X Ray Tube Construction

Question 1

What is the difference between x rays and gamma rays

Answer 1

X rays can be switched on and off as there produced electronically

Gamma rays can’t as they are produced by unstable atoms

Question 2

What are the components of the x ray tube

Answer 2

Cathode assembly
Vaccine
Anode disc
Anode stem
Rotor
Bearings
Focal track filament , and glass envelope
Filament set In focus

Question 3

Give me a brief overview of what takes place in the x ray tube

Answer 3

1) cathode filament is heated
Electron are released by thermionic emissions
Electrons are accelerated by tube potential - at a high voltage
Electrons crash into anode and are slowed down - this deceleration emits x rays
Heat is produced along side 2 types of x rays
Heat must be dissipated
After emissions x rays are collimated and filtered before they reach patients

Question 4

Describe the cathode

Answer 4

Is negative
Filament has a source of electrons
Made up of tungsten

Question 5

What happened at the cathode In step 1

Answer 5

Cathode in heated which increases energy of atoms and electrons are emitted

Question 6

why is a focusing cap needed

Answer 6

We need a focusing cap due to filaments which produce a cloud of electrons.

As theses both are opposite in charge the electrons go outwards when we need them to be closely packed together so we use focusing cap

Question 7

What is a focusing cap

Answer 7

It is negative and on the surface of the cathode

It concentrated the electron beams towards the focal spot of the anode

Made up of nickel

As it is negative it helps accelerate the electrons towards the anode

Focal cap is coiled which gives it a high surface area helping with the coverage of the electrons onto the focal point -(otherwise the electrons would spread out so only some would reach the target)

Question 8

Why do we need a vaccume

Answer 8

As the electrons are accelerate ,if there are particles in the way the electrons will lose energy and scatter

So we remove the particles

Question 9

Why is there a glass envelope in the tube

Answer 9

Contains a vaccume

Question 10

What are the 2 types of x rays

Answer 10

Bremstrahhlung radiation
Characteristic radiation

Question 11

What does the anode look like and why does it look like this

Answer 11

Disk
Slightly angles
Rotates - spreads the area the electrons are hitting so that the heat is distributed - too much heat will damage the tube and melt the wires

Question 12

At the end of x ray production what is more formed

Answer 12

Less than 1% of energy is converted to c rays
And as heat can damage tubing heat dissipation is built into the design- e.g shape of anode

Question 13

What is the equation for heat

Answer 13

W x kVp Xmas

KVe- effective kV- potential difference
W - waveform of voltage
KVp- peak kV
MAs- current exposure time product

Question 14

What is heat capacity and what is desirable for the anode

Answer 14

Temperature rises with heat input

Temp rise = energy applied/ heat capacity

High heat capacity desirable for anode so it wont melt and cool down to another shape

Question 15

What increases heat capacity

Answer 15

Removing more heat

Question 16

What effects heat capacity in terms of equipment in the tube

Answer 16

Rotating anode:
Disk diameter- wider the diameter the larger the circumference you are spreading the electrons

Speed of disk rotation

Target angle
Oil

Question 17

What materials are we looking for when constructing the tube and why

Answer 17

Tungsten
High atomic mass- more electrons to interact with
High melting point
High thermal conductivity- to dissipate heat
Low vapour pressure at working temp- reduces sublimation which damages tube ( transfer of substances from solid state to gas)

Question 18

When d we see a stationery anode

Answer 18

Dental x rays so electrons only hit one area

Question 19

Why does target angle help with heat distribution

Answer 19

Angle increases the area of heat distribution
The focal spot is small as possible - more precise x ray beam so increases image quality

However a small focal spot we won’t dissipate heat as well as a large focal spot

Question 20

What is the target stem ……..

Answer 20

It is a molybdenum disk with tungsten target
This molybdenum stem is a poor heat conductor -present heat from escaping or entering temperature sensitive areas like the bearing

Question 21

Why is the x ray tube submerged in and why

Answer 21

Is housed in oil which dissipates heat
Can leak

Question 22

Why is the casing of the x ray tube important

Answer 22

Protects tube and shields it
So x ray is directed into one specific and wanted place

Question 23

What is the casing made out of

Answer 23

Aluminium or steel
Heat and electrical insulator
Shields from radiation going in wrong direction
Protects components

Question 24

Why is filtration needed

Answer 24

No all energy of x ray beam is useful
So this filtered useless energies
These photons can contribute to dose received to patient but not the image produced as they would be absorbed by patients

So the low ebergues are filtered

Question 25

What is the anode heel effect

Answer 25

Beam on cathode side in larger as it has much higher intensity of photons

There’s not much material however on the anode side the beam goes through a lot of material

Question 26

What are Collimation and what do they do

Answer 26

Restrict the span of x ray beams
Made out of lead shutters
It blocks part of the beam

Question 27

What happened after emission of x rays

Answer 27

X rays are collimated and filtered before they reach the patient

Question 28

Why do we need to cool the anode before the x ray image is taken

Answer 28

Anode still need time to cool otherwise we will worn out anode which can lead to inconsistent out put and bad image quality

Question 29

Why do we need to cool the anode before the x ray image is taken

Answer 29

Anode still need time to cool otherwise we will worn out anode which can lead to inconsistent out put and bad image quality

Question 30

What are the factors that effect x ray production

Answer 30

The potential difference- the faster the electron are accelerated the harder it’ll hit the anode and slow down
Smaller focal spot area make x ray beam more precise

Question 31

What is the light beam diaphragm

Answer 31

Enables you to see where you x ray beam is directed and how it is collimated
Decreases the area exposed results in higher image quality: reduces amount of scatter; reduces dose to patient; affects post processing