Production of xrays Flashcards
What is the history of X-rays?
1895 discovered by roentgen
1896 walkhoff takes first dental X-ray (25 min exposure)
1890s Rollins introduces collimation and filtration
1950s Extraoral imaging technique invented in finland
Now approx 18 million dental X-rays taken per year in UK
What is the charge of a proton?
+1
What is the Mass (amu) of a proton?
1
What is the charge of a neutron?
0
What is the mass (amu) of a neutron?
1
What is the charge of an electron?
-1
What is the mass (amu) of an electron?
1/1840
What does amu stand for?
Atomic mass unit
What is an atomic mass unit (amu)?
1/12 mass of a Carrbon 12 atom
What is the atomic number?
And which letter is it represented by?
The number of protons in the nucleus
z
What is the relationship between the number of protons and electrons orbiting the nucleus?
The same number of each!
What does the number of protons/electrons determine?
The chemical properties of the atom
What is the charge of an atom?
It is electrically neutral
Nucleus +1 and electrons -1 and same no. of each
How are electrons limited?
To certain orbits or shells
What are the letters used to show the electron shell?
K, L, M, N, O, P
Each electron shell has a fixed energy, what is the difference in energy for a shell that is closer to the nucleus?
The closer the shell to the nucleus the lower the energy - needs less energy to hold it in place like sitting on centre of roundabout
What is ionisation?
The removal of an ion (or more) from an atom
What is excitation?
The movement of an ion to another energy shell (of higher energy)
How many electrons does each shell have?
They each have a max number i.e. 2, 8, 18
How do +ve and -ve charges interact?
Opposite charges attract
like charges repel
What is binding energy?
The work needed to overcome the attraction of an electron to the nucleus and remove electron from an atom
Which shell has the greatest binding energy?
k shell (closest to nucleus) = outer shells its smaller as electron is further away from the nucleus = less attraction
What does ionisation create?
An ion pair
What is an ion pair?
+ve and -ve ion = produced simultaneously by the addition of sufficient energy to a neutral atom/molecule to cause it to dissociate into oppositely charged fragments
What are X-rays?
A form of ionising radiation that sits on the electromagnetic spectrum at the high energy end with gamma rays
What is each X-ray?
A photon (a packet of energy/quantum of energy)
How many photons are X-ray beams used in dentistry made up from?
Millions!!
What are the properties of X-rays?
- Invisible and weightless
- Travel in straight lines
- Travel at the speed of light (in a vacuum)
- Range of frequencies and wavelengths 0.01- 0.05nm (different properties)
- Penetrating
- Ionising
- Follows the inverse square law
What is the inverse square law?
When distance from source is doubled, intensity of radiation is 1/4
Essentially what is a radiograph?
A shadow of the body based on the electron densities of materials and how much a tissue absorbs the radiation
How are X-rays produced?
- Produce lots of electrons
- Accelerate them to very high energy
- Smash them into a target
How are electrons accelerated to a v. high energy?
Using a high potential difference (kV) between negative and positive
What material is the target usually made of?
Tungsten
Why does the tungsten target need special features?
Gets very hot and needs to not melt and be destroyed
How does the smashing of electrons into the target produce X-rays?
Rapid deceleration of electrons = interactions with the anode (loss of energy)
What are the different components of an X-ray tube head?
- Glass x-ray tube (filament, copper block and target)
- Step up transformer
- Step down transformer
- Lead casing
- Oil
- Aluminium filtration
- Collimator
- Beam indicating device
What is the role of the step up transformer?
Increases the voltage to accelerate electrons from cathode to anode
What s the role of the step down transformer?
Decreases the voltage heat the filament (cathode) where the electrons come from
What is the role of the lead casing?
Contains X-rays (only use those coming out in the right direction)
What is the role of the oil?
To conduct heat away from the insert (glass X-ray tube)
What is the role of the aluminium filtration?
Filters out low energy X-rays (decrease exposure)
What is the role of the collimator?
Shape the beam = only covers the area we need to on the patient
What is thermionic emission?
electrons produced by filament heating (cathode mA)
What does the extent of filament heating control?
The amount of electrons produce
and here the current flowing between the cathode and anode (tube current in mA)
What properties of tungsten make it ideal as a target?
- High melting point (won’t melt over lifetime of insert)
- High atomic number (74) = lots of atomic electrons for electrons from filament to interact with and produce X-rays
What does the energy lost during smashing against target get transferred into?
99% Heat
1% X-rays
How is the heat produced in smashing removed?
Copper Block
Surrounding Oil
Why are heat producing collisions most common?
There are many outer shell tungsten electrons which can interact and each electron can have many heat producing collisions
What are the three variables of X-ray sets?
kV
mA
Time
What does the kV of an X-ray set determine?
QUALITY of x-ray beam (energy of photons = penetrating power)
Affects film contrast
What does the mA & time of an X-ray set determine?
QUANTITY of x-ray photons
Affects degree of blackening of film (optical density)
What are the two methods of X-ray production?
- Bremsstrahlung radiation
2. Characteristic radiation
What is bremsstrahlung radiation?
Where an incoming electron penetrates the outer electron shells and passes close to the nucleus of tungsten atoms -> almost goes into orbit around nucleus
= slows down -> loss of energy -> electromagnetic wave/ X-ray),
= small interaction with nucleus = small deviation (produces weak x-ray only -> most common = filtered out & deviates to varying degrees = continuous spectrum of x-ray photon energies)
= all energy in electron can be produced i.e. electron can slow down enough it stops
What is characteristic radiation?
Where incoming electrons collide with inner shell electrons displacing it into outer shell/ ejecting it from atom = huge loss of energy (higher energy level electrons jump down to fill the gap = produces X-ray photon… k alpha = L->K, K beta = M->K less frequent but bigger)
=always the same energy between specific shells = predictable
n.b. only electrons from k shell are of diagnostic importance
=bombarding high speed electrons need energy >69.5kV to displace a K shell electron and produce a characteristic line on the spectrum (energy of electron is directly related to the kV - potential difference across the x-ray tube)
