X-ray interaction 1

Question 1

X-ray Photon Interactions cases

Answer 1

Case 1: X-ray photon passes straight through the matter without interacting
- Pass through object but might absorb it.
- Other reaction it can be stopped x-ray detector.
- It can be scattered diverged from its original path and not be able to detect it.
Some materials absorb more x-ray material than others, such as bone will absorb more than soft tissues.

Case 2: X-ray photon is completely absorbed by the matter

Case 3: X-ray photon is scattered from its original path because of interacting with the matter

Question 2

What are X-ray Photon Interactions

Answer 2

Any one of these aforementioned processes is not guaranteed for individual X-ray photons – there is only a probability that one of the processes may occur.
X-ray photon interactions are random statistical processes
On average however when considering many X-ray photon interactions some interactions have a higher probability which is dependent on a number of factors

For example
it is more likely that a thicker object will absorb more X-ray photons than a thinner object made of the same material
it is more likely that a dense object will absorb more X-ray photons than a less dense object

Question 3

What are the 5 processes whereby an X-ray photon can interact with matter?

Answer 3

Photoelectric absorption
Compton scattering
Coherent (Rayleigh) scattering
Pair production
Photodisintegration

Question 4

Photoelectric Absorption

Answer 4

Absorption definition
Dependence on
Atomic Number
Density
X-ray photon energy
Image quality
Radiation dose

Question 5

What is Absorption?

Answer 5

The transfer of energy from the X-ray beam to the medium

Question 6

Photoelectric Absorption

Answer 6

An X-ray photon, with sufficient energy, interacts with a bound electron from the inner shell of an atom in an object.
The X-ray photon must have an energy greater than the binding energy of the inner shell electron to take part in photoelectric absorption

Question 7

Photoelectric absorption: Where does all of the energy of the photon transfer to?

Answer 7

All of the energy of the photon is transferred to the electron
The electron then has enough energy to be freed as a photoelectron and leaves a ‘hole’ or ‘vacancy’ in the shell.

Question 8

Photoelectric Absorption: What energy does it have?

Answer 8

The photoelectron will have a kinetic energy equal to the difference of the incident X-ray photon energy and the binding energy of an inner shell electron.
The photoelectron will only travel a short distance and deposit its energy into the surrounding mater

Question 9

Photoelectric Absorption: Describe the vacancy in the inner shell

Answer 9

The vacancy in the inner shell is filled by electrons from higher shells of the atom.
This produces a photon (characteristic radiation) of usually low energy – so it is not an X-ray photon

The emitted photon is usually absorbed close to the site where it was produced—this deposition of energy contributes to patient dose

The atom is left with a net positive charge as a result of losing an electron

The vacancy in the higher shell is filled by electrons in higher shells
With each transition characteristic radiation is produced further contributing to patient dose
All of the energy of the incident X-ray photon is deposited in the patient and thus contributes to dose

Question 10

The probability of photoelectric effect depends on: (give equation)

Answer 10

E - energy of the x-ray photon
Z - atomic number of the material
p - mass density of the material

𝑃𝐸𝑝𝑟𝑜𝑏 ∝ 𝜌𝑍^3/𝐸^3

Question 11

Photoelectric Absorption: Energy of the X-ray photon

Answer 11

Probability of photoelectric interactions is highest when the X-ray photons energy is slightly above the electron binding energy

If the X-ray photon energy is too low in cannot free the bound electron

If the energy is too high the probability significantly decreases due to the inverse relationship with the cube of the energy (see previous slide)

Question 12

Photoelectric Absorption & Image Quality

Answer 12

Photoelectric effect produces excellent images because it enhances natural tissue contrast and does not produce scatter

X-ray image contrast depends on some tissues absorbing more X-ray photons than other tissues

Contrast is greatest when the difference in absorption between adjacent tissues is greatest

Because the number of photoelectric reactions depends on the third power of the atomic number, the photoelectric effect magnifies the difference in tissues composed of different elements such as bone and soft tissue

Question 13

Photoelectric Absorption & Patient Dose

Answer 13

The photoelectric effect results in patients receiving more radiation dose than from any other type of interaction

All the energy of an incident photon is absorbed by the patient

Effect of photoelectric interaction can be reduced using high-energy (kVp) techniques

Question 14

Photoelectric Absorption: Key Points

Answer 14

Dominates at low photon energy
Produces excellent radiographic contrast
Responsible for majority of patient dose

Question 15

Photoelectric absorption & contrast media

Answer 15

K-edge absorption
Iodine & barium contrast media

As the photon energy increases, there are values where there is a sudden jump in attenuation (absorption increases)

For example, at energies just below the k-edge the X-ray photons don’t have enough energy to free the k-shell electrons

Question 16

K-edge Absorption & Contrast Media: What happens as the energy increases and the probability of the x-ray photon?

Answer 16

As the energy increases to just over the required energy, a much larger number of bound electrons in the object become available for photoelectric absorption

Therefore, the probability of the X-ray photon being attenuated by a photoelectric interaction significantly increases—overall absorption increase significantly

Question 17

K-edge Absorption & Contrast Media: What is it useful for some materials in radiography?

Answer 17

This is particularly useful for some materials used in radiography such as iodine and barium

These materials have k-edge absorption energies within the diagnostic energy range and therefore standout really well on images—they produce excellent contrast.

Iodine contrast agent is often injected into blood vessels to make their structures visible on X-ray images—they would be otherwise invisible.

Barium contrast agent is often administered orally to make gastrointestinal structures visible of X-ray images—they would otherwise be invisible

K-edge: Iodine: 33.2 keV, Barium: 37.4 keV

Question 18

X-ray Interactions with Matter
: Compton Scattering

Answer 18

Scattering
Dependence on
Density
X-ray beam energy
Consequence of Compton scatter on
Patient & staff dose
Image quality
Proportion of Compton scattering compared with photoelectric absorption

Question 19

What is scattering?

Answer 19

A change in the direction of an X-ray photon due to interaction

Question 20

What is Compton Scattering?

Answer 20

An X-ray photon interacts with a loosely bound outer shell electron of the object atom
The bound electron absorbs some of the incident X-ray photon’s energy and is deflected

The X-ray photon having lose some of it’s energy is deflected (scattered)

Compton scattering is also called incoherent scattering as the change of photon energy is not orderly and consistent

The change in energy of the X-ray photon depends on the angle through which it is scattering and not on the scattering medium

Question 21

Compton Scattering: energy transfer

Answer 21

The larger the energy transfer from the incident X-ray photon to the loosely bound outer shell electron the:
lower the energy of the scattered X-ray photon

larger the angle of the deflected X-ray photon

higher the kinetic energy of the deflected electron

Question 22

Compton Scattering: Charge

Answer 22

Compton scattering occurs more often with
Outer shell loosely bound electrons
Compton scattering results in the atom in the medium having a positive charge
This charge is neutralized by absorbing a free electron from the medium

Question 23

Compton Scattering: probability of Compton scattering depends on:

Answer 23

Total number of electrons in absorber
Density
Number electrons per gram
All elements contain approximately the same number of electrons per gram, regardless of atomic number

Therefore, probability of Compton scattering is almost independent of the atomic number Z but does depend on:
the density of the absorber—the higher the density the greater the Compton scattering
the energy of the incident X-ray photon—inversely proportional to photon energy

Probability of a Compton interaction decreases as incident photon energy increases (but is fairly constant across the diagnostic energy range)

Question 24

Compton Scattering: Degrades image quality

Answer 24

Arrives on the image detector from all directions leading to an increase in overall image density with reduced image contrast
At worst the image may be non-diagnostic

Question 25

Compton Scattering: Radiation dose issues

Answer 25

Compton scatter that escapes from the patient or X-ray tube can travel in all directions
Compton scatter can be absorbed by staff working in proximity to the patient
Compton scatter can be absorbed by patient anatomy outside the primary X-ray beam

Question 26

1. Which one of the following cannot happen to an X-ray photon as it traverses matter?
   a. scattering
   b. absorption
   c. transmission
   d. transformation

Answer 26

d. transformation

Question 27

2. The absorption and scattering of X-ray photons by matter is a ______ process where the outcome of each individual interaction is ¬¬¬¬_________. Match the correct pair.
   a. deterministic, guaranteed
   b. random, not guaranteed
   c. a thermal, not guaranteed
   d. statistical, guaranteed

Answer 27

random, not guaranteed

Question 28

3. Which one of the following is not a process whereby X-ray photons interact with matter
   a. photoelectric absorption
   b. Compton scattering
   c. Coherent scattering
   d. photostimulation
   e. Pair production

Answer 28

photostimulation

Question 29

4. Photoelectric absorption involves an incident _______ interacting with an _______ of an ______ shell of an atom. Match the correct triplet of words.
   a. electron, photon, inner
   b. photon, electron, outer
   c. photon, electron, inner
   d. photon, photon, outer

Answer 29

photon, electron, inner

Question 30

5. Characteristic radiation produced during the photoelectric interaction is different than that produced during X-ray production because it is produced by an incident ______ instead of an incident ______ .
   a. photon, electron
   b. electron, photon
   c. electron, scattered photon
   d. proton, photon

Answer 30

photon, electron

Question 31

6. Characteristic radiation produced in the human body as a result of the photoelectric effect have
   a. high photon energy (> 100 keV) and usually escape from the body
   b. high photon energy (> 100 keV) and usually are absorbed by the body
   c. low photon energy (< 4 keV) and usually absorbed by the body
   d. low photon energy (< 5 keV) and usually escape the body

Answer 31

low photon energy (< 4 keV) and usually absorbed by the body

Question 32

7. Photoelectric absorption results in the energy of the incident X-ray photon being…
   a. partially absorbed
   b. totally scattered
   c. totally absorbed
   d. partially scattered

Answer 32

totally absorbed

Question 33

8. The probability of photoelectric absorption is proportional to
   a. mass density & photon energy’3
   b. mass density & atomic number’3
   c. photon energy’3 & atomic number’3
   d. electron density’3 & atomic number’2

Answer 33

mass density & atomic number’3

Question 34

9. The probability of photoelectric interaction at a particular absorption edge energy is greatest
   a. well below the absorption edge energy
   b. just below the absorption edge energy
   c. exactly at the absorption edge energy
   d. just above the absorption edge energy

Answer 34

just above the absorption edge energy

Question 35

10. Photoelectric absorption results in excellent images because it
    a. reduces contrast between elements with different atomic number and reduces scattered radiation
    b. enhances contrast between elements with different atomic numbers and does not produce scattered X-ray photons
    c. reduces X-ray radiation dose and increase contrast between different tissue types
    d. increases scattered radiation which enhances image contrast

Answer 35

enhances contrast between elements with different atomic numbers and does not produce scattered X-ray photons

Question 36

11. Photoelectric absorption is inversely proportional to
    a. Z’3
    b. E’2
    c. E’3
    d. Z’2

Answer 36

E’3

Question 37

12. Common contrast agents used radiography are
    a. lead & copper
    b. silver & iodine
    c. iodine & barium
    d. barium & silver

Answer 37

iodine & barium

Question 38

13. The k-edge absorption energy of iodine and barium are _____ and _____ respectively
    a. 33.2 eV, 37.4 eV
    b. 33.2 keV, 37.4 keV
    c. 37.4 keV, 33.2 keV
    d. 33.2 MeV, 37.4 MeV

Answer 38

33.2 keV, 37.4 keV

Question 39

14. Compton scattering involves an incident ______ interacting with an _____ shell _______. Match the following triplet of terms.
    a. electron, outer, electron
    b. X-ray photon, inner, proton
    c. X-ray photon, outer, electron
    d. electron, outer, electron

Answer 39

X-ray photon, outer, electron

Question 40

15. Compton scattering results in an incident ______ being diverted from its path resulting in it having _______ energy. Match the pair of words.
    a. electron, higher
    b. photon, higher
    c. photon, lower
    d. electron, lower

Answer 40

photon, lower

Question 41

16. Which of the following set of variables does Compton scattering depend on
    a. atomic number, density, energy of incident photon
    b. neutron, energy of incident of incident photon
    c. coulomb force, mass
    d. density and incident photon energy

Answer 41

d. density and incident photon energy

Question 42

17. Over the range of photon energies employed in radiographic imaging Compton scattering
    a. increases rapidly
    b. is approximately constant
    c. decreases rapidly
    d. decreases apart from at k-edges

Answer 42

is approximately constant

Question 43

18. Compton scattering
    a. improves image contrast
    b. degrades image contrast
    c. does not affect image contrast
    d. only improves image contrast of dense objects

Answer 43

degrades image contrast

Question 44

19. Compton scattering
    a. does not escape the patient and so it is not hazard for radiographers
    b. may escape the patient but only in the direction of the primary beam and so is not a hazard for radiographer
    c. may escape the patient at any angle and therefore poses a hazard to radiographers
    d. does not pose a hazard to radiographers as it is of very low energy

Answer 44

may escape the patient at any angle and therefore poses a hazard to radiographers

Question 45

20. Across the diagnostic X-ray photon energy range the probability of Compton scattering
    a. is much greater at low energies compared to photoelectric absorption
    b. is approximately equal compared to photoelectric absorption
    c. is much greater at high energies compared to photoelectric absorption
    d. is a lot less at high energies compared to photoelectric absorption

Answer 45

is much greater at high energies compared to photoelectric absorption

Question 46

21. The effects of Compton scattering on subject contrast can be reduced by
    a. using and anti-scatter grid or an air-gap between the exit of the patient and the entrance of the detector
    b. using a smaller focal spot size
    c. increasing the mA
    d. increasing the exposure time

Answer 46

using and anti-scatter grid or an air-gap between the exit of the patient and the entrance of the detector

Question 47

22. Compton scattering is also called _______ scattering as the change in the photon energy depends on the ______ of scattering. Match the pair of terms
    a. incompetent, magnitude
    b. incoherent, vector
    c. incomplete, angle
    d. incoherent, angle

Answer 47

incoherent, angle