Institute of Fundamental Technological Research

The authors applied two absolute methods for the determination of the intensity of ultrasonic focused beams of 2·5 MHz frequency used in ultrasonograph UG-4 for abdominal visualization: the electrodynamic and the capacitance methods. For the first method a special electrodynamic transducer was constructed which made it possible to measure the acoustic (particle) velocity in the focal point of the ultrasonic beam radiating into water. Hence the focal intensity could be determined.

The two methods gave almost the same intensity values equal to 23 Wcm−2 in peak, (∼ 23 mWcm−2 time average value) although the mean surface value near to the radiating transducer was equal only to 0·59Wcm−2 in peak (∼0·59 MWcm−2 time average value).

Next, the loss of the ultrasonic signal was measured in pregnant women in vivo using for this purpose steel balls as reflecting targets placed in the gravid uterus. The echo amplitudes obtained in the uterus were compared with echoes received from the same steel balls placed in (nonabsorbing) water in the same position relative to the radiating transducer.

In this manner it could be found that intensity levels inside of the uterus were 6–14 dB lower than at the body surface, depending on the anatomical characteristics of the patient.

Thus the maximum (focal) intensities which are to be expected in the gravid uterus in early pregnancy will be between 0·9 and 5·5 Wcm−2 in peak (0·9–5·5 mWcm−2 time average values) when applying the ultrasonograph UG-4.