X-ray Production

Question 1

What are the basics of X-ray production?

Answer 1

Electrons accelerate towards atoms at high speed
On collision, the kinetic energy of the electrons is converted to heat and EM radiation (ideally X-ray photons)
The X-ray photons are aimed at a subject

Question 2

What are the main components of the dental X-ray unit?

Answer 2

Tubehead (with X-ray tube)
Collimator
Positioning arm
Control panel
Circuitry

Question 3

What is found inside the X-ray tube?

Answer 3

Glass envelope with vacuum inside
Cathode (-ve)
Anode (+ve)

Question 4

What does the cathode filament do?

Answer 4

Low voltage, high current electricity is passed through the wire heating it up
Heat and electrons are released

Question 5

What does the cathode focusing cup do?

Answer 5

Repels electrons released at the filament
Focuses the electrons at a small point on the anode target

Question 6

What is the cathode made of?

Answer 6

Filament - tungsten
Focusing cup - molybdenum

Question 7

Describe the relationship between the cathode and anode

Answer 7

High-voltage electricity passes through the X-ray tube
Electrons released from the filament are repelled from the cathode and attracted to the anode
Electrons have high kinetic energy upon colliding with the anode target

Question 8

What do the transformers do?

Answer 8

Take mains electricity supply and convert it by changing the voltage and current

Question 9

What are electron volts (eV)?

Answer 9

Unit used to measure the kinetic energy gained by electrons as they accelerate from cathode to anode
1eV = kinetic energy gained by 1 electron moving across a potential difference of 1 volt

Question 10

What is the anode made from?

Answer 10

Target - tungsten
Heat-dissipating block - copper

Question 11

What is the anode target?

Answer 11

Metal block bombarded by electrons to produce photons and heat

Question 12

What does the anode heat-dissipating block do?

Answer 12

Surrounds the target
Heat produced in target from electron collisions dissipate into the block by thermal conduction to reduce risk of overheating

Question 13

What is the penumbra effect?

Answer 13

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

Question 14

Why is focal spot angulation used?

Answer 14

We need a small focal spot but this means less kinetic energy is converted to photons and more to heat. A larger focal spot solves this but decreases image quality
Angling the target increases the actual surface area where electrons impact (increasing heat tolerance) while reducing the apparent surface area from the where the X-ray beam is emitted (reducing penumbra effect)

Question 15

Describe the glass envelope

Answer 15

Air-tight enclosure to support the cathode and anode while maintaining a vacuum
Made of leaded glass to absorb X-ray photons so only the photons travelling in the desired direction can escape the X-ray tube

Question 16

What are the main components of the tubehead?

Answer 16

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

Question 17

What does filtration do?

Answer 17

Removes lower energy X-rays from the beam - these would increase dose but not contribute to image

Question 18

What does the spacer cone do?

Answer 18

Dictates the distance between the focal spot of target and the patient

Question 19

What sizes of spacer cones are needed?

Answer 19

If <60 kV then 100mm
If >60 kV then 200mm

Question 20

What thickness aluminium is needed for filtration?

Answer 20

If <70 kV then 1.5mm
If >70 kV then 2.5mm

Question 21

What is the collimator?

Answer 21

Lead diaphragm attached to the end of the spacer cone
Crops the X-ray beam to match the size and shape of the X-ray receptor

Question 22

Why is rectangular collimation used?

Answer 22

Reduces effective dose to patient by about 50%

Question 23

Describe continuous radiation

Answer 23

Produces a continuous range of X-ray photon energies
Maximum photon energy matches the peak voltage
Bombarding electron interacts with nucleus of target atom

Question 24

Describe characteristic radiation

Answer 24

Produces specific energies of X-ray photon, characteristic to the element used for the target
Photon energies depend on the binding energies of electron shells
Bombarding electron interacts with inner-shell electrons of target atom

Question 25

Describe the dental X-ray beam spectrum

Answer 25

X-ray beam = continuous radiation + characteristic radiation - filtered photons