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Date May 2012 Marks available 3 Reference code 12M.3.HL.TZ2.20
Level Higher level Paper Paper 3 Time zone Time zone 2
Command term Explain Question number 20 Adapted from N/A

Question

This question is about X-ray absorption.

A parallel beam of X-rays is incident on a section of tissue of thickness x. The constant incident intensity is I0 and the transmitted intensity is It.

 

The half-value thickness of the tissue is 4.0 cm.

On the axes below, sketch a graph to show the variation with tissue thickness x of \(\frac{{{I_t}}}{{{I_0}}}\).

[2]
a.

Calculate the attenuation coefficient of X-rays for this tissue.

[2]
b.

For a different type of tissue, the ratio \(\frac{{{I_t}}}{{{I_0}}}\) is smaller for the same thickness x of material.

Compare the attenuation coefficient of this tissue with that of the tissue in (b).

[2]
c.

Barium has an attenuation coefficient that is much larger than that for human tissue.

Explain why a patient is asked to drink a liquid barium meal to help produce an X-ray image of the digestive system.

[3]
d.

Markscheme

smooth curved decay line beginning at \(\frac{{{I_t}}}{{{I_0}}} = 1\) when x=0;

passing through other three points as shown;

 

a.

\(\mu  = \frac{{\ln 2}}{4}\);
0.17cm-1;
Watch for alternative correct answers such as 17m-1.

b.

e-μx smaller/eμx is larger;
μ larger;

or

it has a shorter half value thickness;
and so μ larger;

 

c.

stomach tissue has similar attenuation coefficient to nearby tissue (so does not show up on X-ray);
barium absorbs X-rays well (and lines the stomach after drinking);
barium allows stomach walls to show as contrast;

d.

Examiners report

 

a.

 

b.

 

c.

 

d.

Syllabus sections

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