The development of standards for ophthalmologic ultrasound diagnosis

Abstract

The performance data of ultrasonic apparatus and transducer probes are decisive in view of the information content of the echograms. Clinically applicable measurement methods, therefore, were developed already in the early sixties for ophthalmic ultrasonography. The sensitivity of each probe apparatus combination was measured first by means of a variable absorption path, the working frequency by counting the oscillations per microsecond in the rf echo signal of a plane reflector. Depth and lateral resolution were determined using monofil filaments mounted to a micrometer gauge. These measurements resulted in marked improvement of safety of diagnosis. In view of the IEC document 854, we developed a test reflector. It reflects a standard echo which can be measured within the control range of most diagnostic machines without need of additional dB-calibrated attenuators. Now, each echo amplitude can be referred to this standard echo (whose difference to the ideal reflector echo is-17 dB). An electronic device (echo simulator) permits additional checking of distinct equipment parts (especially of the dB control, time scale and amplifier dynamics). The entirely empirical use of ultrasonic diagnostic apparatus without measurement of the diagnostically relevant performance data and without readjustment and documentation of comparable examination conditions is, in our opinion, an outdated approach. Repeated measurements at regular intervals and especially after repairs or change of transducer probes are mandatory.