Rheumatoid arthritis  

Abnormal marrow signal is seen in the odontoid process .There is associated abnormal soft tissue (pannus)adjacent to the dens circumferrentially  causing posterior displacement of the transeverse ligament and compression of the cervicomedullary junction at the foramen magnum.

The major role for CT and MRI is in the preoperative assessment of the 2 main indications for surgical intervention, namely neurologic deficit and severe pain. The subluxations that are of a degree likely to result in paralysis need to be identified, as better outcomes are recorded with earlier interventions.

Although CT scanning can document bone damage and alignment abnormalities, especially with more detailed multiplanar reconstruction, MRI has become the preferred modality for evaluation of the spinal cord and neural elements. It demonstrates the presence and effect of pannus on the spinal cord, and is useful in assessing its resolution following posterior fusion and stabilization. On MRI, the direct relationships of the odontoid to the medulla and brainstem can be documented. Spinal cord signal can be assessed; edematous changes in the cord are associated with a poor clinical status, as well as a poor prognosis and a poor postoperative outcome.

The major indications for MRI in RA are abnormal measurements on plain radiographs, unremitting suboccipital/cervical pain, progressive/severe subluxations, symptoms of cord/brainstem compression, and vertebral-artery compression.

Dynamic MRI has been used with the patient in flexed or extended positions and in the traditional neutral position.