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Anatomy & Positioning Atlas

Skull & Facial Bones

The skull has 8 cranial bones and 14 facial bones. Routine skull imaging is a PA axial (Caldwell), an AP axial (Towne) and a lateral; the facial bones and sinuses add the parietoacanthial (Waters) and, when ordered, the submentovertical (SMV).

Overview

Bones

Cranial bones (8)
  1. Frontal
  2. Parietal (2)
  3. Temporal (2)
  4. Occipital
  5. Sphenoid
  6. Ethmoid
Facial bones (14)
  1. Nasal (2)
  2. Lacrimal (2)
  3. Maxillae (2)
  4. Zygomatic (2)
  5. Palatine (2)
  6. Inferior nasal conchae (2)
  7. Vomer
  8. Mandible
Orbit (7 bones form it)
  1. Roof: frontal and lesser wing of the sphenoid
  2. Lateral wall: zygomatic and greater wing of the sphenoid
  3. Floor: maxilla, zygomatic, palatine
  4. Medial wall: ethmoid, lacrimal, maxilla, sphenoid
  5. Supraorbital, infraorbital and lateral orbital margins
  6. Openings: optic canal, superior and inferior orbital fissures
  7. Orbit cone points posteriorly, 37° from the MSP and 30° up from the OML (why the Rhese angles work: MSP 53°, AML perpendicular)
Sutures and junctions
  1. Coronal: frontal and parietals
  2. Sagittal: between the parietals
  3. Lambdoid: parietals and occipital
  4. Squamosal: parietal and temporal squama
  5. Bregma (coronal + sagittal), lambda (sagittal + lambdoid), pterion (frontal, parietal, sphenoid, temporal), asterion
  6. Fontanels in infants: anterior (bregma) and posterior (lambda)
Temporal bone and ear
  1. Squamous, tympanic, mastoid and petrous parts
  2. External acoustic meatus (EAM) and internal acoustic meatus (IAM, in the petrous ridge)
  3. Mastoid process and mastoid air cells (seen on the lateral skull)
  4. Petrous ridge (pyramid): the landmark for Caldwell, Waters and Towne
  5. Styloid process, zygomatic process, mandibular fossa (TMJ)
  6. Middle ear ossicles: malleus, incus, stapes
Positioning landmarks and lines
  1. Glabella, nasion, acanthion, mental point, gonion, EAM
  2. OML: outer canthus to EAM
  3. IOML: infraorbital margin to EAM (7° from the OML)
  4. MML: mentum to EAM · LML: lips to EAM
  5. IPL: line between the pupils · MSP: midsagittal plane

The OML and IOML are 7° apart, so a CR angle changes by 7° with the baseline: Towne is 30° caudad to the OML or 37° to the IOML.

Registry must-knows

  • PA with no angle: the petrous ridges fill the orbits. Caldwell (15° caudad): they drop into the lower third of the orbits.
  • Towne: the dorsum sellae and posterior clinoid processes project within the foramen magnum.
  • Waters: the petrous ridges project just below the maxillary sinuses. An open-mouth Waters also shows the sphenoid sinus.
  • Lateral skull: MSP parallel and IPL perpendicular to the IR, so the orbital roofs, mandibular rami and EAMs superimpose and the sella turcica is in profile.
Show 3 more
  • Sinus series: always erect with a horizontal beam so air-fluid levels show.
  • Rotation check on frontal views: equal distance from the lateral orbital margin to the lateral border of the skull on both sides.
  • Mastoid air cells and the EAM are best on the lateral; the petrous ridges and IAMs are judged on the frontal and axial views.

3D model

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Radiographs 15

Pick a film, then a structure to circle it.

Projections 11

Tap a projection for positioning, central ray and image criteria. See every exam in the Positioning Guide ›

PA axial skull

CaldwellCR: 15° caudad
Position
Prone or erect, forehead and nose on the IR. OML and MSP perpendicular to the IR.
Central ray
15° caudad
Centering
Exits at the nasion
Typical technique
75–85 kVp · 15–25 mAs · 40" SID
Best shows
Frontal bone, frontal and anterior ethmoid sinuses, orbital rims, greater and lesser sphenoid wings

Image criteria

  • Petrous ridges in the lower third of the orbits.
  • Equal distance from each lateral orbital margin to the lateral border of the skull (no rotation).
  • Superior orbital fissures seen within the orbits.

Lateral skull

CR: Perpendicular
Position
Semiprone or erect, side of interest against the IR. MSP parallel, IPL perpendicular, IOML parallel to the transverse axis of the IR.
Central ray
Perpendicular
Centering
2 in (5 cm) superior to the EAM
Typical technique
75–85 kVp · 15–20 mAs · 40" SID
Best shows
Cranium in profile, sella turcica, the side nearest the IR

Image criteria

  • Sella turcica in profile, not rotated.
  • Orbital roofs, mandibular rami and EAMs superimposed.
  • Entire cranium included.

AP axial skull

TowneCR: 30° caudad to the OML, or 37° caudad to the IOML
Position
Supine or erect, back of the head on the IR. OML (or IOML) and MSP perpendicular to the IR.
Central ray
30° caudad to the OML, or 37° caudad to the IOML
Centering
2.5 in (6.5 cm) above the glabella, exiting at the foramen magnum
Typical technique
80–85 kVp · 20–30 mAs · 40" SID
Best shows
Occipital bone, petrous pyramids, foramen magnum

Image criteria

  • Dorsum sellae and posterior clinoids within the foramen magnum.
  • Petrous ridges symmetric.
  • Equal distance from the foramen magnum to the lateral skull borders (no rotation).

Parietoacanthial

WatersCR: Perpendicular
Position
Erect, chin on the IR, head extended so the OML forms a 37° angle with the IR (MML perpendicular).
Central ray
Perpendicular
Centering
Exits at the acanthion
Typical technique
75–85 kVp · 15–25 mAs · 40" SID
Best shows
Maxillary sinuses, orbits, zygomatic bones and arches (facial bones survey)

Image criteria

  • Petrous ridges just below the floor of the maxillary sinuses.
  • Equal distance from each lateral orbital margin to the lateral skull border (no rotation).
  • Maxillary sinuses fully shown.

Submentovertical (SMV)

CR: Perpendicular to the IOML
Position
Head hyperextended, vertex on the IR, IOML parallel to the IR, MSP perpendicular.
Central ray
Perpendicular to the IOML
Centering
Midway between the gonions, about 3/4 in (2 cm) anterior to the level of the EAMs
Typical technique
80–85 kVp · 25–35 mAs · 40" SID
Best shows
Skull base, foramina, sphenoid sinuses, zygomatic arches

Image criteria

  • Mandibular condyles projected anterior to the petrous ridges.
  • Mandibular symphysis superimposed over the frontal bone.
  • Equal distance from the mandible to the lateral skull borders (no tilt).

Modified parietoacanthial (orbits)

Modified WatersCR: Perpendicular
Position
Chin and nose on the IR, OML 55° to the IR (less extension than a Waters); LML about perpendicular.
Central ray
Perpendicular
Centering
Exits at the acanthion (mid-orbits)
Typical technique
75–80 kVp · 10–20 mAs · 40" SID
Best shows
Orbital floors (blowout fractures) and orbital rims

Image criteria

  • Petrous ridges in the lower half of the maxillary sinuses, below the orbital floors.
  • Orbits fully shown and symmetric (no rotation).

Parieto-orbital oblique (optic canal)

RheseCR: Perpendicular
Position
Prone or erect, zygoma, nose and chin on the IR (three-point landing); MSP 53° to the IR, AML perpendicular. Side of interest down; do both sides.
Central ray
Perpendicular
Centering
Through the downside orbit
Typical technique
75–80 kVp · 10–15 mAs · 40" SID
Best shows
Optic canal (foramen) seen end-on in the lower outer quadrant of the orbit

Image criteria

  • Optic canal in the lower outer quadrant of the orbit (too far out or in means wrong rotation; too high or low means wrong AML).
  • Entire orbital rim shown.

Lateral nasal bones

CR: Perpendicular
Position
Lateral, MSP parallel to the IR, IPL perpendicular, IOML parallel to the transverse axis of the IR. Both sides are usually done.
Central ray
Perpendicular
Centering
1/2 in (1.25 cm) distal to the nasion
Typical technique
60–65 kVp · 2–4 mAs · 40" SID
Best shows
Nasal bones and soft tissue of the nose, frontonasal suture and anterior nasal spine

Image criteria

  • Nasal bones in profile with no rotation.
  • Soft tissue of the nose shown (low technique, detail).

PA mandible

CR: Perpendicular
Position
Forehead and nose on the IR, OML perpendicular, MSP perpendicular.
Central ray
Perpendicular
Centering
Exits at the junction of the lips
Typical technique
75–80 kVp · 8–15 mAs · 40" SID
Best shows
Rami and lateral parts of the body

Image criteria

  • Rami and body symmetric on both sides (no rotation).
  • Mandibular body and rami fully included.

PA axial mandible

CR: 20–25° cephalad
Position
As for the PA mandible: forehead and nose on the IR, OML perpendicular.
Central ray
20–25° cephalad
Centering
Exits at the acanthion
Typical technique
75–80 kVp · 8–15 mAs · 40" SID
Best shows
Condyles and rami; condyles elongated, just below the mastoid processes

Image criteria

  • Condyles shown, just below the mastoid processes.
  • Rami symmetric (no rotation).

Axiolateral oblique mandible

CR: 25° cephalad
Position
Lateral, side of interest against the IR, neck extended. Head turned toward the IR: 0° (true lateral) for the ramus, 30° for the body, 45° for the symphysis.
Central ray
25° cephalad
Centering
Exits at the mandibular region of interest
Typical technique
70–80 kVp · 8–12 mAs · 40" SID
Best shows
One side of the mandible (ramus, body or symphysis) free of the other side

Image criteria

  • Area of interest free of superimposition from the opposite side.
  • Ramus not superimposed on the cervical spine.

Technique values are typical starting points; follow your department's technique chart.

Labeled diagrams

Labeled skull diagram (anterolateral): Frontal bone, Nasal bone, Orbit, Maxilla, Mandible, Parietal bone, Temporal bone, Sphenoid bone, Zygomatic bone, Mastoid process, External acoustic meatus.
Skull (anterolateral view) · Diagram: Rad Registry Prep (original illustration).
Labeled skull: lateral (sutures and ear) diagram (lateral): Frontal bone, Parietal bone, Temporal bone (squamous part), Occipital bone, Sphenoid (greater wing), Zygomatic bone, Zygomatic arch, Maxilla, Mandible, Nasal bone, Orbit, Coronal suture, Squamosal suture, External acoustic meatus (EAM), Mastoid process, Styloid process, Mandibular condyle (TMJ), Glabella, Pterion, Lambdoid suture, External occipital protuberance (inion), Nasion, Acanthion, Angle of mandible (gonion).
Skull: Lateral (Sutures and Ear) (lateral view) · Diagram: Rad Registry Prep, restyled and labeled from Gray's Anatomy (1918), public domain.
Labeled skull: frontal (orbits and face) diagram (anterior): Frontal bone, Glabella, Nasion, Supraorbital margin, Supraorbital foramen (notch), Orbit, Superior orbital fissure, Inferior orbital fissure, Infraorbital foramen, Nasal bone, Nasal cavity, Nasal septum, Anterior nasal spine (ANS), Maxilla, Zygomatic bone, Mandible, Mental foramen, Mental protuberance, Sphenoid (greater wing), Temporal bone, Parietal bone, Angle of mandible (gonion), Zygomatic arch.
Skull: Frontal (Orbits and Face) (anterior view) · Diagram: Rad Registry Prep, restyled and labeled from Gray's Anatomy (1918), public domain.

Quick check

1. Which projection of the skull is shown on this radiograph?

Image: Hellerhoff. CC BY-SA 3.0.

2. Why should projections of the paranasal sinuses be taken with the patient erect and a horizontal CR whenever possible?

3. Before a submentovertex (SMV) projection of the cranial base is attempted on a trauma patient, which condition must be ruled out?

4. For lateral nasal bone projections like this one, which technique and comparison are used?

Image: Nevit Dilmen, via Wikimedia Commons. CC BY-SA 3.0.

5. This unilateral image projects one zygomatic arch free of the skull. How is this tangential (May-type) projection positioned?

Image: Hellerhoff. CC BY-SA 4.0.

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