| Date | November 2011 | Marks available | 2 | Reference code | 11N.3.HL.TZ0.16 |
| Level | Higher level | Paper | Paper 3 | Time zone | Time zone 0 |
| Command term | Explain | Question number | 16 | Adapted from | N/A |
Question
This question is about the use of ultrasound for diagnostic imaging.
Outline how ultrasound is produced for use in diagnostic imaging.
In order to look for damage to the chambers of the heart, ultrasound is used to form an image of the heart.
Suggest why it is better to use ultrasound rather than X-rays.
The speed of sound in skin is about five times the speed of sound in air. Given that the density of skin is about 700 times that of the density of air, compare the acoustic impedance of skin to that of air.
Explain, using your answer to (c), why, in using ultrasound for imaging, a layer of gel is placed between the transducer and the skin.
A wide range of frequencies of ultrasound may be used to image internal body organs. The choice of frequency for imaging a particular organ is determined by the depth of the organ beneath the skin.
Outline, with reference to attenuation and resolution, why the depth of the organ determines the choice of ultrasound frequency.
Markscheme
an electric field applied along the axis of a piezoelectric crystal causes the crystal to expand or contract;
a high frequency alternating E field will cause the crystal to oscillate at the frequency of the applied field;
the oscillations are transferred to the surrounding air producing ultrasound;
no contrast between heart and surroundings as X-rays will not be absorbed by the heart;
ultrasound will be reflected/transmitted by different amounts by the slightly differing densities and depths of the heart tissue;
Do not accept answers based on “X-rays are harmful”.
acoustic impedance=speed of sound×density;
acoustic impedance of skin=5×700=3500×acoustic impedance of air;
acoustic impedance of skin is very much greater than that of air so nearly all incident ultrasound is reflected at air/skin boundary;
gel has same acoustic impedance as skin so nearly all ultrasound will be transmitted through the skin;
high frequency ultrasound gives better resolution;
but is subject to more attenuation;
the further the distance the ultrasound travels the more it is attenuated;
the deeper an organ therefore the lower the frequency used in order to minimize attenuation / a compromise has to be made between resolution and attenuation;
