CALIPERS (ULTRASONIC)

Typical Number in Hospital:

Cost Bands:

References: 1

Although this term has an obvious meaning, it arises in medical instrumentation as part of an A- or B-scanner. One use of ultrasonic examination of the body is to measure the size of organs or tumours, and this requires a special facility to measure distances between points on the image.

On the A-scan, calipers usually take the form of bright dots on the A-scan line which can be adjusted in position to mark the two echoes representing the boundaries of the organ in question. This technique has been used extensively in obstetrics to measure the bi-parietal diameter (BPD) of the foetal head to estimate the age of the foetus. It can provide very accurate measurement of distance but it is not suitable where the two points required do not lie on a straight line from the skin surface into the body.

On B-scans, calipers are presented as bright dots, crosses, or a circle, which can be moved by a 'joy-stick' control to mark any two points on the screen. An electronic circuit calculates and displays the distance between these two points. An extension of this method is to draw a complete plan of the boundary of an organ or lesion with a joy-stick or light pen and have the internal computer calculate the circumference or area contained within the boundary.