Head
Paranasal Sinuses
Anatomy
Students should be able to identify the following structures on themselves, the patient or radiographic images:
Frontal Sinuses, Maxillary Sinuses (Antra of Highmore), Ethmoid Air Cells (Ethmoidal Sinuses),
Sphenoid Sinuses, Nasal septum, Orbits, Zygomatic arches, Petrous ridges, Sella turcica, Mandibular condyles and rami.
Positioning Lines and Planes
- Orbitomeatal line (OML) – EAM to outer canthus; reference for Caldwell projection.
- Acanthiomeatal line (AML) – EAM to acanthion; used for Waters projection.
- Infraorbitomeatal line (IOML) – EAM to infraorbital margin; parallel for SMV.
- Midsagittal plane (MSP) – divides skull symmetrically; must be perpendicular to IR.
- Interpupillary line (IPL) – perpendicular to IR for lateral projection alignment
Routine Projections (ARRT Required)
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 (what anatomy must be included and how you verify it’s complete): All four sinus groups, with sphenoid sinus best demonstrated
- Rotation checks (how symmetry or alignment tells you if positioning is correct): Sella turcica in profile, superimposed orbital roofs & mandibular rami
- Motion checks (how sharpness confirms patient cooperation/exposure timing): Sharp bony margins and sinus outlines
- Technique checks (what contrast, density, soft tissue visibility, and artifacts to look for): Contrast sufficient to show soft tissue, air-fluid levels, and bony structures
- Clinical aim (when applicable, e.g., reflux, obstruction, displacement, etc.): 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
Supplemental Projections
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
Anatomy
- How many major sinus groups are there, and what bones contain them?
There are four major groups: frontal (frontal bone), maxillary (maxillae), ethmoid (ethmoid bone), and sphenoid (sphenoid bone). - How are the sinuses arranged relative to one another?
Frontal sinuses lie superior to the orbits, maxillary sinuses lie inferior to the orbits, ethmoid air cells lie between the orbits, and the sphenoid sinuses lie posterior to the ethmoids beneath the sella turcica. - How do the sinuses communicate with the nasal cavity?
Each drains through small ostia into the nasal meatuses — the frontal and maxillary sinuses drain into the middle meatus, and the sphenoid drains into the sphenoethmoidal recess. - Why must the sinuses be developed and aerated before they are visible radiographically?
They begin forming at birth but are not fully pneumatized until adolescence; fluid-filled or undeveloped cavities appear opaque. - How can sinus anatomy help locate fractures or infection?
Opacification, mucosal thickening, or air-fluid levels indicate inflammation or trauma-related hemorrhage within a specific sinus cavity.
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.
Clinical Applications
- Why are sinus series important in evaluating infection or trauma?
They reveal mucosal thickening, fluid levels, or hemorrhage that suggest sinusitis or fracture communication with a sinus cavity. - Why are sinus images sometimes used in pre-operative planning?
They delineate the anatomy of the osteomeatal complex and aid ENT surgeons in image-guided procedures. - Why is differentiation between sinusitis and neoplasm critical?
Both can cause opacification, but neoplasms alter bone margins or cause expansion, requiring further imaging such as CT. - How do upright sinus radiographs complement CT scans in modern practice?
They provide functional information about air-fluid levels at a lower dose and can be obtained rapidly for follow-up or screening. - Why must radiation protection and collimation be emphasized in sinus studies?
The lenses of the eyes are radiosensitive; tight collimation and proper technique minimize unnecessary exposure.