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Head

49 Skull

Anatomy

Topographical Landmarks

  • Acanthion – midline junction of upper lip and nose
  • Alveolar processes – junction of the two maxillary bones and mandible at the midline, between the front two teeth
  • Auricle (pinna) – large flap of cartilage, the external ear
  • Base of orbit – bony rim surrounding each eye
  • External acoustic (auditory) meatus (EAM) – external opening of the ear canal
  • Glabella – smooth, triangular area superior to nasal bridge, between eyebrows
  • Gonion – posterior angle of the mandible, palpable on both sides
  • Infraorbital margin – inferior rim of the orbital base
  • Inion – external occipital protuberance, midline bump at back of skull
  • Inner canthus – medial junction of eyelids, near nose
  • Mental point – midpoint of the chin
  • Midlateral orbital margin – outer rim of orbit near lateral canthus
  • Nasion – depression at the junction of the frontal and nasal bones
  • Superciliary arch – ridge of bone over each orbit under eyebrows
  • Supraorbital groove – depression above superciliary arch; highest level of facial bones
  • Supraorbital margin – superior rim of orbital base
  • Top of ear attachment (TEA) – superior attachment of auricle; corresponds to petrous ridges
  • Tragus – flap of cartilage projecting over the EAM
  • Outer canthus – lateral junction of eyelids
  • Vertex – most superior point of skull

Positioning Lines & Planes

  • Orbitomeatal line (OML) – EAM to outer canthus; standard baseline
  • Infraorbitomeatal line (IOML) – EAM to infraorbital margin; ~7° below OML
  • Glabellomeatal line (GML) – EAM to glabella
  • Acanthiomeatal line (AML) – EAM to acanthion
  • Mentomeatal line (MML) – EAM to mentum
  • Interpupillary line (IPL) – line connecting pupils; perpendicular for lateral skulls
  • Midsagittal plane (MSP) – divides skull into equal halves

Cranial Anatomy

  • Cranial bones (8): frontal, parietals (2), occipital, temporals (2), ethmoid, sphenoid
  • Calvarium (skull cap): frontal, parietals, occipital
  • Cranial floor: temporals, sphenoid, ethmoid
  • Sutures: sagittal, coronal, lambdoidal, squamous
  • Fontanels: “soft spots” in infants — anterior (bregma) and posterior (lambda)
  • Facial bones (14): maxilla (2), zygoma (2), lacrimal (2), nasal (2), palatine (2), inferior nasal conchae (2), vomer, mandible
  • Joints: sutures (immovable), TMJs (movable diarthrodial), gomphoses (teeth)
  • Sinuses (paranasal): frontal, ethmoid, sphenoid, maxillary

Skull Radiography

Routine Skull Series

AP Axial Projection (Towne Method)

Clinical Indications

  • Skull fractures (especially occipital), neoplastic processes, Paget’s disease.

CR Location & Positioning

  • SID: 40 inches (102 cm)
  • Patient position: Supine or erect, remove all metallic objects.
  • Adjustments: Depress chin to bring OML ⟂ IR; if unable, align IOML ⟂ IR and increase CR angle. Align MSP ⟂ midline, avoid tilt/rotation.
  • CR: 30° caudad to OML (or 37° to IOML), entering 2.5 in. above glabella, through foramen magnum.
  • Pt. Instructions: Suspend respiration.
  • Exposure: 80–85 kVp (digital), grid, 10×12 IR lengthwise.

Clinical Tip:

  • If the patient cannot tuck the chin enough to align the OML perpendicular, place a sponge under the head and use the IOML with a 37° caudal angle.

  • Watch for hypersthenic patients — a thick neck can limit flexion, so compensating with the IOML line avoids under-angulation errors.

Evaluation Criteria

  • Coverage: Occipital bone, petrous pyramids, foramen magnum, dorsum sellae, posterior clinoids.
  • Rotation checks: Equal distance from lateral skull borders to lateral foramen magnum margins; symmetric petrous ridges.
  • Motion checks: Sharp bony detail, no blur.
  • Technique checks: Sufficient density/contrast to show occipital bone and sellar region; no artifacts.
  • Clinical aim: Dorsum sellae/posterior clinoids should appear within foramen magnum.

Lateral Projection (Right or Left)

Clinical Indications

  • Skull fractures, neoplastic processes, Paget’s disease.

CR Location & Positioning

  • SID: 40 inches (102 cm)
  • Patient position: Erect or semiprone; side of interest closest to IR.
  • Adjustments: MSP ∥ IR; IPL ⟂ IR; IOML ⟂ front edge of IR.
  • CR: ⟂ IR, centered 2 in. superior to EAM (or midway between glabella and inion for varying skull shapes).
  • Pt. Instructions: Suspend respiration.
  • Exposure: 80–85 kVp (digital), 10×12 IR crosswise.

Evaluation Criteria

  • Coverage: Entire cranium, sella turcica in profile.
  • Rotation checks: Superimposition of orbital roofs, greater wings, TMJs, mandibular rami.
  • Motion checks: Sharp margins of skull and sella turcica.
  • Technique checks: Adequate brightness/contrast to show bony structures and soft tissue margins.
  • Clinical aim: Baseline for skull series; useful in trauma with horizontal beam.

PA Axial Projection (15° Caldwell, or 25°–30° Variation)

Clinical Indications

  • Skull fractures, neoplastic processes, sinus evaluation.

CR Location & Positioning

  • SID: 40 inches (102 cm)
  • Patient position: Erect or prone. Nose and forehead against IR.
  • Adjustments: Flex neck to align OML ⟂ IR; MSP ⟂ midline.
  • CR: 15° caudad, exiting nasion (alternative: 25°–30° caudad to exit nasion).
  • Pt. Instructions: Suspend respiration.
  • Exposure: 80–85 kVp, 10×12 IR lengthwise.

Clinical Tip:

  • If the patient cannot flex the neck enough to bring the OML perpendicular, angle the CR slightly more caudad to project the petrous ridges to the correct level.

  • For trauma patients, use the AP reverse Caldwell (15° cephalic) as an alternative without moving the head.

Evaluation Criteria

  • Coverage: Frontal bone, sphenoid wings, frontal and ethmoid sinuses, superior orbital margins.
  • Rotation checks: Equal distances from lateral orbital margins to lateral skull cortex; symmetric superior orbital fissures.
  • Motion checks: Crisp trabecular markings, no blur.
  • Technique checks: Density adequate to show frontal bone and orbital detail without overexposure.
  • Clinical aim:
    • At 15°: Petrous ridges in lower third of orbits.
    • At 25°–30°: Petrous ridges projected below IOMs, better view of orbital margins, superior orbital fissures, and foramen rotundum.

PA Projection (0° CR)

Clinical Indications

  • Skull fractures, neoplastic processes, baseline frontal bone study.

CR Location & Positioning

  • SID: 40 inches (102 cm)
  • Patient position: Prone or erect.
  • Adjustments: Nose and forehead against IR; OML ⟂ IR; MSP ⟂ midline.
  • CR: ⟂ IR, exiting glabella.
  • Pt. Instructions: Suspend respiration.
  • Exposure: 80–85 kVp, 10×12 IR lengthwise.

Evaluation Criteria

  • Coverage: Frontal bone, sphenoid wings, petrous ridges filling the orbits, dorsum sellae.
  • Rotation checks: Equal distances from lateral orbital margins to skull cortex.
  • Motion checks: Sharpness of frontal sinus and orbital rims.
  • Technique checks: Petrous ridges should fill orbits; no cutoff of superior skull.
  • Clinical aim: Demonstrates frontal bone and petrous ridges at orbital level.

Special Skull Projections

Submentovertex (SMV)

Clinical Indications

  • Basal skull fractures, sphenoid/ethmoid sinuses, foramen ovale and spinosum.

CR Location & Positioning

  • SID: 40 inches (102 cm)
  • Patient position: Supine or erect; head hyperextended, vertex on IR.
  • Adjustments: IOML ∥ IR; MSP ⟂ midline. If unable, angle CR perpendicular to IOML.
  • CR: ⟂ IOML, centered 1.5 in. inferior to mandibular symphysis, midway between gonions.
  • Pt. Instructions: Suspend respiration.
  • Exposure: 80–90 kVp, 10×12 IR lengthwise.

Clinical Tip:

  • Always rule out cervical spine injury before attempting hyperextension.
  • If the patient cannot extend the neck enough to place the IOML parallel, angle the CR perpendicular to the IOML instead.
  • A pillow under the back (supine) or using an upright device can help patients achieve the position more comfortably.

Evaluation Criteria

  • Coverage: Foramen ovale/spinosum, mandible, sphenoid/ethmoid sinuses, petrous ridges, mastoid processes, foramen magnum.
  • Rotation checks: Equal distance from mandibular rami to lateral skull borders.
  • Motion checks: Sharp condyles and sphenoid margins.
  • Technique checks: Contrast sufficient to show foramina and sinus cavities.
  • Clinical aim: Evaluate cranial base and foramina.

PA Axial Projection (Haas Method)

Clinical Indications

  • Alternative to AP Axial (Towne) when patient cannot flex neck sufficiently.

CR Location & Positioning

  • SID: 40 inches (102 cm)
  • Patient position: Prone or erect, nose and forehead against IR.
  • Adjustments: OML ⟂ IR; MSP ⟂ midline.
  • CR: 25° cephalad to OML, centered to pass through EAM, exiting 1.5 in. superior to nasion.
  • Pt. Instructions: Suspend respiration.
  • Exposure: 80–85 kVp, 10×12 IR lengthwise.

Evaluation Criteria

  • Coverage: Occipital bone, dorsum sellae/posterior clinoids within foramen magnum.
  • Rotation checks: Symmetric petrous ridges.
  • Motion checks: Crisp occipital and sella borders.
  • Technique checks: Proper brightness/contrast; collimated to occipital region.
  • Clinical aim: Occipital view with less facial dose, though magnified.

Trauma Skull Projections

Lateral — Horizontal Beam (Cross-Table)

Clinical Indications

  • Basal skull fractures, sphenoid sinus effusion.

CR Location & Positioning

  • SID: 40 inches
  • Patient position: Supine; IR beside head.
  • Adjustments: Do not move head/neck if C-spine injury suspected. Align MSP ∥ IR, IPL ⟂ IR, IOML ⟂ front edge.
  • CR: Horizontal, ⟂ IR, centered 2 in. superior to EAM.
  • Pt. Instructions: Suspend respiration.
  • Exposure: 80–90 kVp, grid.

Evaluation Criteria

  • Coverage: Entire cranium, including sella turcica.
  • Rotation checks: Superimposed orbital roofs, sphenoid wings, mandibular rami.
  • Motion checks: Sharp detail; no patient movement.
  • Technique checks: Adequate contrast for trauma detection.
  • Clinical aim: Detect air-fluid levels, baseline trauma evaluation.

AP 0° Projection

CR Location & Positioning

  • SID: 40 inches
  • Patient position: Supine, head in collar/backboard.
  • Adjustments: MSP ∥ midline.
  • CR: Parallel to OML, centered at glabella.
  • Pt. Instructions: Suspend respiration.

Evaluation Criteria

  • Petrous ridges fill orbits;
  • Frontal bone visualized.

AP Axial 15° (Reverse Caldwell)

CR Location & Positioning

  • SID: 40 inches
  • Patient position: Supine, head in collar.
  • CR: 15° cephalad to OML, centered at nasion.

Evaluation Criteria

  • Petrous ridges in lower third of orbits
  • Orbital rims symmetrical

AP Axial 30° (Trauma Towne)

CR Location & Positioning

  • SID: 40 inches
  • Patient position: Supine, head immobilized.
  • CR: 30° caudad to OML (37° to IOML), entering 2.5 in. above glabella, through foramen magnum.

Evaluation Criteria

  • Occipital bone and dorsum sellae within foramen magnum.

Skull Radiography Quick Reference Table

Projection CR Patient Position Key Evaluation Criteria
AP Axial (Towne) 30° caudad to OML (37° to IOML), entering 2.5 in. above glabella Supine/erect; chin tucked to place OML ⟂ IR (IOML if needed) Occipital bone & foramen magnum demonstrated; dorsum sellae/posterior clinoids in foramen magnum; no rotation (symmetric petrous ridges)
Lateral ⟂ IR, 2 in. superior to EAM Erect/semiprone; MSP ∥ IR, IPL ⟂ IR, IOML ⟂ front edge Entire cranium & sella turcica in profile; orbital roofs, greater wings, TMJs, mandibular rami superimposed
PA Axial (15° Caldwell) 15° caudad, exiting nasion (alternative 25–30° caudad) Prone/erect; nose & forehead on IR; OML ⟂ IR 15°: petrous ridges in lower ⅓ of orbits; 25–30°: ridges below IOMs, better orbital margins/foramen rotundum
PA (0°) ⟂ IR, exiting glabella Prone/erect; nose & forehead on IR; OML ⟂ IR Petrous ridges fill entire orbits; frontal bone, dorsum sellae, and sphenoid wings visualized
SMV ⟂ IOML, centered 1.5 in. inferior to symphysis (midway between gonions) Supine/erect; hyperextend neck until IOML ∥ IR, vertex on IR Foramen ovale/spinosum, sphenoid & ethmoid sinuses, mandible; mandibular condyles anterior to petrous ridges
PA Axial (Haas) 25° cephalad to OML, entering at EAM, exiting 1.5 in. above nasion Prone/erect; nose & forehead on IR; OML ⟂ IR Occipital bone, dorsum sellae, posterior clinoids within foramen magnum; symmetric petrous ridges
Lateral Trauma (Cross-Table) Horizontal beam ⟂ IR, 2 in. superior to EAM Supine; IR vertical at side of head; MSP ∥ IR, IPL ⟂ IR Entire cranium, sella turcica in profile; air-fluid levels in sphenoid sinus may indicate basal fracture
AP 0° (Trauma) Parallel to OML, centered at glabella Supine, immobilized (collar/backboard in place) Petrous ridges fill orbits; frontal bone included
AP Axial 15° (Reverse Caldwell) 15° cephalad to OML, exiting nasion Supine, immobilized Petrous ridges in lower ⅓ of orbits; symmetric orbital margins
AP Axial 30° (Trauma Towne) 30° caudad to OML (37° to IOML), entering 2.5 in. above glabella Supine, immobilized Occipital bone visualized; dorsum sellae/posterior clinoids in foramen magnum

 

Hows & Whys of Skull Radiography

ŸAnatomy

  • How many cranial bones are there? Name them.
    Eight: frontal, right and left parietals, occipital, ethmoid, sphenoid, and right and left temporals.
  • What are the three basic skull shapes?
    Brachycephalic – B= Broad; width is 80% of length or greater; petrous ridges form a 54-degree angle or more.
    Mesocephalic – M= Medium; Average or normal; Width is 75 – 80% of the length; petrous ridges form a 47-degree angle.
    Dolichocephalic – D= Dinky; Longest and most narrow head shape;  petrous ridges form a 40-degree angle or less.
  • How do the petrous ridge angles differ among these skull types?
    Brachycephalic skulls have steeper petrous ridges (closer to 54°), mesocephalic are at the standard 47°, and dolichocephalic skulls have flatter ridges (~40°).
  • How do skull shapes affect positioning?
    Angles of petrous ridges alter CR entry; narrow skulls (dolichocephalic) require slightly more CR angle, broad skulls (brachycephalic) require less, compared to average.
  • What gland sits in the sella turcica?
    The pituitary gland.
  • What are the anterior and posterior parts of the sella turcica called?
    The anterior clinoid processes and the posterior clinoid processes.
  • Which gland may appear calcified on a skull radiograph?
    The pineal gland, sometimes seen through the frontal bone.
  • What are the four main cranial sutures?
    Sagittal, coronal, lambdoidal, and squamous sutures.
  • What are “soft spots” on an infant’s skull called, and what do they become in adults?
    Fontanels. The anterior fontanel becomes the bregma, and the posterior fontanel becomes the lambda.
  • What external landmark helps locate the sella turcica?
    The top of ear attachment (TEA).

Positioning

  • Why is it important to align the midsagittal plane (MSP) perpendicular to the IR in skull imaging?
    To prevent rotation, which is evident when orbital margins, mandibular rami, or petrous ridges appear asymmetric.
  • Why is it especially important to avoid rotation on a PA skull?
    Rotation changes the distance between the lateral orbital margins and the skull cortex, which can mask fractures or mimic pathology.
  • Which positioning lines are most important for the lateral skull?
    The interpupillary line (IPL) must be perpendicular to the IR, and the infraorbitomeatal line (IOML) should be perpendicular to the front edge of the IR.
  • How do you know the interpupillary line (IPL) was perpendicular to the IR on a lateral skull?
    The orbital roofs and greater wings of the sphenoid will be superimposed.
  • Why do we tuck the chin for the Towne projection?
    Depressing the chin places the OML perpendicular to the IR, allowing the dorsum sellae and posterior clinoid processes to project into the foramen magnum without distortion.
  • What error is indicated if the dorsum sellae is projected above the foramen magnum on a Towne?
    The chin wasn’t flexed enough or the CR angle was insufficient.
  • What error is indicated if the posterior arch of C1 is superimposed on the dorsum sellae in a Towne?
    The chin was flexed too much or the CR angle was excessive.
  • Why is the 15° caudad angle used for the Caldwell?
    It projects petrous ridges into the lower third of the orbits, clearing orbital structures.
  • Why might a 25°–30° caudad angle be used for the Caldwell?
    It better demonstrates the orbital floors, superior orbital fissures, and foramen rotundum.
  • How do you know the MSP was perpendicular on a PA or Caldwell?
    The orbital margins are symmetrical side to side.
  • How do you know the MSP was perpendicular on a Waters?
    The orbital margins and nasal septum are symmetrical, with no lateral shift.
  • Why is the submentovertex (SMV) projection only attempted after cervical spine injury has been ruled out?
    Because hyperextension of the neck could worsen an undiagnosed fracture or dislocation.
  • Why is extreme neck extension required for the SMV?
    To place the IOML parallel to the IR, allowing accurate demonstration of the cranial base.
  • How do you know the IOML was parallel to the IR on an SMV?
    The mandibular condyles project anterior to the petrous ridges.
  • Why is the horizontal beam lateral critical in trauma cases?
    It allows imaging without moving the patient’s head or neck and can show sphenoid sinus fluid levels, which may indicate a basal skull fracture.
  • How does body habitus affect skull positioning?
    Hypersthenic patients (short, thick necks) have limited ability to tuck the chin because of neck girth; a sponge under the head helps bring the OML perpendicular. Asthenic patients (long, thin necks) tend to tip backward; a sponge under the shoulders/upper back helps bring the chin down.
  • What is the standard SID for skull projections, and why?
    40 inches — it maintains consistent image geometry and reduces magnification.
  • Why must the MSP be perpendicular on a Waters projection?
    To keep the orbital margins and nasal septum symmetrical and prevent lateral shift.

Technique & Image Evaluation

  • What structures are projected into the foramen magnum on the Towne projection?
    The dorsum sellae and posterior clinoid processes.
  • What structures lie on either side of the foramen magnum in the Towne projection?
    The occipital condyles.
  • Where are the petrous ridges projected on a PA (0°) projection?
    They fill the entire orbits.
  • Where are the petrous ridges projected on a Caldwell (15° caudad) projection?
    They appear in the lower third of the orbits.
  • How do you check for rotation on PA and Caldwell projections?
    The lateral orbital margins should be equal distance from the lateral cranial cortex.
  • How do you know the correct CR angle was used on a Caldwell?
    At 15°, the petrous ridges appear in the lower third of the orbits; at 25°–30°, they are projected just below the IOMs.
  • How do you check for rotation and tilt on an SMV?
    Equal distance between mandibular rami and the lateral cranial cortex indicates no tilt; symmetry of the cranial base confirms no rotation.
  • Where should the mentum be on a proper SMV?
    It should project anterior to the ethmoid sinuses.
  • What happens if the Towne CR angle is under-angled or the chin isn’t flexed enough?
    The dorsum sellae projects above the foramen magnum.
  • What happens if the Towne CR angle is over-angled or the chin is tucked too much?
    The posterior arch of C1 is superimposed on the dorsum sellae, causing foreshortening.
  • What exposure factors are typical for digital skull radiography?
    Generally 75–90 kVp with a grid. Image should show clear bony detail and soft tissue margins, free of motion or artifacts.
  • How do you know a trauma horizontal beam lateral was performed correctly?
    Orbital roofs, sphenoid wings, and mandibular rami are superimposed, and the sella turcica is in profile without tilt.
  • How do you know compensating supports were used effectively in hypersthenic or asthenic patients?
    Proper OML alignment is achieved: petrous ridges fall where expected, and orbital/nasal structures are symmetrical without shift.

Clinical Applications

  • What are common indications for skull radiography?
    Skull fractures, neoplastic processes, Paget’s disease, and pituitary gland evaluation.
  • What is the Haas method, and when is it used?
    A PA axial with a 25° cephalic angle, used as an alternative when the patient cannot flex the neck for the Towne.
  • Why is the Haas method less ideal for occipital bone studies?
    It produces magnification of the occipital region.
  • Which trauma projections are used when the head cannot be moved?
    AP 0°, AP axial 15° (reverse Caldwell), and AP axial 30° (reverse Towne).
  • Which projection is most useful for detecting tripod or complex fractures of the skull base?
    The SMV projection — it visualizes the zygomatic arches and foramen at the skull base.
  • Which projection can show medial or lateral displacement of cranial fractures?
    The AP Axial (Towne).
  • Which projection is critical for assessing paranasal sinus involvement with skull trauma?
    The PA Axial (Caldwell) — it demonstrates the frontal sinuses and anterior ethmoid air cells.

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Radiographic Procedures Review Guide Copyright © 2025 by Carla M. Allen and Taylor M. Otto. All Rights Reserved.

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