A Comprehensive Guide to Dental Radiography Techniques and Principles
Radiology
Radiology is the study and use of radiant energy, including roentgen rays, radium, and radioactive isotopes, as applied to medicine and dentistry.
Roentgenology
Roentgenology is the study and use of roentgen rays (x-rays) as applied to medicine and dentistry.
Radiation Physics
Ionization
Ionization is the process by which a neutral atom or molecule acquires either a positive (+) or negative (-) charge, converting an atom into an ion.
Main Groups of Radiation
Corpuscular/Particulate Radiation
This type consists of bits of matter traveling at high speeds, such as alpha rays, beta rays, and cathode rays.
Electromagnetic Radiation
- Propagation of energy through space accompanied by electric and magnetic force fields.
- Combination of electric and magnetic energy emitted in the form of rays or waves.
- Examples: x-rays, infrared rays, UV rays, radio waves, gamma rays, Grenz rays, cosmic rays.
Production of X-rays
X-rays are a form of radiant energy with the power to penetrate substances ordinarily opaque. They are electromagnetic radiations that travel in a straight line at the speed of light (186,000 miles/sec) and are made of units of energy called quanta or photons.
Two Processes of X-ray Production
1. Bremsstrahlung Radiation
- The majority of x-rays produced by dental x-ray machines.
- Also known as braking radiation because it is produced by the braking or decelerating of high-speed electrons.
- Produced by the direct hit of an electron on the nucleus in the target.
2. Characteristic Radiation
- Occurs when a bombarding electron from the tube current displaces an electron from the shell of a target atom, ionizing the atom.
- The dislodged electron becomes a photon-electron and carries energy.
- It may strike and dislodge other electrons, also a form of scattered/secondary radiation.
Types of Radiations
- Primary: Radiation emanating directly from the focal spot of the target of an x-ray.
- Useful Beam: 1° radiation that passes through the aperture, cone, or collimator.
- Stray Radiation: Radiation emanating from parts of the x-ray tube other than the focal spot.
- Secondary Radiation: Radiation emitted by any substance through which x-rays are passing.
- Scattered Radiation: A form of secondary radiation that has deviated in direction while passing through a substance.
Properties of X-rays
- Penetrate opaque substances.
- Affect the silver halide salts on films.
- Produce fluorescence/glowing on contact with crystals of certain chemical compounds.
- Stimulate or destroy living tissue.
- Produce biologic changes valuable in radiation therapy but necessitate caution in use.
- Differentially absorbed by matter.
- Invisible and weightless.
- Travel in a straight line at the speed of light.
- Have a wide range of wavelengths.
Bisecting-Angle Technique
This technique is based on Cieszynski’s rule of isomerism, a geometric theorem stating that two triangles are equal when they share one complete side and have two equal angles.
Maxillary Central Incisor
Film Placement
Place a No. 1 film directly behind the maxillary central incisors, aligned with the midline of the arch.
Projection
Direct the central ray through the contact point of the central incisors and perpendicular to the bisector of the angle formed by the long axis of the film and the tooth/teeth.
Point of Entry
Midline through the tip of the nose.
Maxillary Lateral
Film Placement
Place a No. 1 film directly behind the maxillary lateral incisor.
Projection
Direct the central ray to pass through the middle of the lateral incisor, opening the mesial contact area. Vertical angulation: about +40 degrees.
Point of Entry
Through the ala of the nose, about 1 cm from the midline.
Maxillary Canine
Film Placement
Place a No. 1 film directly behind the maxillary canine, with the long axis of the tooth superimposed on the central long axis of the film.
Projection
Vertical angulation: about +45 degrees. Horizontal angulation: the beam should pass through the mesial contact of the canine. Do not attempt to open the distal contact.
Point of Entry
Through the ala of the nose, through the canine eminence.
Maxillary Premolar
Film Placement
Place a No. 2 film in the mouth with the long dimension parallel to the occlusal plane. The packet should cover the distal half of the canine, premolars, and the first molar.
Projection
Vertical angulation: +30 degrees. Horizontal angulation: through the interproximal area between the 1st and 2nd premolars.
Point of Entry
Below the pupil of the eye, close to the level of the ala-tragus line.
Maxillary Molar
Film Placement
Position the film far enough posteriorly to cover the 1st, 2nd, and 3rd molar regions. The anterior border should just cover the distal aspect of the 2nd premolar.
Projection
Vertical angulation: +20 degrees. Horizontal angulation: the beam should pass through the interproximal spaces between the molar teeth.
Point of Entry
In line with the outer canthus of the eye, below the zygoma, and on the anteroposterior (AP) level with the second molar.
Mandibular Central/Lateral Incisor
Film Placement
Place a No. 1 film directly behind the mandibular central and lateral incisors.
Projection
Vertical angulation: -15 degrees. Horizontal angulation: the beam should pass through the contact area of the central and lateral incisors.
Point of Entry
Just below the vermilion border of the lip, approximately 1 cm from the midline.
Mandibular Premolar
Film Placement
Place a No. 2 film so that the anterior border extends to cover the distal half of the canine.
Projection
Vertical angulation: -10 degrees. Horizontal angulation: the central ray should pass between the interproximal areas of the 1st and 2nd premolars.
Point of Entry
Usually below the pupil of the eye and approximately 3 cm above the inferior border of the mandible.
Mandibular Canine
Film Placement
Place a No. 1 film just behind the mandibular canine.
Projection
Vertical angulation: -20 degrees. Horizontal angulation: the beam should pass through the mesial contact of the canine.
Point of Entry
Through the canine, approximately 3 cm from the midline.
Mandibular Molar
Film Placement
Place a No. 2 film so that the anterior border extends to the middle of the 2nd premolar.
Projection
Vertical angulation: -5 degrees. Horizontal angulation: direct the central beam through the interproximal spaces between the molar teeth.
Point of Entry
On the cheek, below the outer canthus of the eye, approximately 3 cm above the inferior border of the mandible.
Film Holders
Various holders are available, such as Rinn XCP, Hawe-Neos, Unibite, and Masel Precision. Film holders have three basic components:
- A mechanism to hold the film packet parallel to the teeth, preventing bending.
- A bite block or platform.
- An x-ray beam aiming device.
Advantages of the Paralleling Technique
- Geometrically accurate images with minimal magnification.
- Automatic determination of horizontal and vertical angulations by positioning devices (if placed correctly).
- Accurate aiming of the x-ray beam at the center of the film, preventing cone cuts.
- Reproducible radiographs at different visits and with different operators.
- Consistent relative position of the film packet, teeth, and x-ray beam, regardless of the patient’s head position.
- The shadow of the zygomatic buttress appears above the apices of the molar teeth.
Disadvantages of the Paralleling Technique
- Potentially uncomfortable film packet positioning for the patient.
- Difficult holder placement within the mouth.
- Technique limitations due to mouth anatomy.
- Apices of teeth sometimes appearing near the film edge.
- Challenging holder positioning in the third molar regions.
- Inability to use a short focal spot-to-skin distance.
Advantages of the Bisecting Angle Technique
- Relatively comfortable film packet positioning for the patient.
- Simple and quick positioning.
- Accurate tooth image size (if angulations are correct).
Disadvantages of the Bisecting Angle Technique
- Frequent image distortion due to technique variables.
- Foreshortening/elongation due to incorrect vertical angulation.
- Poor visualization of periodontal bone levels.
- Frequent overlap of the zygomatic buttress shadow over the roots of upper molars.
- Necessity to assess horizontal/vertical angulations for each patient.
- Difficulty obtaining reproducible views.
- Potential for cone cuts.
- Increased scattered x-rays with a plastic short cone.
Bitewing Radiography
Indications
- Detection of proximal dental caries.
- Monitoring the progression of dental caries.
- Assessment of existing restorations and secondary caries.
- Assessment of periodontal status, including alveolar bone height and calculus presence.
Ideal Technique Requirements
- Tab or bite-platform positioned on the middle of the film packet and parallel to its upper and lower edges.
- Posterior teeth and film packet in contact or as close as possible.
- X-ray beam perpendicular to the teeth and film packet, passing directly through all contact areas.
- Vertical angulation of the x-ray tube: approximately 5-8° downwards.
- Reproducible positioning.
Positioning Techniques
- Using a tab attached to the film packet and aligning the tube head by eye.
- Using a film holder with a beam-aiming device to facilitate positioning and alignment.
Positioning Difficulties
Mandibular Third Molars
The main difficulty is placing the film posteriorly enough to capture the entire mandibular molar, surrounding tissues, and the inferior dental canal.
Possible Solutions
- Taking two radiographs with different horizontal tubehead angulations.
- First radiograph: ideal horizontal angulation with the x-ray beam passing between the 2nd and 3rd molars.
Gagging
Possible Solutions
- Having the patient suck on a local anesthetic lozenge before film placement.
- Asking the patient to focus on deep breathing while the film is in their mouth.
Endodontics
Difficulties include film packet placement and stabilization with endodontic instruments, rubber dam, and clamps in position, as well as root canal identification and separation.
Possible Solutions
Using specialized endodontic film holders with a basket in the bite platform to accommodate instrument handles.
Edentulous Alveolar Ridge
: In edentulous patients, the lack of height in the palate, or loss of lingual sulcus depth, contraindicates the paralleling technique and all peri-apical radiograph should be taken using a modified bisecting angle technique
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