Chapter 11 - Merills Skull Projections

Lateral Skull: Positioning and CR

Patient in true lateral (RAO or LAO).

MSP parallel to IR, IOML perpendicular to the front edge of IR.

CR: Perpendicular to 2" above EAM (external auditory meatus).

Lateral Skull: Structures Shown

coronal sutures

orbital roofs

sella turcica

sphenoidal sinus

petrous portion of temporal bone

TMJ

EAM

mandibular rami

Lateral Skull: Image Criteria

✅ Orbital roofs and mandibular rami superimposed

✅ Sella turcica centered and seen in profile

✅ No rotation or tilt (look at rami and orbital roofs)

✅ Proper brightness and contrast to see soft tissues and bone

Lateral Skull: Common Errors

Rotation = orbital roofs/rami not superimposed

Tilt = orbital plates not aligned vertically

Lateral Skull: Used for

Trauma overview

Baseline skull anatomy

PA Skull: Positioning and CR

Forehead and nose against IR.

OML is perpendicular to IR.

MSP perpendicular.

CR:

Perpendicular to exit the glabella.

PA Skull: Structures Shown

Parietal bone

Forntal bone

dorsum sellae

subpraorbital margin

petrous ridge

crista galli

ethmoidal sinus

PA Skull: Image Criteria

✅ Petrous ridges fill the orbits

✅ No rotation (look at equal distance from lateral skull margin to orbits)

✅ Frontal bone and crista galli visible

✅ Clear bony detail

PA Skull: Common Errors

Incorrect OML = distortion of frontal bone

Rotation = asymmetric orbital margins

PA Skull: Used For

Frontal bone evaluation

General skull screening

Caldwell Method (PA Axial): Positioning and CR

OML perpendicular. 15° caudad to exit at nasion.

Caldwell Method (PA Axial): Structures Shown

Frontal Sinus

Crista galli

superior orbital margin

Superior orbital fissures

ethmoidal sinus

petrous ridges

inferior orbital margin

Caldwell Method (PA Axial): Image Criteria

Image Criteria:

✅ Petrous ridges in lower third of orbits

✅ Symmetrical orbits

✅ Frontal sinuses and ethmoid cells visible

✅ Good penetration to show orbital margins clearly

Caldwell Method (PA Axial): Common Errors

Wrong angle = petrous ridges in wrong place

Rotation = asymmetry in orbit distance

Caldwell Method (PA Axial): Used For

Frontal/ethmoid sinus visualization

Orbital trauma

Towne Method (AP Axial): Positioning and CR

30° caudad (OML perpendicular) or 37° caudad (IOML perpendicular)

Directed 2.5" above glabella, passing through foramen magnum.

Towne Method (AP Axial): Structures Shown

Parietal bone

occipital bone

foramen magnum

petrous ridge

posterior clinoid process

dorsum sellae

Towne Method (AP Axial): Image Criteria

✅ Dorsum sellae and posterior clinoids visible inside foramen magnum

✅ No rotation (check petrous ridge symmetry)

✅ Proper angulation—no foreshortening

✅ Occipital bone detail clear

Towne Method (AP Axial): Common Errors

Underangling = dorsum sellae above foramen magnum

Overangling = dorsum sellae below or not seen

Tilt = petrous ridges asymmetric

Towne Method (AP Axial): Used for

Occipital bone

Skull fractures near foramen magnum

HAAS Method (PA Axial, Reverse Towne): Positioning

Forehead and nose on IR.

OML perpendicular

25° cephalad, exiting 1.5" above nasion, entering 1.5" below inion (occipital protuberance)

HAAS Method (PA Axial, Reverse Towne): Structures Shown

Occipital bone

Foramen magnum

petrous ridge

posterior clinoid process

dorsum sellawe

sphenoidal sinus

HAAS Method (PA Axial, Reverse Towne): Image Criteria

✅ Dorsum sellae and posterior clinoids within foramen magnum

✅ Symmetrical petrous ridges

✅ Occipital bone detail shown

HAAS Method (PA Axial, Reverse Towne): Used When

Patient can't lie supine for Towne

Better for patients with limited neck movement

Reverse Caldwell Method (AP Axial): Positioning and CR

OML perpendicular if possible.

15° cephalad to nasion (reverses direction of PA Caldwell).

Reverse Caldwell Method (AP Axial): Structures Shown

Similar to PA Caldwell, but reversed orientation.

Petrous pyramids in lower third of orbits.

Frontal bone, superior orbital fissures.

Reverse Caldwell Method (AP Axial): Image Criteria

✅ Petrous pyramids in lower third of orbits

✅ Frontal bone and orbital margins clear

✅ Symmetry of orbits and skull

✅ Less detail than PA due to increased OID

Reverse Caldwell Method (AP Axial): Used For

Trauma patients who cannot be positioned PA

Orbital and frontal bone trauma