Distance measurement

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Ultrasound machines perform distance measurement to synthesize images from returning echoes. To generate images for an ultrasound scan, machines need to determine the distance of reflective interfaces from the transducer. Simply, the formula used is:

  • distance = (speed x time)/2

Where:

  • distance = the distance between the transducer and the reflective interface
  • speed = assumed propagation speed of sound waves through human tissue  (1540 m/sec)
  • time = time taken for the pulsed sound-wave to reach the interface and the resultant echo to return to the transducer          

The calculation is divided by two as the time measurement refers to the round trip of the pulsed sound wave / returning echo .

This method makes two important assumptions. Firstly, it assumes sound waves travel at a constant propagation speed through tissue. Secondly, it assumes the pulse and resultant echo have traveled a direct path, i.e. with no refraction. The limitations of these assumptions are evidenced by speed displacement artifact and refraction artifact, respectively .