A level: Ultrasound Questions
(a) A piezoelectric ultrasound transducer is made from a thin slice of an artificial ceramic such as lead zirconate titanate (PZT).
(i) Describe what happens when an alternating voltage is applied to a PZT transducer so that ultrasound is produced.
(ii) Under what conditions will maximum energy conversion into ultrasound occur?
(b) The diagram below shows the oscilloscope display of pulse amplitude against time for an ultrasound A-scan through a person's abdomen. Assume that the weaker echoes come from internal organs.
(i) Describe the procedures which are used to obtain this type of scan.
(ii) Explain how the spacing of the pulses is interpreted.
(iii) Give two reasons why the amplitude of the reflected pulses varies.
(iv) If the speed of ultrasound through water and soft tissue is about 1500 ms–1, estimate the distance between the front of the patient's abdomen and the spinal column.
Total 12 marks
(a) Explain how a piezoelectric crystal is caused to generate waves of ultrasound.
(b) In medical applications of ultrasound a short pulse of duration about 1s is often used.
(i) Explain why the pulse of ultrasound must be short.
(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.
(i) Under what conditions is ultrasound reflected strongly at boundaries between two types of material?
(ii) State two physical properties of the materials which determine the proportion of ultrasound which is reflected at a boundary.
(iii) Explain what a coupling medium or gel is and why, and where, it is used.
Total 12 marks
Q3. An ultrasound transducer is used to obtain an A-scan of an internal organ. The A-scan pulses shown on the diagram of the oscilloscope screen were identified as coming from the front and rear surfaces of the organ. The timebase setting was 0.02s/div.
(a) Describe the practical process, including details of the use of the transducer and the adjustment of the oscilloscope, required to produce this A-scan.
(b) From the A-scan, estimate:
(i) the thickness of the organ if the speed of ultrasound in the tissue is 1500 ms–1 (the horizontal scale is 0.02 ms/cm),
(ii) the duration of the first ultrasound pulse.
(c) Give two reasons why the height of the second pulse is smaller than that of the first pulse.
Total 8 marks
Q4. The diagram shows an ultrasound transducer as used in A-scans. The transducer produces short pulses of ultrasound.
(i) Why is it necessary for the pulse to be short?
(ii) Explain, with reference to the diagram, the process by which the transducer produces short pulses.
(b) State one advantage and one disadvantage of ultrasound compared with X-rays in medical imaging.
Total 7 marks
Q5. The figure below shows a transducer used in an ultrasound A scan.
(a) Describe how pulses of ultrasound are produced by the transducer.
(b) In an ultrasound A scan
(i) explain how the received signals are detected,
(ii) state why it is essential to use short pulses of ultrasound.
Total 6 marks
Q6.The diagram shows a cross-section through an ultrasound generator.
(a) Explain the purpose of the backing material.
(b) State the main difference between the probe used in an A-scan and that used in a B-scan.
(c) The diagram belowshows the display on an oscilloscope screen of an A-scan to find the size of an organ. The speed of sound in the organ is 1200 m s-1
(i) Calculate the diameter of the organ.
(ii) State two processes which reduce the strength of the reflected signal received by the probe.
(Total 7 marks)
(a) Describe how a piezoelectric crystal used in an ultrasound probe is made to generate a pulse of ultrasonic waves.
(b) In medical diagnostic imaging, explain why a short pulse of about 1s is used.
(c) Under what condition is ultrasound strongly reflected at the boundary between two media?
(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
(Total 8 marks)
(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.
(ii) Calculate the acoustic impedance of tissue.
(iii) Show that the ratio at an air/tissue boundary is approximately 1.
(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.
(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,
(ii) how the results are used to find the size of an organ.
(Total 10 marks)
(a) State the two physical properties of a material which determine its acoustic impedance.
(b) Under what condition is ultrasound strongly reflected at a boundary between two types of material?
(c) State where a coupling medium or gel is used in an ultrasound scan and explain why it is necessary.
(Total 6 marks)
(a) Explain how and why ultrasound is used to obtain an image of an unborn foetus. You might consider the following points in your answer:
- the method of obtaining the image
- practical considerations for the scan
- safety issues.
The quality of your written communication will be assessed in this question.
(b) Explain why the pulses of ultrasound used in medical imaging must be of short duration.
(8 marks Total)