1. Full-body
CT scans produce detailed 3-D information about a patient and can identify
cancers at an early stage in their development.
(a) Describe
how a CT scan image is produced, referring to the physics principles involved.
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[7]
(b) State and explain two reasons
why full-body CT scans are not offered for regular checking of healthy
patients.
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[3]
[Total 10 marks]
2. In order to take an X-ray
photograph, the X-ray beam is passed through an aluminium filter to remove low
energy X-ray photons before reaching the patient.
(a) Suggest
why it is necessary to remove these low-energy X-rays.
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[1]
(b) The
average linear attenuation coefficient for X-rays that penetrate the aluminium
is 250 m–1. The intensity of an X-ray beam after travelling through 2.5
cm of aluminium is 347 W m–2.
Show
that the intensity incident on the aluminium is about 2 × 105 W m–2.
[3]
(c) The X-ray beam at the filter has a
circular cross-section of diameter 0.20 cm. Calculate the power of the X-ray
beam emerging from the aluminium filter. Assume that the beam penetrates the
aluminium filter as a parallel beam.
power =
............................ W
[2]
(d) The
total power of X-rays generated by an X-ray tube is 18W.
The efficiency of conversion of kinetic energy of the electrons into X-ray photon energy is 0.15%.
The efficiency of conversion of kinetic energy of the electrons into X-ray photon energy is 0.15%.
(i) Calculate
the power of the electron beam.
power = ..................... W
[2]
(ii) Calculate the velocity of the
electrons if the rate of arrival of electrons is 7.5 × 1017 s–1. Relativistic effects may be
ignored.
velocity = .....................
m s–1
[2]
(iii) Calculate
the p.d. across the X-ray tube required to give the electrons the velocity
calculated in (ii).
p.d.= ........................ V
[3]
[Total 13 marks]
. Describe the use of a contrast medium, such as barium, in the imaging of internal body structures. Your answer should include
• how
an image of an internal body structure is produced from an X-ray beam
• an
explanation of the use of a contrast medium
• examples
of the types of structure that can be imaged by this process.
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[Total 8 marks]
