Are xrays longitudinal or transverse?
Transverse
What is the function of the cathode in the xray tube?
Produce electrons
How are electrons produced in the xray tube
Thermionic emission in the xray tube. Current is passed through tungsten filaments –> heat –> valence electron in the tungsten excites to the surface
Why is tungsten used in the cathode?
1) Can machine it very thin so high surface area to have electrons at
2) High melting point
3) Hight atomic number (lots of valence electrons)
What material is used to make the focusing cup in the cathode and why?
Molybdenum or nickel because high melting point but not a good thermionic emitter
What causes the electrons to be accelerated from the cathode to anode?
Large voltage difference between the negative cathode and positive anode.
What is the focal spot?
This is the point on the anode where the electrons hit and produce xrays.
What is the relationship between anode angle and focal spot?
The larger the anode angle, the larger the focal spot
What percentage of electron energy is converted into heat and xrays respectively?
99% heat
1% xray
Name 8 ways anode heat is dealt with.
1) Rotating anode
2) Anode made of tungsten
3) Increased anode angle so larger focal spot (more distribution)
4) Add rhenium to anode surface to prevent cracking
5) Oil in vacuum glass housing
6) Reduce exposure time
7) Reduce filament current
8) Reduce tube potential
What relationship does the line focus principle show?
The relationship between anode angle, actual focal spot and effective focal spot
If you decreased anode angle, would you get a wider or more narrow effective spot
More narrow
Why is a narrow effective focal spot beneficial?
Sharper images
How do you get a more narrow focal spot?
1) Decrease anode angle
2) Select smaller filament on cathode (decreases height of electron beam)
If you want to image a larger body part, what would you do with the anode angle? What would be the compromise?
Increase the anode angle and therefore field size. But wider effective focal spot would mean more geometric blurring.
Where in the field is the effective focal spot?
In the middle
Do you get a wider effective focal spot closer to the anode or cathode?
Closer to the cathode
Do you get more geometric blurring closer to the anode or cathode?
Closer to the cathode
What does the anode heel effect describe?
Difference in xray intensity between anode and cathode side of beam
Are xrays more intense on anode or cathode side? What is this due to?
More intense on anode side because of anode heel effect
If anode angle is increased, what effect does this have on beam intensity? Why?
More intense at anode side due to heel effect because reduced distance through heel.
If anode angle is reduced, what effect does this have on beam intensity? Why?
Less intense at anode side due to heel effect because increased distance through heel.
What is the Source and Image in Source Image Distance?
Source = inside anode
Image = where detector is behind patient
If you increase SID, do you increase or decrease anode heel effect?
Decrease anode heel effect
