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HLT-RADI64-03 · Lesson 10 · 20 clock hours · lab

Skull and Facial Bone Positioning

Objective. Students will position a cooperative partner for the Waters, PA Caldwell, lateral skull, submentovertex, and nasal bone projections, correctly adjusting head alignment using angle landmarks and verification measurements.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed clinician in the field or by a certification or approval body. It is a study draft and may contain errors. Do not use it to make decisions about any patient or medication. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

Skull and Facial Bone Positioning

Accurate skull and facial bone radiography depends on precise head alignment. Even a few degrees of tilt or rotation can project anatomy in a misleading way, hide pathology, or superimpose structures that should appear separate. This lab teaches you five standard projections—the Waters method, PA Caldwell, lateral skull, submentovertex (SMV), and nasal bone views—along with the landmarks and measurements that make each one diagnostically useful.

Why Landmarks Matter

The skull has several reproducible reference lines and planes. The two you will use most are:

Learn to feel and see these landmarks on a real person before you attempt any projection.

The Waters Method (Parietoacanthial Projection)

The Waters view is the primary projection for demonstrating the maxillary sinuses, orbits, and zygomatic arches clear of superimposition by the petrous ridges.

Setup: The patient faces the IR (PA direction). The chin is extended so the OML forms a 37-degree angle with the IR. A quick check: the acanthion (the point at the base of the nose, where the nose meets the upper lip) should rest on the IR surface. The MSP must be perpendicular to the IR—no rotation. The central ray (CR) is perpendicular to the IR, directed to exit at the acanthion.

What to look for: The petrous ridges should project just below the floor of the maxillary sinuses. If the head is under-extended, the petrous ridges rise into the sinuses and obscure them.

PA Caldwell Projection

The Caldwell view demonstrates the frontal sinuses, frontal bone, superior orbits, and anterior ethmoid air cells.

Setup: The patient faces the IR. The OML is perpendicular to the IR (the nose and forehead both touch the IR surface). The CR is angled 15 degrees caudad (toward the patient's feet), exiting at the nasion (bridge of the nose).

What to look for: The petrous ridges are projected into the lower one-third of the orbits. If the petrous ridges fill the orbits completely, the caudal angle was insufficient.

Lateral Skull

The lateral projection shows the sella turcica, cranial vault, and facial bones in profile.

Setup: The patient is positioned in a true lateral, with the MSP parallel to the IR and the interpupillary line (IPL) perpendicular to the IR. The CR is directed perpendicular to the IR, centered 2 inches (5 cm) superior to the EAM. No tilt, no rotation.

What to look for: The orbital plates of the frontal bone should be superimposed (one directly over the other), and the sella turcica should be seen in profile with no double floor. Separation of these structures indicates rotation.

Submentovertex (SMV) Projection

The SMV is used to demonstrate the zygomatic arches, sphenoid sinuses, base of the skull, and mandibular condyles without superimposition.

Setup: The patient's head is hyperextended until the IOML is as parallel to the IR as possible. The chin is the highest point. The MSP must remain perpendicular to the IR. The CR is directed perpendicular to the IOML, through a point midway between the angles of the mandible.

Safety note: This position requires extreme neck extension. Ask about cervical spine injury or limitation before positioning. Never force extension. Stop if the patient reports pain or tingling.

Nasal Bone Projections

Nasal bones are small and thin, requiring careful technique:

Always use the smallest practical field size for nasal bone projections. This reduces patient dose and improves image contrast by minimizing scatter.

Safety and Patient Care in Skull Positioning

Using Your Angle Guide and Protractor

Before each simulated exposure, place your angle guide or protractor alongside the patient's head to verify the OML or IOML angle. A 3-degree error is the maximum acceptable tolerance for a competency check. Develop the habit of measuring before you call the position ready.

Key terms9

Orbitomeatal line (OML)An imaginary reference line drawn from the outer canthus of the eye to the center of the external acoustic meatus; used as the standard baseline for most skull projections.
Midsagittal plane (MSP)The vertical plane that divides the skull into equal right and left halves; must be perpendicular to the IR for PA and AP skull projections to prevent rotation.
Waters methodA parietoacanthial projection of the skull in which the OML forms a 37-degree angle with the IR, projecting the petrous ridges below the maxillary sinuses.
PA CaldwellA PA skull projection with the OML perpendicular to the IR and the CR angled 15 degrees caudad, placing the petrous ridges in the lower third of the orbits to show the frontal sinuses and superior orbits.
Submentovertex (SMV)A projection of the skull base in which the head is hyperextended until the infraorbitomeatal line is parallel to the IR, used to demonstrate zygomatic arches and the skull base.
Petrous ridgesDense bony projections of the petrous portion of the temporal bone that appear as prominent white lines on skull radiographs; their position relative to the orbits or sinuses indicates whether head angulation is correct.
Central ray (CR)The center of the X-ray beam directed to a specific anatomical entry or exit point; its angle and direction are critical to projecting anatomy correctly onto the IR.
CollimationRestriction of the X-ray beam to the smallest area that covers the anatomy of interest, reducing patient dose and scatter radiation.
Infraorbitomeatal line (IOML)A reference line from the inferior orbital rim to the center of the external acoustic meatus, approximately 7–8 degrees inferior to the OML; used primarily for the SMV projection.

Worked examples3

1. A student sets up the Waters projection and measures the angle between the OML and the IR at 42 degrees. How many degrees must the patient's chin be lowered (or raised) to reach the correct 37-degree angle, and in which direction?
  1. Recall the correct OML-to-IR angle for the Waters method: 37 degrees.
  2. Compare the measured angle to the target: 42 degrees measured − 37 degrees target = 5 degrees of difference.
  3. A larger OML-to-IR angle means the chin is extended too far (head tilted back too much). To decrease the angle, the chin must be lowered toward the IR.
  4. Conclusion: lower the chin by 5 degrees to achieve the correct 37-degree OML-to-IR angle.
Answer: Lower the chin by 5 degrees. The OML-to-IR angle will then equal the required 37 degrees for a true Waters projection.
2. For the PA Caldwell, the CR must be angled 15 degrees caudad. The student's tube is set at 0 degrees (perpendicular). The CR indicator shows the tube can tilt toward the patient's feet. If the student mistakenly angles 15 degrees cephalad instead, how many degrees off from correct is the CR direction, and what visual error would appear on the image?
  1. Correct CR: 15 degrees caudad (toward feet).
  2. Incorrect CR: 15 degrees cephalad (toward head).
  3. Total angular error: 15 degrees caudad − (−15 degrees cephalad) = 30 degrees from the correct direction.
  4. A cephalad angle projects structures upward relative to their true position. The petrous ridges would be thrown upward, filling the orbits rather than projecting into the lower third—obscuring the frontal sinuses and making the image non-diagnostic for the intended anatomy.
Answer: The CR is 30 degrees away from the correct angle. On the image, the petrous ridges would fill the orbits, obscuring the frontal sinuses—the opposite of what a correct Caldwell produces.
3. During the lateral skull competency check, the instructor measures the interpupillary line (IPL) and finds it is 8 degrees from perpendicular to the IR. The tolerance is 3 degrees. Does the student pass? How many degrees of correction are needed?
  1. Acceptable tolerance for competency: within 3 degrees of true perpendicular.
  2. Measured deviation of IPL from perpendicular: 8 degrees.
  3. Compare: 8 degrees > 3 degrees tolerance.
  4. Correction needed: 8 degrees − 0 degrees (ideal) = 8 degrees; or phrased as the amount beyond tolerance: 8 − 3 = 5 degrees beyond acceptable range.
  5. The student does not pass this attempt and must adjust the head tilt by approximately 8 degrees to bring the IPL to perpendicular.
Answer: The student does not pass. The IPL is 8 degrees from perpendicular, exceeding the 3-degree tolerance. The head must be adjusted approximately 8 degrees to achieve a true lateral position.
In class: Set up the Waters method (orbitomeatal line 37 degrees to IR), PA Caldwell (orbitomeatal line perpendicular, CR 15 degrees caudad), lateral skull, and SMV on a partner, using a protractor or positioning angle guide to verify head tilt before each simulated exposure; the instructor observes and provides immediate corrective feedback. · Compare student-positioned simulated or practice images to atlas reference images, identifying whether the petrous ridges fall in the correct position relative to the orbits for each projection, and document findings on an image critique form. · Practice nasal bone lateral and PA axial projections on a partner, noting the special care needed for collimation and CR alignment given the small anatomy and low-dose requirements.
Materials: Radiographic positioning unit (energized or simulation), head positioning angle guides, protractor, Merrill's Atlas, positioning sponges and head immobilizer, image critique forms, protective aprons, lead markers
Check: Instructor-observed competency check: student positions for Waters and lateral skull at random; correct head angle (within 3 degrees verified by instructor measurement), CR, collimation, and marker required on both — both must pass.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Radiographic Procedures
1. For the Waters projection, the orbitomeatal line (OML) should form what angle with the image receptor?
Radiographic Procedures
2. On a correctly positioned PA Caldwell image, where should the petrous ridges appear?
Image Evaluation
3. A student positions a patient for the lateral skull and notices the orbital plates of the frontal bone appear separated on the practice image. This most likely indicates:
Radiographic Procedures
4. Which reference line is placed as parallel to the image receptor as possible when positioning for the submentovertex (SMV) projection?
Radiation Protection
5. A student is preparing to radiograph the nasal bones in the lateral projection. Which action is most important for minimizing patient dose and improving image quality?
Patient Care and Safety
6. Before performing the SMV projection, a student must ask the patient about which condition because the position could cause harm?
Radiographic Procedures
7. A student measures the OML-to-IR angle for a Waters projection and finds it is 30 degrees. To correct the position, the student should:
Regulatory Compliance and Ethics
8. Which of the following best describes the role of the State Radiologic License in clinical practice?
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