X-ray Tube Flashcards
What does the X-ray tube cause?
Causes process of ionization to occur
On the X-ray tube, what charges are anode and cathode?
Anode= positive
Cathode= negative
Where does radiation creation start?
At the control panel
What do you need in order to produce x-ray?
- Source of electrons
- Acceleration of electrons (get them moving)
- Decelerate the electrons (stop them)
What does the X-ray tube do?
Provides the electrical productions of x-rays
What are the 4 parts the X-ray tube consists of?
- Cathode (negative side - will provide us electrons)
- Anode (positive side - attracts protons)
- Protective housing
- Evelope (glass)
What are the functions of the cathode/negative side?
- Produce thermionic cloud
- Conduct high voltage to gap between anode and cathode
- Focus the electron stream as they travel through anode side
What’s included in the cathode assembly?
- Filament
- Focusing cup
- Associated wiring
- Filament circuit
- High voltage circuit
What is the main metal of the filament material?
Tungsten (atomic # 74)
What are the properties of tungsten?
- High melting point
- high atomic #
- diffucult to vaporize
- heat conducting ability
What is the melting point of tungsten?
3370
What are other good choices for filament material besides tungsten?
Rhenium and molybdenum
Describe filament
Small coil of tungsten
What does tungsten do?
- Acts as a resistance to flow of electrons
- Produces heat by electrons flowing through the coil
What is thermionic emission?
Boiling off electrons
What is the minimum heat to boil off electrons/for thermionic emission to occur?
2,220°C minimum
Describe thermionic cloud (electron cloud)
Made from electrons boiled off the filament
- Electrons are attracted to the anode (+) and accelerates to the anode side
note: Some electrons flow back to the filament wire. A very small amount of electrons vaporize from the filament line and glass envelope (gassy tube)
How many filament wires does the X-ray tube have?
2
Dual focus
Allows for small and large focal spot size
The smaller the FSS…
The better the detail
What is the focusing cup made out of?
Nickel
What is the purpose of the focusing cup?
- To streamline the thermionic cloud as its driven towards the anode
- house the filament coils
What kind of charge does the focusing cup have?
Negative
What is the space charge effect?
As more and electrons are “boiled” off the filament, their negative charges begin to oppose the emission of additional electrons
How is saturation current achieved?
When there are no further thermionic electrons to be driven towards the anode. As kVp increases, a greater percentage of electrons are driven towards the anode
What are the functions of the anode side (positive side)?
- Attracts the electrons to the anode side
- Serves as a target to stop electron stream
- Source of photons (x-radiation)
- Serves as primary thermal conductor
What are the components of the anode assembly?
- Target
- Stator
- Rotor
What is the job of the target/anode target?
Decelerate electrons
What are the 2 types of targets?
Stationary and rotating
What is the x-ray angle of the target?
7-17° angle — common is 12°
What metals are included in anode layering?
Tungsten (rhenium is added to allow expansion of the focal track when rapidly heated)
&
molybdenum and/or graphite (allows for heat cunducting)
Describe the rotating target
Disc-shaped
5-13 cm in diameter
With a rotating target, the faster the rotation…
The better heat dissipation
What is the average target rpm of the rotating target?
What is the fast speed target rpm of the rotating target?
Average Target= 3200-3600rpm
Fast Speed Target= 10,000-12,000 rpm
What is the temperature of the focal tract after production of photons?
1,000 to 2,000° average
What are the portions where the electrons strike the target called?
- Focal track
- Focal point
- Focal spot
- Source of photons
What is the line focusing principle?
Allows the best resolution (detail) while permitting a large area of the focal track to be struck by electrons
(why? Because a minimum heat 1,000-2,000°C created)
What happens during the stationary line focal principle?
Target angle decreased, effective focal spot size decreased
Same target angle: smaller FSS…
Smaller the EFSS = better detail
What does the line focus principle do?
- Allows for
What is the anode heel effect?
Anode heel effect is more prominent at…
What does the stator do?
Describe stator
When the rotor switch is activated, what happens?
If the stator fails…
Describe rotor
What is the common/average speed of rotor?
What is the highest speed of the rotor?
Common= 3,200-3,600 rpm
high speed= 10,000 to 12,000 rpm
Describe the envelope
- Pyrex glass or metal
- 10” ling
- 6” central diameter
- houses tube part
- the window
- maintains a vacuum
What does the window do?
Allows for photons to pass out of tube
(thinner section of the envelope, less absorption of the photons)
What does the vacuum do?
- Removal of the air permits electrons to flow from cathode to anode without interacting with gas atoms
- increases the radiation output or efficiency
What was created in the late 1930s?
X-ray tube
What type of exam was done in the 1940s?
What does the protective housing do?
- Controls leakage and scatter radiation
- isolates high voltages
- provides means to cool the tube
The housing is made of what to control leakage radiation and scatter radiation?
What is leakage radiation?
Any photons that escapes from the housing except the window
What happens during leakage radiation?
- Radiation emitted from the target isotropically (in all directions)
- primary beam (useful beam) exits out of the window
What must leakage radiation not exceed?
What fills the space between envelope and housing in high voltage isolation and tube cooling?
Dielectric oil
What does dielectric oil act as?
How is off-focus radiation created?
From photons created in the tube somewhere besides on the focal track “ghostly image”
What does the warm up procure do?
- Gradually warms the anode
- prevents cracking
- helps maintain vacuum
Recommendations for extending tube life
