U.S. patent application number 13/614870 was filed with the patent office on 2013-04-18 for piezo micro-markers for ultrasound medical diagnostics.
The applicant listed for this patent is Bruce Towe. Invention is credited to Bruce Towe.
Application Number | 20130096435 13/614870 |
Document ID | / |
Family ID | 33490578 |
Filed Date | 2013-04-18 |
United States Patent
Application |
20130096435 |
Kind Code |
A1 |
Towe; Bruce |
April 18, 2013 |
PIEZO MICRO-MARKERS FOR ULTRASOUND MEDICAL DIAGNOSTICS
Abstract
An imaging system is disclosed that uses piezoelectric markers.
The piezoelectric fields in combination with ultrasound reflections
can be used to construct an image of an otherwise difficult to
detect feature within a subject's body. In one embodiment, the
invention includes a piezoelectric marker, including at least one
piece of piezoelectric material, an ultrasound transducer connected
to an ultrasound pulser and a receiver, a computer sequencing
control connected to the receiver and the ultrasound pulser, a
display connected to the computer sequencing control and electrodes
connected to the computer sequencing control via amplification
circuitry.
Inventors: |
Towe; Bruce; (Mesa,
AZ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Towe; Bruce |
Mesa |
AZ |
US |
|
|
Family ID: |
33490578 |
Appl. No.: |
13/614870 |
Filed: |
September 13, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10557362 |
Dec 12, 2006 |
8282561 |
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PCT/US2004/016417 |
May 24, 2004 |
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13614870 |
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60473242 |
May 23, 2003 |
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Current U.S.
Class: |
600/458 |
Current CPC
Class: |
A61B 8/481 20130101;
A61B 8/0833 20130101 |
Class at
Publication: |
600/458 |
International
Class: |
A61B 8/08 20060101
A61B008/08 |
Claims
1. An imaging system comprising: a piezoelectric marker embedded in
a subject's body, wherein the piezoelectric marker comprises at
least one piece of piezoelectric material; an ultrasound transducer
configured to excite the piezoelectric marker with an ultrasound
signal; and electrodes configured to detect an electric field
generated by the piezoelectric marker.
2. The imaging system of claim 1, wherein the piezoelectric marker
comprises a material selected from the group consisting of PVDF,
PVDF-TRFE, PZT, lithium niobate, quartz, lead metaniobate, lead
titanate, and tourmaline.
3. The imaging system of claim 1, wherein the piezoelectric marker
comprises a plurality of pieces of piezoelectric material connected
in electrical series.
4. The imaging system of claim 1, wherein the piezoelectric
material in the piezoelectric marker is coated with a biocompatible
layer.
5. The imaging system of claim 4, where the biocompatible layer has
an acoustic impedance that is less than the acoustic impedance of
the piezoelectric marker.
6. The imaging system of claim 4, where the thickness of the
biocompatible layer is equal to about one quarter of a wavelength
of the ultrasound signal.
7. The imaging system of claim 1, where the piezoelectric material
has a thickness equal to about half of a wavelength of the
ultrasound signal.
8. The imaging system of claim 1, where the piezoelectric material
is curved.
9. The imaging system of claim 8, where the piezoelectric material
is curved with a radius of curvature that is larger than one half
wavelength of the ultrasound signal.
10. A method of detecting an object embedded in a subject's body,
comprising: exciting a piezoelectric marker embedded in a subject's
body with an ultrasound signal; generating electric fields in the
piezoelectric marker in response to the ultrasound signal; and
detecting the electric fields using electrodes.
11. The method of claim 10, further comprising detecting an
ultrasound signal reflected by the piezoelectric marker.
12. The method of claim 11, wherein the electric fields are
detected before the reflected ultrasound.
13. The method of claim 10, wherein the piezoelectric marker
comprises a material selected from the group consisting of PVDF,
PVDF-TRFE, PZT, lithium niobate, quartz, lead metaniobate, lead
titanate, and tourmaline.
14. The method of claim 10, wherein the piezoelectric marker
comprises a plurality of pieces of piezoelectric material connected
in electrical series.
15. The method of claim 10, wherein the piezoelectric marker is
coated with a biocompatible layer.
16. The method of claim 13, where the biocompatible layer has an
acoustic impedance that is less than the acoustic impedance of the
piezoelectric marker.
17. The method of claim 15, where the thickness of the
biocompatible layer is equal to about one quarter of a wavelength
of the ultrasound signal.
18. The method of claim 10, where the piezoelectric material has a
thickness equal to about half of a wavelength of the ultrasound
signal.
Description
PRIORITY CLAIM
[0001] This is a continuation of U.S. patent application Ser. No.
10/557,362 filed Nov. 18, 2005, claiming priority to PCT Patent
Application PCT/USO4/16417, filed May 24, 2004 and U.S. Provisional
Patent Application Ser. No. 60/473,242 filed May 23, 2003. The
above-referenced disclosures are incorporated herein by reference
in their entirety.
BACKGROUND
[0002] The present invention relates generally to medical imaging
and more specifically to the imaging of foreign objects such as
medical devices that are inserted into the body of a subject.
[0003] Image contrast in conventional medical ultrasound results
from differences in tissue acoustic properties. Small medical
devices made from plastics or polymers are often not easily seen in
ultrasound images because their acoustic properties are similar to
those of the surrounding tissue. Metal objects such as biopsy
needles can also be troublesome to image because they specularly
reflect ultrasound. An alternative to ultrasound imaging is X-ray
radiography. X-ray radiography is routinely used to position
catheters or locate implanted markers, but involves radiation that
can be ionizing.
SUMMARY OF THE INVENTION
[0004] Embodiments of the present invention use piezoelectric
materials to enable the imaging of foreign objects in the body of a
subject using ultrasound. One aspect of the invention includes a
piezoelectric marker, including at least one piece of piezoelectric
material, an ultrasound transducer connected to an ultrasound
pulser and a receiver, a computer sequencing control connected to
the receiver and the ultrasound pulser, a display connected to the
computer sequencing control and electrodes connected to the
computer sequencing control via amplification circuitry.
[0005] In another embodiment, the piezoelectric marker is
constructed from at least one piece of PVDF, at least one piece of
PZT or PVDF-TRFE. In a further embodiment, the piezoelectric marker
is constructed from multiple pieces of piezoelectric material
arranged such that adjacent pieces have alternating polarities. In
yet another embodiment, the piezoelectric material used in the
construction of the piezoelectric marker is coated with a layer of
material having an acoustic impedance that is less than the
acoustic impedance of the piezoelectric marker.
[0006] In a still further embodiment, the computer sequencing
control, the ultrasound pulser, the receiver and the display are
implemented using a conventional ultrasound diagnostic machine.
[0007] One aspect of the method of the invention includes
illuminating the object with ultrasound and forming an image using
information collected from reflect ultrasound and information
collected concerning electric fields. In a further embodiment of
the method, the information collected concerning electric fields is
delayed relative to the information collected from reflected
ultrasound when forming an image. In yet another aspect of the
method of the invention, the object is illuminated using pulses of
ultrasound and the delay is equal to twice the time between the
generation of the most recent ultrasound pulse and the time at
which the electric field is observed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is a schematic view of one embodiment of an imaging
system in accordance with the present invention;
[0009] FIGS. 2A-2F are embodiments of piezoelectric markers in
accordance with the present invention;
[0010] FIGS. 3A and 3B are graphs showing the electric field
generated by piezoelectric markers in response to excitation by
ultrasound;
[0011] FIG. 4 is a graph showing the magnitude of an electrical
waveform generated by a piezoelectric marker that is excited by an
ultrasound wave in accordance with one embodiment of the present
invention and the variation of this magnitude with the conductivity
of the medium surrounding the piezoelectric marker;
[0012] FIG. 5 is a schematic diagram showing an embodiment of an
imaging system in accordance with the present invention;
[0013] FIG. 6A is a reproduction of the output image of a
conventional ultrasound imaging device that is imaging a volume
containing a piezoelectric marker in accordance with an embodiment
of the present invention;
[0014] FIG. 6B is a reproduction of the output image generated by
an imaging system in accordance with the present invention that is
imaging a volume containing a piezoelectric marker in accordance
with an embodiment of the present invention; and
[0015] FIG. 7 is a schematic view of a medical device including a
piezoelectric marker in accordance with one embodiment of the
present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0016] Turning now to the drawings, embodiments of the present
invention include piezoelectric markers that generate electric
fields in response to excitation by ultrasound pressure waves. The
generated electric fields can be detected using electrodes to
provide positional information. In several embodiments, the
positional information can be combined with information from
ultrasound reflections to provide an ultrasound image of a
subject's body that includes the piezoelectric markers, which would
otherwise be difficult to observe.
[0017] An embodiment of an imaging system in accordance with the
present invention is illustrated in FIG. 1. The imaging system 10
includes at least one piezoelectric marker 12 embedded inside a
subject's body 14. An ultrasound transducer array 16 is positioned
external to the subject's body to direct ultrasound pressure waves
into the subject's body and electrodes 18 are attached to the
surface 19 of the subject's body. The ultrasound transducer array
is connected to an ultrasound pulser 20 and to a receiver 22. Both
the ultrasound transducer and the receiver are connected to a
computer 24 that is connected to a display 26. The electrodes can
be connected to a preamplifier 28, which is connected to
amplification and filtration circuitry 30.
[0018] In one embodiment, the ultrasound transducer array generates
pressure waves that are incident on the piezoelectric marker. The
piezoelectric marker is constructed from piezoelectric materials
that generate an electric field in response to excitation by the
ultrasound pressure waves. The electric fields generated by the
piezoelectric markers can then be detected using the electrodes.
The pressure waves can be generated as brief pulses and the
distance of the piezoelectric marker from the ultrasound transducer
can be estimated by timing the delay between the generation of a
pulse and the detection of an electric field by the electrodes.
[0019] The generation of ultrasound pulses by the ultrasound
transducer array can be achieved by the computer prompting the
ultrasound pulser to output a signal capable of driving the
ultrasound transducer array. The generation of an ultrasound image
can be achieved using the receiver and the computer to construct an
image using reflected ultrasound detected using the ultrasound
transducer. An image of the piezoelectric marker can then be
superimposed onto the ultrasound image by interpreting signals
generated by the electrodes. The computer can extract portions of
the electrode signal that are indicative of the electric fields
generated by the piezoelectric markers in response to excitation by
pressure waves. The signals generated by the electrodes are small
and can require pre-amplification prior to amplification and
filtering. By timing the delay between the generation of an
ultrasound pulse and the detection of a signal at the electrodes,
the computer can calculate the distance of the piezoelectric marker
from the ultrasound transducer. Alternatively, the computer can
superimpose the signal from the electrodes over the ultrasound
image by doubling the delay experienced by the electrical signals
received by the electrodes to account for the difference in the
speed at which electrical signals and acoustic waves propagate
through the human body.
[0020] Various constructions of piezoelectric markers can be used
in accordance with the present invention and the particular
construction can depend upon the material that is used in the
construction. A piece of piezomaterial alone may generate a
sufficient electric field to be detected by electrodes on the
surface of the body and hence act as a piezoelectric marker in
accordance with the present invention. Alternatively, coatings may
be required to increase the amount of acoustic energy converted
into charge by the piezoelectric material used in the marker. In
addition, electrodes on the marker may be useful in increasing the
strength of the electric field generated by the piezoelectric
marker. Embodiments of piezoelectric markers in accordance with the
present invention are illustrated in FIGS. 2A-2F.
[0021] A piezoelectric marker 12' in accordance with the present
invention that includes a piece of piezoelectric material 40 is
illustrated in FIG. 2A. In several embodiments, the dimensions of
the piezoelectric material are chosen to generate an electric field
exceeding a predetermined threshold in response to excitation by a
known intensity of ultrasound. Factors that can impact the
generated electric field include the length, thickness and
curvature (if any) of the piece of piezoelectric material. The
factors that impact the choice of the dimensions of pieces of
piezoelectric material that are used in the construction of
piezoelectric markers are discussed in detail below.
[0022] A piezoelectric marker 12'' in accordance with the present
invention that includes a piece of piezoelectric material 40' and
two electrode contacts 42 is illustrated in FIG. 2B. In one
embodiment, the piece of piezoelectric material can be rectangular.
In other embodiments, the piece of piezoelectric material can be
curved and in one embodiment is curved with a radius of curvature
that is larger than one half wavelength of the applied ultrasound.
Curved surfaces can increase the ability of a piezoelectric
material to generate electric fields from ultrasound from a broader
field of view of the transducer array.
[0023] Another piezoelectric marker in accordance with the present
invention is illustrated in FIG. 2C. The piezoelectric marker 12'''
includes a piece of piezoelectric material 40'' that is surrounded
by an electrically conductive material 44. In one embodiment, the
piezoelectric material is rectangular and has a thickness d equal
to half the wavelength of the ultrasound used to excite the
piezoelectric marker. The layer of conducting material surrounding
the piezoelectric marker has a thickness p equal to one quarter of
the wavelength of the ultrasound used to excite the piezoelectric
marker. These dimensions increase the ultrasound that is converted
into electric charge by the piezoelectric material. The conductive
layer can be constructed from any biocompatible electrically
conductive material and is ideally chosen to match the acoustic
impedance of the piezoelectric material with acoustic impedance of
the tissue surrounding the piezoelectric marker (see discussion
below).
[0024] A further embodiment of a piezoelectric marker in accordance
with the present invention is illustrated in FIG. 2D. The
piezoelectric marker 12''' includes a piece of piezoelectric
material 40''' and an electrode 46 that are embedded in an
electrically conductive material 48 that has similar properties to
the electrically conductive material 40 used in the construction of
the piezoelectric marker 12''' shown in FIG. 2C. In one embodiment,
the piece of piezoelectric material has dimensions that are smaller
than one half the wavelength of the ultrasound used to excite the
piezoelectric marker. The spacing of the piezoelectric material and
the electrode using the electrically conductive material can
influence the dipole moment of the marker, with generally greater
detectable signals from the marker with greater spacing.
[0025] An additional embodiment of a piezoelectric marker in
accordance with the present invention that includes multiple pieces
of piezoelectric material that are aligned with alternating
polarities is illustrated in FIG. 2E. The pieces of piezoelectric
material 50 are arranged adjacent each other with alternating
polarities such that the pieces of piezoelectric material appear as
individual generators sensitive to ultrasound impinging from many
directions. The pieces of piezoelectric material are surrounded by
a layer of material 52 that is not electrically conductive
material.
[0026] An embodiment of a piezoelectric marker similar to the
piezoelectric marker illustrated in FIG. 2E except that the pieces
of piezoelectric material are connected in electrical series is
shown in FIG. 2F. The pieces of piezoelectric material 50' are
connected by strips of electrically conductive material 54 and
electrodes 56 are provided adjacent the outermost pieces of
piezoelectric material in the array. The array of piezoelectric
material and the strips of electrically conductive materials are
surrounded by a material 52' similar to the materials shown as 52
in FIG. 2E. The arrangement illustrated in FIG. 2F can enable the
pieces of piezoelectric material to produce a larger aggregate
signal than would be obtained without electrical connections
between the pieces of piezoelectric material.
[0027] As mentioned above, any variety of structures can be used to
construct piezoelectric markers. The following discussion
introduces factors that can impact the electric field generated by
a piezoelectric marker in response to incident ultrasound. An
appreciation of these factors can, therefore, enable the design of
any number of structures that are capable of generating an electric
field in response to a given ultrasound signal that is capable of
detection at the surface of a subject's body.
[0028] In the embodiments of piezoelectric markers described above
piezoelectric materials are used to generate electric fields from
ultrasound pressure waves. Piezoelectric materials are polarized
electrically-attractive materials that generate displacement
currents when pressure is applied to their surface. This class of
materials includes polymers like polyvinylidene fluoride (PVDF),
and ceramics like lead zirconate titanate (PZT). Examples of other
piezoelectric materials that could be used include any
piezoceramic, polyvinylidene fluoride-trifuoro ethylene
(PVDF-TRFE), Lithium Niobate, quartz, Lead Metaniobate, Lead
Titanate, Tourmaline or any other material that will generate a
potential when excited by a pressure wave such as
electron-bombarded plastics.
[0029] As already identified above, the dimensions of any pieces of
piezoelectric material used in the construction of a piezoelectric
marker can influence the electric field generated by the
piezoelectric marker. The dimensions of a piezomaterial required
for its electrical detection at a given depth is clearly a tradeoff
in terms of image resolution and signal to noise ratio. Larger
chips create stronger electrical signals and can be seen at greater
depth. The ratio of the wavelength of ultrasound incident on the
material to the thickness of the piezoelectric material can be
particularly important. For example, a thickness equal to half the
wavelength of the ultrasound pressure waves would tend to increase
the power transfer to the piezoelectric material and increase its
voltage output.
[0030] An additional factor to consider when dimensioning a
piezoelectric material is that the frequency of the evoked
electrical responses from piezoelectric markers generally follows
the ultrasound acoustic frequency, however, under ultrasound pulse
exposure high-Q piezoceramics like PZT will also mechanically ring
at their natural resonant frequency in a way that is largely
determined by their thickness. Therefore, resonance can be used to
increase the magnitude of the electric field generated. In
addition, different resonant frequencies can be used to
individually identify different piezoelectric markers, marker
responding more strongly to a specific ultrasound frequency or
producing an electrical frequency characteristic of its natural
resonance.
[0031] Another important property in selecting a piezoelectric
material for incorporation in a piezoelectric marker in accordance
with the present invention is the acoustic impedance of the
piezoelectric material relative to the tissue in which the marker
is embedded. The relative acoustic impedance determines how much of
the ultrasound energy is used to generate charge. The remainder of
the energy is reflected at the interface between the sound
transport medium and the piezoelectric material. In tissue,
calculations show that approximately 10% of ultrasound energy is
utilized by PZT while about 89% is used by PVDF, because PVDF has
an acoustic impedance much closer to that of tissue. As can be seen
from the embodiments illustrated in FIGS. 2C, the amount of energy
utilized by a piezoelectric material can be increased by coating
the piezoelectric material in a layer of material that has an
acoustic impedance that more closely matches the acoustic impedance
of the tissue. In one embodiment, the layer of material is chosen
to have an acoustic impedance in accordance with the formula (where
Z is the acoustic impedance):
Z.sub.matcg= {square root over (Z.sub.tissueZ.sub.transducer)}
[0032] In another embodiment, the layer has a thickness of one
quarter the wavelength of the ultrasound incident on the
piezoelectric marker. Use of a layer having a thickness of one
quarter the wavelength of the incident ultrasound can increase the
amount of ultrasound energy that is utilized by the piezoelectric
material.
[0033] Another factor that can influence electrical power transfer
is the electrical port impedance of the materials used in the
construction of the piezoelectric marker and how they match to the
electrical impedance of the tissue at the ultrasound frequency.
This characteristic is optimized through selection of the size and
composition of the piezoelectric material. In general small chips
of piezoelectric materials will be assembled in electrical series
to provide an enhanced voltage output in response to an ultrasound
wave.
[0034] When designing an imaging system in accordance with the
present invention, regard should be had to the fact that
piezoelectric ceramics and polymers produce about 5-20 mV across
their thickness when illuminated by 2.5-7.5 MHz ultrasound at 10
mW/cm2 average energy. If these markers are placed in tissue,
several tens to hundreds of microvolts will appear on the skin
surface in response to the above ultrasound pressure waves.
Ultrasound evoked electrical waveforms from pieces of PVDF and PZT
are illustrated in FIGS. 3A and 3B. The graph 60 shown in FIG. 3A
includes a plot 62 indicative of an electric field during a period
of time in which two ultrasound pulses are incident on a piece of
PVDF (one directly from the transducer and the other a reflection
from the back surface of the test tank). The graph shows large
narrow peaks 64 in the detected electric field. The characteristics
of these peaks enable accurate measurements of the distance of the
piezoelectric material from the ultrasound transducer. By contrast,
the signal 66 shown in FIG. 3B shows the peaks in the plot 68 of
the electric field measured when a piece of PZT is excited by
similar ultrasound pulses are not nearly as prominent or distinct.
Despite the greater magnitude of the electrical response of PVDF,
either material may be suitable for use in an embodiment of a
piezoelectric marker in accordance with the present invention.
Choice of material will largely depend on the magnitude of the
electric field that is required to be generated in a particular
application and the accuracy required by the application.
[0035] A factor that can impact the magnitude of the electric field
required to locate a piezoelectric marker in accordance with the
present invention is the conductivity of the material in which the
marker is surrounded. A graph 70 is shown in FIG. 4 that includes a
plot 72 showing the variation on electric field strength with
increased conductivity. As conductivity is decreased, the plot
shows that the electric field increases asymptotically.
[0036] A number of approaches can be taken to constructing an
imaging system in accordance with the present invention that is
capable of displaying the location of piezoelectric markers. One
approach is to construct a custom imaging system in accordance with
the schematic diagram shown in FIG. 1. Alternatively, commercial
ultrasound diagnostic systems have within them circuitry to
sensitively detect, process, and display low-level high frequency
electrical signals such as those detected by ultrasound
echo-receive transducers. If the detected electrical signals
produced by the interaction of sound energy and the piezoelectric
material are introduced into the ultrasound signal-processing path,
then the imaging system can interpret the signals as acoustic
echoes superimposing them on an image generated using actual
ultrasound reflections.
[0037] An embodiment of an imaging system in accordance with the
present invention that is implemented using a commercial ultrasound
diagnostic system is shown in FIG. 5. The imaging system 10'
includes a commercial ultrasound diagnostic system 80 that is
connected to an ultrasound transducer array 82. The ultrasound
transducer 82 is directed toward piezoelectric markers 12 embedded
within a subject's body 14'. Electrodes 18' are placed in contact
with the subject's body and are connected to an amplifier 84, which
is in turn connected to the commercial ultrasound diagnostic
machine via a device 86 capable of introducing a delay between the
detection of a signal at the electrode and the provision of the
signal to the ultrasound diagnostic system.
[0038] Due to the fact that ultrasound reflections propogate more
slowly than electric fields, a device 86 is required to introduce a
delay into the output of the electrodes. Otherwise, the
piezoelectric markers would appear in a location that is
approximately half the actual distance of the piezoelectric marker
to the ultrasound transducer array. The delay can be created by
detecting the generation of the acoustic wave by the ultrasound
transducer and then sampling the output of the electrodes using a
microcontroller with an analog to digital converter. An output can
be generated by the microcontroller and the digital to analog
converter and coupled into the input of the ultrasound diagnostic
system using inductive coupling. The output provided to the
ultrasound diagnostic system is the sampled input delayed by an
amount sufficient to cause the signal to be provided to the
ultrasound diagnostic system at a time after the generation of the
ultrasound pulse equal to twice the time between the generation of
ultrasound pulse and the time at which the sample of the electrodes
was taken. In other embodiments, other components such as discrete
components or application specific circuits can be used to achieve
delays in either the analog or digital domain.
[0039] The delayed output of the electrodes can be provided to a
commercial ultrasound diagnostic system, where the electrode signal
is treated as if it were an ultrasound reflection. When
electrode-detected potentials are added into the signal path of an
ultrasound imaging system, the machine can simultaneously form an
electrical image of the embedded piezoelectric material along with
a conventional acoustic-echo image of the tissue. Contrast for the
piezoelectric marker can appear arbitrarily high in the tissue
image or displayed in a different color or set to blink as an
eye-catching part of the normal diagnostic acoustic image. This is
because the electronic signal-processing pathway for the
piezoelectric information is mostly separated from that of the
acoustic.
[0040] In some embodiments, the electrical signal generated by a
piezoelectric marker is dependent on the instantaneous position of
the ultrasound scanning beam such that the displayed brightness on
a similarly scanning beam on a cathode ray tube used in a display
in accordance with an embodiment of the present invention is
modulated by the detected electrical signal intensity from the
medium. The result of this process is a map of the scanned field
showing in brightness display regions of evoked electrical response
where the ultrasound scanning beam intersects the piezoelectric
marker. This process of creating an image shows the electrical
image as having characteristics that are substantially different
compared to the portion of the image that is generated from
ultrasound acoustic echoes.
[0041] Another approach that can be used to delay the signal
provided to an ultrasound diagnostic system is to apply the
piezoelectric signal to a separate input in video display buffer
and causing it to time scale the piezoelectric signal to match that
of the acoustic signal. One of ordinary skill in the art will
appreciate that each commercial ultrasound diagnostic system is
likely to require a different approach to achieving the required
delay.
[0042] In one embodiment, the commercial ultrasound diagnostic
system is a 3.5 MHz RT-3000 manufactured by GE Healthcare of
Chalfont St. Giles, United Kingdom. In other embodiments, any
commercial diagnostic system possessing a first output capable of
driving an ultrasound transducer, a first input capable of
receiving a signal generated by an ultrasound transducer indicative
of reflected ultrasound and a second input capable of receiving a
signal having characteristics similar to a signal generated by an
ultrasound transducer indicative of reflected ultrasound. In other
embodiments, the first output and the first input can be
implemented using a single physical connection. In addition, the
second input signal can be coupled with the first input signal and
the ultrasound diagnostic system need only have a single input
capable of receiving signals with characteristics similar to
signals generated by an ultrasound transducer in response to
reflected ultrasound pressure waves. Examples of other suitable
systems include a Picker Echoview-80 ultrasound machine
manufactured by Picker International Inc. of Cleveland, Ohio with a
2.25 MHz 8 mm l.f. unit used in A-mode operation. In other
embodiments, almost any commercial ultrasound diagnostic system can
be adapted to image piezoelectric markers in accordance with
practice of the present invention.
[0043] In one embodiment, the electrodes can be implemented using
silver chloride electrodes although in other embodiments other
biocompatible electrodes can be used. In one embodiment the
pre-amplifiers can be implemented using low noise wide bandwidth
amplifiers having a gain of at least.times.1000 and the delay
circuitry can be implemented using digital delay lines.
[0044] The ability of imaging systems in accordance with the
present invention to locate and display images of piezoelectric
markers can be impacted by the configuration of the electrodes used
to detect electric fields generated by piezoelectric markers in
response to excitation by ultrasound pressure waves. Electrical
waves at megahertz frequencies are relatively long in wavelength
compared to body dimensions. This means that the marker can be
considered as a near field electrical source coupled by a complex
impedance to electrodes. The piezoelectric material can be modeled
as an oscillating dipolar current source in a volume conductor with
the ultrasound impacting a small square chip of piezoelectric
material in the direction of its thickness polarization. Induced
displacement currents are flowing around the edges of the chip. The
electrical field lines from the currents extend into an
isotropically conducting medium and ultimately appear at the
surface boundary. An electrode placed on the surface measures a
potential with respect to another remote electrode.
[0045] The amplitude of the potentials detected from imbedded
piezoelectric markers can vary as a function of the orientation and
distance between the markers and the electrodes. With certain
positions of the pickup electrodes relative to the thickness of the
piezoelectric marker, the electrodes cannot detect signals
generated by the piezoelectric markers. In several embodiments,
multiple sets of orthonormal electrodes are placed on the surface
of the skin to enable detection of signals generated by the
piezoelectric markers. In other embodiments, piezoelectric markers
can be used that are polarized in multiple directions.
[0046] In several embodiments, the electrodes can be positioned on
the subject's body next to the ultrasound transducer scanning-head.
In other embodiments, the scanning head and the electrodes are
integrated into a single unit. In other embodiments, other
locations for the electrodes on the body can be used.
[0047] In embodiments where more than two electrodes are utilized,
differences in the strength or other characteristics of the signals
generated by different electrodes can be interpreted to locate the
markers relative to each of the individual electrodes. In several
embodiments, electrode positioning and signal processing can be
used to locate piezoelectric markers in three dimensions. In one
embodiment, three pairs of electrodes are placed on the body
surface in pairs frontally, saggitally and coronally in a way
similar to the well-known placement called the Frank lead system
used for the clinical vectorcardiogram. The electrical signal from
these pairs would be combined by electrical analog or digital
vector addition as known to those in the art, prior to being
introduced into the ultrasound imaging circuitry. The advantage of
using multiple pairs of electrodes positioned according is that the
resulting piezoelectrical signal can have a constant amplitude
regardless of the orientation of the marker within the body.
[0048] A display 90 generated by a commercial ultrasound diagnostic
system when ultrasound pulses are directed towards a piezoelectric
marker in accordance with the present invention is shown in FIG.
6A. This display can be compared with the display 92 shown in FIG.
6B that was generated in accordance with the present invention by
combining information from electrodes with information from the
ultrasound transducer. The conventional display 90 does not show
the piezoelectric marker 94 as clearly as the display generated in
accordance with the present invention.
[0049] In one embodiment, piezoelectric markers can be used as
positional aides for body alignment during medical procedures. In
other embodiments, piezoelectric markers can be included in medical
devices to assist in locating or positioning medical devices within
the body. Such markers can be used as fiducial markers that show up
under ultrasound imaging for stereotactic positioning the head and
other parts of the body in MR, CT and PET imaging systems. Markers
can also be attached to various and multiple parts of body organs
such as the heart to allow visualization of the cardiac motion of
the chambers and to assess cardiac performance.
[0050] In other embodiments, the markers can provide enhanced
operation of the ultrasound diagnostic modes known as M-mode and
Doppler modes. In m-mode, the electrical signals from a moving
piezoelectriccal marker, attached to the heart, for example, will
clearly identify iti in the conventional M-mode operating mode in a
way essentially the same as in conventional M-mode imaging except
that the marker's location will appear bright and in high
contrast.
[0051] Likewise, switching an ultrasound imaging system modified in
accordance with the present invention to Doppler mode can cause the
marker to appear and its velocity of motion to appear on the
display as a marker. This provides a convenient method for
identifying and labeling the piezoelectric markers specific
structures that are in motion. In this operating modality, the
apparent Doppler shifted frequency will be one half of the
comparable values from that of moving tissue acoustic echoes and
so, as in the case of imaging, the Doppler shift in the electrical
channel will be doubled to be comparable to the Doppler shift of
ultrasound reflections caused by moving tissue.
[0052] An embodiment of a biopsy needle in accordance with the
present invention is illustrated in FIG. 7. The biopsy needle 100
is constructed so that a portion of the needle acts as a
piezoelectric marker 102. In one embodiment, the piezoelectric
marker is constructed from PVDF that is polarized radially as it is
extruded to enable detection from numerous orientations of the
electrodes relative to the position of the biopsy needle.
[0053] While the above description contains many specific
embodiments of the invention, these should not be construed as
limitations on the scope of the invention, but rather as an example
of one embodiment thereof Many other variations are possible.
Accordingly, the scope of the invention should be determined not by
the embodiments illustrated, but by the appended claims and their
equivalents.
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