The effect of speed of sound in ultrasound test objects on lateral resolution.

Urethane test objects are available that are more durable than traditional gel-based objects. Their ultrasound (US) propagation speed differs from scanner calibration velocity. This is clearly a disadvantage in checking caliper accuracy, but its effect on other measurements such as resolution is not well known. A test object containing a column of nylon filaments was constructed and filled with water at different temperatures to give US speeds of 1,540 m s(-1) and 1,450 m s(-1). Images of the filaments were digitally captured and analysed to obtain the full-width half-maximum of the target images at each depth. Target image width was greater at 1,450 m s(-1). Measurements were also performed for the two types of tissue-mimicking test objects; results were in close agreement with the water test object. US beam profiles are broader in media with low propagation velocities.