16.3 Skull, Facial Bones, Sinuses & Mandible

Cranial Positioning Lines

Skull and facial positioning is built on radiographic baselines that connect facial landmarks to the external acoustic meatus (EAM). The key lines are the orbitomeatal line (OML) (outer canthus to EAM, the primary skull baseline); the infraorbitomeatal line (IOML, Reid's baseline) (inferior orbit to EAM, about 7 degrees below the OML); the glabellomeatal line (GML); the acanthiomeatal line (AML); the mentomeatal line (MML) (chin to EAM, used for the Waters method); and the interpupillary (interorbital) line (IPL), which is placed perpendicular to the IR for a true lateral. The midsagittal plane (MSP) is set parallel or perpendicular to the IR depending on the projection.

Skull

The lateral skull centers the CR perpendicular to a point 2 inches superior to the EAM, with the MSP parallel and the IPL perpendicular to the IR; it demonstrates the sella turcica, sphenoid sinus, and the superimposed cranial halves. The PA (0-degree) projection with the OML perpendicular fills the orbits with the petrous ridges. The PA axial (Caldwell) projection angles the CR 15 degrees caudad to exit the nasion, dropping the petrous ridges into the lower third of the orbits to reveal the frontal sinuses and anterior ethmoid air cells.

The AP axial (Towne) projection angles the CR 30 degrees caudad to the OML (or 37 degrees to the IOML) with the chin depressed to bring the OML perpendicular to the IR. A correctly angled Towne demonstrates the occipital bone and foramen magnum, and projects the dorsum sellae and posterior clinoid processes within the foramen magnum. Too little angulation throws the dorsum sellae above the rim of the foramen magnum, while excessive angulation foreshortens the occipital bone.

Facial Bones and Orbits

The parietoacanthial (Waters) projection rests the chin on the IR with the mentomeatal line (MML) perpendicular to the IR and the OML forming a 37-degree angle; this elevates the petrous ridges to just below the maxillary sinuses (floor of the maxillary antra) and best demonstrates the maxillary sinuses, orbital floors, and blow-out fractures. The modified Waters places the lips-meatal line (LML) perpendicular with the OML at about 55 degrees, dropping the petrous ridges below the orbital floors for an even cleaner orbital-floor view. When a trauma patient cannot be placed prone, the acanthioparietal (reverse Waters) projection is performed AP (supine) to achieve a comparable image. A lateral facial projection surveys the facial bones as a group.

Paranasal Sinuses

The cardinal rule of sinus imaging: to show air-fluid levels, image the patient erect (upright) with a horizontal central ray because a recumbent projection cannot layer fluid. The routine sinus series uses the erect Waters for the maxillary sinuses, the erect Caldwell for the frontal sinuses and anterior ethmoids, the erect lateral for the sphenoid sinus and all sinuses in profile, and the SMV for the sphenoid and ethmoid sinuses.

Mandible, TMJ, Nasal Bones, and Zygomatic Arches

The mandible is imaged with the axiolateral oblique (the body, ramus, and mentum are shown according to head rotation), the PA (0 degrees) for the rami and lateral body, and the AP axial (Towne) for the condylar processes. Temporomandibular joint (TMJ) studies use bilateral axiolateral obliques (modified Law) in both the open- and closed-mouth positions to assess condylar excursion. Nasal bones are imaged with a lateral (superoinferior tabletop) projection using a low technique, plus the parietoacanthial (Waters) for the overall facial relationship. Zygomatic arches are best shown with the submentovertical (SMV) projection for bilateral arches (IOML parallel to the IR, CR perpendicular to the IOML), the tangential projection for a single depressed arch, and the AP axial (Towne) for bilateral arches. The SMV additionally demonstrates the base of the skull, the sphenoid and ethmoid sinuses, and the mandible.

Evaluation Criteria and the Optic Foramen

Rotation on skull work is judged by symmetry of the petrous ridges. On a straight PA or Caldwell, the distance from the lateral orbital margin to the lateral cranial cortex should be equal on both sides; unequal distances indicate head rotation. On the Towne, the dorsum sellae must sit within the foramen magnum and the posterior arch of C1 should not superimpose it. The optic foramen is imaged with the parieto-orbital oblique (Rhese method): the patient rests the chin, nose, and cheek on the IR with the head rotated so the optic foramen projects into the inferior-outer (lower lateral) quadrant of the orbit. Distinguish the sinus projections by their target: the Waters isolates the maxillary sinuses, the Caldwell the frontal and anterior ethmoid, the lateral the sphenoid, and the SMV the sphenoid and ethmoid air cells plus the zygomatic arches. Remember that a single depressed zygomatic arch is best isolated with the tangential projection rather than the bilateral SMV.