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Date May 2017 Marks available 2 Reference code 17M.3.HL.TZ1.13
Level Higher level Paper Paper 3 Time zone 1
Command term Explain Question number 13 Adapted from N/A

Question

In the context of nuclear magnetic resonance (NMR) imaging explain the role of

Outline why the fracture in a broken bone can be seen in a medical X-ray image.

[2]
a.

The diagram shows X-rays incident on tissue and bone.

The thicknesses of bone and tissue are both 0.054 m.

The intensity of X-rays transmitted through bone is Ib and the intensity transmitted through tissue is It.

The following data are available.

Mass absorption coefficient for bone = mass absorption
coefficient for tissue = 1.2 × 10–2 m2 kg–1
Density of bone = 1.9 × 103 kg m–3
Density of tissue = 1.1 × 103 kg m–3

Calculate the ratio I b I t .

[3]
b.

the large uniform magnetic field applied to the patient.

[1]
c.i.

the radio-frequency signal emitted towards the patient.

[2]
c.ii.

the non-uniform magnetic field applied to the patient.

[2]
c.iii.

Markscheme

bone and tissue absorb different amounts of X-rays
OR
bone and tissue have different attenuation coefficients

so boundaries and fractures are delineated in an image

[2 marks]

a.

I bone I tissue = I 0 e μ b x I 0 e μ t x = e ( μ b μ t ) x

I bone I tissue = e 1.2 × 10 2 × ( 1.9 1.1 ) × 10 3 × 5.4 × 10 2

I bone I tissue = 0.60

[3 marks]

b.

to split the energy level of protons in the body
OR
to cause protons in the body to align with the field / precess at Larmor frequency

[1 mark]

c.i.

to force/excite protons that are in the spin up/parallel state

into a transition to the spin down/antiparallel state

[2 marks]

c.ii.

the emitted radio frequency signal has a frequency that depends on the magnetic field

with a gradient field different parts of the body have different frequencies and so can be identified

[2 marks]

c.iii.

Examiners report

[N/A]
a.
[N/A]
b.
[N/A]
c.i.
[N/A]
c.ii.
[N/A]
c.iii.

Syllabus sections

Option C: Imaging » Option C: Imaging (Additional higher level option topics) » C.4 – Medical imaging (HL only)
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Option C: Imaging » Option C: Imaging (Additional higher level option topics)
Option C: Imaging

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