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BDS3 Radiology > physics basics > Flashcards

physics basics Flashcards

1
Q

radiographs

A

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

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2
Q

use of radiographs

A

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

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3
Q

intraoral dental radiograph views

A

periapical

bitewing

occlusal

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4
Q

extraoral dental radiograph views

A

panoramic

lateral cephalograms

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5
Q

electromagnetic radiation

A

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

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6
Q

properties of all EM radiation types

A

No mass

No charge
Always travels at “speed of light”
3x10^8 ms-1 = 671 million mph

Can travel in a vacuum

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7
Q

em spectrum

A

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

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8
Q

7 main EM groups

A

Gamma, X, ultravitolet, visible, infrared, microwave, radio

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9
Q

wavelength

A

distance over which the wave’s shape repeats

measured in m

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10
Q

wavelength measured in

A

metres, m

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11
Q

frequency

A

many times the wave’s shape repeats per unit time

Measured in hertz, Hz
One hertz = one cycle per second

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12
Q

frequency measures in

A

hertz, Hz

One hertz = one cycle per second

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13
Q

speed =

A

frequency x wavelength

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14
Q

speed of all electromagnetic radiation

A

3x10^8 ms-1

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15
Q

if frequency of EM increases what happens to wavelength

A

decrease

as
speed = frequency x wavelength
3x10^8 ms-1 = frequency x wavelength

shorter waves of greater frequency

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16
Q

if wavelength increases of EM what happens to frequency

A

decrease

as
speed = frequency x wavelength
3x10^8 ms-1 = frequency x wavelength

longer waves of less frequency

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17
Q

photon energy

A

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

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18
Q

1 eV =

A

energy (in joules) gained by 1 electron moving across a potential difference of 1 volt

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19
Q

energy usually measured in

A

electron volts, eV

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20
Q

history of X-rays

A

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

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21
Q

X-ray photon energies

A

~124eV – 124keV

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22
Q

2 types of X rays

A

hard X-rays

soft X-rays

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23
Q

hard X-rays

A

higher energies

Able to penetrate human tissues

  • medical imaging
    (e. g. >5keV)
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24
Q

soft X-rays

A

lower energies

Easily absorbed

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25
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
26
ionisation
displacement of electrons from atoms/molecules
27
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
28
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
29
the atoms (BOHR model)
Atoms are the “building b locks” of matter Central nucleus - Protons (+ve charge) - Neutrons (neutral) Orbiting “shells” - Electrons (-ve charge)
30
neutron charge mass location
0 1 in nucleus
31
proton charge mass location
+1 1 in nucleus
32
electron charge mass location
-1 negligible (0) orbiting shells
33
nucleus
Collection of nucleons Protons & neutrons have similar mass Overall positive charge
34
atomic number (Z) =
number of protons Unique to each element
35
mass number (A) =
number of protons + neutrons
36
hydrogen H atoms atomic number mass number
``` Atomic Number (Z) = 1 Mass Number (A) = 1 ```
37
tungsten W atom atomic number mass number
``` Atomic Number (Z) = 74 Mass Number (A) = 184 ```
38
what does the number of electrons determine
the chemical properties of an atom
39
atom in 'ground state' is
neutral Number of electrons = number of protons
40
ionisation process
removing/adding electron(s) to an atom Atom - e- -> positive ion Atom + e- -> negative ion (negative e outweighs positive p)
41
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
42
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
43
maximum number of electron in K shell
1 2n^2 -> 1 x 1^2 = 1
44
maximum number of electron in L shell
8 2n^2 -> 2x 2^2 = 8
45
maximum number of electron in M shell
18 2n^2 - > 2 x 3^2 - > 18
46
maximum number of electron in N shell
32 2n^2 -> 2 x 4^2 = 32
47
electrostatic force
Orbiting electrons are held within their shells by electrostatic force -ve charge of electrons attracted to overall +ve charge of nucleus
48
what holds orbiting electrons in their shells
electrostatic force -ve charge of electrons attracted to overall +ve charge of nucleus
49
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
50
name for specific amount of energy needed to remove an electron from its shell
binding energy
51
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.
52
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
53
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)
54
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
55
if an electron drops to a more inner shell then
energy is released Possibly in the form of X-ray photons (if sufficient energy)
56
5 basic components of dental X-ray unit
``` Tubehead Collimator Positioning arm Control panel Circuitry ```
57
3 fundamentals of electricity
Current Voltage Transformers
58
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
59
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)
60
unit for alternating current
SI unit: hertz, Hz (cycles per second)
61
direct current (DC)
constant unidirectional flow | e.g. batteries
62
two directions of electricial current
direct current (DC) alternating current (AC)
63
unit for direct current
amp (or ampere), A | Measure of how much charge flows past a point per second
64
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
65
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
66
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
67
electrical supply to X-ray unit
mains electricity ``` Alternating current (≤13 amps) 220-240 volts ```
68
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
69
transformers
alter the voltage (& current) from one circuit to another
70
2 transformers needed for X-ray unit
Mains -> X-ray tube (cathode-anode) Mains -> filament
71
step-up transformer
↑ potential difference across X-ray tube Usually 60,000-70,000 volts (60-70 kV) Current reduced to milliamps (mA)
72
step-down transformer
↓ potential difference across filament ~10 volts ~10 amps
73
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)
74
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)
75
↑ number &/or energy of photons =
↑ intensity
76
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)
77
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
78
intensity ∝
1/distance^2
79
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
80
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

BDS3 Radiology (14 decks)

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