RADT 2721 Review Chapters

Sources of Natural Radiation

• These sources can be divided into 4 components

  • Cosmic Rays

  • Terrestrial Radiation

  • Internally Deposited Radionuclides

  • Radon

Manmade (Artificial) Radiation - Ionizing Radiation Created by Humans

• Sources of artificial ionizing radiation include the following:

  • Consumer products containing radioactive material

  • Air travel

  • Nuclear fuel for generation of power

  • Atmospheric fallout from nuclear weapons testing

  • Nuclear power plant accidents

  • Nuclear power plant accident as a consequence of natural disasters

  • Medical Radiation

Cosmic Radiation

• Result from nuclear interactions that have taken place in the sun and other stars

• The greatest intensity occurs at high altitudes where there is less attenuation due to the low atmospheric density, whereas the lowest intensity occurs at sea level

• Consist predominantly of high energy protons that may be accompanied by alpha particles, atomic nuclei, mesons, gamma rays, and high energy electrons

Terrestrial Radiation

• Radon gas can percolate up through soil and enter a home through holes or crack in its framework, crawl spaces under the living areas, floor drains, sump pumps, and porous cement block foundations

• Present in crust of Earth

• Ex: Uranium-238, radium-226, and thorium-232

• Radon the first decay product of radium is the largest contributor to background radiation. Its colorless, odorless, invisible, heavy radioactive gas that is always present in some degree in the air. Half-life of 3.825 days

• The Environmental Protection Agency (EPA) considers radon to be the second leading cause of lung cancer in the US. Radon is responsible for approximately 20,000 cancer deaths per year

Internal Radiation

• The tissues of the human body contain many naturally existing radionuclides that have been ingested in minute quantities from various food or inhaled as particles in the air. The types of ionizing radiation released by these radionuclides may include the following

  • Alpha particles (helium nuclei)

  • Beta Particles (electrons)

  • Gamma rays (similar to x-rays but usually of higher energy, in the range of a million electron volts MeV)

  • Some types of radioactive decay also affect the distribution of electrons around the atom and result in the emission of x-rays

• Examples of radioactive nuclides that exist in small quantities in the human body are as follows:

  • Potassium-40 (40K)

  • Carbon-14 (14C)

  • Hydrogen-3 (3H; tritium)

  • Strontium-90 (90Sr)

Internal Exposure Routes

• Ingestion

  • From the mouth (swallowing) Absorption through the digestive tract

• Inhalation

  • Incorporation from the respiratory airways

  • Absorption from the lungs and the surface of the airways

• Percutaneous absorption

  • Absorption from the skin

• Wound contamination

  • Contamination from a wound

• Intake of radiopharmaceuticals

  • Injection, oral administration, inhalation of gas

• (Radioactive materials within the body decay as they emit radiation within the body)

• (They may accumulate in some specific organs)

Consumer products containing radioactive material

• Granite

• Clocks/watches

• Ceramics

• Lightbulbs

• Cat litter

• Bananas

Air Travel

• Flights bring humans to higher elevations, in closer contact with high energy extraterrestrial radiation (Cosmic radiation) and increase their exposure

• A flight on a typical commercial airliner results in an EqD rate of 0.005 to 0.01 mSv/hour

• An increase in radiation exposure carries an immeasurable small health risk for those individuals who travel by air infrequently. However, for pilots, flight attendants, frequent flyers, the possibility exists that they may unknowingly be exposed to excessively large doses of radiation

• A commercial flight crew’s actual radiation exposure sometimes exceeds that of workers at nuclear power plants

Nuclear Fuel for the Generation of Power

Nuclear power plants that produce nuclear fuel for the generation of power do not contribute significantly to the annual EqD of the US population during their normal operating cycles

Atmospheric fallout from nuclear weapons testing

• The actual radiation dose to the global population from atmospheric fallout from nuclear weapons test is not received all at once. It is instead delivered over a period of years at changing dose rates

• No atmospheric nuclear testing has occurred since 1980

Nuclear Power Plant Accidents

• This can lead to substantial unplanned radiation exposure for humans and the environment

• 2 Examples

  • Three mile island: Radioactive reactor had a loss of coolant, resulted in severe overheating, greater than 5,000 degrees, because of that radiation escaped, affecting 2 million people

  • Chernobyl: Explosion at a plant in Russia, 200 people received whole body exposure over 1 seivert

Nuclear Power Plant Accidents as a consequence of natural disasters

• Fukushima Daiichi Nuclear Plant Crisis

  • 9.0 mag earthquake that happened in Japan, triggering a tsunami. The tsunami knocked out a nuclear power plant. Six reactors went down. Explosion happened

Medical Radiation

• The number of medical procedures involving the use of ionizing radiation had increase dramatically since the 1980s

• Diagnostic x-ray radiation (Which includes CT scanning, interventional fluoro, and conventional radiography or fluoro) and nuclear medicine procedures are the two largest sources of artificual radiation, and they collectively accounted for 48% of the total collective EfD of the US population as of 2006

• The amount of radiation actually received by a patient from a diagnostic x-ray procedure may be indicated in terms such as the following:

  • Entrance skin exposures (ESE), which includes skin and glandular dose

  • Bone marrow dose

  • Gonadal dose

Target Interactions

• Atomic Structure

  • Atoms

  • Atomic number: number of protons in nucleus

  • Atomic Mass: Protons and neutrons in nucleus

  • Electrons

  • Valence number: how many is in outer shells

  • Ionization-see slide

  • Excitation

  • Isotope

Ionizing Radiation

• X-rays

• Gamma Rays

• Ultraviolet radiation with energy great than 10eV

Ionization

• Happens if the electromagnetic radiation is of a high enough frequency

• It can transfer sufficient energy to some orbital electrons to remove them from the atoms to which they were attached

• Is the foundation of the interactions of x-rays with human tissue

Nonionizing Radiation

• Visible light

• Infrared rays

• Ultraviolet radiation with energy less than 10 eV

• Microwaves

• Radio waves

Target Interactions: Bremsstrahlung Radiation

• The energy of the electrons inside the x-ray tube is generally specified in terms of the eletrical voltage applied across the tube

• This voltage is expresses in thousands of volts, or kilovolts (kV) Because the voltage across the tube fluctuates, it is usually characterized by the kilovolt peak value (kVp)

• 100kVp = 100,000eV = 100keV

• X-Ray photons can have no more than 100 keV

• The units kVp or kV refer to the voltage on the x-ray tube and keV refers to the energy of specific x-rays

Bremsstrahlung X-Ray

• The projectile electron comes under the influence of the nuclear field

• It slows and turns; german for braking

• Energy released as x-ray

• Happens outside the body

Characteristic Radiation

• The projectile electron comes from the filament of the cathode

• The projectile electron knocks out an inner-shell electron

• Outer-shell electrons fill the inner shell void

• And x-ray photon equal to the difference in the binding energies is produced

Inverse Square Law

• X-ray intensity varies inversely with the square of the distance from the x-ray tube target. The relationship is known as the Inverse Square Law

• Formula: I₁/I₂ = (d₂/d₁)²

• Where I1 and I2 are the x-ray intensities

• At distances d1 and d2 respectively

Radiation Quantity

• Refers to radiation intensity or amount of x-ray measured in mR or mGya influenced by mAs

Radiation Quality

• Refers to x-ray beam energy and penetrability, influenced by kVp

• Increase kVp = Increase quality = Increase penetrability

Filtration

• Removes low energy, non-diagnostic x-ray photons from primary beam

• Increase filtration = increase quality

Permanent Inherent Filtration

The combination of the x-ray tube glass wall and the added aluminum placed within the collimator of the x-ray unit

Primary Radiation

Filtered x-ray photon beam

Attenuation

The reduction in the number of primary photons in the x-ray beam through absorption (a total loss of radiation energy) and scatter (a change in the direction of travel that may also involve a partial loss of radiation energy) as the beam passes through the patient in its path

Direct Transmission

When some primary x-ray photons go over the patient without interacting and reach the image receptor

Indirect Transmission

When some scatter x-ray low energy photons go over the patient without interacting and reach the image receptor

Exit or image-formation photons

X-rays that strike the detector

Small angle scatter

An image-formation photon that has a bended path which blurs the lines of the radiographic image

Radiographic Fog

The undesirable exposure caused by small angle scatter. Can be improved via collimating

Frequency

The rate of rise and fall of the electromagnetic photon and is measured in Hertz (Hz)

Wavelength

• The distance between two successive peaks of an electromagnetic photon

• Frequency and wavelength are inversely related; x-ray has very high frequencies and very short wavelengths

Half-Value Layer

Measures filtration and is the thickness of an absorbing material required to reduce x-ray intensity by one-half its original value

Secondary Radiation

Radiation that is emitted from atoms of matter after an x-ray photon from the primary beam interacts with matter

Remnant or Exit Radiation

The portion of the attenuated x-ray beam that emerges from the patient and interacts with the image receptor. This is also known as image forming

Coherent Scattering

• Not important in any energy range

• AKA: Classical scattering, elastic scattering, unmodified scattering, thompson

• Process that results in no loss of energy as x-rays scatter

• Momentary vibration of electrons which creates electromagnetic waves

Photoelectric absorption

• Important in Diagnostic radiology

• An interaction between an x-ray photon and an inner-shell electron (Usually K or L shell)

• Most important mode of interaction between x-ray photons and the atoms of the patient’s body for producing useful images

• To dislodge an inner shell electron from its atomic orbit, the incoming x-ray photon must be able to transfer a quantity of energy as large as or larger than the amount of energy that holds the electron in its orbit

• The now unbound orbital electron, called a photoelectron

• This photoelectron may interact with other atoms in the vicinity, thereby causing excitation (promotion of electrons from lower energy shells to higher energy shells) or ionization (complete ejection of the electron from an atom) until all of its kinetic energy has been spent

Photoelectric Absorption 2

• The “released” energy produced by the movement of the electron shells in the form of a photon is called a characteristic photon, or characteristic x-ray, because its energy is directly related to the shell structure of the atom from which it was emitted

• Auger (awzhay) effect

  • When an inner electron is removed from an atom in a photoelectric interaction, thus causing an inner shell vacancy, the energy liberated when this vacancy is filled can be transferred to another electron of the atom, thereby ejecting that electron, instead of emerging from the atom as characteristic radiation. Such an emitted electron is called an Auger electron

Factors that influence Photoelectric absorption

• As kVp increases the chances of a photoelectric absorption decreases

• As teh atomic number of the atom increases, chance of photoelectric absorption increases

• As body part thickness increases, chance of photoelectric absorption increases

Compton scattering

• Important in Diagnostic radiology and therapeutic radiology

• aka Incoherent scattering, inelastic scattering, modified scattering

• It is responsible for most of the scattered radiation produced during radiologic procedures

• An incoming x-ray photon interacts with a loosely bound outer electron of an atom and dislodges the electron from the outer shell. This photon is now called a Compton scattered photon

• This freed electron is called a Compton scattered electron or secondary or recoil electron. It moves on colliding with other atoms until it recombines with an atom that needs another electron

Pair production

• Important in Therapeutic radiology

• Is a process that does not occur unless the energy of the incident x-ray photon is at least 1.022 million electron volts (MeV)

• The x-ray photon strongly interacts with the electric field surrounding the nucleus of an atom

• The energy of the photon is absorbed and transformed into matter composed of two particles

  • Negatron (an electron)

  • Positron (A positively charged electron)

  • Both have the same mass and magnitude of charge; difference is the “sign” of their electrical charge

Pair Production 2

• Positron will react with closest electron

• The minimum energy to produce an electron-positron pair is 1.022 MeV

• Positrons are classified as antimatter because of its destructive interactions. Usually when interaction occurs both the positron and electron are annihilated producing energy

• Annihilation is the opposite of pair production

• The energy is then carried off by two 0.511 MeV photons that move in opposite directions

Use of Annihilation Radiation in Positron Emission Tomography (PET)

• Radionuclides (which have unstable nuclei due to too many protons compared to neutrons) are injected into patients

• To relieve this instability, a surplus proton is converted in the nucleus into a neutron, and a positron and another particle called a neutrino are ejected from the nucleus

• The emitted positron interacts with a local electron, and the two mutually annihilate, yielding a pair of photons emerging in opposite directions from the electron-positron interaction site

• These annihilation photons are intercepted by a ring of detectors surrounding the patient. The positional information from these detectors is then used to build a cross sectional image of the radioactivity within the patient

Photodisintegration

• Important in therapeutic radiology

• Becomes important at energies exceeding 10 MeV during radiation therapy treatments

• A high-energy photon collides with the nucleus of an atom, which directly absorbs all the photon’s energy

• Creates an instability that in most cases is alleviated by the emission of a neutron or nuclear fragment by the nucleus. Other types of emissions - a proton or proton-neutron combination (deuteron) or even an alpha particle are possible if sufficient energy is absorbed by the nucleus