Medical Physics: X-Rays

Question 1

Nature of X-Rays.

Answer 1

• Electromagnetic Wave.

- Wave length range 10-8 to 10-13 m.

Question 2

Describe how X-Rays are produced.

Answer 2

• High energy electrons.
• Made to bombard with metal target.
• When the electrons are accelerated and/or slowed down, electromagnetic radiation is produced.
• For large accelerations, radiation is in X-ray region.

Question 3

Photoelectric effect.

Answer 3

X-ray photon with energy less than 100KeV is absorbed by electron in the target metal, so that the electron gains enough energy to escape from the atom.

Question 4

Compton scattering.

Answer 4

X-ray photon (with energy of 0.5MeV – 5MeV) loses only a
fraction of it’s energy to an atomic electron in the absorbing material. The interaction is inelastic, causing the scattered X-ray to have less energy than before, thus greater wavelength. Compton Electron goes off in different direction from scattered (X-ray) photon as momentum has to be conserved.

Question 5

Pair production.

Answer 5

High energy X-ray photon (E > 1.02 MeV) passing through electric field of nucleus suddenly produces electron-positron pair; it’s energy appears as the mass of the electron and positron. Positron is soon annihilated after colliding with another electron. Not used in diagnostics as X-ray energies are too low.

Question 6

Intensifying screen.

Answer 6

Screen converts fewer X-ray photons into many visible light
photons which expose the film. Limits patients exposure to harmful rays.

Question 7

Image intensifier.

Answer 7

• Used for fluoroscopy – real time moving image.
• X – rays incident on phosphor which converts X-ray photons into multiple visible photons (amplification).
• Visible light photons incident on photocathode, converts photons to electrons.
• Electrons accelerated (by 25kV) towards output phosphor.
• Electron beam converted back into visible light, sent to camera.
• Image is processed digitally so can be viewed on screen/stored digitally.

Question 8

Contrast media.

Answer 8

Barium meal is taken to image soft tissue (eg in digestive tract).Barium has high absorption coefficient (high μ). This improves contrast of the image.

Question 9

Describe operation of CAT scanner.

Answer 9

• X-ray image produced of target area.
• Image is stored on computer.
• X-ray beam rotated around target so that many images are produced from different angles.
• Computer stores and processes image to give slices through target.
• Process is then repeated for different slices to produced 3D scan of target area.

Question 10

Advantages of CAT scan over standard X-Ray.

Answer 10

• CAT scan produces 3D image meaning it can give a 2D image of any plane through target area and that it can show relationships between different tissues better (due to being 3D)
• Can distinguish between tissues with similar densities (or attenuation coefficients, μ)