Fetus blood-flow detector uses Doppler shift principle for obstetric applications. It measures the fetus heart rate from the blood flow signals and displays them. For obstetric applications, frequency range of 2 to 2.5 MHz is employed. Whereas, to study the blood flow in artery and vein frequency of 5 to 10 MHz is utilized. Maximum amount of ultrasonic energy that is transmitted into body tissue is between 10-15 mW/cm2. Usually a pencil shaped or flat shaped hand-held probes are used for Doppler shift ultrasound method. It has two piezoelectric crystals. Acoustic gel is used to couple probe to skin of the patient. This helps to prevent air from the interface. During early pregnancy, presence of air attenuates the ultrasound. The transmitting crystal produces an ultrasound wave of 2-2.5 MHz. It gets back scattered and receiving crystal receives it. If the reflecting object moves, like fetal heart blood vessels, the frequency of the back scattered ultrasound varies. It will be high when the blood cells approaches the probe and would be less when it moves away from probe. Hence, the frequency shift magnitude is given as follows.
Where f0 is the transmitted frequency, u is the velocity of blood, cos θ is the angle of sound beam and object direction, c is velocity of sound wave travelling in the tissue.
There are two types of ultrasonic transducers namely narrow beam and wide beam types. Narrow beam type uses only one ultrasound transmitter / receiver piezoelectric crystal pair. It is very sensitive but produces good trigger signal to measure heart rate immediately. Wide beam transducers operate in many configurations. It has multiple piezoelectric crystals mounted on it to detect fetal movement on a wider area.
The block diagram of ultrasonic Doppler shift based fetal heart rate is shown below. The setup uses both narrow beam and wide-angle beam transducer. The transmitted signal goes to the receiving path. It acts as local oscillator signal to mix diodes in the demodulator. The value of the reflected signal is 90 to 130 dB lesser than transmitted signal. The receiving signal requires special measures to reduce the effect of interference, low noise distortion, filters to drives amplifier into non-linear operating region. Magnetic and Electrical shields surround the high frequency section of the circuits. Depending on the type of transducer used filters are selected to match the Doppler shifted frequency components.
Monitoring Labour Activity
The uterus muscles start contracting with high intensity during labour. During the expulsion period, the intrauterine pressure reaches 150 mmHg. During spontaneous active labour, a normal patient has uterine contractions at an interval for three to five minutes, 30 to 70 seconds with a peak intensity of 50 to 75 mmHg. Labour activity is recorded either in terms of intra-uterine pressure using catheter or using external transducer. Transducer has a tip that presses against the mother’s abdomen for effective coupling. When the tension in uterus increases, transducer surface is displaced. Inside the transducer housing, a strain gauge converts this displacement into electrical signal.
The above diagram shows the setup for measuring labour activity externally. In an AC amplifier, the output of transducer is amplified. Few low frequency labour activity signals comes from the synchronous detector and reached the DC amplifier. Finally, the activity can be recorded on a chart or displayed in a meter. Passive transducers are used to drive the circuits to obtain electrical indication of pressure.
In internal method, a fluid filled catheter is used to measure intra-uterine pressure (IUP). Tube is inserted into the uterus without rupturing fetal membrane. Amniotic fluid is let to flow in freely to ensure correct placement. Catheter is fitted with a pressure transducer. Any change in the amniotic pressure is transmitted to the gauge using in compressible fluid in catheter. Finally, pressure transducer converts the catheter pressure into electrical signal and displays the result on strip recorder.