RAD Module 2

What are the two parts of an atom?

A central nucleus (protons and neutrons) and orbiting electrons.

Which atomic particles carry a positive charge, a negative charge, and no charge?

Protons are positive, electrons are negative, and neutrons have no charge.

What determines the atomic number of an atom?

The number of protons in the nucleus (equal to the number of electrons in a neutral atom).

What is the name of the paths in which electrons orbit the nucleus, and what letters designate them?

Orbits or shells, designated K, L, M, N, O, P, and Q (K is closest to the nucleus).

What is binding energy?

The electrostatic attraction between the positive nucleus and a negative orbiting electron that holds the electron in its shell.

Which shell has the strongest binding energy, and what is its value in tungsten?

The K shell, at about 70 keV.

What is a molecule?

Two or more atoms joined by chemical bonds, or the smallest amount of a substance that has its characteristic properties.

What is an ion?

An atom that has gained or lost an electron and is electrically unbalanced.

What is ionization?

The production of ions, or the process of converting an atom into ions by removing or adding an electron.

What is an ion pair?

The positive ion (the remaining atom) and the negative ion (the ejected electron) produced when an electron is removed from an atom.

How do radiation and radioactivity differ?

Radiation is the emission and propagation of energy through space or matter as waves or particles; radioactivity is the spontaneous disintegration (decay) of unstable atoms.

Which one is used in dentistry, radiation or radioactivity?

Radiation (specifically x-radiation).

What are the two groups of ionizing radiation?

Particulate radiation and electromagnetic radiation.

Name the four types of particulate radiation.

Electrons (beta particles and cathode rays), alpha particles, protons, and neutrons.

How do beta particles and cathode rays differ?

They differ only in origin: beta particles come from the nuclei of radioactive atoms, and cathode rays come from an x-ray tube.

What is electromagnetic radiation?

The propagation of wavelike energy, without mass, through space or matter, accompanied by oscillating electric and magnetic fields at right angles to each other.

Which electromagnetic radiations are capable of ionization?

Only the high-energy ones: cosmic rays, gamma rays, and x-rays.

What is a photon?

A discrete bundle of energy with no mass or weight that travels as a wave at the speed of light.

How fast do all electromagnetic radiations travel in a vacuum?

The speed of light, 3 × 10^8 meters per second (186,000 miles per second).

Define wavelength.

The distance between the crest of one wave and the crest of the next.

Define frequency.

The number of wavelengths that pass a given point in a certain amount of time.

How are wavelength and frequency related?

Inversely: a short wavelength means high frequency, and a long wavelength means low frequency.

How does wavelength relate to energy and penetrating power?

Shorter wavelength means higher energy and greater ability to penetrate matter.

What is the approximate wavelength of dental x-rays?

About 0.1 nm.

List five properties of x-rays.

Invisible, no mass, no charge, travel at the speed of light, travel in straight lines, cannot be focused, cause ionization, cause fluorescence, and cause biologic changes in living cells.

What are the three visible component parts of a dental x-ray machine?

The control panel, the extension arm, and the tubehead.

What controls are found on the control panel?

On-off switch, exposure button, indicator lights, and time, kilovoltage, and milliamperage selectors.

What is the function of the insulating oil in the tubehead?

It absorbs the heat created by x-ray production and prevents overheating.

What is the function of the tubehead seal?

It lets x-rays exit the tubehead, seals in the oil, and acts as a filter to the beam.

What do the aluminum disks do?

They filter out the longer-wavelength, nonpenetrating x-rays from the beam.

What does the lead collimator do?

It restricts the size of the x-ray beam.

What is the position-indicating device (PID)?

An open-ended, lead-lined cylinder that aims and shapes the x-ray beam; sometimes called the cone.

What are the parts of the cathode and what does each do?

The tungsten filament produces electrons when heated, and the molybdenum cup focuses the electrons toward the target.

What are the parts of the anode and what does each do?

The tungsten target converts electrons into x-ray photons, and the copper stem dissipates heat away from the target.

What memory trick helps you remember cathode and anode?

CAT → NAP: cathode is negative, anode is positive, and electrons travel from cathode to anode.

What is the difference between direct current and alternating current?

In direct current electrons flow in one direction; in alternating current they flow in two opposite directions.

What does it mean that the dental x-ray tube is self-rectifying?

It changes AC into DC while producing x-rays, so current always flows from cathode to anode.

What does the milliamperage (mA) setting control?

The amperage, or number of electrons passing through the cathode filament.

What does the kilovoltage (kV) setting control?

The voltage, or the electrical force that accelerates electrons from cathode to anode.

What voltage does the filament (low-voltage) circuit use?

3 to 5 volts.

What voltage does the high-voltage circuit use?

65,000 to 100,000 volts.

What is the function of a step-down transformer?

It decreases the incoming 110 or 220 volts to the 3 to 5 volts used by the filament circuit.

What is the function of a step-up transformer?

It increases the incoming voltage to the 65,000 to 100,000 volts used by the high-voltage circuit.

What does the autotransformer do?

It acts as a voltage compensator that corrects minor fluctuations in the current.

What is thermionic emission?

The release of electrons from the tungsten filament when current passes through it and heats it.

What happens when the exposure button is pressed?

The high-voltage circuit activates, and electrons in the cloud are accelerated from the cathode to the tungsten target on the anode.

What happens to the kinetic energy of electrons when they strike the tungsten target?

It converts to x-ray energy and heat: less than 1% becomes x-rays and about 99% becomes heat.

What is general (braking) radiation?

X-rays produced when electrons are slowed or stopped by interacting with the tungsten nucleus; it makes up about 70% of the x-ray energy at the anode.

Why does general radiation contain x-rays of many energies?

Electrons slow down by different amounts as they pass near tungsten nuclei, and many electrons undergo multiple interactions.

What is characteristic radiation?

X-rays produced when a high-speed electron dislodges an inner-shell electron and the remaining electrons rearrange to fill the vacancy.

Why does characteristic radiation occur only at 70 kV and above?

The binding energy of the tungsten K-shell electron is about 70 keV.

What is primary radiation?

The penetrating x-ray beam produced at the anode target that exits the tubehead (also called the useful beam).

What is secondary radiation?

Radiation created when the primary beam interacts with matter; it is less penetrating than primary radiation.

What is scatter radiation?

A form of secondary radiation produced when an x-ray is deflected from its path by interaction with matter.

What three things can happen to an x-ray photon that reaches a patient?

It can pass through with no interaction, be completely absorbed, or be scattered.

Why are x-ray photons that pass through the patient without interaction important?

They are responsible for producing the densities (the image) that make dental radiography possible.

Describe the photoelectric effect.

An x-ray photon strikes an inner-shell electron, gives up all its energy, is absorbed, and ejects a photoelectron; ionization occurs.

Describe Compton scatter.

An x-ray photon hits a loosely bound outer-shell electron, gives up part of its energy, ejects a Compton (recoil) electron, and scatters in a new direction at lower energy; ionization occurs.

Describe coherent scatter.

A low-energy photon interacts with an outer-shell electron and changes direction with no loss of energy and no ionization.

Which scatter interaction accounts for most scatter in diagnostic radiography?

Compton scatter (62%).

What are the two mechanisms of radiation injury?

Ionization and free radical formation.

What is a free radical?

An uncharged atom or molecule with a single unpaired electron in its outermost shell; it is highly reactive and unstable.

How are free radicals formed in cells?

An x-ray photon ionizes water, producing hydrogen and hydroxyl free radicals.

What are three things a free radical may do to become stable?

Recombine without causing change, combine with other free radicals and cause changes, or combine with ordinary molecules to form toxins such as hydrogen peroxide.

What is the direct theory of radiation injury?

Cell damage occurs when ionizing radiation directly hits critical targets such as DNA; this occurs infrequently.

What is the indirect theory of radiation injury?

X-ray photons are absorbed by water in the cell, forming free radicals that combine into toxins that damage the cell; this occurs frequently.

Why is indirect injury more common than direct injury?

Cells are 70% to 80% water, so free radical formation is highly likely.

What is the shape of the dose-response curve for radiation?

Linear, nonthreshold.

What does linear nonthreshold mean for radiation safety?

Damage is proportional to dose and no threshold exists, so no dose is perfectly safe; the term "safe" x-rays is discouraged.

What are stochastic effects and what are examples?

Effects whose probability increases with dose, with no threshold and severity independent of dose; examples are cancer, leukemia, and genetic mutations.

What are nonstochastic (deterministic) effects and what are examples?

Effects that have a threshold and increase in severity with dose; examples are skin erythema, hair loss, cataracts, and radiation sickness.

What are the three stages in the sequence of radiation injury?

Latent period, period of injury, and recovery period.

What is the latent period?

The time between exposure to ionizing radiation and the appearance of observable clinical signs.

How do dose and dose rate affect the latent period?

The more radiation received and the faster the dose rate, the shorter the latent period.

List the five determining factors for radiation injury.

Total dose, dose rate, amount of tissue irradiated, cell sensitivity, and age.

Why does a high dose rate cause more damage?

Rapid delivery does not give cells time to repair the damage.

What is the difference between short-term and long-term effects?

Short-term effects follow large amounts of radiation absorbed in a short time; long-term effects follow small amounts absorbed repeatedly over a long period.

Are short-term effects applicable to dentistry?

No.

What is the difference between somatic and genetic effects?

Somatic effects appear in the irradiated person and are not passed on; genetic effects occur in reproductive cells and affect future generations.

Which cell part is more radiosensitive, the nucleus or the cytoplasm?

The nucleus.

What three factors determine a cell's radiosensitivity?

Mitotic activity, cell differentiation, and cell metabolism.

Which cell is the most radiosensitive?

The small lymphocyte.

Name examples of radioresistant cells.

Cells of bone (mature), muscle, and nerve.

Name examples of radiosensitive tissues and organs.

Lymphoid tissue, bone marrow, testes, intestines, and reproductive cells.

What is a critical organ?

An organ that, if damaged, diminishes the quality of a person's life.

Name the critical organs exposed in dental imaging.

Thyroid gland, bone marrow, skin, and lens of the eye.

What does the roentgen measure and what is its limitation?

It measures ionization in air produced by x-rays or gamma rays; it does not measure radiation absorbed by the body.

What does the rad measure?

Absorbed dose, the energy deposited in tissue (100 ergs per gram).

What does the rem measure and how is it calculated?

Dose equivalent; rem = rad × quality factor.

What is the quality factor for x-rays?

1

What are the SI equivalents of the roentgen, rad, and rem?

Coulombs per kilogram (C/kg), gray (Gy), and sievert (Sv).

What are the conversions for rad to gray and rem to sievert?

1 Gy = 100 rad, and 1 Sv = 100 rem.

Which unit is preferred in dentistry for dose equivalent?

The sievert.

What is the average annual radiation exposure for a person in the United States?

About 6.2 mSv (3.1 mSv natural plus 3.1 mSv human-made).

What is the single greatest source of natural background radiation in the U.S.?

Radon gas from the soil.

What is the greatest contributor to human-made radiation exposure?

Medical radiation.

What is the estimated risk of a fatal cancer from dental imaging?

About 3 in 1 million.

What dose is most likely to induce leukemia, and does it occur in dental imaging?

5000 mrad (0.05 Gy) or more; such a dose does not occur in dental imaging.

Which receptor choice is the most effective way to reduce patient exposure?

A digital receptor.

How much does F-speed film reduce exposure compared with D-speed film?

By about 60%.