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BACKGROUND OF THE INVENTION
This invention pertains to the field of examination, material
characterization and imaging of the internal structure of a body by
acoustic techniques. An important application of this field of technology
is in diagnostic medicine, wherein safe, accurate and noninvasive
examination techniques are becoming increasingly important. X-ray and
gamma ray techniques have been used for many years to obtain imagery of
the internal structure of the body. Since the dangers of radiation dosage
buildup have become better understood, the use of such techniques has been
limited to those instances in which the risk represented by the additional
X or gamma ray radiation dosage is outweighed by the need for diagnostic
information.
However, in other fields which involve examination of areas of the body
that are highly susceptible to radiation damage or are statistically prone
to form cancers, alternatives to X-ray or gamma ray radiation have been
sought. This is particularly true where examination is required on a
routine and repeated basis, such as breast examination and in the fields
of obstetrics and gynecology.
Promising developments have been made in the field of ultrasonic
examination and imaging in order to overcome these problems. Although it
is known that very high energy levels of ultrasonic energy are harmful to
the human body, low energy levels of exposure are not known to have any
harmful effects. Unlike the case with X-ray or gamma ray radiation, there
appears to be no reciprocity law (of damage equalizing the product of beam
power and exposure time) concerning exposure to ultrasound, so long as the
doses are kept below a threshold level. Fortunately, workable signal
levels fall well below the danger point. Ultrasonic scanning and imaging
devices thus hold the promise of permitting noninvasive examination of
internal body structures on a repeated and routine basis, without any
presently known, nor suspected, harmful side effects.
Although previously developed ultrasound systems have provided useful
information for physicians, they have left considerable room for
improvement in terms of resolution, repeatability of measurement, and type
and quality of data obtained.
One type of prior art ultrasound device is known as the A-scan which is
widely used in various medical fields, including echo cardiography,
gynecology and in measuring the position of the brain center line. While
not actually providing an image of the tissue, the A-scan provides a
display such as a cathode ray tube (CRT) with one axis representing time
(depth of penetration), and the other axis showing echoes or return
pulses.
Another type of ultrasonic device known as the B-scan is used in obstetrics
and cardiology. The B-scan also uses an echo mode, with the transducer
scanned laterally along the area of interest of the body. The lateral
position of the transducer is displayed on the X-axis, with the depth or
distance of the echo being displayed on the Y-axis, and the strength or
amplitude of the return echo modulating the brightness of the display. The
B-scan is subject to a number of disadvantages resulting from its use of
the pulse-echo mode. The echoes or back scatter from some delicate tissue
structures may be too weak to be received, particularly since the
reflected energy is further attenuated on travelling back through the
tissue to the receiver.
Another disadvantage of the B-scan is that the strength of a return from a
surface within the body is not only a function of the physical properties
of the surface, but also a function of its angular relationship to the
pulses. Thus a portion of the surface which happens to be at nearly right
angles to the beam gives a strong echo, while another portion of the same
surface at a different angle gives a weak return. This same effect makes
it difficult or impossible to accurately calibrate B-scan apparatus in
order to give consistent and repeatable results, from one clinical setting
to another. Slight differences in positioning of the transducer with
respect to the patient will give differing brightness levels for the same
structures within the body. This lack of consistency is due to the fact
that the B-scan does not obtain a quantitative measure of an intrinsic
tissue property, but rather measures a property which is an interaction
between the measurement system and the body. Recent B-scan methods, known
as Compound B-scanning, have employed superimposition of B-scans taken
from a number of different angles in an attempt to overcome the
above-noted limitations, but such techniques per se still do not measure
an intrinsic property of the tissue.
Another prior art acoustic imaging system known as the C-scan involves
forming a two dimensional projection for representation of the body at
right angles to the beam. In some systems, energy transmitted through the
body is received on the other side, and its amplitude serves as a measure
of the attenuation in the body. In some recent work, lensing has been
employed in an attempt to focus on a single plane within the body, but
resolution and contrast are generally poor, and it is not possible to
measure an intrinsic property of the tissue at a point in such systems.
In an echo C-scan, range gating is used to select a desired image plane in
the body, but the systems are subject to intense specular reflections
making it very difficult or impossible to obtain structural data.
Other workers have proposed the use of acoustic holography to produce
attenuation-type two dimensional projections of a body similar to the
C-scan system. However, the present lack of sensitivity of such systems
requires high input acoustic energy levels and therefore raises a possible
question of harmful effects. Such systems to date have produced acoustic
attenuation C-scan images of the reflection and transmission with their
attendant limitations as detailed earlier.
Another acoustic imaging method developed by R. C. Heyser and D. H.
LeCroisette is reported in the IEEE 1973 Ultrasonics Symposium Proceedings
(IEEE Catolog No. 73CHO 807-8SU). This method involves transmitting swept
frequency bursts through a body to a receiver on the opposite side. The
received signal is beat against the original signal, giving a frequency
difference due to the time delay of propagation. Variations in the
difference frequency are then converted to a voltage and painted on a CRT,
to give a fringe contour plot which is essentially another form of two
dimensional projection.
The above prior art systems, although promising in many respects and very
useful in some applications, have not been able to provide quantitative
data of intrinsic properties of the internal structure of a body, nor have
they been able to provide maps or images along sections passing through
the body. This latter type of data and imagery are obtainable in the X-ray
field by means of computerized tomography, tomographic reconstruction,
etc. and systems commonly called scanners. Unfortunately, however, earlier
attempts to apply the same techniques using ultrasonic energy have
produced poor results.
In the case of X-ray scanners, the body is exposed to a plurality of beams
of radiation through a number of different directions in a plane through
which the section is to be imaged, and the amplitude of the received
radiation is measured. With enough sets of data to define a matrix of
small picture elements in the body, algebraic reconstruction techinques
are used to solve for the attenuation coefficient of each picture element.
These data can then be displayed graphically by means of a CRT.
We have previously proposed a system which uses an analogous method, with a
source of ultrasonic pulses and a receiver or an array of receivers on the
other side of the body for receiving beams of ultrasonic energy
transmitted through the body. See J. F. Greenleaf, S. A. Johnson et. al.,
Acoustic Holography, Vol. 5, Plenum Press, New York, 1974, pp. 591-603. In
that system the amplitude of the received pulses are measured, and an
algebraic reconstruction technique is employed to calculate the
attenuation coefficient of the various picture elements. The resolution
and accuracy of such systems are limited by refraction of the beams within
different tissue structures, and to a greater degree by reflection of
energy both at the surface of the body and from tissue structure within
the body. Since the amount of reflection loss from a given beam is unknown
and unpredictable, there is no practical way to discriminate between the
amplitude loss in a received signal due to attenuation losses within the
body, and the losses due to reflection within the body. This imposes a
fundamental limitation upon the resolution of the system, and upon the
accuracy of the attenuation coefficient being calculated.
We have discovered that these limitations of acoustic reconstruction
measurement and imagery can be avoided by measuring the time-of-flight of
acoustic energy pulses through the body, rather than measuring amplitude
losses. Mathematical reconstruction techniques are then employed to
calculate the spatial distribution of acoustic velocities throughout the
plane of measurement in the body. Since only the time of arrival of a beam
is detected, and not its amplitude, the system is immune to reflection
problems.
The velocity data represents actual measurement of intrinsic properties of
the tissue being scanned, and has diagnostic value because different types
of tissue, both normal and abnormal, have characteristic acoustic
propagation velocities or velocity ranges. Further, the data can be
displayed by means of a CRT to give an image or map of values along the
section.
The velocity values thus obtained can be used in conjunction with
attenuation values to achieve a synergestic effect. The combination of
attenuation coefficient data and acoustic velocity data (or index of
refraction data) for each picture element in the section gives the
diagnostician far greater information than from the sets of data
individually.
The index of refraction data obtained on a first calculation can then be
used to recalculate the velocity data, or attenuation data, to take into
account the bending of ray paths through the body due to the changes in
index or refraction from point to point, so as to achieve a higher degree
of resolution.
SUMMARY OF THE INVENTION
The present invention provides a method of, and apparatus for, examining
the internal structure of a body and characterizing the material therein.
Acoustic energy is transmitted through the body from a plurality of
different directions, and the time-of-flight of the acoustic energy
through the body is measured. The acoustic velocities at a plurality of
points within the body are then mathematically reconstructed from the
time-of-flight data. The acoustic velocity, which is uniquely
determinative of the refractive index, gives quantitative information of
an intrinsic property of the material at each point within the body.
The data thus obtained can be visually displayed, as on a cathode ray tube
display or the like, to form a map or image as a sectional view through
the body along a plane of measurement.
Successive applications of the method for different frequencies of acoustic
energy may be obtained, and displayed by different colors on a color CRT
display.
Measurement of the amplitude of the acoustic energy transmitted through the
body can be made along with the time-of-flight measurements, and a
separate reconstruction can be used to calculate attenuation coefficient
data at each point within the body, for a plurality of frequencies. The
velocity data and the attenuation coefficient data can be correlated
against known values as a tool in identifying the material making up the
body at each point for which values are obtained.
The velocity data and the attenuation data can be jointly displayed, for a
fixed frequency or a plurality of frequencies, by means of a color CRT
display, to give a more meaningful map or image of the internal structure
of the body.
The index of refraction data can be used on succeeding reconstructions to
recalculate velocity and/or attenuation data to obtain higher accuracy and
resolution.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a diagrammatic view illustrating a method which can be used to
scan a body according to the present invention;
FIG. 2 is a diagrammatic view of an alternate scanning method which may be
used according to the present invention;
FIG. 3 is a view in perspective illustrating the use of a breast scanner
according to the present invention, with portions thereof broken away for
clarity;
FIG. 4 is an enlarged perspective view, with portions broken away for
clarity of a breast scanner according to the present invention;
FIG. 5 is a view in perspective of an alternate embodiment of a breast
scanner according to the present invention;
FIG. 6 is a diagrammatic view of an alternate breast scanner according to
the present invention employing cylindrical wave fronts;
FIG. 7 is a view in perspective of the manner of use of a body scanning
apparatus according to the present invention;
FIG. 8 is a diagrammatic view in elevation of an alternate embodiment of a
body scanner according to the present invention; and
FIG. 9 is a block diagram of an electrical circuit for use in the present
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Some basic principles involved in the present invention are
diagrammatically illustrated in FIGS. 1 and 2. In FIG. 1, reference
numeral 10 represents the outer periphery of a three dimensional body,
seen in cross section. A source of acoustic radiation 11 is provided on
one side of the body, and a radiation receiver 12 is provided on the other
side of the body. Both source 11 and receiver 12 lie in the plane through
which body 10 has been sectioned in the view of FIG. 1. A scan is made by
transmitting radiation along a plurality of paths 13, with source 11 and
receiver 12 being simultaneously scanned along the body as indicated by
arrows 14. The received radiation is used to measure time-of-flight
through the body as explained more fully hereinafter, and the data is
stored for computation later. After completing one scan, either the body
is rotated through a small angle, or else the source and receiver are
rotated with respect to the body, to a new position indicated by broken
lines and primed reference numbers. From the new position, radiation from
source 11' traces out a plurality of paths 13' through the body to be
received by receiver 12', while the source and receiver scan along the
body as indicated by arrows 14'.
Once the ultrasonic scanning and time-of-flight measurements have been made
according to the present invention, mathematical reconstruction techniques
as are generally known in the prior art are employed for calculating the
velocity values at each point. The general theory of mathematical
reconstruction is well developed in the prior technical literature, for
example, see R. Gordon and G. T. Herman, "Three-Dimensional Reconstruction
From Projections: A Review of Algorithms", International Review of
Cytology, 38:111-151, 1974.
Briefly, it is known that three dimensional functions can be determined
from their two dimensional projections obtained by line integrals along
paths to the three dimensional function. This mathematical process is
known as reconstruction. The line integrals themselves are obtained by
transmission of radiation through the three dimensional body along the
plane of interest, and receiving it on the other side. A matrix of
boundaries defining individual picture elements may be mathematically
established throughout the cross section. In FIG. 1, an individual
representative picture element 15 is shown. The resolution of the system
is established in part by the size and number of the picture elements,
which in turn determines the number and spacing of adjacent beam paths 13.
Each individual path represents a line integral of the value to be
measured. Any given picture element 15 is included in a plurality of such
line integrals, and if a sufficient number of sets of data are taken, the
resulting set of equations can be solved by known mathematical techniques
in conjunction with electronic data processing systems. In the case of
X-ray type scanners, and acoustic attenuation type scanners, the value to
be solved for at each picture element 15 is the absorption coefficient or
attenuation coefficient. In the present invention, the value to be solved
for at each picture element 15 is the acoustic velocity at that point,
derived from time of flight data for each path 13, 13'.
In FIG. 2, an alternative scanning geometry is shown. Instead of a pencil
beam of radiation as shown in FIG. 1, source 21 in FIG. 2 is configured to
produce a fan-shaped beam comprising individual rays 23. Receiver 22,
positioned on the opposite side of body 10 from source 21, comprises an
array of individual sensors 22a, 22b, etc. After a set of data has been
taken and stored, the source and receivers are moved to a new position,
either by rotation of the body of the sensing apparatus. In the new
position, sensor 21' projects rays 23' through the body at a new angle to
the receiver now positioned at 22'. A given picture element 25 is
intersected by a plurality of individual rays 23, 23', etc., and the
resulting line integral equations can be solved for the value at each
picture element. Different equations are involved to describe the
fan-shaped geometry of the beam rather than the parallel geometry of FIG.
1, but the solution to this problem is known in the literature. Many
alternative scanning patterns can be provided in addition to those shown
in FIGS. 1 and 2, with the choice being made largely on the basis of
simplification of hardware design, and accessibility requirements for
various intended areas of the body.
The method and apparatus of the present invention can be used for material
characterization and imaging of the internal structure of any type of body
through which acoustic energy can be propogated. It thus is applicable to
inanimate objects, as well as to biological including human bodies or
parts thereof. For example, the present invention may be useful in various
industries wherein non-destructive testing techniques are required to
ascertain information on the internal structure of an object or product.
As previously mentioned, the most important area of applicability of the
present invention that is presently contemplated is the field of
diagnostic medicine. Accordingly, the embodiments of the present invention
described in detail hereinafter are described in terms of medical
examination and imaging of the human body, but it will be understood that
the same apparatus and methods, with suitable minor modifications, can be
adapted for performing the same functions in other fields.
In the field of human medicine, an important application of the present
invention is in breast scanning. Apparatus for performing such scans is
illustrated in FIGS. 3, 4, 5 and 6. The breast scanner of FIG. 4 includes
a basin or water container having a side wall 30 and a bottom 31. A shaft
32 extends upward through the floor 31 of the basin, and suitable sealing
means, indicated by reference numeral 33 is provided for preventing water
from leaking out. A generally U-shaped transducer mounting bracket has a
horizontal portion 34 attached generally perpendicularly to shaft 32, and
a pair of upright portions 35 and 36. Ultrasonic source transducer 21 is
positioned at the top of upright 35, and a curved array 22 of ultrasonic
receiving transducers is similarly positioned atop upright 36. Array 22
includes a great number of individual transducers (not shown in FIG. 4) as
schematically indicated in the diagram of FIG. 2, with the number and
spacing of individual sensors being chosen according to the picture
element resolution of the system. Source 21 of course is adapted for
transmitting a fan-shaped beam of ultrasonic energy, and the angular
spread of the beam and the corresponding angular extent of array 22 is
selected to fully cover the width of a breast to be scanned, which is
positioned between source 21 and array 22 as indicated by broken line 40.
A stabilizing cup 41 may be provided beneath the breast to help stabilize
and support the breast in position for making a scan. Since several
minutes time may be required to make a complete scan, stabilizing cup 41
may help to prevent relative motion which would of course blur the final
image. If necessary, a small suction or adhesive device could be
incorporated into cup 41 to counteract unwanted buoyancy tendencies in
particular cases. Cup 41 is positioned on a stationary shaft 42 which is
coaxial with shaft 32. Means may be provided for vertical adjustment of
the position of stabilizing cup 41 to accommodate different cases.
The lowermost extension of shaft 32 which extends beneath the water basin
is adapted for angular and elevational adjustment as required for making a
scan. An elevational adjustment device indicated by reference numeral 43
operatively engages shaft 32 by means of any suitable mechanical linkage,
such as cylindrical rack and pinion drive 44, to adjust the elevational
position of source 21 and sensor array 22 according to the desired plane
of interest through the breast. Although the drawing in FIG. 4 suggests
the use of a motor driven rack and pinion arrangement, it will be
appreciated that this is only for illustrative purposes, and in fact any
suitable hydraulic, mechanical or electromagnetic means could be used to
accomplish the necessary vertical adjustment. Similarly, angular
adjustment of shaft 32 is provided by means of an angular adjustment
device 45 which is operatively connected to shaft 32 by means of a
suitable linkage such as gear 46 attached to shaft 32, and gear 47 which
is attached to device 45 and is elongated to accommodate varying
elevational adjustments of the sensors. Again, although a motor and gear
arrangement is suggested in FIG. 4, any means could be provided for
angular positioning of shaft 32 and source 21 and sensor array 22.
Electrical connections for ultrasonic source 21 and the sensors of array 22
may be provided by means of internal wiring in the U-shaped transducer
mount and shaft 32, and a cable 48. Alternatively, a cable may run from
the array 22 and/or transducer 21, to be loosely coiled within the basin
and pass through a sealed opening in the wall. It is understood that some
portion of the electronic support hardware such as preamplifier or
switching networks may be placed within array 22 in close proximity to the
receiving transducers.
The manner of use of the scanning apparatus of FIG. 4 will be understood
with reference to FIG. 3. In FIG. 3, a patient 50 is positioned
horizontally on the surface of a table or bench 51. The surface table 51
has a portion cut away to allow the breast 40 to be suspended in the water
inside the basin of the scanning apparatus between source 21 and sensor
array 22. The water, or saline solution, provides good ultrasonic contact
between the transducers and the skin. In FIG. 4, a portion of the table
surface and the basin of the scanning apparatus are deleted for purposes
of clarity. The patient thus positioned over the scanning apparatus can
lie stationary for the few minutes or less required for a scan.
Stabilizing cup 41 is first positioned as required, then the transducer
mounting bracket is elevationally positioned so that the plane defined by
source 21 and sensor array 22 intersects the breast in a section 10 as
required for diagnostic purposes. Source 21 is then energized and
measurements made by array 22 are recorded, for successive angular
increments around the breast until sufficient data has been obtained. If
it is then desired to take an additional section in another plane, the
transducer mounting bracket can be vertically repositioned to the new
plane of interest.
An alternate implementation of the invention makes use of the desirable
properties of cylindrical waves and cylindrical symmetry in the scanning
or imaging device. Cylindrical waves may be used to advantage in imaging
nearly any object as will be shown below but for purposes of illustration
their use is illustrated in the design of a breast scanner shown in FIG.
6.
FIG. 6 illustrates how use of cylindrical symmetry provides added
flexibility and speed to the design of a breast scanner. The need to
translate the transmitter and receiver transducers is eliminated or
minimized by the use of a two-dimensional cylindrical non-closed surface
for the transmitter and receiver.
Cylindrical waves are generated by the transmitting transducer 80 and pass
into a coupling fluid 81 such as water (or water with additives to better
match impedance or index of refraction of the object studied). Transducer
80 is a non-closed circular cylindrical surface which defines an arc of a
circle of angle denoted by 82. Angle 82 is sufficiently large to produce a
near perfect cylindrical wave at the receiver transducer 85 when no object
is present in the coupling fluid 81. Transducers 80 and 85 have surfaces
which are coaxial and their common axis of cylindrical symmetry 83 acts as
a virtual line of acoustic energy.
Transducers 80 and 85 are fixed relative to each other and can be made to
revolve around a common axis of rotation 84 which is located approximately
in the center of the object 40 to be reconstructed and studied.
Transducer 85 is composed of a multiplicity of individual receiving
transducer elements 85.sub.a1, 85.sub.a2, etc. These transducer elements
(such as 85.sub.a1) are connected to a switching network in a manner
similar to that shown in FIG. 5. Provisions are made for parallel or
serial output or a combination of serial and parallel output from
transducer 85 to a time-of-flight detector or other detection devices.
The multiplicity of rows of elements in the array of elements comprising
transducer 85 provide for data collection to reconstruct the
three-dimension object with only one complete rotation of transducers 80
and 85. Each row of elements, contained in a plane perpendicular to the
rotation axis 84, provides data to reconstruct that plane; for example the
elements 85.sub.k1, 85.sub.k2, . . . 85.sub.kx in row defines a plane
which intersects the object in plane surface 10k. In a like manner, plane
surface 10m is defined by elements 85.sub.m1, . . . 85.sub.mx. In the
figure yx (y times x) elem | | |
