Radiation Physics and Biology: Dental X-ray Fundamentals

Matter

Anything that occupies space and has mass

Energy

The ability to do work and overcome resistance

Potential Energy

Energy stored due to an object's position or state

Kinetic Energy

Energy of an object in motion

Chemical Energy

Energy stored in the bonds of chemical compounds

Electrical Energy

Energy caused by the movement of electrons

Thermal Energy

Energy related to the temperature of an object

Nuclear Energy

Energy released during nuclear reactions

Electromagnetic Energy

Energy that travels through space in the form of electromagnetic waves

Atom

The smallest particle of an element

Ion

An atom that has gained or lost electrons

Ionization

The process by which a radiant energy removes an orbital electron from an atom to form an ion pair

Radiation

The emission and movement of energy through space in the form of electromagnetic radiation or particulate radiation

Ionizing Radiation

Any radiation with sufficient energy to cause ionization of atoms

Particulate Radiation

Radiation consisting of particles, such as alpha and beta particles

Electromagnetic Radiation

Movement of wavelike energy through space as a combination of electric and magnetic fields

Electromagnetic Spectrum

The arrangement of electromagnetic radiations in order according to their energies

Photon

A bundle of energy that travels through space at the speed of light

Wavelength

Distance between two successive crests of a wave, measured in Angstrom units

Frequency

Number of passing waves a point per unit of time

Velocity

Speed of the wave

Characteristics of X-rays

No mass, no weight; travel at 186,000 miles/second; affect the digital sensor; produce biological changes by ionization

Sources of Radiation

Natural/Background and Artificial/Man-made

Radon

The highest source of background radiation

Effective Dose Equivalent

Annual effective dose equivalent of ionizing radiations measured in mSv

Artificial Radiation Sources

Chest x-ray, Nuclear medicine, Dental x-ray, CT scans, Nuclear Fallout

CT scans

24 mSv

Nuclear medicine

12 mSv

Medical/dental x-rays

12 mSv

Consumer products

2 mSv

Occupational

<.1 mSv

Nuclear fuel cycle

<.1 mSv

Total Artificial Radiation

3.1450 mSv, ~50%

Total Background/Natural And Artificial Annual Dose

Artificial + Background = 6.2 mSv

Radiation Exposure Measurement

Exposure, Absorbed dose, Dose equivalent, Effective dose equivalent

Traditional Units of Radiation Measurement

Roentgen, Rad, Rem

Système Internationale (SI) Units

Coloumbs per kilogram, Gray, Sievert

Radiation Exposure

Measurement of ionization in the air produced by x or gamma rays

Traditional unit for Exposure

Roentgen (R)

SI unit for Exposure

Coulombs/kilogram (C/kg), 3,876 R = 1 C/kg

Absorbed Dose

Amount of energy deposited into any form of matter

Traditional unit for Absorbed Dose

Rad

SI unit for Absorbed Dose

Gray, 100 rad = 1 gray

Dose Equivalent

Compares biological effects of various types of radiation

Traditional unit for Dose Equivalent

Rem

SI unit for Dose Equivalent

Sievert, 100 rem = 1 sievert

Effective Dose Equivalent Measurement

Measured in microsieverts (µSv)

Average Dose Of Background Radiation To The U.S. Population

3 mSv/year, 300 mrem/year, (approx. 1 mrem/year)

X-Rays

Part of the electromagnetic spectrum produced from electricity

X-ray production

Controlled by the components of the unit and control panel (your exposure setting; kV, mA).

Electricity

Electrons in motion.

Alternating current (AC)

One type of electricity that can be used to make x-rays.

Direct current (DC)

Another type of electricity that can be used to make x-rays.

Circuit

Path of electrical current.

Filament circuit

Low voltage circuit (3-5 V) that regulates flow of electrical current to the filament of the x-ray tube.

High voltage circuit

Circuit (50,000 to 100,000 V) that accelerates electrons to generate x-rays.

Step down transformer

Decreases voltage to 3-5 V to heat up electrons.

Step up transformer

Increases voltage to 50-100 kVp to accelerate electrons.

Amperage (mA) setting

Controls the heating of the filament and quantity of electrons.

Voltage (kVp) setting

Controls the accelerating potential and penetrating power of x-rays.

Voltage (V)

Measures electrical pressure between two charges, determines speed of electrons, and average wavelength.

KVP

Stands for kilovolt peak, related to the peak voltage in x-ray production.

Exposure time (impulses)

1 impulse = 1/60 sec; 2 impulses = 1/30 sec; 3 impulses = 1/20 sec; 10 impulses = 1/6 sec; 30 impulses = 1/2 sec.

Requirements for x-ray production

An available source of electrons, accelerating potential (high voltage), target, and vacuum environment.

Cathode side

Negative side of the x-ray tube controlled by mA setting.

Anode side

Positive side of the x-ray tube where electrons are accelerated and x-rays are generated.

Thermionic emission

Process where the tungsten wire is heated so hot that electrons are dissociated.

General/Bremsstrahlung radiation

Produced when an electron passes near the nucleus of a target atom and loses energy as radiation.

Characteristic radiation

Produced when a high-speed electron dislodges an inner shell electron from the tungsten atom, resulting in ionization.

Primary radiation

Created at the target inside the tube housing, resulting from General/Bremsstrahlung and Characteristic radiation.

Braking radiation

Another term for General/Bremsstrahlung radiation, accounting for 70% of dental x-rays.

Collimator

The collimator restricts the size and shape of the primary beam to the desired size. It can be no more than 2.75 inches at the entrance of the patient's skin.

Filter

Aluminum disks stop low energy wavelengths. If the unit operates above 70 kV, the aluminum must be 2.5 mm thick. If below 70 kV, it must be 1.5 mm thick.

Secondary Radiation

Results when primary radiation interacts with tissues of the head and neck, which can result in scatter radiation.

Scatter Radiation

A form of secondary radiation that results when x-rays are deflected by matter (patient's tissues).

X-Ray Interactions With Matter

May pass through patient with no interaction, may be completely absorbed by the tissue and cease to exist, or may scatter in different directions and carry no useful information.

Coherent Scatter

A low energy x-ray passes near an atom's outer electron, is scattered without loss of energy, and the vibrating electron will radiate another x-ray photon in a different direction. NO IONIZATION OF THE ATOM OCCURS WITH THIS INTERACTION!

Photoelectric Effect

An all or nothing energy loss where the incoming x-ray photon imparts all its energy to an orbital electron, causing the electron to fly out of orbit, creating an ion pair. IONIZATION OF THE ATOM OCCURS WITH THIS TYPE OF INTERACTION!

Compton Scatter

Similar to photoelectric effect, where the incoming x-ray knocks out an electron but is only a partial energy transfer, resulting in a new, weaker x-ray that can undergo further scattering or may be absorbed.

How Often Does It Happen?

Photoelectric Effect = 30% of the time, Compton Scatter = 60% of the time, Coherent Scatter = 8% of the time, No interaction = 9% of the time.

How X-Rays Are Produced

Electricity from wall outlet supplies power to generate x-rays. The process involves several steps including heating a tungsten filament, activating a high voltage circuit, and filtering long wavelengths.

Radiation Biology

Branch of biology concerned with the effects of ionizing radiation on living systems. Damage to humans/biological tissues is the result of ionization caused from radiation exposure.

Radiology Biology Questions

How much is too much? Risk versus benefit?

Short term effects

Effects seen over minutes, days, or months after exposure.

Acute radiation syndrome (ARS)

Symptoms include erythema, nausea, vomiting, diarrhea, hemorrhage, and depilation.

Long term effects

Effects seen well beyond original exposure, potentially from low levels of exposure over many years.

Free radical

A highly reactive, unstable molecule existing with a single, unpaired electron in its outermost shell.

Direct Effect/Theory

Theory where x-ray photons collide directly with important cell chemicals and ionize them.

Indirect Effect/Theory

Theory where radiation causes damage by ionizing the water in the cell.

Stochastic Effects

Probability of occurring rather than severity of change; for example, cancer.

Deterministic (non-stochastic) effects

Severity of change depends on the dose received; for example, erythema related to sun exposure.

Genetic effects

Effects of radiation do not manifest in irradiated individuals but are seen in the offspring.

Somatic effects

Effects of radiation are seen in the irradiated individual, affecting all cells in the body except reproductive cells.

Latent Period

Period after initial exposure but before the first detectable effects from radiation occur.

Period of Injury

Period after the latent period where certain effects like the stoppage of mitosis occur.

Recovery Period

Period after exposure where recovery takes place, apparent with short-term effects.

Risk Ratio of Cancer Based on Full Mouth Series

2.5 in 1 million chance of developing a fatal cancer from dental radiation exposure.

Everyday Risks

Risks that increase the chance of death in any one year by 1 in 1 million.

Linear/Nonthreshold

Dose-response curve that does not interact with the baseline, indicating any dose has potential to cause a response.

Threshold

The dose below which there is no response.