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Ionising Radiation Physics > X-ray Production > Flashcards

X-ray Production Flashcards

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1
Q

What is Bremsstrahlung radiation?

A

“Breaking radiation” due to interaction with nuclear Coulomb field

Release of photons due to acceleration of charged particle with an amplitude proportional to acceleration

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2
Q

What is the acceleration equation of Bremsstrahlung photon?

A

a ∝ zZ / m

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3
Q

What is the intensity of Bremsstrahlung photons?

A

I ∝ Z^2 / m^2

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4
Q

What are core electrons?

A

Electrons closest to nucleus (inner shell)

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5
Q

What happens after collisional loss?

A

Heating in target material

(recoil, excitation and ionisation)

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6
Q

What happens after radiative loss?

A

X-ray production: Photons produced with set energy or various energy

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7
Q

How are characteristic x-rays produced?

A

Incoming electron interacts with core electron and leaves excited ion behind

Electron from higher orbital falls to lower hole and characteristic photon is released (seen as line in spectrum)

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8
Q

What is the energy of the characteristic x-rays?

A

hv = BEm - BEk

BEm = binding energy electron begins with

BEk = binding energy of electron in new orbital

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9
Q

What does each shell have?

A

Multiple energy states
and not all electron transitions are allowed

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10
Q

What are the selection rules for quantum numbers?

A

Δn ≠ 0
Δl = ± 1
Δj = ± 1; 0 (includes spin of electron, 1/2)

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11
Q

What do increasing principal quantum number mean?

A

Electrons are further away from nucleus as this increases (and higher ΔBE)

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12
Q

What is the energy of x-ray?

A

Difference between the energy levels of the orbits

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13
Q

When is characteristic line present?

A

When the incoming electron energy > electron binding energy

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14
Q

What dictates the energy loss of by e-?

A

The proximity of electron to nucleus

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15
Q

What happens to electron in near nucleus in Bremsstrahlung?

A

Electron feels attraction and deflects, slowing it down and energy is reduced which subsequently emits a photon

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16
Q

What does a greater deflection angle of e- mean?

A

More energy is lost by e- and higher energy Bremsstrahlung x-ray is produced

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17
Q

What is the assumption of a thin target?

A

Target is so thin that no electrons undergo more than one collision (all bremsstrahlung interactions are likely)

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18
Q

What is the assumption of a thick target?

A

That it is made of multiple thin targets so electrons pass through each layer and reduce their energy so new bremsstrahlung x-ray is produced at each layer

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19
Q

What increases the probability of radiative to collisional loss?

A

Increases directly with Z of target and energy

(at low energies: ratio is small so electron looses energy to heat)

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20
Q

What are the components of an x-ray tube?

A

glass envelope

cathode

anode

protective housing

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21
Q

What is the glass envelope made of?

A

Typically Pyrex to withstand tremendous heat and maintain vacuum

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22
Q

What does the tube window in the glass envelope do?

A

Area of envelop that is thinner
Contributes to inherent filtration

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23
Q

What does the cathode do?

A

Negatively charged electrode with two primary parts:

filament
focusing cup

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24
Q

What is in the filament?

A

Coil of metal wire, typically tungsten (current flows to heat filament through thermionic emission)

25
Q

What are the properties of the coil metal wire?

A

High melting point

High thermionic emission (electrons boiled off)

Carry high current to produce high temperature

26
Q

What is the focusing cup used for?

A

Metallic housing filaments which are negatively charged to keep electron cloud together and form a beam

27
Q

What is the anode?

A

Positively charged electrode which is used for x-ray production and heat management

two types: stationary and rotating

28
Q

What are the properties in anode structure?

A

High Z for efficient bremsstrahlung production

High melting point

High conductivity

Small ‘apparent’ source size

Materials: Tungsten, Molybdenum, Rhenium

29
Q

What is protective housing made of?

A

Protective steel

lined with 3mm lead

oil insulation between lead and glass tube

30
Q

What does protective housing prevent?

A

Leakage radiation

Electric shock

Dissipates heat

31
Q

What happens when a high anode-cathode voltage is applied?

A

Electrons are accelerated and then impact anode

32
Q

What is the kilovolts peak (kVp)?

A

Maximum voltage applied across the tube and maximum X-ray energy (kV ∝ eV of electron)

related to maximum energy carried by a thermionic electron across the tube

33
Q

What does Direct AC do to tube?

A

damages it

34
Q

What does the Half-wave (HW) rectification do and lead to?

A

only accepts positive voltage

Poor output -> 50%

35
Q

What does the Full-wave (FW) rectification do and lead to?

A

All negative voltage voltage is made positive

most output is still below kVp

36
Q

What is the 3 phase full wave rectification?

A

the peaks are overlapped at a factor of 3 and output is higher and less variability

37
Q

What does DC voltage produce?

A

Constant potential so constant x-ray production

(used commonly in modern generators)

38
Q

What is the current in the tube?

A

The number of thermionic electrons passing between cathode and anode (mA)

(different from filament current)

39
Q

What is mAs?

A

miliampere seconds

a measure of radiation produced over a set amount of time in an x-ray tube

40
Q

What does continuum emission produce?

A

Bremsstrahlung (peak)

41
Q

What does a discrete emission produce?

A

Characteristic x-ray lines

42
Q

What is the high energy cut off?

A

The edge of the Bremsstrahlung peak

Maximum electron energy dependent on applied kV (x-rays can’t be produced above this)

43
Q

How are characteristic lines produced?

A

Electron can (occasionally) give up all energy to X-ray production

43
Q

What is the low energy cut off?

A

Point at which X-rays cannot penetrate layers as low energy x-ray are easily attenuated

44
Q

What does the filtration of the x-ray do?

A

Attenuation of x-ray beam

45
Q

What effect does quantity have and which parameters are affected by it ?

A

Magnitude changes, shape remains unaltered

current, exposure

46
Q

What effect does quality have which parameters are affected by it ?

A

Non-uniform change in intensities, shape changes

voltage, filtration

47
Q

What is the focal spot size?

A

The area on anode where x-rays are emitted

size is controlled by x-ray target angle and electron beam size (effected by size of filament)

48
Q

What is the anode heel effect?

A

When there is greater attenuation on anode side as x-ray travels through more of the target causing lower intensity on one side (beam hardening)

non-uniform intensity and energy distribution

49
Q

What are the benefits and drawbacks of small focal spot size?

A

Better resolution

Better for smaller parts of body

Heat is concentrated in smaller area

Limit on x-ray flux

50
Q

What are the benefits and drawbacks of large focal spot size?

A

Lower resolution images

Better for imaging large structures

Heat spreads out

Greater flux available so faster exposure (better due to patient movement)

51
Q

What is the space charge effect?

A

Occurs when there is a cloud of electrons at filament

they generate an opposing electric field (negative) and limits the flow of electrons from the cathode to the anode at lower voltages, reducing X-ray intensity (until anode voltage is high enough)

52
Q

In diagnostic x-ray what will define the design of the x-ray tube?

A

Cooling the electron target

53
Q

What are the two types of anode design?

A

Stationary:
Concentrated electron impact

Localised heating

Low power applications only (dental)

Rotating:

Electron impact spread over large area

Energy delivered per unit area reduced

High power applications

54
Q

What is a sign of thermal anode damage?

A

Anode pitting:
small regions of anode surface overheat and liquefied will flow and ‘creep’ to a new location

(effects x-ray spectrum)

55
Q

What does thermal anode damage cause?

A

Beam hardening

X-ray must pass through more material to escape

Increased mean energy

56
Q

How is the life of a tube prolonged?

A

By setting limits on usage (kV, mA, exposure time)

57
Q

What is the effect of of increasing current and voltage?

A

current: more electrons move across gap and counts per second increase linearly

voltage: electrons have higher kinetic energy & increase in electrons per unit time & x-rays have higher energy

58
Q

What is the effect of increasing exposure and filtration?

A

Exposure: more electrons when time increases

Filtration: low cut off edge shifts to right and beam hardens

Ionising Radiation Physics (11 decks)

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