Panoramic and Extraoral Radiographic Techniques — Dental Assisting

Objective. Students operate a panoramic x-ray unit to expose a diagnostically acceptable panoramic radiograph, correctly positioning a patient (or mannequin) and identifying common panoramic errors and their causes.

What Is a Panoramic Radiograph?

A panoramic radiograph (also called a pantomograph or OPG) is a single, curved-plane image that captures the entire maxillofacial region — both jaws, all teeth, the temporomandibular joints (TMJs), the maxillary sinuses, and surrounding bony structures — on one receptor. Unlike the series of small periapical and bitewing films you have mastered for intraoral imaging, panoramic imaging uses a rotating tube-head and a moving receptor that sweep around the outside of the patient's head. The beam is narrow and fan-shaped, exposing only a curved layer of tissue called the focal trough at any moment.

Panoramic radiographs are used to assess impacted third molars, eruption patterns in children, jaw fractures, temporomandibular disorders, cysts, tumors, and overall bone levels. They are not a substitute for periapical or bitewing images for detecting interproximal caries or fine periapical detail, but they provide an invaluable broad survey.

The Focal Trough

The focal trough (also called the image layer) is the three-dimensional curved zone in which structures are recorded with acceptable sharpness. Structures inside the focal trough appear clear; structures outside it appear blurred or magnified. The trough is fixed for a given machine. Your job during positioning is to place the patient's dental arches precisely inside that trough. If the arches are positioned too far forward, too far back, or tilted out of the horizontal, diagnostic quality is lost.

The Three Planes of Alignment

Correct panoramic positioning depends on aligning the patient in three planes.

1. Frankfort Plane (horizontal plane): This imaginary line runs from the inferior border of the orbit to the superior border of the external auditory meatus (ear canal). When the patient is correctly positioned, the Frankfort plane is parallel to the floor. Most units have chin rests and bite guides that help you achieve this. If the chin is tipped too far down, the occlusal plane in the finished image curves upward into an exaggerated smile shape and the anterior teeth appear blurred. If the chin is tipped too far up, the occlusal plane curves downward (reverse smile) and the condyles may be cut off.

2. Midsagittal Plane (vertical plane): The patient's head must not be turned or tilted to either side. The midsagittal plane — the imaginary line dividing the face into equal right and left halves — must be centered and perpendicular to the floor. Lateral tilt or rotation produces asymmetry in the image, making one side appear larger than the other.

3. Focal Trough Position (anterior-posterior plane): The patient bites on a bite guide or positions the teeth in a notched bite block so the anterior teeth are edge-to-edge in the trough. This places the dental arches at the correct depth within the focal layer. If the patient is positioned too far forward (anterior to the trough), the anterior teeth appear narrow and blurred with widened contacts. If positioned too far back (posterior to the trough), the anterior teeth appear wide and blurred, and the spine may project over the image as a prominent artifact.

Positioning Steps

1. Prepare the unit: install a fresh receptor (digital sensor or cassette), disinfect all patient-contact surfaces.

2. Ask the patient to remove all removable prostheses (dentures, partials), orthodontic appliances, earrings, necklaces, hearing aids, and eyeglasses — any metal or dense material in the path of the beam creates ghost images or artifacts.

3. Drape the patient with the panoramic-compatible thyroid-collar lead apron. Note: a standard rectangular thyroid collar is not used for panoramic because it would appear in the image and obscure the lower jaw. Use only the apron style specified by your unit's manufacturer.

4. Guide the patient to the unit. Have the patient stand (or sit) straight.

5. Set the bite guide or have the patient close on the notch; align the Frankfort plane parallel to the floor using the light guides or positioning aids on the machine.

6. Center the midsagittal plane using the mid-sagittal light beam.

7. Ask the patient to close lips around the bite guide, place the tongue flat against the palate, and hold still. The tongue touching the palate eliminates the dark air shadow that would otherwise obscure the roots of the maxillary teeth.

8. Move out of the primary beam path, press and hold the exposure button, and verbally instruct the patient to remain still throughout the rotation cycle.

Common Panoramic Errors

| Error Seen on Image | Cause | Correction |

|---|---|---|

| Smile-shaped (upward-curved) occlusal plane | Chin tipped down | Raise the chin; align Frankfort plane |

| Reverse smile (downward-curved) occlusal plane | Chin tipped up | Lower the chin |

| Narrow, blurred anterior teeth | Patient too far forward in trough | Move patient posteriorly |

| Wide, blurred anterior teeth; spine artifact | Patient too far back in trough | Move patient anteriorly |

| Ghost image (blurry, elevated duplicate) | Metal object on opposite side | Remove all metal objects |

| Dark shadow across roots of maxillary teeth | Tongue not on palate | Instruct patient to place tongue on palate |

| Blurred image overall | Patient movement | Reinforce importance of staying still |

| Asymmetry (one side larger) | Head rotated or tilted | Center midsagittal plane |

Radiation Safety for Panoramic Imaging

Panoramic imaging delivers a relatively low patient dose compared to a full-mouth series, but ALARA principles still apply. Always drape the patient, always stand behind a barrier or at least six feet from the tube-head during exposure, and never expose without a clinical indication documented in the record. Confirm a prescribing dentist's order before any exposure.

Anatomy Visible on the Panoramic Image

Familiarize yourself with structures routinely identified: maxillary sinuses, nasal cavity and septum, nasal conchae, hard palate, TMJ condyles, coronoid processes, mandibular condyles, mandibular rami and body, hyoid bone, cervical vertebrae, styloid processes, and the full dentition including roots and alveolar bone. Recognizing normal anatomy helps you — and the dentist — distinguish normal structures from pathology.

Key terms8

Focal troughThe three-dimensional curved layer in which structures are recorded in acceptable sharpness on a panoramic image; the dental arches must be positioned within this zone.

Frankfort planeAn imaginary horizontal reference line from the inferior border of the orbit to the superior border of the external auditory meatus; must be parallel to the floor during panoramic positioning.

Midsagittal planeThe imaginary vertical plane dividing the face into equal right and left halves; must be centered and perpendicular to the floor to avoid image asymmetry.

Ghost imageA blurry, elevated, and magnified duplicate artifact on a panoramic image caused by a radiopaque object (such as an earring or denture) on the opposite side of the patient from where it actually is.

Smile-shaped occlusal planeA panoramic positioning error in which the occlusal plane curves upward excessively, caused by the patient's chin being tipped too far downward.

Thyroid-collar lead apron (panoramic style)The lead apron design appropriate for panoramic imaging; it protects the patient without wrapping a separate rectangular collar around the neck, which would block the lower-jaw region of the image.

Tongue position artifactA dark (radiolucent) shadow that obscures the roots of maxillary teeth when the patient's tongue is not placed flat against the palate during panoramic exposure, creating an air space in the beam path.

ALARAAs Low As Reasonably Achievable — the guiding radiation protection principle requiring that patient and operator exposure be kept to the lowest level that still produces a diagnostically useful image.

In class: Patient/mannequin positioning demonstration and practice: students rotate through the panoramic unit, positioning a mannequin according to the three planes of alignment (Frankfort plane, midsagittal plane, focal trough position), removing removable prostheses and jewelry, and draping with a lead apron before each exposure. · Panoramic image critique: students receive a set of six panoramic images containing common errors (ghost images, chin tip down causing smile-shaped occlusal plane, spine artifact, patient movement blur, tongue not on palate) and match each error to its cause and correction on a provided error table. · Anatomy on the panoramic image: students label 20 anatomical structures on a printed panoramic radiograph (condyles, coronoid processes, maxillary sinus, nasal septum, hyoid bone, cervical vertebrae, dentition, etc.) and compare findings to the intraoral series from Lesson 8.
Materials: Panoramic x-ray unit, mannequin or volunteer patient (per program clinical protocol), thyroid-collar-compatible lead apron, panoramic error reference table, printed panoramic anatomy labeling worksheet, Iannucci & Howerton panoramic chapter
Check: Instructor observation checklist for positioning: student correctly aligns mannequin in all three planes, removes contraindicated items, applies draping, and initiates exposure; plus 80% score (16 of 20) on anatomy labeling worksheet. Both must be met.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.