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E1791. Air in the Head and Spine: Where Did it Come From?
Authors
  1. Ritu Bordia; Saint Louis University
  2. Adam Held; Saint Louis University
Background
Air is frequently seen on CT scans of head and spine. Although, air within the cranial vault and spinal column does not exist under normal conditions, it is often an expected finding (eg. post-surgical imaging). When air is seen as an unexpected finding, it is extremely important to determine its source, since it could represent pathology with significant implications. The purpose of this exhibit is to provide a comprehensive review of intracranial and intraspinal air in various locations with clues to identifying source and determining its clinical significance.

Educational Goals / Teaching Points
Demonstrate the CT and MRI appearance of air. Discuss mimics of air on CT and MRI. Discuss possible locations of intracranial and intraspinal air. Review clinical scenarios where intracranial air is expected but generally of no consequence. Review scenarios where intracranial air is unexpected and possibly of clinical significance. Review of sources of air in spinal column .

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
The exhibit will include images to demonstrate the following - - CT and MRI appearance of air - emphasizing the non-dependent location and MRI signal characteristics - Mimics on CT - fat and surgical material - Mimic on MRI - hemorrhage - Artifact created by air on MR imaging - Possible locations of intracranial air - scalp, calvarium, epidural, subdural, intra-parenchymal, subarachnoid, intraventricular, intravenous and intra-arterial - Scenarios where intracranial air is expected but generally of no consequence - Post-surgery and post-traumatic (penetrating/ gunshot) - Scenarios where intracranial air is not expected and is not clinically significant- unusual pneumatization patterns of calvarium, intravenous air - Scenarios where intracranial air is clinically significant - Tension pneumocephalus, subtle extra-axial air after blunt trauma with calvarial fractures (particularly around paranasal sinuses/mastoids), air within dural venous sinuses, intraarterial air (air embolism or after thrombectomy), non-traumatic bone defects with CSF leak (tegmen tympani defect), foreign bodies (wooden, plastic), intraventricular air from infection - Air in spinal column due to degenerative changes (in the disc, herniated disc, vertebral body, and facets) - Air in spinal column due to non-degenerative causes –direct penetrating trauma, infection (epidural abscess), iatrogenic (intentional or unintentional), related to air in other compartments of the body

Conclusion
A knowledge of the imaging appearance of intracranial/intraspinal air and its various sources is crucial to determine clinical significance of this common finding.