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 1)
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.
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