Features of Orthopantomogram
Dental Radiology · Panoramic Imaging
TL;DR
The orthopantomogram (OPG) is the most widely used extraoral radiograph in dentistry — a single panoramic image that surveys all teeth, both jaws, the TMJs, the maxillary sinuses, and surrounding structures in one low-dose exposure.
- The orthopantomogram (OPG, OPT, or panoramic radiograph) is a single extraoral image that shows all teeth, both jaws, the TMJs, the maxillary sinuses, and surrounding bone on one film using a rotating tube-receptor assembly
- The image is formed through rotational tomography — only structures within a curved focal trough (image layer) are in focus; structures outside this layer appear blurred
- Normal anatomical structures visible on the OPG include: all teeth and their supporting bone, the maxillary sinuses, nasal fossa, inferior alveolar canal, mental foramen, hard and soft palate, hyoid bone, cervical spine (partially), and both TMJ condyles
- Ghost images are a unique artefact of the rotational mechanism — they are blurred, magnified shadows of contralateral radiopaque structures (dental restorations, hyoid bone, cervical spine, jewellery) that appear on the opposite side of the image
- The OPG is a screening tool: it cannot reliably detect early interproximal caries and should not replace bitewing radiographs or periapical films for detailed tooth and periapical assessment
Key Facts
What Is It?
The orthopantomogram (OPG) — from the Greek “ortho” (correct/straight) + “panto” (all) + “gram” (recording) — is a panoramic radiograph that displays the entire dental and jaw anatomy on a single flat image using the principle of rotational tomography. The X-ray tube and digital receptor rotate around the patient’s stationary head in opposite directions, sweeping through a curved arc that corresponds to the shape of the dental arches. Only the curved layer of tissue within the focal trough (image layer) is in focus; all other structures are blurred through movement.
The OPG is the most widely used extraoral radiograph in general dental practice. It provides a broad survey of the teeth, periodontium, alveolar bone, TMJs, sinuses, and adjacent structures in a single, low-dose exposure — making it invaluable for new patient assessment, third molar evaluation, jaw pathology screening, and orthodontic/implant planning.
Why It Matters
No other single dental radiograph provides as much anatomical information in one exposure as the OPG. Its ability to survey the entire craniofacial complex simultaneously makes it an indispensable screening and planning tool across nearly every dental specialty. At the same time, understanding its inherent limitations — principally those arising from the rotational mechanism itself — is essential for safe and accurate interpretation.
Clinical Relevance
- Broad survey: the OPG provides more anatomical information in a single image than any other routine dental radiograph. It surveys all teeth, jaws, and TMJs simultaneously.
- Third molar assessment: the most common single indication for an OPG — showing third molar position, angulation, root morphology, and relationship to the inferior alveolar canal.
- Jaw pathology screening: cysts, tumours, and bone lesions that would be invisible on intraoral radiographs are routinely detected on OPGs.
- Limitations: OPGs cannot replace intraoral radiographs for caries detection or periapical diagnosis. The resolution is insufficient, and the geometric magnification and distortion make detailed tooth and root assessment unreliable.
- INBDE: OPG anatomy, ghost images, positioning errors, and interpretation are consistently tested topics.
How the OPG Works (Rotational Tomography)
Focal Trough Principle
The OPG unit uses a narrow slit X-ray beam and a curved image receptor that rotate simultaneously around the patient’s head in opposite directions. The rotation centre moves continuously, tracing a path that corresponds to the curvature of the dental arches.
- Only structures within the curved focal trough (image layer) — the layer of tissue whose curvature matches the rotation path — are in focus. Structures outside this layer move relative to the beam and receptor during rotation and are blurred.
- The focal trough is roughly elliptical in shape, matching the average curvature of the dental arches. It is approximately 2.5 cm wide (buccolingual) and follows the arch from condyle to condyle.
- Correct patient positioning is the most important technical step: the dental arches must be positioned within the focal trough. If the patient is too far forward or backward, the anterior or posterior teeth will be blurred.
- The focal trough is fixed for each machine — patients with unusually narrow or wide arches may have regions outside the focal trough.
- Anterior teeth are most vulnerable to being outside the focal trough (narrowest part of the trough).
Normal Anatomical Features of the OPG
A systematic approach to OPG interpretation requires familiarity with all normal anatomical structures and their expected radiographic appearances. The following table covers the key structures encountered on a standard OPG.
| Structure | Location on OPG | Radiographic Appearance | Clinical Notes |
|---|---|---|---|
| All teeth (primary and/or permanent) | Throughout both arches | Varying radiopacity; enamel, dentin, pulp visible | Eruption status, root morphology, restorations |
| Alveolar bone | Surrounding roots | Trabecular radiolucency with corticated crests | Crest height; bone density |
| Inferior alveolar canal (IAC) | Body of mandible, both sides | Radiolucent channel bounded by two thin radiopaque cortical lines | Relationship to lower molar roots; IAN proximity critical for extractions/implants |
| Mental foramen | Below lower premolars, bilaterally | Oval/round radiolucency | May mimic periapical pathology if superimposed on root apex |
| Mandibular condyles | Superior ends of rami, bilateral | Rounded radiopaque structures; general shape assessable | Erosion, flattening, fracture (limited detail — Reverse Towne’s view provides better assessment) |
| Coronoid process | Anterior border of ramus | Triangular radiopaque projection | Hyperplasia; fracture |
| Mandibular rami | Vertical portions of mandible | Broad radiopaque structures | Ramus height; fracture |
| Maxillary sinuses | Posterior maxilla, bilateral | Large radiolucent air spaces | Opacification, mucosal thickening, retention cysts, odontogenic sinusitis |
| Nasal fossa and septum | Central midline, superior to anterior teeth | Radiolucent air space with midline septum | Normal; septal deviation |
| Hard palate shadow | Central superior image | Horizontal radiopaque band | Normal landmark |
| Soft palate | Often projected over upper premolar region | Faint radiolucent band | Normal structure — not pathology |
| Nasal turbinates | Within nasal fossa | Radiopaque soft tissue shadows | Normal; hypertrophy in chronic rhinitis |
| Hyoid bone | Below mandible, midline | U-shaped radiopaque structure | Common ghost image source — appears as bilateral ghost in posterior mandibular area |
| Cervical spine | Behind/below mandible | Radiopaque vertebral segments | Normal; may mimic pathology if ghost projected; osteoarthritis can be noted |
| Styloid process | Behind and below condyle region | Linear radiopaque process descending from skull base | Elongated styloid process (Eagle syndrome) if >25–30 mm |
| Pterygomaxillary fissure | Posterior maxilla | Vertical radiolucent space between pterygoid plates and posterior maxilla | Normal landmark |
| Zygoma | Lateral maxillary region | Curved radiopaque bone | Reference landmark; fractures partially visible |
| Articular eminence/fossa | Above condyle | Radiopaque eminence and fossa | TMJ anatomy; assessment limited on OPG alone |
Ghost Images and Positioning Artefacts
Ghost Images — Mechanism and Recognition
A ghost image is formed when a radiopaque structure is outside the focal trough — typically on the opposite (contralateral) side of the patient — and moves during the rotational scan. Because this structure moves in the same direction as the receptor but at a different speed (due to being out of the focal trough), it projects onto the image as a blurred, magnified, and elevated shadow on the opposite side of the image from where the real structure is located.
Ghost images are always characterised by four features:
- On the opposite side from the real structure
- Blurred (no sharp edges)
- Magnified (larger than the real structure)
- Positioned higher in the image than the real structure
| Ghost Appearance | Real Structure (Source) | Location of Ghost vs Real | How to Identify |
|---|---|---|---|
| Curved radiopaque band across lower molars (bilateral) | Hyoid bone | Ghost is above and opposite the hyoid | Matches shape of hyoid; bilateral; blurred |
| Radiopaque horizontal band | Hard palate | Appears over upper or lower teeth on opposite side | Smooth, parallel edges; bilateral faint band |
| Radiopaque irregular shadow | Cervical vertebra | Appears over ramus/molar area | Vertebral shape; moves with neck position |
| Circular/irregular radiopaque | Earring (jewellery) | Appears on opposite side | Remove before exposure! |
| Dense radiopaque mass | Dental crown or amalgam restoration | Projects as blurred mass on opposite side | Compare with actual restoration location |
| Radiopaque oval | Angle of mandible | Appears above the contralateral condyle region | Bilaterally symmetric; compare with anatomy |
| Soft palate shadow | Soft palate | Faint diagonal shadow across upper premolars | Thin, soft tissue density; disappears if patient swallows or raises tongue during exposure |
Positioning Errors and Their Effects
Correct patient positioning within the focal trough is essential. The following errors are commonly encountered in clinical practice and are high-yield topics for the INBDE examination.
| Positioning Error | Appearance on OPG | How to Identify | Correction |
|---|---|---|---|
| Patient too far forward | Anterior teeth blurred, narrow, overlapping | Anterior region out of focus; teeth narrowed | Reposition patient posteriorly in bite notch |
| Patient too far back | Anterior teeth blurred, wide, magnified | Teeth appear wide; anterior blur | Reposition patient anteriorly in bite notch |
| Chin too low | “Frowning” (downward curved) occlusal plane; hard palate shadow over upper root apices; condyles may be cut off superiorly | Occlusal plane curves downward in centre | Tilt chin upward; align Frankfort plane |
| Chin too high | “Smiling” (upward curved) occlusal plane in centre; lower incisors appear elongated; condyles may be cut off inferiorly | Reverse curve of occlusal plane | Tilt chin downward; align Frankfort plane |
| Head tilted (lateral) | One side of mandible appears longer than the other; asymmetric ramus heights | Ramus height asymmetry | Centre the midsagittal plane |
| Patient moving | Blurred image overall; “jagged” borders to teeth | Global unsharpness | Reinforce patient instructions; shorter exposure |
| Tongue not on palate | Dark (radiolucent) shadow superimposed over upper roots | Radiolucent band over maxillary region | Instruct patient to place tongue on palate before and during exposure |
| Patient not biting on bite notch | Teeth not aligned in focal trough properly | Variable blurring and misalignment | Ensure patient bites on provided notch |
| Lips/teeth apart | Blurred anterior region; dark air gap between arches | Air space between upper and lower teeth anteriorly | Instruct patient to close lips lightly on bite notch |
Clinical Considerations
What the OPG Is Good For — and What It Is Not
Use OPG for: third molar assessment, jaw cysts and tumours, fracture screening, TMJ overview, unerupted teeth, developmental anomalies, implant screening, and periodontal bone level overview.
Do NOT rely on OPG for: early caries detection (use bitewings), periapical assessment (use PA films), fine root canal anatomy, or precise measurements.
Interpretation Sequence
Adopt a systematic approach to OPG interpretation to reduce missed findings. Read in a consistent order: upper arch teeth (right to left) → lower arch teeth (left to right) → periodontium bilaterally → sinuses → TMJs → bone quality → surrounding anatomy.
Maxillary Sinus Assessment
The OPG shows the floors of the maxillary sinuses and their relationship to upper molar and premolar roots. Mucosal thickening ≥2 mm on OPG is a common finding associated with periapical pathology of adjacent teeth — odontogenic sinusitis. Unilateral opacification of a maxillary sinus should always prompt consideration of a dental cause.
Third Molar–IAC Relationship
The relationship between the lower third molar roots and the inferior alveolar canal (IAC) is one of the most important assessments on OPG before lower third molar surgery. Signs of close proximity include:
- Darkening of root where it crosses the canal
- Narrowing/notching of root at the level of the canal
- Diversion of the IAC around the root
- Loss of cortical white lines of the canal at the root interface
If these signs are present, CBCT is recommended before surgery to more precisely define the root–canal relationship in three dimensions.
Bone Quality Assessment
The general trabecular pattern and bone density can be assessed on the OPG. Diffuse demineralisation may suggest osteoporosis and warrant further investigation. Focal radiolucencies or radio-opacities must be characterised and, if clinically significant, further evaluated with targeted intraoral radiography or CBCT.
Common Mistakes & Misconceptions
-
Misconception: “The OPG can replace bitewing radiographs for caries detection.”
Correction: The OPG has insufficient resolution and too much magnification/distortion to reliably detect early interproximal caries. Bitewing radiographs remain the gold standard for caries detection and cannot be replaced by the OPG. -
Misconception: “Ghost images on the OPG represent real pathology on the opposite side.”
Correction: Ghost images are artefacts of the rotational mechanism — blurred, magnified shadows of real structures from the contralateral side. They appear elevated and blurred, without sharp edges. Recognition of ghost image characteristics prevents misdiagnosis. -
Misconception: “A ‘frowning’ occlusal plane on the OPG indicates mandibular prognathism.”
Correction: A downward-curving (“frowning”) occlusal plane on the OPG is a positioning error — the patient’s chin was too low (tilted down). When the chin is too high, the occlusal plane curves upward (“smiling”). Skeletal jaw relationships cannot be reliably assessed from OPG geometry alone. -
Misconception: “OPG measurements can be directly used for implant planning.”
Correction: OPG images have significant and variable magnification (~25–30% horizontal, ~30% vertical). Without calibration correction specific to the machine used, measurements will be inaccurate. CBCT or periapical radiographs with calibration are required for accurate pre-implant measurements. -
Misconception: “The hyoid bone ghost on an OPG is pathological.”
Correction: The hyoid bone almost always produces a bilateral ghost image on the OPG — a U-shaped or oval radiopaque structure appearing over the posterior mandibular body. This is a normal, expected artefact, not a pathological finding.
Related Topics
The OPG sits within the broader context of dental radiology and clinical imaging. The following topics provide essential supporting knowledge.
References & Sources
The following foundational texts and peer-reviewed sources inform this article.
- White SC & Pharoah MJ, 2014. Oral Radiology: Principles and Interpretation. 7th ed. Elsevier Mosby.
- Whaites E & Drage N, 2013. Essentials of Dental Radiography and Radiology. 5th ed. Churchill Livingstone.
- Langland OE, Langlais RP & Preece JW, 2002. Principles of Dental Imaging. 2nd ed. Lippincott Williams & Wilkins.
- Suomalainen A, Vehmas T, Kortesniemi M et al., 2008. Accuracy of linear measurements using dental cone beam and conventional multislice computed tomography. Dentomaxillofacial Radiology, 37(1):10–17.
- Nakayama E, Yoshiura K, Yuasa K & Kanda S, 2001. Clinical use of panoramic and CT findings for the assessment of endosseous implants. Oral Surgery, Oral Medicine, Oral Pathology, 91(2):250–255.
Summary
The orthopantomogram is an irreplaceable broad-survey tool in dental practice. Its diagnostic power and its limitations are both direct consequences of the same underlying mechanism: rotational tomography and the focal trough. Only structures within the focal trough are in focus — this is what makes the OPG useful (broad arch-to-arch coverage in a single image) and also what produces its characteristic artefacts (ghost images, positioning errors, magnification). Mastery of OPG anatomy, ghost image recognition, and positioning error identification is fundamental to clinical competence in dental radiology and is consistently examined at the INBDE level.
Key Takeaways
- Rotational tomography and the focal trough: the OPG uses rotational tomography — only structures within the curved focal trough are in focus. Correct patient positioning within the focal trough is the most important technical step.
- Ghost images: blurred, magnified artefacts of contralateral structures — always on the opposite side, elevated, and without sharp edges. Recognising ghost images prevents misdiagnosis.
- Positioning errors: chin too low → “frowning” occlusal plane, hard palate shadow over roots; chin too high → “smiling” occlusal plane, condyles cut off. These are the most commonly tested OPG positioning errors.
- Screening tool only: the OPG is a screening tool — not a substitute for bitewings (caries) or periapical films (periapical diagnosis). Every finding on an OPG that requires further investigation needs targeted intraoral radiography.
- Third molar–IAC proximity: darkening, notching, canal diversion, or loss of cortical lines around lower third molar roots on OPG are indications for CBCT before lower third molar removal.
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