Production of x-rays Flashcards
function of the collimator
narrows the beam of x-ray
if the incident elecectron hits the nucleus what happens?
- highest energy given off as an x-ray
- energy given off is equivalent to the energy coming in
- called a bremsstrahlung x-ray
what happens when the incidence electron comes close to but does not hit the nucleus?
- the negatively charged electron is attracted to the + nucleus but this does not hit the nucleus.
- the direction is changed and it looses energy.
- moderate to medium x-ray given off
when the incident electron comes further away from the nucleus what happens in bremsstrahlung x-rays?
- energy is lost from deceleration and it bend less than if closer.
- a low energy x-ray is given off
bremsstrahlung x-rays high energy radiation?
High Energy -electrons directly collide with a nucleus, producing a high energy photon (in keV) that is equal in energy to the incident electron.
bremsstrahlung x-rays medium energy radiation
Medium Energy – Produced by electrons passing close to the nucleus causing deflection and deceleration of the electron. This deceleration causes the electron to lose some of its energy in the form of photon radiation.
bremsstrahlung x-rays low energy radiation
Low Energy – Produced when electrons pass at a distance from the nucleus. They are deflected and slowed somewhat, producing a low energy photon.
how are x-rays produced in characteristic radiation?
- the incident electron hits an electron in orbit and it kicks the electron out of the orbit
- the hole in the inner ring is not favorable and an electron from the next electron shell jumps down to fill the gas. - cascade effect starts causing energy to be given off in photons
cathode of x-ray tube is positive or negative?
negative
anode of x-ray tube is positive or negative?
positive
why is tungston used as the target in the x-ray tube?
Has to have a high atomic number, high melting point, high thermal conductivity and low vapor pressure. Tungsten meets all of these requirements.
what is the focal spot?
That point on the target where the stream of x-ray photons originates. Should be as small as possible for the best radiographs yet must be large enough to dissipate heat. Placing target at an angle provides a smaller effective focal spot for sharp images while maintaining a larger actual focal spot for heat dissipation.
-the kinetic energy of electrons produced from the cathode are focused by the molybendeum cup on the focal spot and here they are converted to x rays (at the anode)
A molybdenum focusing cup in an x-ray tube
A. narrows the beam of electrons
B. is located in the cathode
C. directs electrons toward the tungsten target
D. all of the above
D- All the above
function of the cathode?
-Cathode (-) : A tungsten filament and molybdenum focusing cup provide the source and direction of electrons of the cathode ray. Low voltage is applied to heat the filament, producing a cloud of electrons through thermionic emission. The molybdenum focusing cup electrostatically focuses the electrons into a narrow beam.
FYI the current goes from the cathode to the anode.
function of the anode?
-Anode (+): The tungsten target and copper stem convert the kinetic energy of the electron stream of photons to x-rays and heat.
If you wish to increase the energy of the emitted x-ray beam, you would:
A. increase power to the high voltage step up transformer
B. increase power to the low voltage tube current
C. increase the size of the focal spot
D. all of the above
A. increase power to the high voltage step up transformer
doubling the time does what?
doubles the exposure
the quantity of radiation produces is ________ to the tube current?
directly proportional
as tube current (mA) is increased what happens?
more electrons are released to collide with the target
-more collisions = more radiation
increasing mA vs increasing KeV?
- increase mA = increase the number of electrons produced at the filament
- higher KVP= fater the electrons accelerate to the target
what part of x-rays wears out fastest?
-tungston filament burnout = xray machine should be left on. strain on filament when warmed and cooled
increasing the potential between the anode and cathode _____ the energy of each electron when it strikes the target?
increases
_____ KVP _______ x-rap penetration?
increase KVP increases x-ray penetration
filtration absorbs what?
aluminum filter lowers the energy photons to eliminate them so not absorbed by the body.
function of the collimation?
- reduce the size of the x-ray beam
- reduces scatter radiation and patient surface exposed
- lined with lead
source to object distance inverse square law
intensity of x-ray beam is inversely proportional to the square of the distance between the source and the target.
what happens to the number of photons or energy reaching the target when the distance is doubled?
energy cut in 1/4
what happens to the number of photons or energy reaching the target when the distance is tripled?
energy is 1/9
You expose an x-ray and it comes out underexposed. How do you try to correct the exposure on a retake?
A.move tubehead back to increase focal distance
B. increase kVp on the control panel
C. increase exposure area by using a round collimator
D. increase exposure time
B. increase kVp on the control panel - prefered answer
also
D. increase exposure time - to correct expoure
when does coherent scattering occur?
what can it cause?
- occurs when low energy photons are deflected by outer ring electrons.
- Accounts for only about 7% of interactions in dental, so contributes very little to film fog on image.
what is photoelectric absorption?
what does it look like?
- process in which incident photons are absorbed by an atom by interacting with inner shell electrons.
- This is what makes the lighter areas on the film, making radiographic imaging possible.
what does photoelectric absorption depend on?
- density and thickness of the object, thus creates the contrast between different tissues in a film.
- ex metal shows up white, air - black
when does compton scattering occur?
- when a photon interact with an outer orbital electron = lower energy x-ray
- scattered photons darken and degrade the image while carrying no useful information
- accounts for about 49% of interactions in dental radiography
what type of photons are more readily absorbed as the travel through matter?
lower energy photons
what happens as the energy of the photon beam increases in regards to the transmission of the beam?
transmission of the beam through the absorber increases
Which of the following photon interactions make it possible to produce radiographic images:
A. coherent scattering
B. photoelectric absorption
C. Compton scattering
D. all of the above
B. photoelectric absorption
low voltage tube current
heats the tube filament generating a stream of electrons between the cathode and anode
high tube voltage
between the cathode and anode accelerated the electron stream to a level that x-rays can be produced
why is the x-ray device in a vacuum tube?
- no gas molecules for electrons to collide with
- no oxidation or burnout of filament
the tungston target is located at what end of the x-ray machine?
the positive anode
coherent vs compton scattering?
- coherent: -occurs when low energy photons are deflected by outer ring electrons. -Accounts for only about 7%
- compton: -when a photon interact with an outer orbital electron = lower energy x-ray-scattered photons darken and degrade the image while carrying no useful information. accounts for about 49% of interactions in dental radiography
