SMV projection best shows the sphenoid sinus: canonical ARRT positioning

Paranasal sinus anatomy: four groups, four best projections

The paranasal sinuses are air-filled cavities within the skull bones surrounding the nasal cavity. They are named for the bones they occupy. Understanding their location is critical because each sinus group sits in a different anatomic relationship to the skull base, and that relationship determines which projection best visualizes it.

The four groups are:

Frontal sinuses sit in the forehead, above the orbits, deep to the brow ridge. The Caldwell projection (PA axial) is best because it angles down through the sinuses in true basal-axial anatomy.

Maxillary sinuses are the largest, occupying the upper cheekbones below the orbits. The Waters projection (parietoacanthial, chin elevated) projects them with minimal orbital superimposition and is canonical for maxillary work.

Ethmoid sinuses are a collection of small air cells between the orbits. The Caldwell and SMV projections both show them well; SMV is often preferred because it separates them from frontal sinus overlap.

Sphenoid sinuses sit deep in the base of the skull, behind the nasal cavity and below the pituitary fossa. This posterior, basal location is why the SMV projection is best: it angles up through the base of the skull from below, opening up the sphenoid in true basal anatomy.

The key insight is that sinus imaging is about angle and approach. You don’t image all four sinuses the same way. Each requires a different projection because each sits in a different location relative to the skull base.

The common wrong answer (lateral for sphenoid)

Many students and even some study materials mistakenly claim that the lateral projection is best for the sphenoid sinus. This error sticks for one simple reason: the lateral projection does show the sphenoid, and it does show air-fluid levels.

But showing something and being best at showing something are not the same.

The lateral projection is a scout or confirmatory view. When you take a lateral skull or lateral sinus view, all four paranasal sinus groups are in the beam path and visible in profile. The lateral is excellent for detecting air-fluid levels (a horizontal line of fluid in an upright patient) because gravity makes the distinction visible. It is poor at showing the true anatomy of any single sinus group because of overlap and obliquity.

The sphenoid sinus sits posterior and basal (deep and low) in the skull. On a lateral view, it is:

• Obscured by the frontal and maxillary sinuses in front of it
• Projected obliquely, not in true anatomy
• Difficult to evaluate for morphology, septations, or focal lesions

On an SMV projection, the sphenoid is:

• Separated from the other sinuses (anterior/inferior to the viewing angle)
• Projected in true basal anatomy
• Clearly visible for morphologic detail and pathology detection

The ARRT and all canonical radiography textbooks list SMV (or open-mouth Waters) as the best projection for sphenoid evaluation. Lateral is a supplement that shows air-fluid levels but is not primary.

SMV positioning: the mechanics

SMV stands for Submentovertex, sometimes called the full basal projection. It is named for the path the central ray takes: entering below the patient’s chin (sub-mental) and exiting through the vertex (top of the head).

Patient positioning

The patient sits upright (critical for sinus work) with the chin tucked toward the chest. The patient extends the neck backward (hyperextends) so that the infraorbitomeatal line (IOML) is parallel to the image receptor. This positioning requires flexibility and is uncomfortable; the technologist should explain what’s needed and give the patient a moment to settle.

Some patients cannot achieve full SMV positioning due to age, pain, or limited neck mobility. In those cases, a modified submentovertex or a compromise angle may be used.

Central ray direction

The central ray is perpendicular to the IOML, angled 0 degrees (no tube angle). The CR enters below the chin, anterior to the angle of the mandible, and travels straight up through the base of the skull.

Collimation

Collimate to include the entire base of the skull. The image receptor typically shows the skull base, mandible, teeth, and the basal anatomy of all four paranasal sinuses (though the sphenoid is the primary focus).

Evaluation criteria

A properly positioned SMV should show:

• Sphenoid sinuses clearly visible in the center basal view
• Ethmoid sinuses visible as small air cells lateral to the sphenoid
• Maxillary sinus outlines visible
• Mandible symmetric and non-superimposed over the sinuses
• No rotation (mandibular condyles equidistant from spine)

Overprojection (chin not tucked enough) obscures the sphenoid. Underprojection (too much chin tuck) foreshortens the view and distorts anatomy.

Best projection by sinus group: canonical guide

This is the reference table ARRT exam makers use. If the question asks “best projection for sphenoid,” the answer is SMV (or “open-mouth Waters” as an alternative).

Sinus group	Best projection	Alternative(s)	Why this angle
Frontal	Caldwell (PA)	SMV, modified PA	PA axial angles down through the forehead sinuses; frontal area is elevated, not foreshortened
Maxillary	Waters (PAS)	Modified Waters	Chin elevated projects the maxillary sinuses in clear profile with minimal orbital overlap
Ethmoid	Caldwell + SMV	Waters	Both Caldwell and SMV separate ethmoid cells from overlap; Caldwell for superior, SMV overall
Sphenoid	SMV	Open-mouth Waters	SMV angles up through the base of skull, opening the sphenoid in true basal anatomy

Key rule: If the exam question lists multiple projections and asks which is best for the sphenoid, the correct answer is SMV. If it offers only “lateral” or “Waters” and excludes SMV, the question may be testing whether you know lateral is not primary (lateral is not the best).

Why upright is critical for sinus imaging

All paranasal sinus projections should be taken with the patient upright (sitting or standing), not supine or semi-recumbent.

The reason is air-fluid levels. When a patient has acute sinusitis, aspiration, or post-procedural fluid in the sinuses, gravity causes the denser fluid to settle to the bottom of the sinus cavity and air to rise to the top. This creates a sharp horizontal line, the air-fluid level, which is a key radiographic sign of pathology.

Air-fluid levels are visible only when the patient is upright and the sinus is horizontal relative to gravity. If the patient is recumbent, the fluid redistributes and the air-fluid level disappears, potentially masking clinically important findings.

This is why portable sinus radiographs in bedridden or intubated patients are often non-diagnostic or require special positioning (semi-upright if possible).

Why this matters on the ARRT

The ARRT Procedures domain tests sinus positioning in several ways:

1. Direct question: “What is the best projection to visualize the sphenoid sinus?” Answer: SMV (or submentovertex).

2. Comparison question: “Which projection best shows the sphenoid and which best shows the maxillary?” Answer: SMV for sphenoid, Waters for maxillary.

3. Positioning criteria: “In SMV positioning, the infraorbitomeatal line should be…” Answer: parallel to the image receptor.

4. Pathology question: “A patient suspected of sinusitis is imaged supine due to mobility limitations. Air-fluid levels are not visible. Why?” Answer: Air-fluid levels require upright positioning to be visible; supine positioning redistributes the fluid.

5. Projection purpose: “The lateral sinus projection is best for detecting…” Answer: air-fluid levels in all four sinuses, not morphology of any single sinus.

If you’ve memorized “lateral is best for sphenoid,” unlearn that rule before exam day. The ARRT, Bontrager, and every canon source agree: SMV is best for sphenoid.

Quick reference: sinus positioning do’s and don’ts

Do	Don’t
Use upright positioning for all sinus radiographs	Image sinuses supine or recumbent unless medically critical
Include Caldwell and Waters in standard sinus protocol	Assume lateral alone is sufficient for sinus diagnosis
Ask patient to tuck chin for SMV, extend neck backward	Force SMV positioning on patients with cervical mobility limits
Label projections clearly (Caldwell, Waters, SMV)	Mix up Waters and SMV by name
Recognize lateral as a supplement for air-fluid levels	Claim lateral is the primary projection for any sinus

ARRT exam tip

If the question offers SMV as an option and asks for the best sphenoid projection, choose SMV. If the question lists only lateral and Waters and asks about sphenoid, the intended teaching point is usually that lateral is not primary for individual sinus morphology, it’s a supporting view for air-fluid levels.

Memorize the canonical four-sinus table above: Caldwell for frontal, Waters for maxillary, SMV for sphenoid and ethmoid. This is the rule set the ARRT exam enforces.

For a complete review of skull positioning including sinuses, face, and TMJ, see our chapter on skull, face, and TMJ positioning. For the foundations of why angle and anatomy matter, the positioning fundamentals chapter walks through the principle of projection and the impact of central ray angle on visibility.