1 - Physics basics Flashcards
What is a radiograph?
Image created by x-rays projected through an object and interacted with a receptor
What are radiographs used for?
- visualising structures within the body, particularly mineralised structures
- aids diagnosis, treatment planning and monitoring
What are the common intra-oral radiographs?
- periapical
- bitewing
- occlusal
What are the common extra-oral radiographs?
- panoramic
- lateral cephalograms
What are properties of EM radiation?
- no mass
- no charge
- travel at 3x10^8 m/s
- travel in a vacuum
What is the EM spectrum?
- 7 groups with different properties based on energy, wavelength and frequency
- gamma ray, x-ray, UV, visible, infrared, microwave and radio
What is frequency?
- how many times a waves shape repeats per unit time
- measured in hertz
- 1Hz = one cycle per second
What is wavelength?
- distance over which the wave’s shape repeats
- measured in metres
How is energy proportional to frequency?
- directly proportional
- higher frequency = higher energy
How are frequency and wavelength related?
Higher frequency = shorter wavelength
What are photons?
- “packets of energy”
- measured in electron volts, eV
- 1 eV = energy gained by an electron moving across a potential difference of 1 volt
What are the different types of x-rays?
- photon energy varies between 124eV - 124keV
- hard x-rays (>5keV)
- soft x-rays
Which type of x-rays are used in medical imaging?
Hard x-rays >5keV
How are x-rays formed?
- man made
- electrons fired at atoms at high speed
- on collision the kinetic energy is converted to EM and heat
- x-ray photons are aimed at subject
What is an atom made up of?
- protons
- neutrons
- electrons
What is the atomic number?
- Z
- number of protons
- unique to each element
What is the mass number?
- A
- number of protons and neutrons
What are the atomic and mass number of tungsten?
Z = 74
A = 184
What is the ground state of an atom?
When number of electrons = protons
What is ionisation?
Removing or adding of electrons to change the charge of an atom
What is the innermost shell of an electron called?
K
What is the maximum number of electrons in a K shell?
2
What is the maximum number of electrons in a L shell?
8
What is the maximum number of electrons in a M shell?
18
What is the maximum number of electrons in a N shell?
32
How do you calculate the maximum number of electrons in a shell?
2n^2
How are electrons held in the shells?
- negative charge of electrons attracted to positive nucleus
- to remove electrons, binding energy must be broken
What is binding energy?
- energy required to exceed the electrostatic force of the attraction of the electrons to the nucleus
- closer the electron is to the nucleus, the higher the binding energy
What is the K shell binding energy of a tungsten atom?
69.5keV
How does electrons moving between shells release energy?
- an electron moving to a more outer shell releases binding energy
- energy released is equal to difference in energy shells
What is current?
- flow of electric charge
- movement of electrons
- measured in amps, A
What is direct current?
Constant unidirectional flow of electrons (eg batteries)
What is alternating current?
- electron flow repeatedly reverse direction (eg mains)
- number of complete cycles per unit time = frequency
- main frequency = 50Hz
What type of current is used in x-ray machines?
- DC
- powered by AC source, therefore must be rectified
What is voltage?
- difference in electrical potential between 2 points
- how forcefully a charge is push through an electrical field
- measured in volts, V
What voltage is required in an x-ray machine?
- 2 different voltages
- one as high as 10s of thousands of volts
- one around 10V
What is a transformer?
Alters voltage and current from one circuit to another
What transformers are present in an x-ray machine?
- step up transformer, mains to X-ray tube (cathode/anode)
- step down transformer, main to filament
How do x-ray beams travel?
Photons travel in straight lines but diverge from the source
What is the inverse square law?
- intensity of x-ray beam is inversely proportional to square of the distance between the source and point of measurement
- “double the distance, quarter the dose”
