Safety - Radiation Physics

What are the steps to produce X-rays?

1. Source of free electrons

2. Focusing of electrons

3. Acceleration of electrons

4. Deceleration of electrons

The cathode end of the X-ray Tube is the?

Negative Terminal

What is the Filament Coil made of?

Thoriated Tungsten

What is the Dual Focus tube?

A cathode that holds a large and a small filament

What is the small filament used for?

• Produces higher spatial resolution
• Small mA and Heat Capacity
• Smaller/thinner body parts
• 1 cm

What is the large focal spot used for?

• High mA and Heat Capacity
• Thicker Body parts
• 2 cm

What is the process called when mA heats the filament, leading to excitation and production of free electrons?

Thermionic Emission

How is the filament heated for thermionic emission?

3-5A (10-1500mA) and voltage of 4-12V ran through to reach 2000 oC

A cloud of free electrons is called?

Space Charge

How does the Focusing cup work?

A matching negative charge is applied to compress and create a narrow beam of electrons toward the anode

What is the Focusing Cup made of?

Molybdenum

What are Grid Controlled Tubes used for?

A modified focused cup that allows for a pulsating beam for fluoroscopy

The Anode end of the X-ray Tube is the?

Positive Terminal

How are the electrons accelerated?

kVp

What is the kilovoltage Peak (kVp)?

The Voltage difference applied across the cathode and anode. Determines the penetrative quality of X-rays

What is the kVp Range of the X-ray tube?

25-150

What is Tube Potential?

Difference in charge between cathode and anode

What is kiloelectron volt (kEv)?

Energy measurement of X-ray photon

What is the relationship between kVp and kEv?

Proportional, 1:1

• The higher the voltage difference, the higher the Energy (Quality) & intensity (quantity)

What is the purpose of the anode?

• Attracts electrons with a positive charge
• Deccelerates Electrons with a target
• Site for X-ray production

What is the Anode made of?

• Target/Track
• Disk

What is the Anode disk made of?

Molybdenum

What is the Anode Target/track made of?

Tungsten-Rhenium Alloy

Why are high atomic number elements used for the anode?

• Allow for the creation of higher-energy X-rays
• Higher heat capacity & Melting point
• Durability

What are the types of Anode Targets?

• Stationary: low mA & heat, short exposure
• Rotating: Higher heat capacity

What are the types of interactions that occur at the Anode Target?

• Bremsstrahlung Radiation
• Characteristic Radiation

Bremsstrahlung radiation is also known as?

Braking, General or white Radiation

What is Bremsstrahlung radiation?

1) Electron (-) Passes near Nucleus (+) of Tungsten atom
2) Positive Charge slows down and changes electron's path
3) Energy lost in the form of X-rays when turning

A polyenergetic beam is caused by?

Bremsstrahlung Interaction

bremsstrahlung radiation of________ kVp comprises _________% of the beam

• <70 kVp - 100% of all x-rays
• 80 kVp - 90% of all x-rays
• 150 kVp - 70% of all x-rays

Why is characteristic radiation unique?

Radiation energy specific to the atom's shell binding energy

What is Characteristic Radiation?

1) Electron interacts & knocks out K-shell electron
2) Vaccancy is created, requiring an outer shell electron to drop down to stabilize
3) Characteristic energy released

Characterisic radiation of________ kVp comprises _________% of the beam

66.5 kVp - <30% of all X-rays

What is frequency?

the number of waves (Attenuation) that pass a given point per second

What is wavelength?

the distance between corresponding points on adjacent waves

What is the wavelength range of Diagnostic X-rays?

0.1 to 0.5 angstroms (Å)

As frequency increases _______

energy increases

As Wavelength increases _______

energy decreases

Intensity of the Beam is affected by?

• mAs
• SID
• kVp
• Filtration

What is mAs?

milliampere-second(s): amount of mA and the time it takes for the tube to produce Electrons

How is mAs related to Intensity?

Directly proportional

How is kVp related to Intensity?

Proportional

How is SID related to Intensity?

Inverse square relationship, because of beam divergence, the intensity decreases with increased distance

How does Filtration affect the beam?

• Increases average quality
• Decreases Intensity, removes "Soft" x-rays

How does kVp affect beam quality?

Higher the kVp --> greater voltage difference --> greater beam energy

What is the Primary Beam?

the x-ray beam between the collimator and patient

What is the remnant beam?

Attenuated beam, between the patient and IR

Remnant Beam is also known as _____

Stray or secondary Radiation

What makes up the remnant beam

Scatter & Leakage radiation

What is scatter radiation?

x-rays that have interacted with an electron of an atom which causes the direction and energy of the x-ray photon to be altered

What is the inverse square law?

The intensity of radiation is inversely proportional to the square of the distance from the source of radiation

What is the equation for inverse square law?

I1 / I2 = D2^2 / D1^2

Short wavelengths, like x-rays are

More penetrative and invisible

Electromagnetic waves are made of disturbances in the electrical and magnetic fields leading to X-rays being

Possessing no Eletrical charge or magnetic polarity

Size of X-rays means interaction involve _____

Interactions with only electrons limit the atomic or molecular vibration, causing no heat

Diverging Path of Photons

X-rays travel straight from their source unless they interact with matter

Velocity of X-rays are

Speed of light (3 x 10^8 m/second) in a vaccum

What is luminescence?

The ability of a crystal to emit light in response to X-rays.

What is ionizing radiation?

radiation that is capable of producing ions by removing or adding an electron to an atom

What is transmission?

When photons pass through the empty space of atoms without any interaction

What is beam attenuation?

Change in beam intensity after interaction iwthin mass per unit area

What are the types of attenuation?

• absorption
• Scatter
• Divergence

Thicker parts lead to __ and ___ Attenuation

More matter interaction and increased

More dense parts lead to __ and ___ Attenuation

More matter interaction and increased

Higher Atomic # parts lead to __ and ___ Attenuation

More Electron Interaction and Increased

Low kVp leads to __ and ___ Attenuation

Less penetration and increased

What is coherent scattering?

1) X-ray photon is temporarily absorbed by an electron
2) Ejected as scatter, no loss of energy

Coherent Scatter is also known as?

Unmodifed, Thomson, Classical, Elastic, or Rayleigh scattering

Coherent Scattering happens at what kVp?

Low kVp, <30 kEv

How does Coherent Scatter affect image quality?

Decreases contrast & adds noise

How does Coherent Scatter affect occupational dose?

No bystander effects as scatter is forward

How does Coherent Scatter affect patient dose?

No absorption occurs so no ionization occurs

What is the Compton effect?

1) Photon partially absorbed by outer shell electron
2) Rest of the photon travels as scatter
3) Outer Shell electron is ejected as a Recoil electron

How does Compton Scatter affect patient dose?

Ionization occurs due to electron and photon

How does Compton Scatter affect occupational dose?

Outward scatter reaches worker

How does Compton Scatter affect image quality?

Decreases contrast & adds noise

Where are most scatter directed at?

Forward direction from the incident photon, requiring a grid & bucky to absorb

What determines the angle of scatter?

The greater the turn, the more energy is lost doing so. Occupational dose can come from the low-energy scatter

As kVp Increases Compton Effects ____ and Photoelecric Absorption ______

Increases; decreases
But overall interactions decrease as x-rays are more penetrative

As density increases, Comptom interactions ______

Increase

What is the Photoelectric Effect?

!) Photon Absorbed by Inner Shell
2) Ionization occurs and photoelectron is ejected
3) Vacant inner shell causes Cascade Effect
4) Characteristic photon ejected

What are the products of the Photoelectric Effect?

• absorbed photoelectron
• Characteristic Photon

How does the Photoelectric effect affect patient dose?

Ionization occurs due to free electrons

How does the Photoelectric effect affect occupational dose?

No effect

How does the Photoelectric effect affect image quality?

Increases contrast

What is Differential Absorption?

The difference between x-ray photons that are absorbed and those that transmit through

Compton Scatter occurs at ______

Mainly at High kVp

photoelectric effect occurs at _____

Low Energy kVp

Photoelectric Effect only occurs when?

Energy of photon level is equal to or slighlty greater than Orbitals binding energy

Coherent Scattering occurs at _____

Low Energy kVp

What is the Cascade effect?

When outer shell electrons drop down the inner shell to fill a vacancy

In Compton Interactions, the incidentphoton interacts with?

Outer Shell Electron

In Coherent Scatter Interactions, the incident photon interacts with?

The Whole Atom

In Photoelectric Absorption, the incident photon interacts with?

Inner Shell Electron

What is pair production?

1) Absorption of a high-energy photon by an atom's nuclear force field
2) Emission of Electron and Positron

What is the annihilation Reaction?

Interaction between a Positron and an electron leads to 2 gamma photons of .511 MeV

What is photodisintegration?

1) High-energy photon penetrate to Nucleus and is absorbed
2) Nuclear Fragment Ejected