35.1 Radiation Physics, Filtration and Collimation
Key Takeaways
- About 99% of the kinetic energy of accelerated electrons becomes heat and only about 1% becomes X-ray photons.
- Bremsstrahlung radiation produces 70% to 80% of the beam as a continuous spectrum; characteristic radiation contributes 20% to 30%.
- Total filtration must be at least 1.5 mm aluminium equivalent up to and including 70 kVp, and at least 2.5 mm above 70 kVp.
- Rectangular collimation reduces the irradiated area to no more than about 40 by 50 mm and cuts patient dose substantially compared with a 60 mm circular beam.
- Dental sets typically operate between 60 and 70 kV with a focal spot to skin distance of at least 200 mm.
1. Principles of Radiation Physics & X-Ray Production
Diagnostic dental X-rays are high-energy electromagnetic ionizing radiations generated inside a specialized vacuum tubehead.
Dental X-Ray Tubehead Functional Anatomy
│
├── Cathode (-) ────────> Heated Tungsten Filament (Thermionic emission of electrons)
│ Focusing Cup (Molybdenum; repels/focuses electron cloud)
│
├── High Voltage (kVp) ─> Accelerates electrons across vacuum gap (e.g. 60–70 kV)
│
├── Anode (+) ──────────> Tungsten Target (Focal spot where electrons collide)
│ Copper Stem (Rapidly conducts heat away to surrounding oil)
│
└── Filtration & Cone ──> Inherent + Added Aluminium Filtration (Removes soft photons)
Rectangular Collimator (Shapes beam to ≤40 × 50 mm)
Mechanisms of X-Ray Photon Generation
When high-velocity electrons accelerated by the potential difference (kilovoltage peak, $kVp$) strike the tungsten target embedded in the copper anode, approximately 99% of kinetic energy is converted into heat, and only 1% is converted into X-ray photons. Photon generation occurs via two distinct physical interactions:
- Bremsstrahlung (Continuous / 'Braking') Radiation:
- Represents 70% to 80% of the dental X-ray beam.
- High-speed incoming electrons pass near the highly positive tungsten nucleus. The electrostatic attraction exerts a braking force, deflecting and decelerating the electron.
- The kinetic energy lost by the electron during this deceleration is emitted directly as an X-ray photon.
- Deflections range from grazing passes (emitting low-energy photons) to direct nuclear impacts (emitting maximum-energy photons equal to the applied $kVp$). This produces a continuous, heterogeneous energy spectrum.
- Characteristic Radiation:
- Represents 20% to 30% of the dental beam when operating above $69.5\text{ kV}$.
- An incoming high-speed electron collides directly with an inner-shell orbital electron of the tungsten atom (most commonly the K-shell) with sufficient energy to overcome its binding energy ($69.5\text{ keV}$).
- The K-shell electron is ejected, creating an orbital vacancy. An outer-shell electron (e.g., from the L-shell or M-shell) immediately drops into the lower energy vacancy to restore stability.
- The difference in binding energy between the two shells is radiated as a discrete, mono-energetic X-ray photon ($K_\alpha \approx 59.3\text{ keV}$, $K_\beta \approx 67.2\text{ keV}$).
- If the tubehead operating potential is below $69.5\text{ kVp}$, characteristic radiation cannot be produced from the tungsten target.
Beam Filtration: Inherent vs Added Filtration
Low-energy, long-wavelength X-ray photons lack the energy required to penetrate dense oral tissues (teeth and alveolar bone) to reach the image receptor. Instead, they are completely absorbed by facial soft tissues, contributing solely to useless patient skin dose without diagnostic benefit. Beam filtration preferentially absorbs these harmful low-energy photons, a process known as beam hardening.
- Inherent Filtration: Filtration provided by the materials built into the tubehead assembly, including the glass envelope of the X-ray tube, the surrounding insulating oil, and the oil seal / tubehead aperture window. Typically equivalent to 0.5 to 1.0 mm of aluminium (Al).
- Added Filtration: Thin discs of pure aluminium placed in the primary beam path between the tube housing and the collimator.
- Total Filtration: The sum of inherent and added filtration. Statutory requirements under UK IRR17 / IR(ME)R17 mandate:
- For dental X-ray equipment operating at up to and including 70 kVp: Total filtration must be not less than 1.5 mm aluminium equivalent.
- For dental X-ray equipment operating at greater than 70 kVp: Total filtration must be not less than 2.5 mm aluminium equivalent.
Beam Collimation: Rectangular vs Circular Geometry
Collimation restricts the physical dimensions and spatial shape of the primary X-ray beam emerging from the tubehead.
| Collimation Type | Physical Dimensions | Irradiated Surface Area | Clinical & Radiation Protection Characteristics |
|---|---|---|---|
| Circular Collimation | $60\text{ mm}$ diameter circle | $\approx 28.3\text{ cm}^2$ | Irradiates a circular field substantially larger than the standard Size 2 receptor ($31 \times 41\text{ mm} \approx 12.7\text{ cm}^2$). Delivers significant unnecessary radiation to surrounding soft tissues. |
| Rectangular Collimation | $\le 40 \times 50\text{ mm}$ (or $35 \times 45\text{ mm}$) | $\approx 15.7\text{ to } 20.0\text{ cm}^2$ | Matches receptor dimensions closely. Reduces patient absorbed radiation dose by 40% to 50%. Dramatically minimizes scattered Compton radiation, thereby improving image contrast and diagnostic sharpness. UK regulations strongly recommend rectangular collimators for all routine intraoral periapical and bitewing projections. |
Beam Cross-Section Comparison at Patient Face
Circular Collimation (60 mm) Rectangular Collimation (35 x 45 mm)
╭──────────╮ ┌───────────────┐
╭──╯ ╰──╮ │ │
╭─╯ ┌───────┐ ╰─╮ │ ┌─────────┐ │
│ │ Size2 │ │ │ │ Size 2 │ │
│ │Sensor │ │ │ │ Sensor │ │
╰─╮ └───────┘ ╭─╯ │ └─────────┘ │
╰──╮ ╭──╯ │ │
╰──────────╯ └───────────────┘
Dose: 100% (Baseline) Dose: 50%–60% (40%–50% Reduction)
Dose, Risk and Justification
The reason physics appears in a clinical examination is that it underpins justification and optimisation. Radiation effects are divided into deterministic effects, which have a threshold dose and increase in severity above it — skin erythema, cataract, epilation — and stochastic effects, which have no threshold and for which the probability rather than the severity increases with dose, principally cancer induction and heritable effects. All diagnostic dental radiography operates in the stochastic domain, which is why every dose reduction matters however small the individual exposure.
The effective dose of common dental examinations, measured in microsieverts, allows comparison: an intraoral radiograph with a rectangular collimator and a fast digital receptor delivers a very small dose, a panoramic radiograph somewhat more, and a cone beam computed tomography scan substantially more again, with large fields of view approaching or exceeding the dose of many conventional medical examinations. Candidates are expected to know the ranking and the reason for it rather than to memorise exact figures, and to know that cone beam CT must never be used as a routine or screening examination.
Practical Dose Reduction
The measures that reduce patient dose are examinable as a list because each appears as a distractor: rectangular collimation, which reduces the irradiated area and dose by around half compared with a circular beam; the fastest available image receptor, which for digital systems means selecting the correct exposure setting rather than defaulting to a high one; a long focus-to-skin distance with a paralleling technique and beam-aiming device, which reduces divergence and improves geometry; adequate filtration, conventionally at least 1.5 mm aluminium equivalent up to 70 kV and 2.5 mm above it, to remove low-energy photons that contribute dose but not image; appropriate kilovoltage, with higher kV producing a longer scale of contrast and lower patient dose; and above all not taking the radiograph at all unless it is justified. Thyroid collars are used where the thyroid lies in or close to the primary beam, and lead aprons are no longer routinely recommended for dental radiography.