Physics Basics Flashcards
What are radiographs
• They are images created by X-rays which have been projected through an object (usually patient’s teeth and bone) and then interacted with a receptor on the other side
Why are there different shades in radiographs
• The different shades of grey on the image correspond to the different types of tissue and thickness of tissue involved
Why are radiographs useful
They provide the ability to see structures within the body, particularly mineralised tissue
• It can show normal anatomy and pathology
• It aids diagnosis, treatment planning and monitoring
What is electromagnetic radiation
• Electromagnetic radiation is the flow of energy created by simultaneously varying electrical and magnetic fields and it is schematically represented as a sine wave
What are the properties of electromagnetic radiation
○ No mass
○ No charge
○ Always travel at speed of light (671 million mph)
○ Can travel in a vacuum
What is frequency
how many times the wave’s shape repeats per unit time
○ Measured in hertz, Hz
○ (1Hz = 1 cycle per second)
What is wavelength
distance over which the wave’s shape repeats
○ Measured in metres
What is speed
frequency x wavelength
○ BUT speed of all EM radiation is a constant of 3x108 ms-1
Therefore if frequency increases then wavelength must decrease and vice versa
What is photon energy
- EM radiation involves the movement of energy as ‘packets of energy’ known as photons
- Energy is usually measured in electron volts, eV
- 1 eV = energy (in joules) gained by 1 electron moving across a potential difference of 1 volt
What are X-ray photon energies
• X-ray photon energies - 124eV - 124keV
What are the types of X-rays
○ Hard X-rays (higher energies)
§ Able to penetrate human tissues
○ Soft X-rays (lower energies)
§ Easily absorbed but we don’t want this for radiographs because we want the x ray to pass through the body and reach the receptor
What is the property of x rays
• Form of EM radiation
○ No mass, no charge, very fast, can travel in a vacuum etc
• Undetectable to human senses (unlike visible light)
• Man mad (unlike gamma rays which are produced by radioactive decay)
• Cause ionisation i.e displacement of electrons from atoms and molecules
○ This process is what causes the dangerous side effects of X-rays
Describe the basic production of X-rays
- Electrons are fired at atoms at very high speed
- On collision, the kinetic energy of these fast moving electrons is converted to electromagnetic radiation (ideally X-rays) & heat
- The X-ray photons are aimed at a subject
What do atoms consist of
central nucleus
orbiting shells
What does the central nucleus consist of
§ Protons (positive charge)
§ Neutrons (neutral)
What does the orbiting shells consist of
§ Electrons (negative charge)
What is the charge of the nucleus
○ Protons and neutrons have a similar mass
○ There is an overall positive charge as neutrons are neutral
What is the atomic number
(Z) = number of protons and is unique to each element
What is the mass number
(A) = number of protons + neutrons and this can change for the same element
What are the number of electrons
• An atom in its ‘ground state’ is neutral
○ Number of electrons = number of protons
• Ionisation = removing/adding electrons to an atom
What happens if you remove/add an electron
○ If you remove an electron becomes a positive ion (because number of positive now outweigh negative)
○ If you add an electron it becomes a negative ion
What are the 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
• Electros try to fill available spaces in the inner shells first
How is the max number of electrons in a shell calculated
• The maximum number is 2n2 where n is the shell number (K is 1, L is 2 etc)
How are orbiting electrons held in
• Orbiting electrons are held within their shells by electrostatic force - this is due to the negative charge of electrons being attracted to the overall positive charge of the nucleus
How do you remove an electron from its shell
a specific amount of energy is required to overcome this attraction
The binding energy is the additional energy required to exceed electrostatic force
What is binding energy
- The closer to the nucleus, the greater the electrostatic force and therefore binding energy
- K shell electrons have the highest binding energies, then L, M, etc
- The more positively charged the nucleus (i.e higher Z) the greater the electrostatic force
What is the amount of energy required to move from one shell to another
- The specific amount of energy required to move an electron to a more outer shell 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)
What is the energy required to move an electron from K shell to L shell in a tungsten atom
○ Binding energy of K shell - binding energy of L shell
What does the dental x ray unit consist of
○ Tube head ○ Collimator ○ Positioning arm ○ Control panel ○ Circuitry
What are the electricity fundamentals
○ Current
○ Voltage
○ Transformers
What is current
• Flow of electric charge, usually by the movement of electrons
What is the unit for current
• SI unit: amp (or ampere), A, is a measure of how much charge flows past a point per second
What are the different directions of current
direct current
alternating current
What is direct current
constant unidirectional flow e.g batteries
What is alternating current
flow repeatedly reverses direction
§ Flow periodically reverses direction
§ The number of complete cycles (reverse + reverse-back per unit time is the frequency)
What is the unit for alternating current
hertz, Hz (cycles per second)
What is the mains electricity
mains electricity is 50Hz in the UK - this means the current reverses and reverses back in 50 times a second
What is 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 in a process known as rectification
What is voltage
- Difference in electrical potential between 2 points (a negative and positively charged point) in an electrical field
- Related to how forcefully a charge will be pushed through an electrical field (through these 2 points)
What is the unit of voltage
SI unit: volt, V
What is the electrical supply in the mains supply in the uk
○ Alternating current (<13 amps)
○ 220-240 volts
What is the dental x ray unit
○ Required direct current which is solved by rectification
○ Requires 2 different voltages
§ One as high as 10s of thousands of volts
§ One as low as around 10 volts
What are transformers
• Transformers alter the voltage (& current) from one circuit to another
What are the transformers required for X-ray unit
step up transformer
step down transformer
What is a step up transformer
One takes the main supply and changes it so its appropriate for the X-ray tube (cathode-anode)
§ Increases potential difference across the X-ray tube in the unit
§ Usually 60,000-70,000 volts (60-70kV)
§ Current reduced to milliamps (mA)
What is a step down transformer
one takes the main supply and changes it so it is appropriate for the filament
§ Decreases the potential difference across the filament
§ 10 volts
§ 10 amps
What is an 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 (do not travel in parallel)
What is the x-ray beam intensity
- The strength of the X-ray beam is known as intensity
- It is measured as the quantity of photon energy passing through a cross-sectional area of the beam per unit time
- The greater the number and/or energy of photons = greater intensity
- It is proportional to current in the filament (mA) and the potential difference across the X-ray tube (kV)
Why should we be far away from the beam
its divergent
• Dose decreases with distance from X-ray source
• Ensure staff stand a sufficient distance from patient and not in the direction of the primary X-ray beam
What is the inverse square law
- Intensity of X-ray beam is inversely proportional to the square of the distance between the X-ray source and the point of measurement
- Intensity = 1/distance2
- Therefore, doubling the distance will quarter the dose
