Physics Basics Flashcards
What are radiographs?
- Images created by x-rays which have been projected through an object & then interacted with a receptor on the other side
What do the different shades of grey on a radiograph correspond to?
- The different shades of grey on the image correspond to the different types of tissue & thickness of tissue involved
- The x-rays pass through a tooth - the very dense enamel will have a whiter area on the image if the x-rays only pass through the skin of the cheek we will have a very dark image as these tissues aren’t very dense
Why are radiographs useful in dentistry? (3)
Provide the ability to see structured 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 and monitoring
What are the 3 common intra-oral radiographs?
- Periapical
- Bitewing
- Occlusal
What are the 2 common extra-oral radiographs?
- Panoramic
- Lateral cephalograms
What are x-rays a form of?
Electromagnetic Radiation
What is electromagnetic radiation?
- The flow of energy created by simultaneously varying electrical & magnetic fields (schematically represented as a sine wave)
What are the main properties of electromagnetic radiation? (4)
- No mass
- No charge
- Always travels at the speed of light (3x10^8 ms-1 = 671 million mph)
- Can travel in a vacuum
What is the speed of light?
- 3x10^8 ms-1 = 671 million mph
What does the EM spectrum consist of? (3)
- Consists of all different types of Electromagnetic radiation
- Each type has different properties, dependent on its energy/wavelength/frequency
- Typically divided into 7 main groups
How many main groups is the EM spectrum typically divided into?
7
What are the 7 main groups in the EM spectrum?
- Gamma ray
- X-ray
- UV
- Visible
- Infrared
- Microwave
- Radio
How do we determine wavelength?
= divide the number of cycles over the length
How do we determine the frequency?
= divide the number of cycles by the number of seconds
What is frequency?
Frequency = how many times the wave’s shape repeats per unit time
What unit is frequency measured in?
Measured in hertz, Hz
one hertz = one cycle per second
What is wavelength?
Wavelength = the distance over which the waves shape repeats
What unit is wavelength measured in?
Measured in metres, m
What does speed equal?
- Frequency x wavelength
- BUT speed of all electromagnetic radiation is constant (3x10^8ms-1 - speed of light)
- Therefore if frequency increases then wavelength must decrease (& vice versa)
What is energy proportional to?
Frequency
- So if something has a high frequency it also has a high energy
WM radiation involved the movement of energy as ‘packets of energy’. What are these packets of energy known as?
Photons
What unit is energy usually measured in?
- Energy usually measured in electron volts, eV
What does 1eV mean in relation to energy?
= energy (in joules) gained by 1 electron moving across a potential difference of 1 volt
Why were x-rays names x-rays?
Because of their unknown nature
For electromagnetic radiation to be classed as x-ray energy it has to be in a range. What is this range?
Around 124eV - 124keV
What are the 2 different types of x-ray?
- Hard and soft
What are hard x-rays?
- Have higher energies
- Able to penetrate human tissues
What are soft x-rays?
- Have lower energies
- Easily absorbed
- (don’t want soft x-rays if we are taking a radiograph as we want some of the x-rays to pass through the body and reach the receptor therefore medical imaging mostly uses hard x-rays)
Which type of x-ray do we usually use for medical imaging?
- Mostly use hard x-rays (e.g. >5keV)
What are the properties of x-rays? (4)
Form of electromagnetic radiation:
- No mass, no charge, Very fast, can travel in a vacuum
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
Which property of x-rays is this process which is what causes the dangerous side effects that humans can have when they are exposed to too much radiation?
- The fact x-rays cause ionisation
What is the basic production of x-rays? (3)
- Electrons fired at atoms at very high speed
- On collision, the kinetic energy of these electrons is converted to EM radiation (ideally x-rays) & heat
- The x-ray photons are aimed at a subject
What are the ‘building blocks’ of matter?
Atoms (they make up many of the things in the universe)
What do atoms consist of? (3)
Central nucleus:
- Protons (+ve charge)
- Neutrons (neutral)
Orbiting ‘shells’:
- Electrons (-ve charge)
What is the relative charge of a neutron?
0
What is the relative charge of a proton?
+1
What is the relative charge of an electron?
-1
What is the relative mass of a neutron?
1
What is the relative mass of a proton?
1
What is the relative mass of an electron?
Negligible (0)
Where is the location of a proton?
- Nucleus
Where is the location of a Neutron?
- Nucleus
Where is the location of an electron?
- Shells
What is in the nucleus of an atom?
- Collection of nucleons:
- Protons and neutrons have a similar mass
What is the overall charge of the nucleus of an atom?
Positive
What does the atomic number (Z) equal?
Atomic number = number of protons (unique to each element)
What does the mass number (A) equal?
Mass number = number of protons + neutrons (this can change as can have different numbers of neutrons in an atom without changing elements)
What does the number of electrons in an atom determine?
- Determines the chemical properties of the atom
An atom in its ‘ground state’ is neutral. What does this indicate?
Number of electrons = number of protons
What does ionisation mean?
Ionisation = removing/adding electron(s) to an atom
- Atom - e- = positive ion
- Atom +e- = negative ion
Where are electrons positioned in atoms?
- Electrons spin around the nucleus in discrete orbits/shells
- Cannot exist between these shells
What is the name of the innermost electron shell?
- K
- Then L, M, N, O etc
- Electrons try to fill available spaces in the inner shells first
What is the maximum number of electrons that can be held in the first shell (K)?
2
What is the maximum number of electrons that can be held in the second shell (L)?
8
What is the maximum number of electrons that can be held in the third shell (M)?
18
What is the maximum number of electrons that can be held in the fourth shell (N)?
32
How do we work out what the maximum number of electrons in a shell can be?
- Maximum number = 2n^2
- Where n is the shell number
Example: M shell is number 3 -> 2 x 3^2 = 18
What force are orbiting electrons held within their shells by?
Electrostatic force
What is electrostatic force?
-ve charge of electrons attracted to overall +ve charge of nucleus
To remove an electron from its shell, a specific amount of energy is required to overcome this attraction. What is this energy called?
Binding energy = additional energy required to exceed electrostatic forces
The closer the electron is to the nucleus, the greater the electrostatic force (& therefore the binding energy). So, which shell electrons have the highest binding energy?
K shell electrons
How do we calculate the specific amount of energy required to move an electron to a more outer shell (i.e. away from the nucleus)?
This equals the difference in the binding energies of the 2 shells
If an electron drops to a more inner shell, what happens?
- This specific amount of energy is RELEASED
- Possibly in the form of x-ray photons (if sufficient energy)
What is included in a dental x-ray unit? (5)
- Tubehead (creates the x-rays)
- Collimator (the x-rays pass through here)
- Positioning arm (holds the tube head and allows you to position it)
- Control panel
- Circuitry (to power it)
What is ‘current’?
- Flow of electric charge, usually by the movement of electrons
What unit is current measured in?
- SI unit: amp (or ampere), A
- (measure of how much charge flows past a point per second)
Current can be defined by its direction. What are the 2 different directions?
Direct current = constant unidirectional flow
Alternating current = flow repeatedly reverses direction
Give an example of something that uses direct current?
Batteries
What is alternating current?
- Flow periodically reverses direction
What is the frequency of alternating currents?
- Number of complete cycles (reverse + reverse-back) per unit time is the frequency
What unit is used for the frequency of alternating current?
SI unit: hertz, Hz (cycles per second)
Give an example of something that uses alternating current?
Mains electricity (50Hz in UK) - so the current reverses and reverses-back 50 times every second
What is rectification of current?
- X-ray production requires a unidirectional current BUT x-ray units are powered by mains electricity which are alternating currents
- X-ray units have generators which modify the AC so that it mimics a constant unidirectional current (process known as rectification)
What is ‘voltage’?
Difference in electrical potential between 2 points in an electrical field (difference between a negatively charged point in one place and a positively charged point in another)
What is voltage related to?
- Related to how forcefully a charge will be pushed through an electrical field (if it is between these 2 points)
What is the unit used for voltage?
SI unit: volt, V
What is another name for voltage?
Potential difference
What is the mains supply in the UK like?
Alternating current (< or equal to 13 amps)
What is the voltage of the mains supply in the UK?
220-240 volts
Dental x-ray units require 2 different voltages. What are these?
- One as high as 10s of thousands of volts
- One as low as around 10 volts
What is a transformer?
- Transformers alter the voltage (& current) from one circuit to another
Two separate transformers required for x-ray unit:
- Mains -> x-ray tube (cathode-anode)
- Mains -> filament
What do ‘step-up’ transformers do? (3)
- They increase the potential difference across x-ray tube
- Usually 60,000-70,000 volts (60-70 kV)
- Current reduces to milliamps (mA) (because we can’t create electricity out of nothing we also have to reduce the current)
What do ‘step-down’ transformers do? (3)
- decrease the potential difference across filament to approximately:
- 10 volts
- 10 amps
What is an x-ray beam made up of?
- 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)
What is the strength of the x-ray beam called?
- The intensity
What is the x-ray beam intensity measured as?
- Measured as quantity of photon energy passing through a cross-sectional area of the beam per unit time
If you increase the number &/or energy of photons in an x-ray beam what are you also increasing?
The intensity
What is x-ray beam intensity proportional to?
Proportional to current in filament (mA) & potential difference across x-ray tube
What happens to dose as we move away from the x-ray source?
- 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)
What is the inverse square law?
- To work out how much you can cut down the radiation you might receive from a source by standing further away we can use this law
- Remember that if you double the distance that you are standing away from a source you will quarter the dose
What are 3 other types of ionising radiation?
- Alpha particles
- Beta particles
- Gamma rays
(these are all produced by radioactive decay of unstable atoms unlike x-rays which are directly man-made)
