Ultrasound is regular sound at very high frequency, far above the range of human hearing. The frequencies used in our products range from 1,000,000Hz to 5,000,000Hz. (The range of sound heard by the human ear is 20Hz to 20,000Hz.)
The transducer, which we call a probe, contains a treated ceramic disk that flexes when a voltage pulse is applied and which produces a voltage when it is mechanically flexed. This is called the piezoelectric effect. This action produces ultrasound in the same manner as an ordinary speaker produces normal sound.
The pulse of ultrasound emitted by the transducer moves though skin and tissues, until it encounters a type of tissue that it cannot penetrate because the tissue has a high resistance to the particular sound frequency. It is then reflected back toward its source. This reflection is called an echo. The instrument alternately uses the probe as a speaker to send the pulse, and then as a microphone to receive the echo.
A-mode is the type of ultrasound that all RENCO® products use. A pulse of ultrasound is sent out, the echo processed and a determination made on the basis of the time it took to go and return. When a visual display is used, the presentation is a series of spikes rather than a picture.
Doppler instruments use ultrasound to provide an audible indication of movement, such as movement caused by a beating heart or pulsing blood vessel.
Advantages: Assuming a skilled operator, Doppler instruments give fewer false negative readings at earlier times. Also, though not of particular importance from a production point of view, they permit the number of fetuses to be counted.
Disadvantages: Doppler instruments are considerably more expensive and require considerably more time per test, thus increasing cost and providing lower efficiency. They also require a longer time for the operator to develop the necessary skill to differentiate between a beating heart and pulsing blood vessels, and also to be able to mentally set aside high levels of background noise.
For pregnancy detection, the instruments determine if amniotic fluid is present by detecting an echo from the interface between the interior of the far side of the uterus and amniotic fluid. The PREG-ALERT PRO® displays echoes as “spikes” on a TV-like screen. Travel time is displayed in equivalent units of distance. The SONO-GRADER® displays the words “Pregnant” or “Not Pregnant” along with the calibrated scale. The PREG-TONE® and PREG-TONE II PLUS® indicate the pregnancy status by an audible tone. These instruments use an electronic “window” to place upper and lower time limits for accepting a returning echo as valid indication of pregnancy.
For backfat measurement, valid echoes are produced by the interface between fat and the tissue layer separating fat layers and also the interface between the bottom layer and loin muscle. The LEAN-MEATER®automatically determines how many fat layers exist and displays that number along with the total depth including skin. The LEAN-MEATER is not designed to measure loin depth.
The PREG-ALERT PRO and SONO-GRADER visually display all valid echoes from all layers comprising skin, fat and loin muscle.
The SONO-GRADER has several options for making loin muscle measurements at the 10th rib. In all cases, the instrument subtracts the total fat measured at P2. It should be noted that there are numerous factors that can cause variations in readings from time to time even when using the same method; there is no unvarying “absolute”. A “standard” is only that which the user chooses to call a standard. Therefore, provision is made for altering the speed of sound factor individually for fat and loin tissues. This permits the user to adjust the readings to track those of any method the user chooses to use as a “standard”.