A level: Ultrasound Questions
Q7.
(a) Describe how a piezoelectric crystal used in an ultrasound probe is made to generate a pulse of ultrasonic waves.
Apply an alternating voltage across faces of the crystal 
at the natural frequency of the crystal at frequency greater than 20 kHz
(2 marks)
(b) In medical diagnostic imaging, explain why a short pulse of about 1
s is used.
The transducer acts as both emitter and receiver The emitted pulse must have stopped its forced vibration before the reflected pulse is received - due to the small distances involved, the time of the pulse has to be very small
(3 marks)
(c) Under what condition is ultrasound strongly reflected at the boundary between two media?
When the acoustic impedances/densities/speed of sound of the two materials are very different
(1 mark)
(d) Given the following equation, calculate the percentage of ultrasound which is transmitted when travelling from the skull into the brain.
Ir = reflected intensity
Io = incident intensity
Z2 = acoustic impedance of the brain tissue = 1.58 × 106 kg m-2 s-1
Z1 = acoustic impedance of the skull bone = 7.78 × 106 kg m-2 s-1
substitute correct numbers into the equation to get:
reflected ratio Ir/Io = 0.44 so % = 44
and % transmitted = 100 – 44 = 56%
(2 marks)
(Total 8 marks)
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