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Date	November 2019	Marks available	4	Reference code	19N.3.HL.TZ0.14
Level	Higher level	Paper	Paper 3	Time zone	0 - no time zone
Command term	State and Explain	Question number	14	Adapted from	N/A

Question

An X-ray beam, of intensity $I_{0}$ , is used to examine the flow of blood through an artery in the leg of a patient. The beam passes through an equal thickness of blood and soft tissue.

The thickness of blood and tissue is 5.00 mm. The intensity of the X-rays emerging from the tissue is $I_{t}$ and the intensity emerging from the blood is $I_{b}$ .

The following data are available.

Mass absorption coefficient of tissue = 0.379 cm²g^–1
Mass absorption coefficient of blood = 0.385 cm²g^–1
Density of tissue = 1.10 × 10³kg m^–3
Density of blood = 1.06 × 10³kg m^–3

Show that the ratio $\frac{I_{b}}{I_{t}}$ is close to 1.

[3]

a(i).

State and explain, with reference to you answer in (a)(i), what needs to be done to produce a clear image of the leg artery using X-rays.

[4]

a(ii).

In nuclear magnetic resonance (NMR) protons inside a patient are made to emit a radio frequency electromagnetic radiation. Outline the mechanism by which this radiation is emitted by the protons.

[3]

Markscheme

$μ_{t} = 0.379 \times 1.1 \times 10^{3} \times \frac{10^{3}}{10^{6}} = 0.417 « {cm}^{- 1} »$ AND $μ_{b} = 0.408 « {cm}^{- 1} »$ ✔

$\frac{I_{b}}{I_{t}} = \frac{I_{0} e^{- μ_{b} x}}{I_{0} e^{- μ_{t} x}} = e^{- (0.408 - 0.417) \times 0.5}$ ✔

$\frac{I_{b}}{I_{t}} = 1.004$ ✔

a(i).

the difference between intensities is negligible so no contrast ✔

modifying the blood is easier than modifying the soft tissue ✔

increase absorption of X-rays in the blood ✔

by injecting/introducing a liquid/chemical/contrast medium ✔

with large mass absorption coefficient/nontoxic/higher density ✔

a(ii).

«a uniform» magnetic field is applied to align proton spins ✔

proton spins are excited by an «external» radio frequency signal/field
OR
protons change from spin-up to spin-down state due to «external» RF signal/field ✔

«radio frequency» radiation is emitted as the protons relax ✔

NOTE: For MP3 do not allow simplistic “protons emit RF radiation” as this is given in the question