S2 L4 Flashcards
Describe cathode
Has fine and broad focus filament
Makes electrons by thermionic emission
Function of focusing cup
Focus e- on surface of anode so prevents them repelling and stream diverge
Done by applying negative potential
Types of anodes
Fixed
Rotating
Describe fixed anode
Electron focused on focal spot
Only in dental radiology
Describe rotating anode
Disc with thin rim of tungsten
Spins 3k to 10k rpm
Overcome overheating as different areas exposed to e- stream
Why is cathode and anode made of tungsten
Good at emitting elections (high z)
Can make eye thin wires
High melting point
Very strong (withstand rpm)
Low rate of evaporation
Why is anode stem made of molybdenum
Poor heat conductor so prevent heat transmission to metal bearings
Structure and function of tube envelope
Glass (Pyrex) and evacuated metal envelope
Insulate and support anode and cathode
Electrons travel cathode to anode
Why is the glass envelope made of those materials
Filament won’t evaporate
Doesn’t impede electron emission at cathode
Low X-ray absorption
High temp resistance
Electrical insulator
Permeable to radiated heat
What is the window for
X-rays exit through it to expose patient
HT cables in anode and cathode
3 conductors
In anode they’re connected and parallel = 1
In cathode they connect broad and fine focus circuit
Describe X-ray tube induction motor
Rotor is solid bars of copper
Stator is outside and has electromagnetic windings that make a rotating magnetic field inside rotor causing rotation
What is focal spot blurring
Edge becomes blurred due to size of focal spot and object distance to detector
How to decrease focal spot blurring
Lower object to detector distance
Lower focal spot size (this also lowers max exposure)
Lower focal spot to object distance
What is anode heel effect
Photons made at the surface more likely to escape so those made deeper have a lower intensity as more attenuation
Function of oil
Cools via convection
Electrical insulation between anode and cathode
What is inherent filtration
Photons pass through anode, exit window, oil which attenuates
What is minimum total filtration
No less than 2.5mm aluminium
What is additional filtration
Material between tube window and Collimation assembly to achieve total filtration
Why is total filtration good
Eliminates low energy xrays that increase dose but don’t contribute to image
Why is anode angle important
Decrease effective focal spot size so reduce focal spot blurring
Less target material perpendicular to anode so less resorption of X-rays
Larger angle increase field coverage
Pros and cons of large focal spot size
Heat dissipates in larger area so protection for tungsten target
Bad image quality
Effective vs actual focal spot
Actual - where electrons strike anode surface
Effective - area of beam as directed toward patient
What is lime focus principle
Larger anode angle means larger effective focal spot size
