lecture 2- production of x-rays PART 1 Flashcards
control panel options:
time
kVp (voltage using)
mA selectors
then exposure button, indicator light, and audible signal
two things inside tube head:
power supply and x-ray tube
what filtrates the xray beams
aluminum filter
what shapes the final shape of xray
lead collimator
cathode has two elements:
filament and focusing cup
filament is made of
tungstan
focusing cup is made of
molybdenum
this is the source of electrons and is heated with LOW voltage and emits electrons at a rate proportional to its temperature:
this action is called:
filament (of cathode)
and action is called thermionic emission
what is the charge of the focusing cup of cathode and why
negative because need electrons to move away (toward anode)
this focuses the electrons into a narrow beam directed to the focal spot
focusing cup of cathode
what are the two elements of the anode
tungsten target and copper stem
this transforms the kinetic energy of the colliding electrons into x-ray photons.
tungsten target of the anode
what % of the energy from electrons produces x-rays
1%
this works as a thermal conductor to remove the heat from the tungsten avoiding target melting
copper stem of anode
why is tungsten a good target material?
cheap
high atomic number (effic. production of x-rays)
high melting point (withstand heat)
high thermal conductivity (dissipate heat away)
low vapor pressure
(maintain vaccum)
what are the 4 conditions that are necessary for x-ray production
- separation of electrons
- production of high-speed electrons
- concentration of electrons
- sudden stoppage of electron steam
this step is when low voltage filament current heats the tungsten filament to incandescence with result of separation of outer shell electrons from the tungsten atom:
this separation of electrons action is called:
step 1. of the 4 conditions that are necessary for x-ray production: separation of electrons
action: thermionic emission
low voltage supply is from the:
step-down transformer
(6-12 volts)
this step is where high potential difference is produced between the cathode and anode by applying a high voltage between them.
electrons accelerated to about .5 x speed of light
step 2. of the 4 conditions that are necessary for x-ray production: production of high-speed electrons
this step is where the electron beam is focused by the focusing cup (negative charge) and directed towards the focal spot
step 3 of the 4 conditions that are necessary for x-ray production: concentration of electrons
this step is where 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
step 4. of the 4 conditions that are necessary for x-ray production: sudden stoppage of electron steam
what removes the heat from the tungsten in step
copper stem
this is the area on the target to which the focusing cup directs the electrons and from which the x-rays are produced
focal spot
what of the focal spot is important for image quality
size
smaller focal spot, higher spatial resolution, more heat accumulates
what are 2 ways to overcome the focal spot accumulating more heat:
- rotating anodes: allows heat at focal spot to spread out over large surface
- stationary anodes (line-focus principle):
angling the target to get smaller effective focal spot than the actual focal spot= better quality
for line-focus principle, which focal spot size
1. helps with heat
2. helps with image quality
- heat= actual focal spot size
- quality= effective focal spot size
provides a LOW-voltage current to heat the x-ray tube filament aka:
and generates a HIGH potential difference to accelerate electrons from the cathode to the focal spot on the anode aka:
power supply
- step-down transformer
- step-up transformer
electromagnetic device that either increases or decreases the voltage in a circuit
transformer part of power supply
what are the 3 types of transformer in the x-ray unit
- step-down transformer (filament transformer)
- step-up transformer (high voltage)
- autotransformer
the high voltage transformer (step-up) is controlled by
kVp
the filament transformer (step-down) is controlled by
mA selector
for the power supply, this transformer is used to convert 110 or 220 volts to 3-5 volts to heat the filament:
this is also regulated by the:
step-down transformer
regulated by mA switch
increasing mA setting will increase the number of what
number of electrons emitted
for the power supply, this transformer is used to convert 110 or 220 volts to 60,000 - 100,000 to move electrons across the tube:
this is regulated by:
step-up transformer
regulated by kVp setting
increasing the kVp will increase the
energy of the electrons traveling from the cathode to anode
this determines amount of time the high voltage circuit is applied across the x-ray tube (amount of time x-rays are produced)
exposure time
time delay circuit:
1/3 second to heat filament
timing circuit:
actual time of exposure
on the control panel, this controls the acceleration of electrons from cathode to anode
kV
on the control panel, this controls heating of the filament
mA
