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Claims  |
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What is claimed and desired to be secured by U.S. Letters Patent is as
follows:
1. A method of producing an image of an object from acoustic energy that
has been transmitted through and scattered by the object, said image
comprising a high and acoustic absorption at all points within the object,
and said method comprising the steps of:
electronically transmitting an electric signal at multiple frequencies and
transducing said electric signal at each said frequency into acoustic
energy propagated toward said object from a plurality of transducer
transmitter positions;
electronically processing said electrical signal to determine from said
transmitter positions an incident field corresponding to said propagated
acoustic energy, said incident field being stored in the memory of a
central processing unit (CPU) in the form of digitized electric signals;
detecting at a plurality of transducer receiver positions said acoustic
energy transmitted through and scattered by said object;
electronically processing said detected acoustic energy so as to transform
said detected energy into a plurality of digitized electric signals stored
in said memory of said CPU and corresponding to a scattered field detected
at said transducer receiver positions;
said CPU preparing an initial estimate of the scattering potential for said
object and an initial estimate of the internal field of said object at
each pixel of a display screen on which said acoustic image is to be
displayed and storing each said estimate in said memory;
said CPU determining at each said frequency and storing in said memory a
scattered field derived at said frequency from said initial estimates of
the scattering potential and internal field, and thereafter comparing at
said frequency said scattered field detected at said receiver positions to
said scattered field determined by said CPU;
said CPU determining at each said frequency and storing in said memory a
new estimate of said internal field at each said pixel derived from said
incident field, the last estimate of the internal field at each said
pixel, and the last estimate of said scattering potential at each said
pixel, said new estimate of the internal field at said frequency
comprising all orders of scattering;
said CPU determining and storing in said memory a new estimate of said
scattering potential derived from said new estimate of said internal field
and said scattered field detected at said receiver positions;
said CPU continuing to update the estimate for said internal field and
scattering potential until said scattered field determined by said CPU
approximates said scattered field detected at said receiver positions
within a selected range of tolerance; and
said CPU thereafter using the determined scattering potential to
reconstruct and store said image in said CPU memory.
2. A method as defined in claim 1 wherein said step of electronically
transmitting said electric signal at multiple frequencies comprises the
steps of:
positioning a transducer array adjacent said object, said array comprising
a plurality of acoustic transmitters and acoustic receivers;
sending said electric signal at a first frequency to each said transmitter
so that each said transmitter will in turn propagate acoustic energy at
said first frequency; and
thereafter changing the frequency of said signal and sending said
electrical signal at said changed frequency to each said transmitter so as
to sequentially propagate acoustic energy from said said transmitter at
said changed frequency.
3. A method as defined in claim 1 wherein said step of electronically
transmitting an electric signal at multiple frequencies comprises the
steps of:
positioning a transducer array adjacent to said object, said array
comprising a plurality of acoustic transmitters and a plurality of
acoustic receivers;
generating said electric signal in the form of a waveform which is
characterized by a plurality of different frequencies; and
sending said generated waveform in turn to each said transmitter so as to
propagate acoustic energy at said multiple frequencies from each said
transmitter.
4. A method as defined in claims 2 or 3 wherein said transducer array is
configured to encircle said object.
5. A method as defined in claim 1 wherein said step of detecting at a
plurality of transducer-receiver positions said acoustic energy
transmitted through and scattered by said object comprises the steps of:
positioning a transducer array adjacent said object, said array comprising
a plurality of transmitters and a plurality of receivers; and
after acoustic energy is transmitted from one of said transmitters,
sequencing each said receiver so as to detect said scattered acoustic
energy at each said receiver in turn.
6. A method as defined in claim 5 wherein said step of electronically
processing said detected acoustic energy comprises the steps of:
transducing the acoustic energy detected by each said receiver transducer
into a corresponding electric signal; amplifying said corresponding
electric signal; and
thereafter processing each said amplified signal so as to generate two
signals which correspond to mathematical real and imaginary
representations of each said amplified signal.
7. A method as defined in claim 6 wherein the step of processing said
amplified signal from each said receiver transducer so as to generate said
signals corresponding to said mathematical real and imaginary
representations of said amplified signal comprises the steps of:
inputting the amplified signal detected at said receiver transducer to
first and second multiplier circuits and multiplying the amplified signal
input to said first multiplier circuit by the electric signal sent to said
transmitter transducers;
shifting the phase of said electric signal sent to said transmitter
transducers by 90.degree. and thereafter multiplying the amplified signal
input to said second multiplier circuit by said signal that is shifted by
90.degree.; and
filtering the output of each said multiplier circuit with a low-pass filter
circuit and thereafter integrating and digitizing the output of each said
low-pass filter circuit.
8. A method as defined in claim 6 wherein said step of processing said
amplified signals from each said receiver transducer so as to generate
said signals corresponding to said mathematical real and imaginary
representations of each said amplified signal comprises the steps of:
inputting each said amplified signal to a high speed analog-to-digital
converter so as to digitize each said amplified signal; and
inputting each said digitized signal from said high speed analog-to-digital
converter into a parallel processor programmed to take the complex fast
Fourier transform of each said digitized signal.
9. A method as defined in claim 5 wherein said transducer array is
configured to encircle said object.
10. A method as defined in claim 1 wherein said initial estimate of said
scattering potential is zero.
11. A method as defined in claim 1 wherein said initial estimate of said
scattering potential is an average value determined by an estimated
average of density, acoustic speed and acoustic absorption of said object.
12. A method as defined in claim 1 wherein said incident field is used as
said initial estimate of the internal field of said object at each said
pixel.
13. A method as defined in claim 1 wherein said estimate of the internal
field of each said object at each said pixel is determined by said CPU
from said initial estimate of said scattering potential.
14. A method as defined in claim 1 wherein said step of determining said
new estimate of said scattering potential comprises the steps of:
backprojecting said scattered field detected at said transducer-receiver
positions to obtain a blurred image of said scattering potential of said
object; and
deblurring said blurred image of said scattering potential to obtain said
new estimate of the scattering potential.
15. A method as defined in claim 14 wherein said incident field, said
scattered field detected at said transducer-receiver positions, each said
initial and new estimate of said internal field and each said initial and
new estimate of said scattering potential are obtained separately by said
CPU at each said frequency.
16. A method as defined in claim 15 wherein said scattering potential is
formulated using a plurality of frequency-independent components
represented as a vector .GAMMA. multiplied by a frequency-dependent matrix
M, and wherein a least-squares method is applied by said CPU to the
product M.GAMMA. so as to derive therefrom a scattering potential
comprised of frequency-dependent terms whose sum represents a best fit to
said deblurred image of said scattering potential.
17. A method as defined in claim 16 wherein said least-squares method is
applied by said CPU using a conjugate gradient method.
18. A method as defined in claim 1 wherein said incident field, said
scattered field detected at said transducer-receiver positions, and each
said initial and new estimate of said internal field are obtained
separately by said CPU at each said frequency.
19. A method as defined in claim 18 wherein said new estimate of said
internal field and said new estimate of said scattering potential are
obtained by said CPU according to a conjugate gradient method.
20. A method as defined in claim 19 wherein said scattering potential
obtained by said CPU comprises a plurality of frequency-independent
components represented as a vector .GAMMA..
21. A method as defined in claim 19 wherein said conjugate gradient method
is used by said CPU to determine said new estimates of said internal field
and said scattering potential by computing a descent direction wherein
each variable from a set of variables taken from a first vector equation
defining said internal field and a second vector equation defining said
scattered field detected at said receiver positions is allowed to vary
simultaneously.
22. A method of reconstructing an acoustic image of an object using a
central processing unit (CPU) programmed to process data derived from
acoustic energy that has been transmitted at multiple frequencies and
scattered by said object, said method comprising the steps of:
propagating multiple frequency acoustic energy waves toward said object
from a plurality of transmitters positions;
electronically storing a plurality of digitized electronic signals derived
by said CPU from said propagated acoustic energy and said transmitter
positions, said digitized electronic signals representing an incident
field obtained at each said frequency
detecting at a plurality of receivers multiple frequency acoustic energy
waves scattered by said object;
electronically storing a plurality of digitized electronic signals
representing all orders of a scattered field obtained at each said
frequency and derived by said CPU from the scattered acoustic energy waves
detected at said receivers;
said CPU determining (a) an estimate of an internal field at each said
frequency and at each scattering point within said object, and (b) an
estimate of a scattering potential characterized by density, acoustic
speed and acoustic absorption at each said frequency and at each
scattering point within said object, and said CPU electronically storing a
plurality of digitized electronic signals representing said estimates of
said internal field and said scattering potential;
said CPU determining from said estimated internal field and scattering
potential a predicted scattered field at all orders of scattering and at
each said frequency, and said CPU comparing said predicted scattered field
to said scattered field derived from the scattered acoustic energy waves
detected at said receivers;
said CPU updating said estimates of said internal field and said scattering
potential until said comparison results in a predicted scattered field
which approximates said scattered field derived from the scattered
acoustic energy waves detected at said receivers within a selected range
of tolerance; and
said CPU thereafter reconstructing from said estimated scattering potential
an acoustic image of said object, and outputting a visually perceptible
display of said image.
23. A method as defined in claim 22 wherein said step of said CPU updating
said estimate of said internal field comprises the step of said CPU
deriving said updated estimate of said internal field from said incident
field and from said estimate of said scattering potential.
24. A method as defined in claim 23 wherein said step of said CPU updating
said estimate of said scattering potential comprises the step of said CPU
deriving said updated estimate of said scattering potential from said
updated estimate of said internal field and from said scattered field
derived from the scattered acoustic energy waves detected at said
receivers.
25. A method as defined in claim 24 wherein said step of said CPU updating
said estimate of said scattering potential further comprises the steps of:
backprojecting said scattered field derived from the scattered acoustic
energy waves detected at said receivers to obtain a blurred image of said
updated scattering potential; and
deblurring said blurred image.
26. A method as defined in claim 25 further comprising the steps of:
said CPU formulating a frequency-independent vector .GAMMA. comprised of a
plurality of frequency-independent components;
said CPU setting said updated estimate of said scattering potential equal
to the product of said vector and a frequency-dependent matrix M; and
said CPU applying a least-squares method to said product M.GAMMA. so as to
derive therefrom a product comprising a plurality of frequency-dependent
terms whose sum represents a best fit to said deblurred image.
27. A method as defined in claim 23 wherein said updated estimate of said
internal field is obtained by said CPU according to a conjugate gradient
method.
28. A method as defined in claim 27 further comprising the steps of:
said CPU formulating a frequency-independent vector .GAMMA. comprised of a
plurality of frequency-independent components;
said CPU applying a conjugate gradient method to obtain an updated estimate
of said vector from said updated internal field and from said scattered
field derived from the scattered acoustic energy waves detected at said
receivers; and
said CPU determining said updated estimate of said scattering potential
from the product of said updated vector and a frequency-dependent matrix
M.
29. A method as defined in claim 28 wherein said conjugate gradient method
is used by said CPU to determine said updated estimates of said internal
field and said vector by computing a descent direction wherein each
variable from a set of variables defined by (a) a first set of equations
defining said internal field and (b) a second set of equations defining
said scattered field detected at said receivers is allowed to
simultaneously vary.
30. A method as defined in claim 22 wherein said step of propagating said
multiple frequency acoustic energy waves comprises the step of
electronically transmitting an electric signal at multiple frequencies and
transducing said electric signal at each set frequency into said acoustic
energy waves.
31. A method as defined in claim 30 wherein said step of electronically
transmitting said electric signal at multiple frequencies comprises the
steps of:
positioning a transducer array adjacent said object, said array comprising
a plurality of acoustic transmitters;
sending said electric signal at a first frequency to each said transmitter
so that each said transmitter will in turn propagate acoustic energy at
said first frequency; and
thereafter changing the frequency of said signal and sending said
electrical signal at said changed frequency to each said transmitter so as
to sequentially propagate acoustic energy from each said transmitter at
said changed frequency.
32. A method as defined in claim 30 wherein said step of electronically
transmitting an electric signal at multiple frequencies comprises the
steps of:
positioning a transducer array adjacent to said object, said array
comprising a plurality of acoustic transmitters;
generating said electric signal in the form of a waveform which is
characterized by a plurality of different frequencies; and
sending said generated waveform in turn to each said transmitter so as to
propagate acoustic energy at said multiple frequencies from each said
transmitter.
33. A method as defined in claims 30 or 31 wherein said transducer array is
configured to encircle said object.
34. A method as defined in claim 22 wherein said step of detecting at said
plurality of receivers said multiple frequency acoustic energy waves
scattered by said object comprises the steps of:
positioning a transducer array adjacent said object, said array comprising
a plurality of acoustic receivers;
sequencing each said receiver so as to detect said scattered acoustic
energy waves at each said receiver in turn; and
electronically processing said detected acoustic energy waves so as to
transform said detected acoustic energy waves into a plurality of
digitized electric signals.
35. A method as defined in claim 34 wherein said step of electronically
processing said detected acoustic energy waves comprises the steps of:
transducing said acoustic energy waves detected by each said receiver into
a corresponding electrical signal;
amplifying said corresponding electric signal; and
thereafter processing each said amplified signal so as to generate two
signals which correspond to mathematical real and imaginary
representations of each said amplified signal.
36. A method as defined in claim 35 wherein said step of processing each
said amplified signal so as to generate said signals corresponding to said
mathematical real and imaginary representations of said amplified signal
comprises the steps of:
inputting each said amplified signal to first and second multiplier
circuits and multiplying the amplified signal input to said first
multiplier circuit by the electric signal sent to said transmitter
transducers;
shifting the phase of said electric signal sent to said transmitter
transducers by 90.degree. and thereafter multiplying the amplified signal
input to said second multiplier circuit by said signal that is shifted by
90.degree.; and
filtering the output of each said multiplier circuit with a low-pass filter
circuit and thereafter integrating and digitizing the output of each said
low-pass filter circuit.
37. A method as defined in claim 35 wherein said step of processing each
said amplified signals so as to generate said signals corresponding to
said mathematical real and imaginary representations of each said
amplified signal comprises the steps of:
inputting each said amplified signal to a high speed analog-to-digital
converter so as to digitize each said amplified signal; and
inputting each said digitized signal from said high speed analog-to-digital
converter into a parallel processor programmed to take the complex fast
Fourier transform of each said digitized signal.
38. A method as defined in claim 34 wherein said transducer array is
configured to encircle said object. |
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Claims |
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Description |
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BACKGROUND
1. Field of the Invention
The present invention relates to an apparatus and method for acoustic
imaging, which is defined herein to mean electronic reconstruction and
display of the size, shape, and internal elastic and viscous properties
(e.g., density, acoustic speed, and acoustic energy absorption) of an
object or material. More particularly, the present invention relates to an
apparatus and method for acoustic imaging using inverse scattering
techniques.
2. The Prior Art
It has long been known that acoustic waves in the frequency range of a
fraction of a cycle per second up to hundreds of millions of cycles per
second and higher can be propagated through many solids and liquids.
Acoustic energy waves may be partially reflected and partially transmitted
at the interface between two media of different elastic properties. The
product of material density and sonic wave velocity is known as the
acoustic impedance, and the amount of reflection which occurs at the
interface between two media is dependent upon the angle of incidence and
the amount of change in the acoustic impedance from one medium to the
other. This concept of reflection from layers may be generalized to
reflection from small regions of arbitrary shape. If the regions of
differing impedance are of the order of a wavelength or smaller, the
reflection is no longer specular, but diffuse. In this case, the more
general term of scattering is used to include both specular and diffuse
reradiation of energy. It is also seen that scattering is produced not
only by fluctuations in impedance, but also by fluctuations in speed of
sound, compressibility, density, and absorption. The net property of an
object which describes this phenomenon is called the scattering potential.
These principles have been used for imaging reflecting bodies within a
propagation medium. In terms of scattering theory, the direct or forward
scattering problem is concerned with a determination of the scattered
energy or fields when the value and distribution of the elastic or
electromagnetic properties of the body (i.e., scattering potentia) or the
distribution of the particles doing the scattering are known. The inverse
scattering problem consists in the use of scattered electromagnetic and/or
acoustic waves to determine the internal material properties (i.e.,
scattering potential) of objects from the information contained in the
incident and scattered fields. In other words, as defined herein, acoustic
imaging using inverse scattering techniques is intended to mean electronic
reconstruction and display of the size, shape, and unique distribution of
material elastic and viscous properties of an object scanned with acoustic
energy, i.e., reconstruction of that scattering potential which, for a
given incident field and for a given wave equation, would replicate a
given measurement of the scattered field for any source location.
Acoustic imaging through the use of inverse scattering techniques has been
a much studied problem in fields which are as diverse as seismic
geophysical surveying, nondestructive testing, sonar, and medical imaging.
Such inverse scattering techniques would be of particular interest because
of the ability to provide accurate quantitative as well as qualitative
image values when using such techniques. However, the use of complete
inverse scattering techniques in acoustic imaging is generally considered
to be so difficult that it has been common to employ methods which lead
merely to approximations rather than actual image values. For example, one
approach used in holographic imaging and seismic imaging is to "back
propagate" the detector field measurement into the object, usually
assuming as a model a homogeneous or a one-dimensional layered
distribution of wave propagation speed. The images obtained are images of
the source of the scattered fields, and thus only indirectly provide an
indication of internal structure of material properties. Many of such
techniques are described in the Acoustical Imaging series, volumes 1-13,
published by Plenum Press, Inc.
In the case of medical diagnostic imaging, quantitative tissue
characterization based on approximate or theoretically incomplete inverse
scattering techniques are now being investigated for inclusion on clinical
pulse echo scanners, also known as B-scanners. However, tissue
characterization using such scanners is based on 180 degree backscattering
from structural and statistical properties of tissues and not on
determining absolute tissue properties, per se. The statistical properties
of tissues, e.g., texture or the spatial Fourier transform, are often
correlated with the state of health or disease and are therefore valuable,
but they are not easy to measure quantitatively using present incomplete
or approximate inverse scattering techniques. thus, while it is possible
with the present state of the art to obtain some quantitative information
about tissue properties from B-scans, it is not possible to obtain
absolute mechanical properties of such tissues.
In summary, prior art apparatus and methods which have been used to date do
not take into account such problems as multiple orders of scattering
compensation for refraction, frequency dependent effects of density on
scattering properties of the object, changes in acoustic absorption based
on changes in frequency of the acoustic energy, or boundary value
measurements of the incident field, scattered field, and scattering
properties of the object. All of these problems may significantly affect
image quality. Yet the prior art has largely ignored these problems by
using approximations or assumptions which avoid having to account for such
problems when reconstructing the image. Accordingly, it would be an
important advance in the state of the art to be able to provide acoustic
imaging using inverse scattering techniques which provide an image of the
actual material properties of an object (and not just the internal fields)
without use of perturbation or other drastic approximations, such as the
well-known ray optics, single scattering, Born, or Rytov approximations.
Such an apparatus and method are described and claimed herein.
BRIEF SUMMARY AND OBJECTS OF THE INVENTION
The apparatus and method of the present invention provide high-quality
images with high-spatial resolution of an object, including the actual
internal viscous and elastic properties of the object, derived from
acoustic energy propagated through the object and scattered by it. The
apparatus and method include means for sending and receiving acoustic
energy waves and for reconstructing the image using state-of-the-art
electronics to optimize the system's speed and resolution capability.
Improvements to resolution quality of the reconstructed image, including
accurate quantitative imaging of the actual internal elastic and viscous
properties of the object (e.g. density, acoustic speed, and acoustic
absorption),are achieved using high-speed computer-aided data analysis
baded on new inverse scattering techniques.
It is therefore a primary object of the present invention to provide an
improved apparatus and method for acoustic imaging.
Another primary object of the present invention is to provide an apparatus
and method for reconstructing acoustic images of the actual internal
material properties of an object using inverse scattering techniques, but
without degrading image quality through the use of often effectively
drastic approximations, such as geometrical or ray acoustic approximations
or perturbation theories that include the Born or Rytov approximations.
Another object of the present invention is to provide an apparatus and
method for improving the spatial resolution of an image derived from
scattered acoustic energy by more accurately taking into account
refraction and diffraction effects.
A further object of the present invention is to provide an apparatus and
method which is capable of providing high-spatial resolution of elastic
and viscous material properties from scattered energy, even though the
detected energy has undergone multiple scattering events.
Still another important object of the present invention is to provide an
apparatus and method which is capable of providing improved spatial
resolution and actual quantitative material properties of an object
reconstructed from scattered acoustic energy, but which is still capable
of high-speed reconstruction of the images of these properties.
A further object of the present invention is to provide an apparatus and
method for obtaining quantitative images of high-quality and high-spatial
resolution of multiple elastic and viscous material properties in
geometries where the source or receiver locations do not completely
circumscribe the object or where the solid angles defined by the source or
receivers with respect to the body are small.
A further object of the present invention is to provide quantitative images
of speed of sound and attenuation that can be used to correct more
conventional images, such as B-scan or synthetic focus for blurring due to
refractive and attenuation effects.
These and other objects and features of the present invention will become
more fully apparent from the following description and appended claims
taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a side-elevational view shown partially in cross section which
schematically illustrates an acoustic scanner which may be used with the
apparatus and method of the present invention.
FIG. 2 is a perspective view illustrating one type of configuration which
may be used for the transducer arrays employed in the scanner of FIG. 1.
FIG. 3 is a top view of the transducer arrays shown in FIG. 2.
FIGS. 4A-4B are schematic diagrams illustrating one embodiment of an
electronic system that may be used to implement the apparatus and method
of the present invention.
FIG. 4C is a schematic illustration of a circular transducer array showing
a method of electronically multiplexing the array elements to eliminate
the need for mechanical rotation.
FIG. 4D is a further illustration of pure electronic multiplexing of a
linear transducer array.
FIG. 4E is an electrical schematic diagram showing how the elements in
FIGS. 4C and 4D may be switched to act as either a transmitter or receiver
by use of an active switch.
FIG. 4F illustrates how a passive network may be used to allow each element
of an array to be used for both transmitting and receiving.
FIGS. 5A-5B are schematic diagrams illustrating another embodiment of an
electronic system that may be used to implement the apparatus and method
of the present invention.
FIGS. 6A-6F are schematic flow diagrams which illustrate an example of one
presently preferred method by which the electronic systems of FIGS. 4 and
5 are able to rapidly develop a reconstruction of the image of an object
from scattered acoustic energy using inverse scattering techniques.
FIGS. 7A-7E schematically illustrate another method by which the electronic
systems of FIGS. 4 or 5 may implement inverse scattering techniques to
acoustically image an object.
FIGS. 8A-8L schematically illustrate still another method for implementing
inverse scattering techniques using the systems of FIGS. 4 or 5.
FIG. 9 is a photograph of a television display screen showing an image that
simulates a cancer and an actual image obtained as the inverse scattering
solution using the method and a computer simulation of the apparatus of
the present invention.
Reference is now made to the figures wherein like parts are designated with
like numerals throughout.
DETAILED DESCRIPTION
The apparatus and method of the present invention holds promise for many
useful applications in various fields, including seismic surveying,
nondestructive testing, sonar, and medical ultrasound imaging, to name
just a few. For purposes of illustrating the utility of the present
invention, the detailed description which follows will describe the
apparatus and method of the invention in the context of a system for use
in performing ultrasound imaging of human organs, such as the breast.
However, it will be appreciated that the present invention as claimed
herein may be used in other fields, and is not intended to be limited
solely to medical acoustic imaging.
1. The Scanner and Transducer Configuration
Reference is first made to FIG. 1 which generally illustrates one type of
scanner which may be used to implement the apparatus and method of the
present invention for purposes of medical ultrasound imaging of a human
breast or other organs. As shown in FIG. 1, the scanning apparatus
generally designated at 30 includes a fixed base 32. Wheels 38 and 40 are
attached to the underside of a movable carriage base 34. Small shoulders
42-45 formed on the upper surface of cylindrical pedestal 36 define a
track along which the wheels 38 and 40 are guided.
A stepping motor 46 mounted within the fixed base 32 is joined by a shaft
48 to a small pinion gear 50. Pinion gear 50 engages a large drive gear
52. Pillars 54-57 are rigidly joined at one end to the top of drive gear
52 and at the opposite end to the underside of movable carriage base 34.
Bearing block 58 supports drive gear 52 and movable carriage base 34.
Stepping motor 46 may be operated to turn the drive gear 52 which in turn
will cause the movable carriage base 34 to rotate on top of the
cylindrical pillar 6 within the tracks defined by shoulders 42-45. As
hereinafter more fully described, rotation of the movable carriage base 34
may be employed to insure that an object is fully scanned from every
possible angle.
With continued reference to FIG. 1, it will be seen that movable carriage
base 34 has an inner cylindrical wall 60 and an outer cylindrical wall 62.
The outer wall 62 and inner cylindrical wall 60 of movable carriage base
34 define a generally cylindrical chamber 64. Vertical drive motor 66 is
mounted within chamber 64 and is connected by a shaft 68 to a circular
ring of transducer arrays generally designated at 70. Vertical drive motor
66 permits the circular ring of transducer arrays 70 to be vertically
adjusted. Slide bracket 72 is mounted within the chamber 64 and serves to
slidably guide the ring of transducer arrays 70 when it is vertically
adjusted.
The ring of transducer arrays 70 is electrically connected through line 74
to components of an electronic system which may be housed in part within
the chamber 64, as schematically indicated at 76. As hereinafter more
fully described, the electronic system is used to control transmission and
reception of acoustic signals so as to enable reconstruction therefrom of
an image of the object being scanned.
Circular bracket 78 is attached to the top of the outer wall 62 of movable
carriage base 34. A flexible, transparent window 80 extends between
circular bracket 78 and the inner cylindrical wall 60 so as to enclose the
transducer arrays 70 and stepping motor 66 within the chamber 64. The
length of flexible window 80 is greater than the distance between bracket
78 and inner cylindrical wall 60. Window 80 thus serves as a flexible yet
water-tight seal which permits vertical motion of the transducer arrays 70
for purposes of vertical focusing. Acoustically transparent window 80 may
be made of any suitable material, such as plastic or rubber.
A stationary water tank generally designated 86 is adapted to fit within
the movable carriage base 34. Water tank 86 consists of a fixed top plate
88 rigidly attached to vertical support bars 82 and 84. Support bars 82
and 84 are mounted on the fixed based 32. The length of support bars 82
and 84 is chosen such that the fixed top plate 88 of water tank 86 will be
slightly suspended above the bracket 78 of movable carriage 34. Thus, a
space 87 is provided between bracket 78 and fixed top plate 88.
Additionally, a space 89 will be provided between side 94 and bottom 95 of
water tank 86 and cylindrical wall 60 and bottom 61 of movable carriage
34. A third support bar 83 extends through a central hole (not shown)
provided in block 58 and drive gear 52. Support bar 83 also extends
through a watertight opening 84 provided in the bottom 61 of movable
carriage 34. Support bar 83 thus helps to support water tank 86 in spaced
relation from movable carriage 34. Since water tank 86 is suspended in
spaced relation from movable carriage base 34, water tank 86 will remain
stationary as movable carriage 34 is rotated. As hereinafter more fully
described, rotation of the carriage 34 permits the transducer arrays 70 to
scan the object 98 from every possible position around the object 98.
Fixed top plate 88 has a short downwardly extending lip 90 which extends
over the end of circular bracket 78. A rubber-covered window 92 extends
between the lip 90 and side 94 of the water tank. Window 92 encloses
within space 89, water 977, or some other suitable acoustic medium so as
to acoustically couple the transducer array 70 to the water 96 contained
in tank 86. The rubber-covered window 92 also permits acoustic energy
signals to be transmitted therethrough by the transducer arrays 70 and
insures that the patient will be protected in the event window 92 should
be broken.
The scanning apparatus generally described above may be employed to scan
various part os the human anatomy as, for example, a patient's breast, as
schematically illustrated at 98.
Reference is next made to FIGS. 2-3. FIG. 2 generally illustrates one
suitable type of transducer configuration for the transducer arrays of
FIG. 1. As shown in FIG. 2, the transducer configuration consists of eight
transmitter arrays 100-107 and eight corresponding receiver arrays
108-115. The transmitter array 100-107 are thin, cylindrically-shaped
transducer arrays which provide point-source or line-source segment
transmission of acoustic energy. The receiver arrays 108-115 are arcuately
shaped arrays which are interposed between each pair of transmitter arrays
100-107. For purposes hereinafter more fully described, every other
receiver array (e.g.. receiver arrays 108, 110, 112 and 114) has a
shortened arcuate length.
Each of the transducer arrays 100-115 may be any of several well-known
types of transducers. For example, transducers 100-115 may be
piezoelectric transducers which produce ultrasound energy signals directly
from high-frequency electrical voltages applied to the transducer.
Alternatively, the transducer arrays 100-115 may be magnetostrictive
transducers having a magnetic coil (not shown) which receives the
electrical oscillations and converts them into magnetic oscillations which
are then applied to the magnetostrictive material to produce ultrasound
energy signals.
With continued reference to FIG. 1, it will be seen that the transducer
arrays 100-115 are arranged so as to form a cylindrical ring of arrays
which encircles the object 98. By encircling the object with the
transducer arrays 100-115, the arrays 110-115 may be quickly commutated by
either mechanical methods, electronic methods or by a combination of both
methods so as to completely scan the object in a much shorter time. In the
illustrated embodiment, commutation is achieved by both mechanical
rotation by stepping motor 46 and by electronic triggering of transmitter
arrays 100-117 in sequence, as described more fully below.
Commutation of the transmitter arrays 100-107 permits acoustic energy to be
transmitted from every possible position about the object, thus insuring
that the data received (i.e. scattered acoustic energy) is complete.
Commutation of the receiver arrays 108-115 insures that all spaces between
receiver arrays 108-115 (known as "sound holes") will be covered, thus
providing for accurate collection of all acoustic energy that is
transmitted through or scattered by the object 98. However, commutation of
the receiver arrays 108-115 is not necessary where transmitter arrays
100-107 are also used to receive acoustic signals. The circular
configuration of transducer arrays 100-115 permits certain parts of the
body to be scanned which would otherwise be inaccessible because of bones
or other obstructions of the tissue.
The method for commutating the arrays 100-115 is best understood by
reference to FIG. 3. First, each of the transmitter arrays 100-107 is
sequentially triggered so as to transmit acoustic energy. Immediately
after each transmitter array 100-107 is triggered, arrays 108-115 receive
acoustic energy signals that have been either transmitted through or
scattered by the object being scanned. Once this procedure has been
followed for each of the transmitter arrays 100-107, the ring of arrays 70
is then mechanically rotated counterclockwise through a small angle, as
schematically represented by arrow 116. The mechanical rotation is
achieved by the stepping motor 46 (see FIG. 1) which rotates the movable
carriage base 34, as described above.
After rotation of the arrays 100-115 to a second position, each of the
transmitter arrays 100-107 is again sequentially triggered and data are
again collected through receiver arrays 108-115. This procedure is
repeated until acoustic energy has been transmitted at each possible point
about the object.
Where the arrays 100-107 are used only for transmitting acoustic energy, a
second series of rotations must then be effected to cover the sound holes
between each pair of receiver arrays 108-115. For example, by rotating
transmitter array 101 to the position occupied by the transmitter array
100, receiver arrays 109, 111, 113 and 115 will, because of their longer
arcuate length, cover the spaces previously occupied by transmitter arrays
101, 103, 105 and 107. This procedure is repeated until all sound holes
have been covered.
It should be noted that for a fixed circumference by decreasing the length
of each array and increasing the number of arrays, electronic commutation
may be used to reduce the angle through which the ring of transducer
arrays must be rotated to achieve complete collection of both echo and
transmission data.
It should also be noted that, in principal, no mechanical rotation of the
array of detectors is necessary if every element is small enough and can
be made to act as either a receiver or transmitter. Such an arrangement is
more expensive than the technique illustrated in FIGS. 1, 2, and 3.
2. The Electronic System
Reference is next made to FIGS. 4A-4B which schematically illustrate an
electronic system which may be used to implement the apparatus and method
of the present invention. As hereinafter more fully described, the
electronic system generates the acoustic energy that is propagated through
and scattered by the object 98. The electronic system thereafter detects
and processes the acoustic energy signals that are scattered by and
transmitted through the object 98, and then communicates the processed
signals to a computer (CPU) which interprets the signals and outputs the
result in the form of a visual display or printed output.
In the transmission mode, CPU 118 causes an oscillator 128 to output a
waveform which is amplified by a power amplifier 130 before being sent
through multiplexer 132 to one of the transmitters. CPU 118 controls the
multiplexer 132 so as to sequence each transmitter array 100-107 used to
generate the acoustic energy propagated through the acoustic medium and
object. If desired, after it is amplified, the waveform can also be input
to an impedance matching transformer (not shown) and to a series of RC or
RLC networks connected in parallel across the transmitter arrays 100-107
as illustrated and described in U.S. Pat. No. 4,222,274 (hereinafter the
Johnson '274 patent), which is incorporated herein by reference. The
impedance matching transformer may be used to achieve maximum power
transfer while the RC or RLD networks may be used to distribute power
across each transmitter array 100-107 in a way that decreases the side
lobes in the transmitted signal.
Each of the acoustic receivers 108-115 (FIG. 2) are connected through a
multiplexer 134 which is also controlled by CPU 118. In the receive mode,
the detected signals may also be input through a delay line (not shown) to
an analog adder and time variable gain circuit (not shown) to vertically
focus the signals and to compensate for signal attenuation, as shown and
described in the Johnson '274 patent. CPU 118 causes multiplexer 134 to
sequence in turn each of the acoustic receivers 108-115 so as to gather
transmitted or scattered acoustic energy around the entire circumference
of the object. From receiver multiplexer 134, detected acoustic energy
signals are amplified by a preamplifier 136 which may be used to
logarithmically amplify the detected signal to reduce storage space
required for each signal after it is digitized. The amplified signal is
then processed by a phase detector 138.
The operation and components of phase detector 138 are best illustrated in
FIG. 4B. As there shown, p | | |
