OCR G485 Module 4

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X-rays.

X-rays can be polarised.

X-rays are electromagnetic waves.

To produce x-rays a high voltage and a good vacuum in a tube with an anode and a cathode is required.

Electrons are accelerated from the cathode to the positive anode.

  • If they have enough energy some x-rays will be emitted.
  • The cathode is often heated to increase the output of electrons.
  • Only 1% of energy is the electron beam.

X-rays as particles.

X-rays use the wave-particle duality.

x-rays and the photoelectric effect:

  • x-rays can cause emission of photoelectrons.
  • The work function of x-rays are so small it's ignored.
  • The emitted photoelectrons have a maximum kinetic energy equal to the photon energy of x-rays.
  • These electrons cause ionisation.
  • Measuring the energy of the electrons is a method of detecting and measuring the energy of x-rays.

Pair production:

  • Beam of high-frequency x-rays produce a positron and an electron.
  • 1.02MeV would be needed for an x-ray photon.
  • The effect is only seen in high-voltage systems.

The Compton effect.

  • Compton filtered out all wavelengths except those with the strongest sharp peaks.
  • He found some deflected x-rays had a longer wavelength than the initial x-rays.
  • This was explained using the quantum theory.
  • Any photon deflected through a large angle will have lost more energy so its wavelength will be longer.

X-ray intensity

The intensity of a beam of x-rays is defined as the power per unit cross-sectional area.

X-ray beams may be collimated in which the intensity hardly changes with distance.

X-ray absorption.

  • The amount of absorption varies with the frequency of the x-rays.
  • At higher frequencies the compton effect is the main method.
  • At very high frequencies pair production is most important.
  • The distance for halving the intensity is called the half-value thickness.

Image enhancement.

X-ray film is the traditional way of making an x-ray image and requires considerable exposure.

Other methods are used:

  • Use photographic film that is more sensitive to x-rays or put a fluorescent plate behind the film. Both result in less exposure.
  • Use an x-ray absorbing substance as a contrast medium. Patients are given a barium meal before having an x-ray taken of the digestive tract.
  • Use an image intensifier, many radiography departments now use digital methods.

3D X-ray images.

2D x-rays.

  • Traditional x-ray show a shadow of the part of the body which x-rays have passed.
  • If a bone is broken the break shows up as a shadow

Angiograms.

  • An x-ray of the heart is taken and digitised
  • A contrast medium is then injected into the blood stream, and a second x-ray is then taken and digitised.
  • The computer images are then subtracted from one another to only leave changes on the final photo.

Computerised axial tomography (CAT) scan

  • Use x-rays to create a 3D image.
  • The source is shielded so the rays emerge from a point and spread out through the patient.
  • The x-ray beam is fan shaped and has little thickness.
  • Once passed through the patient x-rays are detected by a ring of detectors.
  • The x-ray source…

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