Skull X-Ray: 6 Views — Towne, Caldwell, Lateral & SMV

Why Skull X-Ray Positioning Demands Precision

Skull positioning is technically demanding because dense, bilateral anatomy creates substantial superimposition. Small errors in rotation, tilt, flexion, extension or central-ray (CR) angle change the relationship of the petrous portions, orbits and skull base. Image evaluation should therefore use reproducible anatomic relationships rather than an unsupported millimeter tolerance.

The standard Towne alternatives are 30° caudad to the OML or 37° caudad to the IOML; the baseline and angle must be paired. In the standard Caldwell, the petrous ridges project in the lower third of the orbits, whereas they fill the orbits on a straight PA with the OML perpendicular.

This educational guide describes six named projections using common US teaching conventions. It is not an examination order or a universal protocol: projection selection, detector size/orientation, SID, grid use and exposure factors must follow the imaging department's approved protocol and the patient's condition.

Anatomy of the Skull: What You're Imaging

Before you position, you need to know what you're looking for. The skull is divided into two major parts:

• Neurocranium: 8 bones — frontal, parietal (2), temporal (2), occipital, sphenoid, ethmoid. The calvaria refers more specifically to the skullcap.
• Facial bones: 14 bones — maxilla (2), zygomatic (2), nasal (2), lacrimal (2), palatine (2), inferior nasal conchae (2), mandible, vomer

Depending on the clinical question, radiographs can depict the calvarium, sutures, skull base, paranasal sinus air spaces and radiopaque foreign bodies. Plain skull radiography is insensitive for many intracranial and skull-base abnormalities; it neither excludes intracranial injury nor replaces CT when CT is clinically indicated.

Key Landmarks and Baselines

These reference lines are commonly used; not every projection uses all of them:

Key surface landmarks:

• Nasion — the midline point at the frontonasal junction (Caldwell exit landmark)
• Glabella — the smooth bony prominence between your eyebrows (AP and Towne reference)
• External Auditory Meatus (EAM) — your ear canal opening (lateral centering landmark)
• Vertex — the top-most point of the skull
• External Occipital Protuberance (EOP) — a palpable posterior occipital prominence

Projection 1: Lateral Skull

A well-positioned lateral demonstrates the entire cranium and sella turcica in profile. The side placed against the IR is the side demonstrated with least magnification; bilateral structures remain superimposed, so a lateral skull is not a side-isolating projection.

Patient Position

• Erect or recumbent as the patient's condition and protocol permit; do not rotate or flex a trauma patient's head until cervical injury has been excluded or movement is authorized
• Head in true lateral — the midsagittal plane is parallel to the IR
• Interpupillary line perpendicular to the IR
• Adjust flexion/extension so the IOML is parallel to the transverse axis of the IR

Baselines

• IOML parallel to the transverse axis of the IR; IPL perpendicular to the IR

Central Ray

• Perpendicular to the IR
• Common US convention: center approximately 5 cm (2 inches) superior to the EAM. Some texts or departments use a nearby point referenced to the EAM; follow the approved local protocol.

Technical Factors

• A common adult setup uses a 24 × 30 cm (10 × 12 in) detector and 100–110 cm (40–44 in) SID
• Detector orientation, grid use and exposure factors are equipment- and patient-specific; use the approved technique chart
• Breathing: Suspend respiration

Evaluation Criteria

• Orbital roofs superimposed — indicates no tilt
• Mandibular rami superimposed — indicates no rotation
• Sella turcica in profile near the center of the image
• Temporomandibular joints superimposed
• Both posterior clinoid processes superimposed

Projection 2: PA Skull — Occipitofrontal

When safe and feasible, PA positioning places anterior structures closer to the IR and generally lowers eye-lens dose compared with AP. Whether a PA skull is part of an examination is determined by the order and local protocol; there is no universal modern “routine skull series.”

Patient Position

• Prone (or erect with forehead against the bucky)
• Forehead and nose against the IR/bucky surface
• Arms at sides or under chest for comfort
• Midsagittal plane perpendicular to the IR

Baselines

• OML perpendicular to the IR

Central Ray

• Perpendicular (0°) to the IR
• Exits at the glabella in the standard straight PA skull convention

Evaluation Criteria

• Petrous ridges filling the orbits symmetrically with the OML perpendicular
• Lateral orbital margins equidistant to the lateral borders of the cranium (no rotation)
• Entire cranial vault visualized
• Sagittal suture centered

Projection 3: PA Axial — Caldwell Method

The standard Caldwell uses caudal angulation to project the petrous ridges into the lower thirds of the orbits, demonstrating the frontal bone and anterior ethmoid region. It does not remove the petrous ridges entirely from the orbits.

Patient Position

• Same as the PA skull — forehead and nose against the IR

Baselines

• OML perpendicular to the IR

Central Ray

• Angled 15° caudad
• Exits at the nasion

Evaluation Criteria

• Petrous ridges projected into the lower third of the orbits (not filling the entire orbit as in the PA view)
• Superior orbital fissures and greater/lesser sphenoid wings visible
• Frontal sinus and anterior ethmoid air cells are demonstrated when included; an erect horizontal-beam sinus examination is needed to assess air-fluid levels
• No rotation (symmetric orbital margins)

Projection 4: AP Skull

The AP skull is an alternative when PA positioning is not feasible and the projection is included in the ordered protocol. It is not automatically a “trauma view”: acute head trauma requiring neuroimaging is generally evaluated with noncontrast head CT, and any patient movement must respect spinal precautions.

Patient Position

• Supine
• Bring the OML perpendicular to the IR only when neck flexion is safe; otherwise use the trauma/local protocol without forcing the head
• Midsagittal plane centered to the grid midline

Central Ray

• Perpendicular (0°) to the IR
• Entering at the glabella in the standard straight AP skull convention

Evaluation Criteria

• Petrous ridges symmetric (no rotation)
• Entire cranial vault visible
• Sagittal suture centered

Projection 5: AP Axial — Towne Method

The Towne projection demonstrates the occipital region, foramen magnum, petrous portions, dorsum sellae and posterior clinoids. It is a positioning projection, not an adequate test to exclude posterior-fossa injury or occipital fracture; CT is preferred when acute head injury requires imaging.

Patient Position

• Supine or erect AP, depending on equipment and patient condition
• Flex the neck only when safe: OML perpendicular for the 30° option, or IOML perpendicular for the 37° option

Central Ray

• Angled 30° caudad to the OML OR 37° caudad to the IOML
• CR enters approximately 6 cm (2.5 in) above the glabella
• The CR passes approximately 2 cm (3/4 in) anterior to the EAMs, at the level of the foramen magnum

Technical Factors

• A common adult setup uses a 24 × 30 cm (10 × 12 in) detector and 100–110 cm (40–44 in) SID
• Detector orientation, grid use and exposure factors are equipment- and patient-specific; use the approved technique chart

Evaluation Criteria

• Dorsum sellae and posterior clinoids project within the foramen magnum
• Petrous ridges symmetric
• Occipital bone and posterior fossa well demonstrated
• No rotation

Troubleshooting the Towne View

Projection 6: Submentovertex — SMV

The SMV (basal) projection demonstrates the skull base, including the sphenoid sinus, foramina and zygomatic arches. Its use is protocol- and indication-specific; suspected acute skull-base fracture is generally evaluated with CT rather than an SMV radiograph.

Patient Position

• Supine or erect only if the patient can safely extend the neck
• Hyperextend the neck until the IOML is parallel to the IR
• The vertex rests against the IR/bucky
• Support as needed without forcing the neck; stop if pain, neurologic symptoms or movement restriction is present

Central Ray

• Perpendicular to the IOML
• Center in the midsagittal plane midway between the mandibular angles (a common US convention)
• The CR passes through the skull base at the level of approximately 2 cm (3/4 in) anterior to the EAMs; follow the department's named-projection protocol

Evaluation Criteria

• Sphenoid sinuses and basal foramina demonstrated
• Mandibular condyles project anterior to the petrous portions
• Zygomatic arches visible
• Symmetric basal anatomy

Patient Considerations

The SMV requires marked neck extension. Do not attempt it when cervical-spine injury is suspected or has not been cleared, and do not force extension in a patient with pain or restricted mobility. Ask the radiologist or ordering team for an appropriate alternative; a reverse Waters view is not a general substitute for skull-base CT.

Projection Reference Summary

Protocol note: These six descriptions are a teaching set, not a recommendation to perform six exposures. Perform only the ordered, locally approved projections. For acute head trauma that meets imaging criteria, ACR guidance generally favors noncontrast head CT; skull radiographs are usually not appropriate as a substitute.

Radiation Protection for Skull Radiography

The eye lens can lie in or near the primary beam, depending on projection. The thyroid and embryo/fetus are outside a tightly collimated skull field and receive scatter rather than the primary beam. Optimize each examination rather than quoting a universal dose: patient dose varies with projection, collimation, patient size, equipment, grid and technique.

• Justification and technique: Verify the order, avoid duplicate projections, collimate to the required anatomy, use the approved technique chart and minimize repeats.
• Projection choice: When clinically safe and diagnostically equivalent, PA rather than AP generally reduces eye-lens dose because the lenses are farther from the entrance surface.
• Patient contact shielding: AAPM recommends discontinuing routine gonadal and fetal shielding in x-ray imaging because it provides negligible benefit, can obscure anatomy and can interfere with automatic exposure control. Do not claim that a gonadal shield is required solely because a patient is of childbearing potential. Follow current institutional policy and applicable regulation.
• Thyroid collars: A collar is not universally “standard” for skull work and may enter the field or cause a repeat. Use only if required by local policy and it cannot obscure anatomy or interfere with the examination.
• Pregnancy: Ask and document pregnancy status according to policy. A medically necessary skull radiograph should not be withheld solely because of pregnancy; consult the radiologist/medical physicist when needed and keep the abdomen outside the primary beam.

Positioning Errors: Quick Reference

ARRT Exam Prep: Skull Positioning Quick Review

ARRT publishes content specifications by category, not a guaranteed count of skull questions. Use the current Radiography Content Specifications and your program's required references; the following is a positioning review, not inside information about an examination:

1. Projection selection is protocol-specific: these six named projections do not constitute a universal routine or trauma series
2. Caldwell = 15° caudad, CR exits the nasion, petrous ridges in the lower third of the orbits
3. Towne = 30° caudad to OML (37° to IOML), centering 2.5" above glabella, dorsum sellae in foramen magnum
4. Lateral: common US centering is 2 in (5 cm) superior to EAM; true lateral requires no rotation or tilt
5. PA versus AP: PA is preferred when safe and protocol-equivalent because it reduces anterior-structure magnification and generally lowers eye-lens dose
6. Use the named baseline: do not interchange the OML and IOML without the corresponding angle adjustment
7. SMV requires IOML parallel to the IR and marked extension; never force it or use it before cervical-spine clearance
8. The sella turcica is the central structure visible on a well-positioned lateral skull

Clinical Pearls: Tips from the Techs

• Make your patient comfortable first. Skull positioning is precise, but an uncomfortable patient will move. Pillows, sponges, and clear instructions make the difference between a diagnostic exam and a retake.
• Remove external objects when safe. Hair fasteners, earrings, removable dentures and hearing devices can create superimposed artifacts; document anything that cannot be removed.
• Talk your patient through the breathing instructions. "Take a deep breath in, let it out, then hold" — respiration suspension is standard for skull radiography to prevent motion artifact.
• Check landmarks without forcing movement. Confirm the nasion, glabella, EAM and selected baseline before exposure while respecting pain, postoperative restrictions and spinal precautions.

Authoritative References

• Bontrager's Textbook of Radiographic Positioning and Related Anatomy, 10th ed. (Elsevier) — named-projection positioning and image-evaluation conventions.
• ACR Appropriateness Criteria: Head Trauma — modern modality selection and the limited role of skull radiographs in acute head trauma.
• AAPM Position Statement on the Use of Patient Gonadal and Fetal Shielding — routine patient contact shielding guidance.
• ARRT Examination Content Specifications — official examination scope; ARRT does not promise a fixed number of skull questions.