Gas

Various gases, such as room air, carbon dioxide, and oxygen, have been used as contrast agents in myelography or arthrography.

Gas collections may also be seen on radiographs in pathologic conditions. Submucosal or subserosal gas cysts occur within the bowel wall (pneumatosis) in connective tissues diseases, especially scleroderma. Gas may also be present within the osseous or soft tissue component of an intraosseous ganglion.

At sites of negative pressure in the spine produced by abnormal spaces or clefts, gas collections appear and produce a characteristic radiographic finding, the vacuum phenomenon or Knutsson phenomenon. Although this appearance is a reliable sign of degenerative disease, as such collections are rare in patients with infection, vacuum phenomena are of various types and may be seen in several diseases or conditions, such as intervertebral osteo chondrosis, spondylosis deformans, Schmorls nodes, intraspinal discal herniation, osteoarthritis and ischaemic necrosis. In Legg Calv Perthes disease, an intraepiphyseal vacuum phenomenon occurs in the hip joint from release of gas into the clefts and gaps in the subchondral trabeculae of the femoral head in cases of osteonecrosis.

In some cases gas collections in tissues do occur in infections, but in such instances the gas is formed by the microorganisms themselves, as in gas gangrene or diabetic foot infections. Nonbacterial causes of gas in soft tissues includes visceral rupture, skin lacerations, open fractures, cutaneous ulcerations and chemical exposures. In addition, diastasis of the symphysis pubis with presence of gas may occur during pregnancy.

CT scanning is of value in detecting gas in the medullary canal in cases of osteomyelitis, vacuum phenomenon, or other pathologic conditions. Gas has a very low attenuation coefficient, with air having a CT number of 1000 HU (Hounsfield units). On MR images, gas appears typically as a signal void.

DR