X-ray Production and tube

Question 1

What impacts quality

Answer 1

• Tube voltage (kvp)- the Max photon energy is increased and the intensity is at a higher photon energy when kvp is increased
• Filtration- gets rid of low energy photons and contributes only to dose, there is inherent and added filtration so the quality is improved due to more effective energies
• rectification-

Question 2

What impacts intensity

Answer 2

• the tube current (mA)- the maximum intensity is still at the same point along the energy scale and intensity is directly proportional to the mA
• the tube voltage (kvp)- intensity increases and intensity is directly proportional to kVp2
• filtration- the intensity is reduced due to the fact the low intensity photons are absorbed
• distance from focus- the greater the distance the lower the intensity

Question 3

What does the filament do

Answer 3

Generates the electrons

Question 4

What does the anode do?

Answer 4

Arrests the electrons and produced x-Ray photons

Question 5

What does the tube envelope do?

Answer 5

•it may be glass and maintains the vacuum

Question 6

What does the tube housing do?

Answer 6

Enclosed the glass envelope and contains the tube oil

Question 7

Why tungsten for the target material?

Answer 7

• High atomic number meaning that it’s efficient for X-ray production- especially bremsstrahlung
• high melting point so can deal with the heat
• low vapour pressure so metallic tungsten isn’t released into the coat tube
• malleable so can be easily shaped

Question 8

Why tungsten for the filament metal?

Answer 8

• Low outer shell binding energy so good for producing thermionic emission- It’s a thermionic emmitor
• high melting point so won’t be damaged
• low vapour pressure so doesn’t release metallic tungsten to coat tube
• ductile so can be made into wire easily
• low work/function binding energy meaning lots of electrons can be emitted at low temperatures

Question 9

Why are anode discs being made from molybdenum and carbon fibre (graphite)

Answer 9

Carbon- lighter so less energy to rotate, better heat storage capacity and that the black colour helps with the radiation of heat
Molybdenum- better heat storage and reduced weight for rotation as it has a lower density than tungsten better specific heat capacity which means there is a lower temperature rise

Question 10

Why would you use molybdenum for the anode stem

Answer 10

• As it has a high melting point
• although it has a lower thermal conductivity than tungsten this difference gets smaller the hotter it gets so it plays a smaller factor in the decision here

Question 11

Why is copper used for the anode block

Answer 11

In stationary anodes and it has a better thermal conductivity compared to tungsten meaning that it’s a good conductor of heat. Also the anode has a large mass so it can absorb more heat.
•tungsten is the target as copper has a low melting point and proton number

Question 12

What is the anode heel effect

Answer 12

•Because X-rays are produced below the surface they have to travel through tungsten and as some travel through more the strength of the beam varies slightly

Question 13

What side is more intense due to the anode heel effect

Answer 13

The cathode side side as more of the x-day beam is absorbed towards the anode side of the tube

Question 14

What is the line focus principle

Answer 14

•Used to avoid geometric unsharpness you use the smallest possible apparent focal size
• a=rsin0
Where
a=apparent focus
R= length of real focus
0= anode angle
•but the real focus may be reduced and the tube loading

Question 15

What is the inverse square law?

Answer 15

• Intensity is inversely proportional to distance squared

* The source of radiation is a point source so infinitely small