Haas Method (PA Axial Skull / Reverse Towne’s View) : Skull X-Ray Positioning Notes

The Haas Method (PA Axial Skull / Reverse Towne’s View) is a specialized cranial projection designed to demonstrate the occipital bone, petrous pyramids, foramen magnum, and dorsum sellae.

It functions as the posteroanterior equivalent of the AP Axial (Towne’s) projection. It is primarily indicated when a patient cannot flex their neck backward into the supine/erect AP Towne position (e.g., hypersthenic patients with thick necks or severe dorsal kyphosis), or when significant reduction of radiation dose to the ocular lenses is prioritized.

1. Clinical Indications & Contraindications

Detailed Indications:

• Occipital Bone Pathology & Fractures: Symmetrical evaluation of linear, depressed, or comminuted fractures of the occipital squama and posterior calvarium.
• Alternative to AP Axial (Towne’s): Used when the standard 30⁰ caudal AP Towne’s view is unobtainable due to physical limitations or severe cervical stiffness that prevents chin depression.
• Posterior Fossa & Sellar Structures: Clear visualization of the dorsum sellae and posterior clinoid processes projected inside the radiolucency of the foramen magnum.
• Petrous Pyramids & IAM Evaluation: Comparative assessment of petrous ridges, internal acoustic meatus (IAM) widening, acoustic neuromas, and mastoid air cells.
• Radiation Dose Reduction: Significantly reduces direct radiation exposure to the radiosensitive anterior ocular lenses compared to the standard AP Towne view.

Contraindications & Limitations:

• Unstable Cervical Spine Trauma: Absolute contraindication for prone or forward-facing erect positioning. Prone placement can exacerbate spinal cord injuries.
• Facial & Nasal Trauma: Not suitable if the patient has severe open facial trauma, nasal fractures, or epistaxis preventing forehead-nose contact.
• Magnification of Occipital Region: Because the occipital bone is farther from the Image Receptor (higher Object-to-Image Distance – OID) compared to an AP Towne view, the occiput and foramen magnum show greater geometric magnification and slight loss of spatial resolution.

2. Technical Factors & Exposure Physics

3. Step-by-Step Patient & Part Positioning

                 [ Vertical Bucky Surface ]
                           ║
     (Inion / Vertex) ◄─── ║ ◄── Forehead & Nose touching Bucky
                           ║
                    /──────║──────\
                   │  OML  ║       │ ◄── OML strictly Perpendicular (90°) to IR
                   │ ──────║────── │
                   │  MSP  ║       │ ◄── Mid-Sagittal Plane Perpendicular to IR
                    \      ║      /
                           ║
                          /
                         /  25° Cephalad Tube Tilt
                        ▲
               [ Central Ray: 25° Cephalad ]
           (Enters 1.5" below Inion, Exits 1.5" above Nasion)

• Patient Preparation: Remove all radiopaque materials (hairpins, clips, spectacles, earrings, necklaces, dentures, and hearing aids).
• Base Posture:
  • Erect (Preferred): Patient sits or stands facing the vertical Bucky with shoulders relaxed and leveled.
  • Prone (Alternative): Patient lies prone on the radiographic table with arms resting near the head for support.
• Alignment of Reference Lines & Points:
  1. Forehead & Nose Contact: Rest the patient’s forehead and the tip of the nose firmly against the surface of the upright Bucky.
  2. Orbitomeatal Line (OML): Adjust head flexion/extension until the OML is strictly perpendicular (90⁰) to the plane of the image receptor.
  3. Mid-Sagittal Plane (MSP): Must remain strictly perpendicular to the midline of the vertical grid to ensure zero head rotation.
  4. Interpupillary Line (IPL): Must remain strictly parallel to the floor and receptor plane (ensuring zero lateral head tilt).

4. Central Ray (CR), Tube Angle & Collimation

• Central Ray (CR) Tube Angle: 25⁰ Cephalad (angled upwards toward the vertex).
  • Rationale: A 25⁰ cephalad angle in the PA position corresponds functionally to the 30⁰ caudal angle in the AP Towne position due to natural cranial geometry and divergence of the beam.
• CR Entry & Exit Points:
  • Entry: Centered along the midsagittal plane, entering the posterior neck approximately 4 cm (1.5 inches) inferior to the external occipital protuberance (Inion).
  • Exit: Directed to pass through the level of the external acoustic meati (EAM) and exit 4 cm (1.5 inches) superior to the Nasion.
• Collimation: Four-sided collimation adjusted tightly to the outer skin margins of the skull vault.

5. Positioning Errors & Radiographic Signs

6. Comparison: AP Axial Towne’s vs. PA Axial Haas (Reverse Towne’s)

7. Radiographic Anatomy & Quality Evaluation Criteria

+───────────────────────────────────────────────────────────────────────────+
|                                [ Vertex ]                                 |
|                                                                           |
|                 /───────────────────────────────────────\                 |
|                /       Occipital Bone (Magnified)        \                |
|               |                                           |               |
|               |       Petrous Pyramids                    |               |
|               |        \               /                  |               |
|               |         \  [·]   [·]  /   ◄── IAMs        |               |
|                \        (   [====]   )   /    (Dorsum Sellae in)          |
|                 \        \__________/   /     ◄── Foramen Magnum          |
|                  \─────────────────────/                                  |
|                             [ Occiput ]                                   |
+───────────────────────────────────────────────────────────────────────────+

• Structures Within the Foramen Magnum (Primary Hallmark):
  • The dorsum sellae and posterior clinoid processes are clearly projected within the radiolucency of the foramen magnum.
• Occipital Bone & Posterior Fossa:
  • Complete visualization of the occipital bone, posterior parietal bones, and foramen magnum.
• Symmetry:
  • Symmetrical petrous pyramids on either side of the midsagittal plane.
  • Equal distance from the lateral margins of the foramen magnum to the outer calvarial borders.
• Visual Sharpness: Clear visualization of the internal acoustic canals and inner/outer cortical tables with no motion blur.