Production of X-rays

Question 1

X-rays are produced when what happens?

Answer 1

when fast moving electrons are rapidly decelerated.

Question 2

What are the components of a dental x-ray machine?

Answer 2

• Wall mounted
• Tubehead (contains X-ray tube)
• Jointed, positioning arm
• Control panel

Question 3

What are the components of the X-ray tube head? (9)

Answer 3

• Filament – cathode (negatively charged part)
• Transformer
• Target – anode (positively charged part)
• Target surround
• Evacuated glass envelope
• Shielding
• Filtration
• Collimator
• Spacer cone

Question 4

The filament has what charge?

Answer 4

It is the negative side of the x-ray tube

Question 5

What is the filament?

Answer 5

A small coiled wire made from tungsten

Question 6

What properties of tungsten make it a good material for the filament to be made of and why?

Answer 6

• high atomic number
• High melting point (3410 degrees) which means that the filament can be reused and it will maintain its integrity

Question 7

What are the 2 types of transformers in the x-ray tube head?

Answer 7

• Step-down transformer
• Step-up transformer

Question 8

What does the step-down transformer do?

Answer 8

converts the electricity from the mains electricity to a low voltage current which is passed through the filament circuit

Question 9

What happens to the filament when electricity is passed through it?

Answer 9

• It causes the filament to heat up and incandescence (emit light as a result of being heated).
• This causes electrons to form a cloud around the filament (the electrons have come out from atoms in the filament).

Question 10

What do you need to do to extract the electrons in a cloud around the filament?

Answer 10

Need a very high voltage - this is where the step up transformer comes into play

Question 11

What does the step-up transformer do?

Answer 11

It takes the domestic input from the mains electricity supply (approx. 240eV) and converts it to a high voltage output of 60-70 keV (60-70 thousand).

Question 12

The high voltage that has come from the step-up transformer does what?

Answer 12

causes a huge attraction of the -ve electrons from the cathode (the negative part of the filament) towards to the positive anode (the target).

Question 13

The flow of electrons (current) from the cathode to the anode is what?

Answer 13

7-15 milliamps

Question 14

What is the anode (target) and what is it made from?

Answer 14

• is the positively charged part of the x-ray tube
• tungsten

Question 15

What happens when the electrons reach the anode?

Answer 15

The electrons will interact with the anode and either:

• undergo a reaction that either produces heat
• undergo a reaction that produces x-rays

1% of the electrons will be turned into X-rays and the other 99% will be transformed into heat

Question 16

The heat from the interaction between the anode and electrons is dissipated by what?

Answer 16

The copper stem on the anode

Question 17

Where do the x-rays produced from the electron interation with the anode go?

Answer 17

The x-rays exit the unleaded glass window towards the aluminium disc (filtration) then pass through the collimator

Question 18

What is the glass envelope made from and what does it do?

Answer 18

This is made from evacuated glass.

It acts as a vacuum preventing the risk of interaction of electrons (from the filament) with air atoms prior to meeting the target.

Question 19

What shielding is there within the x-ray tube and why do we have it?

Answer 19

We want the x-rays created to go towards the patient but only the patient

We don’t want the x-rays going anywhere else so there is lead that acts as a shield.

Question 20

Why is lead good for shielding?

Answer 20

Lead has a high atomic number (82) and is very good at absorbing x-rays.

It makes sure the dose rate in the vicinity Is not > than 7.5 uSvh^-1 i.e. it means that people working with the x-rays will not get doses greater than what is legally allowed.

Question 21

Why do we want to filter x-rays?

Answer 21

We only want high energy x-rays to meet the patient as these are able to penetrate the tissues and still reach the image receptor.

Low energy x-rays will just be absorbed by the tissue so we want to filter these out

Question 22

How are x-rays filtered in an x-ray tube head? How does it work?

Answer 22

We use a disk of aluminium

aluminium has a low atomic number meaning it is good at interacting with low energy x-rays but high energy x-rays will still get through it.

Question 23

How thick should the disk of aluminium be for

a) < or + 70kV
b) >70kV

Answer 23

a = 1.5mm thick

b = 2.5 mm thick (panoramics)

Question 24

Describe a collimator (what it does and what its made from).

Answer 24

It is made from lead and is circular or rectangular

It reduces the dose of X-ray’s given to the patient

Question 25

What is the max diameter allowed for circular collimation at the patient end of the spacer cone?

Answer 25

60mm

Question 26

Should rectangular or circular collimation be used and why?

Answer 26

Rectangular as it reduces the dose further (less mm²)

30% reduction in x-ray dose

Question 27

What are spacer cones/beam-indicating devices and what do they do?

Answer 27

These can be circular or rectangular and controls the target (focus) to skin distance (fsd).

Question 28

What do we want the focus to skin distance to be?

Answer 28

10cm for <60kV and 20cm for > or = 60kV (should therefore always be 20cm)

Note: there is an external marker on the cone to measure this

Question 30

Why do you want to use a long x-ray focus-skin distance?

Answer 30

• in order to reduce magnification.
• The longer the distance the more parallel beams so less magnification and distortion.

Note: a ‘long spacer cone’ = a long fsd.