Skull / facial / sinuses

Skull, facial bones & sinuses

Exam category: Procedures → Head, Spine and Pelvis
Why it matters: Three eponym views — Towne, Caldwell, Waters — cover almost everything in this module, and each is defined by a specific structure it's built to isolate from everything around it.

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The big picture (plain English)

The skull is a dense, overlapping 3D structure, so every specialized view here exists to move one specific structure into a clear, unobstructed spot on the image — usually by angling the CR or extending/tucking the chin to shift a dense structure (like the petrous pyramids) out of the way.

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1. The three core eponym views

Routine skull series = Towne + lateral + Caldwell + PA — between them, these four views cover the whole skull from every necessary angle.

2. SMV — skull vs. sinuses (a genuine, important distinction)

Submentovertex (SMV) of the skull shows the base of the skull — this remains a routine skull-series view. SMV of the sinuses is a different clinical application of the same basic projection, used specifically for the sphenoid sinus — don't collapse these into one topic; they're the same projection technique applied to two different diagnostic questions.

3. Facial bones, nasal bones, and orbits

The facial-bone series is the parent routine here, and the other two are variations on it — so learn it first and learn the others by what they change.

The nasal routine drops the Caldwell. It is the lateral plus a Waters, and nothing else — the single most common way this gets answered wrong is by assuming the full facial series applies. The nasal lateral is done on both sides, so the injured side can be read against the normal one — and whichever side lies against the receptor is the one that images best. It centers ½ inch (1.25 cm) below the nasion, and unlike the facial-bone series it is done nongrid — consistent with the general rule that a part this thin doesn't warrant one.

The superoinferior tangential is the displacement view. It exists for one thing a lateral physically cannot show: medial–lateral (side-to-side) displacement of a nasal fracture. The chin rests on the receptor and the CR is angled to run parallel to the glabelloalveolar line, grazing the glabella and the upper front teeth so the mid and distal nasal bones come out in profile. That tangent is unforgiving, and a bad image tells you which way you missed: glabella crowding into the field means the neck was over-flexed; an alveolar ridge showing up means it was over-extended.

Orbits have no routine series of their own. They appear on the facial-bone projections, and two dedicated projections take over when the orbit itself is the clinical question: modified Waters for a blowout fracture (same parietoacanthial family, positioned less steeply than the standard Waters), and the parieto-orbital oblique — Rhese method for the optic foramina.

4. TMJ — motion assessment

TMJ imaging is fundamentally different from the rest of this module: it's not just anatomy, it's range of motion. The Schuller method (axiolateral) evaluates the relationship and range of motion between the condyle and the temporomandibular fossa — typically imaged with mouth open and closed for comparison.

5. Mandible

The mandible reuses the same tricks as the rest of the skull, plus one view of its own:

• Axiolateral oblique — the amount of head rotation toward the IR picks the part you see: true lateral (no rotation) → ramus, 30° → body, 45° → mentum. A horizontal-beam version gets the lateral on a trauma patient who can't be turned.
• AP axial (Towne method) — the same caudad-angled Towne projection that isolates the posterior skull also demonstrates the mandibular condyles (CR 35° caudad to the OML). If a trauma patient can't extend the neck and you line up on the IOML instead, steepen the CR to 42° caudad — the same positioning-line logic as the skull Towne.
• SMV — shows the whole mandible with its coronoid and condyloid processes.
• PA and PA axial projections round out the mandible routine, along with a PA (modified Waters).

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Registry must-know facts

• Towne = occipital bone/foramen magnum. Caldwell = frontal bone/orbits. Waters = facial bones/maxillary sinuses (chin extended to clear petrous pyramids).
• Routine skull series: Towne, lateral, Caldwell, PA.
• Caldwell CR: 15° caudad routinely. On a skull series, 25–30° caudad is an alternative to the 15°; on facial bones, 30° caudad is the one used when the orbital margins are the area of interest. At 15° the petrous ridges sit in the lower third of the orbits.
• Facial bones routine: lateral, Waters, Caldwell. Its special is the modified Waters, for orbital (blowout) fractures and foreign bodies in the eye.
• Nasal bones routine: lateral + Waters only — no Caldwell. The lateral is taken on both sides (whichever side lies against the receptor images best), centered ½ inch (1.25 cm) inferior to the nasion, and done nongrid.
• Superoinferior tangential (axial) nasal bones is the special, and the only projection here that shows medial–lateral displacement. CR runs parallel to the glabelloalveolar line. Glabella crowding the field = over-flexed; alveolar ridge showing = over-extended.
• Orbits have no routine of their own — they show on the facial-bone projections. Modified Waters covers the blowout fracture; the parieto-orbital oblique (Rhese method) covers the optic foramina.
• SMV of the skull (base of skull) and SMV of the sinuses (sphenoid sinus) are the same projection technique used for two different clinical questions — not the same topic.
• Schuller method evaluates TMJ range of motion (condyle-to-fossa relationship), typically open and closed mouth for comparison.
• Mandible: axiolateral oblique rotation picks the part (true lateral = ramus, 30° = body, 45° = mentum), horizontal-beam version for trauma; AP axial Towne method (35° caudad to OML, 42° to IOML) shows the condyles; SMV shows the whole mandible including the coronoid and condyloid processes.

Common trap

Students conflate "the petrous pyramids are in the way" as a generic problem solved the same way everywhere. It's specifically chin extension (Waters) that clears them for facial bones/sinuses — a different structure (like the frontal bone in Caldwell) needs CR angulation instead, not chin position.

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Source: Bontrager, Textbook of Radiographic Positioning and Related Anatomy, 10th ed., Ch. 11. Section 3 is taken from that chapter's per-projection routine/special boxes and "CR", "Part Position" and "Evaluation Criteria" entries rather than summarized from memory: the facial-bone routine (lateral, parietoacanthial/Waters, PA axial/Caldwell) and its modified-Waters special with the blowout-fracture and eye-foreign-body indications; the nasal-bone routine of lateral plus Waters only, with both sides imaged so they can be compared and the receptor-side one shown to best advantage, the CR ½ inch (1.25 cm) inferior to the nasion, and the nongrid technical factor; and the superoinferior tangential (axial) nasal projection — its medial-lateral-displacement indication, the CR parallel to the glabelloalveolar line, and the over-flexion/over-extension tells read off the glabella and alveolar ridge. The Caldwell angles are from the same chapter: 15° caudad routinely with a 25–30° alternative on the skull series page, and the 30° caudad note for orbital margins on the facial-bone page — two separate entries, not one range. Orbits have no dedicated routine page in that chapter; the optic foramina are covered by the parieto-orbital oblique (Rhese method).