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HLT-RADI64-08 · Lesson 4 · 25 clock hours · lab

Advanced Radiographic Positioning: Spine, Skull, and Facial Bones

Objective. Students produce correctly positioned and collimated spine and skull images on a phantom, explaining the anatomical landmarks and centering points used for each projection.

⚠ 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.

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Advanced Radiographic Positioning: Spine, Skull, and Facial Bones

Accurate positioning of the spine, skull, and facial bones is one of the most technically demanding skills in radiologic technology. Small errors in patient alignment, central ray angulation, or collimation can obscure pathology or require repeat exposures that increase patient dose. This lesson builds on foundational positioning skills and prepares you to perform and critique complete spine and skull series independently.

The Cervical Spine Series

The standard cervical spine series typically includes the AP axial (Ottonello or standard AP), the AP open-mouth odontoid, bilateral obliques, and a lateral projection.

AP axial projection: The patient is supine or erect, chin slightly raised, midsagittal plane centered to the image receptor (IR). The central ray (CR) is angled 15–20° cephalad and directed to the level of C4 (thyroid cartilage). This angulation opens the intervertebral joint spaces and projects the mandible superiorly off the spine.

AP open-mouth (odontoid) projection: The patient opens the mouth wide so the upper and lower incisors are superimposed on the occiput and base of skull, placing C1 and C2 (atlas and axis, including the dens/odontoid process) in clear view. The CR is perpendicular, directed through the open mouth to the mid-C1–C2 level. Careful positioning is essential; the dens must not be cut off by the upper incisors or occipital bone.

Lateral projection: Performed erect (upright) whenever possible. The midsagittal plane is parallel to the IR, the interpupillary line is perpendicular to the IR, and the CR is horizontal at the level of C4. For trauma patients, a cross-table lateral is performed without moving the patient.

Posterior obliques (LPO/RPO) or anterior obliques (LAO/RAO): Oblique projections demonstrate the intervertebral foramina. Posterior obliques: the body is rotated 45°; the foramina closest to the IR are demonstrated. The CR is angled 15–20° cephalad to C4. Anterior obliques demonstrate the opposite side's foramina.

Thoracic and Lumbar Spine

Thoracic spine AP: Patient supine, midsagittal plane centered, CR perpendicular at the level of T7 (3–4 inches below the jugular notch, or at the inferior angle of the scapulae). Collimate to include T1–T12.

Thoracic spine lateral: Patient lateral, coronal plane parallel to IR, CR horizontal at T7. Using a breathing technique (long exposure while patient breathes quietly) blurs overlying rib and lung structures and improves vertebral body visualization.

Lumbar spine AP: Patient supine, knees flexed to flatten the lumbar lordosis, midsagittal plane centered, CR perpendicular at the level of L3 (approximately 1–1.5 inches above the iliac crest). Collimate to include the full lumbar spine and sacrum.

Lumbar spine lateral: CR horizontal at L3–L4. The lumbar obliques (45° rotation) demonstrate the zygapophyseal (facet) joints; the side down is demonstrated on posterior obliques.

Skull and Facial Bone Projections

PA Caldwell projection (15° caudad CR): Demonstrates the frontal bone, anterior ethmoid sinuses, and orbits. The orbitomeatal line (OML) is perpendicular to the IR; the CR is angled 15° caudad, exiting at the nasion. This projects the petrous ridges to the lower one-third of the orbits.

Lateral skull projection: Midsagittal plane parallel to IR, interpupillary line perpendicular. CR perpendicular, directed 2 inches superior to the external acoustic meatus (EAM). Demonstrates the sella turcica, clinoid processes, and cranial vault profile.

Waters projection (parietoacanthial): The primary view for facial bones and maxillary sinuses. The head is extended so the OML forms approximately a 37° angle with the IR. The CR is perpendicular, directed to exit at the acanthion (base of the anterior nasal spine). The petrous ridges are projected below the maxillary sinuses.

Key Anatomical Landmarks

Memorize the surface landmarks used to locate centering points: the glabella (between the brows), nasion (bridge of the nose), acanthion (base of nose/upper lip junction), gonion (angle of the mandible), EAM, mastoid tip, and the orbital-meatal line (line from outer canthus of eye to EAM).

For the spine: the jugular notch (T2–T3), sternal angle of Louis (T4–T5), xiphoid tip (T9–T10), iliac crest (L4–L5), ASIS (S1), and the thyroid cartilage (C4–C5).

Collimation, Shielding, and Radiation Safety

Collimate the beam to the area of clinical interest only — never larger than the IR. For lumbar spine AP, collimate to include L1 through S1 and the SI joints, keeping beam width within the body margins. Proper collimation reduces scatter, improves image contrast, and limits patient dose.

Gonadal shielding is used whenever the gonads lie within or near the primary beam AND shielding will not obscure essential anatomy. For lumbar spine imaging, ovarian shielding in female patients can be challenging because the ovaries are close to the structures of interest; clinical judgment and institutional protocol guide this decision. For male patients, a contact shield is placed over the testes during lumbar and pelvis exposures whenever feasible. Thyroid shielding (a thyroid collar) should be used during cervical spine and skull imaging to reduce dose to this radiosensitive gland. Always document shielding use.

Quality Criteria Summary

Every image must show: correct anatomy included, proper alignment (no rotation unless oblique intended), appropriate density and contrast, and collimation margins visible. Use Merrill's Atlas as your reference for every projection's specific quality criteria before and after each exposure.

Key terms10

Orbitomeatal line (OML)An imaginary line extending from the outer canthus of the eye to the center of the external acoustic meatus (EAM); a standard reference line for skull positioning.
Central ray (CR)The center of the x-ray beam, directed to a specific anatomical point; its angle and entry point determine which structures are demonstrated and how they are projected.
Odontoid process (dens)The peg-like superior projection of C2 (axis) that articulates with the anterior arch of C1 (atlas); visualized on the AP open-mouth projection.
Zygapophyseal jointsThe facet joints between superior and inferior articular processes of adjacent vertebrae; demonstrated on oblique projections of the spine.
AcanthionThe surface landmark at the base of the anterior nasal spine where it meets the upper lip; used as the CR exit point for the Waters (parietoacanthial) projection.
CollimationRestricting the x-ray beam to the smallest field necessary to cover the area of clinical interest, reducing scatter radiation and patient dose while improving image contrast.
Caldwell projectionA PA skull projection with a 15° caudad CR angulation that projects the petrous ridges to the lower third of the orbits, demonstrating the frontal bone and anterior ethmoid sinuses.
Breathing techniqueA long-exposure method used on the lateral thoracic spine in which the patient breathes quietly during the exposure, blurring overlying ribs and lungs to improve vertebral body visibility.
Intervertebral foraminaOpenings between adjacent vertebrae through which spinal nerve roots exit; best demonstrated on oblique projections of the cervical and lumbar spine.
Gonadal shieldingLead or equivalent shields placed over the gonads to reduce radiation dose when the gonads are in or near the primary beam and shielding will not obscure essential anatomy.

Worked examples4

1. Calculate the field size (in centimeters) projected at the image receptor for a lumbar spine AP when the source-to-image distance (SID) is 100 cm, the source-to-object distance (SOD) is 80 cm, and the collimator opening at the collimator face is set to 15 cm × 35 cm.
  1. Step 1 — Identify the magnification factor (MF): MF = SID ÷ SOD = 100 cm ÷ 80 cm = 1.25.
  2. Step 2 — Calculate the projected field width at the IR: 15 cm × 1.25 = 18.75 cm.
  3. Step 3 — Calculate the projected field length at the IR: 35 cm × 1.25 = 43.75 cm.
  4. Step 4 — Confirm the projected field (18.75 cm × 43.75 cm) fits within the IR size typically used (e.g., 35 cm × 43 cm landscape or 30 cm × 35 cm), and ensure it covers L1 through S1 without unnecessary extension beyond the body margins.
Answer: The projected field size at the IR is approximately 18.75 cm × 43.75 cm, meaning the beam has been magnified by a factor of 1.25 relative to the collimator setting. If the IR is smaller than the projected field, reduce collimator opening so the field does not exceed the IR.
2. A student sets up for a lateral cervical spine at 183 cm (72-inch) SID. The anatomy (C-spine) is centered at mid-neck, approximately 10 cm from the IR surface (OID = 10 cm). What is the source-to-object distance, and what magnification factor should the student expect?
  1. Step 1 — Calculate SOD: SOD = SID − OID = 183 cm − 10 cm = 173 cm.
  2. Step 2 — Calculate magnification factor: MF = SID ÷ SOD = 183 ÷ 173 ≈ 1.058.
  3. Step 3 — Interpret: The image will be magnified approximately 5.8% relative to true anatomical size. This is acceptable for routine diagnostic purposes; using a longer SID (72 inches is standard for erect cervical spine) minimizes this magnification compared to shorter distances.
Answer: SOD = 173 cm; MF ≈ 1.058. The cervical spine structures will appear approximately 5.8% larger on the image than their true size, which is within acceptable limits at 72-inch SID.
3. For a PA Caldwell projection, the patient's OML must be perpendicular to the IR and the CR angled 15° caudad. If the technologist instead forgets the angulation and uses a perpendicular CR, which structures will be affected and why?
  1. Step 1 — Recall the purpose of the 15° caudad angle: it projects the petrous ridges inferiorly so they fall in the lower one-third of the orbits, allowing the orbital rims and frontal sinuses to be seen clearly.
  2. Step 2 — Without the caudad angle (CR perpendicular), the petrous ridges are not projected downward and will instead superimpose on the mid-orbit region.
  3. Step 3 — This results in a PA skull projection (0° — sometimes called a simple PA), which obscures the orbital floors and ethmoid sinuses with the dense petrous pyramids.
  4. Step 4 — The corrective action is to re-angle the CR 15° caudad toward the feet, or to reposition the patient's head to compensate if the unit does not allow tube angulation.
Answer: Without the 15° caudad angulation, the petrous ridges superimpose over the mid-orbits rather than the lower third, obscuring pathology in the orbital floors and ethmoid sinuses. The CR must be angled 15° caudad for a true Caldwell projection.
4. A lumbar spine AP field on the collimation guide is measured as 18 cm wide × 40 cm long at the IR. The IR used is 35 cm × 43 cm. Is the field within the IR borders, and is unnecessary irradiation occurring on any side?
  1. Step 1 — Compare field width to IR width: 18 cm field vs. 35 cm IR width. The field is well within the IR's 35 cm width — 8.5 cm of unexposed IR on each side.
  2. Step 2 — Compare field length to IR length: 40 cm field vs. 43 cm IR length. The field fits within the IR's 43 cm length with 1.5 cm to spare.
  3. Step 3 — Assess whether the field width is appropriately restricted: 18 cm for a lumbar spine AP is clinically reasonable for an average adult; the beam should not extend beyond the body margins laterally.
  4. Step 4 — Conclusion: The field is within IR borders (no beam beyond IR, no film cut-off). Verify the field does not extend beyond the patient's lateral skin margins — if the IR is much wider than the patient, further collimation of the width is appropriate to minimize dose.
Answer: The 18 cm × 40 cm field fits within the 35 cm × 43 cm IR. No anatomy is cut off. The technologist should verify the beam does not exceed the patient's lateral body margins and tighten collimation further if it does.

The math in this lesson1

Learners multiply collimated field size by magnification factor 1.25 to find the projected field at the IR.
G-SRT.5
Brush up in the math halls at newSchoolWorks.com:
In class: Perform a complete cervical spine series (AP, odontoid, obliques, lateral) and thoracic and lumbar spine series on a positioning phantom, verbalizing each centering point and landmark aloud as each image is taken. · Use provided skull radiographs to complete an identification exercise labeling 20 anatomical structures on PA Caldwell, lateral, and Waters projections, then compare answers against an atlas and self-score. · Discuss the clinical relevance of gonadal and thyroid shielding during spine and skull imaging: calculate field size for a lumbar spine AP and demonstrate proper collimation and shield placement on the phantom.
Materials: Radiographic unit, skull and spine phantom, anatomy labeling worksheet, Frank, Long & Smith's Merrill's Atlas of Radiographic Positions and Radiologic Procedures (current edition), gonadal shields, collimation guide
Check: Practical: each spine series and skull projection scored on 10-point checklist at 8/10 or higher; anatomy labeling exercise scored at 80% or higher — pass 80%.

Practice check8

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

Radiographic Procedures
1. For the AP open-mouth (odontoid) projection, the mouth is opened wide so that which two structures are superimposed?
Radiographic Procedures
2. A student performs a lateral thoracic spine projection using a long exposure while the patient breathes quietly. What is the PRIMARY purpose of this breathing technique?
Radiographic Procedures
3. On a Waters (parietoacanthial) projection, the OML is angled approximately 37° from the IR. What is the PRIMARY anatomical benefit of this positioning?
Patient Care and Education / Radiation Protection
4. During a lumbar spine AP projection on a male patient, the technologist places a contact gonadal shield over the testes. Which statement BEST reflects the standard practice guideline for this action?
Radiographic Procedures
5. On a PA Caldwell projection, the CR is angled 15° caudad. This projects the petrous ridges to which position within the orbits?
Radiographic Procedures
6. Which surface landmark is used to locate the centering point for an AP axial cervical spine projection?
Radiographic Procedures
7. For posterior oblique projections of the cervical spine (LPO and RPO positions), which intervertebral foramina are demonstrated?
Radiographic Procedures
8. A technologist collimating for a lumbar spine AP should set the field length to include which anatomical range?
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