production of X rays Flashcards
Control panel:
contains (3)
Exposure time, kVp and mA selectors
Exposure button (dead-man type)
Indicator light and audible signal
Tube head: includes (2)
Power supply + x-ray tube
Tube head
within (6)
◦Glass envelope ◦Insulating oil ◦Metal housing ◦Tube window ◦Aluminum filters ◦Lead collimator
Cathode (-) (2)
◦ Filament
◦ Focusing cup
Filament:
3
◦ 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
3
◦ Made of Molybdenum ◦ Negatively charged ◦ Focuses the electrons into a narrow beam directed to the focal spot (+).
Anode (+) (2)
◦ Tungsten target
◦ Copper stem
Tungsten target:
2
◦ Transform the kinetic energy of the colliding electrons into x-ray photons. ◦ Highly inefficient: produces 99% heat and 1% x-rays
Copper stem:
1
◦ Works as a thermal conductor
to remove the heat from the
tungsten avoiding target
melting.
Why is tungsten a good
target material?
(4)
- High atomic number
- High melting point
(3422°C) - High thermal conductivity
- Low vapor pressure
- High atomic number:
efficient production of x-
rays
- High melting point
(3422°C):
to withstand the
heat 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
(4)
- Separation of electrons
- Production of high-speed electrons
- Concentration of electrons
- Sudden stoppage of electron steam
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
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
Electron beam is focused by the — and directed towards
the —
focusing cup (negative charge) focal spot.
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.
— removes the heat from the tungsten
◦ Cooper stem
Focal spot
◦ 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 →
more heat accumulates
The smaller the focal spot →more heat accumulates.
◦ 2 ways to overcome this problem:
- Rotating anodes
2. Stationary anodes (line- focus principle)
Rotating anode
2
• Allows heat at the focal spot to spread out over a large surface (dark band) • Medical devices
Line-focus principle
(stationary anode)
(2)
• 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 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 — than the
actual focal spot.
smaller
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 or
decreases the voltage in
a circuit.
3 types of Transformer in the x-ray unit:
◦ Step-down transformer (filament
transformer)
◦ Step-up transformer (high voltage)
◦ Autotransformer
Step-down transformer
3
◦ 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
3
◦ 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:
◦ 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 (kilovoltage):
controls
the acceleration of
electrons from cathode to
anode
mA (milliamperage):
controls heating of the
filament
Exposure time:
controls the
time in which x-rays are
produced
