Head
55 Paranasal Sinuses
Anatomy
Students should be able to identify the following on sinus projections:
- Frontal sinuses (above the frontonasal suture, within frontal bone)
- Maxillary sinuses (antra of Highmore) (largest, within maxillae)
- Ethmoid air cells (anterior, middle, posterior groups, within ethmoid bone)
- Sphenoid sinuses (posterior, within body of sphenoid)
- Nasal septum
- Orbits
- Zygomatic arches
- Petrous ridges
- Sella turcica (seen on lateral)
- Mandibular condyles and rami (as positioning checks on lateral and SMV)
Projections
Lateral Projection
CR Location & Positioning
- SID: 40 inches (72 inches if pre-op measurement is required)
- Patient position: Upright (preferred) or dorsal decubitus if unable to sit/stand
- Adjustments: MSP parallel to IR, IPL perpendicular, IOML horizontal (perpendicular to front edge of IR)
- CR: Horizontal beam, directed ½–1 inch posterior to outer canthus
- Pt. Instructions: Suspend respiration, keep still
- Exposure: 75–85 kVp, low mAs, grid
Evaluation Criteria
- Coverage: All four sinus groups, with sphenoid sinus best demonstrated
- Rotation checks: Sella turcica in profile, superimposed orbital roofs & mandibular rami
- Motion checks: Sharp bony margins and sinus outlines
- Technique checks: Contrast sufficient to show soft tissue, air-fluid levels, and bony structures
- Clinical aim: Detect air-fluid levels and survey all sinus groups
PA Axial Projection – Caldwell Method
CR Location & Positioning
- SID: 40 inches
- Patient position: Upright
- Adjustments (angled IR): OML perpendicular to angled IR, nose & forehead resting on grid, MSP perpendicular
- Adjustments (vertical IR): Extend neck slightly, nose only on grid, OML at 15° to horizontal CR
- CR: Horizontal, exiting nasion (15° caudad relationship to OML maintained)
- Pt. Instructions: Suspend respiration, hold still
- Exposure: 75–85 kVp, grid
Evaluation Criteria
- Coverage: Frontal sinuses above frontonasal suture, anterior ethmoid air cells inferior to frontal sinuses
- Rotation checks: Equal distance skull-to-orbit laterally; symmetric petrous ridges
- Motion checks: Clear bony margins of frontal and ethmoid sinuses
- Technique checks: Petrous ridges in lower third of orbits, exposure shows both bone and possible air-fluid levels
- Clinical aim: Best for frontal sinuses and anterior ethmoid air cells
Parietoacanthial Projection – Waters Method
CR Location & Positioning
- SID: 40 inches
- Patient position: Upright
- Adjustments: Chin extended so OML is 37° from IR, MML perpendicular, MSP perpendicular
- CR: Horizontal, exiting acanthion
- Pt. Instructions: Suspend respiration, hold still
- Exposure: 75–85 kVp, grid
Evaluation Criteria
- Coverage: Maxillary sinuses (best demonstrated) with orbits and nasal structures included
- Rotation checks: Equal orbital margins, MSP aligned with IR
- Motion checks: Crisp outline of sinus walls and nasal septum
- Technique checks: Petrous ridges immediately below maxillary sinuses, contrast sufficient to show sinus fluid if present
- Clinical aim: Gold standard for maxillary sinuses; can also show foramen rotundum
Open-Mouth Waters Projection
CR Location & Positioning
- SID: 40 inches
- Patient position: Upright
- Adjustments: Chin extended as in Waters, OML 37° to IR, MSP perpendicular; patient opens mouth widely while holding position
- CR: Horizontal, exiting acanthion
- Pt. Instructions: Open mouth only when instructed; suspend respiration
- Exposure: 75–85 kVp, grid
Evaluation Criteria
- Coverage: Sphenoid sinuses projected through open mouth, along with maxillary sinuses
- Rotation checks: Equal distance lateral skull to orbits, symmetric orbits/maxillae
- Motion checks: Sharp sinus outlines
- Technique checks: Petrous ridges inferior to maxillary sinus floors; exposure shows both bone and potential air-fluid levels
- Clinical aim: Best alternative to SMV for sphenoid sinuses
Submentovertical Projection (SMV)
CR Location & Positioning
- SID: 40 inches
- Patient position: Upright (preferred) or supine if necessary
- Adjustments: Hyperextend neck until IOML parallel to IR; MSP perpendicular
- CR: Horizontal, perpendicular to IOML, entering MSP ~1 inch anterior to EAM
- Pt. Instructions: Keep mouth closed, suspend respiration
- Exposure: 80–90 kVp, grid
Evaluation Criteria
- Coverage: Sphenoid and ethmoid sinuses, anterior cranial base
- Rotation checks: Equal distance skull border to mandibular condyles; condyles symmetric
- Motion checks: Sharp detail of ethmoid and sphenoid air cells
- Technique checks: Mandibular condyles projected anterior to petrous ridges; mentum projected anterior to ethmoid sinuses
- Clinical aim: Shows sphenoid and ethmoid sinuses; evaluates base of skull for pathology
Paranasal Sinuses – Quick Reference Table
| Projection | CR Location & Positioning | Patient Position / Adjustments | Key Evaluation Points |
|---|---|---|---|
| Lateral | • CR horizontal, 0° • Center 0.5–1″ posterior to outer canthus |
• Erect preferred • Side of interest closest to IR • MSP ∥ IR, IPL ⟂ IR • IOML ∥ transverse axis of IR |
• All four sinus groups included (sphenoid best seen) • Sella turcica in profile • Superimposed orbital roofs & mandibular rami • Sharp margins, no motion |
| PA Axial (Caldwell) | • CR horizontal, exiting nasion • OML forms 15° angle with horizontal CR |
• Erect preferred • Forehead & nose or tip of nose against IR (angled grid or extended neck technique) • MSP ⟂ IR |
• Frontal sinuses above frontonasal suture • Anterior ethmoids visible • Petrous ridges in lower 1/3 of orbits • Symmetry of orbits (no rotation) |
| Parietoacanthial (Waters) | • CR horizontal, exiting acanthion | • Erect preferred • Chin against IR • MML ⟂ IR (OML ~37° to IR) • MSP ⟂ IR |
• Maxillary sinuses best seen • Petrous ridges just below maxillary floors • Symmetric orbits/maxillae (no rotation) • Sharp margins, no motion |
| Parietoacanthial (Open-Mouth Waters) | • CR horizontal, exiting acanthion | • Same as Waters, but patient opens mouth while holding position | • Sphenoid sinuses projected through open mouth • Maxillary sinuses also seen • Petrous ridges below maxillary floors • Symmetry of orbits and sinuses |
| Submentovertical (SMV) | • CR horizontal, perpendicular to IOML, centered 1.5–2″ below mandibular symphysis (midway between gonions) | • Erect preferred • Hyperextend neck, vertex on IR • MSP ⟂ IR, IOML ∥ IR |
• Sphenoid & ethmoid sinuses best seen • Mandibular condyles anterior to petrous ridges • Equal distance from mandible to lateral skull borders • No tilt or rotation |
Hows & Whys of Sinus Radiography
Positioning
- Why are sinus projections performed upright whenever possible?
To demonstrate air-fluid levels and differentiate fluid from other pathology. - Why must the central ray always be horizontal in sinus imaging?
A horizontal beam ensures air-fluid levels can be detected. - Why are both lateral sides of the sinuses not required, unlike nasal bones?
All four sinus groups can be demonstrated on a single lateral image. - Why is the Waters projection considered the “gold standard” for maxillary sinuses?
It projects the petrous ridges just below the maxillary floors, giving an unobstructed view. - Why is the open-mouth Waters projection performed?
It shows the sphenoid sinuses projected through the open mouth, an alternative to SMV when hyperextension is not possible. - Why is the SMV projection difficult for some patients?
It requires extreme neck extension to bring the IOML parallel to the IR. - Why must the MSP be perpendicular in sinus imaging (Caldwell, Waters, SMV)?
To prevent rotation, which distorts symmetry of orbits, nasal septum, or sinuses. - Why is the angled-grid Caldwell preferred over the vertical-grid version?
It brings the IR closer to the patient, reducing OID and improving resolution.
Technique & Image Evaluation
- How do you know the IPL was perpendicular in the lateral sinus projection?
The orbital roofs and mandibular rami are superimposed. - How do you confirm the IOML was positioned correctly on the lateral?
The sella turcica is in profile, and no tilt is present. - How do you know the MSP was perpendicular on the Caldwell?
The orbits and frontal sinuses appear symmetric side to side. - What indicates the correct CR angle and positioning on the Caldwell?
Petrous ridges are projected in the lower third of the orbits, and frontal sinuses are above the frontonasal suture. - How do you know the chin was extended enough on a Waters projection?
The petrous ridges are projected just below the maxillary sinuses. - What error is indicated if the petrous ridges appear too high in a Waters?
The chin was not extended enough. - How do you check for rotation on a Waters projection?
The orbits and maxillary sinuses should appear symmetric, with nasal septum aligned at midline. - How do you know the patient was positioned correctly for the open-mouth Waters?
Sphenoid sinuses are clearly projected through the open mouth, and petrous ridges lie immediately below the maxillary sinuses. - What error is indicated if the mandible obscures the ethmoid sinuses on an SMV?
The neck was not extended enough to make the IOML parallel with the IR. - How do you confirm correct extension on the SMV?
The mandibular condyles project anterior to the petrous ridges, and the mentum projects anterior to the ethmoid sinuses.