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

Parameter	Recommended Specification	Radiographic Rationale
Patient Posture	Erect (Seated/Standing) or Prone	Erect positioning is easier for kyphotic patients to maintain
Image Receptor (IR)	24 x 30 cm (10 × 12 inches), Portrait	Captures elongated posterior fossa structures
SID	100 to 115 cm ( 40 to 44 inches)	Standard distance; account for tube angulation
Grid Ratio	8:1 to 12:1 focused grid	Essential to absorb scatter from dense cranial bones
Tube Potential (kVp)	75 – 85 kVp	Adequate penetration through the dense occiput and petrous bones
mAs Range	25 – 35 mAs	Calibrated based on cranial thickness; center AEC cell active
Focal Spot	Small (0.6 mm)	High spatial sharpness to compensate for the higher OID of the occiput
Breathing	Suspended respiration	Eliminates breathing and motion blur

3. Step-by-Step Patient & Part Positioning

Haas View (Reverse Towne) Positioning Diagram

Haas Method (PA Axial / Reverse Towne’s)

Cranial Positioning

║ Vertical Bucky Surface ║

◄── Forehead & Nose Contact

OML ⟂ Image Receptor (90°)

MSP ⟂ Image Receptor (90°)

▲ [ Central Ray: 25° Cephalad ]

Enters 1.5″ below Inion ➔ Exits 1.5″ above Nasion

Patient Contact Rule

Forehead and tip of the nose rest firmly against the vertical Bucky along the midline.

OML Alignment

Orbitomeatal Line (OML) must be strictly perpendicular (90°) to the plane of the image receptor.

Mid-Sagittal Plane (MSP)

MSP is aligned perpendicular to the grid midline to eliminate any lateral head rotation or tilt.

25° Cephalad Trajectory

Projects dorsum sellae & posterior clinoid processes inside the radiolucency of the foramen magnum.

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

Error Type	Radiographic Sign on Film	Clinical Impact	Correction
Under-Angulation (< 25⁰ Cephalad)	The dorsum sellae is projected superior to the foramen magnum, overlapping the dense occiput.	Fails to isolate the sellar floor within the foramen magnum.	Increase the cephalad tube angle to a full 25⁰.
Over-Angulation (> 25⁰ Cephalad)	The posterior arch of the atlas (C1) is projected inside the foramen magnum, obscuring the dorsum sellae.	Superimposition masks foramen magnum margins.	Decrease the cephalad tube angle back to 25⁰.
Head Rotation (MSP not 90°)	Asymmetric distance from lateral margins of foramen magnum to lateral skull walls; uneven petrous ridges.	Mimics pathological petrous bone destruction.	Realign the Mid-Sagittal Plane (MSP) perpendicular to the IR.
Head Tilt (IPL not parallel)	Petrous pyramids and internal acoustic canals are slanted at different vertical levels.	Creates structural asymmetry.	Realign the Interpupillary Line (IPL) strictly parallel to the floor.

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

Parameter	AP Axial (Towne’s Method)	PA Axial (Haas / Reverse Towne’s)
Patient Orientation	AP (Back of head against IR)	PA (Forehead & nose against IR)
Central Ray Angle	30⁰ Caudad (to OML)	25⁰ Cephalad (to OML)
Lens Radiation Dose	High (Direct ocular beam entry)	Significantly Reduced (Lenses on exit side)
Occipital Magnification	Minimal (Low OID, occiput near IR)	Increased (Higher OID, occiput away from IR)
Primary Indication	Standard routine protocol	Alternative for kyphotic / thick-necked patients

7. Radiographic Anatomy & Quality Evaluation Criteria

Haas View Anatomical Evaluation Diagram

Haas Method: Radiographic Anatomy & Quality Criteria

PA Axial Skull

Haas Projection Trans-Foraminal Alignment

Key Anatomical Evaluation Criteria

Sella

Dorsum Sellae in Foramen Magnum

Dorsum sellae and posterior clinoids must project symmetrically inside the radiolucency of the foramen magnum.

Pyramids

Symmetric Petrous Ridges & IAMs

Equal distance from lateral skull borders to foramen magnum; internal acoustic meati visible bilaterally.

Margins

Foramen Magnum Clearance

No superimposition of the posterior arch of C1 over the sella (verifies correct 25° cephalad angle).

Occiput

Magnified Occipital Squama

Complete visualization of posterior parietal bones and occipital squama with sharp cortical margins.

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.