14.1 Skull Positioning: Towne, Caldwell, Lateral & SMV Projections

14.1 Skull Positioning: Towne, Caldwell, Lateral & SMV Projections

Positioning Lines & Cranial Landmarks

Radiographic examination of the cranium requires precise alignment of anatomical landmarks and positioning lines to ensure reproducible projections and clear visualization of complex bony structures. The cranium consists of eight bones—the frontal, two parietal, two temporal, occipital, sphenoid, and ethmoid bones—enclosing the cranial cavity.

Key Positioning Lines

1. Orbitomeatal Line (OML): Extends from the outer canthus of the eye to the center of the external auditory meatus (EAM). Serves as the primary baseline for most cranial positioning.
2. Infraorbitomeatal Line (IOML / Reid's Base Line): Extends from the inferior orbital margin (infraorbital rim) to the EAM. There is an anatomical angular difference of 7° to 8° between the OML and IOML.
3. Acanthomeatal Line (AML): Extends from the acanthion (the point at the junction of the upper lip and nasal septum) to the EAM.
4. Mentomeatal Line (MML): Extends from the mentum (mental point of the chin) to the EAM.
5. Lips-Meatal Line (LML): Extends from the junction of the lips to the EAM.
6. Interpupillary Line (IPL): An imaginary line connecting the outer canthi or pupils of both eyes. In true lateral skull positioning, the IPL must be strictly perpendicular to the image receptor (IR).
7. Mid-Sagittal Plane (MSP): Divides the head into equal left and right halves. Depending on the projection, the MSP is aligned either perpendicular or parallel to the plane of the IR.

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Standard Cranial Projections

1. AP Axial Projection (Towne Method)

• Clinical Purpose: Visualizes the occipital bone, dorsum sellae, posterior clinoid processes, foramen magnum, and petrous pyramids.
• Patient & Part Position: Seated or supine. Align the mid-sagittal plane (MSP) perpendicular to the midline of the grid/IR. Flex the patient's neck to place the Orbitomeatal Line (OML) perpendicular to the IR. If the patient cannot flex their neck sufficiently, place the Infraorbitomeatal Line (IOML) perpendicular to the IR.
• Central Ray (CR) Angulation & Centering:
  • If OML is perpendicular: Angle the CR 30° caudal.
  • If IOML is perpendicular: Angle the CR 37° caudal.
  • Direct the CR to enter 2.5 inches (6.5 cm) superior to the glabella (passing midway through the level of the EAMs) to exit through the foramen magnum.
• Diagnostic Criteria & Radiographic Evaluation:
  • The dorsum sellae and posterior clinoid processes must be clearly visualized within the shadow of the foramen magnum.
  • Symmetric petrous ridges extending bilaterally.
  • Absence of rotation indicated by equal distances from the lateral margin of the foramen magnum to the lateral margin of the cranium on both sides.

2. PA Axial Projection (Caldwell Method)

• Clinical Purpose: Demonstrates the frontal bone, anterior ethmoid air cells, frontal sinuses, and superior orbital rims.
• Patient & Part Position: Seated or prone. Place the patient's forehead and nose against the upright grid/table. Align the MSP perpendicular to the IR and adjust head flexion to place the OML perpendicular to the IR.
• Central Ray (CR) Angulation & Centering:
  • Standard 15° Caudal Angle: Direct the CR 15° caudal to the OML, exiting at the nasion.
  • Alternative 25° to 30° Caudal Angle: Direct the CR 25° to 30° caudal to project the petrous ridges completely below the inferior orbital rims, demonstrating the superior orbital fissures, inferior orbital rims, and foramen rotundum.
• Diagnostic Criteria & Radiographic Evaluation:
  • On the standard 15° Caldwell projection, the petrous pyramids/ridges are projected into the lower one-third (1/3) of the orbits.
  • Equal distances from the lateral orbital borders to the lateral margins of the cranium confirm absence of head rotation.

3. PA Skull Projection (0° Central Ray)

• Clinical Purpose: Evaluates the frontal bone, petrous ridges, and cranium without angular displacement.
• Patient & Part Position: Forehead and nose against the IR. MSP perpendicular to IR; OML perpendicular to IR.
• Central Ray (CR): Direct the CR 0° (perpendicular) to the IR, exiting at the nasion.
• Diagnostic Criteria & Radiographic Evaluation:
  • The petrous ridges completely fill the orbits, lying at the level of the superior orbital margins.
  • Frontal bone demonstrated in full projection without spatial distortion.

4. Lateral Skull Projection

• Clinical Purpose: Evaluates cranial fractures, sella turcica morphology, anterior and posterior clinoid processes, and metabolic bone disease.
• Patient & Part Position: Seated oblique or prone in a semi-paretic position. Place the affected side of the head flat against the IR.
  • Adjust the head so that the Mid-Sagittal Plane (MSP) is parallel to the IR.
  • Align the Interpupillary Line (IPL) perpendicular to the IR.
  • Adjust chin elevation/depression to place the Infraorbitomeatal Line (IOML) parallel to the transverse axis of the IR.
• Central Ray (CR): Direct the CR perpendicular to the IR, entering 2 inches (5 cm) superior to the External Auditory Meatus (EAM) (or midway between glabella and inion).
• Diagnostic Criteria & Radiographic Evaluation:
  • Sella turcica demonstrated in true profile, including the tuberculum sellae, hypophyseal fossa, dorsum sellae, and posterior clinoid processes.
  • Superimposed orbital roofs, mandibular rami, and temporomandibular joints confirm absence of tilt or rotation.

5. Submentovertex (SMV) Projection (Schuller Method)

• Clinical Purpose: Demonstrates the base of the skull, sphenoid and ethmoid sinuses, foramen ovale, foramen spinosum, and petrous pyramids.
• Patient & Part Position: Seated or erect (or supine with chest elevated). Hyperextend the patient's neck until the vertex of the skull touches the IR.
  • Adjust the neck so that the Infraorbitomeatal Line (IOML) is parallel to the plane of the IR.
  • Align the MSP perpendicular to the IR.
• Central Ray (CR): Direct the CR perpendicular to the IOML, entering along the MSP 1.5 inches (3.8 cm) inferior to the mandibular symphysis (midway between the angles of the mandible).
• Diagnostic Criteria & Radiographic Evaluation:
  • Foramen ovale and foramen spinosum clearly visualized bilateral to the sphenoid sinus.
  • Mandibular condyles projected anterior to the petrous pyramids.
  • Symmetric petrous pyramids and basilar structures confirm absence of tilt or rotation.

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Cranial Positioning Lines & Projection Criteria Summary