Simple device for the calibration of ultrasonic calipers

✍ Scribed by D. H. Swingler; J. M. Piggins

• Publisher: John Wiley and Sons
• Year: 1974
• Tongue: English
• Weight: 239 KB
• Volume: 2
• Category: Article
• ISSN: 0091-2751
• DOI: 10.1002/jcu.1870020413

✦ Synopsis

Electronic calipers, now included as standard in most commercial ultrasonic scanners, are used for a variety of measurements including fetal cephalometry ( 1 ) . They operate by generating two electronic pulses which brighten the A-scan or B-scan display, or both. The position on the time-base at which each pulse occurs may be selected manually. When the bright-up pulses coincide with the leading edges of the echoes from two structures, the time interval between the pulses is a measure of the separation of these structures (2). On the basis of a suitable mean value for the velocity of sound, the actual separation may be calculated. The conversion of time intervals t o units of distance is normally made electronically, within the caliper, and the result displayed in digital form.

The reading given by a caliper of this type is subject to errors from two sources. First, the relationship between the time intervals and the indicated distance may deviate from linearity. Second, the slope of the distanceltime relationship-that is, the sound velocity for which the caliper is calibrated-may be in error. Most commercial calipers provide a facility for adjusting the value of the velocity, and some include means for optimizing the linearity.

Correct settings for these adjustments may be determined by comparing caliper readings with the time interval between the bright-up pulses measured by a precision electronic timer. This involves dismantling the caliper t o gain access to *Polymethylmethacrylate. Trade mark, Imperial Chemical Industries Limited, England.