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BDS3 Dental Radiology > X-ray production > Flashcards

X-ray production Flashcards

1
Q

How are x-rays made?

A
  • In a machine - they are not naturally occurring energy source that we can harness, we have to create them
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2
Q

What is processing of an x-ray?

A
  • Conversion of a latent image to permanent visible image by computer technology or chemical
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3
Q

How are x-rays produced?

A
  • X-rays are produced when fast moving electrons are rapidly decelerated
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4
Q

What is an electron?

A
  • Negatively charged particle in an atom (-ve)

- Conceptually sited in orbits around the nucleus (+ve)

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5
Q

What does an x-ray machine consist of? (4)

A
  • Wall mounted
  • Tubehead - contains x-ray tube
  • Jointed, positioning arm
  • Control panel
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6
Q

What is contained within a tubehead? (3)

A
  • Tubehead
  • Spacer cone
  • Rectangular collimator - built in or inserted
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7
Q

What are the components of a tube head? (9)

A
  • Filament - cathode
  • Transformer
  • Target - anode
  • Target surround
  • Evacuated glass envelope
  • Shielding
  • Filtration
  • Collimator
  • Spacer cone
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8
Q

What is the negatively charged part of the x-ray tube?

A
  • Filament - cathode
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9
Q

What is the positively charged part of the x-ray tube?

A
  • Target - anode

- This is where we are going to bombard electrons at

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10
Q

What is the filament (cathode) made from?

A
  • Tungsten
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11
Q

What is the filament?

A
  • A tiny piece of coiled wire which is really difficult to see because it is embedded in a focussing cup
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12
Q

What happens in the filament?

A
  • There is an electrical circuit which runs through the filament and follows the current coming out of the wall going through a step down transformer to produce a low voltage, high current which is then going to make something happen at the filament
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13
Q

Why do we use Tungsten as the filament?

A
  • One of the reasons we use this is it has a really high melting point so we can use this filament several times without it melting and also it maintains its integrity
  • Also has quite a high atomic number (74) so that is determined by the number of protons in the nucleus and because it is a stable atom it will have the same number of electrons
  • This is going to be useful as this high number is going to help us
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14
Q

What is the filaments function once the electrons are produced?

A
  • So we’ve got our electrons but they are still at the filament - they are around it
  • So a little like thinking of steam coming off of a hot cup of coffee but if you hold it underneath an extractor fan it will pull the air towards it
  • So what we want to do is to do something that will pull the electrons over to the positive side of the x-ray tube
  • To do that we have to have a very high voltage
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15
Q

What kind of transformer is in an x-ray tubehead?

A
  • A step-up transformer
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16
Q

The transformer in the tubehead produces a high voltage output. What is the advantage of this?

A
  • Huge attraction of -ve electrons (mA) from cathode towards positive anode (target)
  • We apply the high voltage to our x-ray tube so it creates the potential difference between the negative filament and the positive target which creates this powerful force which is going to attract the negative electrons to the positive target
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17
Q

What is the current of the flow of electrons from the cathode towards the anode?

A

Around 7-15 mA

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18
Q

Why do we want a DC output as opposed to an AC output (Which is our normal electricity)?

A
  • Because it means we get a more standard output and will have shorter exposures
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19
Q

What range of kV should new equipment operate to?

A
  • Within the range of 60 to 70 kV
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20
Q

What is the target (anode) made of?

A
  • Tungsten
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21
Q

What can the target (anode) be referred to as?

A
  • Referred to also as focus or focal spot
22
Q

Why is the target anode at a slope?

A
  • This is important because that is a way of having a target that is a rectangle but we refer to it as a square
  • We call it the effective area - it is a way of increasing the efficiency of what we are doing (it is about 0.7mm^2)
23
Q

What is the effective area of the target (anode)?

A

0.7mm^2

24
Q

How much heat is produced when our electrons bombard the target?

A

99%

25
Q

What happens when our electrons bombard the target (target interactions)?

A
  • 99% heat production
  • <1% x-ray production
  • So this is a very inefficient way of producing x-rays but its the way that it has to be done - there is not a more efficient way
26
Q

What does spectrum mean?

A
  • Energy values in the final output of x-rays
27
Q

What are the 2 ways that our electrons can bombard a target?

A
  • Continuous spectrum

- Characteristic spectrum

28
Q

How do we solve the heat production problem in an x-ray machine?

A
  • The heat issue:
  • Going to talk about what happens when an electron in that stream of electrons coming from the filament reaches the target
  • In this case when we think about 1 incoming electron it is deflected by the outer shell electrons of the Tungsten atoms in the cut target
  • Or it might bump into an outer shell electron and move it a little bit
  • The result of this is a small loss of energy - not a lot (this energy is in the form of heat)
  • The heat is a problem - we want to get rid of it so the tungsten target is set into t a block of copper and copper is very good at conducting heat away
  • And then the x-ray tube is surrounded by oil which is also good at conducting heat away and then there is also the air around the x-ray tube head
  • So can get quite warm near x-ray machines if used again and again and again
29
Q

What is the target surround made of?

A
  • Copper
30
Q

What is the purpose of the target surround?

A
  • IT is an effective heat conductor so the purpose for it is to get rid of the heat
31
Q

Does the continuous spectrum result in the production of x-rays?

A
  • Yes
32
Q

Explain the continuous spectrum for x-ray production?

A
  • Incoming e- passes close to nucleus of a target atom (so gets through the outer cloud)
  • e- rapidly decelerated and deflected (it is these 2 things that give an indication of how much energy is lost from that electron which came in)
  • Amount of deceleration and deflection proportional to e- loss (there is a whole range - can be from a little bit to a lot of energy)
  • E loss in form of electromagnetic radiation as a continuous spectrum of energies)
  • Maximum E is applied kV (e.g. 70kV) (so if we have a 70 kV the max energy would be 70 and the min energy would be down to just above 0 - we can plot this and show it on a continuous spectrum)
33
Q

Why are lower energy x-ray photons of no use to us in image production?

A
  • because they don’t have enough energy to get through the tissues and help to create the image so we are going to have to get rid of them later
  • It is the higher energy ones that we want
34
Q

Explain the characteristic spectrum in producing x-rays?

A
  • The other mechanism for producing x-rays is the characteristic spectrum
  • Characteristic = means characteristic to a particular element - in this case Tungsten
  • So, the incoming electron gets through outer shell but doesn’t get to the nucleus - it collides with an inner shell electron in one of the target atoms
  • Because it collides with it, it will either displace it where it just goes a bit further away from the nucleus
  • Or it might actually knock it right out
  • The target atom - whether it has lost the electron or it is just on the wrong place - it is unstable
  • And atoms don’t like being unstable so they are going to re arrange the electrons almost instantly to get back their stability and it is then that they produce x-rays
  • So the various orbiting electrons will rearrange themselves automatically in order to have the correct number of electrons in the electron shells
  • Or if the atom had completely lost an electron there will be some loose electrons in the vicinity and one of them will drop into the outside
  • This arrangement means that the electrons are in shells that have different binding energy values and that difference in the energy is released and this is what is called characteristic radiation
  • This is very similar to one of the interaction mechanisms - called photo electric absorption
35
Q

What are the characteristic radiation values of Tungsten? (3)

A

Approximately:

  • 8 kV - L shell
  • 58 kV - K shell
  • 68 kV - K shell
36
Q

What is the glass envelope made of?

A
  • Evacuated glass

- Inside the glass is a vacuum

37
Q

What is the purpose of the glass envelope?

A
  • The vacuum in the glass is really important so that the electrons from the filament don’t have any interactions before they reach the target because that’s where we want the action to happen
38
Q

What does the shielding part of the x-ray tubehead do?

A
  • At that target x-rays are actually generated which go off in all different directions
  • We only want the x-rays in the direction of the patient, in the direction that we choose and we don’t want x-rays going elsewhere
  • So there is also lead surrounding to absorb x-rays that are trying to get out of the tube anywhere else
39
Q

What is the shielding part of the tubehead made of?

A

Lead

40
Q

Why is the shielding part of the tubehead made of lead?

A
  • Lead is a very good absorber of x-rays
  • This is down to it having a high atomic number
  • The reason for this shielding is to ensure that the dose rate is not more than 7.5
  • If the dose rate is not more than this then this means that people who work with x-ray machines will not get more radiation than they are legally allowed to
41
Q

We use lead shielding in the tubehead to ensure the dose rate in the vicinity is low. What is the dose rate?

A

cannot be greater than 7.5 uSvh^-1 (Sv = Sievert)

42
Q

which part of the x-ray tube head is going to get rid of the low energy x-rays that we don’t want?

A
  • Filtration
43
Q

What is the filtration part of the tubehead made of?

A
  • Aluminium
44
Q

How does the filtration part of the tubehead work?

A
  • This is where we are going to get rid of the low energy x-rays that we don’t want
  • This is done using another metal that this time has a very low atomic number
  • This means that it is going to be good at interacting with low energy x-rays but the high energy x-rays are going to get through it
  • This is important as we want the high energy x-rays as they produce the usable beam
  • This picture is as if we are looking down a spacer cone and in the middle there is a shiny silver looking bit - this is the aluminium
  • It is right in the middle where the x-ray beam is coming out
  • IT is surrounded by something that is dark grey - this is lead
  • The law controls how thick this disc of aluminium should be
  • If have a machine that operate up to or including 70kV it must be 1.5mm
  • The panoramic machine that can use higher voltages must use s thicker disc of aluminium (2.5mm)
45
Q

What is the collimator part of the x-ray tubehead made of?

A
  • Lead
46
Q

What shape is the collimator of the x-ray tubehead?

A
  • Circular or rectangular diaphragm
47
Q

What is the maximum beam diameter using the collimator?

A
  • 60mm at patient end of spacer cone
48
Q

What is the collimator of the x-ray tube head?

A
  • The collimator is the component that is going to control the shape of the x-ray beam and depending on how far it is from the patient it will also determine the size of the beam at the patient
  • So important to use machine correctly and having everything at the right distance
  • Most machines start off by producing a circular shape of an x-ray beam
  • So the x-ray beam coming out would obviously be a circle
  • But we want to use a rectangular beam to take bitewings and periapicals and also often occlusals so you then have to have a rectangular shaped lead collimator as well
  • Sometimes this is removable but sometimes it is fixed
  • When talking about a circular beam it must be no more than 60mm measured at the patient end of the spacer cone
  • Would do that by taking the x-ray tube head and would put the spacer cone directly onto something like an occlusal image receptor and could make an exposure and then could measure it
49
Q

What does the spacer cone do?

A
  • This is the part that helps us line up our x-ray tube head correctly
  • All about knowing which direction to point the beam in
  • Originally they were cone shaped (they are not cone shaped anymore)
  • They might be circular or rectangular
  • They might be removable or fixed
  • But if they are fixed you need to be able to rotate them round
  • The spacer cone is the important component which helps to control the distance between the focus (which is target where the x-rays were made) and the skin
  • And that distance is controlled by law - it is related to kV but is not the same kV that filtration is related to
  • Anything at 60kV or more it must be at 200mm
  • Thing that matters is the focus to skin distance
50
Q

What does the spacer cone of the tube head control?

A
  • Controls focus-skin distance (fsd)

- Measure from external marker to patient end of cone

51
Q

we use a long x-ray focus to skin distance to reduce what?

A
  • Magnification

- fsd needs to be at least 20cm

BDS3 Dental Radiology (13 decks)

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