A-level Physics - Medical imaging
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How are x-rays produced?
Electrons are produced by thermionic emission, accelerated by an anode and strike a metal plate where some of their energy is given off as x-rays.
Why do x-ray tubes need cooling?
Only about 1% of the electrons energy is converted to x-rays. The remainder is converted to heat.
Name the 3 ways that x-rays interact with matter
- Photoelectric effect
- Pair production
- The Compton effect
Describe the process of pair production with regard to x-rays
X-rays collide with a particle and spontaneously produce a positron and an electron
Energy of x-ray photon =
What is the Compton effect with regard to the interaction of x-rays with matter?
- X-rays collide with free electrons and bounce off them. Any photon deflected through a large angle will have lost more energy so will have a longer wavelength.
By which method do low energy x-rays interact with matter?
The photoelectric effect
By which method do very high energy x-rays interact with matter?
What does this equation represent?
The intensity of x-rays after passing through a distance x of material with an attenuation coefficient
and with an initial intensity of
What is meant by the "half-value thickness" ?
The thickness of material that will result in a halving of the intensity of incident x-rays
In the equation
What does the letter
The attenuation coefficient of the material
State 3 ways of enhancing x-ray images
- Any of:
- Use more sensitive photographic film
- Place a fluorescent plate behind the x-ray film
- Use a contrast medium (e.g. barium sulphate)
- Use an image intensifier
State to advantages of digital x-rays
- The ability to zoom in on detail
- Easy to share with remote specialists
- Subsequent x-rays can be easily compared with previous x-rays
- X-rays can be enhanced e.g. use of false colour
Angiograms use a method called the "subtraction technique". How does this work?
- An x-ray is taken and digitised
- A contrast medium is injected into the bloodstream and a 2nd x-ray taken and digitised
- The two x-ray images are then subtracted from one another, thereby only showing the contrast medium
Describe the principles of Computerised Axial Tomography scans (otherwise known as CAT or CT scans)
- A fan shaped beam of x-rays irradiates a thin slice of the patient and are detected by a ring of detectors
- The x-ray source is rotated around the patient and moves up the patient slightly
- The process is repeated until the patient has been completely scanned
- A computer constructs a 3-D image from all the x-ray slices
Why are gamma-ray sources used as medical tracers rather than alpha or beta sources?
- Alpha and beta would be absorbed by the body
- Alpha and beta are too ionising
Name the principal components of the gamma camera
- Collimator-a block of lead with lots of vertical holes through it
- Scintillator-sodium iodide crystal which emits a flash of light when it absorbs a gamma photon
- Photomultiplier tubes-multiply the effect of the flash of light
- Computer-to interpret the signals and produce an image
What is the purpose of the collimator in a gamma camera?
To ensure that only photons travelling vertically are detected by the camera. This ensures that the direction from which the photons have come is known.
What is the relevance of half life when considering radioactive sources to be used in medicine?
The half life needs to be long enough such that the source remains radioactive for the duration of the treatment, yet not so long that it continues to emit radiation long after the treatment has finished.
Describe the principles of positron emission tomography (PET)
- It is used to detect abnormal metabolic or chemical activity within the body.
- Radio-labelled glucose is injected into the bloodstream and is absorbed into the tissues
- When a positron is emitted it and an electron are annihilated and 2 gamma photons emitted in opposite directions simultaneously.
- The gamma photons are detected by a ring of detectors
Describe the principles of Magnetic Resonance Imaging (MRI)
- Apply a very strong magnetic field to make spinning protons precess. (The frequency of precession is called the Larmor frequency)
- Apply a magnetic field alternating at the Larmor frequency.
- The protons will absorb a relatively large amount of energy and flip over to a higher energy state
- Switch off the alternating magnetic field- let the proton relax back to the low energy state
- As it relaxes it will emit radio frequency radiation
- This radio frequency radiation can be detected and amplified
What is the significance of the relaxation time in MRI scans?
Relaxation times for hydrogen nuclei vary according to their surroundings. Tumour tissue has a different relaxation time to brain tissue.
Why do MRI scanners need a very large coils of wire and elaborate cooling systems
Large currents are needed to produce the large magnetic fields. The coils are kept near absolute zero, using the principles of superconductivity to reduce the resistance of the wires in the coil.
State advantages of MRI scans
- No ionising radiation involved
- High quality image
- Good distinction between different types of tissue
- Radio waves can penetrate the skull easily
- No side-effects
State disadvantages of MRI scans
- Metallic objects heat up when scanned
- Very expensive equipment
- Scans take a longer time therefore patient throughput is limited
What is the significance of the Larmor frequency in MRI scans?
- It is the frequency at which hydrogen nuclei precess when in a magnetic field.
- It is the frequency at which an alternating magnetic field will cause the hydrogen nuclei to absorb lots of energy and flip
- It is the frequency of radiation emitted when the nuclei relax
What does the Larmor frequency depend upon?
Magnetic field strength
Name 3 non-invasive techniques
- Ultrasound scans
- MRI scans
What is Doppler ultrasound used for?
To measure the speed and direction of blood flow
How does the frequency of ultrasound change when reflected by an object moving towards the transducer?
The wavelength reduces therefore the frequency increases
What is the piezoelectric effect?
- When certain crystals have a potential difference applied across them they contract.
- When these crystals are compressed a potential difference is induced across them
Why is ultrasound transmitted in pulses during ultrasound scans?
- To prevent the transmitted ultrasound interfering with the reflected ultrasound
- To allow time for the reflected pulse to be received by the transducer
Describe the principles of ultrasound scans
- An ultrasound pulse is transmitted
- It is (partially) reflected at the boundary between two media
- The reflected pulse is detected by the transducer
- The time taken between the pulse being transmitted and the reflection being received is used to determine the distance travelled or the type of media through which the pulse has travelled
What is ultrasound?
A longitudinal wave with a frequency above 20 kHz
What is the advantage of using very high frequency ultrasound for scans?
The higher the frequency of ultrasound, the higher the resolution of the scan i.e. the more detail that can be seen
What is the difference between an ultrasound A-scan and a B-scan?
- A-scan : used to measure distances and uses ultrasound travelling in only one direction
- B-scan : Uses an array of transducers to produce a fan shaped beam, producing an image
What is the relevance of impedance matching in relation to ultrasound scans?
If two materials are not of similar acoustic impedance, too much ultrasound will be reflected at the boundary. (This is why gel is used at the boundary between air and skin)
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