Normal Fischgold’s Bimastoid Line Size on X-ray, CT, MRI
Fischgold’s bimastoid line is a craniometric reference line drawn between the tips of the two mastoid processes on frontal (AP) skull or cervical spine imaging. It serves as a landmark for assessing the vertical position of the odontoid process (dens) relative to the foramen magnum. Accurate measurement is essential in detecting basilar invagination, a potentially life-threatening craniocervical junction abnormality.
Normal Reference Values
| Orientation | Measurement |
|---|---|
| Odontoid | <10 mm |
Clinical Significance
In normal individuals, the tip of the odontoid process lies less than 10 mm above Fischgold’s bimastoid line. When the odontoid tip projects 10 mm or more above this line, basilar invagination should be strongly considered. This upward migration of the dens can compress the cervicomedullary junction, leading to myelopathy, lower cranial nerve deficits, or sudden neurological deterioration.
It is important to distinguish basilar invagination (a primary or secondary structural anomaly) from platybasia (flattening of the skull base), as the two conditions may coexist but require separate evaluation. Secondary causes of an elevated odontoid relative to this line include inflammatory and destructive processes affecting the craniocervical junction.
- Basilar invagination — congenital (e.g., Chiari malformation, Klippel-Feil) or acquired
- Rheumatoid arthritis — atlanto-axial instability with cranial settling
- Paget’s disease — bony softening and skull base deformity
- Osteomalacia / rickets — axial skeletal softening
- Osteogenesis imperfecta — congenital bony fragility
Reference: Greenberg MS. Handbook of Neurosurgery. Thieme. (2010).
Imaging Notes
On plain radiography (AP open-mouth or Towne’s view), the bimastoid line is drawn between the most inferior tips of the mastoid processes, and the perpendicular distance from this line to the odontoid tip is measured. Patient positioning must be true AP to avoid parallax error that can falsely elevate or depress the apparent odontoid position.
On CT and MRI, coronal reformatted or coronal sequences allow precise identification of both mastoid tips and the odontoid apex in a single plane. MRI offers the additional advantage of directly visualizing cervicomedullary compression and associated soft-tissue or ligamentous pathology, making it the preferred modality when neurological symptoms are present. Ensure imaging is performed in neutral head position, as flexion or extension can alter apparent relationships at the craniocervical junction.