Figure 6 illustrates this effect. Instruments utilizing the Doppler effect typically utilize some electronic filtering to remove signals returning from stationary objects and then amplify the shifted signals. For most instruments the operator listens to an amplified, audible signal, so that the human ear becomes the final discriminator of significant signals. Ultrasound systems which utilize the Doppler effect are typically used to monitor blood flow of the fetus. The blood cells are the reflecting bodies and fetal heart rate, which is faster than the maternal heart rate, is the dominant factor controlling their movement.
Some general rules apply to use of ultrasound equipment regardless of type of instrument being used.
First, the instruments contain fragile electronic components and should be protected from sharp blows by animals or other equipment. This is particularly true of the transducers which transmit and receive the reflected ultrasound signal. Secondly, an air-tight connection must be made between the transducer and the animal. Ultrasound waves are not propagated through gases so a lubricant is necessary between the probe and the animal's skin to assure effective transmission and reception. Alternatively, if the probe is being located rectally for a B-scan of the reproductive tract, it is important to evacuate the rectum to assure that neither feces nor gases interfere with signal transmission.