Fetal heart detector

A self contained hand held fetal detector comprises an ultrasonic receiver and transmitter (23). Selectively actuable ultrasonic receiving and transmitting circuits (25,21) are connected to the ultrasonic receiver and transmitter (23) with a controller (24) for actuating either the ultrasonic receiving circuit (25) or the ultrasonic transmitting circuit and a loudspeaker (29) connected to the ultrasonic receiving circuit (25) for generating an audio output signal. The detector comprises a single unit (13) and the controller (24) modifies at least one of the transmitted and received signals, to prevent interference in the received ultrasound.

FIELD OF THE INVENTION 
This invention relates to a fetal heart detector, in particular to a hand 
held detector. 
DESCRIPTION OF THE PRIOR ART 
Conventional fetal heart detectors include hand held, audible detectors 
which are formed in two parts connected via cabling. The first part is an 
ultrasonic probe having two angled ultrasonic crystals, one for 
transmission and one for reception. An ultrasonic signal is continuously 
transmitted typically at a frequency of 2 MHz and this causes reflections 
from surfaces in the medium through which it passes. The receiving crystal 
continuously senses reflected signals. 
If an object, off which the signals are reflected, moves this causes a 
doppler shift in the frequency which is detected. This doppler shift is 
typically in the audio range when caused by movement of a fetal heart wall 
and the signal can be amplified and output directly through an audio 
loudspeaker in the second part. The main advantage of an audio output to a 
pocket monitor is that it enables the mother to hear the fetal heartbeat, 
thus reassuring the mother. Additionally in view of its focused ultrasonic 
beam it allows earlier detection of the fetal heart beat than a bedside 
monitor where a larger transducer having a divergent beam is used. 
A hand held detector may be used by a mid-wife in the home or in a 
hospital, or by consultants to give the heart rate of the fetus. The heart 
rate may be displayed directly on the monitor. 
It is desirable that the cost of manufacture of such detectors should be 
kept as low as possible and that they should be easy to use and to carry. 
Conventional hand held detectors rely on continuous wave transmissions of 
ultrasonic signals. A continuous wave system has no limit on the distance 
of travel of a wave before its reflected wave can be detected in the 
receiving crystal. This means that movement of the loudspeaker in the 
output section will be detected by the receiving crystal unless there is 
some form of separation between the transducer and the output section. 
Typically, this is achieved by housing the transducer in a probe and using 
a separate audio unit connected to the probe by means of a cable. 
Other proposed solutions to this problem include providing the probe with a 
transmitter to transmit information via telemetry to a completely separate 
audio unit such as described in EP-A-0367251, or using infrared 
transmission. An alternative is to use headphones rather than a 
loudspeaker but this does not overcome the disadvantages inherent in two 
piece units which require additional connectors and separate manufacturing 
of the parts. The inclusion of connectors and cables automatically 
provides an undesirable level of unreliability and cost. Testing the 
complete unit requires the separately manufactured parts to be put 
together adding to the cost. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a self contained hand held fetal 
detector comprises ultrasonic receiving and transmitting means; 
selectively actuable ultrasonic receiving and transmitting circuits 
connected to the ultrasonic receiving and transmitting means; control 
means for actuating either the ultrasonic receiving circuit or the 
ultrasonic transmitting circuit; and a loudspeaker connected to the 
ultrasonic receiving circuit for generating an audio output signal, the 
detector comprising a single unit and the control means including means 
for modifying at least one of the transmitted and received signals, to 
prevent interference in the received ultrasound. 
Preferably, the modifying means comprises means for causing a pulsed signal 
to be transmitted. 
The ultrasonic receiving circuit and ultrasonic transmitting circuit are 
actuated with a time delay, between the end of a transmission period and 
the start of a reception period, such that an area ahead of the transducer 
is provided in which everything reflected within that distance is not 
detected by the receiving circuit thereby cutting out near field 
interference and feedback due to the reflections affected by moving parts 
of the loudspeaker. This allows a single piece monitor to be manufactured. 
However, at high gain settings, an additional form of unwanted feedback can 
occur in the presence of strong reflections from large stationary objects 
within the depth range of the gated ultrasound signal. Vibrations from the 
loudspeaker once initiated by any means, will be mechanically coupled to 
the ultrasound transducer causing it to move microscopically relative to 
objects within its ultrasonic field. Such movements will cause reflections 
from stationary objects to be Doppler shifted and appear as audio on the 
loudspeaker. The process is then repeated resulting in an audio "positive 
feedback" tone when there should be no signal present. 
Preferably, the detector further comprises shift means to shift the 
frequency or phase of the audio signal by a non-integer multiple of the 
audio-frequency. 
Thus recirculating signals due to reflections within the depth range of the 
ultrasonic circuit, such as large organs of the mother, do not reinforce 
one another. 
A device which can fulfil this function is a "pitch shifter" which can be 
inserted into the audio amplifier section of the circuit. Such devices 
produce an output signal which is a frequency shifted version of any audio 
signal presented to the input. In the present application the signals of 
interest are already the result of a frequency shift (the Doppler effect) 
so provided the signals remain in the audio bandwidth the effect of being 
shifted once by a few semitones is of little significance. However, by 
using the device to give a frequency shift which is a non-integer multiple 
of the input frequencies, the effect on potential feedback signals is to 
change their frequency and hence their phase each time they re-circulate 
through the system so they are unable to reinforce and are quickly shifted 
out of the audio bandwidth preventing unwanted tones. 
Preferably, the audio signal is full wave rectified. This allows a smaller 
loudspeaker to be used. 
Preferably, the detector further comprises display means to display a 
digital output relating to a frequency of a signal received by the 
ultrasonic receiving circuit. For example, a digital output of the heart 
rate, may be displayed at the same time as the audio output, for the 
benefit of the midwife or consultant using the monitor. 
Preferably, the detector further comprises a sealed housing and wherein the 
loudspeaker is covered by an impermeable membrane. 
The seal and membrane are waterproof and dustproof, making cleaning easier 
and allowing the detector to be used during water births.

EMBODIMENT 
In a conventional two piece system, such as that shown in FIG. 1, a 
loudspeaker 9 is housed 40 separately from a transducer circuit 41. In the 
circuit 41, a transmitter 1 is driven by an oscillator 2 to excite a 
transmitting crystal 3. The transmitting crystal is angled relative to a 
receiving crystal 4 such that when an ultrasonic signal emitted from the 
crystal 3 impinges on a moving object and is reflected back it is 
reflected towards the receiving crystal 4 with a differential frequency in 
the audio range. A signal received in receiver 5 is rectified 6 and 
filtered in a filter 7 to obtain the differential audio frequency output 
which is then amplified by an amplifier 8. The amplified signal is output 
to the loudspeaker 9 via a connection cable 44. 
In FIG. 2, a detector in accordance with the present invention is shown. A 
loudspeaker 11, control switches and digital display 12 are provided 
within a single body 13 which provides a waterproof cover, together with 
their associated circuitry (not shown). Operation of the detector is via 
the pair of control switches 14,15 for controlling the volume of the 
output and a single ON switch 16 for switching on the monitor. An 
ultrasonic transducer 17 is provided in one end of the body 13 of the 
monitor. 
FIG. 3 shows a block diagram of one example of the circuits mounted within 
the detector 13 of FIG. 2. A transmitter 21 is connected between an 
oscillator 22 and a single crystal 23. The oscillator also provides a 
signal to a gate timing circuit 24 and mixer 26. A receiver 25 is 
connected between the crystal and the mixer 26. Outputs of the gate timing 
circuit are connected to the mixer 26, receiver 25 and transmitter 21. An 
output from the mixer 26 is filtered in a set of filters 27, optionally 
full wave rectified in a rectifier 28 and pitch shifted in pitch shifter 
44, then amplified by amplifier 30 and output through an audio loudspeaker 
29. Automatic gain control is optionally applied to maximise audio 
performance and improve dynamic range. Only one of the transmitter 21 and 
receiver 25 operate at any one time. This timing is controlled by the gate 
timing circuit 24. A passive redirector directs any signal coming from the 
transmitter to the crystal, outward from the transmitter and any signal 
coming into the crystal, inward to the receiver. 
The set of filters 27 removes the switching frequency between transmit and 
receive and a low frequency difference signal from the mixer 26, typically 
100 hertz, is output to the pitch shifter 44 after filtering. The pitch 
shifter digitises the analogue signal, changes the sample rate by digital 
means and uses the samples to reproduce an analogue signal at the output. 
The output signal is similar to the input signal but with the frequencies 
shifted by a non-integer multiple. This signal is used to drive the 
loudspeaker through an audio amplifier 30. In this example, the rectifier 
28 doubles the frequencies from the pitch shifter 44 in order that a small 
loudspeaker which does not work so well at very low frequencies, may be 
used. Typically, the ultrasonic frequency is of the order of 2 MHz. 
The maximum depth of a reflected signal which can be detected is defined by 
the period for which a transmit pulse is transmitted t.sub.1, a delay 
period t.sub.2 -t.sub.1 and a receive period t.sub.3 -t.sub.2 as 
illustrated in FIG. 4. For a fixed delay between the end of the transmit 
period and the start of the receive period the depth from which the 
leading part of the transmitted pulse may be reflected and still be 
detected at the trailing end of the receive period is the maximum depth 
for detection. The minimum depth which can be detected by the receiver 25 
is determined by the delay period between the end of the transmit pulse 
and the start of the receive period. This avoids feedback due to movement 
of the loudspeaker being detected because it is within the dead zone 
resulting from delay between the end t.sub.1 of the transmission period 45 
and the start t.sub.2 of the reception period 46. 
In another example of the present invention shown in FIG. 5, the oscillator 
22 is incorporated in a microprocessor 31. The gate timing circuitry is 
divided between the microprocessor 31 and the gate timer 24 and a correct 
oscillation frequency is achieved by a divide by two circuit 38. The 
doubled frequency signal output from the full wave rectifier 28 is input 
to a low pass filter 32, then converted to a digital signal by the A/D 
converter 33. Within the processor 31 the digitized signal is matched with 
stored data patterns to obtain a value for the heart rate which is then 
output to the digital display 36 via a display driver 35. The processor 
may be controlled by the operator via buttons 34. A power source 37 
connected to the processor 31 provides for the monitor to be 
self-contained. 
If a single crystal is used, it has the advantages of ease of manufacture 
and cost reductions although two parallel crystals could equally be used, 
one connected to each of the transmit and receive circuits. 
Typically, the volume and ON switches 14,15,16 will be membrane switches. 
Under processor control the monitor will automatically switch off a fixed 
time after displaying a value for the heart rate unless the ON switch is 
depressed again.