Overview
This lecture covers proper skull positioning for radiographic imaging, focusing on anatomical landmarks, positioning lines, patient setup, and key projection techniques. It also addresses the pathology each projection can reveal and the criteria for a diagnostic image.
Key Positioning Landmarks & Lines
- Anatomical landmarks:
- Glabella (between eyebrows)
- Nasion (bridge of nose)
- Canthus (inner and outer corners of eyes)
- Infraorbital margin
- Mental point (chin)
- Gonion (angle of mandible)
- External acoustic meatus (EAM)
- Positioning lines:
- MSP (Mid-sagittal plane): Divides skull into right and left halves; must be straight for accurate imaging.
- IPL (Interpupillary line): Perpendicular between pupils; used to check for head tilt.
- OML (Orbitomeatal line): Outer canthus to EAM.
- IOML (Infraorbitomeatal line): Infraorbital margin to EAM.
- GML (Glabellomeatal line): Glabella to EAM.
- AML (Acanthiomeatal line): Canthus to EAM.
- MML (Mentomeatal line): Mental point to EAM.
Patient Positioning Considerations
- Preferred position: Upright, unless patient condition requires supine.
- Body habitus:
- Hyposthenic/asthenic: Support under chest to align cervical spine and MSP.
- Hypersthenic: Support under head to prevent upward tilt of MSP.
- Alignment: Always ensure MSP and IPL are straight, regardless of patient orientation.
- Avoid using the nose as a reference due to possible deviation; use bony landmarks from glabella to mental point.
Preparation & Patient Protection
- Clean hair and skin to prevent artifacts from oil or debris.
- Remove all items: Jewelry, dentures with metal, hearing aids, prosthetic eyes (if not glass), wigs, hairpieces, hairpins, barrettes.
- Shielding: Use thyroid and breast shields when possible, especially for female patients.
- Collimation: Always collimate to the area of interest to reduce exposure and improve image quality.
Major Skull Projections & Techniques
Lateral Skull
- Position: Patient upright or recumbent; MSP parallel and IPL perpendicular to image receptor; IOML parallel to cassette.
- CR: Perpendicular, entering 2 inches superior to EAM; use 10x12 cassette.
- Pathology: Demonstrates fractures, neoplasms, and air-fluid levels (e.g., from trauma or bleeding).
- Criteria:
- Superimposed orbital roofs, EAMs, and mandibular rami.
- No tilt (top-to-bottom shift) or rotation (front-to-back shift).
- Sella turcica in profile.
- Entire skull visualized, including frontal and occipital bones.
PA & PA Axial (Caldwell) Projections
- Position: Patient prone or seated; forehead and nose on table; OML perpendicular to IR; MSP centered.
- PA: CR perpendicular, exits at nasion.
- PA Axial (Caldwell): CR angled 15° caudad, exits at nasion.
- Pathology: Shows frontal and ethmoid sinuses, frontal bone, and orbits; useful for detecting fractures and sinus disease.
- Criteria:
- PA: Petrous ridges fill the orbits.
- Caldwell: Petrous ridges in lower 1/3 of orbits.
- Symmetric orbits and cranium; no rotation or tilt.
- Frontal bone and superior orbital margins clearly seen.
AP & AP Axial Projections
- Position: Used if PA is not possible; patient supine or upright; MSP and OML perpendicular to IR.
- AP: CR perpendicular, entering nasion.
- AP Axial: CR angled 15° cephalad, entering nasion.
- Pathology: Similar to PA/PA Axial but with magnified orbits; used for trauma or non-ambulatory patients.
- Criteria:
- Petrous ridges fill orbits (AP) or in lower 1/3 (AP Axial).
- Symmetry of skull structures.
- Orbits appear larger due to magnification.
AP Axial (Towne’s Method)
- Position: Supine or seated; MSP and OML (or IOML) perpendicular to IR.
- CR:
- 30° caudad to OML or 37° caudad to IOML.
- Enters 2.5 inches above glabella, passes through foramen magnum.
- Pathology: Visualizes occipital bone, foramen magnum, dorsum sellae, and posterior clinoids; used for detecting occipital fractures and basilar skull pathology.
- Criteria:
- Dorsum sellae and posterior clinoids projected within foramen magnum.
- Symmetric petrous ridges.
- Entire occipital bone and parietal bones included.
PA Axial (Haas Method)
- Position: Patient prone; forehead and nose on table; OML perpendicular to IR.
- CR: 25° cephalad, entering 1.5 inches below external occipital protuberance, exiting 1.5 inches above nasion.
- Pathology: Alternative to Towne’s for patients who cannot flex neck; shows occipital region and foramen magnum.
- Criteria:
- Dorsum sellae and posterior clinoids within foramen magnum.
- Symmetric petrous ridges.
- Occipital bone and posterior skull visualized.
Submentovertex (SMV/Schuller Method)
- Position: Patient upright or supine with torso elevated; IOML parallel to IR; MSP perpendicular.
- CR: Perpendicular to IOML, entering MSP between mandibular angles (gonions), passes 3/4 inch anterior to EAM.
- Pathology: Demonstrates cranial base, zygomatic arches, sphenoid and ethmoid sinuses, and mandible; used for detecting basal skull fractures and zygomatic arch injuries.
- Criteria:
- Mandibular condyles anterior to petrous ridges.
- Symmetric cranial base structures.
- No tilt or rotation; entire cranial base included.
Key Terms & Definitions
- MSP (Mid-sagittal Plane): Divides skull into right/left halves; must be straight for imaging.
- IPL (Interpupillary Line): Line between the pupils; used to check head tilt.
- OML (Orbitomeatal Line): Outer canthus of eye to EAM.
- IOML (Infraorbitomeatal Line): Infraorbital margin to EAM.
- GML (Glabellomeatal Line): Glabella to EAM.
- AML (Acanthiomeatal Line): Canthus to EAM.
- MML (Mentomeatal Line): Mental point (chin) to EAM.
- EAM (External Acoustic Meatus): Ear canal opening, key landmark.
- CR (Central Ray): X-ray beam’s direction and angle.
- Petrous Ridges: Dense bones seen in cranial base on images; their position helps assess projection accuracy.
Action Items / Next Steps
- Memorize and practice identifying all positioning lines and anatomical landmarks.
- Study the appearance, anatomy, pathology, and image criteria for each skull projection.
- Complete the assigned image analysis workbook.
- Prepare for the upcoming quiz on skull positioning.