Radiographic Procedures · ARRT Radiography
Skull, Facial Bones & Sinuses for the ARRT Radiography Exam
The cranial positioning lines, Towne and Caldwell angles, Waters vs modified Waters, petrous ridge placement, and the upright sinus rules for the ARRT Radiography exam.
Overview
Skull, Facial Bones and Sinuses is built on lines and ridges. The skull holds 22 bones, 8 cranial and 14 facial, and positioning starts from lines drawn from facial landmarks to the ear: the orbitomeatal line (OML) for the PA, Caldwell and Towne, the mentomeatal line for the Waters, the lips-meatal line for the modified Waters. The OML and IOML sit 7 to 8 degrees apart, so swapping lines moves the tube angle by that much: the AP axial Towne runs 30 degrees caudad to the OML or 37 to the IOML.
The PA, Caldwell and Waters views are judged by where the dense petrous ridges land. A straight PA at 0 degrees fills the orbits with them; the 15 degree Caldwell drops them to the lower third of the orbits; a 25 to 30 degree PA axial puts them at the infraorbital margin; the Waters, with the OML at 37 degrees, clears them below the maxillary sinuses. The modified Waters (OML at 55 degrees) leaves the ridges in the lower half of the sinuses but sets the orbital floors perpendicular to the receptor, which is why Bontrager and Merrill's both name it for blowout fractures. On a correct Towne, the dorsum sellae sits inside the shadow of the foramen magnum.
Sinus rules: upright position, horizontal beam, no tube angle on the routine projections. The horizontal beam is what catches an air-fluid level, so a patient who cannot sit up is still shot with one, and a patient moved from recumbent to erect waits at least 5 minutes before exposure. The lateral is the routine projection the textbooks credit with all four sinus groups at once.
The mandible axiolateral oblique picks its target by rotation: 0 degrees for the ramus, 10 to 15 for a general survey, 30 for the body, 45 for the mentum. TMJs are imaged closed and open mouth, never open on a suspected fracture. Procedures supplies 66 of the 200 scored ARRT questions to 28 February 2027, and 72 from 1 March 2027.
What you’ll learn in this chapter
The 35 lessons in this chapter break down as follows. The full lesson content is unlocked when you start a free account.
Skull, Facial Bones & Sinuses
- Skull, Facial Bones and Sinuses
Anatomy
- The Eight Cranial Bones
- The Temporal Bone
- Sutures, Fontanels and Skull Shape
- The Fourteen Facial Bones
- The Orbits
- Quiz
- The Paranasal Sinuses
- Mandible and TMJ Anatomy
Landmarks
- Landmarks and Positioning Lines
- Quiz
Petrous ridges
- Where the Petrous Ridges Land
- Quiz
Technique
- Technique, Shielding and the Five Errors
Skull series
- AP Axial Projection: Towne Method
- Quiz
- Lateral, PA and PA Axial Skull
- Towne or Caldwell, From the Image Alone
Facial bones
- Waters and Modified Waters
- Quiz
- The Facial Bone Routine
Sinuses
- Sinus Rules: Erect, With a Horizontal Beam
- Quiz
- Which Projection Shows Which Sinus
Mandible
- The Mandible
TMJ
- Temporomandibular Joints
- Quiz
Small parts
- Nasal Bones, Optic Foramina and Zygomatic Arches
Image critique
- Critiquing a Head Film
Recap
- What to Carry Out of This Chapter
- Quiz
- Quiz
- Quiz
- Quiz
- Quiz
Key terms in this chapter
These are the 8 terms most likely to appear on the ARRT registry from this chapter. Use them as a flashcard pre-quiz.
- Orbitomeatal Line (OML)
- The positioning line from the outer canthus to the midpoint of the EAM, the reference for the PA, PA axial Caldwell and AP axial Towne. It sits 7 to 8 degrees from the IOML.
- Towne Method
- AP axial skull projection, with the CR angled 30 degrees caudad from the OML, or 37 degrees when the IOML is used. On a correct image the dorsum sellae and posterior clinoids sit inside the shadow of the foramen magnum.
- Caldwell Method
- PA axial projection with the OML perpendicular and a 15 degree caudad angle exiting at the nasion, projecting the petrous ridges into the lower third of the orbits.
- Waters Method
- Parietoacanthial projection with the mentomeatal line perpendicular, putting the OML at 37 degrees. The petrous ridges clear below the maxillary sinuses, so it serves facial bones and the maxillary sinuses.
- Modified Waters
- Less neck extension: the lips-meatal line is perpendicular and the OML sits at 55 degrees, placing the orbital floors perpendicular to the receptor. Bontrager and Merrill's both name it for orbital blowout fractures.
- Petrous Ridges
- The dense upper borders of the petrous pyramids, at the level of the top of ear attachment. The frontal facial bone and sinus projections, such as the Caldwell and the Waters, are judged by where they land on the image.
- Sinus Positioning Rules
- Upright patient and horizontal beam, with no tube angle on the routine projections, so trapped fluid can layer into an air-fluid level; a patient who cannot sit up still gets a horizontal beam. Wait at least 5 minutes after moving a patient from recumbent to erect.
- Axiolateral Oblique Mandible
- The mandible projection where head rotation selects the part: 0 degrees for the ramus, 10 to 15 for a general survey, 30 for the body, 45 for the mentum.
Sample practice question: Head, Spine, and Pelvis
One free sample from the 256-question Head, Spine, and Pelvis bank. See the format, the rationale style, and the difficulty before you sign up.
A 45° posterior oblique projection of the lumbar spine demonstrates the Scotty Dog pattern. A defect appearing as a 'collar' on the dog's neck represents which of the following pathologies?
Show answer and rationale
A, Incorrect: Spondylolisthesis is forward slippage of one vertebra on another, best assessed on lateral imaging.
B, Correct: Correct. The Scotty Dog's neck represents the pars interarticularis. A 'collar' on the neck indicates spondylolysis, a defect or fracture of the pars. This finding is pathognomonic on the 45° lumbar oblique.
C, Incorrect: Disk herniation is not directly visible on plain radiographs and requires MRI for diagnosis.
D, Incorrect: Compression fractures involve loss of vertebral body height and are best demonstrated on lateral views.
Hands-on
Practice the positioning
Open the Positioning Lab to drill body position, central ray, anatomy, and common errors for each projection in this chapter.
- Skull Lateral True lateral cranium. SID 40". CR perpendicular to IR.
- Skull AP axial (Towne) AP axial / Towne. Occipital bone, dorsum sellae. SID 40", CR 30° caudad.
- Skull PA axial (Caldwell) PA axial / Caldwell. Frontal bone, frontal & ethmoid sinuses. CR 15° caudad to OML.
- Skull Waters (parietoacanthial) Waters method. Maxillary sinuses, facial bones. MML perpendicular to IR.
- Skull AP AP cranium. SID 40". CR perpendicular to IR.
- Skull PA Forehead and nose on IR. OML perpendicular. CR exits nasion.
- Skull SMV (Cranial Base) Vertex on IR. IOML parallel to IR. CR perpendicular to IOML.
- Facial Bones Parietoacanthial (Waters) Waters method. Maxillary sinuses, facial bones. MML perpendicular to IR.
- Facial Bones Lateral True lateral facial skeleton. SID 40". CR perpendicular to IR.
- Facial Bones PA Axial (Caldwell) Caldwell method. Frontal sinuses, ethmoid, facial bones. CR 15° caudad.
- Nasal Bones Lateral True lateral nasal bones. SID 40". CR perpendicular to IR.
- Mandible PA (Rami) PA mandible. Body and rami. CR perpendicular to IR.
- Mandible PA Axial (Rami) PA Axial mandible. Rami and body. CR 20-25° cephalad.
- Mandible Axiolateral and Oblique Axiolateral Axiolateral & oblique axiolateral mandible. CR 25° cephalad.
- Mandible Submentovertical (SMV) SMV mandible. Entire mandible. IOML parallel to IR.
- TMJ Axiolateral Axiolateral TMJ. Joint space, condyle. CR 25-30° caudad.
- TMJ Axiolateral Oblique Axiolateral oblique TMJ. Joint space clear. CR 15° caudad.
- Sinuses Lateral Upright lateral. CR horizontal. Demonstrates air-fluid levels.
- Sinuses SMV Upright SMV. Vertex on IR. IOML parallel. Sphenoid + ethmoid sinuses.
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Frequently asked questions
What does the ARRT Radiography Radiographic Procedures category cover?
Skull, Facial Bones and Sinuses is built on lines and ridges. The skull holds 22 bones, 8 cranial and 14 facial, and positioning starts from lines drawn from facial landmarks to the ear: the orbitomeatal line (OML) for the PA, Caldwell and Towne, the mentomeatal line for the Waters, the lips-meatal line for the modified Waters. The OML and IOML sit 7 to 8 degrees apart, so swapping lines moves the tube angle by that much: the AP axial Towne runs 30 degrees caudad to the OML or 37 to the IOML.
How many lessons are in the Skull, Facial Bones & Sinuses for the ARRT Radiography Exam chapter?
This chapter contains 35 lessons across 13 sections, plus a knowledge-check quiz at the end. The full lesson content is unlocked with a Premium subscription. The free tier includes 3 complete chapters.
Is this chapter aligned with the current ARRT Content Specifications?
Yes. Every chapter on this site maps directly to the ARRT Radiography Content Specifications in effect for your exam window. This chapter falls under the Radiographic Procedures domain of the official ARRT exam blueprint.