Basic Concepts of EM Radiation

Question 1

What unified electricity and magnetism in 1873?

Answer 1

Scottish physicist James Clerk Maxwell developed a unified theory of electromagnetism in 1873.

Question 2

What are the four main electromagnetic interactions?

Answer 2

1. Force between electric charges - force of attraction and repulsion inversely proportional to the square of the distance between them
2. Magnetic pole interaction
3. Magnetic field produced by electric current in a wire (whose direction depends on the direction of the current)
4. Relationship between moving electric fields and magnetic fields. (Produce each other)

Question 3

How is electromagnetic radiation created?”

Answer 3

EM radiation is created when an atomic particle, such as an electron, is accelerated by an electric field, causing it to move, thus producing oscillating electric and magnetic fields in a photon.

Question 4

What are the characteristics of electromagnetic waves?

Answer 4

Frequency, wavelength, and energy are characteristics used to describe electromagnetic waves.

Question 5

How fast do photons travel ?

Answer 5

• the fastest speed possible in the universe : 186,282 miles per second (299,792,468 meters per second) in a vacuum (the speed of light)

Question 6

What is a wavelength?

Answer 6

The distance between two consecutive peaks of a wave, usually measured in meters (m) or fractions thereof.

Question 7

What is frequency?

Answer 7

Frequency is the number of wave cycles per second, measured in hertz (Hz).

Question 8

How does wavelength relate to frequency?

Answer 8

A shorter wavelength corresponds to a higher frequency, as one cycle can pass in a shorter time; a longer wavelength has a lower frequency because each cycle takes longer to complete.

Question 9

What is the classification of radiation?

Answer 9

Radiation is classified as non-ionizing (longer wavelength/lower frequency) and ionizing (short wavelength/high frequency with higher energy).

Question 10

What effects can ionizing radiation have on matter?

Answer 10

Ionizing radiation can produce ions in matter at the molecular level, potentially causing significant damage to DNA and proteins in humans.

Question 11

What is kerma?

Answer 11

Kerma stands for “kinetic energy released per unit mass” or “kinetic energy released in matter”

and

represents the sum of initial kinetic energies of charged particles liberated by uncharged ionizing radiation in a sample of matter, divided by the sample’s mass.

• uncharged same as indirect ionizing radiation - electrically neutral and do not interact with atomic electrons through coulomb forces eg. Photons and neutrons)

Question 12

What is the SI unit of kerma?

Answer 12

The SI unit of kerma is the gray (Gy), equivalent to joule per kilogram (J/kg).

Question 13

What distinguishes kerma from absorbed dose?

Answer 13

Kerma can differ from absorbed dose due to the escape of some energy from the absorbing volume in the form of X-rays or fast-moving electrons, which is not counted as absorbed dose, especially at higher energies.

Question 14

What does exposure refer to in the context of x-rays?

Answer 14

Exposure refers to the concentration of x-rays in air at a specific point and represents the ionization produced in a specific volume of air

described by the formula E=Q/m, where E is exposure, Q is the quantity of charge on the ions, and m is the unit mass of air.

Question 15

What decreases with the square of the distance from an x-ray source?

Answer 15

Exposure decreases with the square of the distance from an x-ray source, following the inverse square law.

SI unit - Coulomb per kilogram (roentgens old). 1 roentgen = 2.58 x 10-4 C kg-1

Question 16

What is exposure ?

Answer 16

The ability of x-ray photons to ionize air (cannot be used for protons, neutrons or electrons)

Question 17

What is the relationship between absorbed dose and biological effect?

Answer 17

Absorbed dose alone is not a good indicator of the likely biological effect; different types of ionizing radiation can have varying biological effects even at the same absorbed dose.

Question 18

What is absorbed dose ?

Answer 18

A measure of the energy deposited in a medium by ionizing radiation

Equals the energy deposited per unit mass of a medium - Joules per kilogram - Gray adopted name. 1 Gray = 1 J/kg

Question 19

What can be applied to reflect the different relative biological effect of radiation to find the equivalent dose ?

Answer 19

Tissue weighting factors

Question 20

What is equivalent dose?

Answer 20

Equivalent dose measures the radiation dose to tissue, accounting for the different relative biological effects of different types of ionizing radiation.

It’s measured in sieverts (Sv) or rem (old)
1 Sv = 100 rem

Question 21

What is the purpose of effective dose?

Answer 21

The effective dose is used to compare the stochastic risk of non-uniform exposure to radiation and calculates the risk of developing fatal cancer in a specific tissue.

Effective dose = Equivalent dose HT of individual organ x Tissue weighting factor WT (products of each organ added)

Question 22

What is the tissue weighting factor (WT)?

Answer 22

The tissue weighting factor is a relative measure of the risk of stochastic effects resulting from irradiation of specific tissues, accounting for their variable radiosensitivity to ionizing radiation.

Question 23

What are the new tissue weighting factors according to the ICRP in 2007 (International Commission on Radiological Protection)?”

Answer 23

High risk (WT = 0.12): stomach, colon, lung, red bone marrow, (breast - mod risk by EU)

Gonads (WT = 0.08) * 0.2 by EU - risk of genetic effects included

Moderate risk (WT = 0.04 * EU 0.05): urinary bladder, esophagus, liver, thyroid

Low risk (WT = 0.01): (bone surface, skin, brain by EU) , salivary glands.

EU low WT insufficient data 0.05 : adrenals, brain,kidney,muscle, small bowel, pancreas, spleen, thymus, uterus

Sum of weighting factors = 1

Question 24

What is the half-value layer (HVL)?

Answer 24

The half-value layer is the width of a material needed to reduce the air kerma of an x-ray or gamma ray to half its original value and quantifies the polyenergetic beams’ energy or hardness.

Measured in millimeters of aluminum

• narrow beam geometry - broad beam will have more scatter which underestimates attenuation
• linear and mass attenuation coefficients for mono-energetic beams

Question 25

What does a lower HVL indicate?

Answer 25

A lower HVL indicates low photon energy in the x-ray or gamma ray beam.

Question 26

What is the relationship between HVL and photon energy?

Answer 26

After filtration by one HVL, subsequent HVLs will be higher as the filtered photons have higher energy, requiring thicker material to attenuate half of the penetrating beams.

Question 27

What models exist to predict the effects of radiation exposure?

Answer 27

linear-no threshold - cancer induction increases linearly with no threshold dose - ICRP accepted

linear-quadratic - cancer induction increases in a quadratic-linear function

adaptive-dose response - hormesis - adaptive response to stress - low doses are protective and high doses are detrimental

bystander effect - epigenetic - modification of gene expression-

related to damage that cells not directly irradiated suffer due to molecular signals (ROS) nitrogen species and cytokines) that arise from nearby irradiated cells in the same or separate tissue

attempt to predict the stochastic effects of radiation exposure.

Question 28

What is the nature of stochastic effects of radiation exposure?

Answer 28

Stochastic effects of radiation exposure increase in probability with dose but do not change in severity; the individual will either develop cancer or not.

Question 29

What are non-stochastic effects of radiation exposure?”

Answer 29

Non-stochastic effects require a threshold dose before biological effects occur, such as radiation-induced skin injuries in radiotherapy or IR fluoro procedures.

Acute > 2 Gy - erythema
> 7 - permanent epilation
> 12 - delayed skin necrosis

• damage from absorbed dose of area of skin receiving most exposure in exams

Question 30

What are methods to control skin dose during radiological procedures?

Answer 30

Maintain low current and higher kV

ensuring close detector proximity

tight collimation

minimizing screening time

adapting grid use for smaller patients or air gap techniques.

Question 31

What are ways of detecting radiation?

Answer 31

Radiation can be detected using photographic film in devices like film badges, which turn from transparent to dark when exposed to ionizing radiation.

Question 32

What is the purpose of film badges in radiation monitoring?

Answer 32

Film badges worn by personnel exposed to radiation regularly are checked to determine the type and amount of radiation exposure they’ve encountered.

Question 33

What is the significance of x-ray beam collimation (limiting the primary X-ray beam ?

Answer 33

X-ray beam collimation is vital for patient dose reduction and image quality, limiting the volume of irradiation and minimizing radiation risk while improving subject contrast and image quality (as less volume irradiated means less scatter incident on the detector

Question 34

Why is x-ray beam filtration necessary?

Answer 34

To filter out low energy photons from a polyenergetic spectrum, as they contribute nothing to the production of the radiograph. Aluminum absorbers are used to filter out these photons after collimation. This reduces patient exposure.

Question 35

What is the process by which radiation is detected by the photographic film in film badges ?

Answer 35

• The film inside the badge is coated in a paper
• Through 6 windows on each side of the badge, beta and gamma rays are allowed through
• Windows 2 and 3 being plastic of different thickness allow detection of beta radiation
• Windows 4, 5 and 6 being metals including lead and tin allow detection of gamma rays and other ionising radiation

Question 36

What is electromagnetic radiation?

Answer 36

A stream of quanta (photons, particles) or waves that arises from oscillating electric and magnetic fields.

Question 37

Define Amplitude (A)

Answer 37

Amplitude is the peak field strength.

Question 38

Define Time (T)

Answer 38

Time is the time between successive peaks.

Question 39

Define Velocity (c)

Answer 39

Velocity is the speed calculated as the distance travelled by a peak in one second.

Units = m (metres)

Question 40

What is the equation for velocity in terms of frequency and wavelength?

Answer 40

Velocity = frequency x wavelength

v = f x λ

Question 41

What are photons in the context of EM radiation?

Answer 41

Photons are small packets of energy that travel in straight lines.

Question 42

What is the unit of electron-volt (eV)?

Answer 42

1 ev = 1.6 x 10^-19J

Question 43

What is Planck’s constant?

Answer 43

Planck’s constant (h) is 6.63 x 10^-34 m^2kg/s

Question 44

What is the equation for photon energy?

Answer 44

E = hf or E = hc/λ

• energy - power derived from the utilization of physical or chemical resources, especially to provide light and heat or to work machines.
  “nuclear energy”

E = photon energy, h = Planck’s constant, f = frequency
c = speed of light

Question 45

What is the relationship between frequency and energy of the wave?

Answer 45

As the frequency increases, the energy of the wave also increases (directly proportional).

Question 46

What is the relationship between wavelength and energy of the wave?

Answer 46

As the wavelength increases, the energy of the wave decreases (inversely proportional).

Question 47

What is Photon fluence?

Answer 47

Photon fluence is the number of photons per unit area at a given time and given cross-section of beam.

Question 48

What is Energy fluence rate?

Answer 48

Energy fluence rate (aka beam intensity) is the total energy per unit area passing through a cross section per unit time (watts/mm2).

Question 49

What is the Inverse square law?

Answer 49

As the beam moves further from the source, the area of the beam increases. The intensity is inversely proportional to the area.

Question 50

What is the Electromagnetic Spectrum?

Answer 50

The range of wavelengths and frequencies over which electromagnetic radiation extends.

Question 51

Describe the process of ionization and excitation with a thermoluminescent (TLD) dosimeter

Answer 51

As ionizing radiation passes through the thermoluminescent dosimeter electrons in the material are moved into dosimetric traps and held there until the detector is heated up.

The photographic film will turn from being transparent to dark once exposed to ionizing radiation

A photographic film is placed inside the film badge holder. The film is already in a paper covering.

On each side of the badge are various ‘windows’ - numbered 1-6 in the picture above.

The open window allows all types of radiation through, therefore showing exposure to Beta and Gamma radiations.

Windows 2 and 3 are plastic of different thicknesses allowing the person examining the film to tell what kinds of Beta radiation they have been exposed to. Windows 4,5 and 6 are of different metals (including lead and tin) allowing the detection of gamma rays and other ionising radiations.

The final exposure of the film will show how much and what kind of radiation the worker has been exposed to.

Film badges worn by medical personnel dealing with radiation are checked regularly.

Question 52

What is mass attenuation coefficient

Answer 52

The MAC is a measure of the rate of energy loss by a photon beam as it travels through an area of material. By dividing LAC by the den- sity of the material the effect of density is re- moved. The MAC is, therefore, independent of density and depends only on the atomic num-ber of the material and the photon energy.

MAC = μ / ρ
Key: μ = LAC, units: cm-1 MAC units : cm2g-1
ρ = density

depends only on atomic number of material and photon energy

Question 53

What is linear attenuation coefficient

Answer 53

The probability of a material to attenuate an x-ray beam. It can also be expressed as the amount of energy transferred to the ma- terial per unit of track length of the particle.

The LAC (μ) is calculated by:
μ = 0.693 / HVL Key: μ = LAC, units: cm-1

Question 54

What factors affect attenuation of photons

Answer 54

the energy of the photons
the thickness of the bodily tissue
atomic number
density of the tissue

Question 55

Describe the process of attenuation

Answer 55

Attenuation is the reduction of the intensity of an x-ray beam as it traverses matter. By photoelectric effect- causes an atomic detail - varying shades of grey - reflections of linear attenuation coefficient of various tissues within patient

The reduction may be caused by absorption or by deflection (scatter) of photons from the beam

and can be affected by different factors such as beam energy and atomic number of the absorber.

Question 56

State the attenuation of a polychromatic X-ray beam as related to HVL

Answer 56

Half-value layer (HVL) is the thickness of a material required to reduce the air kerma of an x-ray or gamma ray to half its original value. It applies to narrow beam geometry only, as with broad-beam geometry, a greater amount of scatter will reach the detector, overestimating the degree of attenuation. It is used to quantify polyenergetic beams instead of linear and mass attenuation coefficients used for monoenergetic beams.

It is a surrogate measure of the penetrability of an x-ray spectrum, also referred to as beam quality. A lower HVL indicates low photon energy. HVL is conventionally measured in millimetres of aluminium. After filtration by one HVL, the subsequent HVL will be higher as the filtered photons have higher energy (thicker material is required to attenuate half of the penetrating beams). This is because the lower energy photons are preferentially absorbed via the photoelectric effect. The result of this is that the 1st HVL < 2nd HVL.

Question 57

State the attenuation of a monochromatic X-ray beam as related to linear and mass attenuation coefficient

Answer 57

In the case of a monochromatic radiation (i.e., all transmitted photons having the same energy), the linear attenuation coefficient is the result of four physical processes: the photoelectric effect, the coherent Raleigh scattering, the incoherent Compton scattering, and pair generation.

• constant describing the fraction of photons removed from a monochromatic x-ray beam by a homogeneous absorber per unit mass.