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Description  |
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This invention relates to the technique of ultrasonic echoscopy of objects
and in particular to means for decreasing the time required for
examination of an object using the pulse-echo ultrasonic technique and for
improving the clarity and hence the utility of the examination results. It
is particularly, but not solely, directed to the use of this technique in
medical diagnostic examination.
Ultrasonic echoscopy provides information about an examined object which
may be displayed in the form of an ultrasonic echogram. Such an echogram
consists of a display of acoustic impedance discontinuities or reflecting
surfaces in the object. It is obtained by directing a short pulse of
ultrasonic energy, typically in the 1-30 MHz frequency range, into the
examined object where any acoustic impedance discontinuities in the object
reflect and return some of the energy in the form of an echo. This echo is
received, converted into an electric signal and displayed as an echogram
on a cathode ray oscilloscope, a film, a chart or the like.
The echogram may constitute either a one dimensional or a two dimensional
representation and in both cases the information is contained in the
position and magnitude of the echo displayed. In a one dimensional
display, the position along a base line is used to indicate the distance
to the reflecting surface whilst the magnitude of the echo is displayed
for example as a deflection of the base line "A mode" or as an intensity
change "B mode". In a two dimensional display, the position along a base
line is used to indicate the distance to the reflecting surface as in a
one dimensional display, and the direction of the base line is used to
represent the direction of propagation of the acoustic energy. The two
dimensional display is obtained by changing this direction of propagation
of the acoustic energy and by instituting a similar but not necessarily
identical movement of the base line of the display. The magnitude of the
echo is displayed as for a one dimensional display, for example, as a
deflection of the base line or as an intensity change.
The technique of ultrasonic echoscopy is used in medical diagnosis to
obtain information about the anatomy of patients. The application of this
technique is now widely investigated and is described, for example, by D.
E. Robinson in Proceedings of the Institution of Radio and Electronics
Engineers Australia, Vol. 31, No. 11, pages 385-392, November, 1970: "The
Application of Ultrasound in Medical Diagnosis". As pointed out in this
article, ultrasonic echoscopy may be used to produce displays resembling
anatomical cross-sections which have proved clinically useful when the
desired information concerns physical dimensions, shapes of organs or
structures or the like. Utransonic echography has proved of particular
value as a diagnostic aid in the abdomen and pregnant uterus, eye, breast,
brain, lung, kidney, liver and heart, these being areas of soft tissue
with little bone and air. In general, the technique is considered to
complement other techniques to provide a more complete picture of the
patients' condition, however, particularly in pregnancies, ultrasonic
echoscopy may be useful in place of X-rays where the latter may not give
sufficient information or may be dangerous. In medical use, a pulse of
ultrasonic energy is transmitted into a patient in a known direction and
echoes are received from reflecting surfaces within the body. The time
delay between a transmitted pulse and the received echo depends on the
distance from the transmitter to the reflecting surface and the distance
information so obtained may be displayed in a suitable way for
interpretation and clinical use as a one dimensional range reading or as a
two dimensional cross section as previously described.
In one presently known form of ultrasonic diagnostic examination, a single
transducer is used and it is physically moved to various positions around
the patient. At each of these positions the beam is swept with an
oscillatory motion while constrained to remain within a single plane by
mechanical oscillation of the transducer, to obtain the required scan
pattern. By the use of suitable deflection circuits, for example, in a
cathode ray display tube, a line is caused to follow the motions of the
beam axis and echoes within the part examined are thus displayed in their
correct geometrical positions. By way of example, for transverse sections,
the transducer may be moved horizontally in a 150.degree. arc around a
patient who is substantially erect while undergoing .+-.15.degree.
oscillations and for longitudinal sections the transducer may be moved
vertically while undergoing .+-.30.degree. oscillations.
It has, however, been found that in such systems where the transducer is
physically moved around the patient this movement leads to a limitation on
the examination time of between ten and twenty seconds for each
cross-sectional visualization due to mechanical inertia and, in the case
where the transducer is coupled to the patient via a coupling medium such
as water, the generation of turbulance by the transducer when it moves
quickly in the coupling medium.
Several alterative forms of ultrasonic examination equipment have been
devised which will avoid the limitations discussed above and thus enable a
speeding up of the time required for each cross-sectional visualization.
It will be apparent that a reduction in examination time of a patient will
lead to a technical improvement in the resultant echograms as the effects
of movement of the part under examination will be reduced. In addition, a
reduction in examination time of a patient will have the economic
advantage that more cross-sectional visualizations and hence more
examinations will be able to be performed in a given time.
In the currently used two dimensional B mode systems of ultrasonic
examination, a single transducer is either mechanically driven or manually
operated to obtain a echogram. To enable examination of a region of
interest, for example of a patient, the plane of scan is adjusted as
required, an echogram obtained in that plane, and then the plane of scan
moved, and so on, usually in such a way as to examine parallel sections of
the region of interest. As each cross-section visualization using the
known systems requires from ten to twenty seconds, the entire examination
may take up to half an hour or so. Such a long examination time is
undesirable for a number of reasons. Firstly, there can be considerable
movement during the examination period, particularly of the fetus in a
pregnant uterus, blurring individual echograms and causing difficulty in
interpreting a full set of cross-sectional visualizations over a region of
interest. Furthermore, a prolonged examination is often not possible or is
at least most undesirable when the technique is used in the examination of
sick patients. Of course, a lengthy examination time precludes the use of
the technique as a screening test, and the reduced throughput of patients
has an obviously deleterious effect on the economics of the technique.
It is therefore an object of the present invention to provide apparatus for
ultrasonic examination of an object by the pulse-echo technique which is
capable of performing a complete ultrasonic examination in a time
considerably less than that possible at present. As will be appreciated
from the above, any reduction of the time necessary for the examination
will have the further benefit that the quality of the echograms which are
obtained will be improved since the effects of patient movement will be
reduced.
According to the present invention there is provided apparatus for the
ultrasonic examination of an object comprising a housing enclosing a
coupling medium, said housing being provided with an aperture therein, and
transducer means contained within said housing and immersed in said
coupling medium, said transducer means comprising means for transmitting
pulses of ultrasonic energy through said aperture into the object and
means for receiving echoes of said pulses of ultrasonic energy reflected
through said aperture by acoustic impedance discontinuities within the
object.
Preferably, the housing consists of a bath filled with water as coupling
medium. The aperture is provided in the top of the bath which otherwise
completely encloses the coupling medium. Preferably, a flexibe coupling
membrane such as a polythene membrane is provided to seal the aperture so
as to ensure satisfactory contact between the coupling medium and the
object such as a patient to be examined without loss of the coupling
medium through the aperture. In one convenient arrangement particularly
suitable for medical diagnostic examination, the bath is constructed in
the form of a couch upon which a patient may be positioned with the body
region to be examined over and in contact with the coupling membrane.
Preferably also, the coupling medium is temperature controlled. Whilst the
use of the flexible coupling membrane as desribed above is preferred as a
matter of convenience, particularly where a large number of patients are
to be examined, it will be apparent that use of this membrane may also be
dispensed with thereby allowing direct contact of the coupling medium with
the patient's skin.
As previously described, in accordance with the present invention the
transducer means is immersed in the coupling medium contained within the
housing. In known ultrasonic examination systems, coupling from the
transducer to the patient has been achieved either by skin contact or by
use of, for example, a water delay bath. Use of a water delay bath is
recognised as introducing possible ambiguities due to multiple reflection,
although these may be avoided by so arranging the system that the distance
between the transducer and the skin surface of the patient is greater than
the largest depth of penetration to be used. Nevertheless, while skin
contact systems in general result in greater comfort for the patient, the
resulting echograms are of less clarity, and the use of water delay bath
provides better quality echograms. The present invention provides
apparatus whereby these better quality echograms may be conveniently
achieved.
In order to obtain good coupling between the transducer means and the
flexible coupling membrane, or skin surface of the patient, in accordance
with the present invention the housing is filled with the coupling medium
and preferably the housing is provided with a header tank to ensure that
the housing remains filled during flexing of the membrane, for example
during positioning of the patient on the apparatus. Fluid pressure exerted
by coupling medium stored in the header tank maintains pressure on the
coupling membrane also, thereby helping to maintain close contact between
the flexible membrane and the skin of the patient.
In a particularly important aspect of the present invention, the transducer
means comprises a plurality of transducers mounted in a single arm, the
arm being mounted within the housing by a supporting mechanism for
movement thereof with respect to the object to be examined to facilitate
examination thereof in any desired plane. Preferably, this supporting
mechanism provides movement in the "x", "y" and "z" directions with
respect to the object and also provides for rotation and tilting of the
arm with respect to the object, thereby allowing focusing of the
transducers of the arm on the desired region of the object and scanning in
any desired plane. Preferably also, the arm supporting the transducers is
curved thereby enabling a degree of mechanical focusing of the transducers
at a point within the object to be examined. The curve of the arm may be
circular, however linear arms or other non-linear arms may be used if
desired.
The use of a plurality of transducers has been found to enable a speeding
up of the time required for each cross-sectional visualization. In a
typical operation the transducers are spatially positioned in a single
plane relative to each other and to the object under examination as by
mounting in an arm described above and the beam from each transducer is
steered to a plurality of angular directions in this plane, for example by
oscillation of the transducers in the plane of the arm. The transducers
may then be energized sequentially, the time of energizing each of the
transducers being set so that the whole set of transducers is energized
before the appropriate beam from each transducer has moved a significant
distance. In this way an entire scanned cross-section may be formed in one
cycle of the transducers.
Oscillatory motion of the beam axes from the plurality of transducers in
order to build up a complete cross-sectional visualization may be provided
by two alternative means. The first means of obtaining oscillatory motion
of the beam axes is by mechanically scanning all of the plurality of
transducers either independently or simultaneously. In this case, although
mechanical movement of the transducers does introduce a limitation on the
scanning rate, the effect of this limitation may be minimised by providing
suitable switching means which require the transducers to scan only once
while obtaining a complete cross-sectional visualization. Thus, each
transducer is activated in turn to direct a pulse of ultrasonic energy
along the beam axis, the rate at which the transducers are activated being
sufficiently fast, compared with the rate of mechanical oscillation of the
transducers, that each transducer oscillates only a small distance between
successive activations thereof. The final result achieved by this method
of operation is that at the end of a single mechanical scan, each of the
transducers has been activated whilst it's beam was directed in all
required directions. It will be apparent that monitoring of the direction
of the beam axes will be necessary in order to build up the complete
visualization.
The alternative means of obtaining oscillatory motion of the beam axes is
by use of transducer arrays at each transducer position, the arrays being
appropriately designed as to be capable of being steered electronically.
In such a system there are no moving parts and the scanning rate
obtainable with this system is limited only by considerations of
electronic switching speeds and the rate of acquisition of ultrasonic
information by the transducer after each transmitted pulse. Since such an
array may be electronically steered to direct its beam in all required
directions at a rate much faster than that possible when mechanical
oscillation of the transducer is required, it is possible to operate this
system by steering the beam from each transducer array to each of the
required directions to measure the reflected echoes before activating the
next transducer array and steering the beam from it to each of the
required directions, and so on. It will, however, be appreciated that this
plurality of transducer arrays capable of being electronically steered may
also be operated in a manner similar to the operation of the mechanically
oscillated transducers previously described.
In a further important aspect of the present invention, however, the
individual transducers of an array which is being mechanically scanned may
be energized in a "scattered" operation rather than in a sequence of
energizing adjacent transducers. Thus, instead of energizing transducer
No. 2 after transducer No. 1, and then transducer No. 3, transducer No. 4
and so on, in accordance with this invention transducer No. 5 in the array
may be energized after transducer No. 1, then transducer No. 2 followed by
transducer No. 6 and so on. It has been found that such "scattered"
operation of a plurality of transducers enables even further speeding up
of the visualization since it enables one transducer to be energized
without having to wait for multiple reflections arising from energization
of the previous transducer to die down, this being possible since the
transducer is to transmit its pulse and receive reflections along a beam
which is spaced substantially from the beam of the previous transducer.
Since the "wait period" which can be avoided by such "scattered" operation
may be of the order of 500 .mu.sec, a significant speeding up of the
examination is thereby achieved.
In yet another mode of operation of the apparatus of the present invention
the individual transducers of an array may be energized in such a manner
that the time of display of echoes approximates the total available time.
In the "scattered" operation described above in which the circuit
conventionally includes a single deflection generator circuit for the
display means, the time of display of echoes is approxmately half of the
total available time, the other half of the available time being taken up
by time for transmission of the pulses through the coupling medium. In the
present alternative mode of operation, however, it has been found that the
use of two deflection generator circuits enables on pulse to be
transmitted through the coupling medium during the period of display of
echoes from the preceding pulse so that the display of echoes from the one
pulse may commence almost immediately after conclusion of the display of
echoes from the preceding pulse.
Where the plurality of transducers are mounted in a single arm in
accordance with the present invention for oscillation in the plane of the
arm, it is preferred that the transducers be mechanically linked for
simultaneous oscillation. Thus a single motor may be utilised for
oscillation of all of the transducers, and, if desired, a single
monitoring device may be utilised to monitor the oscillation of all
transducers since their respective beams will be fixed relative to one
another. In accordance with a preferred aspect of the present invention
the single arm is mounted on a supporting mechanism which provides for
both rotation and tilting of the arm with respect to the object under
examination. Rotation of the arm of course enables different
cross-sectional visualizations to be obtained without repositioning the
object while tilting of the arm enables the transducers to be directed to
a single point from different angles, thereby enabling inclined sections
to be obtained. Appropriate drive means may be provided to perform this
rotation and tilting, and monitoring means provided to measure the
position of the transducers. Similarly, drive means and monitoring means
are provided to move the arm in the "x", "y" and "z" directions and to
monitor this movement.
Other features of the present invention are illustrated in the accompanying
drawings in which:
FIGS. 1 to 3 illustrate schematically apparatus in accordance with the
present invention;
FIG. 4 illustrates in more detail the mechanism for oscillation of the
transducers of the apparatus of FIGS. 1 to 3;
FIG. 5 shows a basic block diagram of one form of the electronics for
ultrasonic examination apparatus in accordance with the present invention;
FIG. 6 shows a transmitter and receiver switching block diagram for the
circuit of FIG. 5;
FIG. 7 shows a block diagram of an angle and origin switching network for
the circuit of FIG. 5;
FIG. 8 graphically depicts a "scattered" mode of operation of the apparatus
utilising the circuit of FIG. 5;
FIG. 9 shows a modification of the circuit of FIG. 5; and
FIG. 10 graphically depicts an alternative mode of operation of the
apparatus utilising the modified circuit of FIG. 9.
As shown in FIGS. 1 to 3 of the drawings, the apparatus of the present
invention comprises a housing 19 in the form of a couch upon which a
patient 17 can be positioned. The patient 17 is positioned with the region
to be examined placed in contact with a flexible coupling membrane 18
which seals an aperture in the top of the housing. Housing 19 is filled
with a coupling medium such as water and is provided with a header tank as
at 24 to maintain proper contact between the patient and the membrane.
Located within the housing 19 and immsersed in the coupling medium is a
plurality of transducers 2 spatially mounted in a main supporting arm 1 so
as to be capable of oscillating in the plane of the arm, thereby directing
the respective pulses of ultrasonic energy along beams which are steerable
in said plane. Eight transducers are shown in the Figures by way of
example, however it will be appreciated that this number may be increased
or decreased as desired. As shown in greater detail in FIG. 4, each
transducer 2 is arranged to oscillate about pivot center 3 by action of
oscillator arm 4. The respective oscillator arms 4 of each of the
transducers 2 are coupled by links 5 which are provided with adjusters 6
to enable accurate positioning of the transducers 2 with respect to each
other and the arm 1. One of the oscillator arms 4 is provided with a
sector gear 7 which meshes with a geared output 8 of motor 9, attached to
arm 1. It will be apparent that rotation of the shaft 8 of the motor 9
will result in simultaneous pivoting of each of the transducers 2 about
its pivot center 3. The position or angle of each transducer is monitored
by means of monitoring potentiometers 10.
Referring again to FIG. 1, main arm 1 is mounted on a main frame which is
slidable towards and away from the patient 17 to enable positioning of the
transducers mounted on arm 1 relative to the patient. The main frame is
slidable on pillars 11 and movement along these pillars is controlled by
motor 12, for example, by means of a rack and pinion drive. The main frame
is also mounted on a carriage 25 which slides along a track 15 controlled
by motor 22 (FIG. 3), again by means of a rack and pinion drive or the
like. As depicted, the apparatus enables movement of carriage 25 along
track 15 transverse to the patient 17 in the position depicted.
Longitudinal movement of the arm 1 with respect to the patient is effected
by movement along track 16 provided by motor 23 (FIG. 3) again, for
example, by means of a rack and pinion drive.
As best seen in FIG. 2, arm 1 is mounted on a frame 20 which is supported
by bearings or the like on carriage 25 for tilting motion with respect to
the patient. Thus frame 20 is provided with an arcuate track section which
engages the shaft of motor 13, for example in a rack and pinion type
drive, for tilting of the arm. Frame 20 is in turn mounted on turntable 21
which provides rotary motion of the arm 1 relative to the carriage 25,
this rotary motion being controlled by motor 14. It will thus be apparent
that by provision of appropriate mechanisms supporting arm 1, this arm can
be positioned transversely, longitudinally, or toward or away from the
patient. Further, the arm may be tilted or rotated relative to the patient
to enable any desired plane of scan of the transducers 2. By way of
example, the arm may be rotated in increments of 1.degree. through
180.degree. to give transverse, longitudinal or oblique echograms, and it
may be tilted in increments of 1.degree. through an angle of
.+-.30.degree. to give inclined section. The tilting movement of the arm
is designed to pivot about the patients skin line minimising the
translational movement of the scanning plane normally obtained with
tilting. The arm may also be translated in the "x", "y" and "z" directions
in increments ranging from 1 mm to 2 cm to give automatic acquisition of a
number of parallel echograms separated by the selected distance. With the
water coupling method all of these sections are obtained without changing
the coupling to the patient thus allowing reliable cross correlation of
detail in echograms obtained in different planes.
Preferably, all movement of the transducer means is powered by stepper
motors. These have the advantage that their operation is controlled by the
application of electrical pulses. For oscillatory motion of the
transducers the rate of application is gradually built up, then kept
constant during the echogram forming period and then gradually slowed down
giving a reproduceable even motion necessary for the attainment of a
consistent grey scale and reducing the vibrations set up during
acceleration or deceleration of the transducers. Stepper motors are also
amenable to control by computers and all of the motions of the scanner may
be placed under computer control.
The rapid rate of acquisition of high quality echograms has obvious
advantages. It reduces blurring of detail caused by movement of structures
during the echogram forming period and allows quasi real time viewing of
the same section. Alternatively it allows the taking of many views from
different angles making the examination somewhat similar to fluoroscopy.
The total time necessary for the total examination is also significantly
reduced allowing the same instrument to handle a much greater clinical
load. The instrument also has the advantage that it utilises the minimum
number of pulses to acquire a compound scan echogram and no tissue need be
irradiated more than for example eight times. To minimise the irradiation
dosage the attenuator settings of the echoscope control the level of the
transmitted energy and not the gain of the receiver. The echoscope of this
invention may also have the provision to switch off any set of transducers
and also allow a simple scan to be obtained from any transducer.
The many degrees of movement of the apparatus of this invention makes it
very versatile and it can be used in a variety of other modes of
operation. For instance by combining a translational movement in the
length direction of the arm, a compound scan echogram is obtained where
the origin of oscillation of the transducers is moved. This mode of
operation is useful in circumstances where it is desirable to view around
some overlying shadowing structures such as ribs. This translation
movement also reduces the Moires interference pattern obtained in the
stationary mode of operation. The translation movement in this case need
not be large, a movement corresponding to the inter transducer distance
being adequate and this movement may be achieved in the same time as that
taken for the single oscillatory movement.
In another mode of operation all of the transducers may be made to function
as a single transducer corresponding to the size of the arm, and focusing
techniques of the type used in annular phased array transducers may be
used to generate a highly focused line of sight along the axis of the arm.
A linear scan may then be formed by translating the arm in the length
direction.
The multiple transducer configuration of the echoscope may be used to
examine tissues in the single transmit-multi receiver mode of operation
analogous to the multi-channel receiver operation used in seismology.
Cross-correlation of information received by the multiple transducer may
then be used to reduce multiple reflection artifacts in tissue and to
measure local values of velocity in the various visualised tissues.
The echo scattering cross-section of tissue may also be obtained by two
methods with the echoscope. In the first method, the tilting motion of the
echoscope is used, the echo scattering cross-section being obtained from
tissue at a depth corresponding to the projection from the transducer to
the axis of rotation of the tilting motion. For a transducer facing
vertically up this depth is equal to the radius of curvature of the
tilting motion. Alternatively, by pointing the transducers to cross at the
center the echo scattering cross-section of tissue lying at the radius of
curvature of the arm may be obtained by rotating the arm. The first method
gives the echo scattering cross as a function of the angle in a fixed
plane while the second method gives the value of this parameter when
viewed at constant angle a in variable plane. In both methods the multiple
transducer nature of the echoscope also allows a relatively coarse two
dimensional dependence of the parameter.
One mode of operation of the apparatus in accordance with this invention is
depicted in FIGS. 5 to 7.
As shown in FIG. 5, the master clock 65 provides basic time impulses to
initiate multi-channel clock 64 and to drive motors 52, 54 via the motor
controller 57, 58, 59.
The multi-channel clock 64 outputs, in turn, trigger pulses to each channel
upon receiving a pulse from the master clock 65.
The channel selector 60 counts trigger pulses and sends a binary channel
address code to transmitter and receiver switching network 55 and to angle
and origin switching network 56.
The signal processer 62 processes echoes from transmitter and receiver
switching network 55, which echo signals are fed together with blanking
pulses to the intensity modulation input of the display unit 66.
Deflection generator 61 generates X and Y deflection voltages from signals
received from angle and origin switching network 56.
FIG. 6 shows the transmitter and receiver switching network in greater
detail. As seen in FIG. 6, transmit and receive decoder 73 decodes channel
selector output signals from channel selector 60 to activate the correct
diode switch drive 72 and transmitter drive 71. The multi-channel clock
output then triggers the transmitter drive 71 to energize the transmitter
70.
Receiver diode switch 74 is energized by the decoder 73 via diode switch
drive 72 which allows echo signals to pass to the preamplifier 76.
In FIG. 7, the angle and origin switching network 56 of FIG. 5 is more
fully disclosed. As seen in FIG. 7, angle and origin decoder 82 decodes
the channel selector output to activate the correct switch drive 81 and
hence turn on the correct potentiometer supply switch 80. The supply
switch 80 supplies reference voltages to the sine cosine angle monitoring
potentiometer 83 and to the origin network 84.
The X and Y origin coordinates from the origin network 84 and the sine and
cosine angle output from the potentiometer 83 are fed via their respective
OR gates to the deflection generators 61 (see FIG. 5) within the
processing electronics.
FIG. 8 illustrates graphically the "scattered" mode of operation which can
be adopted using the circuits of FIGS. 5 to 7. It will be apparent from
FIG. 8 that by timing the pulses such that pulse 1 is followed by pulse 5,
rather than directly by pulse 2, pulse 5 may be transmitted through the
water acting as the coupling medium after a brief lapse of time, the
settling time, following completion of the display of echoes from pulse 1.
As described above, in this "scattered" mode of operation, transmission of
pulse 5 through the coupling medium need not be delayed until multiple
reflections from pulse 1 die down as the beam of pulse 5 is substantially
spaced from the beam of pulse 1. Of course, when pulse 2 is transmitted
after the display of echoes from pulse 5 and a brief settling time the
multiple reflections from pulse 1 will by then have died down. FIG. 8
illustrates that by use of this mode of operation a display time
approximating half the total available time can be achieved.
FIG. 9 shows a modification of the circuit of FIG. 5 in which deflection
generator 61 thereof is replaced by deflection generator 1, deflection
generator 2 and a deflection selector. Separate angle and origin switching
networks, are also provided for each deflection generator. Other integers
of FIG. 9 which are not shown are as shown in FIG. 5.
FIG. 10 illustrates an alternative mode of operation of the apparatus of
the present invention, utilising the modified circuit of FIG. 9. In this
alternative mode of operation, the existence of a second deflection
generator enables pulse 5 to be transmitted through the coupling medium at
substantially the same time as the display of echoes from pulse 1 is
effected using the first deflection generator. Thus, display of echoes
from pulse 5 can be effected almost immediately after completion of
display of echoes from pulse 1. At substantially the same time, pulse 2
can be transmitted through the coupling medium utilising the first
deflection generator. FIG. 10 illustrates that by use of this mode of
operation a display time approximating the total available time can be
achieved.
From the foregoing description it will therefore be appreciated that the
present invention enables more rapid and complete scanning of an object
subject to ultrasonic examination. While the invention has been described
with reference to illustrative embodiments, it will be generally
understood by those skilled in the art that various changes may be made
and equivalents be substituted for elements thereof without departing from
the true spirit and scope of the invention.
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