The importance of rigorous testing of circumference measuring callipers.

On-screen circumference measurements are frequently made in obstetric ultrasound and are also made in other ultrasound specialties. The accuracy of electronic callipers is traditionally checked by linear measurement in a test object; this does not adequately test callipers used for nonlinear measurement. This technical note describes a simple test object designed for both linear and nonlinear calliper checks. Results show one machine to generate consistently large overmeasurements (15%). The clinical consequences of such overmeasurement may be serious. Appropriate quality control is essential.