====== Retro-odontoid pseudotumor diagnosis ======

===== Radiographic features =====

Retro-[[odontoid]] [[pseudotumor]] is defined by [[soft tissue]] thickening at the [[atlantoaxial junction]] around the expected location of the [[transverse band]], posterior to the [[dens]].

MRI and CT have an important role in the detection and diagnosis of retro-odontoid pseudotumor. However, due to a wide range of imaging characteristics and ambiguous etiology, it is a frequently misunderstood entity
((Shi J, Ermann J, Weissman BN, Smith SE, Mandell JC. Thinking beyond pannus: a review of retro-odontoid pseudotumor due to rheumatoid and non-rheumatoid etiologies. Skeletal Radiol. 2019 Oct;48(10):1511-1523. doi: 10.1007/s00256-019-03187-z. Epub 2019 Mar 13. PMID: 30868232.))

===== CT =====

Mineralization within the pseudotumor may be present in calcium pyrophosphate deposition disease (chondrocalcinosis, linear/arciform), hydroxyapatite deposition disease (cloudlike), gout (faintly), fracture callus, and ossification of posterior longitudinal ligament (by definition).

Joint or pressure erosion may be present in RA, CPPD, gout, and tenosynovial giant cell tumor.

==== MRI ====

Signal characteristics vary by etiology. In general, there are no reliable imaging features that distinguish [[rheumatoid arthritis]]-related [[pannus]] from non-rheumatoid [[retro-odontoid pseudotumor]].

T1: usually low

T2: variable, often heterogeneous