What is an atom?
The smallest part of an element that still has the properties of that element
Atoms consist of a nucleus surrounded by electrons.
What is the ground state of an electron?
The lowest energy state that an orbital electron may inhabit
Any states other than this are known as excited states.
What is ionisation?
When an orbital electron is removed from an atom
What is electromagnetic radiation?
Comes in seven types, travels as waves at the speed of light
As frequency increases, wavelength decreases.
What is a photon?
A particles of electromagnetic radiation
Photons exhibit wave-like properties such as wavelength and frequency
The energy of a photon is given by the Planck equation: E = hf. What do the variables represent?
- E: photon energy
- h: Planck constant
- f: frequency
The energy of a photon is directly proportional to its frequency.
What is ionising radiation?
Radiation that has enough energy to ionise atoms
Ionisation can damage biological molecules, such as DNA.
What is projectional radiography?
A technique where X-rays are passed through a sample to form a two-dimensional image
This technique relies on partial absorption of X-rays
What does the cathode do in an X-ray tube?
The cathode is heated until it releases electrons via thermionic emission
What does the anode do?
The anode absorbs electrons and releases X-ray photons
What effect does increasing the voltage between the anode and cathode have on X-rays?
Increases the maximum energy and penetrating power of the X-ray photons
However, it can also decrease the contrast in the final image
Why are the anode and cathode placed into a vacuum?
To prevent the electrons from interacting with gas molecules before arriving at the anode
Why is there a very high voltage between the anode and cathode
To accelerate the electrons towards the anode
If the voltage wasn’t high enough, then the photons released would have lower energies, and would no be X-rays (so they could be UV, visible or infra-red photons)
What happens when you increase the current in an X-ray tube?
More electrons are emitted per second, meaning that more X-rays are produced, increasing the brightness of the final image
This has no effect on the contrast
What is attenuation?
The reduction in intensity of a signal or wave as it travels further from a source
What two mechanisms cause attenuation in an X-ray beam?
Absorption and Scattering
Why is absorption the most important attenuation mechanism for diagnostic X-rays?
The amount of absorption varies depending on tissue type
What is linear attenuation coefficient?
a measure of how easily a material can absorb or scatter a beam of radiation
What is the mass attenuation coefficient?
The ratio of linear attenuation coefficient to density
It helps in understanding how different materials affect X-ray beams.
What is density?
Mass per unit volume
It has units of kilograms per metre cubed
