X-ray Interaction With Matter

Question 1

How is the energy of the electrons inside a diagnostic x-ray tube expressed?

Answer 1

• expressed in terms of electrical voltage applied across the tube
• in radiography, this is expressed in thousands of volts of kilovolts (kV)
• voltage across the tube fluctuates; its expressed in kilovolt peak (kVp)

Question 2

What type of energy does a photoelectron have?

Answer 2

Photoelectron has kinetic energy equal to the incident photon minus the binding energy of the electron shell

Question 3

What is the minimum energy required to produce an electron-positron pair?

Answer 3

1.022 MeV

Question 4

What is the effective atomic number of compact bone?

Answer 4

13.8

Question 5

What is the term used for energy absorbed by the patient per unit mass?

Answer 5

Absorbed dose

-the unit of measurement for absorbed dose is Gray (Gy) Milligray (mGy)

Question 6

Define differential absorption

Answer 6

Relationship between transmitted x-rays, photo-electrically absorbed x-rays and Compton scattered x-rays resulting in the image

Question 7

What are the 5 types of interaction between x-radiation and matter? Identify the photon energy range for each types and which are important to radiology?

Answer 7

• COMPTON: 50 kVp-2 Mev
• PHOTOELECTRIC: 1-50 kVp
• COHERENT, CLASSIC, UNMODIFIED: typically less than 10 keV
• PAIR PRODUCTION: begins at 1.022 MeV, important at 10 Mev, predominant at 50 MeV and greater
• PHOTO DISINTEGRATION: greater than 10 MeV
• • Compton and photoelectric are important in radiology

Question 8

List the products of photoelectric interaction and the significance of each

Answer 8

Photoelectron:
-quickly absorbed
-doesn’t travel far, just micrometers
-has kinetic energy which is equal to the difference between the energy of the incident photon and binding energy of the electrons shell
-interacts with atoms until all kinetic energy has been spent
-can cause ionization or excitation
-results in increased patient dose and contributes to biologic damage of tissues
Positive ion:
-photoelectron leaves the atom, resulting in one less electron, therefore, becoming a positive ion
Characteristic radiation:
-given off when the outer shell electron drops to fill empty spot from photoelectron

Question 9

Describe the photoelectric effect

Answer 9

• Photon interaction in which its not scattered but totally absorbed
• photoelectron is the released from the atom

Question 10

When does photoelectric effect increase?

Answer 10

• using low kVp (lower energy of incident x-ray photon)
• high atomic number
• increased thickness of matter
• increased density of matter

Question 11

List 3 rules of photoelectric interaction

Answer 11

• incident photon energy must be greater than the binding energy of the inner shell electron
• PE interaction is more likely to occur when the photon energy and electron BE are close to one another
• PE interaction is more likely to occur with an electron that is tightly bound in its orbit

Question 12

What impact does photoelectric interaction have on radiography?

Answer 12

• differences in absorption properties of matter creates the diagnostic images
• greater differences in absorption, the greater contrast between structures
• absorption increase, so does potential for biologic damage
• PE interaction creates the light areas depending on absorption
• dark areas, less absorption, more radiation hits the IR
• decreased kVp gives increase in PE effect, more absorption
• increase photoelectric effect gives increased contrast and increases dose

Question 13

Outline the circumstances that increase the probability of Compton scatter

Answer 13

• dominates at 50 kVp and higher

- higher the energy of the x-ray photon, more Compton scattering that occurs

Question 14

Describe Compton interactions

Answer 14

-interaction between the incident x-ray photon and outer shell electron
-incident photon has enough energy to eject the electron from its outer shell resulting in ionization of the atom
-incident photon is scattered in a different direction called a Compton scattered “photon”
-ejected electron is called: Compton, scattered, secondary or recoil “electron”
-“compton scattered photon” the initial x-ray photon
-“compton scattered electron” the electron that was kicked out of the atom
-energy of the incident photon is distributed between the scattered photon and ejected electron
-two factors determine the energy obtained by the photon:
Initial energy
🔹angle of deflection: dominated at 50 kVp and higher, higher the energy of the incident photon, more Compton scattering occurs
🔹scattered radiation: diagnostic exam consists mainly of “Compton effect” scatter

Question 15

What impact does Compton scatter have on radiography?

Answer 15

• free radicals can cause biologic damage
• damage is depended on what the electron binds with
• can form “free radicals” can cause harmful combinations
• probabilities of occurrence of Compton scatter compared to PE increase as photon energy increases
• Compton scattered photons can decrease the image contrast by adding undesirable “radiographic fog”
• provides the largest safety hazard for technologist!

Question 16

Outline the two factors that determine the energy of a Compton interaction photon

Answer 16

Deflection: greater the angle of deflection, more energy is given to the recoil electron and less stays with the scattered photon. Higher energy photons are more difficult to deflect. Greater probability of making it to the IR degrading the image
Backscatter: photon deflected back towards the source, in the opposite direction of the incident photon. Small angle scatter in a forward direction is most common

Question 17

Characterize incident photons and recoil electrons with respect to Compton scatter

Answer 17

Incident photon: called Compton scattered photon, continues in a new direction causing more Compton or is absorbed, can leave patient, hitting the IR causing fog, degrading the image or scattering outside causing a dose for staff
Recoil electron: can go on to ionize other atoms with its kinetic energy, can collide numerous times with a number of atoms, energy is exhausted it recombines with an atom needing an electron
*** all of this occurs usually a few micrometers of the original interaction

Question 18

Define “coherent, classical, elastic or unmodified scatter”

Answer 18

• aka “Rayleigh scattering”
• all means the same thing
• occurs between very low energy (10 keV or less)
• photon changes direction by less than 20 degrees with no energy loss
• incident photon emits the same amount of energy absorbed but in a different direction
• low energy incident photon interacts with an atom may transfer some of its energy
• causes some of the electrons in the atom to vibrate momentarily
• electrons vibrate, releases some energy in the form of electromagnetic waves
• waves combine to form a scattered wave representing a “scattered photon”
• wavelength of the scattered wave and incident photon are the same, so no energy has been exchanged

Question 19

What impact does coherent scatter have on radiography?

Answer 19

• coherent scattering occurs in the diagnostic range
• small amount of fog produced is insignificant
• affects visible light more than x-rays that’s why we see different colors in the sky and sun

Question 20

Describe photo-disintegration

Answer 20

• interaction is between extremely high energy photon (10 MeV) and the nucleus
• all the energy is absorbed by the nucleus exciting it
• excited nucleus responds by emitting a nuclear fragment
• part of the nucleus that is ejected may be a neutron, proton or a combination
• extra energy makes the nucleus unstable and sends out a neutron, a proton or a combo of both
• depending on how much incident energy was absorbed by the nucleus
• end result is that the nucleus has been made radioactive, in the stable state, nothing has happened, but the emission of particles classes it as radioactive

Question 21

Where in diagnostic imaging would photo disintegration have the most impact?

Answer 21

In therapeutic, they care about pair production and photo disintegration

Question 22

Identify and describe the particles formed by pair production. When a photon gives up its entire energy what 2 particles are formed?

Answer 22

Negation: normal electron, quickly absorbed by nearby atoms
Positron (antimatter): extremely volatile, finds a negative electron almost instantly, pair combines and disappears, producing two photons that move in opposite directions, each with energy of 0.51 MeV, called an “annihilation reaction” because matter is being turned back into energy

Question 23

Does pair production have any impact on radiography?

Answer 23

• does not occur in the diagnostic range
• not a significant reaction until in the levels of about 10 MeV
• most often seen with PET
• in therapeutic, they care about pair production and photo-disintegration

Question 24

Two types of interactions in the anode can produce x-ray photons

Answer 24

• characteristic (narrow range of energies)

- brem (most x-ray interactions in diagnostic range)

Question 25

Characteristic radiation

Answer 25

• characteristic x-rays are produced after ionization of a k-shell electron
• when an outer-shell electron fills the vacancy in the k shell, an x-ray is emitted

Question 26

Bremsstrahlung radiation

Answer 26

• results from the interaction between a projectile electron and a target nucleus
• electron is slowed and its direction is changed

Question 27

What happens in the patient?

Answer 27

When radiation enters matter it can be:
1. Attenuated- interact with atoms in the patient
2. Transmitted-pass through without interaction
Factors affecting photon interactions include: incident photon energy, density, thickness and atomic # of the matter, tissue or organ in which it interacts

Question 28

Radiation protection

Answer 28

Absorption: patient dose (image contrast)
Scatter: technologist dose (image quality) or occupational dose or to whoever is in the room

Question 29

Compton and photoelectric interactions

Answer 29

In these interactions incident photons either:

• interact and are absorbed by the atom (photoelectric interaction)
• interact and change direction, called “scattering” (Compton interaction)

Question 30

Photoelectric absorption

Answer 30

• responsible for patient dose and image contrast
• results when a photon interacts with a k-shell electron
• total absorption of the incident photon
• occurs if the energy is equal to or slightly greater than the binding energy of the atom

Question 31

Photoelectric effect facts

Answer 31

Increase probability of PE effect:

• decrease of energy of the incident photon
• higher atomic number of irradiated tissue
• good for image quality, increased contrast
• lower kVp=more photoelectric interaction
• higher atomic number= more PE absorption
• low dose and high atomic numbers= increase in patient dose