Unit 2, Part B: Posterior Paralleling, Radiation History and Production

Fundamental Radiographic Terminology

• Radiology: The scientific study and application of radiation, including its uses in diagnosis and treatment.
• Radiation: Emission and propagation of energy through space or a material medium in the form of waves or particles.
• X-Radiation: High-energy, ionizing electromagnetic radiation produced by the collision of a beam of electrons with a metal target in an x-ray tube.
• Radiographs: Image representations (either traditional film-based or digitally captured) recording anatomic structures produced by the passage of x-rays through an object.
• Radiographer: A trained, certified health professional who exposes an anatomical area of a patient to ionizing radiation for diagnostic purposes.

History and Pioneers of Dental Radiography

• Wilhelm Conrad Roentgen:
  • Discovered x-rays on November 8, 1895, while experimenting with vacuum tubes.
  • Historically, x-rays were termed roentgen rays, the discipline was referred to as roentgenology, and radiographic images were called roentgenographs.
  • Honored with the first Nobel Prize in physics in 1901.

• Otto Walkhoff: Produced the first dental radiograph in history.
• Dr. C. Edmund Kells: New Orleans dentist credited with making the first practical clinical application of x-rays in dentistry in 1896.

Posterior Paralleling Technique and Exposure Sequences

• Rationale for Exposure Sequence:

  • Always expose the premolar views prior to the molar views in each quadrant.
  • Comfort: Premolar receptor placement is significantly easier for the patient to tolerate.
  • Gag Reflex Control: Premolar placement is less likely to trigger the gag reflex compared to posterior molar placement.

• Standard 8-Receptor Posterior Paralleling Sequence:

  • The posterior paralleling protocol utilizes $8$ total receptor placements: $4$ maxillary exposures and $4$ mandibular exposures.
  • Step 1 (Maxillary Right Quadrant):
  • Assemble the posterior Extension Cone Paralleling (XCP) instrument for the maxillary right region.
  • Expose the premolar view (covering teeth $14$ and $15$) first.
  • Expose the molar view (covering teeth $16$, $17$, and $18$).
  • Step 2 (Mandibular Left Quadrant):
  • Keep the XCP instrument assembled as is (no reassembly required between maxillary right and mandibular left).
  • Expose the premolar view (covering teeth $34$ and $35$) first.
  • Expose the molar view (covering teeth $36$, $37$, and $38$).
  • Step 3 (Maxillary Left Quadrant):
  • Reassemble the posterior XCP instrument for the maxillary left region.
  • Expose the premolar receptor (covering teeth $24$ and $25$) first.
  • Expose the molar receptor (covering teeth $26$, $27$, and $28$).
  • Step 4 (Mandibular Right Quadrant):
  • Keep the XCP instrument assembled as is.
  • Expose the premolar receptor (covering teeth $44$ and $45$) first.
  • Expose the molar receptor (covering teeth $46$, $47$, and $48$).

• Receptor Coverage Criteria:

  • Maxillary Premolar View: Must display all crowns and roots of the first and second premolars and first molar, including root apices, alveolar crests, contact areas, and surrounding bone.
  • Maxillary Molar View: Must display all crowns and roots of the first, second, and third molars, including root apices, alveolar crests, contact areas, surrounding bone, and the maxillary tuberosity region.
  • Mandibular Premolar View: Must display all crowns and roots of the first and second premolars and first molar, including root apices, surrounding bone, and the distal contact area of the mandibular canine.
  • Mandibular Molar View: Must display all crowns and roots of the first, second, and third molars, including root apices and surrounding bone.

• Mandibular Premolar Technique Modifications:

  • The floor of the mouth in the mandibular premolar area can be extremely sensitive.
  • Placement Procedure: The receptor must be placed underneath the tongue.
  • Comfort Technique: The lower edge of the film or digital receptor binding can be gently softened or curved to avoid tissue impingement.

• Advantages of the Paralleling Technique:

  • Accuracy: Yields an image free of dimensional distortion that accurately represents true anatomical tooth dimensions.
  • Simplicity: Eliminates guesswork in determining custom horizontal and vertical angles.
  • Duplication: Radiographs are easily standardized, making serial comparative examinations highly valid.

• Disadvantages of the Paralleling Technique:

  • Receptor Placement: Can present technical placement challenges in pediatric patients, individuals with small oral cavities, or patients with shallow palatal vaults.
  • Discomfort: The required receptor-holding device can cause soft tissue impingement and patient discomfort.

Atomic Structure, Physics, and Ionization

• Matter and Energy:

  • Matter: Anything that occupies space and possesses form or shape. Matter consists of specific atomic arrangements called molecules.
  • Energy: The defined capacity to perform work.

• Atomic Architecture:

  • An atom comprises two main parts: a central dense nucleus and surrounding orbiting electrons.
  • Atoms are identified by the precise composition of their nucleus and the spatial arrangement of orbiting electrons.

• The Nucleus:

  • Composed of subatomic particles: Protons (carrying positive electrical charges) and Neutrons (carrying no electrical charge).
  • Dental x-rays do not alter or disrupt the tightly bound atomic nucleus; they only experience directional changes or scattering.
  • Dental x-rays cannot induce radioactivity in matter; therefore, patients do not emit radiation after exposure ceases.

• Electrons and Shells:

  • Electrons are tiny, negatively charged subatomic particles with minimal mass.
  • Electrons travel around the nucleus in designated orbital paths known as electron shells (e.g., K orbit, L orbit).
  • Each shell holds a strict maximum capacity of electrons.
  • Orbiting electrons are maintained in position by electron binding energy.

• Mechanism of Ionization:

  • Electrons remain stable within their orbital shells until impacted by external energy, such as x-ray photons.
  • Photon: A minute bundle of pure energy possessing no weight or mass.
  • Ion: An atom or molecule that gains or loses an electron, becoming electrically unbalanced.
  • Ionization: The process of converting an atom into ions. When an x-ray photon collides with an orbital electron, it ejects the electron from its shell. This produces an ion pair: the ejected electron becomes a negative ion, and the remaining atom becomes a positive ion.

Characteristics and Wave Dynamics of Electromagnetic Radiation

• Electromagnetic Spectrum:
  • Electromagnetic radiation consists of photon energy travels through space in a straight line at the speed of light ($3 \times 10^8 accession/s$) with a wavelike motion.
  • Encompasses gamma rays, x-rays, ultraviolet light, visible light, infrared light, microwaves, and radio waves.

• Characteristics of X-Rays:
  • Invisible and undetectable by human senses.
  • Have no mass or weight.
  • Possess no electrical charge.
  • Travel at the speed of light.
  • Travel in short-wavelength, high-frequency waves.
  • Travel in straight lines and can be deflected or scattered.
  • Absorbed by matter during passage.
  • Induce ionization in matter.
  • Cause certain substances to fluoresce.
  • Produce latent images on photographic film or digital sensors.
  • Cause biological changes in living cells.

• Wavelength and Frequency Dynamics:
  • Wavelength: The distance measured from the crest of one wave to the crest of the next.
  • Short Wavelength / High Frequency: Waves crest close together. High frequency delivers high energy and high penetration capability (diagnostic x-rays).
  • Long Wavelength / Low Frequency: Waves crest far apart. Low frequency delivers low energy and weak penetration capability.

Step-by-Step Production of X-Rays

1. Electrical Supply: The x-ray machine is plugged into an electrical outlet. Activating the machine routes current to the control panel.
2. Path to Tubehead: Electric current travels from the control panel through electrical wires inside the extension arm to the x-ray tubehead.
3. Step-Down Transformation: Current passes through a step-down transformer that reduces incoming voltage down to $3\text{ to }5\text{ V}$.
4. Filament Heating: The low voltage enters the cathode filament circuit, heating the tungsten filament.
5. Thermionic Emission: Heat causes the liberation of electrons from the tungsten surface, forming an electron cloud around the cathode filament.
6. Exposure Activation: Pressing the exposure button activates the high-voltage circuit.
7. Electron Acceleration: High potential difference accelerates the negatively charged electron cloud rapidly across the evacuated tube from cathode to anode.
8. Electron Focusing: The negatively charged molybdenum cup inside the cathode directs and concentrates the electron stream onto the target area of the anode.
9. Impact and Conversion: Accelerated electrons strike the tungsten target on the anode. Their kinetic energy is converted into radiation and heat:
   • Heat Generation: Less than $1\%$ of the kinetic energy is converted into x-rays; the remaining $99\%$ is converted into heat.
   • Heat Dissipation: Heat is conducted away from the target through a solid copper stem and absorbed by insulating oil surrounding the tube in the tubehead.
10. Beam Filtration: Generated x-rays travel through the unleaded glass window, through the insulating oil, and through the tubehead seal to the aluminum filter.
11. Filter Function: The aluminum filter absorbs and screens out non-diagnostic, low-energy, long-wavelength x-rays.
12. Collimation: The filtered beam passes through a lead collimator, restricting the size and shape of the primary beam.
13. Beam Exit: The collimated beam travels down the lead-lined Position Indicating Device (PID) and exits at the open end.

Radiation Classifications and Patient Interactions

• Primary Radiation:
  • The original, penetrating x-ray beam that emerges directly from the target of the x-ray tube.
  • Referred to as the useful beam or primary beam.
• Secondary Radiation:
  • X-radiation created instantly when the primary beam interacts with matter (such as human facial tissues or patient bones).
  • Less penetrating than primary radiation.
• Scatter Radiation:
  • A specific sub-form of secondary radiation resulting from an x-ray photon being deflected from its initial trajectory by interaction with matter.
  • Travels to all parts of the patient's body and all areas of the dental operatory.

Visual Characteristics of Diagnostic Radiographs

• Radiolucent Structures:

  • Structures that readily permit the passage of x-ray beams through them.
  • Appear dark gray or black on processed radiographs.
  • Examples include air spaces, soft tissue masses, periodontal ligament spaces, and dental pulp chambers.

• Radiopaque Structures:

  • Dense structures that absorb or resist the passage of x-rays.
  • Appear bright white or light gray on processed radiographs.
  • Examples include metallic restorations (amalgam, gold), dental enamel, dentin, and dense cortical bone.