28.2 Dental Radiography: Paralleling and Bisecting Techniques, Bitewing, Occlusal, Panoramic & Cephalometric
Key Takeaways
- The paralleling technique places the receptor parallel to the long axis of the tooth with the central ray perpendicular to both, using a holder and a long cone, and it produces the most dimensionally accurate periapical image.
- The bisecting-angle technique directs the central ray perpendicular to an imaginary line bisecting the angle formed by the long axis of the tooth and the plane of the receptor, and it is more prone to elongation and foreshortening.
- Excessive vertical angulation causes foreshortening and insufficient vertical angulation causes elongation, while incorrect horizontal angulation causes overlapping of the interproximal contacts.
- Bitewing radiography uses roughly a positive 10 degree vertical angulation to demonstrate the crowns and interproximal surfaces of the upper and lower teeth plus the alveolar crests on a single image, and is the principal projection for interproximal caries and periodontal bone level.
- Panoramic radiography requires the midsagittal plane vertical, the Frankfort horizontal plane parallel to the floor, the tongue pressed against the hard palate, and the incisors biting in the groove of the bite block; violating any of these produces a recognisable, named artifact.
28.2 Dental Radiography: Paralleling and Bisecting Techniques, Bitewing, Occlusal, Panoramic & Cephalometric
The Enhanced TOS assigns 4 items to Dental Radiography, with competencies to show the skills in preparation and proper positioning technique, illustrate positioning principles, indications and contraindications, and patient preparation for the various dental radiographic procedures, and to distinguish diagnostic features. Dental radiography is examinable because a Philippine radiologic technologist may staff a dental clinic, a hospital dental service, or a mobile unit, and because it demonstrates geometric principles in their purest form.
1. The Families of Dental Radiography
| Family | Receptor location | Principal projections |
|---|---|---|
| Intraoral | Inside the mouth | Periapical, bitewing (interproximal), occlusal |
| Extraoral | Outside the mouth | Panoramic (orthopantomogram), lateral cephalometric, posteroanterior cephalometric, temporomandibular joint projections, lateral oblique mandible |
2. Intraoral Periapical Radiography
The periapical image shows the entire tooth from the occlusal or incisal surface to about 2-3 mm beyond the apex, plus the surrounding periodontal ligament and alveolar bone. It is the projection for periapical pathology, root morphology, and endodontic assessment.
Two techniques exist and are examined against each other constantly.
Paralleling (long-cone, right-angle, extension-cone) technique
- The receptor is placed parallel to the long axis of the tooth, held away from the teeth in the palate or floor of mouth by a beam-alignment holder.
- The central ray is directed perpendicular to both the tooth and the receptor.
- Requires a long cone — a source-to-object distance of about 20-40 cm — to offset the magnification caused by the increased object-to-receptor distance.
| Advantages | Disadvantages |
|---|---|
| Most dimensionally accurate; minimal elongation or foreshortening | Receptor placement can be uncomfortable, especially with a shallow palate or a strong gag reflex |
| Reproducible, because the holder fixes the geometry | Requires holders, which must be sterilised between patients |
| Alveolar crest levels are shown accurately, which matters in periodontics | Longer source-to-object distance means longer exposure or higher output |
| Less superimposition of the zygomatic process on maxillary molar roots | Not always possible in children or in anatomically restricted mouths |
This is the preferred technique wherever it can be applied.
Bisecting-angle technique
- The receptor is placed directly against the crown and the palate or floor of the mouth, so it forms an angle with the long axis of the tooth.
- An imaginary line bisects the angle between the long axis of the tooth and the plane of the receptor.
- The central ray is directed perpendicular to that bisector, which makes the projected image length approximately equal to the true tooth length (the rule of isometry).
- A short cone is typical, with the receptor held by the patient's finger or a bisecting-angle holder.
| Advantages | Disadvantages |
|---|---|
| Comfortable; usable with shallow palates, in children, and with a strong gag reflex | Dimensional distortion is common |
| Shorter exposure time with the short cone | Alveolar crest height is unreliable, so it is poor for periodontal assessment |
| No specialised holder strictly required | Patient's finger in the beam if no holder is used |
Vertical and horizontal angulation errors — the highest-yield content
| Error | Cause | Appearance |
|---|---|---|
| Foreshortening | Excessive (too steep) vertical angulation | Teeth appear shorter than they are |
| Elongation | Insufficient (too flat) vertical angulation | Teeth appear longer than they are |
| Overlapped contacts | Incorrect horizontal angulation — the central ray is not perpendicular to the curvature of the arch through the interproximal spaces | Interproximal surfaces superimpose, hiding early caries |
| Cone cut | The beam is not centred on the receptor | An unexposed clear border on one edge |
| Herringbone (tyre-track) pattern with a light image | The film packet was placed backwards, so the beam passed through the lead foil first | Faint image with an embossed pattern |
| Apices cut off | Receptor not placed far enough into the palate or floor of mouth | The area of clinical interest is missing |
Standard vertical angulations for the bisecting technique (positive means the tube head is angled downward, negative upward):
| Region | Maxillary | Mandibular |
|---|---|---|
| Incisor | about +40 to +45 degrees | about -15 to -20 degrees |
| Canine | about +45 to +50 degrees | about -20 degrees |
| Premolar | about +30 to +35 degrees | about -10 to -15 degrees |
| Molar | about +20 to +25 degrees | about -5 degrees |
3. Bitewing (Interproximal) Radiography
- The patient bites on a tab or holder so that a single receptor records the crowns of both the maxillary and mandibular teeth on one image, together with the alveolar crests.
- The vertical angulation is about +10 degrees to compensate for the slight lingual tilt of the receptor and the curve of the arch.
- Horizontal angulation must be exactly perpendicular to the interproximal contacts, or the contacts overlap and early caries is hidden.
Indications: interproximal (proximal surface) caries, recurrent caries beneath restorations, alveolar bone level in periodontal disease, and overhanging restoration margins. Bitewings do not show the apices, so they never replace a periapical when periapical pathology is suspected.
Vertical bitewings, with the receptor oriented long-axis vertically, are used when bone loss is advanced and more alveolar height must be captured.
4. Occlusal Radiography
The receptor is placed on the occlusal plane and the patient bites gently to hold it. It demonstrates a broad area of the arch in cross-section.
| Projection | Central ray | Demonstrates |
|---|---|---|
| Maxillary topographic (anterior) occlusal | About +65 degrees through the bridge of the nose | Anterior maxilla, supernumerary and impacted teeth, palatal cysts |
| Mandibular topographic occlusal | About -55 degrees | Anterior mandible |
| Mandibular cross-sectional (true) occlusal | Perpendicular to the receptor, directed from beneath the chin | Sialoliths in Wharton's duct, buccolingual expansion, mandibular fractures |
The mandibular cross-sectional occlusal is the classic projection for a submandibular salivary calculus — an examination favourite that links this section back to sialography in Section 15.4.
5. Panoramic Radiography (Orthopantomography)
Principle. The tube head and the receptor rotate around the patient's head in opposite directions about a moving centre of rotation, exposing the receptor through a narrow slit. Structures lying within the focal trough (image layer) — a curved zone shaped to the dental arch — are rendered sharply; structures outside it are blurred or distorted. A typical exposure lasts about 12 to 20 seconds.
Indications: third molar assessment, trauma to the mandible and maxilla, extensive dental caries survey, developmental and eruption assessment in children, temporomandibular joint overview, jaw lesions and cysts, and pre-surgical implant planning.
Preparation: remove all earrings, necklaces, hairpins, spectacles, dentures, orthodontic removable appliances, hearing aids and tongue or lip piercings. Apply the lead apron without a thyroid collar shaped to obstruct the beam path, because a conventional thyroid collar will project as a large opaque artifact across the mandible.
Positioning rules and the artifact each violation produces
| Rule | Violation | Named artifact |
|---|---|---|
| Frankfort horizontal plane parallel to the floor | Chin tipped too high | Flat or reverse smile occlusal plane; hard palate superimposed on maxillary roots; loss of maxillary detail |
| Chin tipped too low | Exaggerated smile (excessively curved) occlusal plane; mandibular incisor roots obscured; condyles may be cut off | |
| Midsagittal plane vertical and centred | Head rotated or shifted laterally | Unequal magnification — the side farther from the receptor is magnified, the near side narrowed |
| Incisors biting edge-to-edge in the bite-block groove | Positioned too far forward | Anterior teeth narrowed and blurred |
| Positioned too far back | Anterior teeth widened and blurred; spine may be superimposed | |
| Tongue pressed firmly against the hard palate throughout | Tongue not raised | Radiolucent air shadow (palatoglossal air space) across the apices of the maxillary teeth |
| Spine straight, patient standing tall, shoulders down | Slumped posture | Pyramidal opaque shadow of the cervical spine in the centre of the image |
| Remove all metal | Object left in place | Ghost image — appears on the opposite side, higher, larger and more blurred than the real object |
The ghost image rule is examined every administration: opposite side, higher, larger, blurrier. It arises because the object is struck twice by the beam, once on each side of the rotation.
Limitations: panoramic images have lower spatial resolution and inherent magnification and distortion compared with intraoral films, and they cannot reliably show early interproximal caries. Panoramic radiography complements, but does not replace, bitewing and periapical imaging.
6. Cephalometric Radiography
Used chiefly in orthodontics, orthognathic surgery planning and craniofacial growth studies, cephalometry produces a standardised, reproducible image using a cephalostat with ear rods that fix the head.
| Projection | Position | Use |
|---|---|---|
| Lateral cephalogram | Midsagittal plane parallel to the receptor; Frankfort plane parallel to the floor; teeth in centric occlusion; lips relaxed; a soft-tissue filter over the facial profile | Skeletal and dental relationships, growth analysis, airway assessment; landmark tracing of sella, nasion, points A and B, gonion and menton |
| Posteroanterior cephalogram | Midsagittal plane perpendicular to the receptor; OML perpendicular to the receptor | Facial asymmetry, transverse discrepancies |
A long SID (typically about 150-180 cm) is used to minimise magnification, and the magnification factor is recorded so measurements can be corrected.
7. Radiation Protection in Dental Radiography
- Rectangular collimation rather than round reduces the irradiated area by roughly 60%, and is the single most effective dental dose-reduction measure.
- Use the fastest receptor system consistent with diagnostic quality — F-speed film or digital sensors rather than D-speed film.
- Long cone (20-40 cm) reduces entrance skin dose compared with a short cone.
- Lead apron for the patient; a thyroid collar for intraoral work, and specifically not an obstructing collar for panoramic work.
- The operator stands at least 2 metres away and at 90 to 135 degrees to the primary beam, or behind a protective barrier; a hand-held unit must have its own shield.
- Never hold the receptor or the patient's head during exposure. If a receptor cannot be retained, a guardian who is neither pregnant nor an occupationally exposed worker assists, wearing a lead apron.
- Select images by clinical need, not by routine — the professional standard is that radiographs follow a clinical examination and a specific question.
A periapical radiograph of the maxillary molars shows the teeth appearing significantly shorter than their true clinical length. What error caused this?
On a panoramic radiograph, a diffuse radiolucent band is seen crossing the apices of the maxillary teeth. What positioning fault caused this?
Which statement correctly compares the paralleling and bisecting-angle intraoral techniques?
A dentist asks which projection best demonstrates a suspected calculus in Wharton's duct. Which intraoral projection is indicated?