Radiographic Prescription, Exposure & Safety

Radiographic Prescription, Exposure, and Safety

Radiographic imaging is a vital diagnostic tool in dental hygiene, but it carries inherent biological risks due to ionizing radiation. To ensure patient and operator safety, dental hygienists must adhere strictly to evidence-based guidelines for radiographic prescription, understand the parameters governing radiation exposure, and execute comprehensive protective protocols.

FDA/ADA Guidelines for Prescribing Radiographs

Dental radiographs must never be prescribed as a routine administrative procedure or at predetermined calendar intervals. Instead, they are prescribed only after a thorough clinical examination and a review of the patient's medical and dental history. The Food and Drug Administration (FDA) and the American Dental Association (ADA) have established guidelines that categorize patients by age, developmental stage, and risk status to determine the appropriate type and frequency of radiographic examinations.

New Patients (Initial Assessment)

• Child (Primary Dentition): Selected periapical or bitewing views if proximal surfaces cannot be examined clinically.
• Child (Transitional Dentition): Posterior bitewings with panoramic exam, or posterior bitewings with selected periapical views.
• Adolescent (Prior to Eruption of Third Molars): Posterior bitewings and panoramic exam, or posterior bitewings and selected periapical views. A full mouth series (FMX) is preferred when there is clinical evidence of generalized dental disease or a history of extensive dental treatment.
• Adult (Dentate or Partially Edentate): FMX or select bitewings with a panoramic radiograph. FMX is indicated when there is generalized oral disease or a complex treatment history.
• Edentulous Adult: Selected intraoral views or a panoramic exam to evaluate for retained roots, cysts, or residual pathology.

Recall Patients (Active Caries or High Risk)

• Child (Primary/Transitional): Posterior bitewings at 6-to-12-month intervals if proximal surfaces cannot be examined clinically.
• Adolescent: Posterior bitewings at 6-to-12-month intervals.
• Adult: Posterior bitewings at 6-to-18-month intervals.

Recall Patients (No Active Caries and Low Risk)

• Child (Primary Dentition): Posterior bitewings at 12-to-24-month intervals.
• Child (Transitional Dentition): Posterior bitewings at 12-to-24-month intervals.
• Adolescent: Posterior bitewings at 18-to-36-month intervals.
• Adult: Posterior bitewings at 24-to-36-month intervals.

Recall Patients with Periodontal Disease or History of Treatment

• Clinical Judgement: Select periapicals and/or bitewings to assess bone levels. The frequency depends on disease activity, severity, and stability, typically ranging from 12 to 24 months.

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The ALARA Principle and Radiobiology

The ALARA principle stands for As Low As Reasonably Achievable. This principle is founded on the non-threshold, cumulative nature of ionizing radiation, which assumes that any dose of radiation, no matter how small, can produce biologic damage (stochastic effects, such as cancer induction or genetic mutations). To implement ALARA, dental professionals must minimize exposure time, maximize distance from the source, and utilize appropriate shielding.

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Exposure Factors: Quality, Quantity, and Density

Three primary exposure factors control the characteristics of the x-ray beam: kilovoltage peak (kVp), milliamperage (mA), and exposure time.

To maintain optimal image density, if mA is increased, exposure time must be proportionally decreased.

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Patient Radiation Protection Measures

To minimize patient exposure to ionizing radiation, several engineering controls and safety equipment are employed:

1. Collimation: The collimator (usually made of lead) restricts the size and shape of the primary x-ray beam.
   • Circular Collimation: Restricts the beam to a diameter of 2.75 inches (7 cm) at the patient's skin.
   • Rectangular Collimation: Restricts the beam to slightly larger than a size 2 sensor. It reduces the area of skin exposure by 60% to 70% compared to circular collimation and is highly recommended by the ADA.
2. Filtration: Aluminum filters are placed in the path of the x-ray beam to absorb low-energy, long-wavelength, non-penetrating x-rays that would otherwise be absorbed by the patient's skin without contributing to the diagnostic image.
   • Total Filtration: Operating at or below 70 kVp requires a minimum of 1.5 mm aluminum equivalent. Operating above 70 kVp requires 2.5 mm aluminum equivalent.
3. Position Indicating Device (PID):
   • A long, rectangular PID (16 inches) is preferred over a short PID (8 inches). The longer PID produces less beam divergence, resulting in a smaller area of exposure and reduced tissue volume irradiated, as well as less image magnification.
4. Lead Apron and Thyroid Collar:
   • The apron must have a minimum of 0.25 mm lead equivalent to shield the patient's gonads and blood-forming tissues from scatter radiation.
   • The thyroid collar shields the highly radiosensitive thyroid gland during intraoral exposures.
   • Exam Trap: The thyroid collar must never be used during panoramic radiography because it blocks the rotating beam and produces a large, radiopaque artifact on the final image.
5. Fast Film and Digital Receptors:
   • Digital sensors (CCD, CMOS, or PSP) are the most effective way to reduce patient radiation, requiring 50% to 90% less radiation than conventional film.
   • If film is used, F-speed film (the fastest film speed) should be selected. It reduces exposure by 60% compared to D-speed film.

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Operator Safety Protocols

The operator must avoid the primary beam and minimize exposure to scatter radiation.

1. Position and Distance Rule:
   • The operator must stand at least 6 feet (2 meters) away from the x-ray tubehead during exposure.
   • If distance is not possible, a protective barrier (e.g., lead-lined wall or lead glass window) must be used.
   • The operator must stand at an angle of 90 degrees to 135 degrees relative to the primary beam.
2. Receptor and Tubehead Handling:
   • The operator must never hold a receptor in the patient's mouth during exposure. If a patient cannot hold the holder (e.g., a child or disabled patient), a parent or caregiver wearing a lead apron may assist.
   • The operator must never hold the PID or tubehead housing during exposure. If the tubehead drifts, it must be serviced; holding it exposes the operator's hand to leakage radiation.
3. Radiation Monitoring:
   • Personnel monitoring devices (dosimeter badges) measure the accumulated exposure of the operator. Badges must be worn at waist level and never during personal medical/dental exposures.