2 - X-ray production

Question 1

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

Answer 1

• tube head with X-ray tube
• collimator
• positioning arm
• control panel
• circuitry

Question 2

What are the components of the X-ray tube?

Answer 2

• glass envelope (vacuum inside)
• cathode
• anode

Question 3

What are the components of the cathode?

Answer 3

• negative end
• filament
• focussing cup

Question 4

What are the components of anode?

Answer 4

• positive end
• target
• heat dissipating block

Question 5

Describe how the filament works.

Answer 5

• coiled metal wire
• low voltage, high current electricity passed through wire
• heats up to 2200c
• electrons released from wire by thermionic emission
• cloud of electrons form around cathode

Question 6

What is the filament made of?

Answer 6

Tungsten

Question 7

What are the properties of tungsten?

Answer 7

• high melting point (3422c)
• high atomic number (74)
• malleable and ductile

Question 8

Describe the focussing cup.

Answer 8

• curved metal plate around filament
• negatively charged so directs electrons at target
• made of molybdenum

Question 9

Describe the relationship between the cathode and anode.

Answer 9

• electrons formed at the cathode are directed at the anode by the focussing cup
• electrons with high kinetic energy collide the anode target

Question 10

Describe the target.

Answer 10

• located in the anode
• metal block made of tungsten
• bombarded with electrons to produce photons
• angled increase SA but direct photons in correct direction

Question 11

Describe the heat dissipating block.

Answer 11

• made of copper
• lost of excess heat produced by electrons hitting target
• reduces risk damage to target

Question 12

What are the properties of copper?

Answer 12

• high melting point (1085c)
• high thermal conductivity (removes heat away from target)

Question 13

Describe the emission of x-ray photons.

Answer 13

• omnidirectional
• divergent

Question 14

What is the penumbra effect?

Answer 14

• blurring of a radiographic image due to focal spot not being a single point
• x-rays diverge from several points
• reduced by reducing size of focal spot

Question 15

Describe the glass envelope.

Answer 15

• air tight vacuum enclosure
• supports cathode and anode
• made of leaded glass to absorb x-ray photons, unleaded window in desired direction of photons
• vacuum prevents air molecules interfering with movement of electrons

Question 16

What are the components of the tubehead?

Answer 16

• X-ray tube
• metal sheilding
• aluminium filtration
• oil
• spacer cone

Question 17

Describe the aluminium filtration.

Answer 17

• removes the lower energy x-rays
• aluminium is able to absorb photons (attenuate)
• aluminium must be at least 1.5mm when x-ray machine operates at < 70 kV

Question 18

Why are lower energy x-rays problematic?

Answer 18

• non diagnostic
• cannot travel through desired tissues to receptor
• photons will still be absorbed by tissues and therefore increased dose

Question 19

Describe the spacer cone.

Answer 19

• dictates distance between focal spot and patient
• focus to skin distance (fsd)

Question 20

What is the required fsd?

Answer 20

> 60 kV = 200mm (modern technology)

Question 21

Describe the collimator.

Answer 21

• lead diaphragm attached to the end of the spacer cone
• reduces patent dose
• crops x-ray beams to size and shape of receptor

Question 22

How does rectangular collimation reduce x-ray dose?

Answer 22

• reduces dose by nearly 50%
• can lead to collimation errors although these are avoided by good technique

Question 23

What are the components of the control panel?

Answer 23

• on/off switch
• electronic timer
• exposure time selector and presets
• warning light and noise
• kV selector

Question 24

What are the outcomes of electrons bombarding the target?

Answer 24

• heat production (99% of interaction)
• x-ray production (1%)

Question 25

How is the heat energy dissipated in the X-ray tube?

Answer 25

- tungsten target - copper block - oil in tubehead - air

Question 26

What are the x-ray producing interactions?

Answer 26

- continuous radiation interaction (majority) - characteristic radiation interaction

Question 27

Describe continuos radiation interactions.

Answer 27

- bombarding electrons pass close to nucleus of target atom - electron rapidly decelerates and is deflected - releases lost kinetic energy as x-ray photons

Question 28

Describe the continuous radiation spectrum.

Answer 28

- the closer the electron passes to the nucleus, the greater the energy released - greater proportion of lower energy electrons - greatest energy achieved when electron collides with nucleus and stops completely

Question 29

Describe characteristic radiation interactions.

Answer 29

- bombarding electron collides with electron in shell and displaces - can displace into different shell (excitation) - can displace entirely (ionisation) - energy released based on electrons dropping down the lower shell, values depending on atom