Chapter 27 Medical imaging Flashcards
Who discovered X-rays?
Wilhelm Rontgen
How does an x-ray tube work?
A tube is evacuated and a cathode electrode is placed at one end which is heated with electricity. An anode is placed at the other end of the tube and is made from tungsten. A high voltage supply gives each electrode a charge and the heat from the cathode causes electrons to be released (called thermionic emission) and these are accelerated towards the tungsten target. When they RAPIDLY DECELERATE on impact, x-rays are produced. These are filtered out of a window so the x-rays only cover a certain area
What happens to the tungsten in the x-ray tube and why does this happen?
It becomes very hot and needs to be cooled with oil or by rotation the tungsten target so that the heat is spread out.
This happens because only 1% of the energy of the electrons is turned into x-rays and most is turned into heat
How many electrons does it take to release 1 x-ray photon?
One electron produces one x-ray photon
What is the equation that is used for x-rays?
λ = hc / eV λ = wavelength of x-rays produced h = planks constant c = speed of light e = elementary charge V = potential difference
Maximum energy of x-ray photon = …
= maximum kinetic energy of electron
Increasing the current…
… Increases the intensity of the x-rays
What does attenuation mean?
This is the change in the intensity of the electromagnetic waves as it passes through matter
What are the 4 ways that x-ray photons interact with matter?
Simple Scatter
Photoelectric effect
Compton Scattering
Pair production
Describe x-ray Simple scattering
An x-ray photon collides with an electron of an atom and the x-ray gets scattered because the x-ray has not enough energy to remove the electron. The energy of the photon needs to be in the range 1 - 20 keV
Descrive x-ray Photoelectric effect
An x-ray photon collides with an electron and this causes the electron to be removed from the atom because it has enough energy to escape the attractive force of the nucleus. The energy of the photon needs to be less than 100keV
Describe x-ray Compton scattering
An x-ray photon collides with an electron and this causes a low energy photon to be scattered and the electron to be removed from the atom. The energy of the x-ray needs to be between 0.5 - 5 MeV. Some of the energy goes into removing the electron and some of the energy goes into producing the low energy photon
Describe x-ray Pair production
This is when an x-ray photon interacts with the nucleus of the atom and disappears and produces an electron and a positron. The energy of the x-ray needs to be equal to or greater than 1.02MeV
How does that thickness of a substance change a photons intensity?
It falls exponentially
What is the equation for the change in intensity of a photon passing through a substance?
I = I0e^-µx I = intensity I0 = Starting intensity (before it entered the substance) µ = The attenuation coefficient x = thickness of the substance
