U.S. patent application number 09/808833 was filed with the patent office on 2002-09-19 for imaging based on the electroacoustic effect.
Invention is credited to Diebold, Gerald J..
Application Number | 20020129655 09/808833 |
Document ID | / |
Family ID | 25199874 |
Filed Date | 2002-09-19 |
United States Patent
Application |
20020129655 |
Kind Code |
A1 |
Diebold, Gerald J. |
September 19, 2002 |
IMAGING BASED ON THE ELECTROACOUSTIC EFFECT
Abstract
The present invention discloses a method and device for imaging
based on the electroacoustic effect. The electroacoustic effect
takes place when an ultrasonic wave passes through an electrolyte
or colloidal suspension. The method and device of the present
invention produces images whereby a sound wave is generated at the
surface of an object, and, as the wave progresses through the body
a voltage is generated in time corresponding to the electroacoustic
response of the body at a point in space and time corresponding to
the position of the ultrasonic wave in the body. As pulses are
launched into the body at different points in space, the signal
sensed by an amplifier is used to generate an image.
Inventors: |
Diebold, Gerald J.;
(Barrington, RI) |
Correspondence
Address: |
BARLOW, JOSEPHS & HOLMES, LTD.
101 DYER STREET
5TH FLOOR
PROVIDENCE
RI
02903
US
|
Family ID: |
25199874 |
Appl. No.: |
09/808833 |
Filed: |
March 15, 2001 |
Current U.S.
Class: |
73/606 |
Current CPC
Class: |
G01N 2291/0222 20130101;
G01N 2291/02466 20130101; G01N 29/032 20130101; G01N 2291/02416
20130101 |
Class at
Publication: |
73/606 |
International
Class: |
G01N 009/24 |
Claims
What is claimed:
1. A method of creating an image of a body comprising: generating
an ultrasonic vibration; directing said ultrasonic vibration into a
body to be imaged, thereby generating an ultrasonic vibration
potential within said body; measuring said ultrasonic vibration
potential; and processing said electronic vibration potential
measurement to generate an image of said body.
2. The method of creating an image of a body in claim 1, wherein
said steps of generating an ultrasonic vibration and directing said
ultrasonic vibration further comprises, generating and directing
said ultrasonic vibration using an ultrasonic transducer
device.
3. The method of creating an image of a body in claim 1, wherein
said step of measuring said ultrasonic vibration potential
comprises, measuring the magnitude of said ultrasonic vibration
potential using electrodes placed adjacent to said body to be
imaged.
4. A method of creating an image of a body comprising: generating
an ultrasonic vibration; directing said ultrasonic vibration into a
body to be imaged at a controlled location, thereby generating an
ultrasonic vibration potential within said body; measuring the
magnitude of said ultrasonic vibration potential; repeating said
steps of generating said ultrasonic vibration, directing said
ultrasonic vibration and measuring said ultrasonic vibration
potential at a plurality of controlled locations, thereby
generating an ultrasonic vibration potential measurement for each
controlled location; and processing said electronic vibration
potential measurement from each of said controlled locations to
generate an image of said body.
5. An apparatus for creating an image of a body comprising: a
device for generating and directing an ultrasonic vibration into a
body to be imaged at a controlled location, thereby generating an
ultrasonic vibration potential within said body; a device for
measuring the magnitude of said ultrasonic vibration potential; and
a device for processing said magnitude measurement of said
vibration potential to generate an image of said body.
6. The apparatus for creating an image of a body of claim 5
wherein, said device for generating and directing an ultrasonic
vibration is an ultrasonic transducer.
7. The apparatus for creating an image of a body of claim 6
wherein, said ultrasonic transducer can be controllably directed to
a plurality of points within said body to be imaged.
8. The apparatus for creating an image of a body of claim 5
wherein, said device for measuring the magnitude of said ultrasonic
vibration potential comprises at least one electrode.
Description
BACKGROUND OF THE INVENTION
[0001] The field of the present invention relates to imaging of
bodies, examples being ultrasonic, nuclear magnetic resonance, and
x-ray imaging which are used commonly as diagnostic techniques for
medicine, non-destructive testing, and quality control in
industry.
[0002] In 1933, Debye predicted that passage of an ultrasonic wave
through a solution of electrolytes would result in the generation
of a voltage. The potential generated has become know as the
ultrasonic vibration potential. After experimental confirmation of
the effect, it was found that colloidal suspensions produced a
large signal. The effect is thus also known as the colloidal
vibration potential.
[0003] The principle of the effect, as well as experimental and
theoretical findings have been reviewed by Zana and Yeager, by
Povey in his text, and O'Brien, Cannon, and Rowlands. The theory of
the electroacoustic effect can be found in these reviews, the
references therein, and in the early paper by Hermans. O'Brien and
coworkers over the years have developed a detailed theory of the
effect; much of his theory can be found in the papers
disclosed.
[0004] The electroacoustic effect takes place when an ultrasonic
wave passes through a fluid containing electrolytes or colloids. In
the case of an electrolyte, the different inertias of the ionic
species in an electrolyte solution cause them to move to a greater
or lesser extent in response to the fluid motion that constitutes a
sound wave. The microscopic charge separation that follows from
their different dynamic response, when added over the interaction
region of the sound wave, results in the macroscopic voltage.
Further details can be found in the original paper of Debye.
Voltages are also produced when a sound wave passes through a
colloidal suspension. Consider the usual case of a colloidal
suspension of particles in a fluid where the particles have a
higher density than that of the surrounding fluid. Here, the higher
mass of a particle relative to that of the fluid volume it
displaces means that when an ultrasonic wave passes, the particle
motion does not exactly follow the fluid motion, but rather lags it
with both a smaller displacement in space and velocity. The
different motions of the fluid and particles are described by the
equations of fluid dynamics and follows as a result of the higher
inertia of a dense particle relative to that of an equivalent
volume of the fluid. Colloidal particles are charged bodies with a
so-called "zeta" potential, surrounded by a cloud of the opposite
charge. The solution thus has overall charge neutrality. The
presence of the ultrasonic wave gives rise to a charge separation
that arises from distortion of the charge cloud around the particle
when the particle fails to move in phase with the fluid. When an
ultrasonic wave passes, the fluid carries along the counter charge
but the particle and its charge remains more stationary in space.
The result of the different motions of the particle and surrounding
fluid is that a dipole is generated at the site of each particle,
which, when added over a half cycle of an acoustic wave (where the
velocity is unidirectional) adds to give a macroscopic voltage, the
frequency of the voltage being governed by the frequency of the
ultrasonic acoustic wave.
[0005] The magnitude of the vibration potential generated in a
colloidal suspension, as given by O'Brien, or as summarized in the
literature from Matec, Inc., is proportional to the density
difference between the particle and the fluid, the volume fraction
of the particles, the dynamic mobility, the inverse of the
conductivity, and the magnitude of the ultrasonic velocity. Zana
and Yeager give somewhat different expressions that involve
relaxational parameters, the thickness of the ionic atmosphere, the
number density of particles, the particle charge, the dielectric
constant, and solvation volume. The magnitude of the signal
produced as the ultrasonic wave traverses the body depends on the
above quantities; however, the exact details of the theory are not
important for the operation of the imaging device described here.
Note that in the case of particles with densities lower than that
of the fluid, the motion of the particle is opposite to that
described above. Again, a voltage is produced, but, for the same
relative charge of the particle and fluid, the opposite polarity
dipole and overall voltage are produced on each acoustic cycle.
[0006] Practical application of the ultrasonic vibration potential
to characterization of suspensions has been reported. Freeman
describes a device useful in industrial chemical processes for
detecting particles or the change in concentration of particles
during a chemical process based on the ultrasonic vibration
potential. Likewise, Oja, Petersen and Cannon describe a device for
characterizing the bulk properties of particulate suspensions using
the vibration potential. The Matec Inc. sales literature describes
a commercially available device for fluid characterization.
[0007] It is to be noted parenthetically that there is another
electroacoustic effect that is essentially the reverse of that
described above, whereby a voltage is applied to a fluid and an
ultrasonic wave is produced. The acoustic wave magnitude is
referred to as the "electrokinetic sonic amplitude", as described
in the brochure by Matec, Inc. cited above. It is possible to
determine colloidal properties of a bulk sample placed in a cell
with the device manufactures by Matec that measures the
characteristics of sound wave produced following application of a
voltage to the cell.
[0008] Description of the principle of imaging through the
ultrasonic vibration potential has been given by Diebold and
Beveridge. An image formed using the ultrasonic vibration potential
is thus a map of the response of electroacoustic signal in space,
which, in turn, is dependent on the quantities that appear in the
theories of the effect as given above. Mixtures of electrolytes and
colloidal particles would have a response that is a combination of
the responses of the electrolytes and colloidal particles. Whether
the body contains electrolytes or colloids alone, or has mixtures,
for a given ultrasonic wave of a given frequency and amplitude,
there is some voltage response to the wave in fluid. The voltage
produced has a magnitude that depends on the ultrasonic wave and
the fluid properties. Hereinafter the word colloid will be used for
colloids, electrolytes, or mixtures of electrolytes and
colloids.
[0009] The present document summarizes some of the theoretical
aspects of the problem given in the paper by Diebold and Beveridge,
and adds new ideas concerning principles and the operation of an
actual device. It is to be noted that although the invention uses
ultrasonic waves, it is fundamentally different than the well-known
method of ultrasonic imaging since the latter records a reflected
wave as the means of imaging while the invention described here
records a voltage produced by the object itself in response to the
ultrasonic waves.
[0010] It is therefore an object of the present invention to
provide a means for formation of an image of a body. Imaging is
carried out for the same diagnostic purposes as in x-ray imaging,
NMR imaging, ultrasound imaging, or photoacoustic imaging, namely,
for visualizing the inside of bodies. Such information is useful
for diagnostic purposes, as in medicine and nondestructive
testing.
[0011] It is also an object of the present invention to provide a
means of image formation based on a different principle, namely,
the electroacoustic effect, which will have properties unique to
the method, in particular contrast, based on a completely new
principle for imaging, namely, the ultrasonic vibration
potential.
SUMMARY OF THE INVENTION
[0012] Consider a one-dimensional response, which in the case of a
colloidal suspension, would be a concentration of the suspension
that varies as a function of distance from the launching point of
an ultrasonic wave. Take a 10 cm thick layer with the launching
point of the ultrasonic wave at x=0, and with non-colloidal "inert"
fluid extending throughout the 10 cm region except between points 3
and 5 cm from the origin where a colloidal suspension is found. The
region between 3 and 5 cm will be referred to as the active region.
Electrodes are placed at the points x=0 and x=10 cm where the
voltage is recorded. It is assumed that the body is a weak
conductor of electricity and that the voltage generated at its
endpoints can be sensed with a high input impedance amplifier. The
pulsed ultrasonic wave in this example is considered as having a
wavelength much smaller than 1 cm; say 0.1 mm, and a pulse width of
a duration that corresponds to 1 mm, so that about ten cycles are
in the pulse. On launching the ultrasonic wave no signal is
produced until the first half cycle of the wave reaches the 3 cm
point, enters the colloid, and produces a uni-polar voltage. When
the second half cycle of the wave reaches the interface, it begins
to generate a voltage of the opposite polarity, subtracting from
that generated by the first part of the wave. The process repeats
itself as subsequent cycles of the ultrasonic wave enter the active
region containing the colloid giving an alternating voltage at the
electrodes. Further, for the simple case presented here, it can be
seen that the output voltage is proportional to the integral of the
acoustic velocity in the pulse from the point x=3 to the point x=5.
When the pulse is totally inside the active region, the integral of
the pulse velocity is zero and no voltage is produced between the
points x=3 and 5, and of course, x=0 and 10 where the electrodes
are placed. Next, the pulse begins to exit the active region. As
the pulse reaches x=5 cm, the part of the pulse immediately
extending outside the active region gives no signal, but the part
remaining in the active region produces a voltage. The result is
that an overall voltage is again recorded with the electrodes. An
alternating voltage is produced again as the pulse moves in space
until all of the acoustic pulse exits the active region. When the
pulse is wholly in the region x>5, no voltage is produced. When
the magnitude of the ac voltage signal versus time data are plotted
as Voltage (magnitude) versus Z (distance), a plot such as shown in
FIG. 2 would be generated. The active region between Z=3 cm and Z=5
cm, where the vibration potential is generated, can be seen on the
plot as defined by the peaks at these two points.
[0013] It can be seen that a recording of the voltage gives a
profile of the colloid spatial distribution within the body, in
this case the size of the ac voltage indicating that the pulse is
entering or leaving the active region containing colloid. Knowledge
of the sound speed permits transformation of the voltage vs. time
profile into a colloid "response" vs. distance profile. Of course
response involves all of the factors given by theory. Such a
profile can locate objects within a body. When sharp boundaries are
not present between the colloid and its inert surroundings,
description of the voltage production must include the concept of a
"response gradient", which would describe the gradient of the
colloid concentration in space. The details of the voltage
generation process can be described as proportional to the integral
of the response gradient of the medium with the ultrasonic pulse.
Note that plots of signal vs. distance in the sample can be given
meaning through empirical methods by correlating the features of
plots with known features in control samples. A detailed knowledge
of the theory of signal production is not essential for obtaining
useful information.
[0014] Simple examples of the use of the device would be for
determining the location of blood or blood vessels within the human
body, or the presence of colloidal fluids in opaque, weakly
conducting objects. Chodorow has shown that blood can be detected
through use of the ultrasonic vibration potential. In the case of
blood veins, for instance, the voltage response of the
electroacoustic imaging device would be greater for the blood vein
than for muscle or fatty tissue since the blood contains both
electrolytes and cells that are colloidal in nature. Thus, a large
contrast in the image would be expected at the site of blood or
lymph in proximity to tissue with a low fluid content. A possible
application of the imaging method would be in detecting the
dimensions of arteries and veins, and the early detection of
tumors, which are known to have increased vascularization. With
regard to living tissue, the device would be to a certain extent a
blood detector. From the standpoint of nondestructive testing, the
presence of a fluid in a within solid or porous material would be
detectable since the solid cannot produce a substantial
electroacoustic effect, but the fluid can.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The novel features which are characteristic of the present
invention are set forth in the appended claims. However, the
invention's preferred embodiments, together with further objects
and attendant advantages, will be best understood by reference to
the following detailed description taken in connection with the
accompanying drawings in which:
[0016] FIG. 1 is a schematic diagram of the device of the present
invention;
[0017] FIG. 2 is a graph of the output generated by the device of
the present invention; and
[0018] FIG. 3 is a schematic diagram of an alternative embodiment
of the device of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0019] The present invention comprises an ultrasonic transducer
that is driven by a pulse generator, electrodes for picking up the
voltage generated by the ultrasonic vibration potential at the
surface of the body, a receiver that detects the minute voltage
produced, a means of moving the transducer along the surface of the
body, and a recording device. In the simplest version of the device
the voltage is displayed on an oscilloscope as signal vs. time,
which can be converted into signal vs. distance through knowledge
of the sound speed. In a second embodiment of the device, the
transducer is scanned in two dimensions along the surface of the
body as voltage vs. time data is collected. A device such as a
computer converts the signal at each point in the XY scan with the
Z amplitude dependence into a three-dimensional image.
[0020] Turning first to FIG. 1, the present invention is shown. A
voltage vs. time signal is generated by placing a transducer 1,
such as a conventional PZT (lead zirconate titanate) or PVDF
(polyvinylidene fluoride) transducer, in contact with the object 2
and exciting the transducer 1 with an electrical pulse by means of
a conventional ultrasonic pulse generator 3. The transducer 1 can
be of a geometry such that it focuses the ultrasonic radiation
inside the object 2 under investigation. The frequency at which the
transducer 1 is driven depends on the characteristics of the
colloid within the body 2 under investigation. Generally a
frequency is chosen so that the wave produces the largest signal
and can traverse the body 2 under investigation without severe
attenuation and at the same time gives the best resolution. Several
factors determine the optimal frequency, but frequencies in the
kilohertz or megahertz range are expected to be the most useful
since such frequencies are commonly used in generation of the
ultrasonic vibration potential.
[0021] An acoustic delay line 4 can be placed between the
transducer 1 and the body 2 that transmits the ultrasound to the
body 2 thus giving a time delay in the production of the signal.
Electrodes 5,6 are attached to the body 2 and the voltage generated
is sent to a sensitive receiver 7 operating at the same frequency
as the ultrasonic transducer 1. The electrodes 5,6 are required to
transmit the signal from the body 2 under study to wires or cables
that are fed to the input of the receiver 7. The electrodes 5,6 can
be conducting films, plates, or foils applied to the surface of the
body 2 whose functioning may be enhanced by the use of conducting
pastes or liquids. The signal generator 3 can produce a burst of ac
voltage, or it can produce a voltage spike. The function of the
electrical signal is that it drives the transducer 1 to give an
ultrasonic burst that is transmitted into the body. The received
signal from the burst would be an ac signal of the same frequency
of the pulse generator 3. The magnitude of the ac signal can be
detected giving the magnitude of the ac signal and passed on as its
output. The output of the receiver 7 recording the ac signal is fed
to an oscilloscope 8 or other equivalent device to produce a plot
of voltage vs. time, which corresponds to voltage versus distance
in the body. For purposes of viewing, the amplitude of the signal
would typically be "detected" giving a dc voltage proportional to
the magnitude of the ac signal. Through use of a computer 9 to
store the signal from the receiver 7, this device would produce a
one-dimensional picture of the body, that can be called an "alpha"
scan where signal vs. depth is determined. The depth can be
assigned the Z coordinate.
[0022] The signal-to-noise ratio in the signal from the receiver 7
can be improved by a standard processing method known as time, or
signal averaging, where signals from successive pulses are added in
the computer 9 at each point in time. The signal adds coherently
with that from previous pulses; the noise, on the other hand, adds
incoherently, averaging to zero in the long time limit. FIG. 2
shows what might be a typical signal from a hypothetical body with
symmetry on one dimension, as described in the first paragraph of
the Summary of the Invention.
[0023] Turning to FIG. 3, a second embodiment of the present
invention is shown, the transducer and electronics are the same,
but the transducer 101 is scanned in two other dimensions, X and Y,
to make a plot of the voltage vs. time at various positions of the
transducer 101 along the X and Y directions. Such a scan can be
referred to as a "beta" scan. The only modification of the
instrument in FIG. 1 is to add a two-dimensional scanning device
110 that moves the transducer 101 in two directions along the
surface of the body 102. Again, a data acquisition instrument such
as a computer 109 with appropriate inputs for taking data from both
the receiver 107 and scanner 110. The data acquisition device 109
is used to store data and to plot the signal magnitude as a
function of the coordinates. It is possible to carry out the same
scanning of the body 102 by using composite transducers that scan
the ultrasound inside the body using conventional phased array
techniques. The method of scanning the ultrasound is not important,
only that the ultrasonic radiation be moved in space across or
around the object to probe different parts of the body 102 while
the signal is acquired as a function of space inside the body 102.
The signal acquired by moving the ultrasound within the body 102
permits a three dimensional image of the voltage produced at
coordinates X, Y, and Z to be formed, recalling that the Z
coordinate is equivalent to the time after launching the pulse. In
one embodiment of the invention, color would indicate the magnitude
of the signal.
[0024] The exact method of scanning or geometry of scanning is
variable. The transducer 101 can be rotated around the body 102 and
signals acquired at different angles. Such rotation is facilitated
by the use of a coupling agent (not shown) that permits the
ultrasound to pass into the body 102 so that the transducer 101 is
not in direct contact with the body 102 of interest. Such signals
can be used to construct an image of the body 102 through computer
reconstruction algorithms, again using the position of the
transducer 101 and the signal versus time information.
[0025] Different frequencies can be used in forming the image so
that different images are acquired at different frequencies. Such
different images can be subtracted giving a contrast based on the
frequency dependence of the vibration potential. In practice,
driving PZT at overtones can give the higher frequencies so that
the transducer would not have to be changed in forming an image
based on an overtone of the original frequency.
[0026] It would be appreciated by those skilled in the art that
various changes and modifications can be made to the illustrated
embodiments without departing from the spirit of the present
invention. All such modifications and changes are intended to be
covered by the appended claims.
* * * * *