8.4 Panoramic, CBCT & Selection Criteria for Advanced Imaging
Key Takeaways
- Radiographs are prescribed based on individual clinical need after a history and examination, never as a routine screening series at fixed intervals
- A ghost image on a panoramic radiograph appears on the opposite side, higher than the real object, and is magnified horizontally
- Cone beam computed tomography delivers a substantially higher dose than intraoral imaging and must be justified case by case rather than used routinely
- The effective dose of a full-mouth intraoral series with photostimulable phosphor plates and rectangular collimation is far lower than the same series taken on D-speed film with round collimation
- For children, the ALADA principle requires the smallest field of view and the lowest exposure settings that answer the clinical question
Panoramic, CBCT & Selection Criteria for Advanced Imaging
Why this matters on the INBDE: Ordering an image is a prescription. The examination tests whether you can justify a projection against a specific clinical question and against dose — a decision that sits at the intersection of Clinical Content area 10 and the Practice and Profession component.
Selection Criteria: The Governing Principle
The ADA and U.S. Food and Drug Administration selection criteria are built on one rule: radiographs are prescribed for the individual patient after a history and clinical examination, based on the patient's signs, symptoms, and risk — never as an automatic series at fixed calendar intervals.
| Patient situation | Typical recommendation |
|---|---|
| New patient, adult dentate | Individualized examination; posterior bitewings with selected periapicals, or a full-mouth series when there is generalized disease or a history of extensive treatment |
| New patient, primary dentition with closed proximal contacts | Posterior bitewings |
| Recall, caries risk HIGH | Posterior bitewings at 6 to 12 month intervals |
| Recall, caries risk LOW, adult | Posterior bitewings at 24 to 36 month intervals |
| Recall, periodontal disease | Selected periapical or bitewing images of areas with clinical evidence of disease |
| Growth and development assessment, adolescent | Panoramic or periapical images to assess third molars and eruption |
| No clinical signs or symptoms | No routine screening images; a panoramic radiograph is not a substitute for an examination |
ALARA — as low as reasonably achievable — is the dose principle. Applied to imaging decisions specifically, the current formulation is ALADA: as low as diagnostically acceptable. For pediatric patients this is stated as ALADAIP, as low as diagnostically acceptable being indication-oriented and patient-specific.
Panoramic Radiography
Panoramic imaging uses tomography: the source and receptor rotate about the patient around a moving center of rotation, so structures within the focal trough (image layer) are sharp and structures outside it are blurred or distorted.
Strengths: broad coverage of both jaws, the temporomandibular joints, the maxillary sinuses, and the inferior alveolar canals in a single low-dose image; excellent for impacted third molars, jaw fractures, large lesions, and eruption assessment; well tolerated by patients who cannot manage intraoral receptors.
Limitations: magnification (roughly 10% to 30%, and unequal vertically and horizontally), superimposition of the cervical spine in the midline, and inadequate resolution for interproximal caries and crestal bone detail. A panoramic radiograph never replaces bitewings for caries.
Panoramic positioning errors
| Error | Appearance |
|---|---|
| Patient positioned too far forward (anterior to the focal trough) | Anterior teeth narrow and blurred; spine may appear at the edges |
| Patient positioned too far back | Anterior teeth wide and blurred; condyles crowded toward the edges |
| Chin tipped up (occlusal plane flat or reversed) | "Frown" appearance; hard palate superimposed on roots; condyles may be cut off |
| Chin tipped down | Exaggerated "smile" curve; mandibular incisors blurred; hyoid superimposed |
| Head rotated | One side magnified, the other narrowed; asymmetric ramus widths |
| Tongue not on the palate | Radiolucent air shadow obscuring maxillary apices |
| Slumped posture | Radiopaque cervical spine shadow in the midline |
Ghost images
A ghost image occurs when a dense object — a lead apron collar, an earring, a dense metal restoration, the mandibular ramus itself — is struck by the beam twice. The ghost is:
- On the opposite side of the image from the real object,
- Higher than the real object, and
- Magnified and blurred horizontally.
Recognizing the pattern prevents misinterpreting a ghost as pathology and prevents an unnecessary repeat exposure.
Cone Beam Computed Tomography (CBCT)
CBCT acquires a volumetric data set with a cone-shaped beam and a single rotation, producing undistorted, 1:1 multiplanar reconstructions in axial, coronal, and sagittal planes.
Established indications:
- Implant site assessment — ridge width, height, and the position of the inferior alveolar canal, mental foramen, and maxillary sinus.
- Localizing impacted teeth and assessing associated root resorption of adjacent teeth.
- Complex endodontic anatomy, suspected vertical root fracture, resorptive defects, and persistent apical pathosis after treatment.
- Maxillofacial trauma and pathology requiring three-dimensional extent.
- Temporomandibular joint osseous morphology.
- Orthognathic and airway analysis, and craniofacial anomaly planning.
Limitations and cautions:
- Dose is substantially higher than intraoral or panoramic imaging — a small field-of-view scan is comparable to a handful of intraoral images, while a large field-of-view scan can approach the dose of dozens.
- Poor soft-tissue contrast; CBCT does not replace medical CT or MRI for soft-tissue disease, and MRI remains the study of choice for the TMJ articular disc.
- Beam-hardening and metal streak artifacts near restorations and posts.
- The clinician who orders a CBCT is responsible for interpreting the entire imaged volume, including incidental findings in the sinuses, airway, and cervical spine, or for obtaining a radiologist's report. Failure to review the whole volume is a documented source of liability.
- CBCT is not indicated as a routine screening tool, nor for routine caries or periodontal diagnosis.
Relative Dose Context
| Examination | Relative effective dose |
|---|---|
| Single intraoral image, digital, rectangular collimation | Lowest |
| Four-image bitewing set | Low |
| Panoramic | Low, roughly comparable to a small number of intraoral images |
| Full-mouth series, digital with rectangular collimation | Moderate |
| Full-mouth series, D-speed film with round collimation | Several times higher than the digital rectangular equivalent |
| Small field-of-view CBCT | Higher than a full-mouth series |
| Large field-of-view CBCT | Substantially higher again |
| Medical head CT | Highest of those listed |
For perspective, natural background radiation in the United States averages roughly 3 mSv per year, and dental imaging represents a small fraction of a patient's total medical exposure — a fact worth conveying accurately to anxious patients without implying that dose is irrelevant.
Special Populations
- Pregnancy: dental radiography with a thyroid collar delivers negligible fetal dose, and necessary imaging should not be withheld. Elective imaging may reasonably be deferred. Documentation of the justification matters.
- Children: tissues are more radiosensitive and life expectancy is longer, so the relative risk per exposure is greater. Use the smallest receptor that answers the question, rectangular collimation, reduced exposure factors, and the smallest CBCT field of view when a scan is genuinely indicated.
- Thyroid collars should be used for all patients when they do not interfere with the examination, and are especially important for children and patients of reproductive age. A lead apron is not required when other dose-reduction measures are in place under current guidance, but a thyroid collar remains recommended for intraoral imaging.
Decision rule to carry into the exam: ask what clinical question the image will answer and whether the answer will change management. If the answer is no, the image is not justified, regardless of how low its dose is.
A panoramic radiograph shows a diffuse radiolucent band obscuring the apices of the maxillary teeth. What positioning error produced this?
An asymptomatic 32-year-old presents as a new patient with no clinical signs of disease and a low caries risk. Which imaging approach is consistent with ADA and FDA selection criteria?
A dentist orders a large field-of-view cone beam scan for implant planning. Which statement describes the clinician's interpretive responsibility?
A patient in the second trimester of pregnancy presents with facial swelling and severe pain from a suspected periapical abscess. What is the appropriate imaging decision?