Physics Basics And X Ray Production Flashcards
What is binding energy?
Additional energy required to exceed electrostatic force (remove an electron from its shell)
This increases as you get closer to the nucleus
What are the 5 elements of a dental x ray unit?
Tube head
Collimator
Positioning arm
Control panel
Circuitry
What type of currents do X ray units require and how is this achieved?
X ray units require a unidirectional current so have generators which modify the AC to mimic a constant DC (rectification).
What is the purpose of a transformer in an X ray unit?
Alters the voltage and current from one circuit to another.
One alters mains -x ray tube (step up)
Another alters mains - filament (step down)
(Need 2 different voltages)
What is the inverse square law?
Intesity of the x ray beam is inversely proportional to the square of the distance between the x ray source and point of measurement
Eg. Doubling the distance will quarter the dose
What are the components of the x ray tube?
Glass envelope- vacuum inside to prevent entry of air, leaded glass absorbs photons travelling in undesired direction
Cathode (+ve) - filament and focussing cup
Anode (-ve) - target and heat dissipating block
What is the filament made of?
Tungsten
What is the focusing cup made of?
Molybdenum
What is the relationship between cathode and anode?
Cathode is negatively charged, electrons are repelled away from cathode and attracted to anode- hit target with high kinetic energy
What is the anode made of?
Tungsten
What is the heat dissipating block made of?
Copper
Why is the target at an angle to the filament?
Increases the actual surface area where electrons impact
Reduces the apparent surface area from where the x ray beam is emitted (reduced penumbra effect- slight blurring due to more than one focal point)
What is the purpose of aluminium filtration and what is the minimum thickness required?
Removes lower energy x rays (non-diagnostic) from the beam (this would increase pt dose with no added effect)
<70 kV- 1.5mm
What is the purpose of a spacer cone?
Dictates the distance between focal spot and patient (focus to skin distance- fsd)
This is usually 200mm
What is a collimator?
Lead diaphragm attached to end of spacer to reduce patient dose (crops beam to match size and shape of receptor)
Use rectangular with size 2 receptors
Why is rectangular collimation recommended?
Can reduce patient dose by approx 50%
What are the 2 interactions taking place at the tungsten target?
Heat production - bombarding electrons reach tungsten outer electrons and are deflected. This loss of kinetic energy produces heat which is dissipated out of the tube. This is 99% of the interactions
X ray production - continuous radiation interactions (mostly) and characteristic radiation interactions
What is continuous radiation?
Bombarding electrons pass close to target nucleares, causing it to be rapidly decelerated and deflected (lost kinetic energy released as X ray photons) - the amount of energy of the x ray photons depends on how close it came to the electron.
Eg. If collides directly with electron - 70KeV photon produced.
Some are of low energy and are removed via aluminium filter
What is characteristic radiation?
Bombarding electron collides with inner shell electron and either displaces it into more peripheral shell (excitation) or removes it completely (ionisation).
Remaining orbiting electrons rearrange themselves to fill space - when an electron drops to a lower shell, it loses energy (difference in binding energy) which is emitted as a photon of specific energy (depends on element)
