'A' Level Medical Option Questions - X-Rays
(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'.
(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.
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
(7 marks total)