![]() Autocorrelation is a process of successively comparing waveforms to identify their similarities. To assist in converting these raw data into useful information, the monitor employs a method referred to as autocorrelation. These filtered waveforms are irregularly shaped and lack an obvious common distinguishing element from which the timing of the occurrence of one waveform could be compared to that of the next ( Figure 2-3). To facilitate determination of the factor t, the waveform is filtered to reduce the amount of high-frequency component. The original signal is “subtracted” from the reflected signal, and the resulting waveform is representative of the motion of the fetal heart ( Figure 2-2). These data are created by electronically comparing the reflected ultrasound signal to the transmitted signal. The raw data gathered by the ultrasound transducer do not contain such distinct reference points from which the factor of t may be figured. Determining the factor of t is very straightforward with internal monitoring-it is the time elapsed between the clearly identifiable, successive R peaks of the QRS complex. The external mode of monitoring requires different processes for identifying the factor of t for use in the equation, FHR = 60/ t, than that which is used with internal monitoring. Because of the timing or frequency built into each crystal, the signal emitted by the ultrasound transducer for the purpose of evaluating one fetus will not be received by the ultrasound transducer that is tracking the second fetus. ![]() Pulse Doppler ultrasound makes it possible to monitor more than one fetus simultaneously. ![]() The crystals are then reactivated as receivers for the ultrasound signal as it is returned for processing. ![]() Each of the crystals inside the transducer is timed to transmit sound waves and then “stand by” for a predetermined period (the amount of time it takes for the signal to be transmitted and reflected). Pulse Doppler ultrasound allows all of the crystals in the transducer to both send and receive sound waves. Continuous wave ultrasound was used in early versions of the ultrasound transducer until it was replaced by pulse Doppler technology. This is accomplished by having certain crystals dedicated to emitting sound waves, whereas others only receive the reflected signal. The Doppler shift frequency can be used to produce the audible sound that is recognized as the fetal heart beat and also provides the raw data from which the FHR can be determined by using the equation, FHR = 60/ t.Ĭontinual transmission and reception of sound waves to and from the ultrasound transducer is known as “continuous wave” ultrasound. ![]() The difference between the frequency of the sound wave that is transmitted and that which is reflected back is referred to as the Doppler shift. The reflected sound wave is converted back into electrical energy at a frequency altered from the original transmitted frequency. Meets the fetal heart, its frequency becomes either compressed or stretched by the beating motion of the fetal heart and is reflected back to the crystals. This is accomplished in the ultrasound transducer by the application of voltage to the crystals, which causes them to vibrate at a predetermined frequency. The piezoelectric effect is the conversion of electrical energy into mechanical sound wave energy and vice versa. The ultrasound transducer contains crystals that transmit and receive high-frequency sound waves using the piezoelectric effect. When properly placed on the maternal abdomen, the ultrasound transducer detects the movements of the fetal heart that occur with each beat. The FHR may also be determined from the signal acquired through use of external sensors. ![]()
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