x-ray tube structure and operation Flashcards
outer tube housing
- x-ray tube housing
- ceiling suspended
- movable (x.y.z)
- collimators
- light beam diaphragm
- filtration
- ionisation chamber
- pre locked positions for bucky centring
filters and collimation
- continuous x-ray spectrum means it contains low energy x-ray photons (will not contribute to the image only to dose= removed)
- achieved thru using 2.5mm Al filter (has low atomic number so attenuate low level x-ray photons
- collimator consists of light beam diaphragm which allows the operator to view the area that will be irradiated by the primary x-ray beam
- Lead shutters attenuate the primary beam outside of the area of visible light
- light is directed out of the collimator by a mirror, so it does not interfere with the x-ray
ionisation chamber
- gas filled
- proportion of incident photons cause ionisation which is collected at charged anode of the chamber
- measures dose area product (DAP) in cGy.cm2
tube housing
•Physical protection
•Radiation protection
•Electrical protection
•Steel +/- lead lining
•Exit window
•Oil filled
•Heat dissipation
•Electrical Insulation
x-ray tube
•function of the X-ray tube is to take the energy from accelerated electrons and convert this into X-rays
•The electrons are produced at the cathode
•The X-rays are produced at the anode target
cathode
•Source of electrons therefore on negative side of tube
•Made up of two components:
•Filament(s)
•Focusing cup
cathode filament
- source of electrons
- produduced by thermionic emissions from tungsten filament heated by an electric current in cathode
- 6 x 10^17 e-s travel from cathod to anode of the x-ray tube every second
- accelerated from cathode to anode across high voltage
broad and fine focus
- dual filaments offer 2 effective spot sizes
broad: - required for imaging larger textures as it disperses the e- beam across greater area of anode
- cause better heat dispersion but negatively affects image sharpness
fine: - preferable as produces less image unsharpness
- greater thermal loading of anode so used for small areas
fine focus
- high detail required
- low patient attenuation
- hand/ foot
broad focus
- high patient attenuation
- less detail required
- abdomen/ lumbar spine
focussing cup
•Controls stream of electrons hitting the anode
•Negative charges repel each other
•As the focusing cup will hold the filament as the current passes through it, it needs to be designed in such a way that it will not also participate in thermionic emission, this is
- achieved by constructing it from either nickel or stainless steel which both have high melting points and high thermionic work functions
anode
• anode target is the component in which the X-rays are produced.
- anode itself is a basically large piece of metal (and/or graphite) that connects to the positive side of the electrical circuit.
The anode assembly has three primary functions:
• convert electron energy into X-rays (at the target track)
• dissipate the heat created in the process (thermal conductor)
• act as an electrical conductor
• the material for the anode has to be selected carefully
anode targets
• Site of X-ray photon production
Materials:
• Tungsten (often alloyed with Rhenium as this can reduce thermal damage)
• Molybdenum also for lower photon energies
Properties:
• High atomic number (74) increases probability of electron interaction
• High melting point (>3000C)
• Thermal conductivity for fast cooling
• The specific area on the anode where electrons strike is called the focal spot or focal area
anode x-rays and thermal energy
• Less than 1% of the electron energy is converted into X-ray photons.
• 99% is converted
The amount of the total electron energy that is converted into X-rays at the anode depends on 2 factors:
•1. The atomic number (Z) of the anode material
•2. The energy of the electrons
anode angle
•Smaller real focus: finer effective focus, less heat dissipation
•Smaller anode angle: finer effective focus, smaller beam field size possible
anode heel effect
• Emitted X-rays pass through different thickness of tungsten depending on angle of target
• 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
envelope and vacuum
• anode and cathode must be contained within a vacuum because the presence of gases in the X-ray tube would interrupt the electron beam
• Must be strong, rigid and withstand heating and high voltage environment
• Glass, metal or ceramic
• Contains anode and cathode within a vacuum
• vacuum improves efficiency as electrons are not impeded by air molecules on their way to the target
• vacuum flashover is where there is gas within the tube which causes arcing which can impact tube output
basics of operation
kV - voltage
mA- current the flows thru tube
s - exposure time in seconds
kV
- voltage applied to tube
- established the voltage of the e-s as they reach anode target
- photon can’t be created with an energy greater than that of e-
- max e- energy referred to as kilovolt peak (determine how much radiation will be produced for a given number of e-s hitting the anode)
- important because it determines their kinetic energy and efficiency of x-ray production by bremsstrahlung process (will be increased with increasing kinetic energy)
- high energy end of the spectrum is determined by kV applied
mA
electrical current that passes thru tube
s
exposure time in seconds
x-ray generators
•Mains Socket
•Power from Electric Company
•240 volts
•Oscillating (50Hz)
•AC – Alternating Current
•Required Voltage is 100,000 volts
•Constant Voltage
•DC – Direct Current
•High Frequency
high voltage transformer
Step-up Transformer
- Converts 240 volts to 100,000 volts
•Voltage Increase is Proportional to Turn Ratio
•High Voltage Generator converts low voltage AC from electric company into kilovoltage of the required DC
voltage rectification
Voltage Rectifier
- Converts oscillating voltage to constant voltage
- Move the current in only one direction
Mains power is three phase so this produces
- a “voltage ripple” approaching a constant DC
- supply across the x-ray tube
High frequency generators use high speed switch generators to reduce this voltage ripple (only about 3-4% ripple) so is much more efficient
at exposure
at prep:
- filament current applied to facilitate thermionic emission
at exposure:
- kV potential applied
- enough voltage for cathode e-s to jump to anode and escape cathode
directed by focussing cup
