False Assumptions

Question: What are some of the most common false assumptions?

• Waves travel in straight lines
• The ultrasound beam is infinitely thin
• The beam has a uniform width
• The speed of sound is constant in all tissues at 1540 m/sec, (there are subtle changes but the machine assumes all speed is same)
• The amplitude of the return echo is proportional to the echogenicity in the image
• The echo comes from the last pulse sent out
• The echo has followed the shortest possible route

There are a number of features of both ultrasound transducer design and sound propagation that violate these basic assumptions and result in image artefacts. The origins of some of the more common artefacts are outlined in the following pages.

For each artefact, consider where it is likely to occur, how it may affect image interpretation and how technique could be adapted to either utilise or avoid it.