How are x-rays produced? Flashcards
What part of an x-ray machine produces the x-ray beam?
The glass xray tube
What is the purpose of the step-up transformer?
Step-up (increase) mains voltage of 240 volts to 60-70 kV (60,000-70,000 volts)
What is the purpose of the spacer cone?
Indicate direction of beam and set ideal distance from pt’s skin
What is the purpose of the surrounding oil?
To facilitate heat removal
What letters represent electron shells?
K, L, M, N, O
What is the atomic number?
Number of protons in the nucleus = greater density = bigger atom
What is the filament made out of?
Tungsten wire
What is the x-ray target made out of?
Tungsten
What happens in the x-ray tube when the exposure button is pressed?
An electric current is passed through the cathode and warms up the tungsten filament at the end of the cathode.
This then causes the release of electrons from the filament which then travels to the target.
For the electrons to be able to travel through the gap to the target it requires the 60-70 kV voltage so it has energy to reach target. Lots of heat is produced (& travels down copper anode) as well as x-rays.
Why is there leakage?
Because some photons can stray from target
Is the x-ray process considered efficient?
No, because only 1% produces x-rays, the rest is heat
How does the electron and tungsten produce x-rays?
- A negatively charged electron is accelerated towards the atoms of tungsten
- The e- passes near to the positively charged nucleus, and it is deflected
- This causes the electron to slow down enough to lose some of it’s energy (the more it slows = the more energy lost = the bigger the photon)
- This energy is then emitted as an x-ray
What is said to form by the electron-nucleus interaction?
Emitted x-rays produce a continuous spectrum
Which type of photons are more dangerous?
Low energy
Which type of photons are more diagnostically useful?
The high energy photons
What is photon energy mainly determined by?
It is determined by the energy of the incoming electron
What determines the energy of the incoming electron?
The size of the voltage applied (kV)
What is E.max?
It is the highest possible energy a photon can have
Why is a continuous spectrum produced?
Due to the incoming electrons slowing down and deflected to varying degrees
What happens to the continuous spectrum and Emax if you increase the votlage?
The Emax increases and more photons are produced in the continuous spectrum
What is another way x-rays can be produced with the tungsten atom?
The incoming electron can instead collide with an orbiting electron.
This causes it to displace.
The incoming electron is then deflected.
This destabilised the atom due to the electron vacancy. So an electron from adjacent shell/energy level drops to the inner shell and so on.
This movement of electrons causes energy to be released in the form of x-ray photons.
Eventually free electron is captured to stabilise atom
What spectrum is produced when electrons are rearranged resulting in the emission of x-rays?
Characteristic spectrum
(e.g. characteristic of tungsten)
The photons produced are only of specific size or energy
Why does the x-ray equipment have to operate at 70 kV minimum with tungsten?
Because any lower voltage would not have enough energy to displace the inner electrons of the tungsten atom
How are the lowest energy photons removed from the beam to prevent hitting the patient?
The beam is passed through a sheet of aluminium which absorbs and filters out lowest energy photons.
What thickness of aluminium is recommended for machines operating at <70 Kv?
1.5mm
What thickness of aluminium is recommended for machines operating at >70 Kv?
2.5 mm
Which setting can you usually always change on an x-ray machine?
The time (s)
e.g. by choosing which tooth you’re irradiating
What does the kV determine?
Determines energy of photons or quality of x-ray beam
determines penetrating power of photons
affects film contrast (can alter contrast by changing kV)
What does the mA (current) and time together determine?
Determines quality of x-ray photons.
affects degree of blackening of film (overexposed film will look too dark, for example)
If an x-ray machine uses AC (alternating current), when do the x-rays get produced - in the negative phase or positive phase?
How do you overcome this?
Positive phase
(travels in opposite direction in negative phase)
The current has to be rectified
What is the simplest way to rectify an AC ?
Half-wave rectification, where negative phase is removed. so electron flow becomes pulsatile (in pulses). so inefficient and exposure takes longer and photons are lower energy
What are the 3 ways AC can be rectified?
Half- wave rectification: negative phase removed.
Single phase, full wave rectification: negative phases converted to positive phases (still pulse but closer together)
Constant potential rectification: not pulses but creates ripple effect:
What is the definition of the FSD (focal to skin distance)
Distance from focal spot (where x-rays are reduced) on the target to the end of the spacer cone that touches the patients face
Why is a long FSD necessary?
Because the beam disperses and irradiates a larger surface area
What is the current recommendations for FSD?
200mm
What is a collimator?
It is the device that limits the beam size
What is the purpose of the spacer cone?
It determines the distance of the target to the patient and directs the beam
What is the current recommendations for collimation?
the recommend a rectangular collimator of 40mm x 50mm (same as standard IO film size)
What is the maximum diameter of circular collimators?
60mm
What are the current dental x-ray equipment recommendations? (8)
- Operating range of 60-70 kV
- Adjustable kV, mA and exposure time
- 1.5mm aluminium filtration
- DC or constant potential
- Rectangular collimator (40mmx50mm)
- 200mm FSD or long cone
- Audible and visual warning signs of exposure
- Exposure timer switches continuously depressed by operator
