X-Ray Production

Question 1

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

Answer 1

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

Question 2

what is the x-ray tube composed of?

Answer 2

• glass envelope
• cathode
• anode

Question 3

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

Answer 3

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

Question 4

explain the electricity that passes through the cathode?

Answer 4

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

Question 5

what is the filament in the cathode made of?

Answer 5

tungsten
- high melting point (3422degrees celsius)

Question 6

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

Answer 6

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

Question 7

what does a higher current in the filament lead to?

Answer 7

• more heat
• more electrons

Question 8

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

Answer 8

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

Question 9

What is the focusing cup of the cathode?

Answer 9

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

Question 10

Why is the focusing cup a specific shape?

Answer 10

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

Question 11

What is the focusing cup of the cathode made of?

Answer 11

Molybdenum

Question 12

What is the relationship between the cathode & anode?

Answer 12

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

Question 13

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

Answer 13

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

Question 14

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

Answer 14

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

Question 15

What transformers are present in the tubehead?

Answer 15

• step up transformer
• step down transformer

Question 16

What is the function of the step up transformer?

Answer 16

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

Question 17

What is the function of the step down transformer?

Answer 17

lowers potential difference across filament to 10V

Question 18

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

Answer 18

Electron volts (eV)

Question 19

What does 1eV mean?

Answer 19

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

Question 20

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

Answer 20

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

Question 21

What is the focal spot of the target?

Answer 21

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

Question 22

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

Answer 22

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

Question 23

What is the heat dissipating block made of?

Answer 23

Copper

Question 24

What is the Penumbra effect?

Answer 24

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

Question 25

How can the penumbra effect be minimised?

Answer 25

shrinking size of focal spot

Question 26

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

Answer 26

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)

Question 27

What is the function of the glass envelope?

Answer 27

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

Question 28

What are the main components of the tubehead?

Answer 28

- X-ray tube - Metal shielding - Aluminium filtration - Oil - Spacer cone

Question 29

What is used for metal shielding?

Answer 29

Lead - absorbs X-rays

Question 30

Why is filtration necessary?

Answer 30

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

Question 31

What is used to filtrate low energy x-rays?

Answer 31

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

Question 32

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

Answer 32

1.5mm

Question 33

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

Answer 33

2.5mm

Question 34

What is the function of the spacer cone?

Answer 34

- Dictates distance between focal spot (of target) & patient (focus to skin distance) - Indicates direction of beam

Question 35

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

Answer 35

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

Question 36

What occurs if fsd is increased?

Answer 36

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

Question 37

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

Answer 37

200mm

Question 38

What is the collimator?

Answer 38

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

Question 39

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

Answer 39

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

Question 40

What are the consequences of the electrons bombarding the target?

Answer 40

- Heat production - X-ray production

Question 41

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

Answer 41

heat production = 99% of interactions X-ray production = <1%

Question 42

how is continuous radiation produced?

Answer 42

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

Question 43

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

Answer 43

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)

Question 44

how is characteristic radiation produced?

Answer 44

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

Question 45

In characteristic radiation, what does the photon energy equal?

Answer 45

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

Question 46

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

Answer 46

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

Question 47

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

Answer 47

K shell electrons