Procedures · Head, spine and pelvis2 min read5-question check
Skull baselines and routine skull projections
Know the baselines; the petrous ridges then show if the angle was right: filling the orbits or lower third.
The baselines
Skull positioning uses lines on the face. The orbitomeatal line, OML, runs from the outer canthus of the eye to the external acoustic meatus. The infraorbitomeatal line, IOML, runs from the infraorbital margin to the EAM, and it sits about 7 degrees from the OML. The glabelloalveolar line runs from the glabella to the alveolar process of the maxilla, and the mentomeatal line runs from the chin to the EAM. The interpupillary line connects the pupils and is used to check tilt.
Central ray
For the PA skull, the forehead and nose touch the IR, the OML is perpendicular to the IR and the CR is perpendicular, exiting the nasion. For the PA axial Caldwell, the head stays the same and the CR is angled 15 degrees caudad to exit the nasion. For the AP axial Towne, the patient is supine with the chin tucked, and the CR is angled 30 degrees caudad to the OML or 37 degrees caudad to the IOML, entering about 2.5 inches above the glabella. The lateral skull places the midsagittal plane parallel to the IR and the interpupillary line perpendicular, with the CR perpendicular about 2 inches above the EAM. All use 40 inches.
What a good image shows
On the PA, the petrous ridges fill the orbits. On the Caldwell, they lie in the lower third of the orbits. On the Towne, the dorsum sellae and posterior clinoids project within the foramen magnum and the occipital bone is shown. On the lateral, the sella turcica is in profile, and the orbital roofs and mandibular rami are superimposed. No rotation on PA views means equal distance from the lateral orbital margins to the lateral skull margins.
Fixing it
On a lateral, structures separated front to back mean rotation; structures separated up and down, like the orbital roofs, mean tilt, so recheck the interpupillary line. On a Towne, if the dorsum sellae is above the foramen magnum, add more angle or tuck the chin more. On PA views, unequal orbit-to-skull distances mean rotation; correct the midsagittal plane.
Clinical Example
A patient cannot tuck the chin for a Towne, so the IOML, not the OML, is perpendicular. You increase the caudad angle to 37 degrees. The dorsum sellae now projects inside the foramen magnum.
Common Mistakes
- Using 30 degrees when the IOML, not the OML, is perpendicular to the IR.
- Confusing the Caldwell result with the PA; only the PA fills the orbits.
- Correcting tilt by turning the face, which adds rotation.
- Angling cephalad for a Towne; the Towne angle is caudad.
Why It Matters
Correct baselines avoid repeats in a radiosensitive area near the lenses. The registry tests the 7 degree OML to IOML difference and where the petrous ridges should fall.
Key Relationships
- OML and IOML differ by about 7 degrees
- PA 0 degrees: petrous ridges fill the orbits
- Caldwell 15 degrees caudad: ridges in lower third of orbits
- Towne 30 degrees to OML or 37 to IOML: dorsum sellae in foramen magnum
- Lateral: interpupillary line perpendicular, MSP parallel