Topic 3: X-ray tube

Question 1

How is X-ray produced?

Answer 1

• High energetic e- interact with matter
• Conversion of KE > EM = 99% energy lost to heat

Question 2

What does a device that produces X-rays in diagnostic energy range contain?

Answer 2

• e- source
• Evacuated path for e-acceleration
• Target electrode
• External power source = provide high voltage > accelerate e-

Question 3

What does an X-ray tube insert contain?

Answer 3

• e- source
• Target in evacuated glass/metal envelope

Question 4

What does the tube housing provide?

Answer 4

• Protective radiation shielding
• Cools X-ray tube insert

Question 5

What does an X-ray generator do?

Answer 5

• Supplies voltage = accelerate e-
• Permits control of X-ray bean characteristics

Question 6

Define collimators

Answer 6

• Define size + shape of X-ray field

Question 7

What are the qualities of the produced X-ray photons?

Answer 7

• Well-defined intensity
• Penetrability
• Spatial distribution

Question 8

What are the qualities of X-rays?

Answer 8

• Invisible
• Can not be heard
• No odour
• Not effected by electric/magnetic fields

Question 9

What type of photons are X-rays?

Answer 9

• Ability to ionize other substances
• Cause atoms that they pass through > eject e- from their e- shells
• Accounts for imaging properties + harms

Question 10

How can X-rays be detected?

Answer 10

• Ability to ionize other substances
• Cause fluorescence
• Color change in substances e.g. barium platinocyanide
• Produce changes visible in film

Question 11

When are rotating/stationary anode X-ray tubes used?

Answer 11

• Rotating: most X-ray equipment + major radiology equipment
• Stationary: only in dental equipment + small portable machines

Question 12

2 components of X-ray tube

Answer 12

1) Insert = evacuated + where X-ray produced
2) Tube shield = supports insert + responsible for electrical + radiation safety

Question 13

Function of X-ray tube

Answer 13

1) Provide beam from as near point source as possible
2) Dissipate heat produced = prevent damage to tube
3) Provide consistant quality + quantity of radiation
4) Allow X-ray to emerge only from window of tube
5) Provide electrically safe environment for practitioner
6) Tube securely supported = easy movement into any position + maintain position

Question 14

What is the tube housing made from?

Answer 14

Steel construction lined with lead

Question 15

What is the port made from?

Answer 15

Plastic/beryllium

Question 16

What is the insert/tube envelope made from?

Answer 16

Metal/ceramic/borosilicate glass

Question 17

What is the filament/cathode assembly focuser made from?

Answer 17

Nickel/stainless steel

Question 18

What is the filament made from?

Answer 18

Tungsten

Question 19

What is the anode disk made from?

Answer 19

• Molybdenum alloy/graphite disc
• Tungsten 90% + rhenium 10% = alloy prevents crazing effect

Question 20

What is the anode stem made from?

Answer 20

Molybdenum

Question 21

What are the stator windings made from?

Answer 21

Copper

Question 22

What is the additional filtration made from?

Answer 22

Aluminium + copper

Question 23

Function + structure of the insert

Answer 23

• Maintains vacuum X-ray production
• Contains: anode + cathode assembly
• Metal component = earthed = no static buildup
• Cathode + anode assembly = fixed to envelope = supports electrodes in correct alignment + distance

Question 24

Describe the seals + poles of the insert

Answer 24

• Carefully chosen to match expasnsion coefficients = heat of different parts
• Reduces risk of damage to insert

Question 25

Structure of anode assembly

Answer 25

- Anode disc + focal track - Anode stem - Rotar assembly - HT connection for +ve side of tube circuit - Rotating anode = tungsten rhenium disc = 90-150mm diameter - Large disc = increases tube rating + thermal capacity

Question 26

What is the structure of the anode disc?

Answer 26

- Tungsten + rhenium focal track - Graphite/molybdenum backing - Bevelled edge = forms anode angle + focal track = to decrease size of apparent focal spot

Question 27

What is the typical angle for rotating X-ray tubes?

Answer 27

16-20 degrees

Question 28

What is the structure of anode stem?

Answer 28

- Anode attached to molybdenum stem = connects to rotors - Stem = small cross-section = long as possible = restrict conduction of heat to bearing assembly - Stem connected > copper rotor assembly

Question 29

What speed do rotors rotate the anode at?

Answer 29

- Rotors use induction to rotate at 3000-9000 r.p.m

Question 30

Structure of cathode assembly

Answer 30

- Filaments - Focusing cup = -ve charge - Electrical supply + connections = filament supply + HT supply to -ve side of X-ray tube circuit

Question 31

What is the purpose of the cathode?

Answer 31

- Produce thermionic emission of e- = focused on + attracted to anode - Thermionic emission achieved by supplying variable mA = heats filament

Question 32

Describe the filament

Answer 32

- Tightly coiled wire - Increase mA = increase temp of filament - Increases number of e- in e- cloud - 2x mA from 100 - 200 = 2x e-

Question 33

What is the relationship between mA and number of X-ray?

Answer 33

mA directly proportional to number of X-ray produced

Question 34

Describe the focusing cup

Answer 34

- Filament encased by nickel housing = focuses e- onto focal track of anode - Achieved by having sharp edges > focusing cup - Edges = -ve charge when tube voltage applied - Sharp edges = concentrate -ve charge - Narrows e- bean directed > anode