Summary
Summary A2 Alevel Physics Medical imaging Notes
- Module
- Medical Physics (9702)
- Institution
- CIE
Best notes to go through to get in depth understanding made with questions and answers from exam papers
[Show more]
Seller
Follow
Content preview
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
The benefits of buying summaries with Stuvia:
Guaranteed quality through customer reviews
Stuvia customers have reviewed more than 700,000 summaries. This how you know that you are buying the best documents.
Quick and easy check-out
You can quickly pay through credit card for the summaries. There is no membership needed.
Focus on what matters
Your fellow students write the study notes themselves, which is why the documents are always reliable and up-to-date. This ensures you quickly get to the core!
Frequently asked questions
What do I get when I buy this document?
You get a PDF, available immediately after your purchase. The purchased document is accessible anytime, anywhere and indefinitely through your profile.
Satisfaction guarantee: how does it work?
Our satisfaction guarantee ensures that you always find a study document that suits you well. You fill out a form, and our customer service team takes care of the rest.
Who am I buying these notes from?
Stuvia is a marketplace, so you are not buying this document from us, but from seller tshepangmolatedi. Stuvia facilitates payment to the seller.
Will I be stuck with a subscription?
No, you only buy these notes for £7.99. You're not tied to anything after your purchase.
Can Stuvia be trusted?
4.6 stars on Google & Trustpilot (+1000 reviews)
56326 documents were sold in the last 30 days
Founded in 2010, the go-to place to buy revision notes and other study material for 14 years now