Radiology 3 Flashcards
In domestic energy supply, what is the typical voltage?
220/240 volts
The current in domestic energy supply travels in a wavelength form of…
50 Hertz
Typical domestic energy supply current
13 amp current (however, circuits can be fused at 3, 5 or 13 amps).
Where is the domestic electricity supply converted to the x-rays needed in dentistry?
The tube head
What can you do on the control panel?
- the time of the exposure
- the area of the exposure
- the type of film we are taking (e.g bitewing etc)
A
Cathode connection
B
Filament
C
Electron beam
D
Tungsten target
E
Glass housing
F
Anode connection
G
Bremsstrahlung ((German: “braking radiation”), and characteristic X-rays
H
X-rays
I
Filament focusing cup
The filament - cathode
- Fine wire (tungsten)
- Current gets passed along it (8-10mA), electrons are excited, wire gets hot (may give off light).
- Electrons are lost from outer shell/orbits round the nucleus. Electron cloud forms around the cathode (focusing cup helps bring this together).
The anode
- Small tungsten target embedded in copper
- Potential difference between anode and cathode (e.g. 70kV)
- Step up transformer gives rise to the change in voltage
- Due to the large potential difference, negatively charged electrons from the cathode are pulled over to the anode (attracted to the positive charge of the anode nuclei).
- Then they come to a sudden stop/decelerate and get rid of most of the energy that they contain.
- 99% of this energy is transferred into heat, 1% in x-rays.
Function of the step up transformer in the tube
The step up transformer (coils of copper) facilitates the necessary increase in voltage:
Domestic mains supply = 240V
———–>
X-ray production voltage required = 70,000V
Two types of x-ray spectra
- Continuous spectrum
- Characteristic spectrum
Continuous spectrum
Bremsstrahlung / breaking radiation: electromagnetic radiation produced by a sudden slowing down or deflection of electrons. They display a wide range of photon energies. Moving round the nucleus loses energy - gradually, some of the electrons with less energy will move off to react with other atoms (much of the energy given off is as heat, however some of it is Bremsstrahlung radiation).
Characteristic spectrum
Depends on material used in anode. Emitted by loss of electrons from K & L shells.
Why is filtering out necessary?
- Small deflections are the most common, producing many low energy photons.
- These have little penetrating power and will not contribute to the useful x-ray beam.
- Filtration is necessary to remove radiation that would otherwise be absorbed by the body.
What gives rise to high energy photons and how common are they?
Large deflections (which are less likely meaning there are few high energy photons).
Maximum photon energy is directly related to…
kV across the X-ray tube
What must happen for characteristic radiation to occur…
An incident electron has a direct hit with an electron in the K shell, it knocks the K shell electron out, and this is called the ejected orbital electron (the initial electron may also be ejected after interaction ‘bounces off’ - potentially with less energy than when it came in).
What occurs once an incident electron has knocked an electron from the K-shell?
- The shell is no longer stable with electrons, therefore an electron from an outer shell drops down into the K-shell (going on until the outer shell and all inside shells are stable).
- However, that electron has too much binding energy, gets rid of the excess energy as an x-ray photon.
Characteristic spectrum: which shell’s line spectra is of diagnostic importance?
K shell (58-69.5kV)
as those belonging to L shell are less than 10kV which would be absorbed within the X-ray tube
X-ray tube needs to be operating at above____ for characteristic spectrum to be produced.
69.5kV
If you were using a X-ray tube that works between 60-65kV we would only have ____________ radiation
bremsstrahlung
Non-ionising radiation
Ionising radiation
Scattering
random change in direction after hitting something
Absorption
deposition of energy in tissues
Intensity
number of x-ray photons in a defined area of the beam
Attenuation
reduction in intensity of beam due to scattering & absorption
Ionisation
removal of electron from neutral atom to give -ve (electron) & +ve (atom) ions
Penetration
the ability of photons to pass through or into tissues/materials
What are the 4 main outcomes for an x-ray when it interacts with something?
- Completely scattered with no loss of energy
- Absorbed with total loss of energy
- Scattered with some absorption & loss of energy
- Transmitted unchanged
Internal scatter
Radiation passes downwards into the patient’s body.
Properties of x-rays
- Not detectable by human senses
- Produce a latent image on film emulsion
- Cause ionisation
- Can cause biological damage
- Cause certain salts to fluoresce & emit light
Radiation dose
The radiation dose is the amount of radiation absorbed by the patient. Low energy photons often are absorbed by soft tissues
