X-ray interactions

Question 1

What is the photoelectric effect?

Answer 1

When a photon knocks an innner-shell electron out of its shell

Question 2

What is the name of the electron knocked out of its shell due to the photoelectric effect?

Answer 2

a photoelectron

Question 3

What normally happens to an electron knocked out of its shell in the photoelectron effect?

Answer 3

Absorbed almost immediately by surround matter

Question 4

What end products are produced by the photoelectric effect?

Answer 4

Characteristic radiation (as an electron drops down a shell and releases it’s excess energy in the form of a photon)

A negative ion (the photoelectron)

A positive ion (atom deficient one electron)

Question 5

What occurs to the characteristic radiation x-rays produced by the photoelectric effect?

(when x-rays interact with the body)

Answer 5

They are usually absorbed almost immediately?

Question 6

What increases the chance of the photoelectric effect occurring?

Answer 6

If the energy of the x-ray is close to the binding energy of a given electron (although it must have more energy than the binding energy)

The tighter an election is bound in its shell the more likely it is be involved in the photoelectric effect

Photo electric effect is proportional to:
atomic number cubed/ incident photon energy cubed

Question 7

Why are characteristic radiation x-rays produced in the body more likely to be absorbed, than those produced in an x-ray machine?

Answer 7

They have a lot less energy.

Instead of high energy electron hitting inner shells of high atomic number atoms it’s x-rays hitting inner shells of average atomic number atoms.

Question 8

What are the main types of interactions that x-rays have with tissues?

Answer 8

Photo-electric effect
Compton Scatter
Coherent (Rayleigh) scattering

Question 9

Does photoelectric effect have more of on important role in x-rays of soft tissues (muscle, fat) or hard tissues (bone)?

Answer 9

soft tissues

helps to create contrast int these tissues

Question 10

What is the disadvantage of relying on the photoelectric effect for x-ray images?

Answer 10

increased patient radiation, all the energy from the x-rays is absorbed

(not the case with Compton scatter)

Question 11

What occurs in Compton scatter?

Answer 11

The x-ray interacts with an electron in the outer shell., displacing it.
A new x-ray is created, travelling in a slightly different direction.

Question 12

What factors increase the amount of energy absorbed from a photon during Compton scatter

Answer 12

The more of an angle the photon is ‘deflected’ the more energy is lost.
Lower energy photons also lose a greater proportion of their energy for a given angle of scatter.

Question 13

Is compton effect more or less likley with higher keV?

Answer 13

Less likley, although only slightly.

However, it becomes more relevant than the photoelectric effect, which has a vanishingly small chance of happening.

Question 14

What affect the probability of Compton Scatter occurring?

Answer 14

Slightly less likley with increasing keV photons.

Electron density.

Although all elements contain roughly the same electron density so nothing really.

Question 15

What metric is used to measure the amount of scattered radiation in an x-ray?

Answer 15

Scatter to primary ratio

Question 16

What are methods to reduce scatter in an x-ray image?

Answer 16

limit the x-ray field (collimation)

limit the irradiated thickness (e.g. squish breasts for mammography)

anti-scatter grids

Question 17

Luminescence

Answer 17

Materials absorbing energy and converting it to light

Question 18

Flourescence

Answer 18

Materials absorbing energy and converting it to light within 10^-8 seconds

Question 19

Phosphorescence

Answer 19

Materials absorbing energy and converting it to light in > 10^-8 seconds

Question 20

What does CR stand for in x-ray technology?

Answer 20

Computed Radiography

Question 21

What is a phosphor

Answer 21

a chemical compound that emits light when exposed to light of a different wavelength.

Question 22

How is an x-ray CR cassette read?

Answer 22

laser light (red) stimulates emission of trapped energy.

Visible light (blue) is released from the plate

The visible light strikes a photo-multiplier tube and produces an electric signal.

Question 23

What is thermoluminescence

Answer 23

where heating liberates energy, which is released as light

Question 24

What uses are there for thermoluminescence in radiology?

Answer 24

personal dosimetry
patient dose measurement
measurement of scatter

Question 25

What is Rayleigh scattering also called?

Answer 25

Coherent scattering

Question 26

What is Coherent scattering also called?

Answer 26

Rayleigh scattering

Question 27

What occurs on Rayleigh scattering?

Answer 27

incident photon excites a total atom.

a scattered photon is released with the same energy/ wavelength as the incident photon

Question 28

Is there ionisation in Ryleigh scattering?

Answer 28

No

Question 29

Is there ionisation in Compton scattering?

Answer 29

Yes

Question 30

Is there ionisation in Photo-electric interactions?

Answer 30

Yes

Question 31

What units are used to measure attenuation in x-rays?

Answer 31

Linear attenuation co-efficient (μ):
A quantitative measurement of attenuation per cm of the absorber

Mass Attenuation Coefficient
divide linear attenuation coefficient by material density

Question 32

What factors influence the degree of attenuation of an x-ray beam?

Answer 32

The spectrum of x-rays

The density of the material

Question 33

What is the unit of measurement for linear attenuation co-efficient?

Answer 33

μ

OR

cm^-1

Question 34

What word is used to describe an x-ray beam that has a spectrum/ range of energies? (2)

Answer 34

polyenergetic

polychromatic

Question 35

What are the main x-ray contrast materials used? (2)

Answer 35

Barium

Iodine

Question 36

Through what mechanism do phosphors work usually?

Answer 36

The photo-electric effect from a high atomic number component.

Question 37

Does the photo-electric effect produce scattered radiation?

Answer 37

no

Question 38

What’s the fundamental difference between the photoelectric effect and Compton scattering

Answer 38

Photo electric effect knocks an inner shell electron out of orbit

Compton scatter knocks an outer shell electron out of orbit

Question 39

Photoelectric effect more likely if electron energy much higher than the binding energy of an electron?

Answer 39

No, photoelectric effect is more likely if the energies are similar (although electron energy still needs to be a little higher than the binding energy)

Question 40

Compton Scatter more likely if electron energy much higher than the binding energy of an electron?

Answer 40

Yes, more likely as electron energy increases