Intraosseous pneumatocysts are gas-containing cystic structures seen inside the bone.
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Pathology
They are of uncertain etiology. These cysts do not communicate with the joint.
Location
Most common site of involvement is sacroiliac joints. Other sites involved are cervical spine, scapula and rarely other bones.
Radiographic features
CT
Appear as a very low density (i.e. air at approximately -1000 HU) cystic lesion with thin sclerotic margins, usually abutting a joint.
MRI
Low signal intensity on both T1 and T2 weighted MR images.
History and etymology
Intraosseous pneumatocysts were first reported by Ramirez in 1984 4.
Differential diagnosis
Imaging differential considerations include:
osteomyelitis from gas-producing organism
osteoblastic metastases on MRI
Practical points
There is a risk of erroneously misdiagnosing intraosseous lipomas as intraosseous pneumatocysts, or vice-versa, if the lesion is only eye-balled on a soft tissue window. Therefore if unsure, one should either measure an ROI of the lesion to clarify if gas or fat density, or review the lesion on a lung window.
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