Module 6 X-rays

Question 1

Typical wavelength of x rays?

Answer 1

1x10^-10 m

Question 2

What are the four main components of a x ray tube?

Answer 2

Cathode, Acceleration tube, Target anode, Thin sheet of aluminium

Question 3

What is the function of the cathode in generating x rays?

Answer 3

Emits a stream of electrons through thermionic emission

Question 4

What is thermionic emission?

Answer 4

Where electrons are emitted from a metal by gaining KE from a PD

Question 5

Describe how a cathode causes thermionic emission.

Answer 5

Cathode filament is heated by a low voltage. Electrons gain KE according to W=QV. And escape the filament by thermionic emission.

Question 6

What is a typical voltage which is used in a cathode?

Answer 6

6V

Question 7

What is the typical potential difference between the cathode and target anode?

Answer 7

50-100kV

Question 8

Why is a vacuum used in an acceleration tube?

Answer 8

To prevent electrons colliding with air particles and losing energy

Question 9

What proportion of colliding electrons cause the emission of x rays?

Answer 9

<1%

Question 10

Explain why electrons accelerate in the acceleration tube.

Answer 10

Large potential difference between cathode and anode

Question 11

Is an anode positive or negative?

Answer 11

Positive

Question 12

Is a cathode negative or positive?

Answer 12

Negative

Question 13

What is the target anode usually made of?

Answer 13

Tungsten

Question 14

What are the properties of tungsten which make it useful for x ray emission?

Answer 14

High density, High melting point

Question 15

Explain why the anode is sometimes rotated.

Answer 15

To distribute heat evenly and prevent damage

Question 16

Explain what happens at the anode which causes emission of x rays.

Answer 16

Some collisions between electrons and tungsten atoms cause electrons to lose large amounts of KE by emitting an X ray photon.

Question 17

What is the function of the thin sheet of aluminium?

Answer 17

Filters out low energy electrons

Question 18

Why is it important that a thin sheet of aluminium is used when generating x rays?

Answer 18

Removes low energy x rays which do not contribute to the image produced and are potentially harmful to the patient.

Question 19

Briefly describe the function of each of the four parts of an x ray generator.

Answer 19

Cathode: electrons gain KE through a low voltage and are emitted through thermionic emission. Acceleration: electrons are accelerated in a vacuum through a large potential difference by more KE = QV. Anode target: most electrons collide and KE is transferred to heat the target. Some electrons lose KE rapidly by emitting x rays.

Question 20

What colour is a photographic plate originally?

Answer 20

White

Question 21

What do x ray photons do to a photographic plate?

Answer 21

Darken it

Question 22

Explain why when in air, the photographic plate turns black.

Answer 22

Air does not absorb x rays well.

Question 23

Explain why something like bone is visible on x rays?

Answer 23

Strongly absorbs x rays. They can’t reach the plate so it remains white.

Question 24

What are the benefits and risks to x rays?

Answer 24

Non invasive, Ionising

Question 25

What does attenuation mean?

Answer 25

Reduction in intensity of x-rays as they pass through a material

Question 26

What is μ?

Answer 26

Attenuation constant

Question 27

What are the units for the attenuation constant?

Answer 27

m^-1 (can have any prefix next to it as long as they match x)

Question 28

Name the four ways x rays can interact with matter.

Answer 28

Simple scatter, Photoelectric effect, Compton effect, Pair production

Question 29

What is the energy range for x rays to interact with matter by simple scatter?

Answer 29

1-20keV

Question 30

Describe what happens during simple scatter.

Answer 30

Low energy x ray photons (1-20keV) bounce off electrons orbiting an atom, changing direction but with the same energy.

Question 31

Explain why simple scatter is considered an elastic collision.

Answer 31

Low energy photons bounce off electrons orbiting an atom and change direction but same speed and therefore same KE.

Question 32

What is the energy range for x rays to interact with matter through the photoelectric effect?

Answer 32

50-100keV

Question 33

Describe what happens during the photoelectric effect with x rays.

Answer 33

X ray photons of energy 50-100keV are absorbed by an orbiting electron. Photon disappears and orbiting electron gains its KE. Electron is emitted from the atom leaving it ionised.

Question 34

Why during the photoelectric effect and Compton effect is momentum conserved?

Answer 34

It's a typical collision.

Question 35

In a conceptual sense, what is the Compton effect?

Answer 35

Same as photoelectric but photon is not completely absorbed and instead just loses some energy.

Question 36

Outline the Compton effect.

Answer 36

X ray photons collide with an orbiting electron and change direction but at a lower energy (lower wavelength). Electrons gain PE to escape from the atom with KE.

Question 37

What does ionisation mean?

Answer 37

Removal of electrons

Question 38

What is the energy range of x ray photons for the Compton effect?

Answer 38

500keV-5MeV

Question 39

Explain how pair production of x rays takes place.

Answer 39

X ray photon collides with a nucleus. Photon disappears and is converted to an electron positron pair.

Question 40

Show how you find the minimum energy of a photon required for pair production.

Answer 40

E = mc^2 + mc^2 (where m is the mass of an electron)

Question 41

What is the main process which contributes to the attenuation of x rays?

Answer 41

Photoelectric effect

Question 42

What is the probability of the photoelectric effect taking place proportional to?

Answer 42

Z^3 (proton number cubed)

Question 43

Why can x rays not differentiate between most soft tissues?

Answer 43

Most soft tissue have similar values of Z. Attenuation is similar producing minimal contrast in images.

Question 44

What are the two main contrast media?

Answer 44

Barium meal (barium sulphate), Iodine

Question 45

Explain why contrast media work.

Answer 45

Have a high Z value so increase the probability of photoelectric effect taking place. Produces large contrast as they absorb a lot of x ray photons.

Question 46

Describe how a barium meal can be used as a contrast media.

Answer 46

Patient drinks a solution containing barium sulphate before an x ray of the digestive system. Produces very strong absorbance so the digestive system shows clear white images.

Question 47

Describe how iodine can be used as a contrast media.

Answer 47

A solution of iodine is injected into the blood, producing very strong absorbance in the blood vessels which remain white against a grey background.

Question 48

What does CAT stand for?

Answer 48

Computerised axial tomography

Question 49

How are CAT scans also abbreviated?

Answer 49

CT scans

Question 50

What is a gantry?

Answer 50

A part of a CT scanner that houses the X-ray tube and detector array. A circular machine or tube.

Question 51

Describe how a CT scan works.

Answer 51

Patient lies on a gantry that has an x ray tube. The X ray tube and detectors rotate around the patient. X ray tube produces thin fan shaped beams which are attenuated by the patients tissues and the remaining x rays are detected. Detectors send electrical signals to a computer which produces a 3D image that can be viewed from any angle.

Question 52

What are the advantages of CAT scans?

Answer 52

Can distinguish between different types of soft tissues of similar attenuation coefficients. 3D image produced can be viewed from any angle.

Question 53

What are the disadvantages of CAT scans?

Answer 53

Expensive to buy and to run. Patients receive more ionising radiation from CAT scans as patients must be kept still for approximately 30 minutes for a complete scan.

Question 54

Describe the structure of a CAT gantry.

Answer 54

A rotating circular tube with an X ray tube at one side and a detector array at the other.