Head
53 Orbits
Anatomy
The orbits are cone-shaped, bony-walled cavities located symmetrically on either side of the midsagittal plane of the skull. Each orbit is formed by seven bones: three cranial (frontal, ethmoid, sphenoid) and four facial (lacrimal, palatine, maxillary, and zygomatic).
Key structural features:
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Base: the quadrilateral anterior opening, easily palpable.
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Apex: the posterior point at the optic foramen, where the optic nerve enters.
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Axis: directed posteriorly and medially at ~37° from the midsagittal plane and ~30° superior from the OML.
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Fissures:
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Superior orbital fissure: between sphenoid wings, allows passage of cranial nerves.
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Inferior orbital fissure: between sphenoid, zygomatic, and maxilla; allows vessels/nerves to pass.
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Because the orbital walls are thin, they are subject to blowout fractures, typically of the orbital floor. The Waters and Modified Waters projections are especially important in evaluating these fractures.
Projections
Lateral Orbits (Right or Left)
CR Location & Positioning
- SID: 40 in (102 cm)
- Patient position: erect or recumbent; side of interest closest to IR
- Adjustments: MSP ∥ IR; IPL ⟂ IR; IOML ⟂ front edge of IR
- CR: perpendicular through outer canthus
- Pt. instructions: gaze straight ahead, eyes still
- Exposure: digital, 70–80 kV, nongrid high resolution
Evaluation Criteria
- Coverage: entire orbit(s) demonstrated
- Rotation: orbital roofs superimposed, mandibular rami and sella turcica aligned
- Motion: sharp margins, no blur of soft tissue or bony detail
- Technique: adequate contrast/density to visualize both bony orbit and eye
- Clinical aim: foreign body localization, fractures
PA Axial Orbits (Caldwell Method)
CR Location & Positioning
- SID: 40 in
- Patient position: erect preferred, forehead and nose against IR
- Adjustments: MSP ⟂ IR; OML ⟂ IR
- CR: 30° caudad to exit at mid-orbits (projects petrous ridges below orbital shadows)
- Pt. instructions: eyes closed, hold gaze steady
- Exposure: digital, 75–85 kV
Evaluation Criteria
- Coverage: entire orbit region included
- Rotation: symmetric orbits, equal distance lateral margins → cortex
- Motion: sharp orbital margins
- Technique: petrous ridges projected below inferior orbital margins, allowing orbit detail
- Clinical aim: screening for metallic foreign body prior to MRI, orbital trauma
Parietoacanthial Orbits (Waters Method)
CR Location & Positioning
- SID: 40 in
- Patient position: erect preferred, chin on IR
- Adjustments: MSP ⟂ IR; MML ⟂ IR; OML forms 37° angle with IR
- CR: perpendicular to exit at acanthion
- Pt. instructions: eyes still, no movement
- Exposure: digital, 75–85 kV
Evaluation Criteria
- Coverage: orbits, maxillae, and zygomatic arches included
- Rotation: symmetric distances from lateral orbital margins to skull cortex
- Motion: sharp detail of bony orbit and adjacent soft tissue
- Technique: petrous ridges just below maxillary sinuses
- Clinical aim: demonstrates blowout fractures, maxillary sinus involvement
Modified Parietoacanthial Orbits (Modified Waters Method)
CR Location & Positioning
- SID: 40 in
- Patient position: erect preferred, chin and nose on IR
- Adjustments: MSP ⟂ IR; LML ⟂ IR; OML ~55° to IR
- CR: perpendicular to exit at acanthion
- Pt. instructions: eyes closed, hold steady gaze
- Exposure: digital, 75–85 kV
Evaluation Criteria
- Coverage: orbital floors and rims with reduced distortion
- Rotation: equal orbital margins on both sides
- Motion: sharp bone/soft tissue margins
- Technique: petrous ridges projected in lower half of maxillary sinuses
- Clinical aim: best for orbital floor (“blowout”) fractures
Parieto-Orbital Oblique Orbits (Rhese Method) – Supplemental
Clinical Indications
- Demonstrates bony abnormalities of the optic foramen.
- Used to show the lateral margins of the orbits and detect foreign bodies.
- Both sides are typically imaged for comparison.
- CT is generally preferred today for detailed evaluation, but this projection remains important for positioning practice and ARRT review.
CR Location & Positioning
- SID: 40 in
- Patient position: erect or prone preferred, chin, cheek, and nose resting on IR.
- Adjustments: MSP rotated 37° toward affected side (53° between MSP and IR). AML ⟂ IR.
- CR: perpendicular through mid-orbit of the downside eye.
- Pt. instructions: eyes closed, no movement during exposure.
- Exposure: digital, 75–85 kV; nongrid high-resolution technique.
Evaluation Criteria
- Coverage: nondistorted view of the optic foramen.
- Rotation: optic foramen projected into the lower outer quadrant of the orbit (indicates correct rotation and AML alignment).
- Motion: sharp bony orbital margins, no blur.
- Technique: adequate contrast/density to show foramen detail without overpenetration.
- Clinical aim: localize optic foramen and orbital canal; supplemental view for orbital pathology.
Clinical Tips
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For orbital trauma patients, Modified Waters is often preferred over Caldwell because it avoids distortion of the orbital floor.
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For foreign body localization, immobilization of the eyes (patient fixates on an object) is critical to eliminate motion blur of intraocular structures.
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Symmetry checks (orbital margins, orbital roofs, nasal septum) are the best way to confirm correct MSP alignment across all orbit projections.
Quick Reference Table
| Projection | CR Location & Angle | Patient Position | Key Evaluation Points |
|---|---|---|---|
| Lateral | Perpendicular to outer canthus | Erect/recumbent; IPL ⟂ IR; MSP ∥ IR | Orbits superimposed; orbital roofs aligned; sharp detail |
| PA Axial (Caldwell) | 30° caudad, exiting mid-orbits | Erect preferred; forehead & nose on IR; OML ⟂ IR | Petrous ridges below orbital shadows; symmetric orbits |
| Waters | ⟂, exiting acanthion | Erect; chin on IR; MML ⟂ IR (OML 37°) | Petrous ridges below maxillary sinuses; symmetric orbital margins |
| Modified Waters | ⟂, exiting acanthion | Erect; chin & nose on IR; LML ⟂ IR (OML 55°) | Orbital floors with minimal distortion; petrous ridges in lower ½ maxillary sinuses |
| Rhese (Supplemental) | ⟂, through mid-orbit downside eye | Erect/prone; AML ⟂ IR; head rotated 37° toward affected side | Optic foramen in lower outer quadrant; sharp orbital detail |
Hows & Whys of Orbit Radiography
General Anatomy & Basics
- How many bones form each orbit? Name them.
Seven bones: frontal, ethmoid, sphenoid, lacrimal, palatine, maxillary, and zygomatic. - What shape are the orbits, and what is the direction of their long axis?
Cone-shaped; directed posteriorly and medially at ~37° to the MSP and superiorly at ~30° from the OML. - What is the apex of the orbit, and what structure is located there?
The apex corresponds to the optic foramen, where the optic nerve passes. - What are the two major fissures of the orbit, and what do they transmit?
The superior orbital fissure (between greater and lesser sphenoid wings) transmits cranial nerves and vessels; the inferior orbital fissure (between sphenoid, maxilla, and zygoma) transmits vessels and nerves to the face.
Positioning
- What are the routine projections for the orbits?
PA Axial (Caldwell), Waters, Modified Waters, and lateral. - Which projection best demonstrates blowout fractures of the orbit?
The Modified Waters projection, because it places the orbital floor perpendicular to the IR. - Which projection is best to demonstrate the optic foramen?
The Rhese method (parieto-orbital oblique). - Why is MRI contraindicated in orbital foreign body cases?
Because magnetic fields could dislodge metallic fragments, causing hemorrhage or further injury. - Why do you position the interpupillary line (IPL) perpendicular to the IR on the lateral orbit projection?
To prevent tilt and ensure the orbital roofs and floors are superimposed. - Why is the 30° caudal CR angulation used on the PA Axial (Caldwell) for orbits instead of 15°?
To project the petrous ridges completely below the orbital shadows, giving a clear view of the orbits. - Why must the chin be extended for the Waters projection of the orbits?
To place the petrous ridges below the maxillary sinuses so the orbits are unobstructed. - Why is the OML positioned at a 55° angle to the IR for the Modified Waters?
This makes the orbital floors perpendicular to the IR, which is essential for evaluating blowout fractures. - Why is the AML positioned perpendicular to the IR in the Rhese (optic foramina) method?
To correctly project the optic foramen into the lower outer quadrant of the orbit.
Technique & Image Evaluation
- How do you know that the interpupillary line was perpendicular to the IR on a lateral projection of the orbits?
The orbital roofs will be aligned and superimposed. - What indicates that the midsagittal plane was parallel to the IR on the lateral projection?
The posterior margins of the orbits and mandibular rami are superimposed. - Why do you use a 30° caudal angle on the PA Axial (Caldwell) projection of the orbits?
To project the petrous ridges completely below the orbital shadows for optimal orbital visualization. - How do you know that the midsagittal plane was perpendicular to the IR on the PA Axial (Caldwell)?
The orbits and superior orbital fissures appear symmetrical. - How can you tell that the patient’s chin was extended correctly for the Waters projection?
The petrous ridges appear immediately below the maxillary sinuses. - How can you tell that the midsagittal plane was perpendicular to the IR on the Waters projection?
The orbital margins are symmetrical on both sides. - What error is indicated if the petrous ridges are projected too high within the maxillary sinuses on a Waters projection?
The chin was not extended enough. - What error is indicated if the mentum appears distorted or the nasal septum is not aligned with the midline on a Waters projection?
The patient’s head was rotated, and the midsagittal plane was not aligned perpendicular to the IR. - How do you know the OML was angled correctly on a Modified Waters projection?
The petrous ridges are projected in the lower half of the maxillary sinuses. - What error is indicated if the petrous ridges appear above the orbital floor on a Modified Waters projection?
The patient’s chin was not extended enough, or the OML was not correctly angled at 55°. - On the Rhese (optic foramina) projection, how do you know the patient’s head was rotated correctly?
The optic foramen appears in the lower outer quadrant of the orbit. - What error is indicated if the optic foramen is projected too close to the nasal side of the orbit on the Rhese projection?
The head was rotated too little toward the affected side. - What error is indicated if the optic foramen is projected too lateral in the orbit on the Rhese projection?
The head was rotated too much toward the affected side. - What error is indicated if the optic foramen is projected too high or too low within the orbit on the Rhese projection?
The AML was not perpendicular to the IR (flexion/extension error).