The Anteroposterior (AP) View of the Skull (0° AP Cranium) is a standard diagnostic projection used primarily to evaluate the anterior cranial vault, frontal bones, orbital margins, and cranial symmetry when a patient cannot assume the standard Posteroanterior (PA) position.

1. Clinical Indications & Contraindications

Detailed Indications:

• Major Head Trauma & Polytrauma: Detection of linear, comminuted, or depressed fractures of the frontal bone, parietal bones, or supraorbital ridges when the patient cannot be turned prone.
• Cervical Spine Clearance Protocol: Performed on polytrauma patients on a spinal board wearing a rigid cervical collar (C-collar) without manipulating the head or neck.
• Pediatric & Geriatric Assessments: Used for uncooperative children, bedridden patients, or elderly individuals with severe kyphosis who cannot lie prone or maintain the PA erect position.
• Pathological & Metabolic Bone Conditions:
  • Evaluation of lytic or blastic lesions (e.g., Multiple Myeloma, metastatic disease, “salt-and-pepper” appearance in Hyperparathyroidism).
  • Paget’s disease (osteitis deformans) showing cotton-wool appearance of the calvarium.
  • Cranial osteomyelitis or fibrous dysplasia.
• Suture & Structural Assessment: Evaluation of premature closure of cranial sutures (craniosynostosis) and macrocephaly/microcephaly in pediatric cases.
• Intracranial Calcifications & Foreign Bodies: Localization of radio-opaque foreign bodies, metallic fragments, or calcifications within the pineal gland or choroid plexus.
• Post-Surgical Follow-up: Verification of craniotomy bone flap alignment, burr hole margins, or intracranial hardware/shunt placement.

Contraindications & Limitations:

• Unstabilized Cervical Spine Injury: Never forcefully flex, extend, or rotate the head to align reference lines (like OML) until the C-spine is cleared radiologically. The projection must be adapted using central ray tube angulation instead.
• Elective Non-Trauma Cases (Relative): For cooperative, ambulatory patients, the standard PA Skull projection is strongly indicated over the AP projection to reduce radiation exposure to the radiosensitive ocular lenses (radiation cataracts) and eliminate geometric magnification of the orbits.
• Severe Facial Bone Fractures (Stand-alone): AP 0° Skull is not optimal for isolated zygomatic arch, nasal bone, or blowout fractures due to superimposition of the petrous ridges over the orbits; specialized projections (Waters/Caldwell) or CT scans are indicated instead.

2. Technical Factors

3. Step-by-Step Patient & Part Positioning

• Patient Preparation: Remove all radiopaque foreign objects from the head and neck region, including earrings, necklaces, hairpins, wigs, dentures, glasses, hearing aids, and dental prostheses.
• Basic Posture:
  • Table/Recumbent: Place the patient supine on the radiographic table with arms resting by their sides and shoulders in the same transverse plane to avoid tilting.
  • Erect/Upright: Seat or stand the patient facing the X-ray tube with the posterior aspect of the skull (occiput) touching the upright Bucky.
• Alignment of Reference Lines & Planes:
  1. Mid-Sagittal Plane (MSP): Align the MSP strictly perpendicular to the midline of the grid/table. Stand at the patient’s head to confirm the nose, philtrum, and chin are aligned without any left/right head rotation.
  2. Orbitomeatal Line (OML): Gently tuck the patient’s chin to bring the line connecting the outer canthus of the eye to the external auditory meatus (EAM) perpendicular to the plane of the IR.
  3. Interpupillary Line (IPL): Ensure the imaginary line connecting the pupils is strictly parallel to the plane of the IR; this confirms zero lateral tilt.
  4. Vertex Coverage: Ensure the top of the image receptor extends approximately 2 to 3 cm beyond the cranial vertex.

Positioning Adjustments for Trauma/Restricted Mobility:

• If the patient is immobilized in a rigid C-collar or has severe kyphosis and cannot achieve OML perpendicular:
  • Align the Infraorbitomeatal Line (IOML) (infraorbital margin to EAM) perpendicular to the IR instead.
  • Angle the Central Ray 3∘ to 5∘ Caudad to match the 7∘ anatomical angle difference between OML and IOML.

4. Central Ray (CR) & Collimation

• CR Angle: 0∘ (perpendicular to the image receptor and parallel to the OML).
• CR Entry Point: Directed straight to the Glabella (or mid-nasion level, roughly 2 cm superior to the nasofrontal junction).
• Collimation: Four-sided collimation adjusted strictly to the outer skin margins of the skull vault (vertex superiorly, mental protuberance/chin base inferiorly, and lateral skin edges).

5. Radiographic Anatomy & Quality Evaluation Criteria

• Petrous Ridge Superimposition: The petrous ridges of the temporal bones must completely fill the lower third to middle third of the orbits (the standard hallmark of true 0∘ OML alignment).
• Rotational Symmetry:
  • Equal distance measured from the lateral cranial margin (inner and outer table) to the lateral orbital margins on both sides.
  • The crista galli, nasal septum, and anterior nasal spine lie along the central anatomical midline.
• Absence of Lateral Tilt: Both orbital roofs and petrous ridges are at identical horizontal planes without asymmetry.
• Visualization: Complete calvarium including frontal bone, crista galli, frontal sinuses, ethmoid air cells, and coronal/sagittal sutures with distinct bony detail and no motion blur.