Production of X-rays part 1 Flashcards
What are the two parts of the tubehead?
x-ray tube
power supply (generator)
What buttons are on the control panel?
Exposure time, kVp and mA selectors
What type of exposure button do we use?
dead-man type (if you take finger off it stops exposure)
What three things are required on the control panel?
- dead-man type exposure button
- indicator light
- audible signal
What is in the tube head?
Power supply + x-ray tube
What transformer is on the cathode (-) side?
step-down transformer
What transformer is on the anode (+) side?
step-up transformer
X-rays are produced by the anode or cathode?
anode
Electrons are produced by the anode or cathode?
cathode
What does the step-up transformer do?
raises voltage
What does the step-down transformer do?
lowers voltage
What are the parts of the tube head besides the power supply and x-ray tube?
◦ Glass envelope
◦ Insulating oil
◦ Metal housing
◦ Tube window
◦ Aluminum filters
◦ Lead collimator
What does the insulating oil in the tube head do?
disperse heat
What does the lead collimator do in the tube head?
determines final shape of the x-rays
What do the aluminum filters do in the tube head?
filters the x-rays that are not useful (don’t go to patient)
What is the space called inside the x-ray tube?
vacuum
What are the two main parts of the cathode (-)?
filament
focusing cup
What is the filament made of?
tungsten
What is the focusing cup made of?
molybdenum
What is the source of electrons?
filament
How does the filament generate electrons?
Is heated with a low voltage source and emits electrons at a rate proportional to its temperature (thermionic emission)
What is the charge of the focusing cup?
negative charge
What does the focusing cup do?
Focuses the electrons into a narrow beam directed to the focal spot (+)
What is thermionic emission?
emits electrons at a rate proportional to its temperature
What are the two parts of the anode (+)?
- tungsten target
- copper stem
What does the copper stem do?
Works as a thermal conductor to remove the heat from the tungsten avoiding target melting
What does the tungsten target do?
Transform the kinetic energy of the colliding electrons into x-ray photons
What produces the x-rays?
x-rays produced on the focal spot on the tungsten target (anode)
Is the tungsten target efficient or inefficient at producing x-rays?
Highly inefficient: produces 99% heat and 1% x-rays
Why is tungsten a good target material?
- High atomic number: efficient production of x-rays
- High melting point
- High thermal conductivity: to dissipate the heat produced away from the target
- Low vapor pressure: helps maintain vacuum in the tube at high temperatures.
- Cheap
What are the conditions necessary for x-ray production?
1.Separation of electrons
2.Production of high-speed electrons
3.Concentration of electrons
4.Sudden stoppage of electron stream
What allows for the separation of electrons?
Low voltage filament current heats the tungsten filament to incandescence with resultant separation of outer shell electrons from the tungsten atom (thermionic emission)
What allows for the production of high-speed electrons?
High potential difference is produced between the cathode and anode by applying a high voltage between them
What allows for the concentration of electrons?
Electron beam is focused by the focusing cup (negative charge) and directed towards the focal spot
What allows for the sudden stoppage of electron stream?
Upon striking the focal spot in the anode, the electron stream is stopped abruptly, and kinetic energy of the electron stream undergoes conversion to greater than 99% heat and less than 1% x-radiation
What is the focal spot?
Area on the target to which the focusing cup directs the electrons and from which the x-rays are produced
Size of the _________ is important for image quality
focal spot
smaller focal spot, _____ spatial
resolution
higher
smaller the focal spot → _____ heat accumulates
more
Inclining the tungsten target does what to do the focal point?
smaller focal point
What are two ways to alter the focal point heat problem?
- Rotating anodes
- Stationary anodes (line- focus principle)
What is a rotating anode?
Allows heat at the focal spot to spread out over a large surface (medical devices mostly)
What is the line-focus principle (stationary anode)?
Angling the target to achieve a smaller effective focal spot than the actual focal spot
- Results in sharper radiographic image while maintaining the original surface area of the target for heat dissipation
What is the actual focal spot?
the projection of the focal spot perpendicular to the target
What is the effective focal spot?
the projection of the focal spot perpendicular to the electron beam
Effective focal spot is always _____ than the actual focal spot
smaller
What is the average size of the actual focal spot and effective focal spot?
actual - 3 mm
effective - 1 mm
What are the two functions of the power supply?
◦ Provide a low-voltage current to heat the x-ray tube filament (step-down transformer)
◦ Generate a high potential difference to accelerate electrons form the cathode to the focal spot on the anode (step-up transformer)
What does a transformer do?
Electromagnetic device that either increases or decreases the voltage in a circuit
What are the 3 types of transformers?
◦ Step-down transformer (filament
transformer)
◦ Step-up transformer (high voltage)
◦ Autotransformer
Used to convert 110 or 220 volts to 3-5 volts to heat the filament: step-up or step-down transformer?
Step-down transformer
Which transformer is regulated by the mA switch?
Step-down transformer
increasing mA setting will _______
the number of electrons emitted
increase
Used to convert 110 or 220 volts to 60,000-100,000 volts to move the electrons across the tube: step-up or step-down transformer?
Step-up transformer
Which transformer is regulated by the kV setting?
Step-up transformer
Increasing the kV will ________ the energy of the electrons traveling from the cathode to anode
increase
What does exposure time do?
Determines amount of time the high voltage circuit is applied across the x-ray tube (amount of time x-rays are produced)
What is the time delay circuit?
delay of ½ second to heat the filament
What is the timing circuit?
Actual time of exposure
What does the kV (kilovoltage) do?
controls the acceleration of electrons from cathode to anode
What does the mA (milliamperage) do?
controls heating of the filament
What does the exposure time do?
controls the time in which x-rays are produced
glass envelope
x-ray tube
power supply - step up transformer
aluminum filter
aiming cylinder
power supply - step down transformer
collimator
insulating oil
anode
cathode
cathode
tube window
anode
copper stem
vacuum
glass envelope
focal spot on tungsten target
Electron focusing cup
filament and electron cloud
filament circuit
aluminum discs
postion indicating device
lead collimator
unleaded glass window of x-ray tube
insulating oil
metal housing of x-ray tube head
anode
step-up transformer
x-ray tube
step-down transformer
cathode
