Skull X-Ray Positioning: Lateral, PA, Caldwell, Towne, and AP Views
Why Skull X-Ray Positioning Demands Precision
Skull radiography is one of the most technically demanding areas of radiographic positioning — and one of the most heavily tested on the ARRT registry. Unlike extremity work where you have generous tolerance for centering errors, skull positioning requires precision within millimeters because the bony anatomy is dense and overlapping structures can obscure fractures if the angle is off by just a few degrees.
Every year, rad tech students struggle with the same questions: Is it 30° or 37° for the Towne view? Where do the petrous ridges fall on a Caldwell vs a straight PA? What's the centering point for the lateral skull? These aren't just trivia — they directly affect image quality and diagnostic accuracy.
Whether you're a student preparing for clinicals or a working tech refreshing your cranial positioning skills, this guide covers the six essential skull projections with Clark's-verified centering points, evaluation criteria, and ARRT exam tips you won't find in a standard textbook summary.
📖 What You'll Learn
By the end of this guide, you'll know the centering points, tube angles, IR sizes, and evaluation criteria for all six major skull projections: lateral, PA, PA axial (Caldwell), AP, AP axial (Towne), and submentovertex (SMV). You'll also understand the specific ARRT exam angles for each projection and how to troubleshoot common positioning errors.
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:
- Cranium (calvarium): 8 bones — frontal, parietal (2), temporal (2), occipital, sphenoid, ethmoid
- Facial bones: 14 bones — maxilla (2), zygomatic (2), nasal (2), lacrimal (2), palatine (2), inferior nasal conchae (2), mandible, vomer
On a clinical level, you're assessing bone density, fracture lines, suture diastasis, vascular markings, and the paranasal sinuses depending on the projection. Pneumocephalus, foreign bodies, and mass lesions may also be identified — but your job as the technologist is to produce a diagnostic image that makes those findings visible.
Key Landmarks and Baselines
Getting skull positioning right comes down to mastering three anatomical lines. Every projection uses one of these as its reference baseline:
| Baseline | Landmarks | Used For |
|---|---|---|
| Orbitomeatal Line (OML) | Outer canthus of eye → center of EAM | PA, AP, Caldwell (primary baseline) |
| Infraorbitomeatal Line (IOML) | Infraorbital margin → upper border of EAM | Towne (37° variant), SMV, CT head alignment |
| Meatomental Line (MML) | EAM → mental point (tip of chin) | Lateral positioning verification |
Key surface landmarks:
- Nasion — the depression at the bridge of your nose (primary centering landmark for PA and Caldwell)
- 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) — the bump at the base of your skull (posterior landmark)
Clinical Pearl — OML vs IOML
A common mistake students make is confusing the OML with the IOML when setting up the Towne view. The difference is 7° — and on the ARRT, that's the difference between a pass and a fail on technique. Remember: OML = canthus to ear; IOML = infraorbital rim to the upper margin of the ear canal.
Projection 1: Lateral Skull (Clark's, p190–192)
The lateral skull is usually the first projection in a routine skull series and the one that sets the standard for head positioning. A well-positioned lateral demonstrates the sella turcica in profile, the vascular grooves of the calvarium, and the entire cranium from vertex to base.
Patient Position
- Erect (preferred for comfort) or recumbent (trauma)
- Head in true lateral — the midsagittal plane is parallel to the IR
- Interpupillary line perpendicular to the IR
- Chin slightly elevated to avoid superimposition of the mandible on the cervical spine
Baselines
- IOML perpendicular to the IR (parallel to the transverse axis of the IR)
Central Ray (Clark's, p190)
- Perpendicular to the IR
- Entering 2.5 cm (1 inch) superior and 2.5 cm (1 inch) anterior to the EAM (the side closer to the IR)
Technical Factors
- IR size: 24 × 30 cm (10 × 12 in), portrait
- SID: 100 cm (40 in)
- Grid: Yes
- 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
| Structure to Check | Pass/Fail Indicator |
|---|---|
| Orbital roofs | If not aligned → tilt error |
| Mandibular angles | If doubled → rotation error |
| Sella turcica outline | Should be crisp, not blurred |
| Vascular grooves | Should be visible throughout |
ARRT Exam Tip — Lateral Skull
The ARRT frequently tests the centering point for the lateral skull. Remember it as "one and one" — 1 inch (2.5 cm) superior AND 1 inch (2.5 cm) anterior to the EAM. A common distractor on the registry is "2.5 cm above the EAM only" (missing the anterior component). Both coordinates are required for proper centering.
Projection 2: PA Skull — Occipitofrontal (Clark's, p198)
The PA projection is preferred over the AP because it places the orbits and facial bones closer to the IR, reducing magnification and providing better bony detail. It is the standard for non-trauma skull examinations.
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 (Clark's, p198)
- Perpendicular (0°) to the IR
- Directed to the nasion
Evaluation Criteria
- Petrous ridges filling the orbits symmetrically — this is the key criterion. If the petrous ridges are not filling the orbits, the OML was not 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 (Clark's, p200)
The Caldwell view uses a slight caudal angulation to shift the petrous ridges out of the lower orbits, providing an unobstructed view of the orbital margins, superior orbital fissures, and the frontal/ethmoid sinuses.
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 and ethmoid sinuses well demonstrated
- No rotation (symmetric orbital margins)
| View | CR Angulation | Petrous Ridge Position |
|---|---|---|
| PA (straight) | 0° — perpendicular | Fills lower two-thirds of orbits |
| Caldwell (15°) | 15° caudad | Lower third of orbits |
| 20° caudad (rare) | 20° caudad | Below orbits entirely |
ARRT Exam Tip — Caldwell Petrous Ridge Logic
A classic registry question asks: "How much caudal angulation is used for the Caldwell view?" — 15° caudad. But the trick variant asks: "Where should the petrous ridges appear on a properly positioned Caldwell?" — In the lower third of the orbits. If they fill the entire orbit, the angle is too shallow (closer to 0°). If they're below the orbits, the angle is too steep (>15°). This "petrous ridge logic" reappears across multiple skull projections on the ARRT exam.
Projection 4: AP Skull (Clark's, p197)
The AP skull is used when the patient cannot assume a prone position — trauma, post-surgical, or unconscious patients. It's also the standard for portable skull radiography in the emergency department.
Patient Position
- Supine
- Neck flexed to bring the OML perpendicular to the IR
- Midsagittal plane centered to the grid midline
Central Ray (Clark's, p197)
- Perpendicular (0°) to the IR
- Entering at the nasion (or mid-orbital level if the vertex must be included)
Evaluation Criteria
- Petrous ridges symmetric (no rotation)
- Entire cranial vault visible
- Sagittal suture centered
Projection 5: AP Axial — Towne/Grashey Method (Clark's, p202)
The Towne view is arguably the most important skull projection for occipital bone fractures and posterior fossa evaluation. It is a required view in most trauma skull series and is frequently tested on the registry.
Patient Position
- Supine
- Neck flexed to bring the IOML perpendicular to the IR (or OML perpendicular for the 30° variant)
Central Ray
- Angled 30° caudad to the OML OR 37° caudad to the IOML
- CR enters approximately 5–6 cm (2.5 in) above the glabella
- The CR passes through the foramen magnum
Technical Factors
- IR size: 24 × 30 cm (10 × 12 in), portrait
- SID: 100 cm (40 in)
- Grid: Yes
Evaluation Criteria
- Dorsum sellae must overlie the foramen magnum — the most critical evaluation point
- Petrous ridges symmetric
- Occipital bone and posterior fossa well demonstrated
- No rotation
Troubleshooting the Towne View
| Finding on Image | Problem | Fix |
|---|---|---|
| Dorsum sellae projects above foramen magnum | Insufficient angle | Increase caudad angle |
| Anterior arch of C1 visible in foramen magnum | Excessive angle | Decrease caudad angle |
| Asymmetric petrous ridges | Rotation | Reposition head (no tilt) |
ARRT Exam Tip — The Dorsum Sellae Rule
If you see a question that asks "What structure must be seen within the foramen magnum on a Towne view?" — the answer is the dorsum sellae and posterior clinoid processes. Remember it as "dorsum in the foramen." Another common question: "What adjustment should you make if the dorsum sellae projects above the foramen magnum?" — the answer is increase the caudad angle.
Projection 6: Submentovertex — SMV (Clark's, p204–206)
The SMV (also called the "base view" or "full basal view") provides a bird's-eye perspective of the skull base. It is less commonly performed but critical for evaluating the sphenoid sinuses, basal foramina, and zygomatic arches.
Patient Position
- Supine (or erect in a wheelchair for patients who cannot hyperextend)
- Hyperextend the neck until the IOML is parallel to the IR
- The vertex rests against the IR/bucky
- Shoulders supported with pillows for patient comfort
Central Ray (Clark's, p204)
- Perpendicular to the IOML
- Entering at the midpoint of the line between the mental point (chin tip) and the EAM
- The CR passes approximately 1.5 cm (0.6 in) below the mandibular symphysis at the level of the sella turcica
Evaluation Criteria
- Sphenoid sinuses and basal foramina demonstrated
- Mandibular body superimposed on petrous pyramids (no rotation)
- Zygomatic arches visible
- Symmetric basal anatomy
Patient Considerations
This is not a comfortable position — patients with cervical spine injuries, severe arthritis, or limited neck mobility should not be positioned for this view. The reverse Waters (or similar alternative projection) may be substituted if the SMV is contraindicated. Take your time explaining the position — patient cooperation is essential.
Routine Skull Series Summary
| # | Projection | Key Angle | Centering Point | IR Size | Primary Purpose |
|---|---|---|---|---|---|
| 1 | Lateral | 0° (true lateral) | 1" superior & anterior to EAM | 10×12 P | Overall skull, sella, fractures |
| 2 | PA (Occipitofrontal) | 0° perpendicular | Nasion | 10×12 P | Frontal bone, orbits, sutures |
| 3 | PA Axial (Caldwell) | 15° caudad | Exits at nasion | 10×12 P | Sinuses, orbital fissures |
| 4 | AP (supine) | 0° | Nasion | 10×12 P | Trauma, portable exams |
| 5 | AP Axial (Towne) | 30° to OML (37° to IOML) | 2.5" above glabella | 10×12 P | Occipital bone, posterior fossa |
| 6 | SMV | Perp. to IOML | Midpoint mental-EAM | 10×12 P | Skull base, sphenoid sinuses |
Note: A complete skull series may not require all 6 projections. Clinical indications determine the series. A trauma series typically includes lateral, AP, and Towne. A routine series adds PA and Caldwell.
Radiation Protection for Skull Radiography
Skull radiography exposes the lens of the eye and the thyroid gland — both radiosensitive tissues.
- Gonadal shielding: Not required for skull exams, but always use if the patient is of childbearing age (follow your department's policy)
- Thyroid shielding: Use a thyroid shield whenever it does not obstruct the anatomy of interest
- Lead contact shielding: AAPM/ACR/NCRP guidelines have moved away from routine gonadal shielding — but thyroid collars remain standard practice for skull radiography
- Collimation: Tight collimation to the skull is the most effective dose-reduction technique. Always collimate to skin margins
Positioning Errors: Quick Reference
| Error | Likely Cause | Correction |
|---|---|---|
| Orbital roofs not superimposed (Lateral) | Tilt (side-to-side) | Align interpupillary line perpendicular to IR |
| Mandibular rami doubled (Lateral) | Rotation | Check midsagittal plane parallel to IR |
| Petrous ridges fill orbits but patient rotated (PA) | OML not perpendicular + rotation | Recheck OML perpendicular and center MS plane |
| Petrous ridges below orbits (Caldwell) | CR angle too steep (>15°) | Reduce to standard 15° caudad |
| Dorsum sellae above foramen magnum (Towne) | Angle too shallow | Increase to 30° (OML) or 37° (IOML) |
| C1 arch visible (Towne) | Angle too steep | Decrease angle |
| SMV sphenoid sinuses asymmetrical | Rotation or tilt | Check midsagittal plane |
ARRT Exam Prep: Skull Positioning Quick Review
The ARRT registry examination typically includes 8–12 questions on skull and facial bone positioning. Here are the highest-yield facts to remember:
- The routine skull series includes: lateral, PA (or AP for trauma), PA axial (Caldwell), AP axial (Towne), and optional SMV
- Caldwell = 15° caudad, nasion centering, petrous ridges in lower third of orbits
- Towne = 30° caudad to OML (37° to IOML), centering 2.5" above glabella, dorsum sellae in foramen magnum
- Lateral = 1" (2.5 cm) superior and anterior to EAM, true lateral no rotation or tilt
- PA preferred over AP for non-trauma patients (less orbit magnification, better bone detail)
- OML is the primary baseline for most skull projections (not IOML)
- SMV requires IOML parallel to the IR with full neck hyperextension
- The sella turcica is the central structure visible on a well-positioned lateral skull
ARRT Exam Tip — Identifying Positioning Errors
On the registry, you will be asked to identify positioning errors from an image. Learn to recognize: rotation (doubled margins), tilt (non-superimposed orbital roofs), and incorrect angle (petrous ridges in wrong position). These three errors account for the majority of skull positioning questions.
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 everything from the hair. Bobby pins, hair clips, earrings, dentures — all cause artifacts that can mimic fractures. This is especially important in trauma where every subtle finding matters.
- 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.
- If you're not sure about your centering, double-check your landmarks. Palpate the nasion, glabella, and EAM before every exposure. A few extra seconds of palpation saves a retake.