel disease, and diabetic ketoacidosis. In pneumomediastinum (also known as mediastinal emphysema), extraluminal air accumulates in the mediastinum causing increased intrapulmonary pressure that manifests with chest pain and labored breath. On chest radiographs and CT scans, the abnormal collection of air is readily seen in the mediastinum, and it sometimes outlines the heart. Pneumomediastinum usually resorbs slowly. Pneumorrhachis is defined as intraspinal air collection, either epidural or subarachnoid, which may be benign and self-resolving, or it can have an ominous prognosis. Subcutaneous emphysema indicates the entrapment of air in subcutaneous soft tissue that is usually self-limited. Interestingly, these pneumatoses may be detected on imaging studies either as isolated conditions or in combinations that reflect the etiology, i.e., the underlying anatomic disorder and pathological mechanism.

Educational Goals / Teaching Points
Following a brief review of the most common routes by which free air may occur, we focus on the imaging diagnosis of thoracic, spinal, and craniocervical pneumatosis manifesting as pneumomediastinum, pneumorrhachis, and subcutaneous emphysema.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
Air is radiolucent, and as such it can be readily identified on radiographs. Gas has a known Hounsfield unit number less than zero. Unlike radiography, CT can detect small foci of gas or extensive collections of air that may be present in a single body compartment or different sites. Although MRI is not necessary in all cases of pneumatosis, major indications involve the detection of air in the spine.

Conclusion
In the clinical context of pneumatosis, radiography provides important information that can help guide patient management. CT is most useful in diagnosing the presence of free air in a body compartment or in the soft tissues. In addition to the documentation of gas, CT provides information regarding the regional extent of pneumatosis and the presence or absence of related abnormalities, such as trauma and infection. MRI is superior to both imaging methods in allowing assessment of the spinal canal and the spinal cord.