10. X-RAYS: QUALITY ASSURANCE

Question 1

1. What influences the quality of the Radiation?

Answer 1

• the beam’s overall energy

Question 2

2. What influences the intensity of the beam?

Answer 2

• the number of X-Ray photons
• determined by the mass or the Kilovolts

Question 3

3. What does the intensity of the beam represent?

Answer 3

• the quantity of the radiation produced

Question 4

4. What can be said about the height of the Bremsstrahlung curve?

Answer 4

• it is directly proportional to the intensity of the X-Ray
  beam

Question 5

5. What does the quality of the X-Ray beam measure?

Answer 5

• it measures how readily the beam will penetrate any
  given material

Question 6

6. What does HVT stand for?

Answer 6

• the Half- Value Thickness of a beam of radiation
• this is also known as the Half- Value Layer (HVL)
• it affects the quality of the beam

Question 7

7. What is the Half-Value thickness of a beam of radiation?

Answer 7

• it is the amount of a given material that will reduce
  the intensity of the X-Ray beam by 50%
• it is the amount that will half the quality of the beam
• it affects the energy and the penetrability of the beam

THE MEASURE OF THE HVT:
- provides a guide to the penetrating power of the
beam

Question 8

8. What is Beam Attenuation?

Answer 8

• it is the change in the number of photons as a beam
  passes through a medium of material

IT IS DESCRIBED BY THE EQUATION:
N = N₀ x e to the power of µx

Question 9

9. In the equation, what is µ?

Answer 9

• this is the linear attenuation coefficient
• this is the fraction of the photons that interact per unit
  thickness of the attenuator
• this coefficient exists to describe how different materials
  can absorb radiation differently

Question 10

10. In the equation, what is N?

Answer 10

• this is the number of photons after they have passed
  through the material
• it is the intensity of the beam after it has passed through
  the material

Question 11

11. In the equation, what is N₀e?

Answer 11

• this is the number of photons before they have passed
  through the material
• it is the intensity of the beam before it has passed through
  the material

Question 12

12. In the equation, what is x?

Answer 12

• the thickness of the material
• this is in metres or millimetres

Question 13

13. How do we work out the HVL?

Answer 13

• this is the Half-Value Layer
• it is a special thickness that attenuates the beam to
  50%

Question 14

14. What is the quality of any beam of X-Rays proportional to?

Answer 14

• the HVT for any given material
• the maximum beam energy is often referred to as E-Max

IN DIAGNOSTIC X-RAY BEAMS:
- the E-Max will always be equal to the generating voltage
(kVp) of the X-Ray beam
- the generating voltage is the voltage that the operator
gives to the voltage supply

Question 15

15. What is the mA?

Answer 15

• it is the measure of the current flowing across the X-Ray
  tube
• this is often called the Tube Current

Question 16

16. What is current?

Answer 16

• this is the flow of electrons
• its value is in mA
• this is determined by the number of electrons flowing per
  unit time

Question 17

17. What happens to the number of electrons if we double the current from 200 to 400 mA?

Answer 17

• there will be twice as many electrons making up the tube
  current

Question 18

18. What does it mean for the electron that it is subjected to the same potential difference?

Answer 18

• it will have the same chance of creating an X-Ray Photon

THIS MEANS THAT DOUBLING THE MA (CURRENT)
- will double the number of X-Rays produced
- this will not affect the energy range of the X-Rays
- this means that it does not affect the quality of the
X-Ray beams

Question 19

19. What would happen if the mA was halved?

Answer 19

• the number of X-Rays would be halved

THE ASSOCIATION BETWEEN THE mA AND THE X-RAY BEAM
- the beam intensity is proportional to the mA (current)
- the beam quality is unaffected by changes in the mA

Question 20

20. What will an increase in the mAs (current per second) result in?

Answer 20

• this would increase the background blackness
• this would increase the image contrast

Question 21

21. What does the X-Ray tube voltage (keV) determine?

Answer 21

• it determines the potential difference between the
  cathode and the anode of the X-Ray tube
• this determines the kinetic energy that is acquired by each
  of the electrons as the current flows across the X-Ray tube

Question 22

22. What is the Peak-Kilovoltage (kVp)?

Answer 22

• this is the maximum cathode-anode voltage
• it is measured in Kilovolts

Question 23

23. What can be inferred if kVp is equal to 150 kV?

Answer 23

• the electrons will have a kinetic energy that is equal to 150
  keV just before they hit the anode

Question 24

24. What happens as the keV is increased?

Answer 24

• the speed a which the electrons impact on the target
  anode is increased
• there are more opportunities for the conversion of energy
  into X-Rays and heat

Question 25

25. What will happen when we have more energy available at the interaction process at the anode?

Answer 25

1. The maximum photon energy achievable will increase
2. There will be an increase in the average energy of each
   photon of X-Rays
3. If the total number of photons increases:
   - the intensity of the X-Ray beam must increase

Question 26

26. How can we mathematically write down the associations between beam quality, beam intensity and keV?

Answer 26

• Beam Quality ∝ (keV)²
• Beam Intensity ∝ keV

Question 27

27. Does this graph make sense?

Answer 27

• yes

Question 28

28. Does this image make sense?

Answer 28

• yes

WHEN WE DECREASE THE KVP:
- the photoelectric effect is reduced

Question 29

29. Does this diagram make sense?

Answer 29

• yes

AN INCREASE IN KVP LEADS TO AN INCREASE IN PENETRANCE:
- this leads to a decrease in Contrast