A level: Ultrasound Questions
Q2.
(a) Explain how a piezoelectric crystal is caused to generate waves of ultrasound.
Two faces of a thin slice of crystal (coated with a thin layer of silver) act as electrodes. 
The electrodes are connected to a high frequency (several MHz) source of e.m.f. As the applied e.m.f. alternates it applies an alternating (rapidly reversing direction) electric field across the slice of crystal between the electrodes. The crystal expands and contracts at the same frequency as the applied e.m.f. The vibrations of the faces of the crystal slice produce ultrasound pressure waves
4 marks
(b) In medical applications of ultrasound a short pulse of duration about 1
s is often used.
(i) Explain why the pulse of ultrasound must be short.
The pulse needs to be short compared with the transit time. Pulses are used for timing echoes which give measurements of depth in the body therefore the pulse must be short enough to ensure the leading edge of the pulse returns well after the trailing edge departs
(ii) Short voltage pulses applied to the piezoelectric crystal make it vibrate and emit short pulses only if the crystal assembly is modified. Explain the modification which is necessary.
Behind the crystal a vibration-absorbing backing material is attached. This damps the vibrations - stops the vibrations quickly - after the electrical signal is stopped, ensuring that the pulse is short
3 marks Max
(c)
(i) Under what conditions is ultrasound reflected strongly at boundaries between two types of material?
When there is a large difference in acoustic impedance [or significant change in density or significant change in elasticity or texture of tissue]
(ii) State two physical properties of the materials which determine the proportion of ultrasound which is reflected at a boundary.
Tissue density and tissue elasticity/texture.
(iii) Explain what a coupling medium or gel is and why, and where, it is used.
Ultrasound is reflected back at boundaries with big difference in density. Therefore air pockets between the skin and transducer would prevent the ultrasound from entering the skin - most of it would be reflected. Gel (of similar density to body tissue) between transducer and skin prevents loss of signal due to boundary reflection Acoustically well -matched gel gives good transmission (with minimum reflection at skin boundary)
5 marks
Total 12 marks
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