X-ray production

Question 1

What is Bremsstrahlung x-ray production?

Answer 1

X-rays created from the conversion of the electrons kinetic energy into electromagnetic radiation when they are decelerated by interaction with a target material

Question 2

What is Bremsstrahlung radiation?

Answer 2

Electrons undergo inelastic collisions; their path is deflected by the +ve charged nucleus and there is kinetic energy loss. The radiation acompanying the KE loss is Bremsstrahlung (braking) radiation

Question 3

What is Bremsstrahlung emission per atom proportional to?

Answer 3

Z^2 / m^2

Z = atomic number of absorber / m = mass of incident particle

Question 4

Why is the xray spectrum continuous?

Answer 4

Bremsstrahlung x-ray photon energies can be any value up to and including the entire KE of the deflected electron. Many electrons undergoing Bremsstrahlung interactions = continous spectrum of x-ray energies

Question 5

What voltages are used in DR?

Answer 5

20-150kV

Mammo 25-40kV; diagnostic imaging 40-150kV

Question 6

What does the x-ray tube contain?

Answer 6

• electron source (cathode)
• target (anode)
• rotor/stator
• glass or metal envelope
• tube port (output window)
• cable sockets (out to the housing, then out of the housing)

Housing = lead for shielding and cooling; tube is evacuated

Question 7

How does x-ray generation work?

Answer 7

1. Appy potential difference between 2 electrodes (anode held at +ve voltage WRT cathode)
2. Electrons travel from cathode to anode, accelerating due to voltage, gaining KE
3. Electrons hit anode target
4. KE is converted into other forms of energy
5. Heat mostly, also Bremsstrahlung and characteristic xrays

Question 8

What type of interactions occur at the anode?

Answer 8

• Mostly collisional; energy exchange with the target gives rise to heat
• Small fraction interact with nucleus +ve field, get deflected (Bremsstrahlung)
• Some electrons interact with inner shell (K) of bound electrons, ejecting one if their KE > binding energy (characteristic)

Question 9

Which force is at play during Bremsstrahlung radiation?

Answer 9

Coulombic (electrical) force

electrons come within proximity of +ve nucleus, get influenced by its field.
Force attracts the electron, decelerating it, changing its direction, causing a loss of KE.
KE is emitted as an xray photon of equal energy

Question 10

How does Coulomb force depend on distance?

Answer 10

1/(distance)^2

Question 11

The amount of energy through loss of KE (causing Bremsstrahlung radiation) depends on what?

Answer 11

The distance between the incident electron and the target nucleus

Question 12

If an electron interacts very near to the nucleus, what energy will the Bremsstrahlung radiation be?

Answer 12

close to nucleus = force acting on electron increases = greater deceleration = more KE lost = higher energy radiation emitted

Question 13

What happens if an electron nearly directly impacts on a target nucleus?

Answer 13

Nearly all the KE will be lost, so the highest possible energy Bremsstrahlung xray radiation will be produced

this is rare!

Question 14

How does the probability of an interaction resulting in a Bremsstrahlung xray radiation change with interaction distance r?

interaction distance = distance between nucleus and electron

circumference = 2.pi.r

Answer 14

Probability is proportional to r

Increasing distance from nucleus r = increased circumference = probability of interaction increases (but xray energy decreases)

Question 15

If the probability of interaction increases, what happens to the emitted xray energy?

Answer 15

Energy decreases [as interaction probability increases]

Question 16

How does the number of xrays produced scale with the energy they will have?

Answer 16

No. xrays produced decreases linearly with energy

(up to the maximum xray energy = incident electron energy)

Question 17

A Bramsstrahlung spectrum shows….

Answer 17

the probability distribution of xray photons as a function of photon energy (keV)

Question 18

For an unfiltered Bremsstrahlung spectrum, how does the number of xrays produced scale with their energy?

Answer 18

inverse linear relationship, i.e. lots produced but mostly low energy

Question 19

For a filters Bremsstrahlung spectrum: when is the maximum number of xrays produced?

Answer 19

When E = 1/3 max

the number produced will decrease to zero as the energy increases to the

Filtered spectrum has no xrays < 10keV

Question 20

What determines the highest energy an xray can have?

Answer 20

The peak voltage applied across the tube (kV)

Question 21

What is xray filtration?

Answer 21

Removal of xrays by attenuation in materials that are inherent in the tube (e.g. glass window of insert) and materials placed in the beam to remove lower energy xrays (aluminium, copper)

Question 22

Why would you want to filter an xray beam?

Answer 22

To remove lower energy xrays and adjust the spectrum for optimal low-dose imaging. If the energy is too low, the xrays would contribute to the patient dose but not to image formation process

Question 23

What affects the efficiency of xray production?

Answer 23

• atomic number of the target material
• KE of the incident electrons

energy conversted to heat or low energy e-m radiation is not useful!

Question 24

Name 3 common target materials

Answer 24

• tungsten W (Z=74)
• molybdenum Mo (Z=42)
• rhodium Rh (Z=45)

Mo and Rh used in mammography