Topic Menu

Cyber-chess a chess instruction site

A level: Ultrasound Questions

Q10.

(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.

The candidate’s writing should be legible and the spelling, punctuation and grammar should be sufficiently accurate for the meaning to be clear. The candidate’s answer will be assessed holistically. The answer will be assigned to one of three levels according to the following criteria.

The explanation expected in a competent answer should include a coherent selection of the following points concerning the physical principles involved and their consequences in this case.


Method of obtaining the image

  • Each transducer emits an ultrasound pulse in turn and receives the echoes from the interfaces directly in line with it.
  • Ultra sound is reflected at the interface between two materials of different acoustic impedances. The greater the differernce the greater the strnegth of the reflection.
  • Each echo displayed as a bright spot on screen - the bigger the aplitude of the echo the brighter the spot.
  • The y position (depth of the point) is determined by the time taken from transmission to the time the echo is received back.
  • The x position is determined by the position of the transducer
  • Images are produces at about 25 per second and therefore appear as a real time moving image on a computer screen.

Practical considerations

  • A probe has line of transducers (approx 100 in typical probe used to look at a foetus)
  • A high frequency ac pulse is applied to each tranducer in turn
  • Each transducer has piezoelectric crystal to generate the ultra sound wave
  • Use of gel between probe and skin to eliminate air is important otherwise the difference in impedance between air and tissue would mean the ultrasound would virtually all reflect - very little entering the body.
  • Transducer acts as receiver of the echo, changing the ultrasound energy into an electrical signal which is transmitted to a computer.

Safety

  • No harmful side effects known
  • Does not use ionising radiation so is safe for the unborn child.
  • Is not invasive.


High Level (Good to excellent): 5 or 6 marks
 

The information conveyed by the answer is clearly organised, logical and coherent, using appropriate specialist vocabulary correctly. The form and style of writing is appropriate to answer the question.

The answer will discuss

  • multi-array of transducers in a linear formation and the
  • use of gel between the skin and the probe will be explained.
  • There will be mention of the transducers acting as receivers and why ultra sound echoes occur. There will be some discussion of the processing of the received signal to produce an image.

The fact that this is non-ionising and thus has no known side effects will be included.
Intermediate Level (Modest to adequate): 3 or 4 marks
 

The information conveyed by the answer may be less well organised and not fully coherent. There is less use of specialist vocabulary, or specialist vocabulary may be used incorrectly. The form and style of writing is less appropriate.

The answer will contain at least one property of the probe and either the use of gel or the transducer acting as a receiver should be discussed.

The processing of the signal will be sketchy, but the reason that ultrasound is safe is likely to be mentioned.
Low Level (Poor to limited): 1 or 2 marks
 

The information conveyed by the answer is poorly organised and may not be relevant or coherent. There is little correct use of specialist vocabulary. The form and style of writing may be only partly appropriate.

There will be a few of the guidance points mentioned, but there will be little cohesion in the writing.

(6 marks)

 

(b) Explain why the pulses of ultrasound used in medical imaging must be of short
duration.

The transducer to be damped (made to stop oscillating) before the echo returns to allow the transducer to act as a receiver.

(This time is very short) as distances travelled are short

Emitted pulse must cease before echo arrives so that there is no overlapping at the transducer/ no interference

(2 marks)

(8 marks Total)