production of X Rays pt 1 Flashcards
Control panel
can adjust?
type of button?
requires?
Control panel:
Exposure time, kVp (energy) and mA (#xrays) selectors
Exposure button (dead-man type)
Requires Indicator light and audible signal
Tube head components
Tube head: Power supply + x-ray tube
◦Glass envelope
◦Insulating oil
◦Metal housing
◦Tube window
◦Aluminum filters
◦Lead collimator
power supplies within the tube head
step up (anode) and step down transformers (cathode)
what is the state inside the x ray tube?
vacuum
why do we need insulating oil
the heat produced by x-ray production must be dissipated
purpose aluminum filter
remove low E x rays, not useful for imaging
cathode of the x ray tube
negative charge (repel e), 2 parts: filament and focusing cup, uses low V power supply
◦ Filament:
◦ Made of tungsten
◦ Source of electrons
◦ Is heated with a low voltage source and emits electrons at a rate proportional to its
temperature (thermionic emission).
◦ Focusing cup
◦ Made of Molybdenum
◦ Negatively charged
◦ Focuses the electrons into an arrow beam directed to the focal spot (+).
anode of x ray tube
positively charged= attracts e, high V power source, 2 main parts
◦ Tungsten target:
◦ Transform the kinetic energy of the colliding electrons into x-ray photons.
◦ Highly inefficient: produces 99% heat and 1% x-rays
◦ Copper stem:
◦ Works as a thermal conductor to remove the heat from the tungsten avoiding target
melting.
Why is tungsten a good target material?
- High atomic number: efficient production of x-rays
- High melting point (3422°C): to withstand theheat produced
- High thermal conductivity: to dissipate the heat produced away from the target
- Low vapor pressure: helps maintain vacuum in the tube at high temperatures
Conditions necessary for X-ray production in order
1.Separation of electrons
2.Production of high-speed electrons
3.Concentration of electrons
4.Sudden stoppage of electron steam
Separation of electrons from the filament, also called?
◦ Low voltage filament current heats the tungsten filament to incandescence with
resultant separation of outer shell electrons from the tungsten atom (“boiling off”). This
separation of electrons is called thermionic emission
Production of high-speed electrons
approx acceleration?
◦ High potential difference is produced between the cathode and anode by applying a high voltage between them.
◦ Electrons are accelerated to approx. 0.5 X speed of light increasing the voltage differential can increase the acceleration
Concentration of electrons
◦ Electron beam is focused by the focusing cup (negative charge) and directed towards the focal spot.
Sudden stoppage of electron steam
◦ Upon striking the focal spot in the anode, the electron stream is stopped abruptly, and the kinetic energy of the electron stream undergoes conversion to greater than 99% heat and less than 1% x-radiation.
◦ Cooper stem removes the heat from the tungsten
focal spot, size implication for image quality?
◦ Area on the target to which the focusing cup directs the electrons and from which the
x-rays are produced.
◦ Size if the focal spot is important for image quality (smaller focal spot, higher spatial
resolution)
◦ The smaller the focal spot →heat? how can this be overcome?
◦ The smaller the focal spot →more heat accumulates.
◦ 2 ways to overcome this problem:
1. Rotating anodes
2. Stationary anodes (line- focus principle)
Rotating anode
Rotating anode
• Allows heat at the focal spot to spread out over a large surface (dark band)
• Medical devices
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
actual and effective focal spots of the line focus principle
• Actual focal spot: the projection of the focal spot perpendicular to the target
• Effective focal spot: the projection of the focal spot perpendicular to the electron beam
• Effective focal spot is always smaller than the actual focal spot.
Power supply 2 principal functions:
◦ 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)
◦Transformer
◦Electromagnetic device that either increases (step-up) or decreases (step-down) the voltage in a circuit.
transformers of the x ray unit
◦ Step-down transformer (filament transformer)
◦ Step-up transformer (high voltage)
◦ Autotransformer
step down transformer
◦ Used to convert 110 or 220 volts to 3-5 volts to heat the filament (low voltage circuit)
◦ Regulated by the mA switch
◦ increasing mA setting will increase the number of electrons emitted
step up transformer
◦ Used to convert 110 or 220 volts to 60,000-100,000 volts to move the electrons across the tube (high voltage circuit)
◦ Regulated by the kV setting
◦ Increasing the kV will increase the energy of the electrons traveling from the cathode to anode
Exposure timer, time delay circuit, and timing circuit
◦ Determines amount of time the high voltage circuit is applied across the x-ray tube (amount of time x-rays are produced)
◦ Time delay circuit: delay of ½ second to heat the filament
◦ Timing circuit: Actual time of exposure
kV on control panel
controls the acceleration of electrons from cathode to anode
mA on control panel
controls heating of the filament
exposure time on control panel
controls the time in which x-rays are produced
