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BACKGROUND OF THE INVENTION
A. Field of Invention
This invention relates to a direction-finding system for locating
reflecting interfaces, in particular of concretions in the biologic tissue
of the human body. Such concretions may include, for example, gall,
bladder, and kidney stones. The direction-finding system has an ultrasound
pulse source, which pulses are readily transmitted through some
substances, such as liquids (e.g. water) and biological tissues, and
reflected by the interfaces between the liquids and other substances, and
are recorded by ultrasound receivers. Additional devices transform the
received ultrasonic echoes into stereophonic audible signals.
B. Description of the Prior Art
Image-forming ultrasonic display units are frequently used in the field of
medicine. Other devices are known which evaluate the movement of a
reflecting interface in the body (e.g. heart muscle movement of the
embryo) by the Doppler effect have monophonic audio signals, as an output
possibly including a threshold circuit which suppresses echo signals that
are too weak, and hence not usable.
These ultrasound echo devices known in medical technology, however, permit
only a non-dimensional, monophonic acoustic identification (e.g. a beep)
or they indicate the output signal of a Doppler effect detector.
Problems in locating acoustically reflecting objects in the human body
arise in particular in extracorporal acoustic lithotripsy wherein
image-forming x-ray and ultrasonic methods are used.
As the conditions of propagation of x-rays and of sound waves in the human
body are different, a congruent localization of the concretion using both
systems does not result. The radiological observation of a stone furnishes
shadow images usable for locating said stone, but the correlation of the
stone position to the focus point of the shock wave generator is rendered
difficult when the focusing dimension is small. A useful depth and lateral
localization can therefore not be carried out with satisfactory certainty.
If image-forming ultrasonic methods are used for locating stones, the
representations produced according to the present state of the art are
interpretable only by highly specialized personnel, and reflections not
produced by the stone may easily be misinterpreted. This problem is
particularly prominent in distinguishing between a stone, a bone, or, a
gas-containing area.
A stereophonic signal presentation in connection with position-finding
problems of the blind has been proposed already in ultrasonic
blind-guiding devices (DAGA '80--Fortschritte der Akustik, VDE-Verlag,
Berlin 1980, pages 767 ff.). The problems of ultrasound transmissivity of
biologic tissue and of water were of no importance in that field. Hence
that description of a stereophonic ultrasonic direction-finding system
does not teach the design of an apparatus applicable in medical
technology.
Ultrasonic direction-finding systems are also known in other fields. Thus,
for example for the localization of ships ultrasound pulses of short
duration are emitted, the reflected echos are detected by a receiver
("transducer") and the transit time of the echo pulses is measured.
The graphic representation of sonar signals is described in U.S. Pat. Nos.
2,528,725 to 2,528,730.
SUMMARY OF THE INVENTION
The object of the present invention is the development of an ultrasonic
direction-finding system useful in the medical arts for the localization
of concretions in the living body which will enable the treating physician
by means of acoustically perceptible signals to confirm, reject or at
least question the location indicated by other known image-forming
ultrasonic and radiological methods. This system must be able, therefore,
to pick up and process the existence of a reflecting object within a space
present at a certain distance from the ultrasound receiver, as well as the
size and nature of the reflection produced by an object located there.
The assumption made here is that any normally hearing person is able to
determine with sufficient precision the direction of origin of a sound.
This ability is utilized by the apparatus according to the invention to
simulate an audible and localizable phantom sound source with the aid of
transformed ultrasonic echo signals. For two-channel stereo reproduction,
a differential range of 0 to about 20 dB, is used with a time delay of 0
to about 600 microseconds for earphone reproduction and about 20 ms for
loudspeaker reproduction.
The system according to the invention consists of at least one ultrasonic
pulse transmitter controllable with respect to its direction of radiation,
having at least two likewise orientable ultrasonic receivers. Ultrasonic
transmitters are known e.g. as piezoelectric, electromagnetic, inductive,
magnetostrictive transducers (transceivers) as well as explosion or spark
gap transducers. The ultrasound frequencies suitable for stone detection
are in the range of 100 kHz to about 5 MHz. The ultrasound pulse
repetition frequency is about 5 Hz, the pulse duration being in the range
of 0.5 to 100 oscillation periods.
If the object reflecting the sound is equidistant from the receivers, the
reflected echo signals arrive at them simultaneously; their transit time
difference is zero. The echo amplitudes are the same if the reflecting
object disperses the pulses in an approximately symmetrical fashion. Due
to the spheroidal shape of concretions, this is usually the case. The
stereophonic reproduction equipment here produces a phantom sound source
which is in the stereophonic center position.
If the object is not equidistant from the receivers, transit time
differences will result in the received echo signals. A receiver located
at a shorter distance from the object, detects a signal first and at a
higher amplitude. The stereophonic reproduction equipment produces in
response a phantom sound source which is offset from the stereophonic
center position.
As the entire direction-finding field is in liquid or in biologic tissue,
the resulting differential transit times of the received signals are as a
rule extremely small and therefore cannot be evaluated for
direction-finding directly by the human ear.
The signals picked up by the receivers are transformed into audible,
stereophonic signals, which seem to be coming from a phantom sound source.
According to the invention, therefore, the locating system contains
devices or circuits which store the intercepted signals, determine
therefrom transit time differences, amplitudes, thresholds, reflection
characteristics (polarity of the reflection) and preferably also the time
position of the echoes, and transform the stored signals into audible
frequency ranges in analog form. It is possible to realize the devices as
algorithms of computer programs rather than as electronic circuits.
Thus audible signals are produced which permit not only a lateral
localization, but also by their acoustic character and their intensity an
estimate of the type of reflecting object (e.g. tissue/tissue, tissue/bone
or tissue concretion interfaces).
The direction-finding system according to the invention contains for each
ultrasound receiver a respective gate circuit as a time window. The
opening time and opening duration are chosen so that only the echo signals
to be expected from an area to be examined can pass. Each gate circuit is
appropriately combined with an adjustable amplifier, the gain of which can
increase with the distance of the area to be examined in such a manner
that on the output side a constantly high echo signal occurs if the area
to be examined contains a spherical reflector.
The gate circuits are connected to a transient storage device for picking
up ultrasonic echo signals with storage locations assigned to each
receiver. Means are provided which subject the stored data to direct sweep
magnification, as explained more fully below.
By sweep magnification it is possible to convert inaudible ultrasonic
pulses into the audible frequency range in correct curve form to obtain
stereophonic audio signals.
However not in all cases will a given sweep magnification scale transfer
the received ultrasonic signals simultanously into the optimum mean audio
frequency range of about 400 Hz to about 1 kHz and into the transit time
difference range optimum for spatial perception of from 0 to about 600
microseconds. For instance, the ultrasonic pulses can contain very high
frequency components (about 10 MHz) or the targeted concretions could be
at a distance from the receivers such that the transformed differential
echo transit time assume values which are too high (for headsets >1 ms, or
for loudspeakers >10 ms) to allow stereophonic phantom-source
localisation.
The direction-finding system according to the invention therefore comprises
not a simple sweep magnification device for the received signals with
respect to the transmission pulse synchronizing the two records, but means
which adjust two transformation scales independently of each other.
One transformation scale (ML) makes it possible to select the "locating
precision", that is, to freely decide which ultrasound differential echo
transit time is to lead to what transit time offered to the ear in the
audio reproduction. For headset reproduction the transferred transit time
differences must be 0 to 600 microseconds.
Another transformation scale (MF) makes it possible to decide into what
audio frequency range one wishes to place the converted ultrasound echo
signals. This is of special practical importance when the tone quality of
the echo is to contribute to the evaluation. It has in fact been found
empirically that this makes possible a distinction between tissue echo and
stone echo.
The memory reception channels carrying the reflected ultrasonic signals,
triggered by the transmission pulse, are activated to receive after an
inactive time following the pulse transmission. By automatic polling of
the memory contents it can now easily be determined which signal arrives
later and by what time period. The delay time is known with reference to
the number of idle samples preceeding the arriving echo signals and this
can be related to the transformation scale ML, while the signals
themselves are related to the transformation scale MF. These
transformations can be realized in the manner known in itself by
variations of the sampling rate of the memory contents.
The ultimate goal of the audio direction finding and locating aimed at,
namely to document the existence of a reflecting obstacle in a small space
present at a certain distance from the ultrasound receivers, is thus
possible by the mentioned sweep magnifications of the ultrasound signals
storable in a time window.
A solution especially favorable for the locating consists in normalizing
the two echo signals to equal levels. Thereby the audio direction-finding
depends most exclusively on the differential transit time. Erroneous
lateral deviations of the phantom sound source, which occur for example
due to echo level differences (caused by direction-dependent reflections),
are effectively suppressed.
A preferred form of the direction-finding system according to the invention
contains in the ultrasound receiver section, when one transmitter is
present, four receivers at the corners of an imaginary rectangle, since a
receiver pair permits direction finding in one plane. Thus, in the case of
square or rhombic arrangement, it becomes possible to take bearings in two
planes perpendicular to each other, and hence to detect the existence of a
reflecting obstacle on one axis.
The output stereo phonic system is realized in conventional manner with a
two-channel arrangement. The preferred stereophonic presentation occurs
via a headset which is cordlessly coupled with the signal output device
via a transmitter. The connection is obtained e.g. with radio frequency
means, inductively or by infrared technology.
To the stereophonic signals are added in an especially preferred variant
additional identifying marks, such as tones, tone sequences, keyed tones,
wobble tone, voice text outputs, which as additional information, describe
the qualitative properties of the echo signals, e.g. amplitude, time
position in relation to the transmission signal, transmit time difference,
threshold and polarity.
Such identifying marks can also be preceptible, separately from the
audio-phonic section, via visual signs (light monitors) or tacticle
stimulators (vibrators) controlled by the acoustic transceiver system.
The system according to the invention permits ultrasonic detection as well
as audio-signal direction-finding, in the medical field for concretions
which brings about a significant increase in reliability and certainty, in
cooperation with a reflector position evaluation obtainable from known
image-forming ultrasonic methods. The direction-finding equipment of the
invention thus helps to find the target more easily, more reliably and
faster and furnishes via the acoustic, optical or tactile signal
transmitters, additional useful information about the position, size and
nature of the reflecting body.
BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 shows a schematic diagram for a direction-finding system constructed
in accordance with the invention;
FIG. 2 shows a graph for various values of the constant MF;
FIG. 3 shows a log/log graph of four different transforming ML values;
FIG. 4 shows a linear/linear graph of four different transforming ML
values;
FIGS. 5(a), 5(b), 5(c), 5(d), 5(e) and 5(f) show various waveforms
developed within the device of FIG. 1; and
FIG. 6 shows in block diagram the signal processing required to obtain the
waveforms of FIG. 5.
DETAILED DESCRIPTION OF THE INVENTION
In FIG. 1, a direction-finding system is realized with an ultrasonic
transmitter 18 and a quad receiver system 2, 3, 4, 5. The stereophonic
output of the receiver system is detected via a headset 15. A cordless
receiver head set 15 is coupled to system 8 by radio frequency or infrared
signals propagating through antennas 13 and 14.
The stereophonic signals are also sent to the loudspeakers 9 and 10 and are
indicative of the location of a stone. Thus the signals provide the
listener with a subjective directional indication of a phantom sound
source and (upon evaluation of the tone quality of the audio echo signals)
also a qualitative estimation of the reflecting object 24.
The sterophonic signals contain in this example additional identifying
characteristics which are displayed via a light-emitting display 16 or an
tactile stimulator 17. The latter is shown in the sketch at the bottom as
a separate component, however, it is possible to generate tactile
stimulations directly at the actuating element 23 controlling the position
of the acoustic transceiver system.
More specifically, the stereophonic system comprises an ultrasonic
transmitter 18 and four individual receivers 2, 3, 4 and 5 which are
arranged respectively at the corners of a rhombus as shown. Transmitter 18
preferably comprises a hemispherical bowl of about 40 cm in diameter. The
bowl is made up of a large number (up to 10000) of ceramic disks which are
arranged on the bowl to generate ultrasonic waves by the application of a
high-voltage electric field. Ultrasonic waves generated by the individual
disks coincide at the focal point of the bowl.
Preferably, the transmitter 18 is disposed below a treatment table
supporting the patient and the ultrasonic waves are transferred into the
patient's body through a water-filled opening made in the table. The
schematic diagram of FIG. 1 shows how the subject system is used to locate
an object 24. For the purposes of this diagram, the treatment table and
the patient have been omitted, however it should be understood that object
24 is preferably a concretion produced within a patient's body.
If object 24 reflecting emitted ultrasonic pulses 1, which is equidistant
from these receivers 2-5 the reflected echo signals 11, 12 and 21, 22,
arrive at the same time to the receivers 4, 5 and 3, 2 respectively. Hence
the transit time difference is zero and the echo amplitudes are
practically identical if the object reflects approximately symmetrically.
If the object 24 is at a different distance from the receivers, there
result transit time differences in the received echo ultrasound signals 11
and 12 or respectively 21 and 22. The receivers closest to the object
detect the signals earlier and at a higher amplitude.
The received ultrasound echo signals 11, 12 and 21, 22 are windowed by four
separate gate circuits 25, are stored in the four-channel transient memory
6, and supplied to the sweep magnifier 7 via the amplifier 26.
For separate, independent variation of the transit times via the
transformation scale ML and of the audio frequency range via the
transformation scale MF, the memory contents are retrieved from memory 6
singly and, allowing for a delay after the transmitting pulse, it is
determined which signal arrives later and by what time period. The delay
time is now known on the basis of the different number of idle samples
preceeding the echo signals and can be detected through the transformation
scale ML, while the signals themselves are interpreted with the
transformation scale MF.
After these values are adjusted, it is possible by way of the control 23,
control unit 19 and the connection 20 to the servomotors (not shown) to
move the entire transducer system 18 in three directions and to pivot it
back and forth until, via the display unit, the exact position and the
nature of the reflecting object 24 have been determined.
Selectively singly or in combination with one another, the outputs of the
transient memory 6 can be linked with detectors for determining the
polarity (hard/soft reflection) (27), the echo amplitudes (28), the time
position (29) of the echoes 11, 12, 21, 22 with respect to the
transmission signal 1, the space position (30) of the concretion 24, as
well as the value and/or sign of the transit time differences (31). These
detectors control one or more signal output devices 8, which process the
data and provide signals to loudspeakers 9, 10, headset 15, display 16, or
tactile stimulator 17.
As shown in FIG. 2, preferably transformation MF is linear whereby the
ratio between the ultrasonic frequency US of the waves produced by
transmitter 18 and the frequency of the audisounds produced by device 8 is
a constant MF. In FIG. 2, curves A, B and C correspond to constant MF
having values of 500, 1000 and 2000 respectively. It has been found that
the system operates best if the audio signals of device 8 are selected in
the range of 200 to 1600 HZ, and preferably in the range of 400 to 800 HZ.
Once a frequency for the audio signals, and the constant MF is selected,
the frequency of the ultrasonic waves may be found from FIG. 2. Audio
signals above 1600 Hz or below 200 HZ are less favorable.
FIGS. 3 and 4 show four ways for transforming the differential transit
times of the ultrasonic echoes (the ordinate axis) to corresponding
differential transit times for audio signals. In FIG. 3 it is indicated
that for earphone reproduction, the audio signals transit time must be in
the range of 0-640 microseconds. Different transformation scales ML result
in different "precision" in the quality of the audio signals.
The signal processing required to transform the signals in the ultrasonic
range into signals in the audio range shall now be described with
reference to FIGS. 5 and 6. FIG. 5 illustrate various wave forms within
the system as a function of time while FIG. 6 shows how these waveforms
are derived.
Initially, transmitter 18 of FIG. 1 generates a burst of ultrasonic waves.
These waves may be in the frequency range of 100 KHZ to 10 MHZ. A device
using a frequency of 480 KHZ was operated successfully. The burst is
relatively short. For example the burst may be in the range of 1.5-2
periods. The response from receivers 2 and 3 are fed respectively to
memory 6 through two gate circuits 25. (FIG. 6). Within memory 6 these
analog signals are sampled at a sampling frequency of S.sub.us and stored
to form data fields 2 and 3 respectively. The sampling rate is dependent
on the frequency of the waves produced by generator 1. As is well known
from communication theory, this frequency must be at least twice the
frequency of the ultrasonic waves. Thus, S.sub.us may be in the range of
200 KHz-20 MHz. In order to insure the accuracy of the results, the
sampling rate could be increased to a range of 10-100 MHz corresponding to
a sampling interval of (signal to 1/S.sub.us) of 0.1 microsec to 10
nanosec. A short sampling time interval insures that the differential
transit time between two received waves is detected accurately.
Typically, a differential transit time of up to .+-.20 microsec, may be
stored in memory, which permits at 100 MHZ sampling rate the detection of
an object which is of .+-.3 cm from the focal point. If the sampling rate
is reduced by a factor of two, the detector range of the device is from
.+-.0.06 mm to .+-.6 cm corresponding to a differential time delay between
the echo signals of 40 nanosec to 40 microseconds.
The wave shapes of FIG. 5 were obtained by generating an ultrasonic wave of
480 KHz for 1.5 time periods. After delay at t=0 the gate circuits (25)
are opened. The signal bounces off concretion 24 and is detected by
receiver (2) four time intervals later (FIG. 5a) and by receiver (3) six
time intervals later (FIG. 5b). Therefore, the differential time delay
between the two signals is two sampling intervals.
As shown in FIG. 6, the intervals N.sub.F and N.sub.D indicative of the
detector of the echo signals by receivers 2 and 3 respectively are used to
calculate the differential time delay from the formula:
t.sub.us =(N.sub.D -N.sub.F)/S.sub.us
This time delay tus (separating the ultrasonic time delay) is used by the
sweep magnifier 7 to generate a corresponding audio time delay t.sub.d
from the formula
t.sub.d =t.sub.us (1/ML*MF-1)
Datafields 2 and 3 are amplified by amplifier 26. Datafield 2 remains
essentially unchanged as indicated by FIG. 5(c). Datafield 3 is delayed by
t.sub.d to generate a datafield 3', as shown in FIG. 5(d) wherein t.sub.d
=8 sampling intervals. N+t.sub.d /U.sub.us samples of datafields 2 and 3'
are stored (N being the number of samples for the wave in FIG. 5(a), i.e.
27). These values are played backed at a much slower rate S.sub.LF
=S.sub.us /MF thereby effectively expanding the two waves in time as shown
in FIGS. 5(e) and 5(f) respectively. The differential time delay between
these expanded waves is given by
T.sub.LF =(t.sub.d +t.sub.us)MF or
T.sub.LF =[t.sub.us (1/ML*MF-1)+t.sub.us ]MF or
T.sub.LF =t.sub.us /ML
Thus the delay T.sub.LF is independent of MF, and therefore the two
variables ML and MF can be chosen arbitrarily.
It should be understood that the waveshapes of FIGS. 5(a)-(f) and the
numerical values given above have been described or given for illustrative
purposes only and are not to be interpreted as limitations on the claims.
The values N.sub.D, N.sub.F and t.sub.us are preferably determined and
calculated as follows:
N.sub.F and N.sub.D by the time position detector (29),
t.sub.us by the transit time detector (31),
t.sub.d by the sweep magnifier (7), shown in FIG. 1.
Obviously, numerous modifications may be made to the invention without
departing from its scope as defined in the appended claims.
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