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Description  |
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BACKGROUND OF THE INVENTION
This invention relates to ultrasonic systems and, more particularly, to
apparatus for imaging sections of a body by transmitting ultrasonic energy
into the body and determining the characteristics of the ultrasonic energy
reflected therefrom.
During the past two decades ultrasonic techniques have become more
prevalent in clinical diagnosis. Such techniques have been utilized for
some time in the field of obstetrics, neurology and cardiology, and are
becoming increasingly important in the visualization of subcutaneous blood
vessels including imaging of smaller blood vessels.
Various fundamental factors have given rise to the increased use of
ultrasonic techniques. Ultrasound differs from other forms of radiation in
its interaction with living systems in that it has the nature of a
mechanical wave. Accordingly, information is available from its use which
is of a different nature than that obtained by other methods and it is
found to be complementary to other diagnostic methods, such as those
employing X-rays. Also, the risk of tissue damage using ultrasound appears
to be much less than the apparent risk associated with ionizing radiations
such as X-rays.
The majority of diagnostic techniques using ultrasound are based on the
pulse-echo method wherein pulses of ultrasonic energy are periodically
generated by a suitable piezoelectric transducer such as a lead
zirconate-titanate ceramic. Each short pulse of ultrasonic energy is
focused to a narrow beam which is transmitted into the patient's body
wherein it eventually encounters interfaces between various different
structures of the body. When there is a characteristic impedance mismatch
at an interface, a portion of the ultrasonic energy is reflected at the
boundary back toward the transducer. After generation of the pulse, the
transducer operates in a "listening" mode wherein it converts received
reflected energy or "echoes" from the body back into electrical signals.
The time of arrival of these echoes depends on the ranges of the
interfaces encountered and the propagation velocity of the ultrasound.
Also, the amplitude of the echo is indicative of the reflection properties
of the interface and, accordingly, of the nature of the characteristic
structures forming the interface.
There are various ways in which the information in the received echoes can
be usefully presented. In one common technique, the electrical signal
representative of detected echoes are amplified and applied to the
vertical deflection plates of a cathode ray display. The output of a
time-base generator is applied to the horizontal deflection plates.
Continuous repetition of the pulse/echo process in synchronism with the
time-base signals produced a continuous display, called an "A-scan", in
which time is proportional to range, and deflections in the vertical
direction represent the presence of interfaces. The height of these
vertical deflections is representative of echo strength.
Another common form of display is the so-called "B-scan" wherein the echo
information is of a form more similar to conventional television display;
i.e., the received echo signals are utilized to modulate the brightness of
the display at each point scanned. This type of display is found
especially useful when the ultrasonic energy is scanned transverse the
body so that individual "ranging" information yields individual scan lines
on the display, and successive transverse positions are utilized to obtain
successive scan lines on the display. This type of technique yields a
cross-sectional picture in the plane of the scan, and the resultant
display can be viewed directly or recorded photographically or on magnetic
tape.
While successes have been achieved in the field of ultrasonic imaging,
there are a number of problems which need to be overcome in obtaining high
quality ultrasonic images in a convenient, reliable and cost-effective
manner. Regarding problems which have been partially overcome, it is
known, for example, that ultrasound is almost totally reflected at
interfaces with gas. This has led to the use of coupling through a fluid
such as water or the use of a direct-contact type of transducer. The
latter technique may give rise to problems when attempting to image
structures such as arteries which may be only a few millimeters below the
skin surface, the contact imaging causing aberrations in the near field of
the transducer. Coupling through a fluid offers advantage over
direct-contact in this respect, but leads to various design problems and
cumbersome generally non-portable structures which are inconvenient to
use, especially when attempting to register them accurately on a patient.
Some techniques involve immersing the patient in water or obtaining
appropriate contact of the body part with a bulky water tank wall.
The need to scan the ultrasonic beam in two dimensions has added to the
problems of conciseness and cost-effectiveness. One class of techniques
utilizes electronic beam steering wherein a multi-element array is
utilized in conjunction with dynamic focusing circuitry employing variable
delay elements. This technique is generally complex, especially when used
to achieve scanning in two dimensions. Also, at the necessary bandwidths,
it is found that the delay elements in such circuitry, including numerous
fixed delay elements, become physically cumbersome and add cost, size, and
weight to the scanning units.
The above factors, inter alia, contribute to the difficulty in obtaining an
ultrasonic imaging apparatus which is convenient to use, portable,
relatively cost-effective, and has these attributes without sacrificing
necessary operational characteristics. It is among the objects of this
invention to provide such an apparatus and to offer solution to prior art
problems as set forth.
SUMMARY OF THE INVENTION
The present invention is utilized in an ultrasonic apparatus for imaging
sections of a body by transmitting ultrasonic energy into the body and
determining the characteristics of the ulrasonic energy reflected
therefrom. Such an apparatus typically includes timing means for
generating timing signals, energizing/receiving means alternately
operative in response to the timing signals, and dislay/record means,
synchronized by the timing signals, for displaying and/or recording
image-representative electronic signals from the energizing/receiving
means.
In accordance with the invention there is provided a portable scanning
module, suitable for being hand held, which comprises fluid-tight
enclosure having a scanning window formed of a flexible material. A
transducer is provided for converting energy from the energizing/receiving
means to periodic ultrasonic energy and for converting reflected
ultrasonic energy to electrical signals, the transducer having a plurality
of defined segments. A variable delay means is coupled between the
segments of the transducer and the energizing/receiving means, the
variable delay means being responsive to the timing signals to effect
electronic focusing of signals coupled therethrough. A focusing lens is
coupled to the transducer. Fluid means, such as water, is contained in the
enclosure in the volume between the focusing lens and the scanning window.
A reflective scanning means is disposed in the fluid means between the
lens and the window, and driving means, synchronized with the timing
signals, is provided for moving the scanning means in periodic fashion.
In a preferred embodiment of the invention the focusing lens has a general
contour which effects focusing at or from a point in a range of foci, the
lens having a number of geometrical steps in its general contour
substantially overlaying the positions of at least some of the transducer
segments. In this embodiment, the geometrical steps introduce fixed
propagation delays as a function of their step height and as a function of
the index of refraction of the lens material with respect to the index of
refraction of the fluid.
In accordance with a further feature of the invention, the reflective
scanning means comprises a supportive surface and a thin sheet of
ultrasonically-transmissive material mounted over the surface. A gaseous
region is located between the supportive surface and the material. The
transmissive material is typically a thin transparent foil and the
supportive surface is sufficiently rough such that the material contacts
the surface only at a relatively small percentage of its surface area. In
this manner, the gaseous region, typically a microscopic layer of air,
causes virtually total reflection of the ultrasound regardless of
considerations of critical angle at incidence.
Further features and advantages of the invention will become more readily
apparent from the following detailed description when taken in conjunction
with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates the operation of a scanning apparatus which employs the
improvements of the invention.
FIG. 2 shows a cross-sectional view of a portion of the scanning module
along with diagrams of portions of circuitry therein and in the
accompanying console.
FIG. 3 illustrates, in further detail, the focusing lens 90 shown in FIG.
2, illustrated in conjunction with the segmented transducer and variable
delay circuitry.
FIG. 4 shows a microscopic view of a portion of the reflective scanner of
FIG. 2.
FIG. 5 illustrates an embodiment of the invention wherein a pair of
transducers and lenses are utilized in a scanning module to obtain a beam
having an even larger total aperture.
FIG. 6 illustrates a portion of a further embodiment of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, there is shown an illustration of a scanning apparatus
which employs the improvements of the invention. A console 10 is provided
with a display 11 which may typically be a cathode ray tube
television-type display, and a suitable control panel. A video tape
recorder or suitable photographic means may also be included in the
console. The console wll also typically house power supplies and portions
of the timing and processing circuitry of the system, to be described. A
portable scanning module or probe 50 is coupled to the console by cable
48. In the present embodiment the probe is generally cylindrical in shape
and has a scanning window 51 near one end, the scanning window being
defined by protruding flexible material, which may typically be silicone
rubber. During operation of the apparatus, the probe 50 is hand held to
position the scanning window over a part of the body to be imaged. For
example, in FIG. 1 the probe is positioned such that a cross section of
the heart will be obtained. Imaging of other portions of the body is
readily attained by moving the probe to the desired position and
orientation, the relative orientation of the scanning window determining
the angle of the cross section taken.
Referring to FIG. 2, there is shown a cross-sectional view of a portion of
the scanning module or probe 50 along with diagrams of portions of the
circuitry therein and in console 10 used in conjunction therewith. An
enclosure, which may be formed of plastic, consists of a front cover 52
which defines the fluid-tight compartment, and a rear cover 53 which
houses at least a portion of the system electronics. Both covers are
generally cylindrical in shape and fit, with suitable seals, over a
cylindrical inner housing 54 having an annular rim 55. The inner housing
holds a segmented transducer 80 and a focusing lens 90, to be described.
The scanning window 51 is defined by a generally rectangular opening in
the side of cover 52, the opening having a slightly protruding lip on
which is mounted a flexible material 56, which may be a silicone or
neoprene rubber membrane. The volume of the enclosure defined by front
cover 52 is filled with a fluid 57, for example water. The membrane 56
will accordingly conform in shape to the surface of a body portion with
which it is placed in contact, thereby minimizing the possibility of an
undesirable reflective liquid-air interface.
A reflective scanner 70, which is flat in the present embodiment but which
may be curved to provide focusing if desired, is disposed in the fluid 57
between the lens 90 and the scanning window 51. The scanner 70 is mounted
on a shaft 71 (perpendicular to the plane of the paper) which passes
through a suitable seal in cover 52 and is connected to a small electric
motor 72 which is mounted on the outside of cover 52 and provides the
desired ocsillatory or sawtooth motion. The motor 72, which is shown in
dashed line in the FIGURE, may be provided with separate small cover (not
shown) which constitutes a protrusion on the cover 52 or an irregular
outer shape can be avoided by providing a secondary larger outer shell
(not shown).
The segmented transducer 80 is coupled to variable delay circuitry 100
which includes variable delay elements (and may include some fixed delay
elements, as will be described hereinbelow) whose values are controlled by
phase control circuitry 120. A pulser/receiver circuit 130 alternately
provides energizing pulses to and receives echo signals from the segmented
transducer 80, both these operations being coupled through the variable
delay circuitry 100 which provides dynamic focusing, in known manner. The
receiver portion of circuit 130 includes a preamplifier which is coupled
through gain control amplifier 140 to display 11 and recorder 160, which
may be any suitable recording or memory means, such as a video tape
recorder. The gain control circuitry may include interactive gain
compensation ("IGC"), which is shown as block 141 and described in detail
in copending U.S. Application Ser No. 569,185 filed Apr. 18, 1975, now
U.S. Pat. No. 4,043,181, and assigned to the same assignee as the present
application. This circuitry compensates the amplitude of later arriving
signals for attenuation experienced during passage through body tissue and
losses due to prior reflections. Timing circuitry 170 generates timing
signals which synchronize operation of the system; the timing signals
being coupled to the previously described circuiry and also to sweep
circuitry 180 which generates the signals that control the oscllations of
scanner 70 and the vertical and horizontal sync signals for the display 11
and recorder 160.
In broad terms, operation of the system is as follows: Upon command from
the timing circuits the pulser in circuitry 130 generates pulses which
excite the segments of transducer 80 via a portion of the variable delay
circuitry 100 (or a second separate set of delay circuitry -- not shown)
which is, in turn, controlled by phase control circuitry 120. If all
segments of the transducer were excited at the same time (i.e., in phase),
the ultrasonic energy would be focused approximately at a point determined
by the focusing properties of lens 90; e.g. the point P in the body 5 as
shown in FIG. 2, with the dashed lines depicting the beam outline. As is
known in the art, the depth of focus can be varied electronically by
imparting predetermined delays or phase changes to different segments of
the transducer 80. When the ultrasound pulse is launched the variable
delay circuit is set so that the transmitted beam is focused at the point
Q which is the deepest (furthest) point at which the image is being
sought.
When the focused beam for a given scanline has been transmitted toward the
body, the timing circuitry causes the pulser/receiver 130 to switch into a
"receive" or "listen" mode and also activates a reverse cycle of the phase
control circuitry 120. Now, the transducer segments serve to convert the
received ultrasound energy into electrical signals which are combined in
proper phase relationship for focusing on particular reflection
origination points in the range of depths being investigated. For example,
at an instant of time t after the pulse was transmitted, it is known that
the arriving echoes will be coming from a depth of about t/2C (where C is
the propagation velocity -- assumed, for ease of illustration, to be the
same in the body as in the fluid 57). For such echoes the arrival phases
at each of the transducer segments can be predetermined. Thus, at a time t
the phase control circuitry provides these time delays at each transducer
segment. For a "B-scan" display, a sweep over the range of depths
corresponds to a horizontal scanline of the display, so the timing signals
from circuitry 170 synchronize the horizontal sync of the display with the
phase control circuitry 120 providing appropriate delays as the successive
echoes arrive from increasing depths until one entire "scan line" is
completed, typically from the patient's skin up to a fixed preselected
depth in the body. The second dimension of the desired cross-sectional
image is attained by a slower mechanical scan of scanner 70 which is
synchronized with the vertical sweep rate of the display and recorder by
the sweep circuitry 180. The mechanical scanning range is illustrated by
the double-headed arrow 7.
The received signals are coupled through the preamplifier in
pulser/receiver 130 and through gain control amplifier 140 to display 11
wherein the signals modulate the brightness of the scanning raster to
obtain the desired cross-sectional image. The signals are also recorded on
the video tape recorder 160.
Referring to FIG. 3, there is illustrated in further detail the focusing
lens 90 in accordance with the invention, shown in conjunction with the
segmented transducer 80 and a portion of the variable delay circuitry 100,
also in further detail. The transducer 80, typically formed of a
piezoelectric ceramic, is segmented into a circular center disc 81 and
isolated annular rings 82, 83 and 84. Segmented transducers of this form
are known in the art and, while only four segments are shown for ease of
illustration, it will be appreciated that a larger number of segments can
be employed. The lens 90, which preferably has a relatively flat surface
opposing the transducer, may be a bonded lens or attached to the
transducer using a thin layer of gel. The lens is preferably formed of a
plastic material selected in accordance with the principles set forth in
co-pending U.S. application Ser. No. 515,352 filed Oct. 16, 1974, now U.S.
Pat. No. 3,958,559, assigned to same assignee as the present application.
As disclosed therein, by selecting the lens material in accordance with
specified parameters, "apodization" is achieved; i.e., undesired output
side lobes, caused by factors such as finite transducer size, are
minimized. Further, in accordance with the last referenced co-pending U.S.
application, the lens 90 has a generally elliptical contour, as depicted
by dashed line 91, which provides desired focusing in the manner described
in said co-pending application.
The variable delay circuitry 100 includes variable delay units 101, 102 and
103 connected in a series arrangement. Each unit may comprise conventional
components for obtaining variable delay including, for example, a varactor
diode. The illustrated series arrangement constitutes a multiple-entry
analog tapped delay line with lines 101A, 102A, 103A and 104A being
coupled from the points shown to metalization electrodes on the respective
transducer segments 81, 82, 83 and 84. Lines 101B, 102B and 103B, which
originate from phase control circuitry 120, are coupled to the units 101,
102 and 103 and serve to control the amount of delay provided by each
unit. The summed output is taken at one end (either at line 101A), as
shown in the FIG., or at 104A).
To better understand the nature of this improved feature of the invention,
fixed delay units 106, 107 and 108 are shown as being present in the lines
102A, 103A and 104A, respectively, consistent with the output being at
101A. During operation, as previously described, the phase control
circuitry 120 generates signals, which appear on the lines 101B, 102B and
103B, these signals causing appropriate variation of the delay units 101,
102 and 103, in known manner, to provide electronic focusing of the beam.
In the prior art, the fixed electronic delays 106, 107 and 108, are
generally required in conjunction with the illustrated variable delays.
The size of these fixed delays will depend on various system parameters,
with typical values being in the range of tens of nanoseconds to a few
microseconds. At the necessary bandwidth (of typically several megahertz
in frequency), these fixed delays involve substantial size and cost,
especially where a relatively large number of transducer segments are
utilized. In the present invention, as seen in the FIGURE, the need for
some or all of the fixed electronic delays is eliminated by introducing
appropriate ultrasonic wave propagation delays which are achieved by
providing discrete geometrical steps in the general contour of the lens.
Each step overlays the position of one of the transducer segments and
produces a delay which depends on the step height and the index of
refraction of the lens material with respect to the propagation medium.
Stated another way, since the velocity of ultrasound in the propagating
fluid 57 (water, in this case) is less than the velocity of ultrasound in
the lens material (plastic, in this case), delay is introduced by causing
the ultrasonic energy to travel further in the slower medium. Of course,
depending on the choice of lens material and propagating medium utilized,
as well as whether "positive" or effective "negative" delay is desired,
the steps in the lens can rise toward the center (as shown) thereof or
toward the outer edge. In FIG. 3 the transducer segments 82, 83 and 84
respectively have one, two and three units of delay associated with them.
For a particular design configuration, each unit step was calculated as
providing 150 nanoseconds of delay, so the "fixed" delays associated with
the lines 106, 107 and 108 for this case would be 150 ns., 300 ns. and 450
ns., respectively.
Referring to FIG. 4, there is shown a microscopic view of a portion of the
surface of the reflective manner 70 of FIG. 2. The scanner is designed in
a way which insures total reflection of the ultrasound energy,
notwithstanding whether or not the angle of incidence of such energy is
beyond the critical angle for total reflection. Typically, where a
reflective coating is utilized, a problem can arise when "dual" or
"multiple" reflection occurs from both the outer surface of the reflective
coating and the interface between the inner surface thereof and its
substrate; the problem being manifested by resultant multiple transmit
pulses. One way to avoid this is to design the equipment such that the
ultrasound is incident on the reflector at an angle greater than the
critical angle whereby all the sound energy gets reflected from the front
surface of the reflector and no sound energy goes into the reflector to
cause secondary reflections. It is desirable to not be constrained to
incidence angles greater than the critical angle, especially since the
ultrasound beam has a substantial range of incidence angles over the
reflector surface, and the angle is constantly changing at any particular
point thereon as the reflector is scanned. Accordingly, applicant provides
a reflector which makes use of the total reflection of ultrasound at a
liquid/gas interface.
In the present embodiment the reflector comprises a flat piece of substrate
material 74, typically a metal such as steel having a finish which is not
optically polished and has a roughness which is, for example, of the order
of one to ten microns. An exceedingly thin layer of ultrasonically
transparent material 75, such as a plastic foil having a thickness of
about 1-2 mils (much smaller than the wavelength of the ultrasound),
overlays the substrate. As can be seen from the microscopic view, during
fabrication of the reflector a thin layer of air 76 is trapped in the
asperities on the surface of the substrate 74; i.e., between the substrate
surface and the layer 75. Applicant has discovered that the incident
ultrasound is completely reflected at what operates effectively as a
liquid/air interface, so that considerations of critical angle are
obviated.
Referring to FIG. 5, there is shown an embodiment of the invention wherein
a plurality of segmented transducer/lens combinations, 80A and 90 A, as
well as the previously described 80 and 90, are utilized to obtain a
larger total aperture ultrasound beam. The additional combination may be
mounted parallel to the elements 80 and 90 (as shown), or at an angle
thereto. The accompanying circuitry for each combination can be
essentially the same as shown in FIG. 2, with provision being made for
integrating the scanning beams. The beams are directed toward the
reflective scanners 70 and 70A, as shown, and operation is generally the
same as that described in conjunction with FIG. 2. A single scanner could
also be employed.
Referring to FIG. 6, there is shown a portion of an embodiment of the
invention wherein a separate transducer designated 80B is utilized to
transmit the ultrasound beam from the position of the reflector 70. This
technique is applicable when electronic variable focusing is provided in
only the "receive" or "listening" mode, as is generally the case. A
coupling medium 95 couples the transducer to the reflector, and energizing
signals from the pulser/receiver (see e.g. FIG. 3) are coupled to the
transducer 80B. In other respects, the system is similar to the embodiment
shown in FIG. 2, with the lens 90 and transducer 80 (and associated
circuitry) processing reflected echoes as previously described. The
transmitting transducer 80B, which moves with the reflector 70, scans one
scan line at a time and echoes are reflected toward the receiving
transducer 80, as shown by the dashed lines and arrows. In this
embodiment, the total ultrasound travel path is smaller so that successive
pulses (for each scan line) can be transmitted at a higher rate with an
accompanying desirable increase in the overall imaging rate.
The invention has been described with reference to particular embodiments,
but variations within the spirit and scope of the invention will occur to
those skilled in the art. For example, some or all of the timing or sweep
circuitry of FIG. 2 may be housed in the scanning module 50, if desired,
the basic consideration being the desire to maintain portability of the
module while minimizing the noise-susceptibility of low-level signals
passing through cables between the scanning module and the console. Also,
it will be understood that transmit focusing could be used, if desired.
Finally, it can be noted that the present invention is particularly
applicable to ultrasonic surgery techniques wherein, after imaging and
observing a diseased tissue such as a tumor growth, it is desired to
"eliminate" diseased tissue by exposing it to very high intensity
ultrasound. In such a case, after the imaging sequence, the scanner can be
electrically aligned with the tumor and the transmitted power of the
transducer increased, such as by a factor of 10,000. The phase coding
circuitry is controlled so that the high power beam is focused only at the
tumor site and defocused at other sites so that thermal damage of healthy
tissue does not occur. The equipment can be periodically switched back to
the imaging mode so as to monitor the effects of treatment during the
surgery.
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