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BDS3 CLINICAL Radiology > X-ray Production > Flashcards

X-ray Production Flashcards

1
Q

How are X-rays produced

A
  • X-rays are produced when fast moving electrons are rapidly decelerated
    • They are produced inside the X-ray tube
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2
Q

What is an electron

A
  • It is a negatively charged particle in an atom and it is negative
    • It conceptually sits in orbits around the positive nucleus in what is known as the Bohr model
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3
Q

What does an X-ray tube head consist of

A

○ Tube head (has several components to it)
○ Spacer cone - it is the cylinder seen at the front
○ Rectangular collimation - built in or inserted
Controls the shape of the beam

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4
Q

What are the components of the tube head

A 
○ Filament - cathode
		○ Transformer
		○ Target - anode
		○ Target surround
		○ Evacuated glass envelope
		○ Shielding
		○ Filtration 
		○ Collimator
		○ Spacer cone
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5
Q

What is the cathode/filament

A
  • The filament is a tiny bit of coiled wire which we can’t see as it is embedded in a focussing cup
    • It is made of tungsten
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6
Q

How does the filament work

A
  • There is an electrical circuit which runs through the filament and follows the current coming out the wall which goes through the step down transformer to produce a low voltage and high current
    • The low voltage current is passed through the filament circuit and the filament heats up to incandescence and electrons form a cloud around the filament but we want something to pull away the electrons over to the positive side of the X-ray tube and to do that we have to have a very high voltage and that is where the transformer is required
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7
Q

What is tungsten

A
  • Symbol W
    • Atomic number is 74 so it is a stable atom
    • Melting point of 3410 which is very useful as we can use the filament again ad again and it wont degrade or melt
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8
Q

What is the transformer

A

• It is inside the tube head and in front of the tube and it has a hollow centre so when we make our X-rays they can go through it and interact with the transformer itself

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9
Q

What is the function fo the transformer

A
  • We start off with the electricity supply which is 240 eV in this country but the dental machine requires 60-70KeV
    • It creates a huge attraction of negative electrons (mA) from the cathode towards the positive anode (target)
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10
Q

What is the flow of electrons

A

7-15 mA

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11
Q

What is the target anode

A
  • Positive
    • Beyond the filament
    • Made of tungsten
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12
Q

What is the function of the anode

A

○ The whole anode is not included in X-ray production, the X-rays are produced ona rather small rectangular surface called the focal spot
• It is on a 20 degree slope
• It is referred to as the focus or the focal spot

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13
Q

What are the target interactions

A

• When the electron beam hits the anode, interactions of the electrons with the target material produces x-ray beams as well as heat

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14
Q

How much is heat

A

99%

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15
Q

Describe how heat is produced and how we prevent its impact

A

○ When an incoming electron is deflected by the outer shell electrons of the target, it may move it a bit and the result of that is a small loss of energy in the form of heat
○ This is undesirable so the tungsten is set into a block of copper which is good at conducting the heat away to the surrounding oil and then into the air around the X-ray tube

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16
Q

How are x-rays produced

A

continuous spectrum

characteristic spectrum

17
Q

What is the continuous spectrum

A

® X rays are produced when high velocity electrons strike the target material of a high atomic number and due to these forces, the electrons get deflected from their original path
® As a result of this the electrons are decelerated and hence the energy of the electron decreases continuously

18
Q

What is the characteristic spectrum

A

® Characteristic x-rays are emitted when outer-shell electrons fill a vacancy in the inner shell of an atom, releasing X-rays in a pattern that is ‘characteristic’ to each element

19
Q

What is the target surround

A
  • The target surround is made of copper
    • It has an atomic number of 29
    • It has a melting point of 1080
    • It is an effective heat conductor
20
Q

What does the continuous spectrum refer to

A

• Continuous X-ray spectrum refers to the range of photon energies produced in an X-ray tube due to the properties of Bremsstrahlung radiation

21
Q

Describe how the continuous spectrum results in x-rays

A
  • The incoming electron gets through the outer cloud of electrons and gets close to the nucleus and due to these forces, the electron gets deflected from its original path and becomes decelerated and it changes direction and it is these two things together that dictate how much energy is lost from that electron
    • The amount of deceleration and deflection is proportional to the energy loss
    • The energy loss is in the form of electromagnetic radiation as a continuous spectrum of energies
    • The maximum energy is the applied kV
22
Q

What electrons do we want from the continuous spectrum

A
  • There are more X-ray photons in the low energy part, and there are fewer in the high energy part
    • Low energy X-rays aren’t useful in image production, they don’t have enough energy to get through tissue and help create the image so we have to get rid of them as it is the higher ones we want
23
Q

Why is the characteristic spectrum characteristic

A

It is characteristic to a single element

24
Q

Describe how the characteristic spectrum results in electrons

A
  • The incoming electrons gets through the outer shell but not as far as the nucleus and it collides with an inner shell (orbit) target electron
    • When it collides with it, it either displaces it to an outer shell or the electron is completely lost from the atom
    • This means that the target atom is now unstable so orbiting electrons rearrange to fill vacant orbital slots to return the atom to neutral state
    • The difference in energy between orbits is released as characteristic radiation, of known energy values
    • It is the same mechanism as photoelectric absorption
25
Q

What is an evacuated glass envelope

A

• It consists of evacuated glass

The vacuum prevents risk of interaction of electrons with air atoms prior to meeting target

26
Q

What is the shielding

A
  • Consists of lead - Pb
    • Lead has an atomic number of 82
    • It gets rid of low energy x-rays that we don’t want
    • It is to ensure the dose rate in vicinity is not > 7.5 sievert
27
Q

What is the filtration

A
  • Made of aluminium
    • Atomic number of 13
    • It is surrounded by lead
    • The law dictates how thick the aluminium disc should be
28
Q

How thick should aluminum be for machine that operates up to 70kv

A

○ A machine that operates up to 70kV including 70kV must be 1.5mm so all dental x-ray machines

29
Q

How thick should aluminum be for machine that operates above 70kv

A

○ For panoramic which is greater than 70kV the disc has to be 2.5mm

30
Q

What is the collimator

A
  • It controls the shape of the X-ray beam
    • Made of lead
    • Can be circular or rectangular diaphragm
31
Q

What is the maximum beam diameter

A

• The maximum beam diameter is 60mm at the patient end of the spacer cone - this is measured by putting it directly on the occlusal image receptor and measuring the exposure

32
Q

Why do we use rectangular collimation

A

• Rectangular collimation should be provided on new equipment and retrofitted to existing equipment as it reduces the beam size and thus dose to the patient and if you do that for every intra oral then it adds up

33
Q

What is the spacer cone /BID for

A
  • Direction indicating device or beam indicating device - helps line up the tube head corrently
    • It controls the target (Focus)-skin distance
34
Q

What should the Fed be for less than 60kV

A

100mm

35
Q

What should the Fed be for greater than 60kV

A

200m

36
Q

What is the FSD

A
  • We use long X-ray focus-skin distance to reduce magnification, it should be at least 20cm
    • By having a longer distance, we make the different in our beam more near parallel which reduces magnification and distortion
    • (long spacer cone = long fsd)

BDS3 CLINICAL Radiology (16 decks)

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