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Radiation Safety
Dental x-rays are a form of ionizing radiation that can damage living cells, so every exposure must be justified and kept as small as possible. This chapter covers how x-rays are produced, how radiation affects tissue, and the specific steps that protect the operator and the patient.
How X-Rays Are Produced
X-rays are high-energy electromagnetic waves with no mass and no charge that travel in straight lines at the speed of light. Inside the tube head, electrons boil off a heated filament in the cathode, accelerate across a vacuum, and strike a tungsten target in the anode. Most of that energy becomes heat and only a small fraction becomes the x-ray beam that exits through the position indicating device.
Ionizing radiation
X-rays carry enough energy to knock electrons out of atoms, and that ionization is what makes them capable of damaging cells.
Cathode and anode
The cathode contains the tungsten filament that supplies electrons; the anode contains the tungsten target and copper stem that convert the electron stream into x-rays and carry away heat.
Primary, secondary, and scatter
The primary beam leaves the tube head, secondary radiation is created when the beam strikes matter, and scatter radiation is deflected in all directions and is the main occupational hazard to the operator.
Inverse square law
Beam intensity drops as the square of the distance, so doubling the distance from the source reduces intensity to one quarter.
Filtration and collimation
Aluminum filtration removes low-energy photons that would only add patient dose, and the lead collimator restricts the beam size.
Biological Effects of Radiation
Radiation damage happens when photons ionize water or DNA inside a cell, and injury may not appear until long after the exposure. Doses add up over a lifetime, which is why a small dose today still matters. Cells that divide rapidly and are poorly differentiated are the most sensitive.
Cumulative dose
The body does not fully repair all radiation injury, so every exposure adds to a lifetime total that can never be subtracted.
Somatic versus genetic effects
Somatic effects appear in the exposed person and are not passed on, while genetic effects damage reproductive cells and can be transmitted to future generations.
Latent period
Time passes between exposure and the first visible sign of damage, and that latent period may be days for high doses or many years for the low doses used in dentistry.
Radiosensitive tissues
Bone marrow, reproductive cells, the lens of the eye, the thyroid gland, and the lining of the intestines are highly sensitive, while muscle, nerve, and mature bone are relatively resistant.
Direct and indirect theory
A direct hit on DNA is rare; most damage is indirect, created when radiation ionizes water in the cell and forms toxic free radicals.
Units of measure
The sievert measures dose equivalent and the gray measures absorbed dose in the SI system, replacing the older rem and rad.
ALARA and Operator Protection
ALARA stands for as low as reasonably achievable, the guiding principle that every dental radiograph must have a diagnostic reason and be taken with the smallest dose that produces a usable image. The operator faces repeated small exposures every working day, so distance, shielding, and position are the three defenses.
ALARA principle
Expose a patient only when the diagnostic benefit justifies the dose, and use every reasonable method to keep that dose as low as possible.
ALARA PrincipleDistance rule
Stand at least six feet from the patient and the tube head during the exposure when there is no protective barrier.
Position rule
Stand between 90 and 135 degrees to the primary beam, which is the zone where the patient's head absorbs most of the scatter.
Shielding
Stand behind a fixed wall or lead barrier whenever one is available, and never stand in the path of the primary beam.
Never hold anything
The operator must never hold the sensor, film, or tube head during an exposure; if a patient cannot hold the receptor, a parent or caregiver who is not pregnant does so while wearing a lead apron.
No pointed cone
Use an open-ended, lead-lined, rectangular or round position indicating device, because pointed plastic cones increase scatter radiation.
Patient Protection
Patient dose is reduced by shielding the body, by shortening the exposure needed for an image, and by narrowing the beam so it covers only the receptor. The single largest dose reduction usually comes from the choice of image receptor and collimator.
Lead apron and thyroid collar
Place a lead apron on every patient and add a thyroid collar for all intraoral exposures; the collar is generally omitted for panoramic images because it blocks the beam.
Fast film or digital sensors
Use the fastest available intraoral film, such as F speed, or a digital sensor, which can cut exposure substantially compared with older D speed film.
Rectangular collimation
A rectangular collimator shaped to the receptor reduces the irradiated area, and therefore patient dose, far more than a round collimator.
Beam-alignment devices
Use a receptor holder with an external aiming ring so the beam is centered on the receptor, which prevents cone cuts and the retakes that double a patient's dose.
Selection criteria
Order radiographs based on the individual patient's history and clinical examination rather than on a fixed schedule.
FDA and ADA Dental Radiographic Examinations: Recommendations for Patient Selection and Limiting Radiation ExposureProper technique
Correct receptor placement, angulation, and processing avoid retakes, and every avoided retake is a full exposure the patient never receives.
Exposure Factors and Monitoring
Three settings control the beam: kilovoltage peak, milliamperage, and exposure time. Understanding what each one changes lets the operator adjust an image without adding unnecessary dose, and personal monitoring documents the operator's own accumulated exposure.
Kilovoltage peak
kVp controls the energy and penetrating power of the beam and determines image contrast; higher kVp produces a longer scale of contrast with more shades of gray, while lower kVp produces short-scale, high-contrast images.
Milliamperage
mA controls the number of electrons and therefore the quantity of x-rays produced, which governs image density or overall darkness.
Exposure time
Time also controls the quantity of radiation; mA multiplied by time gives the total exposure, so the two can be traded against each other to keep density constant.
Too light or too dark
An image that is too dark was overexposed or overdeveloped, and one that is too light was underexposed or underdeveloped; correct density by adjusting mA or time, not kVp alone.
Dosimeter badges
Wear a personal monitoring badge at waist or collar level on the outside of the lead apron, exchange it on the service schedule, and never wear it home or leave it near a radiation source.
Occupational dose limits
The recognized whole-body limit for an occupationally exposed adult is 50 millisieverts per year, and the limit for a member of the general public is far lower at 1 millisievert per year.
NCRP Report No. 145Kiểm tra kiến thức của bạn
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