X-Ray Tube Flashcards
The two types of x-ray production
Characteristic
Bremsstrahlung
Parts of an x-ray tube
Cathode
Anode
Evacuated glass tube
Vacuum Housing
Requirements of x-ray tube
Controllable
Safe
Point source
Thermionic emission
Electric current causes heating
Above 4 amps electrons boiled off
Cloud of electrons forms around filament
Contained by space charge effect
Why tungsten?
Good thermionic emitter
High melting point (3410 degrees C)
How does the focusing cup work?
Controls stream of electrons hitting anode
Negative charges repel each other
>3kV relative to filament will alter electron flow
Charge can be altered to change focal spot size
What are the functions of the anode and the two types ?
Support target material: source of x-ray photons
Electrical conductor
Thermal conductor
- Stationary and rotating
When would a stationary anode be used?
Dental- low output needed
99.5% of electron energy converted to heat
Benefits of a rotating anode
Increase heat dissipation
Increase anode load
How efficient is the x-ray process?
99.5% of energy lost as heat
Only a small percentage of photons produced exit the window
Of those photons that make up the beam, most are attenuated by the patient
Total energy put into process- 1.3x10-12% creates the image
Structure of the envelope and vacuum of the anode?
Must be strong, rigid and withstand heating and high voltage environment
Glass, metal or ceramic
Contains anode and cathode within vacuum
Vacuum improves efficiency as electrons are not impeded by molecules
Functions of tube housing (protections)
- Physical protection
- Radiation protection
- Electrical protection
Properties of tube housing
- Steel/lead lining
- Exit window
- Oil filled- heat dissipation and electrical insulation
What happens at prep and exposure?
Thermionic cloud developed by current
Rotating anode begins to spin
A large voltage is applied across the c- and a+
Potential energy of electrons becomes kinetic
Cathode repels electron cloud to anode
Problems with pressing the button to expose
If the anode is not up to speed and fully prepped, exposure won’t trigger on full press
Prepping too long can prematurely age the filament as thermionic cloud is maintained
When would Fine focus be used?
Used when image quality is limited by geometric factors
Energy focussed on small area
Need to be aware of tube loading to prevent damage
When would Broad focus be used?
Used when image quality is limited by patient attenuation
Increase heat dissipation
Permits higher tube loading
Anode angle- real and effective
- Effective spot determined by real focus size and anode angle
- Anode angle 7-15 degrees
- Effective spot ~3 times smaller than real focus spot
What does a ‘smaller real focus’ mean?
- Finer effective focus
- Less heat dissipation
What does a ‘smaller anode angle’ mean?
Finer effective focus
Smaller beam field size possible
Explain the ‘anode heel effect’
Emitted X-rays pass through different thickness of tungsten depending on anode angle
X-ray B travels greater distance through target than X-ray A
Results in greater image intensity on cathode end of image
Increases as anode angle is reduced
Light beam diaphragm and collimator benefits
Control dimensions of primary beam
Uses light and mirror to centre rays
Lead shutters
May auto cone to receptor when inserted in bucky
What does filtration do?
Enriches beam with high energy photons by absorbing lower energy x-rays
Usually preset into protocols
‘Beam hardening’
Higher average energy- greater average energy, reduce skin dose
Inherent filtration examples
Envelope
Tube housing
LBD
Mirror
etc
Added filtration examples
For normal x-ray work ~2.5mm aluminium or 0.3mm copper
Compensation filtration examples
Wedge filter
Bow tie filter
What are Ionisation chambers?
Gas-filled ionisation chamber
A proportion of incident photons cause ionisations which are collected at the charged anode
Measures dose area product cGy.cm2
High voltage generator
- High voltage transformer- step up 240V-100kV
- Voltage rectifier- converts AC to DC
- High frequency generator- 50Hz to 500-5,000Hz
