Production of X-Rays Part I Flashcards
what is on the control panel
- exposure time, kVp, and mA selectors
- exposure button
- indicator light and audible signal
what makes up the tube head
power supply and x ray tube
where is the insulating oil located
outside the X ray tube and transformers
what is inside the xray tube
a vacuum
what is in the tube head
- glass envelope
- insulating oil
- metal housing
- tube window
- aluminum filters
- lead collimator
what does the aluminum filter do
filters X ray and removes low energy X rays from the whole beam, so only those that have sufficient energy can exit the tube head
what is the x ray tube made of
glass
what is the tube head made of
metal
what does the oil do
dissipates the heat generated in the tube head
what is in the cathode
filament and focusing cup
what is the filament made of
tungsten
what is the source of electrons
the filament
how are electrons emitted from the cathode
filament is heated with a low voltage source and emits electrons at a rate proportional to its temperature - called thermionic emission
what is the focusing cup made of
molybdenum
what is the charge of the focusing cup
negative
what does the focusing cup do
focuses the electrons into a narrow bean directed to the focal spot
what charge is the focal spot
positive
what is in the anode
tungsten target and copper stem
what does the tungsten target do
- transform the kinetic energy of the colliding electrons into Xray photons
how efficient is the tungsten target
very ineffective. produced 99% heat and 1% Xrays
what does the copper stem do
works as a thermal conductor to remove the heat from the tungstem 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 heat in tube
- 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
- separation of electrons
- production of high speed electrons
- concentration of electrons
- sudden stoppage of electron steam
what happens in separation of electrons
- thermionic emission
- step down transformer provides the voltage
how much voltage is supplied from the step down transformer
6-12 volts
describe the production of high speed electrons
- high potential difference is produced between the cathose and anode by applying a high voltage between them
- electrons are accelerated to approximately 1/2 speed of light
what does the kVp stand for
difference between the anode and the cathose
what is usually the kVp
60-90
describe the concentration of electrons
- electron beam is focused by the focusing cup (negative charge) and directed towards the focal spot
describe the sudden stoppage of electron steam
- upon striking the focal spot in the anode the electron steam is stopped and kinetic energy of the electron steam undergoes conversion to greater than 99% heat and less than 1% x-radiation
what removes the heat from tungsten
copper stem
the smaller the focal spot the _____ of the image
higher resolution
what is the focal spot
the area of the target to which the focusing cup directs the electrons and from which the xrays are produced
the smaller the focal spot the more ___ accumulates
heat
what are 2 ways to overcome heat accumulation with a smaller focal spot
- rotating anodes
- stationary anodes - line focus principle
what does the rotating anode do and where is it found
- allows heat at the focal spot to spread out over a large surface
- medical devices
what is the line focus principle (stationary anode)
- angling the target to achieve a smaller effective focal spot than the actual focal spot
what does a stationary anode result 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
how does the effective focal spot compare to the actual focal spot in size
the effective is always smaller
what are the 2 functions of the power supply
- provide a low voltage current to heat the x ray tube filament - step down transfomer
- generate a high potential difference to accelerate electrons from the cathose to the focal spot of the anode- step up transformer
what is the transformer
electromagnetic device that either increases or decreases the voltage in a circuit
what are the 3 types of transformers in the xray unit
- step down transformer - filament transformer
- step up transformer - high voltage
- autotransformer
what does the mA control
the amount of energy in the step down transformer
what happens if you increase the mA
more energy through the transformer the higher voltage through the filament and the higher number of electrons -> modifies the AMOUNT of X rays produced
what does the kVp modify
step up transformer and ENERGY of the X ray
what happens if you increase kVp
greater difference between anode and cathose -> electron will be more attracted to the anode -> electron has more energy -> Xray will be more energetic -> modified ENERGY of X ray
what does the step down transformer do
used to convert 110 volts to 3-5 volts to heat the filament
what does the step up transformer do
used to convert 110 volts to 60,000-100,000 volts to move the electrons across the tube
what does the exposure timer determine
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 1/2 second to heat the filament
what is the timing circuit
actual time of exposure
what affects number of X rays
time and mA
what affects energy of x rays
kVp
what does kV control
the acceleration of electrons from cathode to anode
what does mA control
the heating of the filament
what does exposure time control
the time in which xrays are produced
