U.S. patent application number 12/110901 was filed with the patent office on 2008-08-28 for method and apparatus for non-invasive ultrasonic fetal heart rate monitoring.
This patent application is currently assigned to General Electric Company. Invention is credited to Ralph T. Hoctor, Kai E. Thomenius.
Application Number | 20080208057 12/110901 |
Document ID | / |
Family ID | 35240327 |
Filed Date | 2008-08-28 |
United States Patent
Application |
20080208057 |
Kind Code |
A1 |
Hoctor; Ralph T. ; et
al. |
August 28, 2008 |
METHOD AND APPARATUS FOR NON-INVASIVE ULTRASONIC FETAL HEART RATE
MONITORING
Abstract
A continuous, non-invasive fetal heart rate measurement is
produced using one or more ultrasonic transducer array patches that
are adhered or attached to the mother. Each ultrasound transducer
array is operated in an autonomous mode by a digital signal
processor to obtain data from which fetal heart rate information
can be derived. Each ultrasonic transducer array patch comprises a
multiplicity of subelements that are switchably reconfigurable to
form elements having different shapes, e.g., annular rings. Each
subelement comprises a plurality of interconnected cMUT cells that
are not switchably disconnectable. The use of cMUT patches will
provide the ability to interrogate a three-dimensional space
electronically (i.e. without mechanical beam steering) with
ultrasound, using a transducer device that is thin and lightweight
enough to stick to the patient's skin like an EKG electrode. The
ultrasound device can track the fetal heart in three-dimensional
space as it moves due to the mother's motion or the motion of the
unborn child within the womb.
Inventors: |
Hoctor; Ralph T.; (Saratoga
Springs, NY) ; Thomenius; Kai E.; (Clifton Park,
NY) |
Correspondence
Address: |
GENERAL ELECTRIC COMPANY (PCPI);C/O FLETCHER YODER
P. O. BOX 692289
HOUSTON
TX
77269-2289
US
|
Assignee: |
General Electric Company
Schenectady
NY
|
Family ID: |
35240327 |
Appl. No.: |
12/110901 |
Filed: |
April 28, 2008 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10838799 |
May 4, 2004 |
|
|
|
12110901 |
|
|
|
|
Current U.S.
Class: |
600/453 |
Current CPC
Class: |
A61B 8/08 20130101; A61B
8/02 20130101; B06B 1/0292 20130101; A61B 8/4483 20130101; A61B
8/488 20130101; A61B 8/4236 20130101; A61B 8/0866 20130101 |
Class at
Publication: |
600/453 |
International
Class: |
A61B 8/02 20060101
A61B008/02 |
Claims
1. A system for autonomous monitoring of heart rate in a fetus,
comprising an array of ultrasonic transducer elements, data
processing means, and means for delivering signals derived from the
output of said array to said data processing means, wherein said
data processing means are programmed to perform the following
steps: (a) controlling said ultrasonic transducer array to scan the
abdomen of the pregnant woman by transmitting beams of pulsed
ultrasonic wave energy into the abdomen, said beams being steered
at different angles; (b) acquiring acoustic data by transducing
ultrasound wave energy returned from the abdomen of the pregnant
woman after each transmission in step (a), the acoustic data being
acquired using relatively long range gates; (c) detecting a pattern
in the acoustic data acquired in step (b), said pattern
representing a fetal heart signature; (d) controlling said
ultrasonic transducer array to transmit beams of pulsed ultrasonic
wave energy into the abdomen that are steered at an angle equal to
or approximately equal to the angle of a beam transmitted in step
(a) that resulted in the detection of said pattern in said acquired
data; (e) acquiring acoustic data by transducing ultrasound wave
energy returned from the abdomen of the pregnant woman after each
transmission in step (d), the acoustic data being acquired using
relatively short range gates; and (f) calculating a fetal heart
rate based at least in part on the acoustic data acquired in step
(e).
2. The system as recited in claim 1, wherein step (c) comprises the
step of recognizing an oscillating mean frequency of the Doppler
shift in a certain frequency range.
3. The system as recited in claim 1, wherein said data processing
means are further programmed to perform the step of adjusting the
range gate in step (e) to optimize the signal-to-noise ratio.
4. The system as recited in claim 1, wherein said ultrasonic
transducer array comprises micromachined ultrasonic
transducers.
5. The system as recited in claim 1, wherein said data processing
means are further programmed to perform the following steps: (g)
controlling said first ultrasonic transducer array to transmit
beams of pulsed ultrasonic wave energy into the abdomen that are
steered at respective different angles which are respectively
displaced relative to the angle of the beams transmitted in step
(d); (h) acquiring acoustic data by transducing ultrasound wave
energy returned from the abdomen of the pregnant woman after each
transmission in step (g); (i) determining the signal-to-noise ratio
of the acquired acoustic data for each different beam steering
angle in step (g); (j) controlling said first ultrasonic transducer
array to transmit additional beams of pulsed ultrasonic wave energy
into the abdomen that are steered at the beam steering angle which
produced a maximum in the signal-to-noise ratio in step (i); (k)
acquiring acoustic data by transducing ultrasound wave energy
returned from the abdomen of the pregnant woman after each
transmission in step (j); and (l) calculating a fetal heart rate
based on the acoustic data acquired in step (k).
6. A system for autonomous monitoring of heart rate in a fetus,
comprising an array of ultrasonic transducer elements, said array
comprising a multiplicity of micromachined ultrasonic transducer
cells built on or formed in a first substrate, data processing
means, and means for delivering signals derived from the output of
said array to said data processing means, wherein said data
processing means are programmed to perform the following steps: (a)
controlling said array to transmit beams of pulsed ultrasonic wave
energy; (b) beamforming acoustic data output from said array in
response to impinging ultrasound wave energy transmitted to and
returned from the fetal heart; and (c) computing the fetal heart
rate based on the beamformed acoustic data.
7. The system as recited in claim 6, further comprising CMOS
electronics built on or formed in a second substrate, said first
and second substrates being bonded together to form a patch
suitable for adhesion to a patient.
8. The system as recited in claim 7, wherein said data processing
means comprise a digital signal processor, and said signal
delivering means comprise a cable.
9. The system as recited in claim 6, wherein said data processing
means are further programmed to perform the step of detecting a
pattern in the beamformed acoustic data, said pattern representing
a fetal heart signature,
10. The system as recited in claim 6, wherein said data processing
means are further programmed to perform the step of adjusting the
range gate to optimize the signal-to-noise ratio.
11. The system as recited in claim 6, wherein said ultrasonic beams
are transmitted in step (a) using a generally circular active
aperture.
12. The system as recited in claim 6, wherein step (a) comprises
the step of activating concentric, generally annular transducer
elements with beamforming delays.
13. A method for monitoring fetal heart rate, comprising the
following steps: (a) attaching a first ultrasonic transducer array
comprising micromachined ultrasonic transducers to a first area on
the exterior of an abdomen of a pregnant woman; (b) controlling
said first ultrasonic transducer array to transmit beams of pulsed
ultrasonic wave energy into the abdomen that are steered at an
angle that causes the beams to intersect a fetal heart inside the
abdomen; (c) acquiring acoustic data by transducing ultrasound wave
energy returned from the abdomen of the pregnant woman after each
transmission in step (b), the acoustic data being acquired using
relatively short range gates; and (d) calculating a fetal heart
rate based at least in part on the acoustic data acquired in step
(c).
14. The method as recited in claim 13, further comprising the
following steps performed after step (a) and prior to step (b): (e)
controlling said first ultrasonic transducer array to scan the
abdomen of the pregnant woman by transmitting beams of pulsed
ultrasonic wave energy into the abdomen, said beams being steered
at different angles; (f) acquiring acoustic data by transducing
ultrasound wave energy returned from the abdomen of the pregnant
woman after each transmission in step (e), the acoustic data being
acquired using relatively long range gates; and (g) detecting a
pattern in the acoustic data acquired in step (f), said pattern
representing a fetal heart signature, wherein the beams transmitted
in step (b) are steered at an angle equal to or approximately equal
to the angle of a beam transmitted in step (e) that resulted in the
detection of said pattern in said acquired data in step (g).
15. The method as recited in claim 14, wherein step (g) comprises
the step of recognizing an oscillating mean frequency of the
Doppler shift in a certain frequency range.
16. The method as recited in claim 13, further comprising the step
of adjusting the range gate in step (c) to optimize the
signal-to-noise ratio.
17. The method as recited in claim 13, further comprising the
following steps performed prior to step (a): (e) controlling said
first ultrasonic transducer array to transmit a beam of pulsed
ultrasonic wave energy; (f) acquiring acoustic data by transducing
ultrasound wave energy returned from the abdomen of the pregnant
woman after each transmission in step (h); (g) converting said
acquired acoustic data into audible signals; and (h) adjusting the
position of said first ultrasonic transducer array relative to the
abdomen of the patient until the audible signal corresponds to the
fetal heart beat, said first ultrasonic transducer array being
attached in step (a) at the location where this audible event
occurs.
18. The method as recited in claim 13, further comprising the
following steps: (e) controlling said first ultrasonic transducer
array to transmit beams of pulsed ultrasonic wave energy into the
abdomen that are steered at respective different angles which are
respectively displaced relative to the angle of the beams
transmitted in step (b); (f) acquiring acoustic data by transducing
ultrasound wave energy returned from the abdomen of the pregnant
woman after each transmission in step (e); (g) determining the
signal-to-noise ratio of the acquired acoustic data for each
different beam steering angle in step (e); (k) controlling said
first ultrasonic transducer array to transmit additional beams of
pulsed ultrasonic wave energy into the abdomen that are steered at
the beam steering angle which produced a maximum in the
signal-to-noise ratio in step (g); (l) acquiring acoustic data by
transducing ultrasound wave energy returned from the abdomen of the
pregnant woman after each transmission in step (k); and (m)
calculating a fetal heart rate based on the acoustic data acquired
in step (l).
19. The method as recited in claim 18, wherein in step (f) the
acoustic data is acquired using different range gates for the same
receive beam steering angle.
20. The method as recited in claim 13, further comprising the
following steps: attaching a second ultrasonic transducer array to
a second area on the exterior of the abdomen of said pregnant
woman; and performing steps similar to steps (b) through (d),
except using said second ultrasonic transducer array instead of
said first ultrasonic transducer array.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 10/838,799, entitled "METHOD AND APPARATUS FOR
NON-INVASIVE ULTRASONIC FETAL HEART RATE MONITORING", filed May 4,
2004, which is herein incorporated by reference.
BACKGROUND
[0002] The present invention generally relates to methods and
apparatus for determining the heart rate of a subject, and
particularly relates to methods and apparatuses for determining the
beat-to-beat heart rate of a fetus.
[0003] Fetal monitoring (i.e., monitoring of the fetal condition
during gestation and at birth) usually comprises monitoring uterine
activity and the fetal beat-to-beat heart rate. The fetal heart
rate, which provides an indication of whether the fetus is
sufficiently supplied with oxygen, is preferably calculated from
beat to beat.
[0004] To obtain a signal indicative of the fetal heart rate prior
to rupture of the membranes, a noninvasive monitoring technique
must be used. The most widely adopted measurement technique
involves measuring the Doppler shift of an ultrasound signal
reflected by the moving fetal heart.
[0005] In accordance with a known ultrasonic detection technique,
an ultrasound transducer array is placed externally on the pregnant
woman's abdomen and oriented such that the transmitted ultrasound
waves impinge upon the fetal heart. The reflected ultrasound waves
are received either by the same or by a different ultrasound
transducer array. The Doppler shift of the reflected ultrasound
wave is directly related to the speed of the moving parts of the
heart, e.g., the heart valves and the heart walls.
[0006] To extract the Doppler component from the received
ultrasound signal, typically the latter is demodulated. Further
processing depends on the specific application. One technique for
acquiring an acoustic indication of the heart beat uses
autocorrelation. In accordance with the autocorrelation technique,
the Doppler signal or the envelope of the Doppler signal is
correlated with itself, thus providing significant peaks in time
intervals which correspond to periodic components of the Doppler
signal that are caused by the fetal heart beat. Such techniques are
necessary because the received ultrasound signals contain noise
originating from various physiological sources, such as the
maternal aorta, movement of the fetus as a whole, and the like.
[0007] In prior art devices, a peak trigger device or an equivalent
circuit detects the peaks in the autocorrelation function. The
beat-to-beat heart rate, computed as the inverse of the time
interval between two successive heart beats, is then available for
further processing, display and/or recordation.
[0008] In clinical applications, the ultrasound transducer array is
placed on the maternal abdomen such that the ultrasound waves will
impinge upon the fetal heart. The transmitted ultrasound energy is
typically in the form of bursts of high-frequency (e.g., 1 MHz)
waves. The ultrasound waves reflected by the fetal heart and other
fetal tissue are then received by ultrasound transducer array and
fed to a demodulator, which produces a signal that is indicative of
the Doppler shift of the received ultrasound signal relative to the
transmitted signal. This Doppler shift is caused by the moving
parts of the fetal heart, in particular the heart walls and the
heart valves. The output of the demodulator is then fed to a
bandpass filter that removes unwanted components in the Doppler
signal. This filtered signal is then fed to an envelope detector,
which generates the envelope of the peaks of the Doppler signal.
Later the envelope of the Doppler signal is fed to an
autocorrelation circuit that correlates the received signal with
itself. The result of the autocorrelation function is that periodic
components in the received signal are amplified, while non-periodic
or stochastic signals (such as signals caused by movement of the
fetus or the mother) are largely suppressed.
[0009] The output of the autocorrelation function is used to
calculate the fetal heart rate. The significant peaks in the
amplitude of the autocorrelated signal correspond to fetal heart
beats and successive peaks are separated by time intervals that are
approximately equal. The fetal heart rate is calculated as the
inverse of the time interval between two successive peaks in the
autocorrelated signal. Alternatively, the heart rate could be
calculated by, e.g., computing it over a longer time interval and
averaging the time intervals or averaging the heart rate.
[0010] One challenge in clinical applications is ensuring that the
ultrasonic transducers are properly positioned relative to the
fetal heart. Known instrumentation uses a relatively cumbersome
array of single-element ultrasound transducers designed to cover
the entire maternal abdomen. Unfortunately, as the result of fetal
or maternal motion, the alignment of these transducers is such that
the transducers fail to capture some echoes from the fetal heart
and there will be a loss of heart rate information. When this
occurs, the attending nurse must readjust the positioning of the
transducer array. The resulting frustration experienced by hospital
staff members because of this inconvenience is apparently to the
degree that the instrument is often ignored and the mother and
fetus do not receive the benefit of monitoring or the assessment of
fetal distress. Additionally, in many instances the mother is
instructed not to move, so that the fetal heart rate monitor does
not lose the reflected ultrasound signal. This is contrary to good
practice, since the mother's motion is important in causing the
onset of heavy labor. If the mother were able to move around and
still have a working fetal monitor, labor could be shortened in
many instances.
[0011] There is a need for a method and means for continuously
monitoring the fetal heart rate without any operator intervention.
The method and apparatus should be designed so that the mother is
allowed free motion while being monitored. It is also desirable
that a more reliable heart rate measurement be provided as compared
to current systems.
BRIEF DESCRIPTION
[0012] The present invention overcomes the aforementioned drawbacks
by using ultrasound to provide input data for the fetal heart rate
calculation. In a disclosed embodiment, a continuous, non-invasive
fetal heart rate measurement is produced using one or more
ultrasonic transducer array patches that are adhered or attached to
the mother. Each ultrasound transducer array is operated in an
autonomous mode by a digital signal processor to obtain data from
which fetal heart rate information can be derived.
[0013] In accordance with the disclosed embodiment, each ultrasonic
transducer array patch comprises a multiplicity of subelements that
are switchably reconfigurable to form elements having different
shapes, e.g., annular rings. Each subelement comprises a plurality
of interconnected capacitive micromachined ultrasonic transducer
(cMUT) cells that are not switchably disconnectable. The use of
cMUT patches will provide the ability to interrogate a
three-dimensional space electronically (i.e. without mechanical
beam steering) with ultrasound, using a transducer device that is
thin and lightweight enough to stick to the patient's skin like an
EKG electrode.
[0014] Thus the present invention applies cMUT technology to the
monitoring of fetal heart rate in a labor and delivery or ICU
setting. The ultrasound device can track the fetal heart in
three-dimensional space as it moves due to the mother's motion or
the motion of the unborn child within the womb. This will allow the
use of a narrower beam and a smaller sample volume than is used by
prior art devices, so that less interference will be present in the
Doppler signal that is processed to derive the fetal heart rate.
While this tracking is taking place, the fetal heart rate can be
derived by standard means from the Doppler shift of the ultrasound
reflected from the fetal heart.
[0015] One aspect of the invention is a method for monitoring fetal
heart rate, comprising the following steps: (a) attaching an
ultrasonic transducer array comprising micromachined ultrasonic
transducers to an area on the exterior of an abdomen of a pregnant
woman; (b) controlling the ultrasonic transducer array to transmit
beams of pulsed ultrasonic wave energy into the abdomen that are
steered at an angle that causes the beams to intersect a fetal
heart inside the abdomen; (c) acquiring acoustic data by
transducing ultrasound wave energy returned from the abdomen of the
pregnant woman after each transmission in step (b), the acoustic
data being acquired using relatively short range gates; and (d)
calculating a fetal heart rate based at least in part on the
acoustic data acquired in step (c).
[0016] Another aspect of the invention is a method for monitoring
fetal heart rate, comprising the following steps: (a) attaching an
ultrasonic transducer array to an area on the exterior of an
abdomen of a pregnant woman; (b) controlling the ultrasonic
transducer array to scan the abdomen of the pregnant woman by
transmitting beams of pulsed ultrasonic wave energy into the
abdomen, the beams being steered at different angles; (c) acquiring
acoustic data by transducing ultrasound wave energy returned from
the abdomen of the pregnant woman after each transmission in step
(b), the acoustic data being acquired using relatively long range
gates; (d) detecting a pattern in the acoustic data acquired in
step (c), the pattern representing a fetal heart signature; (e)
controlling the ultrasonic transducer array to transmit beams of
pulsed ultrasonic wave energy into the abdomen that have a
relatively short range gate and are steered at an angle equal to or
approximately equal to the angle of a beam transmitted in step (b)
that resulted in the detection of the pattern in the acquired data;
(f) acquiring acoustic data by transducing ultrasound wave energy
returned from the abdomen of the pregnant woman after each
transmission in step (e); and (g) calculating a fetal heart rate
based at least in part on the acoustic data acquired in step
(f).
[0017] A further aspect of the invention is a method of tracking a
moving fetal heart inside an abdomen of a patient, comprising the
following steps: (a) attaching an ultrasonic transducer array
comprising micromachined ultrasonic transducers to an area on the
exterior of an abdomen of a pregnant woman; (b) scanning a volume
inside the abdomen with beams of pulsed ultrasonic wave energy; (c)
acquiring acoustic data by transducing ultrasound wave energy
returned from the abdomen after each transmission in step (b), the
acoustic data being acquired using range gates centered in
different sample volumes; (d) determining the signal-to-noise ratio
of the acquired acoustic data for each different range gate; and
(e) determining which one of the different range gates had the
acoustic data which produced a maximum signal-to-noise ratio.
[0018] Yet another aspect of the invention is a system for
autonomous monitoring of heart rate in a fetus, comprising an array
of ultrasonic transducer elements, the array comprising a
multiplicity of micromachined ultrasonic transducer cells built on
or formed in a substrate, data processing means, and means for
delivering signals derived from the output of the array to the data
processing means, wherein the data processing means are programmed
to perform the following steps: (a) controlling the array to
transmit beams of pulsed ultrasonic wave energy; (b) beamforming
acoustic data output from the array in response to impinging
ultrasound wave energy transmitted to and returned from the fetal
heart; and (c) computing the fetal heart rate based on the
beamformed acoustic data.
[0019] A further aspect of the invention is a system for autonomous
monitoring of heart rate in a fetus, comprising an array of
ultrasonic transducer elements, data processing means, and means
for delivering signals derived from the output of the array to the
data processing means, wherein the data processing means are
programmed to perform the following steps: (a) controlling the
ultrasonic transducer array to scan the abdomen of the pregnant
woman by transmitting beams of pulsed ultrasonic wave energy into
the abdomen, the beams being steered at different angles; (b)
acquiring acoustic data by transducing ultrasound wave energy
returned from the abdomen of the pregnant woman after each
transmission in step (a), the acoustic data being acquired using
relatively long range gates; (c) detecting a pattern in the
acoustic data acquired in step (b), the pattern representing a
fetal heart signature; (d) controlling the ultrasonic transducer
array to transmit beams of pulsed ultrasonic wave energy into the
abdomen that are steered at an angle equal to or approximately
equal to the angle of a beam transmitted in step (a) that resulted
in the detection of the pattern in the acquired data; (e) acquiring
acoustic data by transducing ultrasound wave energy returned from
the abdomen of the pregnant woman after each transmission in step
(d), the acoustic data being acquired using relatively short range
gates; and (f) calculating a fetal heart rate based at least in
part on the acoustic data acquired in step (e).
[0020] Yet another aspect of the invention is a system for tracking
a fetal heart, comprising an array of ultrasonic transducer
elements, data processing means, and means for delivering signals
derived from the output of the array to the data processing means,
wherein the data processing means are programmed to perform the
following steps: (a) scanning a volume inside the abdomen with
beams of pulsed ultrasonic wave energy; (b) acquiring acoustic data
by transducing ultrasound wave energy returned from the abdomen
after each transmission in step (a), the acoustic data being
acquired using range gates centered in different sample volumes;
(c) determining the signal-to-noise ratio of the acquired acoustic
data for each different range gate; and (d) determining which one
of the different range gates had the acoustic data which produced a
maximum signal-to-noise ratio. Other aspects of the invention are
disclosed and claimed below.
DRAWINGS
[0021] These and other features, aspects, and advantages of the
present invention will become better understood when the following
detailed description is read with reference to the accompanying
drawings in which like characters represent like parts throughout
the drawings, wherein:
[0022] FIG. 1 is a block diagram of a fetal heart rate measurement
system in accordance with one embodiment of the invention.
[0023] FIG. 2 is a drawing showing a cross-sectional view of a
typical cMUT cell.
[0024] FIG. 3 is a drawing showing an isometric view of the cMUT
cell shown in FIG. 2.
[0025] FIG. 4 is a drawing showing an isometric view of a "daisy"
subelement formed from seven hexagonal cMUT cells having their top
and bottom electrodes respectively electrically interconnected in a
manner that is not switchably disconnectable.
[0026] FIG. 5 is a drawing showing a mosaic array comprising eight
annular elements.
[0027] FIG. 6 is a drawing showing the construction of a cMUT patch
probe employed in the disclosed embodiment of the invention. In the
depicted example, the cMUT cells have been surface micromachined on
the substrate. However, the cMUT cells could alternatively be bulk
micromachined on the substrate, in which case the solid line at the
interface of the cMUT and substrate layers could be changed to a
dashed line.
[0028] FIG. 7 is a drawing showing scanning of an ultrasound beam,
which can be repeated frequently to generate a rectilinear
two-dimensional image.
[0029] FIG. 8 is a drawing showing steering of an ultrasound beam
away from the normal direction.
[0030] Reference will now be made to the drawings in which similar
elements in different drawings bear the same reference
numerals.
DETAILED DESCRIPTION
[0031] In accordance with one embodiment of the present invention,
one or more cMUT patches are adhered to the mother's abdomen. Each
cMUT patch comprises a two-dimensional ultrasound transducer array
that is steerable in three-dimensional space. The cMUT patches are
electrically connected to a bedside instrument.
[0032] When the instrument is powered on, a search is performed to
acquire the Doppler signal of the fetal heart. An initial coarse
search, using wide beams and long range gates, will be followed by
higher-resolution searches that will successively locate the fetal
heart to a greater precision. Once the fetal heart has been located
precisely, its position will be tracked using multiple sample
volumes surrounding the heart's location. The sample volume that is
directly over the heart's location will produce a Doppler waveform
that can be processed to extract the fetal heart rate.
[0033] Based on acoustic data from the cMUT patch probes, a
processor incorporated in the bedside instrument autonomously
computes various parameters, including the estimated fetal heart
rate. In addition, the processor forms and steers ultrasound beams
over the volume of space in front of each cMUT patch probe. The
cMUT patch probes can be electrically coupled to the processor (or
processors) by means of cables. Alternatively, a wireless
electrical coupling could be provided.
[0034] The foregoing concepts can be easily extended to monitor
multiple-birth patients using multiple cMUT patches. Even for a
single birth, the use of multiple cMUT patches will produce
redundant fetal heart signatures, allowing for enhanced
reliability. Note that multiple cMUT patches can be employed in a
multiple-birth situation in a mode where each cMUT patch is
tracking multiple fetal hearts. This is possible because electronic
beam steering allows the ultrasound "look directions" to be changed
without any delay.
[0035] In accordance with one embodiment of the invention shown in
FIG. 1, the DSP 34 controls two cMUT patch probes 30 and 32. The
reason for having two probes is to have some redundancy in
measuring the fetal heart signal. If the mother moves, it is hoped
that one or the other probe will remain locked onto the fetal
heart. More than two cMUT patch probes can be used. However, it
should be understood that the broad concept of the invention also
encompasses the use of a single cMUT patch probe.
[0036] Referring to the embodiment shown in FIG. 1, the cMUT patch
probes 30 and 32 will each operate in a pulsed-wave (PW) Doppler
mode. When acquiring the fetal heart, the range gates will be long;
this is the standard mode of operation of known ultrasound fetal
heart rate monitors. However, the use of cMUT patch probes in
conjunction with an instrument operating in the PW Doppler mode
provides a beam steering capability that is absent from known fetal
heart rate monitoring systems. The fetal heart signature will be
obtained by scanning the beams laterally until the fetal heart is
detected. This detection occurs because the valves of the fetal
heart cause a Doppler shift in the reflected ultrasound. The fetal
heart rate is much higher than the maternal heart rate, which makes
it easy to recognize. In accordance with one embodiment of the
invention, the DSP 34 is programmed to perform an automatic
recognition algorithm for the fetal heart signature. For example,
the DSP can be programmed to detect an oscillating mean frequency
of the Doppler shift in a certain frequency range.
[0037] Lateral scanning could be accomplished by translating the
transmit aperture or by changing the beam direction from a
stationary transmit aperture or a combination of both. However, the
use of translation but not beam steering would require the cMUT
patch be large enough for a translated aperture to cover a
sufficient range of positions.
[0038] Once both cMUT patch probes 30 and 32 have acquired the
fetal heart, the DSP 34 will begin to decrease the length of the PW
range gates. The basic mode of operation will be to decrease the
length on one probe until that probe starts to lose signal. A
smaller range gate encompassing the heart will give a better
signal-to-noise ratio (SNR).
[0039] It is possible to use the beam steering angles from the
multiple patch probes to guide the process of decreasing the range
gate length. For example, if two cMUT patch probes are properly
positioned on the mother's abdomen so that two ultrasound beams
originate from two points, with the beams at an angle relative to
each another and with both beams producing a respective fetal heart
signal, then the fetal heart will be located within the spatial
intersection of the beams. (The two probes do not have to be
transmitting simultaneously in order for this to happen.) If the
DSP (or other processor) knew their respective beam steering angles
and their relative positions, then the distance of the fetal heart
from either probe could be computed, at least approximately, from
the geometry of the data collection set-up. The estimated range to
the fetal heart could then be used to define a shorter range gate
centered around the newly estimated range. However, using the beam
steering angles from multiple patch probes to guide the process of
decreasing the range gate length is not necessary to practice of
the broad concept of the present invention.
[0040] The process of adjusting the range gate to optimize the SNR
will be in continuous operation as a method of tracking fetal heart
range. The method would be a search algorithm based on the Doppler
signal power. The range gate would be modified in both size and
depth, and changes that resulted in a higher Doppler signal power
would be retained. The instrument can identify Doppler signal power
because it is the power of that part of the input signal that gets
past the high-pass filter used for stationary clutter
rejection.
[0041] Lateral tracking will occur on one probe at a time. The beam
will be steered away from the initial beam steering angle in eight
directions, i.e., the eight range gates will surround the range
gate of the initial beam in a 3.times.3 spatial relationship. That
direction and range giving the best signal (i.e., highest SNR) will
be the new beam steering angle and range gate depth. Extrapolating
this principle, one could steer 27 successive beams to acquire data
from a cube of range gate positions surrounding the nominal range
rate position in three-dimensional space. The implementation of
this scheme would require formation of three range gates for each
of nine (eight plus the nominal) beam steering angles.
[0042] More specifically, the cMUT patch probes can be used to
track the location of a fetal heart using a method comprising the
following steps: (a) scanning a volume inside the abdomen with
beams of pulsed ultrasonic wave energy; (b) acquiring acoustic data
by transducing ultrasound wave energy returned from the abdomen
after each transmission in step (a), the acoustic data being
acquired using range gates centered in different sample volumes;
(c) determining the signal-to-noise ratio of the acquired acoustic
data for each different range gate; and (d) determining which one
of the different range gates had the acoustic data which produced a
maximum signal-to-noise ratio. This determined range gate will then
be used to acquire additional acoustic data from which the fetal
heart rate will be calculated.
[0043] Typical frequencies for existing fetal heart rate monitoring
probes lie in the range of 1 to 2 MHz. In accordance with one
embodiment of the present invention, higher frequencies in the
range of about 3 to 5 MHz could be used.
[0044] In some known monitors, a speaker is provided and the probe
is manually manipulated until the operator hears the fetal heart.
The cMUT patch probes disclosed herein can used in conjunction with
an instrument operating in such an audio mode. This feature can be
utilized to locate the patch probes more accurately at the time of
their attachment to the patient's abdomen. For example, the
processor may be programmed with a mode whereby the cMUT patch
probe is controlled to transmit a series of beams of ultrasonic
wave energy in a direction normal to the face of the probe.
Acoustic data is acquired by transducing the ultrasound wave energy
returned from the abdomen of the patient after each beam
transmission. The acquired acoustic data is then converted into
audible signals using a speaker 38 (see FIG. 1) connected to the
processor 34. The clinician or attending nurse can then attach each
cMUT patch probe to the abdomen of the patient at a respective
location where the respective audible signals produced using each
probe sound like or mimic the fetal heart beat.
[0045] In accordance with a further embodiment of the invention,
multiple cMUT patch probes can be used to monitor more than one
fetus, e.g., twins, inside a patient. The system will monitor twins
by assigning a subset of the probes to one fetus and the rest of
the probes to the other fetus. If only two cMUT patch probes are
used, then each probe will be placed as nearly as possible in
positions overlying the respective fetal hearts. When two probes
have acquired the same fetal heart, their Doppler mean-frequency
traces will be in phase. In general, a second fetal heart will
result in a mean-frequency trace that is out of phase.
[0046] The disclosed embodiment will operate on complex baseband
signals, and will thus be able to track the fetal heart valve
motion through a complete cardiac cycle. For example, the DSP will
be programmed to compute the mean Doppler frequency on a continuous
(every pulse) basis, as is done in color M-mode and using the same
computations as those used by a color flow mapper (imaging system).
This will produce a simple signed function of time representing the
motion that is being sensed within the range gate.
[0047] As seen in FIG. 1, the instrument further comprises a
display screen 36 for displaying the fetal heart rate and an image
of a PW Doppler flow signal taken from the fetus. The spectrogram
is similar to that acquired from an adult heart, except that the
signal will evidence a higher heart rate. The PW Doppler
spectrogram shows frequency versus time, in which the mean
frequency and bandwidth of the signal vary in a periodic manner.
The DSP 34 is programmed to locate the fetal heart signal in space
by steering the successively transmitted beams until a Doppler
signal is acquired that is sufficiently close to being a match of
the "signature" signal. There is some ambiguity because the shape
of the spectral Doppler output will change with the angle at which
the beam intersects the heart, but the rate will always be higher
than that of the mother and accordingly it is believed that an
automated search would be feasible.
[0048] Because the cMUT patches will be adhered to the mother's
body, and because the fetal heart is tracked in three-dimensional
space as it moves relative to the cMUT patches, the fetal heart
rate should be continuously available without any operator
intervention. This arrangement has the following advantages. First,
operator intervention is eliminated for operation of the fetal
heart rate monitor. Second, the mother is allowed free motion while
being monitored. Third, the heart rate measurement should be more
reliable because the relatively smaller (as compared to current
systems) Doppler sample volume will allow a higher Doppler
signal-to-noise ratio in the measured ultrasound data. Although
similar capabilities will also be available with standard
(non-cMUT) two-dimensional arrays and real-time three-dimensional
imaging, the cost of these devices is likely to be so high as to
rule them out for this application. Additionally, such transducers
are too bulky and heavy to be made to adhere to the skin.
Furthermore, the use of cMUTs permits a relatively easy and
low-cost implementation of concepts such as the mosaic annular
array.
[0049] The use of ultrasound measurements in the continuous fetal
heart rate monitoring application is enabled by micromachined
ultrasonic transducer patch probe technology, which allows
ultrasound data to be taken using a thin, lightweight probe that
adheres to the patient's skin. Recently semiconductor processes
have been used to manufacture ultrasonic transducers of a type
known as micromachined ultrasonic transducers (MUTs), which may be
of the capacitive (cMUT) or piezoelectric (pMUT) variety. cMUTs are
tiny diaphragm-like devices with electrodes that convert the sound
vibration of a received ultrasound signal into a modulated
capacitance. For transmission the capacitive charge is modulated to
vibrate the diaphragm of the device and thereby transmit a sound
wave.
[0050] One advantage of MUTs is that they can be made using
semiconductor fabrication processes, such as microfabrication
processes grouped under the heading "micromachining". As explained
in U.S. Pat. No. 6,359,367: [0051] Micromachining is the formation
of microscopic structures using a combination or set of (A)
Patterning tools (generally lithography such as projection-aligners
or wafer-steppers), and (B) Deposition tools such as PVD (physical
vapor deposition), CVD (chemical vapor deposition), LPCVD
(low-pressure chemical vapor deposition), PECVD (plasma chemical
vapor deposition), and (C) Etching tools such as wet-chemical
etching, plasma-etching, ion-milling, sputter-etching or
laser-etching. Micromachining is typically performed on substrates
or wafers made of silicon, glass, sapphire or ceramic. Such
substrates or wafers are generally very flat and smooth and have
lateral dimensions in inches. They are usually processed as groups
in cassettes as they travel from process tool to process tool. Each
substrate can advantageously (but not necessarily) incorporate
numerous copies of the product. There are two generic types of
micromachining . . . 1) Bulk micromachining wherein the wafer or
substrate has large portions of its thickness sculptured, and 2)
Surface micromachining wherein the sculpturing is generally limited
to the surface, and particularly to thin deposited films on the
surface. The micromachining definition used herein includes the use
of conventional or known micromachinable materials including
silicon, sapphire, glass materials of all types, polymers (such as
polyimide), polysilicon, silicon nitride, silicon oxynitride, thin
film metals such as aluminum alloys, copper alloys and tungsten,
spin-on-glasses (SOGs), implantable or diffused dopants and grown
films such as silicon oxides and nitrides.
[0052] The same definition of micromachining is adopted herein. The
systems resulting from such micromachining processes are typically
referred to as "micromachined electromechanical systems"
(MEMS).
[0053] The use of a MUT patch allows the obstetrician to stick the
transducer to the mother's skin. The MUT patch is lightweight and
flat. In accordance with one embodiment of the invention, the cMUT
patch probes can be attached to the patient's skin with an
acoustically transparent layer of adhesive. For the purpose of
illustration, a transducer patch will be described that is made up
of capacitive micromachined ultrasonic transducers (cMUTs).
However, it should be understood that the patch could instead
employ pMUTs. The concept of the invention can be extended to cover
piezoceramics as well as piezoelectric materials. An embodiment
will now be described that incorporates a MUT patch. However, it
should be understood that the present invention encompasses not
only a device, but also methods for continuous non-invasive fetal
heart rate monitoring.
[0054] Referring to FIG. 2, a typical cMUT transducer cell 2 is
shown in cross section. An array of such cMUT transducer cells is
typically fabricated on a substrate 4, such as a heavily doped
silicon (hence, semiconductive) wafer. For each cMUT transducer
cell, a thin membrane or diaphragm 8, which may be made of silicon
nitride, is suspended above the substrate 4. The membrane 8 is
supported on its periphery by an insulating support 6, which may be
made of silicon oxide or silicon nitride. The cavity 16 between the
membrane 8 and the substrate 4 may be air- or gas-filled or wholly
or partially evacuated. Typically, cMUTs are evacuated as
completely as the processes allow. A film or layer of conductive
material, such as aluminum alloy or other suitable conductive
material, forms an electrode 12 on the membrane 8, and another film
or layer made of conductive material forms an electrode 10 on the
substrate 4. Alternatively, the bottom electrode can be formed by
appropriate doping of the semiconductive substrate 4.
[0055] The two electrodes 10 and 12, separated by the cavity 16,
form a capacitance. When an impinging acoustic signal causes the
membrane 8 to vibrate, the variation in the capacitance can be
detected using associated electronics (not shown in FIG. 2),
thereby transducing the acoustic signal into an electrical signal.
Conversely, an AC signal applied to one of the electrodes will
modulate the charge on the electrode, which in turn causes a
modulation in the capacitive force between the electrodes, the
latter causing the diaphragm to move and thereby transmit an
acoustic signal.
[0056] The individual cells can have round, rectangular, hexagonal,
or other peripheral shapes. A cMUT cell having a hexagonal shape is
shown in FIG. 3. Hexagonal shapes provide dense packing of the cMUT
cells of a transducer subelement. The cMUT cells can have different
dimensions so that the transducer subelement will have composite
characteristics of the different cell sizes, giving the transducer
a broadband characteristic.
[0057] Unfortunately, it is difficult to produce electronics that
would allow individual control over such small cells. While in
terms of the acoustical performance of the array as whole, the
small cell size is excellent and leads to great flexibility,
control is limited to larger structures. Grouping together multiple
cells and connecting them electrically allows one to create a
larger subelement, which can have the individual control while
maintaining the desired acoustical response. So a subelement is a
group of electrically connected cells that cannot be reconfigured.
For the purpose of this disclosure, the subelement is the smallest
independently controlled acoustical unit. One can form rings or
elements by connecting subelements together using a switching
network. The elements can be reconfigured by changing the state of
the switching network. However, individual subelements cannot be
reconfigured to form different subelements.
[0058] For the purpose of illustration, FIG. 4 shows a "daisy"
transducer subelement 24 made up of seven hexagonal cMUT cells 2: a
central cell surrounded by a ring of six cells, each cell in the
ring being contiguous with a respective side of the central cell
and the adjoining cells in the ring. The top electrodes 12 of each
cell 2 are electrically coupled together by connections that are
not switchably disconnectable. In the case of a hexagonal array,
six conductors 14 (shown in both FIGS. 3 and 4) radiate outward
from the top electrode 12 and are respectively connected to the top
electrodes of the neighboring cMUT cells (except in the case of
cells on the periphery, which connect to three, not six, other
cells). Similarly, the bottom electrodes 10 of each cell 2 are
electrically coupled together by connections that are not
switchably disconnectable, forming a seven-times-larger capacitive
transducer subelement 24.
[0059] Subelements of the type seen in FIG. 4 can be arranged to
form a two-dimensional array on a semiconductive (e.g., silicon)
substrate. These sub-elements can be reconfigured to form elements,
such as annular rings, using a switching network. FIG. 5 is a
drawing showing a mosaic array 40 comprising eight annular
elements. The drawing has been simplified by representing each
subelement as a square, the squares being aligned in mutually
orthogonal rows and columns. However, it should be understood that
the subelements could be of the type shown in FIG. 4, in which case
the subelements would be aligned along three axes separated by
60-degree angles.
[0060] Reconfigurability using silicon-based ultrasound transducer
sub-elements was described in U.S. patent application Ser. No.
10/383,990. One form of reconfigurability is the mosaic annular
array, also described in that patent application. The mosaic
annular array concept involves building annular elements by
grouping subelements together using a reconfigurable electronic
switching network. The goal is to reduce the number of beamforming
channels, while maintaining image quality and improving slice
thickness. To reduce system channels, the mosaic annular array
makes use of the fact that for an unsteered beam, the delay
contours on the surface of the underlying two-dimensional
transducer array are circular. In other words, the iso-delay curves
are annuli about the center of the beam. The circular symmetry of
the delays leads to the obvious grouping of those subelements with
common delays and thus the annular array is born. The
reconfigurability can be used to step the beam along the larger
underlying two-dimensional transducer array in order to form a scan
or image.
[0061] In accordance with one embodiment of the present invention
shown in FIG. 6, an array of cMUT subelements is built on one
silicon wafer and conventional complementary metal oxide
semiconductor (CMOS) switches and preamplifier/buffer circuits are
formed on a second silicon wafer to provide a cMUT patch having
reconfigurable beamforming elements. An acoustic backing layer 18
is preferably sandwiched between the cMUT wafer 2/4 and the CMOS
wafer 20 with vias 22 for passage of electrical connections between
the wafers.
[0062] The acoustic backing material 18 should have a composition
that is acoustically matched to the cMUT substrate 4, to prevent
reflection of the acoustic energy back into the device. In the case
where the substrate is made of silicon, one example of a suitable
backing material comprises a mixture of 96.3% (by mass) tungsten
(of which 85% was 10 micron and 15% was 1 micron particle size) and
3.67% polyvinyl chloride (PVC) powders, as disclosed in U.S. patent
application Ser. No. 10/248,022 entitled "Backing Material for
Micromachined Ultrasonic Transducer Devices". The person skilled in
the art will recognize that the composition of the acoustic backing
material can be varied from the example given above, However, the
acoustic impedance of the resulting backing material should be
matched to that of the substrate material. For example, if the
substrate is silicon, the acoustic impedance should be
approximately 19.8 MRayls.+-.5%.
[0063] The CMOS electronics preferably includes the transmit and
receive circuits (including a respective transmit/receive switch
for each cMUT subelement) and at least a portion of the beamforming
circuits. The CMOS electronics also include switches that enable
reconfiguration of the subelements, allowing an aperture to be
translated over the two-dimensional active area of the transducer.
The shape of the apertures is determined by the desired steering
angle for the ultrasound beam.
[0064] The beams can be translated across the cMUT patch by
translating the annular array of activated subelements (seen in
FIG. 5) across the patch as shown in FIG. 7. A uniform translation
of the beamforming coefficients produces a new beam at a different
location. Repeated frequently, this generates a rectilinear
two-dimensional image. An additional bilinear term in the
beamforming coefficients produces a beam directed away from the
normal, as seen in FIG. 8.
[0065] While the invention has been described with reference to
preferred embodiments, it will be understood by those skilled in
the art that various changes may be made and equivalents may be
substituted for elements thereof without departing from the scope
of the invention. In addition, many modifications may be made to
adapt a particular situation to the teachings of the invention
without departing from the essential scope thereof. Therefore it is
intended that the invention not be limited to the particular
embodiment disclosed as the best mode contemplated for carrying out
this invention, but that the invention will include all embodiments
falling within the scope of the appended claims.
[0066] As used in the claims, the term "data processing means"
means one or more digital signal processors, one or more
microprocessors, one or more computers, one or more computer or
processor chips, any combination thereof, and any functionally
equivalent circuitry, as well as any associated memory device or
memory chip for storing executable instructions or data. As used in
the claims, the term "different sample volumes" means sample
volumes that either overlap only partially or do not overlap (i.e.,
occupy the same space) at all.
[0067] While only certain features of the invention have been
illustrated and described herein, many modifications and changes
will occur to those skilled in the art. It is, therefore, to be
understood that the appended claims are intended to cover all such
modifications and changes as fall within the true spirit of the
invention.
* * * * *