LECTURE 3: X RAY TUBES 1 Flashcards
What are the 3 types of support for x-ray tubes? Why do we need them?
Ceiling, floor, c-arm. Need them because the x-ray tube itself is very light but the HOUSING for it is very heavy
What is the term used to describe the direction of x-ray emission? What does it mean?
“ISOTROPICAL” - X-rays are produced in 360deg direction
What is the purpose of housing the x-ray tube in heavy material?
This metal/material will absorb x-rays that shoot in uneeded direction
Is the housing for x-ray tubes 100% absorbant?
No- there is still some leakage radiation
How is leakage radiation regulated (normal x-ray)
Leakage radiation measured at 1m from the x-ray source must not exceed 0.1% of the exposure rate at the same distance.
How is leakage radiation regulated (mammography)?
Leakage radiation measured at 5cm from housing and averaged over a detection area of 100cm2 must not exceed 20mGy/hour
There is oil in the x-ray tubes - what does it serve as?
Electrical INSULATOR and heat DISSIPATOR
What is an electrical insulator?
Does not allow e- to move freely through - i.e. oil in x-ray tube
How is heat and pressure maintained in an x-ray tube?
Oil is used as an indicator; heat will cause oil to expand and when this expansion becomes too great, a micro-switch will be activated to inhibit further exposures
How are large and small focal spot connected to each other? What does this mean?
In parallel; meaning when one is turned on the other must be off
What type of glass is used for x-ray tubes? Why? What is a special feature?
Pyrex - can withstand large amounts of heat and does not melt; the glass is very ROUND and there are no sharp edges so it cannot ‘crack’
Where does the pressure come from?
the OUTSIDE as the x-ray tube is in VACUUM so there is no pressure iNSIDE
What is a limitation of using a glass tube?
Metallic coating - glass tubes are affected by this film - can cause arcing
What is arcing? When does this happen?
Arcing means electrical discharge - can occur when metallic film due to heating attaches to glass enclosure
What is wrong with metallic film on glass enclosure?
Can also attenuate x-ray beams, which decreases image quality and affects tube efficiency
Where are the metal-ceramic type enclosures located in an x-ray tube?
- anode, cathode, anode stem
Why are metal envelopes preferred over glass ones?
They are unaffected by the thin films of tungsten formed on their walls and allows for high tube current at shorter exposure times
Why do we prefer rotating anodes?
Gives us more surface area to hit which gives us more interactions; also allows for dispersal of heat
Filament current and tube current - where and which is higher
Filament current is also called the cathode current. Tube current is when there is an exposure. Filament current is measured in Ampere and tube is mA. Filament is higher as it comes directly from high voltage generator
What is the cathode?
The negative terminal and is the source of electrons
How thick and long is a filament tungsten wire?
0.2mm in diameter and 1cm long (coiled)
Why do we use THIN filaments?
We want high resistance to produce heat: more e- produced - THERMIONIC EMISSION
What is thermionic emission?
The generation of e- due to heat generated in the filament giving e- enough thermal energy to move a small distance, heat is due to high resistance because of the filament wire being thin
What is the Edison Effect?
A cloud of e- around the filament due to repulsion of e- to each other; this is following thermionic emission
How can we counteract the edison effect?
Place 2 negative poles beside the filament so that e- is repelled and pushed towards anode
Why is tungsten chosen as filament material?
- High melting point -3k
- Little tendency to vaporise (metallic film)
- Long life expectancy
- Can be draw into a thin strong wire
What is a limitation of tungsten filament? How to counteract?
Not efficient in emitting thermal electrons - because you have to heat up to 2k deg. Fix this by using THORAITED TUNGSTEN (Alloy)
What is the main aim of thoraited tungsten?
To lower the amount of heat required for thermionic emission - this in turn reduces vaporisation of tungsten
What is Emission current density?
The number of e- emitted from a metal depends on temperature … and other things (see formula)
Why is filament wire coiled?
To increase surface area - increases efficiency to emit e-
What is Emission Current Density directly proportional to?
T^2
What is temperature proportional to? Why do we care?
IR^2. So thin wire means more resistance which means higher temp. Higher temp means higher ECD which means more e- emission
What is bad about tungsten coat/metallic film?
- Can cause arcing
- Attenuates x-rays: reduces image quality
What is Standby Current?
Pre-heating the x-ray tube
What is Space Charge Effect?
Cloud of negative charge prevents other e- from being emitted from the filament until they have acquired sufficient thermal energy to overcome the Coulomb force caused by the space charge
What is Space Charge Effect Equilibrium?
Since so many e- leaving filament, filament becomes positively charged and attracts the e- back; we reach a state of EQ where number of e- in space charge remains constant
What are the other terminals of a cathode?
Focusing cup and connecting wires (for voltage and current)
Why is tube current less than filament current?
Because tube current is from heating and filament current is supplied by high voltage generator - 99% is lost to heat! so tube current quite small (mA)
T or F: Number of x-rays produced is proportional to number of e- flowing from cathode to anode
TRUE!!! 99% if heat and 1% x-rays
What is saturation?
At high voltages all available e- have been emitted from around the filament.
Tube current remains constant with increasing voltage
What happens to tube current at saturation?
It remains constant and now only depends on SA!
How can the focusing effect of a focusing cup be enhanced?
By increasing voltage potential
What does the focusing cup acting as a grid mean?
Grid pulsed operation - meaning it can act as a switch to turn x-rays on and off; often used in fluroscopy
Why is filament wire wounded?
To increase its length without widening the bombarded area on the anode
The number of electrons emitted from a metal depends on the metal’s temperature, geometrical dimensions and the material constants.
Correct!
True
To obtain currents in the range of 0.1 and up to 2 A necessary for useful x-ray production for imaging, emitter temperatures around ____ Kelvin are needed
2700
The anode is responsible for thermionic emission
NO- THIS IS THE CATHODE!!!
At saturation the current depends on the cathode temperature and consequently on the filament power.
True - note that this is current, NOT tube current
At saturation, the TUBE current depends only on the emitter’s surface area
True
