Note: Descriptions are shown in the official language in which they were submitted.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is a further development in the
field of acoustical imaging, with particular application to
the imaging of anatomical organs.
2. Description of the Prior Art
Acoustical imaging of anatomical organs of the
human body has heretofore generally been accomplished by the
pulsed-echo technique, whereby an electrical pulse excites
an acoustical transducer to launch a compressional wave into
the body. As the compressional wave passes from one region
of the body to another region having a different acoustical
impedance, part of the wave energy is reflected at the
interface between the two regions back toward the transducer. -
The remainder of the wave energy is transmitted deeper into
the body tissues until another acoustical impedance discontin-
uity is reached, whereupon another partial reflection and
partial transmission of the wave energy occur. The reflected
acoustical signals are converted by the transducer into
electrical signals, which are amplified as necessary. These
electrical signals can be processed to generate an image of
the organ being examined. From a knowledge of the velocity of
a compressional wave in the various tissues of the body, it is
possible with the pulse-echo technique to measure the depths
at which the various reflections occur within the body by
relating the times of arrival at the transducer of the
reflected signals to the time of the initially emitted pulse.
In the contact scanner system, which is a particular
application of the pulsed-echo technique, a single transducer
is used to launch a parallel beam of ultrasonic wave pulses
into the body and to receive any waves that may be reflected
from impedance discontinuities. The position and orientation
,,;
.
of the ultrasonic beam transducer are determined for the
particular organ to be scanned by suitable linkages to
position-determining transducers that are coupled to a
storage-type oscilloscope. In operation, the ultrasonic
beam transducer is moved over the surface of the body, and
an image of the internally reflecting surfaces within the
body is built up on the screen of the oscilloscope. Typically,
a period of approximately 20 seconds is used to move the
transducer over the body surface so as to form a suitable
image for display and analysis. A significant disadvantage
of the contact scanner system is the relatively long time
required to form an image. Such a long time is likely to
result in loss of resolution due to movement of the patient
or involuntary movement of the bodily organ being imaged
(e.g., by the beating of the heart).
It was also known to the prior art to use a
linear array of acoustical transducers. In such an arrangement,
each transducer would be automatically time-multiplexed so
that only one transducer at a time would emit a pulse. Since
multiplexing can be done much faster than any corresponding
mechanical movement of a contact scanner as described above,
an image can be formed from an array of 30 transducers in a
total of about 30 milliseconds. Such a short time period
makes it possible to observe the movements of an internal
organ such as a beating heart. A major disadvantage of such
a linear array system, however, is that the length of the
array must be equal to a linear dimension of the object to
be examined. Consequently, with a linear array of transducers,
a large ultrasonic aperture in the body is required in order
to image an internal organ. Unfortunately, ultrasonic
absorption by bone tissue is extremely high in comparison
with ultrasonic absorption by soft tissues. Consequently,
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bone tissue will shadow any soft-tissue structure located
behind it. Where the organ to be imaged is located within
the rib cage (e.g., the heart), the overlying rib cage
presents an obstacle to the imaging of the organ by a linear -~ -
array of transducers. Furthermore, the inherent divergence
of an ultrasonic beam emanating from a linear array of
transducers severely limits the resolution obtainable for
objects located deep within the body.
In order to circumvent the limitation on reso-
lution inherent in a linear array of transducers, it was known
to use an acoustical lens in combination with such a linear -
array of transducers. The acoustical lens serves to focus
the ultrasonic beam from each transducer of the array onto a
particular point on a focal surface within the body. The
acoustical lens in such prior art imaging systems was physically
separated from the transducer array, typically by an inter-
vening water bath. The intervening water bath caused such
imaging systems to be heavy and mechanically complex, and
thereby effectively precluded the design of a convenient hand-
held lens system.
Acoustical imaging systems known to the prior
art did not use a homocentric lens, and hence were troubled
by off-axis aberrations. Furthermore, acoustical imaging
systems known to the prior art were troubled by the rever-
berations of ultrasonic waves within the various media
located between the transducers and the lens focus. The
water bath between the array of transducers and the ultra-
sonic lens was a particular source of such reverberations.
SUMMARY OF THE INVENTION
The acoustical imaging system of this invention
comprises an array of ultrasonic transducers affixed to the
outer surface of an acoustical lens. In a particular embodi-
13;~
ment of the invention, the lens is a spherical homocentric lens.
The inner surface of the lens is maintained in contact with
an object having an acoustical refractive index approximating
that of water, such as the skin overlying a soft-tissue
portion of the human body. If necessary to prevent an air
gap, a suitably shaped container of liquid such as water is
disposed intermediate the inner surface of the lens and the
contact surface of the skin. The lens is made of a material
having an index of refraction less than that of water so
that the ultrasonic waves emitted by the transducers will
converge at a focal surface within the tissues under the
skin. The waves from the various transducers pass into the
portion of the body to be examined through a common acoustic
aperture located at the common center of curvature of the
lens surfaces. By arranging this aperture to be located in
a region of the body such as at an intercostal space in the
rib cage, object points distributed over a relatively large
solid angle on the other side of the aperture (i.e., an
anatomical organ within the rib cage) can be imaged without
obstruction.
It is therefore an object of this invention to
provide an acoustical lens system for focusing ultrasonic
waves so that a large object can be imaged through a
relatively small acoustical aperture. It is a particular
object of this invention to provide a small hand-held instru-
ment, having a simple mechanical structure and requiring
uncomplicated electronic circuitry, for providing ultrasonic
echograms of internal organs of the human body. A special
application of this invention would be the acoustical imaging
of a human heart through an acoustic aperture taken at an
intercostal space in the rib cage.
--5--
It is also an object of this invention to provide
an acoustical lens system comprising an array of ultrasonic
transducers and a lens for converging the waves generated by
these transducers, where the transducers are affixed to a
surface of the lens in order to preclude reverberations
between the lens and the transducers.
It is likewise an object of this invention to
provide an acoustical lens system for ultrasonic imaging,
wherein the lens comprises a converging homocentric lens. In
an alternative embodiment, the lens comprises a converging
lens, each surface of which has a different radius of curvature
in each of two mutually orthogonal directions. As a special
case of this alternative embodiment, the lens has the shape
of a simple cylindrical section.
A further object of this invention is to produce
an ultrasonic imaging apparatus capable of providing high-
resolution images of moving objects immersed in media of
differing acoustical impedances. In particular, it is an
object of this invention to produce an apparatus capable of
providing a continuous acoustical image of a beating heart.
Another ~bject of this invention is to provide
in combination an ultrasonic lens system comprising a linear
array of ultrasonic transducers affixed to a converging
acoustical lens, means for converting reflected ultrasonic
signals into electrical signals, and means responsive to
such electrical signals for displaying an image of structures
causing reflection of such ultrasonic signals.
In accordance with the foregoing objects,
there is provided a probe assembly for examining a wide sector
of the interior of a body with ultrasonic waves through a
small acoustic aperture comprising: an array of transducers
providing ultrasonic beams and translating reflected ultra-
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sonic energy into electrical signals; a homocentric acousticlens for converging ultrasonic waves and to which said
transducer array is affixed; and means for housing said lens
and array and for spacing said lens from said acoustic
aperture by a distance approximately the radius of curvature
of said lens, whereby upon said assembly being brought into
contact with said body adjacent said acoustic aperture, and
upon said transducers being activated to produce sequentially
a plurality of ultrasonic beams, said beams may be converged
to pass through said aperture and interrogate said wide
interior sector.
There is also provided a probe assembly for
ultrasonic examination of a wide sector of the interior of a
body through a small acoustic aperture, comprising: a lens
for converging ultrasonic waves, said lens having a surface
which is curved to define a segment of a circle in at least
one cross-section transverse to said surface; an array of
transducers providing ultrasonic beams and translating re-
flected ultrasonic energy into electrical signals representative
thereof, said transducers being affixed to said surface along
said cross-section; and spacer means attached to said lens
and array for maintaining said lens spaced from said body by
a distance approximating the radius of curvature of said
surface upon said assembly being brought into contact with
said body, whereby said ultrasonic beams are converged to
pass through said aperture and interrogate said wider sector
of said body interior.
There is further provided in an ultrasonic
system for imaging a wide sector of the interior of a body
through a small acoustic aperture; a converging acoustic
lens having a surface which is curved to define a segment of
a circle in at least one plane transverse to said surface
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said lens being spaced from said acoustic aperture by a
distance approximating the radius of curvature of said surface;
an array of transducers aligned along the intersection of said
surface and said plane; and means for activating said trans-
ducers to transmit and receive through said lens a series of
ultrasonic beams, each over a respective separate time period,
and from a different position in said array; whereby said
wide interior sector of said body may be interrogated despite
said small acoustic aperture.
Other features and advantages of the present
invention will become apparent upon perusal of the following
specification in conjunction with the accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 shows a cross-sectional view of an
acoustical lens according to this invention, with the lens in
position to provide a visual display of an anatomical organ,
viz., the human heart, in situ.
FIG. 2 shows an acoustical lens and an
electronic circuit in block form for providing a visual
display of an anatomical organ.
FIG. 3 shows an alternative embodiment of an
acoustical lens system of this invention comprising a double
set of transducers, one for transmitting and one for receiving
ultrasonic waves.
FIG. 4 shows an alternative embodiment of an
acoustical lens according to this invention, wherein each
surface thereof has a different radius of curvature in each
of two mutually orthogonal directions.
FIG. 5 shows an aperture stop for use in
conjunction with the acoustical lens of this invention.
--8--
.
lV91;~37
DESCRIPTION OF THE INVENTION
The acoustical imaging system of this invention
is particularly adapted to provide a visual display of an
anatomical organ in situ within the human body. FIG. 1
illustrates the system with its lens 40 positioned to provide
an image of the heart 10 in sagittal plane. Overlying the
heart is a layer of skin 20. Under the skin may be found
a layer of subcutaneous fat of varying thickness, breast
tissue in females, a layer of connective tissue covering
the pectoralis major muscle, the pectoralis major muscle
itself which may be quite thick in muscular subjects, some
slips of the pectoralis minor muscle with its associated,
connective tissue, and the bony thorax including the ribs 21.
It is desirable to be able to provide a visual display of
the heart 10 by processing ultrasonic compressional waves
reflected from the various structures of the heart. Such a
display is called an echocardiogram. -
Ultrasonic waves are provided by transducers
31 located externally of the skin in an array. The configura-
tion of the array will be discussed hereinafter. Thetransducers 31 may be of piezoelectric material such as lead
zirconium titanate. The ultrasonic waves are launched by
applying a voltage pulse, as of 100 volts, directly across
the transducer material. Suitable transducers for the
practice of this invention are marketed by Clevite Corporation
of Bedford, Ohio under the designation PZT5A.
Ultrasound can be transmitted only poorly
through bone because of the high acoustical absorption of bone
tissue. Consequently, it is necessary to focus the
compressional waves produced by the transducers 31 in such
a way that the waves enter the middle mediastinum, wherein the
heart is located, via an acoustic window between the ribs.
~ i --9 _
`` 10~3i337
It has been found that the region inside the chest which is
most readily accessible to ultrasonic imaging comprises the
volume bounded anteriorly by the sternum, posteriorly by the
posterior pericardial wall, inferiorly by the diaphragm,
superiorly by the great blood vessels leading from and to
the heart, and laterally on either side by the lung margins.
In order to obtain an echocardiogram of the heart without
having to radiate compressional waves through the lungs, it
is advantageous to focus the waves by means of the converging
lens 40 through an acoustic window located in the second,
third, fourth or fifth intercostal space between the left
sternal margin and a line 3 to 4 centimeters left of that
margin. These waves are focused onto a focal surface 44
located within the rib cage.
At the present time, there is no unanimity
in the literature concerning the precise rates of energy
attenuation that will be experienced by an ultrasonic beam
as it passed through the various tissues of a human body.
However, an attenuation value of about 4.4 decibels per
centimeter of depth at a frequency of 2.5 megahertz is an
approximate measure of the attenuation of an ultrasonic beam
passing successively through skin, fat, muscle and blood
tissue. Signals reflected from the posterior wall of the
h~art at a depth of 13 centimeters would show an attenuation
of 57 decibels, according to this approximation. Since
reflected signals from a water/muscle interface show a
relative intensity of about 23 decibels below what would be
expected from a perfect reflector, the total intensity drop
or attenuation of an ultrasonic signal reflected from the
posterior wall of the heart would be about 80 decibels. From
such energy transfer considerations, it appears that an
initial peak power density of 100 milliwatts per square
--10--
3.37
centimeter would be entirely suitable for medical examining
procedures, and would not cause adverse physiological effects
upon a patient being examined. At this power level, the
intensity of the attenuated reflected signal can be readily
detected.
In FIG. 1, the transducers 31 are all affixed
as by epoxy cement, in a two-dimensional array on the external
surface 41 of a spherical homocentric lens 40. A homocentric
lens is defined as one in which both of the lens surfaces
that intersect the acoustical axis of the lens (analogous to
the optical axis of an optical lens system) have the same
center of curvature. Thus, the outer surface 41 and the
inner surface 42 of the lens 40 all have a common center of
curvature 43. The lens 40 is made of a solid material, either
metal or plastic, such as aluminum or polystyrene, and has an
acoustical refractive index less than the acoustical
refractive index for water.
A homocentric lens has the property that the
lens configuration is identical for each line or ray joining
an image point to its corresponding object point. The image
points for the lens 40 in FIG. 1 are the locations of the
transducers 31 on the inner lens surface 41. All such rays
intersect the outer lens surface 42 at an angle perpendicular
to the surface 42. Furthermore, the radius of curvature
of the surface 42 is identical at all points of intersection
of such rays with the surface 42, thereby making the focusing
properties of the lens 40 along any given one of such rays
identical to the focusing properties along any other one of
such rays. Thus, all such rays connecting corresponding
image and object points are equivalent. An ordinary lens
system has good focusing properties only very near the
symmetry axis of the system, while off-axis rays do not
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- lV91~3;~7
experience equivalent focusing properties. A system having
a homocentric lens, however, has wide-angle focusing capa-
bility because all the rays passing through the homocentric
lens experience identical focusing properties.
A number of factors are of importance in
selecting the lens material. First, the acoustical refractive
index of the material (i.e., the ratio of the velocity of
sound in water to the velocity of sound in the material) is
important because this factor controls the refractive power
of the lens. The acoustical impedance of the material is also
important because this factor determines the ratio of
reflected power to transmitted power at the lens surfaces.
The density of the material is likewise a significant factor
in determining the utility of the material for the lens of a
hand-held instrument. For the lens materials suggested
above, the refractive index of aluminum for ultrasonic
compressional waves is 0.24 whereas the refractive index of
polystyrene is 0.65, where the refractive index of water is
taken as unity. Thus, aluminum provides greater refractive
power than polystyrene, so that a smaller lens can be
designed with aluminum. This is important where overall
size is a prime consideration. However, the density of rolled
aluminum is 2.7 whereas the density of polystyrene is
only 1.06. Thus, where weight is a prime consideration, --~
polystyrene might be preferred over aluminum.
The lens 40 may be either supported by a
flexible supporting structure or mounted in a hand-held
instrument. In operation, the inner lens surface 42 is
positioned so that the center of curvature 43 lies in a
desirable acoustic window in the rib cage, e.g., in the
fourth intercostal space. The lens 40 may be pressed tightly
against the skin 20 so that no air gap remains between the
-12-
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1t)9i33~
inner lens surface 42 and the contacting surface of the skin,
or else a suitably sized container of water or other liquid can
be inserted between the inner lens surface and the skin.
In FIG. 1, rays indicating the direction of
forward motion of transmitted wave fronts are shown for two
transducers of the array. The wave fronts initially travel
divergently from the transducers 31 through the lens 40. At -
the inner surface 42, the wave fronts are caused to converge
to a focal surface 44 located within the rib cage on the
other side of the acoustic window. The focal surface 44, as
indicated by dashed lines in FIG. 1, is a section of a sphere
having the same center of curvature 43 as the outer and inner
surfaces (41 and 42, respectively) of the homocentric lens 40. ~
Each transducer 31 in the array has a unique focal point 45 ~-
on the focal surface 44. Although the transducers 31 may
collectively cover a large total area on the outer surface 41
of the lens 40, and although the focal surface 44 may extend
over a large area within the chest cavity, nevertheless all
rays passing through the acoustic window are converged to a
small intercostal space area centered about the center of
curvature 43 and bounded by the adjacent ribs. The focal
surface 44 defines the depth within the body for which the
sharpest focus is obtainable. For purpo5es of echocardiography,
where the maximum dimension of an adult human heart is on -
the order of 12 centimeters, adequate depth of field is
obtainable with the technique described herein. Formulas given
by A.E. Conrady in Applied Optics and Optical Design, Dover
Publications, Inc., Part I (1957) and Part II (1960), can be
used for analogous acoustical systems to estimate the focal
range and the effects of spherical aberration for an acoustical
homocentric lens system. The focal range is determined by
the acoustical wavelength and by the angle subtended by the
-- lV91;~7
acoustic aperture as viewed from a point on the focal surface
44. In an acoustical imaging system, depth resolution is not
determined by the depth of focus as would be the case with
optical imaging; but instead acoustical depth resolution is
determined by the pulse length of the transmitted acoustical
wave and the time resolution of the amplifier and the detector
of the receiving components of the system.
After passing through the acoustic window,
the ultrasonic wave fronts travel through a succession of
different types of tissue (e.g. fat, muscle, and blood) which
have no significant effect on the velocity of propagation of
the wave, until the wave fronts strike the anterior wall 11 of
the heart. In a thin adult male, the anterior wall of the heart
may be located about 2 centimeters under the skin. In females
and obese males, the anterior wall of the heart may be about
5 or 6 centimeters under the skin. A significant advantage of
using a homocentric lens imaging system is that uniform
resolution of all points on a heart wall can be obtained --
independently of the angle of ultrasonic wave transmission
with respect to the central axis of the lens.
The disposition of the transducers 31 on the
outer surface 41 of the lens 40 makes it possible to obtain an
image through a relatively large solid angle. In a preferred
embodiment of this invention, an array of 1024 transducers
are affixed in a grid-like pattern of thirty-two rows and
thirty-two columns on the outer surface 41 of the lens 40. The
lens 40 with its associated transducers is mounted in for
example, a probe structure housing 39. Possibly some compon-
ents of the signal processor 48, as shown in block form in
FIG. 1, could also be mounted within the probe structure
housing 39. For an aluminum lens, the radius of curvature
of the outer surface 41 would be approximately 10 centimeters,
-14-
jl
~J9i;~37
and the radius of curvature of the inner surface 42 would be
approximately 6.2 centimeters. For an instrument of this
preferred size, an angular sector image through an aperture
angle of approximately 90 degrees would be possible. The over-
all dimensions of an adult heart are typically about 12
centimeters in length, 8 to 9 centimeters in width at the
broadest extent, and 6 centimeters in depth. Consequently,
echocardiograms taken through two or three different
intercostal space could provide a composite picture of the
entire heart.
The wave front from any particular transducer
31 can be represented by rays drawn orthogonally to that wave
front. Rays are shown from two particular transducers in
FIG. 1 to illustrate the focusing properties of a lens
according to this invention.
It is desirable to eliminate sources of inter-
nal reflections and reverberations as much as possible from an
ultrasonic imaging system, in order to reduce the overall
noise level. Every improvement in the signal-to-noise ratio
permits a corresponding reduction in the level of transmitted
power required to produce unambiguous reflected information-
bearing signals. As a general principle, it is always desirable
to minimize the power level of diagnostic radiation of any
sort -- including ultrasonic radiation -- incident upon a
human patient. In this invention, the transducers 31 are
affixed directly to the outer surface 41 of the lens 40; and
the remaining portion of the surface 41 which is not covered
by transducers 31 is covered by an acoustical absorbing
material 49. The acoustical absorbing material 49 should
have an acoustical impedance closely matching that of the
lens 40, and in addition should have a relatively hiah
acoustical attenuation. Suitable materials for use as
~ ; .
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~.091;~37
absorbing material 49 include soft rubber-like materials and
composite substances comprising plastics loaded with metal
particles. Tungsten vinyl composite materials are particularly
suitable for absorbing material 49. The properties of and
techniques for fabricating tungsten vinyl composite materials
are discussed by Lees, Gilmore and Kranz in an article entitled
"Acoustic Properties of Tungsten-Vinyl Composites" published
in IEEE Transactions on Sonics and Ultrasonics, SU-20, pages 1-2,
January 1973.
By affixing the transducers 31 directly to the
outer surface 41 of the lens 40 and by contacting the remaining
portion of the lens surface 41 with the acoustical absorbing
material 49, it is possible to preclude reflections from the
surface 41 and to eliminate reverberations between the surface
41 and the transducers 31 because substantially all of the
energy reflected from the inner lens surface 42 will be absorbed
either by one of the transducers 31 or by the absorbing
material 49. In the prior art, acoustical absorbing material
could not be used to damp out reverberations within the lens
system, because the transducers were physically separated from
the lens. The elimination of sources of internal reverberations
with concomitant improvement in signal-to-noise ratio for the
information-bearing reflected signals, and the reduction in
the number of internal reflecting surfaces with concomitant
improvement in image contrast, are significant advantages
inherent in the present invention.
With reference to FIG. 1, as a compressional
wave from a transducer 31 passes from the lens 40 into a
medium of different acoustical impedance (e.g., perhaps into
a quantity of water disposed externally of the skin 20, or
perhaps directly through the skin into the underlying blood,
fat and muscle tissues), there will be a first partial
-16-
F
lU91~37
reflection and partial transmission of the wave energy at
the surface 42. The transmitted portion of the wave energy
will thereupon proceed deeper into the body until the next
subsequent acoustical impedance discontinuity is encountered,
at which time there will be a second partial reflection and
partial transmission of wave energy. In FIG. 1, the second
partial reflection and partial transmission will take place
at the anterior wall 11 of the heart 10. There will
subsequently be further partial reflections from and partial
transmissions through the various internal structures within
the heart and the posterior heart wall 12. Ultimately, the
transmitted wave energy will be completely attenuated within
the body tissues posterior to the heart.
The signals reflected from the first reflecting
surface 42 will arrive back at the transducers 31 before the
arrival of subsequent echo signals reflected from more distant
reflecting surfaces located within the patient's body. Con-
sequently, reflections from the surface 42 are readily
discernable, and can be rejected by the processor 48 so as
not to appear in the image which is built up on a display
device 82. An electronic circuit for providing such an image
is discussed below in connection with FIG. 2.
A temporal pattern of the reflections of the
various wave fronts from the anterior wall 11 provides an
image, which is displayed electronically on the display device
82, of the movements of the anterior wall 11. Similarly, a
temporal pattern of the reflections from internal structures
within the heart, or of reflections from the posterior wall 12,
can provide an image of the movements of these structures.
An echocardiographic examination of a given patient may be
especially concerned with a particular internal structure
13 of the heart, such as a section of the myocardium or
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alternatively the aortic valve. In this case, the processor
48 can be programmed to provide a display only of reflected
signals whose arrival time at the transducers 31 indicates
that they originated at a particular depth within the body
corresponding to the approximate location of the structure 13
under examination. For such a program, all reflecting structures
located at the selected depth would appear on the display device
82, with the images of the left and right margins of the heart
(indicated by the reference numbers 52 and 52', respectively,
in FIG. 1) appearing on the left and right sides, respectively,
of the display screen, and with the superior and inferior
regions of the heart being imaged at the top and bottom,
respectively, of the display screen. This type of display
is commonly called the C-scan display mode.
If the transducers 31 are arranged in a two-
dimensional grid-like array on the outer surface 41 of the
lens 40, it is possible to obtain C-scan signals whereby a
section of the human body perpendicular to the direction of
propagation of the transmitted ultrasonic waves can be imaged
and displayed visually on the display device 82. Such C-scan
imaging is achieved by programming the processor unit 48 to
select only those reflected signals which originate from
reflecting surfaces (i.e., impedance discontinuities) located
at a particular predetermined depth within the body. C-scan
imaging provides a two-dimensional image that is representative
of the reflections occurring at a particular depth, from
either a planar or curved reflecting surface, within the body.
In the C-scan display mode, each transducer 31 supplies
information for only one picture element 89 ~n the overall
display. The position on the display image of any particular
picture element, as indicated by reference number 89, depends
upon the position on the lens of the particular transducer 31
-18-
~()91;33~7
which generates that picture element. Each picture element 89
corresponds to a point in the patient's body lying on a line
which extends from a particular transducer 31 through the center
of curvature 43 on the lens and on through the point in the body
that is being imaged. The position of the picture element 89
along such a line will depend upon the timing electronics ~-
within the processor 48. The greater the number of transducers
31 in the array, the greater will be the resolution of the image
and consequently the finer will be the detail of the picture -
displayed on the display device 82.
A curvilinear arrangement of the transducers
31 on the outer surface 41 of the lens 40, as shown in FIG. 2,
provides a B-scan image of the organ being examined. It is
noted that the linear array of transducers for B-scan imaging
may be a selected set of transducers within the two-dimensional
array provided for C-scan imaging. In other words, a single
lens and transducer array combination may be designed to be ~ -
selectively operable in either the C-scan or the B-scan mode.
Alternatively, a linear array of transducers could be mounted
on a truncated spherical lens as shown in FIG. 2 to provide a
more compact and electronically simpler acoustical lens system
than would be possible with a two-dimensional transducer array.
For B-scan imaging, the outer surface 41 of the lens 40 need
have a width that is sufficient merely to accommodate one row
of transducers 31. Thus, the surface 41 could be a section of
a cylinder whose axis passes through the center of curvature
43 of the circular cross section of the cylinder. The surface
41 could also be a section of a sphere having a center at 43.
B-scan imaging provides a two-dimensional display corresponding
to reflections from an impedance discontinuity lying in a plane
which contains the linear array of transducers 31 and the
center of curvature 43.
-- 19 --
.
337
An electronic circuit for producing a B-scan
image from a linear array of transducers 31 is shown in block
form in FIG. 2. A master programmer 83 programs a transmit-
receive multiplexer 60 to select one particular transducer
element from the array and to electrically connect the
selected transducer element to a transmitter 50 and to an
amplifier 70. The programmer 83 then activates the transmitter
50 to produce a voltage pulse that is applied to the selected
transducer element. In response to this pulse, the selected
transducer element emits an ultrasonic wave pulse that travels
through the lens 40 and on into the patient's body. Compres
sional wave reflections from the various acoustical impedance
discontinuities within the body thereupon travel back to the
same transducer element, thereby generating a separate voltage
pulse in that transducer element for each reflection, the various
voltage pulses generated in the transducer element being
separated in time in accordance with the respective depths of
the various reflecting surfaces within the body. Each voltage
pulse is amplified by an amplifier 70, and is detected by a
detector 75. Further amplification is provided by a video
amplifier 81. The output of the video amplifier 81 is used to
modulate the brightness of the image displayed on the display
device 82. The master programmer 83 also activates a sweep
generator 84 to produce a radial line 86 on the display device
82. The brightness of any given point 89 along the radial line
86 corresponds to the magnitude of the reflected signal produced
by a given acoustical impedance discontinuity. Referring to
FIG. 2, the point 87 from which all radial lines in the display
emanate corresponds to the center of curvature 43.
The above-discussed imaging process occurs
during a quiescent state of pulse transmission. The programmer
83 further programs the multiplexer 60 to select a second
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337
transducer element from the array and to electrically connect
this second transducer element to the transmitter 50 and to
the amplifier 70 after a sufficient time has elapsed to per- -
mit all reflections from the first transmitted pulse to re-
turn to the initially selected transducer element. An in-
terval of one millisecond between consecutive pulses would
be sufficient to allow all reflected signals to return to ~ -
the transducer array before a new transmitted pulse is
transmitted. The transmitter 50 then produced a voltage
pulse that is applied to the second selected transducer.
Reflections from the second transmitted pulse are then dis-
played on the display device 82 as points along another
radial line 88 emanating from point 87. Each transducer in
the array is activated in sequence; and a display of bright
points 89 is consequently built up along a series of lines
which appear as a radial sweep over the image screen of the
display device 82. After all the transducers in the array
have been activated, each in turn, by the master programmer
83, a complete frame of the anatomical organ being imaged is
displayed on the display device 82. For a linear array of
thirty-two transducers, a complete frame could be produced
approximately every 32 milliseconds for a programmed quie-
scent of one millisecond between consecutive pulses. Approxi-
mately thirty complete frames could be displayed per second,
which is sufficiently fast to provide a visually continuous
moving picture of the movements of the organ under examina-
tion--e.g., the beating of the human heart.
An alternative embodiment of this invention, as
shown in FIG. 3, uses a double set of transducers in order
to minimize the effect of reverberations. The circuitry of
FIG. 3 is identical to that of FIG. 2, except that separate
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.~
multiplexers, designated by reference numbers 61 and 62 res-
pectively, are used for transmitting and receiving. One set
of transducers 31 is used for transmission of the initial
ultrasonic signals, and another set 31' is used to receive
reflected signals. These two sets of transducers could be
affixed in two parallel rows to the outer surface of the
homocentric lens. Alternatively, the lens could be split
in half, with a sound absorbing material separating the two
halves. The major advantage of this embodiment is that
reverberations excited by the pulse transmitter would not be
directly coupled into the reflected wave receiver.
The lens 40 of FIG. 1 has heretofore been described
as a spherical homocentric lens, wherein the lens surfaces
41 and 42 are segments of spheres having a common center. The
radius of curvature of the surface 41 is constant for all
portions of thereof, and the radius of curvature of the sur-
face 42 is likewise constant for all portions thereof. An
alternative embodiment of the lens 40 is shown by reference
number 90 in FIG. 4. In this alternative embodiment, the
lens surfaces 91 and 92, rather than being segments of
spheres, are instead segments of ellipsoids. As with the
lens 40 in FIG. 1, the outer surface 91 of lens 90 is convex
and the inner surface 92 is concave in order to focus the
ultrasonic energy emitted by the transducers 31 which are
affixed to the outer surface 91. The inner lens surface 92
has a radius of curvature rl in a lateral direction orthogonal
to the orientation of the transducer array, and a different
radius of curvature r2 in the direction parallel to the
orientation of the transducer array. Similarly, the outer
lens surface 91 may have different radii of curvature for
each of two mutually orthogonal directions on the surface
thereof.
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37
With a linear array of tranducers 31 as used in
the B-scan display mode, ellipsoidal geometry may be more
advantageous than spherical geometry for the acoustic lens
because it may be desirable for the ultrasonic beam to be
broader in the direction orthogonal to the orientation of
the curvilinear transducer array than in the direction
parallel to the orientation of the curvilinear transducer
array. By choosing the radius of curvature rl of the surface
91 to be greater in the direction orthogonal to the orienta-
tion of the curvilinear transducer array affixed thereto,
ellipsoidal or reactangular transducers may more easily be
attached to the surface 91. The corresponding radius of
curvature for the inner surface 92 can be selected to provide
the desired focusing properties. The deviation in this em-
bodiment from the homocentric design is not serious because
wide aperture angles are not required in the narrow dimension
of the lens 90. The area of the ellipsoidal or rectangular
transducers 31 on the outer ellipsoidal surface 91 of the
lens 90 can be greater for ellipsoidal geometry than for
spherical geometry. The greater surface area for the crystals
results in a corresponding decrease in their electrical
impedance, thereby permitting the crystals to be driven by a
lower electrical voltage which is more compatible with
present-day solid state electronics.
As a special case of the above-described ellipsoidal
geometry, the radius of curvature of the outer surface 91 of
the lens 90 in the lateral direction orthogonal to the
orientation of the transducer array can be made infinite.
This, in effect, allows the lens to assume a cylindrical
shape, with the axis of the cylinder being orthogonal to the
acoustic axis of the lens. This cylindrical configuration
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F
.
~U~ 3~
greatly facilitates the manufacturing process for such a
lens, and provides additional sensitivity at the expense of
some degredation of lateral resolution. For B-scan imag-
ing, where laterial resolution is of no relevance, the
cylindrical lens configuration is preferred.
In the usual case/ where the outer lens surface 91
is of cylindrical configuration, the inner lens surface 92
can also advantageously be made of cylindrical configuration.
However, for particular purposes, it may be desirable to
provide a cylindrical configuration for the outer surface
91 while providing an ellipsoidal or even a spherical con-
figuration for the inner surface 92. An ellipsoidal or
spherical inner surface 92 would allow desired focusing of
the ultrasonic beam in two dimensions by appropriate choices
of values for rl and r2, while a cylindrical outer surface
91 would provide simpler transducer geometry and therefore
lower-impedance transducer elements than would be possible
with an ellipsoidal or spherical surface. The use of a
cylindrical outer surface and an ellipsoidal or spherical
inner surface for the acoustical lens therefore provides the
advantages of simplified lens manufacture, low electrical
impedance transducer elements, and increased sensitivity
resulting from large-area transducer elements.
FIG. 5 shows an aperture stop 94 placed at the
center of curvature 43 of the inner lens surface 42. The
aperture stop 94 limits off-axis reflected rays from entering
the lens 40, thereby preventing the spherical aberration
that would otherwise result. The aperture stop 94 is part-
icularly useful when scanning regions of the abdomen or other
parts of the body where the transsonant aperture is not
particularly small.
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~._
i()~ .3'7
Reverting to FIG. 1, it can be seen that each
receiving transducer 31 receives a sequence of relatively
discrete reflected signals, each signal being indicative of
the depth beneath the skin of a particular ultrasonic wave
reflecting structure within the heart 10. It should be
noted that it is not nesessary that all the transmitting
transducersbe capable of operating in a receive mode. In
fact, as discussed above, in connection with FIG. 3, it is
possible for the transmit and receive operations to be
performed separately by two different sets of transducers.
It will be appreciated that the greater the number of
receiving transducers 31 there are in the array, the more
detailed will be the overall image that can be constructed
from the reflected signals.
While the invention as described herein is part- -
icularly adapted to provide an acoustical image of an -- -anatomical organ in situ, nevertheless the principles
upon which this invention is based are of general applica-
bility with respect to acoustical imaging, and are applic-
able for nondestructive testing procedures. Therefore, the
preferred embodiment described is to be considered as
illustrative of the invention, and the scope of the invention
is limited only by the following claims.
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