Radiographic Procedures · ARRT 2025
Paranasal Sinus Radiography
The four-view sinus series, Caldwell, Waters, lateral, SMV, and the erect/horizontal-beam rule for demonstrating air-fluid levels for the ARRT exam.
Overview
Paranasal Sinuses is a small, focused chapter, but the ARRT registry tests it consistently because the rule set is unforgiving. Two principles dominate: the patient must be erect, and the central ray must be horizontal. Both are required to demonstrate air-fluid levels (the diagnostic finding that distinguishes acute sinusitis from chronic mucosal thickening).
The four-view sinus series: Caldwell PA (CR 15° caudad to OML, frontal and anterior ethmoid sinuses), Waters parietoacanthial (CR perpendicular, maxillary sinuses), lateral (sphenoid sinuses and overall sinus profile), and SMV submentovertical (sphenoid and ethmoid in axial profile). Every view must be performed with the patient erect and the central ray horizontal, no exceptions. If the patient cannot stand or sit upright, document the reason and discuss with the radiologist before substituting recumbent views.
Why the rule matters: fluid in a sinus settles by gravity, creating a horizontal air-fluid level only when imaged with a horizontal beam in an erect patient. Recumbent imaging or angled beams obscure this finding. Sinus radiography has largely been replaced by CT in the modern ED workflow, but the ARRT continues to test the conventional projections because they remain in the Content Specifications and may be used in resource-limited settings or screening contexts.
What you’ll learn in this chapter
The 7 lessons in this chapter break down as follows. The full lesson content is unlocked when you start a free account.
Foundations
- The Four Paired Sinuses
- Critical Principle: Erect Position, Horizontal CR
The Four-View Sinus Series
- Sinus Series: Comparative Guide
Knowledge Check
- Question 1 of 4 Quiz
- Question 2 of 4 Quiz
- Question 3 of 4 Quiz
- Question 4 of 4 Quiz
Key terms in this chapter
These are the 7 terms most likely to appear on the ARRT registry from this chapter. Use them as a flashcard pre-quiz.
- Erect Position
- Patient sitting or standing upright. Required for sinus radiography to demonstrate air-fluid levels.
- Horizontal Beam
- Central ray traveling parallel to the floor. Required to demonstrate air-fluid levels when the patient is erect.
- Air-Fluid Level
- Horizontal interface between air and fluid in a body cavity. Diagnostic of acute sinusitis when present in a sinus.
- Caldwell PA
- PA sinus projection with 15° caudad CR to OML. Demonstrates frontal and anterior ethmoid sinuses.
- Waters Sinus
- Parietoacanthial projection. OML 37° to the IR. Demonstrates the maxillary sinuses without petrous ridge superimposition.
- Frontal Sinus
- Air-filled cavity in the frontal bone above the orbits. Best demonstrated on the Caldwell.
- Maxillary Sinus
- Largest paranasal sinus, located in the maxilla below the orbit. Best demonstrated on the Waters.
Sample practice question: Head, Spine, and Pelvis
One free sample from the 68-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.
Read the full chapter, free.
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Frequently asked questions
What does the ARRT Radiography Radiographic Procedures category cover?
Paranasal Sinuses is a small, focused chapter, but the ARRT registry tests it consistently because the rule set is unforgiving. Two principles dominate: the patient must be erect, and the central ray must be horizontal. Both are required to demonstrate air-fluid levels (the diagnostic finding that distinguishes acute sinusitis from chronic mucosal thickening).
How many lessons are in the Paranasal Sinus Radiography chapter?
This chapter contains 7 lessons across 3 sections, plus a knowledge-check quiz at the end. The full lesson content is unlocked with a Premium subscription. The free tier includes the first chapter complete.
Is this chapter aligned with the ARRT 2025 Content Specifications?
Yes. Every chapter on this site maps directly to the ARRT Radiography Content Specifications effective 2025. This chapter falls under the Radiographic Procedures domain of the official ARRT exam blueprint.