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BDS3 RADIOLOGY > X-Ray Production > Flashcards

X-Ray Production Flashcards

1
Q

what are the main components of a dental x-ray unit?

A
  • tubehead (with X-ray tube)
  • collimator
  • positioning arm
  • control panel
  • circuitry
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2
Q

what is the x-ray tube composed of?

A
  • glass envelope
  • cathode
  • anode
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3
Q

what is the cathode? (found in the x-ray tube)

A

The filament
- coiled metal wire
- low voltage, high current electricity passed through wire

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4
Q

explain the electricity that passes through the cathode?

A

Low voltage, High current
- heats up until emits light (2200°C)

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5
Q

what is the filament in the cathode made of?

A

tungsten
- high melting point (3422degrees celsius)

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6
Q

What is passed through the filament of the cathode? what happens during this?

A

Low-voltage, high-current electricity
- heats up until incandescent (emits light)
- electrons released from atoms in wire by thermionic emission
- cloud of electrons from around cathode

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7
Q

what does a higher current in the filament lead to?

A
  • more heat
  • more electrons
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8
Q

What is the filament wire made of? what are some properties of this?

A

Tungsten
- high melting point
- high atomic number (lots of electrons per atom)
- malleable

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9
Q

What is the focusing cup of the cathode?

A

Metal plate shaped around filament
- negatively charged —> repels electrons released at filament

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10
Q

Why is the focusing cup a specific shape?

A

to focus the electrons at a small point on the anode target

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11
Q

What is the focusing cup of the cathode made of?

A

Molybdenum

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12
Q

What is the relationship between the cathode & anode?

A

High potential difference between negative cathode & positive anode
- when high voltage electricity passed through the x-ray tube electrons released at filament are REPELLED FROM CATHODE and ATTRACTED TO ANODE

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13
Q

What occurs if the potential difference between the anode & cathode is increased?

A
  • higher acceleration of electrons towards target
  • more kinetic energy produced when they collide with target
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14
Q

What is the function of transformers in the X-ray machine?

A

They take mains electrical supply (220-240V) & convert it by changing the voltage and current

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15
Q

What transformers are present in the tubehead?

A
  • step up transformer
  • step down transformer
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16
Q

What is the function of the step up transformer?

A

Increases potential difference across X-ray tube to 60,000 to 70,000V

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17
Q

What is the function of the step down transformer?

A

lowers potential difference across filament to 10V

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18
Q

What is the unit used to measure the kinetic energy gained by electrons as they accelerate from cathode to anode?

A

Electron volts (eV)

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19
Q

What does 1eV mean?

A

kinetic energy gained by 1 electron moving across a potential difference of 1 volt

20
Q

What is the target on the anode? what is it made of?

A

A metal block bombarded by electrons that produces photons
- made of tungsten

21
Q

What is the focal spot of the target?

A

precise area on target where electrons collide & X-rays are produced

22
Q

Why is the target embedded in a larger block of metal?

A

Heat produced in target (by electron collisions) dissipates into this block by thermal conduction
- reduces risk of overheating

23
Q

What is the heat dissipating block made of?

A

Copper

24
Q

What is the Penumbra effect?

A

Blurring of radiographic image due to focal spot not being a single point (but rather a small area)

25
Q

How can the penumbra effect be minimised?

A

shrinking size of focal spot

26
Q

If we need a small focal spot, what problems arise from this and how are they solved?

A

PROBLEM = small focus spot size —> better image quality BUT higher heat concentration

SOLUTION = use an angled target
- increases the actual surface area where electrons impact (better heat tolerance)
- reduces the apparent surface area from where the X-ray beam is emitted (lowered penumbra effect)

27
Q

What is the function of the glass envelope?

A

Air-tight enclosure supports the cathode & anode
- maintains a vacuum allowing the electrons to travel from cathode to anode unhindered

28
Q

What are the main components of the tubehead?

A
  • X-ray tube
  • Metal shielding
  • Aluminium filtration
  • Oil
  • Spacer cone
29
Q

What is used for metal shielding?

A

Lead
- absorbs X-rays

30
Q

Why is filtration necessary?

A

Removes lower energy (non-diagnostic) X-rays from beam

31
Q

What is used to filtrate low energy x-rays?

A

Aluminium absorbs these photos so resulting X-ray beam contains mostly diagnostic X-ray photons

32
Q

What is the minimum thickness of aluminium required for an X-ray of <70kV

A

1.5mm

33
Q

What is the minimum thickness of aluminium required for an X-ray of >70kV ?

A

2.5mm

34
Q

What is the function of the spacer cone?

A
  • Dictates distance between focal spot (of target) & patient (focus to skin distance)
  • Indicates direction of beam
35
Q

What will alteration of the fsd (focus to skin distance) cause?

A

Affects the degree of divergence of X-ray photos in X-ray beam

36
Q

What occurs if fsd is increased?

A

Reduced divergence of X-ray beam & reduced magnification of image

37
Q

What is the typical size of a spacer cone used in modern equipment?

A

200mm

38
Q

What is the collimator?

A

Lead diaphragm attached to end of spacer cone to help reduce patient dose

39
Q

Why is the use of rectangular collimation strongly recommended? does it have any drawbacks?

A

Can reduce effective dose to the patient by approx 50%
- can increase the risk of collimation errors but this is minimised by good radiographic technique

40
Q

What are the consequences of the electrons bombarding the target?

A
  • Heat production
  • X-ray production
41
Q

What percentage of interactions between the electrons and target produce heat production vs xray production?

A

heat production = 99% of interactions

X-ray production = <1%

42
Q

how is continuous radiation produced?

A

Bombarding electron passes close to target nucleus, causing it to be rapidly decelerated & deflected —> lost kinetic energy released as X-ray photon

43
Q

Why is the production of continuous radiation described as being a spectrum?

A

Photons are produced over a wide range of energies:

greater proximity of electron to nucleus —> more deceleration & deflection —> more energy released (greater proportion of lower energy photons)

44
Q

how is characteristic radiation produced?

A

Bombarding electron collides with an inner-shell electron & either displaces it into a more peripheral shell or removes it completely

The remaining orbiting electrons rearrange themselves to re-fill the innermost shells

When electron drops to a lower shell it loses energy which is emitted as a photon of specific energy

45
Q

In characteristic radiation, what does the photon energy equal?

A

The difference in the binding energies of the 2 shells involved (which are specific to that element)

46
Q

As the target is made up of tungsten atoms, what are the different binding energies of the shells?

A

K shell binding energy = 69.5keV
L shell binding energy = 10.2keV
M shell binding energy = 2.5keV

47
Q

As dental x-ray tubes often operate at 70kV, what shells are typically displaced?

A

K shell electrons

BDS3 RADIOLOGY (8 decks)

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