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Date May 2015 Marks available 5 Reference code 15M.3.HL.TZ1.19
Level Higher level Paper Paper 3 Time zone Time zone 1
Command term Deduce Question number 19 Adapted from N/A

Question

This question is about ultrasound scanning.

Outline how ultrasound is generated for medical diagnostic purposes.

[2]
a.

When ultrasound of intensity I0 travels in a medium of acoustic impedance Z1 and is incident on a medium of acoustic impedance Z2, the intensity IR that is reflected at the interface is given by the following equation.

\[{I_R} = {\left( {\frac{{{Z_1} - {Z_2}}}{{{Z_1} + {Z_2}}}} \right)^2}{I_0}\]

The following data are available.

Use the data to deduce why a layer of gel must be used between a transducer and the patient’s skin in medical ultrasound imaging.

[5]
b.

In medical scanning, practitioners have the option of using A-scans or B-scans. Distinguish, with reference to the techniques used to produce the scans, between an A-scan and a B-scan.

[3]
c.

Markscheme

use of piezoelectric crystal / quartz;

crystal changes shape/vibrates when an alternating electric field is applied to it;
crystal dimensions are cut to achieve resonance at required frequency;

a.

(acoustic impedance of air = ) 330 × 1.3 = ( =430 (kgm-2 s-1));
(acoustic impedance of skin
=) 1500 × 1000 = ( =1.5×106(kgm-2 s-1));
\(\frac{{{I_R}}}{{{I_0}}} \approx 1/0.999\);
little/no transmission (because the reflected intensity is approximately equal to the incident intensity);
gel (has similar impedance to skin and so) is required as it excludes air from the interface (so transmission occurs);
Accept first two marking points if implied in the third marking point.

b.

A-scan is a “graph” of returned intensity against time but B-scan is a 2-D image of section through patient;
A-scan: ultrasound probe is fixed in position;
B-scan: operator moves ultrasound generator and computer records echo returns;

 

c.

Examiners report

[N/A]
a.
[N/A]
b.
[N/A]
c.

Syllabus sections

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