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Summary A2 Alevel Physics Medical imaging Notes
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A2 CIE PHYSICS NOTESMEDICAL IMAGING
X--R-AYS
The nature and production of X-rays
• They have wavelengths in the range 10−8m to 10−13m and are effectively the same as gamma-rays (γ-rays), the difference being in the way
they are produced
○ X-rays are produced when fast-moving electrons are rapidly decelerated. As the electrons slow down, their kinetic energy is
transformed to photons of electromagnetic radiation.
○ γ-rays are produced by radioactive decay. Following alpha (α) or beta (β) emission, a gamma photon is often emitted by the decaying
nucleus
•
Usually for medical applications, soft X rays are used, as they energy is not very great.
X rays have wave particle duality and thus we can talk about their photons.
X-rays are formed in a Vacuum or evacuated tube
• The cathode- The heated filament acts as the cathode and gives rise to thermionic emission of electrons
• Anode: The rotating anode is made out of hard metals such as tungsten and these act as the target material.
• Part of the outer casing, the window, is thinner than the rest and allows X-rays to emerge into the space outside the tube
• The width of the X-ray beam can be controlled using metal tubes beyond the window to absorb X-rays. This produces a parallel-sided beam
called a collimated beam.
• Most of the incident energy is transferred to the anode, which becomes hot. This explains why the anode rotates; the region that is heated
turns out of the beam so that it can cool down by radiating heat to its surroundings
X- Ray Spectrum
The X-rays that emerge from an X-ray tube have a range of energies
Each spectrum has two components
• broad background ‘hump’ of braking radiation
• a few sharp ‘lines’ of characteristic radiation
Medical Imaging X rays Page 1
, When an electron is emitted and strikes the metal, the electron interacts with nuclei of atoms until it loses its energy until it comes to a halt.
The X rays emitted in this process contribute to the background breaking radiation.
An electron also may cause rearrangement of the electrons in an anode atom , causing electrons to drop from high energy levels to lower
ones. This forms line spectra excite n de-excite.
WHY A CONTINOUS SPECTRUM
• EM is produced when moving electrons are rapidly decelerated
• This happens when the electrons hit the metal target
• Larger deceleration, larger photon energy
• Range of decelerations so continuous range of frequencies
• Thus there is a continuous spectrum of X rays with different wavelengths
Max E is proportional to the voltage applied
The lowest energy X-rays will not have sufficient energy to penetrate through the body, so will have no effect on the resulting image.
However, they will contribute to the overall X-ray dose that the patient receives.
These X-rays must be filtered out; this is done using aluminum absorbers across the window of the tube.
Controlling intensity and hardness
The intensity of an X-ray beam is a measure of the energy passing through unit area
○ To increase the intensity of a beam, the current in the X-ray tube must be increased. More electrons per second, more X-ray photons
per second.
Hardness [ENERGY]
hardness measures the penetration of the beam
greater hardness, greater penetration
Hard X ray is one that has a short wavelength and great energy
Soft X-rays are less penetrating so they contribute more to patients exposure to harmful radiation.
The hardness of an X-ray beam can be increased by increasing the anode voltage across the X-ray tube
Another method is to use a filter which absorbs the lower energy soft X-rays so that the average energy of the X-rays is higher
X-Ray Attenuation
Good absorbers like bones absorb X- rays and little radiation arrives at the photographic films.
The better the absorber the greater the attenuation coefficient.
ATTENUATION
The gradual decrease in the intensity of a beam of X-rays as it passes through matter is called attenuation
IMPROVING X RAYS
• REDUCING DOSAGE
X-rays, like all ionizing radiation, can damage living tissue, causing mutations which can lead to the growth of cancerous tissue. It is therefore
important that the dosage is kept to a minimum.
○ intensifier screens
These are sheets of a material that contains a phosphor,
a substance that emits visible light when it absorbs X-ray photons.
The film is sandwiched between two intensifier screens .
Each X-ray photon absorbed results in several thousand light photons
which then blacken the film.
This reduces the patient’s exposure by a factor of 100–500
image intensifiers
Medical Imaging X rays Page 2
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