X-ray attenuation

Question 1

What are the four major interaction processes:-

Answer 1

Unmodified, elastic or classical scatter
Photo-electric process
Compton process
Pair production

Question 2

What are the types of attenuation?

Answer 2

Scatter

Absorption

Question 3

What is Classical scatter?

Answer 3

Occurs when the energy of the incident photon is less than the binding energy of the orbiting electron in the irradiated material
The photon collides with the electron and is scattered without loss of energy

Question 4

What is PHOTO-ELECTRIC PROCESS?

Answer 4

This occurs mainly between 40kV and 80kV
The process occurs when the photon energy is equal to or slightly greater than the binding energy of the orbiting electron.
It ejects the orbiting electron, (known as a Photo-electron), pure absorption
Gives of a photon with the binding energy of the electron.

Question 5

What is the COMPTON effect? Also known as modified scatter

Answer 5

Occurs when the energy of the photon is much greater than the binding energy of the orbiting electron.
It gives kinetic energy to the “recoil electron”
The scattered photon has a longer wavelength than the incident photon because it has given up some of its energy to the electron.

Question 6

What does COMPTON effect lead to?

Answer 6

Decreased in Image receptor density.

Reduction in contrast.

Question 7

What is pair production?

This process occurs at energies above 1.02 MeV i.e. in radiotherapy machines

Answer 7

The photon interacts with the electrical and magnetic fields of the nucleus.
The photon is transformed into 2 particles:-
A positron (+ve)
A negatron (-ve)
The positron combines with a normal electron to make annihilation radiation. Photon/wavelength given off.

Question 8

What are the two main factors affecting attenuation?

Answer 8

Changes in the atomic number of the irradiated material

Changes in the photon energy

Thickness/density of the material

Question 9

How does atomic number affect the photo-electric process?

Answer 9

Higher atomic numbers imply a greater positive charge on the nucleus (due to the number of protons)
This means that the binding energies of the orbiting electrons will be higher.

Question 10

How does atomic number affect absorbed dose?

Answer 10

The absorbed dose increases with higher atomic numbered materials e.g. Bone

Question 11

How do you work out The photo-electric mass attenuation coefficient (PMAC)

Answer 11

PMAC ∝ Z³

Z=proton number

Question 12

How does photon energy affect The photo-electric process

Answer 12

The photo-electric effect is unlikely to occur if the photon energy is much greater than that of the binding energies of the orbiting electrons.
The photo-electric effect is greatest at low photon energies (Low kV)

Question 13

What’s the formula to work out the effect photon energy has on the photo-electric effect?

Answer 13

PMAC ∝ 1/kV³

Question 14

The photo-electric effect produces features known as Absorption Edges. What do these absorption edges mean?

Answer 14

Each ‘edge’ results from the absorption of photons by the inner shell electrons thro’ the PE effect.
It needs more energy than the binding energies to make “K” absorption edge and “L” absorption edge

Question 15

How does atomic number affect Compton Mass Attenuation Coefficient (CEMAC)

Answer 15

Determined by electron density

Electron density is relatively constant regardless of atomic number (except in Hydrogen)

Question 16

How does atomic number affect Pair production mass attenuation coefficient (PPMAC)

Answer 16

PPMAC ∝ Z
Pair production mass attenuation coefficient (PPMAC) is directly proportional to atomic number
This is because the strength of the electromagnetic field of the nucleus increases with atomic number

Question 17

How does photon energy affect Compton Mass Attenuation Coefficient (CEMAC)

Answer 17

CEMAC ‘falls off’ or decreases as photon energy increases

Fall off is gradual and considered as constant over the diagnostic range

Question 18

How does photon energy affect Pair production mass attenuation coefficient (PPMAC)

Answer 18

This process never occurs in the diagnostic range but does occur in radiotherapy beam energies
Produced by megavoltage radiotherapy units
Cannot occur below 1.02 MeV
PPMAC increases with photon energy

Question 19

What is Total Mass Attenuation Coefficient? (TMAC)

Answer 19

Comprises of all the mass attenuation coefficients added together to give an overall curve or range of values.

Question 20

During the photoelectric effect the incident photon gives all of its energy to the orbiting electron.

Select one:
True
False

Answer 20

True

Question 21

The Compton effect mass attenuation coefficient has the following relationship with energy and atomic number

A. Independent of Z and proportional to E

B. Independent of Z and inversely proportional to E

C. Inversely proportional to Z3 and inversely proportional to E3

Answer 21

Independent of Z and inversely proportional to E

Question 22

When the classical scattering process takes place, which of the following is correct?

A. The scattered electron has more energy that the incident photon.

B. The scattered electron has a long wavelength.

C. There is no scattered electron in this process.

Answer 22

C. There is no scattered electron in this process.

Question 23

The energy of the particles that result from the Pair Production effect are equal to

A. 5.10 eV

B. 51 KeV

C. 510 Kev

D. 5.10 MeV

Answer 23

C. 510 Kev