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Description  |
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BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to non-destructive testing and particularly
to the non-invasive examination of soft tissue and body organs. More
specifically, this invention is directed to medical ultrasonic equipment
and particularly to pulse-echo body scanners. Accordingly, the general
objects of the present invention are to provide novel and improved methods
and apparatus of such character.
(2) Description of the Prior Art
While not limited thereto in its utility, the present invention is
particularly well suited for use in diagnostic medicine. Apparatus and
techniques which permit the non-invasive examination of soft tissue organs
are, for obvious reasons, of considerable interest. Presently available
techniques for performing "imaging" of soft tissue organs include x-ray,
nuclear medicine, thermography and, to a much lesser extent, diagnostic
ultrasound. Nuclear medicine is, of course, an invasive technique,
thermography has very limited utility and the degree of information which
can be provided by conventional x-rays is limited; i.e., x-rays are not
well suited for the imaging of soft tissues. Further, with imaging
techniques other than diagnostic ultrasound, there may be some restriction
to repeating the test if inconclusive results are obtained. In the case of
nuclear medicine, for example, an inconclusive or unsatisfactory
radioisotope scan may require the patient to be subjected to the
reinjection of the radioisotope. As an additional disadvantage thereto,
radioisotope scans and x-rays are notoriously expensive procedures.
Ultrasonic diagnostic techniques, because of the very high benefit to risk
ratios for the patient and the ability to perform imaging of soft tissue
organs that no other modality can provide, are attracting ever increasing
interest. Thus, ultrasonic diagnosis has found applicability in obstetrics
and gynecology, cardiology, neurology, ophthalmology and urology in
addition to crossing over medical disciplines with the imaging of various
internal body organs. In some situations invasive techniques for studying
the heart, such as cardiac catheterization and angiography, can be
replaced by ultrasonic techniques. Similarly, ultrasonic diagnosis has
found use in the diagnosis of mitral stenosis. The widespread utility
notwithstanding, the adoption of this modality has been impeded by
inherent limitations in the equipment previously available.
Ultrasonic diagnostic instruments operate on either a pulse-echo or Doppler
principle. The pulse-echo principle, which is primarily used for the
imaging of soft body tissue, involves the transmitting of short bursts of
ultrasonic energy and recording echoes reflected from anatomic structures
within the body. Since the time required for an emitted pulse to return as
an echo indicates the distance of the target structure from the
transducer, the "echo gram" provides both a picture of the object and a
graphic recording of any changes in the objects position. Thus, ultrasonic
diagnosis is based on the reflection of ultrasonic waves which occur at
the boundaries between different tissues within the body. A fraction of
the incident energy is reflected if there is a change in characteristic
impedence at such a boundary; impedence being defined as the product of
the density of the tissue multiplied by the velocity of sound. Although
the echoes which correspond to soft tissue boundaries have very small
amplitudes, these echoes can be detected by a receiver having the
requisite sensitivity. Energy which is not reflected travels beyond the
boundary, and may be reflected at deeper boundaries. The maximum
penetration is limited by the attenuation of the ultrasonic wave in
passing through the tissues; attenuation being defined as the decrease in
intensity of the sound pulse per unit of distance as it propagates in the
medium and loses energy as the result of absorption and scattering.
Ultrasonic diagnostic instruments employ a transducer which converts
electrical signals into acoustic pulses which are coupled into the tissue
of the patient. The transducer may also serve the dual function of
receiver for detecting the reflected pulses from within the patient. The
transducers employed in ultrasonic body scanners are typically
piezoelectric elements comprised of ceramic materials such as synthetic
lead zirconate titanate. An ultrasonic diagnostic instrument will also
comprise an oscillator which establishes the pulse repetition frequency
and a linear power amplifier which excites the transducer through a
coupling circuit. A decoupler permits the transducer to be used as both a
transmitter and a receiver. The received pulses; i.e., the echoes returned
from within the patient's body; are converted into electrical signals in
the manner known in the art, these electrical signals are processed and
the processed signals are presented on a display. The display will
typically be a cathode ray tube and the oscillator which controls the
transducer may also be employed to generate a time base trace for the
display.
In order to obtain maximum utility from the instrument, two-dimensional
images of various organs or body regions of interest must be generated.
This can be accomplished by "scanning" wherein the transducer is moved
back and forth. In the prior art the most common method of scanning
involves contact scanning in which the transducer is placed directly on
the patient's skin and moved, through a type of compound scan, in stepwise
fashion. The information obtained must be optimized through coordinated
movement of the transducer to achieve a meaningful image. Accordingly, a
high degree of skillful operator interaction with the instrument is
essential for a successful ultrasonic examination employing prior art
equipment and it has been exceedingly difficult to duplicate initial test
results since repeatability was almost totally dependent upon operator
placement of the transducer.
The high degree of operator skill required and the extreme difficulty in
repeating test results have, in part, been a consequence of the use of
small size contact transducers; this small size resulting from the
necessity of fitting the transducer to the contour of the skin. Prior art
ultrasonic body scanners, as a consequence of their use of small contact
transducers, were also characterized by slowness of use since the ability
to find the area of interest was limited to trial and error scans. It is
to be noted that the small size of the transducers, the slowness of the
procedure and the difficulty in obtaining repeatability was also
attributable to the fact that the prior transducers and associated
apparatus lacked both the ability to electronically focus the "beam" of
ultrasonic energy over the entire examination depth of interest and the
ability to easily aim the "beam".
Prior ultrasonic diagnostic equipment has also been characterized by
insufficient resolution over the desired examination range in the body;
this examination range or field of examination typically being on the
order of 20 centimeters. In order to be practical, an ultrasonic
diagnostic device must have the ability of providing real time images of
high resolution. The required characteristics, which result only from
minimizing beam width and side lobes, have been lacking in the prior art.
A further deficiency of prior art ultrasonic body scanners has resided in
their poor dynamic range. The returns or echoes from the signal propagated
into the body may vary over a range of 100 db. It is impossible to record
the 10.sup.6 shades of gray which correspond to a 100 db range. It is,
accordingly, prior practice to compress the signals generated by echoes
through either the use of logarithmic amplifiers or by simple time gain
compensation circuits. Using the prior art compression techniques,
however, important information contained in the received signals has been
lost.
With particular respect to the transducers employed in the prior art,
briefly noted above the transducers previously used have not been capable
of being focused electronically to achieve variable examination depth.
Prior art transducers have typically been of a flat contour; i.e., have
had no natural focus; and accordingly have been characterized by large
magnitude side lobes which give spurious signals from off-axis targets and
also result in ambiguity in range measurements because of the different
path lengths for the echoes from the same object.
SUMMARY OF THE INVENTION
The present invention overcomes the above briefly discussed and other
deficiencies and disadvantages of the prior art and in so doing provides
"echo grams" which are real time images of high resolution. The apparatus
which permits the achievement of such high quality images is characterized
by the ability to dynamically focus the beam of ultrasonic energy and to
scan the beam in multiple planes with a single placement of the ultrasound
transducer relative to a body to be examined. Dynamic focusing and
multiple plane scanning, in turn, provide repeatability of results since
the results of an examination are not totally dependent on operator
placement of the transducer.
In a preferred embodiment, a generated beam of ultrasonic energy will be
focused at a preselected depth and will be scanned at a first rate along a
line or plurality of "parallel" lines. When an area of interest is
observed, the beam will be caused to rescan such area while its focal
length is varied.
Also in accordance with a preferred embodiment, the ultrasonic energy is
coupled to the patient via a liquid path. Scanning of the beam is
accomplished by inserting a steerable acoustical mirror in the liquid
path.
Apparatus in accordance with the invention may also employ a transducer
having a natural focal length; i.e., a shaped piezoelectric crystal; which
is actively driven. The crystal, in accordance with one embodiment, has a
plurality of coaxial annular electrodes which are individually driven.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention may be better understood and its numerous objects and
advantages will become apparent to those skilled in the art by reference
to the accompanying drawing wherein like reference numerals refer to like
elements in the various figures and in which:
FIG. 1 is a perspective view of a first embodiment of an ultrasonic body
scanner in accordance with the present invention;
FIGS. 2A and 2B are respectively front plan and cross-sectional side
elevation views of a first embodiment of a transducer crystal which may be
employed in the apparatus of FIG. 1;
FIG. 3 is a rear plan view of a second embodiment of a transducer crystal
which may be employed in the apparatus of FIG. 1;
FIGS. 4A and 4B are respectively front and rear plan views of a further
embodiment of a transducer crystal which may be employed in the apparatus
of FIG. 1;
FIG. 5 is a cross-sectional side elevation view of the transducer head of
the apparatus of FIG. 1;
FIGS. 6 and 7 are respectively top and side elevation views of the beam
scanning control mechanism of the apparatus of FIG. 1;
FIGS. 8A-8C inclusive comprise a functional block diagram of an electrical
control circuit for the apparatus of FIG. 1;
FIG. 9 is a schematic illustration of the electrical focusing of an
ultrasonic transducer in accordance with the present invention; and
FIG. 10 is a wave form diagram related to the control circuitry of FIG. 8.
DESCRIPTION OF THE PREFERRED EMBODIMENT
With reference now to FIG. 1, an ultrasonic body scanner in accordance with
a first embodiment of the present invention is shown. The body scanner
includes a head assembly, indicated generally at 10, which houses a
transducer and means for controlling the scanning of the beam generated by
the transducer. The present invention employs a transducer which
preferably has a comparatively large area. Also, the transducers employed
in the present invention are preferably also characterized by having a
natural focal length. Since a large area transducer will not normally
conform to the contour of the patient's body, flexible means must be
provided to couple the ultrasonic energy from the transducer into the
patient. In the disclosed embodiment the coupling means comprises a
flexible bladder 12 filled with a liquid such as water and/or other liquid
with low sound absorption qualities. As will be seen from FIG. 5, the
front face of the actual transducer element is immersed in the fluid
within bladder 12. This offers the important advantage of permitting high
speed sector scanning without repositioning the head with respect to the
patient. Also, separation of the transducer from the body permits focusing
of the beam of ultrasonic energy close to the patient's skin line.
Head 10 is supported on the free end of an articulated arm which has been
indicated generally at 14. The construction of arm 14 is such that head 10
may be raised, lowered, moved toward or away from the support column 16
for arm 14 and pivoted about the axis of the support column 16. Thus, head
10 has six degrees of freedom of motion whereby any plane in the body can
be visualized and objects such as veins can be tracked as they course
through the body. Head 10 is mounted from arm 14 via a pair of yokes 22,
24 which permit a limited degree of movement of the head with respect to
the arm so as to permit optimizing the contact between bladder 12 and the
patient. A control panel 18 is also mounted on arm 14 immediately above
head 10. Through use of controls on panel 18 the operator may select the
scanning mode of the transducer within head 10 and particularly the rate,
area and depth of the scan. The operator may also, via control panel 18,
control the taking of photographs of regions of interest. The movements of
head 10 are physically controlled by the operator; i.e., the position of
the scanning head on the patient is manually changed by steering the head
through use of a pair of handles 20, 20'.
A monitor 26, which may be a conventional television receiver, is mounted
from the support column for the arm mechanism 14 as shown. The monitor 26
is positioned so as to provide the operator with a visual presentation of
the area being scanned whereby the operator will be assured that the head
is properly positioned and will be given information which will enable him
to change scanning modes, for example from a fast to a slow scan, and to
activate the camera.
The power supplies and control circuitry necessary for operation of the
body scanner, as well as the circuitry for processing received signals,
are mounted in a pair of equipment cabinets 28 and 30. Cabinet 30
includes, in addition to a main control panel, an "A" scan scope which is
typically a cathode ray tube. Scan scope 32 displays the raw signal
produced by the transducer in head 10 in response to the receipt of
echoes; i.e., the scope 32 displays echo amplitude versus time (depth).
The "A" scan provides information to the operator which is initially
employed for adjusting the gain controls of the apparatus so as to achieve
equal amplitude for signals commensuate with echoes received over the
entire range of depth of examination. The "A" scan also provides
information, not easily ascertainable from photographs, as to the
magnitude of the received echoes. This information may be of interest in
interpreting the results of an examination. In response to the information
provided on scope 32, typically at the beginning of an examination, the
operator will adjust a time gain control which is actually a curve shaping
control. Equipment cabinet 30 includes a further cathode ray tube and a TV
camera tube which are employed to produce and transmit a two-dimensional
image back to the TV monitor 26. The signal displayed on the cathode ray
tube in cabinet 30 will be the raw signal from the transducer processed so
as to give a two-dimensional body scan. A camera 34 is mounted on cabinet
30 so as to permit the making of a permanent record of the results of the
scan; camera 34 taking a picture of the display on the cathode ray tube
and being controlled from panel 18 as discussed above.
Referring now to FIGS. 2-4, various transducers which may be employed in
the practice of the present invention are depicted. The transducers are
fabricated from a wafer or disc of piezoelectric material, typically a
ceramic such as lead zirconate titanate, and preferably have the common
characteristic of a concave front or emitting surface and a convex rear
surface as may be seen from FIG. 2B. The transducer could, however, be a
flat crystal having a concave acoustical lens bonded to the front surface
thereof. In such case the lens would typically be fabricated from a
plastic including a suitable filler which gives the desired propagation
velocity. The lens material should also have an absorption coefficient of
zero, an impednace which is matched to that of the crystal and a
refractive index which is not unity. Thus, the transducers employed in the
preferred embodiment of the invention have a natural focus which will
typically be 30 centimeters from the center of the transducer. In order to
permit energization of the transducers by the application of an electrical
signal, to thereby generate a sound pulse, all or portions of the opposed
faces of the ceramic wafer must be coated with electrically conductive
material. The presence of such electrodes on the transducer also permits
the sensing of electrical signals generated by the piezoelectric material
in response to pressures applied to the material commensurate with
received echoes.
FIG. 2 depicts a transducer electrode configuration particularly well
suited for the imaging of tumors. In the FIG. 2 embodiment the entire
front surface of the tuned piezoelectric wafer 40 is coated with a layer
of conductive material 42 as may be seen from FIG. 2B. As best seen from
FIG. 2A, the opposite or back surface of wafer 40 is, with the exception
of a pair of annular regions, also completely coated with the electrode
material as indicated at 43. These two annular regions define, in their
centers, a pair of discrete electrodes 44 and 46. Electrode 44 is disposed
on the axis of the transducer while electrode 46 is displaced from the
axis. The discrete electrodes 44 and 46 are employed only in the receiving
mode.
FIG. 3 depicts, in a plan view, an electrode configuration which may be
employed for both the front and back surfaces of the shaped piezoelectric
wafer. The electrode of FIG. 3 is characterized, extending outwardly from
the axis thereof, by a plurality of concentric rings 49 of electrode
material; the circles of conductive material being aligned on the opposed
faces of the transducer. Use of a plurality of concentric circles of
electrode material on at least the back surface of the piezoelectric wafer
permits the electronic focusing of the transducer whereby the examination
depth may be varied about the natural focal length of the transducer. Also
in accordance with the FIG. 3 embodiment, the outer electrode ring 47 is
segmented. The segmented electrode is employed for receiving purposes
only. It has been found that, to obtain optimum results, the width of ring
47 should be less than ten wavelengths of the transmitted ultrasonic
energy and should preferably be about three wavelengths.
As an alternative to the electrode arrangement depicted in FIG. 3, it is
possible to employ an annular lead zirconate titanate crystal, having a
width which corresponds to one-half the wavelength of the transmitted
ultrasonic signal, on which the segmented electrodes are deposited. A
separate transmitting transducer would, in this case, be mounted on the
axis of such an annular transducer.
Considering FIG. 4, a further embodiment of a transducer for use in
accordance with the present invention is depicted. In the FIG. 4
embodiment, as shown in FIG. 4A, the entire front surface of the tuned or
focused piezoelectric wafer is covered with electrode material. The rear
surface of the transducer, as may be seen from FIG. 4B, is identical to
the electrode configuration of FIG. 3 with the exception that the
segmented outer electrode is omitted. The focal length of the transducer
of FIG. 4 may be varied electronically about the natural focal length.
The discrete electrodes of the FIG. 2 embodiment may be incorporated into
any of the other disclosed transducer embodiments.
To discuss further the above described transducer configurations, in pulsed
ultrasonic body scanning, and particularly in B-mode scanning wherein the
object is to intensify reflection points on a cathode ray tube, a short
burst of ultrasonic energy will be transmitted by the piezoelectric
transducer into the body in the form of a narrow beam. As this beam
progresses into the body it encounters tissue substances and interfaces
between tissue structures which scatter back some of the incident energy.
The returning signals or echoes are converted back into electrical signals
by a receiver transducer which may be the same element as the transmitting
transducer. The nature of the echoes scattered back in the direction of
the receiver transducer is extremely varied and depends upon the nature of
the tissues responsible for the scattering. A tissue structure which is
small compared to the wavelength of the insonifying energy will scatter
energy equally in all directions; i.e., there will be diffuse
reflectances. Tissue structure which is large and flat, however, will
function as a mirror and reflect the incident energy in one direction;
i.e., specular reflectances will result. Generally, body tissue will
exhibit a combination of diffuse and specular reflectance patterns. It has
long been desired in the art to enhance the diffuse reflectances relative
to the specular reflectances. Such enhancement of the diffuse reflectances
will produce an improved image as the specular echoes tend to be one or
two orders of magnitude larger than the diffuse reflectances. Enhancement
of diffuse reflectances has not previously been successfully accomplished
in ultrasonic scanners intended for medical purposes partly because the
large specular echoes overpower and obliterate the weaker diffused echoes
in both the penetration depth; i.e., range; and asmuth directions. This
obliteration, in part, is a result of the presence of side lobes in the
transmitted signal and thus in the echoes; side lobes being inherently
present in a diffraction limited energy beam and resulting from fringing
effects. Thus, ultrasound directivity patterns not only have a central
main lobe, through which most of the energy returns, but also include side
lobes through which off-axis signals may enter. If the main lobe is
directed towards a weak echo, a stronger off-axis signal such as a
specular echo may enter the side lobes and completely obliterate the small
signal that is on-axis. It is also to be noted that very large echoes can
saturate the amplifiers coupled to the receiver transducer thereby
resulting in the "blooming" of the display on the cathod ray tube with the
resultant obliteration of small signals.
In accordance with one embodiment of the present invention the diffuse
reflectance may be enhanced through the use of a separate receiver in the
form of a segmented annulus. As discussed in the description of FIG. 3,
the electrode annulus representing the receiver may be divided into
several elements by segmenting an outer electrode ring on both sides of
the transducer into corresponding sections. The electrodes are connected
cyclically; i.e., a "first" rear electrode is connected to a "second"
front electrode, the "second" rear electrode is connected to a "third"
front electrode, etc. The electrodes are thus electrically interconnected
in a series aiding configuration. When reflected energy is received
equally by all regions of the annulus, characteristic of a diffuse
reflectance, voltages which add together are generated. A specular echo,
if it strikes the annulus at all, will strike only one or two regions
where segments of the receiver ring are present. Thus, by way of example,
if there are ten elements in the receiver, a diffuse echo would generate a
voltage ten times higher than an equal magnitude specular echo impinging
on a single segment.
It is to be understood that, rather than connecting all of the elements in
series, each pair of front and rear electrodes may be coupled to a
separate amplifier and the amplified signals processed as desired. By way
of example, the amplified signals from all of the electrodes could be
averaged to produce a result which would be the same as a series aiding
configuration. Also, if more complex signal processing is desired, signals
commensurate with all of the echoes being of approximately the same
magnitude could be accepted for display while the display would be
inhibited if there was a wide discrepancy in the amplitude of the signals
received at the segments around the annulus.
In accordance with another embodiment, which has achieved exceptionally
good results, all of the electrode rings of either of the transducers of
FIGS. 3 and 4 may be used as both transducing and receiving elements. The
use of all electrode rings for focusing in a transducing mode and for
receiving is particularly advantageous when employing the transducer
configuration of FIG. 4.
Referring now to FIG. 5, the transducer portion of the head 10 of the
ultrasonic body scanner depicted in FIG. 1 is shown in cross-section. The
principal elements of the transducer subassembly are the sonic generator
and a scanning mirror. The sonic generator includes a focused
piezoelectric crystal 40 while the mirror has been indicated at 50. Both
the sonic generator and mirror are mounted within a housing 52 which is
preferably molded from an elastomer or other plastic having suitable sound
absorption characteristics. The lower end of housing 52, as the apparatus
is shown in FIG. 6, will be mated to the bladder 12 which is brought into
contact with the patient. Housing 52 will be filled with a suitable sound
transmitting liquid and the front or transmitting face of crystal 40 and
the face of mirror 50 will be immersed in this liquid. The "tubular" lower
portion of housing | | |
