physics basics Flashcards
radiographs
Images created by X-rays which have been projected through an object & then interacted with a receptor
The different shades of grey on the image correspond to the different types of tissue & thicknesses of tissue involved
- Enamel – white, soft tissue - greyer
use of radiographs
Provide ability to see structures within the body, particularly mineralised tissues
- Many dental-related conditions affect the mineral content of tissues
Can show normal anatomy & pathology
Aid diagnosis, treatment planning, & monitoring
intraoral dental radiograph views
periapical
bitewing
occlusal
extraoral dental radiograph views
panoramic
lateral cephalograms
electromagnetic radiation
X-rays are a form of electromagnetic radiation
The flow of energy created by simultaneously varying electrical & magnetic fields
Schematically represented as a sine wave
properties of all EM radiation types
No mass
No charge
Always travels at “speed of light”
3x10^8 ms-1 = 671 million mph
Can travel in a vacuum
em spectrum
Consists of all the different types of electromagnetic radiation
Each type has different properties, dependent on its energy/wavelength/frequency
Typically divided into 7 main groups
- Gamma, X, ultravitolet, visible, infrared, microwave, radio
7 main EM groups
Gamma, X, ultravitolet, visible, infrared, microwave, radio
wavelength
distance over which the wave’s shape repeats
measured in m
wavelength measured in
metres, m
frequency
many times the wave’s shape repeats per unit time
Measured in hertz, Hz
One hertz = one cycle per second
frequency measures in
hertz, Hz
One hertz = one cycle per second
speed =
frequency x wavelength
speed of all electromagnetic radiation
3x10^8 ms-1
if frequency of EM increases what happens to wavelength
decrease
as
speed = frequency x wavelength
3x10^8 ms-1 = frequency x wavelength
shorter waves of greater frequency
if wavelength increases of EM what happens to frequency
decrease
as
speed = frequency x wavelength
3x10^8 ms-1 = frequency x wavelength
longer waves of less frequency
photon energy
EM radiation involves the movement of energy as “packets of energy” known as photons
Energy usually measured in electron volts, eV
1 eV = energy (in joules) gained by 1 electron moving across a potential difference of 1 volt
1 eV =
energy (in joules) gained by 1 electron moving across a potential difference of 1 volt
energy usually measured in
electron volts, eV
history of X-rays
1895: officially discovered by German physicist Wilhelm Röntgen
- Awarded Nobel Prize in Physics
1896: X-rays used in medicine & dentistry
Named “X-rays” because of their unknown nature
X-ray photon energies
~124eV – 124keV
2 types of X rays
hard X-rays
soft X-rays
hard X-rays
higher energies
Able to penetrate human tissues
- medical imaging
(e. g. >5keV)
soft X-rays
lower energies
Easily absorbed
properties of X-rays (4)
Form of electromagnetic radiation
- No mass, no charge, very fast, can travel in a vacuum, etc.
Undetectable to human senses
Man-made
- Note: gamma rays are identical except that they occur naturally (& generally have higher energies)
Cause ionisation
- i.e. displacement of electrons from atoms/molecules
ionisation
displacement of electrons from atoms/molecules
gamma rays compared to X-rays
gamma rays are identical
to X-rays
except that they occur naturally (& generally have higher energies) whereas X-rays are man made
basic production of X-rays
Electrons fired at atoms at very high speed
On collision, the kinetic energy of these electrons is converted to electromagnetic radiation (ideally X-rays) & heat
The X-ray photons released and are aimed at a subject
the atoms (BOHR model)
Atoms are the “building b locks” of matter
Central nucleus
- Protons (+ve charge)
- Neutrons (neutral)
Orbiting “shells”
- Electrons (-ve charge)
neutron
charge
mass
location
0
1
in nucleus
proton
charge
mass
location
+1
1
in nucleus
electron
charge
mass
location
-1
negligible (0)
orbiting shells
nucleus
Collection of nucleons
Protons & neutrons have similar mass
Overall positive charge
atomic number (Z) =
number of protons
Unique to each element
mass number (A) =
number of protons + neutrons
hydrogen H atoms
atomic number
mass number
Atomic Number (Z) = 1 Mass Number (A) = 1
tungsten W atom
atomic number
mass number
Atomic Number (Z) = 74 Mass Number (A) = 184
what does the number of electrons determine
the chemical properties of an atom
atom in ‘ground state’ is
neutral
Number of electrons = number of protons
ionisation process
removing/adding electron(s) to an atom
Atom - e- -> positive ion
Atom + e- -> negative ion (negative e outweighs positive p)
electron shells
Electrons spin around the nucleus in discrete orbits/shells
- Cannot exist between these shells
Each shell is labelled alphabetically
- Innermost shell is K, Then L, M, N, O, etc.
Electrons try to fill available spaces in the inner shells first
maximum number of electrons per shell
= 2n^2
Where “n” is the shell number
K is 1, L is 2, etc.
Example: M shell is number 3 -> 2 x 32 = 18
maximum number of electron in
K shell
1
2n^2
-> 1 x 1^2
= 1
maximum number of electron in
L shell
8
2n^2
-> 2x 2^2
= 8
maximum number of electron in
M shell
18
2n^2
- > 2 x 3^2
- > 18
maximum number of electron in
N shell
32
2n^2
-> 2 x 4^2
= 32
electrostatic force
Orbiting electrons are held within their shells by electrostatic force
-ve charge of electrons attracted to overall +ve charge of nucleus
what holds orbiting electrons in their shells
electrostatic force
-ve charge of electrons attracted to overall +ve charge of nucleus
binding energy
additional energy required to exceed electrostatic force
To remove an electron from its shell, a specific amount of energy is required to overcome this attraction
name for specific amount of energy needed to remove an electron from its shell
binding energy
closer electron is to nucleus then…
the greater the electrostatic force (& therefore binding energy)
K shell electrons have the highest binding energies
- Then L, then M, etc.
the more positively charged the nucleus (i.e. high Z/atomic number) then
greater the electrostatic force
Carbon (Z=6): K shell binding energy = 0.28 keV
Tungsten (Z=74): K shell binding energy = 69.5 keV
electron movement between shells
The specific amount of energy required to move an electron to a more outer shell (i.e. away from the nucleus) equals the difference in the binding energies of the 2 shells
Conversely, if an electron drops to a more inner shell then this specific amount of energy is released
- Possibly in the form of X-ray photons (if sufficient energy)
The specific amount of energy required to move an electron to a more outer shell (i.e. away from the nucleus) =
difference in the binding energies of the 2 shells
Binding energy of tungsten K Shell = 69.5 keV
Binding energy of tungsten L shell = 10.2 keV
69.5 – 10.2 = 59.3keV
if an electron drops to a more inner shell then
energy is released
Possibly in the form of X-ray photons (if sufficient energy)
5 basic components of dental X-ray unit
Tubehead Collimator Positioning arm Control panel Circuitry
3 fundamentals of electricity
Current
Voltage
Transformers
current
Flow of electric charge, usually by the movement of electrons
SI unit: amp (or ampere), A
Measure of how much charge flows past a point per second
Direction
- Direct current (DC) = constant unidirectional flow
- Alternating current (AC) = flow repeatedly reverses direction
alternating current (AC)
Flow periodically reverses direction
Number of complete cycles (reverse + reverse-back) per unit time is the frequency
SI unit: hertz, Hz (cycles per second)
e.g. mains electricity (50Hz in UK)
unit for alternating current
SI unit: hertz, Hz (cycles per second)
direct current (DC)
constant unidirectional flow
e.g. batteries
two directions of electricial current
direct current (DC)
alternating current (AC)
unit for direct current
amp (or ampere), A
Measure of how much charge flows past a point per second
rectification of current
X-ray production requires a unidirectional current
- But X-ray units are powered by mains electricity (AC)
X-ray units have generators which modify the AC so that it mimics a constant DC
what type of current do X-rays need
unidirectional current
but powered by mains electricity (AC) so therefore need rectification of current by generators to mimic DC
voltage
Difference in electrical potential between 2 points in an electrical field
Related to how forcefully a charge will be pushed through an electrical field
SI unit: volt, V
Note: “potential difference” synonymous with voltage
electrical supply to X-ray unit
mains electricity
Alternating current (≤13 amps) 220-240 volts
voltage needed of dental X-ray unit
unidirectional (AC from mains rectified)
One as high as 10s of thousands of volts
One as low as around 10 volts
transformers
alter the voltage (& current) from one circuit to another
2 transformers needed for X-ray unit
Mains -> X-ray tube (cathode-anode)
Mains -> filament
step-up transformer
↑ potential difference across X-ray tube
Usually 60,000-70,000 volts (60-70 kV)
Current reduced to milliamps (mA)
step-down transformer
↓ potential difference across filament
~10 volts
~10 amps
X-ray beam
Made up of millions of X-ray photons directed in the same general direction
Photons effectively travel in straight lines but diverge from the X-ray source (i.e. do not travel in parallel)
X-ray beam intensity
quantity of photon energy passing through a cross-sectional area of the beam per unit time
↑ number &/or energy of photons = ↑ intensity
proportional to current in filament (mA) & potential difference across X-ray tube (kV)
↑ number &/or energy of photons =
↑ intensity
divergence of X ray beam
dose decreases with distance from X-ray source
ensure staff stand a sufficient distance from patient (& not in the direction of the primary X-ray beam)
inverse square law
Intensity of X-ray beam is inversely proportional to the square of the distance between the X-ray source & the point of measurement
Intensity ∝ 1/distance^2
Therefore, doubling the distance will quarter the dose
intensity ∝
1/distance^2
e.g. if a patient standing in the X-ray beam gets a dose of 4 grays at a distance of 1 metre (from the X-ray source), what will the dose be at 4 metres?
Double the distance twice (1 to 4m) so ¼ grays twice ((½)^2then (½)^2)
1 = intensity x distance^2
intensity(a) x distance(a)^2 = 1 & intensity(b) x distance(b)^2 = 1
intensity(a) x distance(a)^2 = intensity(b) x distance(b)^2
4 x 12 = ? x 42
4 = ? x 16
? = 4/16
? = 0.25 Gy
other types of radiation
Alpha particles
Beta particles
Gamma rays
All produced by radioactive decay of unstable atoms
- Unlike X-rays which are directly man-made
