 'A' Level Medical Option Questions - X-Rays Q12.

(a) Explain the term contrast enhancement when applied to X-ray photographic imaging and explain how such enhancement is achieved.

The clearest images are obtained when there is a large difference in density/proton number between the part of the body being investigated and the surrounding parts. You can improve contrast by introducing a high proton number material into a bady cavity, such as barium sulphate - this is done as a 'barium meal' or 'barium enema'. (2 marks)

(b) The half-value thickness of lead for 90 keV X-ray photons is 12 mm.

(i) What is meant by half-value thickness?

Half-value thickness is the thickness of material needed to reduce the transmitted intensity of the X-rays to half of the intensity incident on the material. (ii) Calculate the linear attenuation coefficient, μ, for these X-ray photons in lead. Your equations do not 'give' you an 'equation Bingo' solution to this problem. You have to use the definition to work it out!

The intensity level drops to a half. so,

0.5 = e-μx

Taking logs we get:

ln 0.5 = -μx

or ln 2 = μx

μ = (ln 2)/0.012 = 58 m-1 (iii) It is required to reduce the intensity of X-radiation escaping in unwanted directions from a 90 kV X-ray tube to 5% of its full intensity. Calculate the thickness of lead shielding needed to achieve this reduction.

ln (0.05) = - 58 × d d = 0.052 m (5 marks)

(7 marks total)