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Background
Emphysematous osteomyelitis (EO) is an exceedingly rare and lethal bone infection caused by gas-forming bacteria. Initially described in 1981, there is a dearth of scientific literature surrounding this unique variant of osteomyelitis. Early detection of EO with prompt initiation of intravenous antibiotics and/or surgical intervention is paramount to patient survival as recent studies have described a greater than 30% mortality rate. Diagnosis relies on radiologic evidence of intra-osseous gas locules, presenting specific challenges when reviewing radiographic, computed tomography (CT) and magnetic resonance imaging (MRI) studies. This educational exhibit aims to review key imaging features and familiarize radiologists with the advantages and limitations of different imaging modalities in the detection of this potentially fatal entity.

Educational Goals / Teaching Points
The teaching points of this exhibit include an overview of the pathogenesis, clinical presentation, and management of EO to provide a context for the critical role for diagnostic imaging. A case-based review of key imaging features of EO on x-ray, CT, and MRI is presented. Additionally, the novel use of positron emission tomography-magnetic resonance imaging (PET-MRI) in diagnosing EO and localizing infection spread is demonstrated. Advantages, limitations, and important pitfalls of each study modality will be explored to guide the radiologist in making recommendations for further radiologic testing.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
Radiography of EO may demonstrate a combination of focal osteopenia, periosteal reaction, bony erosions and sclerosis. Intraosseous gas may not be radiographically visible due to the size of the subcentimeter air locules or due to bowel gas overlying the lumbar spine/pelvis, where the majority of cases occur. CT is the most sensitive modality for intraosseous gas and should be performed if EO is suspected on x-ray or MRI. The characteristic CT finding is a pumice stone pattern of irregular 2 - 5mm intramedullary gas locules. MRI is usually the modality of choice for osteomyelitis, but the intra-osseous gas causes susceptibility artifact, which limits evaluation for bone marrow edema and/or replacement of normal fatty marrow signal seen in typical osteomyelitis. This susceptibility should not be confused for poor imaging technique, calcifications, hemosiderin, or metallic foreign body. PET-MRI with fluoride-18-deoxyglucose (FDG) demonstrates localized susceptibility artifact due to intra-osseous gas, and increased radiotracer uptake signifies local inflammation such as abscess formation.

Conclusion
EO is a lethal condition requiring prompt radiologic identification of intra-osseous gas. This exhibit reviews key imaging findings of EO and evaluates the advantages and limitations of available imaging modalities to guide radiologic decision making.