A level: Ultrasound Questions
Q8.
(a) The acoustic impedance, Z, of a medium is equal to the product of the medium's density and the speed of sound in that medium.
When sound is incident on the boundary between two media of acoustic impedances Z1, and Z2 respectively, some sound is reflected and some transmitted.
The ratio of the reflected intensity, Ir, to the incident intensity, Ii, is given by the equation:
- speed of sound in air = 330 m s–1
- speed of sound in tissue = 1540 m s–1
- density of air = 1.3 kg m–3
- density of tissue = 1100 kg m–3
(i) Calculate, giving the appropriate unit, the acoustic impedance of air.
Zair = 330 x 1.3 = 430 kg m-2 s-1 
(ii) Calculate the acoustic impedance of tissue.
Ztissue = 1540 x 1100 = 1.7x 106 kg m-2 s-1
(iii) Show that the ratio at an air/tissue boundary is approximately 1.
Use of the equation gives:
= 0.999
(3 marks)
(b) Use your answer to part (a)(iii) to explain why a coupling gel is needed between an ultrasound probe and a patient's skin. State and explain what the ideal value of the acoustic impedance would be for such a gel.
Without gel, air between probe and tissue reflects nearly all the ultrasound or very little enters the body.
With gel air excluded and require Ir = 0
Therefore Zgel = 1.7 x 106 or equals that of skin/tissue
(3 marks)
(c) An A-scan is used to find information about the depth and size of organs within a patient's body. Explain
(i) the basic physical principles behind the A-scan,
The transmitter produces short pulses
At an internal boundary some reflected, rest transmitted to next boundary
The reflected pulse received by probe and signal sent to oscilloscope
The oscilloscope sweep is started when pulse is first transmitted
(ii) how the results are used to find the size of an organ.
Time taken between pulses from front and back of organ (from oscilloscope)
distance = speed x time/2
(4 marks)
(Total 10 marks)
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