Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
This invention relates to the technique o~
ultrasonic echoscopy of objects and in particular to an
extension of known techniques of ultrasonic echoscopy
to provide more useful information concerning the
examined objects. It is particularly, but not solely,
directed to the more effecti.ve acquisition of data in
medical diagnosis utilising this technique.
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 MXz frequency range, into the
~m; ned 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 di~played as an
echogram on a cathode ray oscilloscope, a film, a chart
or the like.
The echogram may constitute either a one
~ s~cional or a two dimensional representation and in
both cases the in~ormation is contained in the position
and magnitude of the echo displayed. In a one
~; n~ional display, the posi~ion along a base line is
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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 or as an
intensity change. 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 magni~ude of the echo is displ~yed
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.~. Robinson in Proceedings of the
Institution of Radio and Electronics Engineering 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
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which have proved clinically useul when the desired
information concerns physical dimensions, shapes of
organs or structures or the like. Ultrasonic echography
has proved of particular value aq 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 echoscspy may be useful in place of x-rays
when 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
~;m~ional range reading or as a two ~; -nqional cross
section as previously described.
This known system suffers from a disadvantage
due to the extent of the transducer beam in a direction
a~ right angles to the beam axis. ~his beam wid~h is
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large compared wi~h th~ length of the ultrasonic pulse
along the axis and farms the limitation on the resolution
of ultrasonic visualisation systems. The beam width
can be reduced by geometrically focusing the transducer
S by physically curving the face of the transducer,
alternatively an acoustic lens can be placed between
the transducer and the object to be examined. The
degree of focus can be altered by changing the size of
the transducer and the focal length of the focusing
system with a larger transducer and shorter focal length
giving rise to a more sharply focused system. This ~
process of resolution improvement by focusing is limited
by diffraction effects to the so called "diffraction
limited resolution". With the sharper focusing the
distance over which the beam width is small is restricted
and it has been customary in pulse echo visualisation to
use transducers which are weakly or medium focused to
afford the best compromise between beam width resolution
and usable range or depth of field. Theoretical
discussion of these effects is given in a paper "Design
of Narrow Beam Width Transducers", G. Kossoff, J. Acoust.
Soc. Amer.v. 36, n. 6, pages 905-902, June, 1963~
An alternative method of achieving a focusing
action is to use a planar transducer surface with a
plurality of array elements each connec~ed to a separa~e
electronic circuit. In particular, the array elements
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may be composed of a number of concentric rings. The
focusing action is achieved by sequentially delaying the
siynals from appropriate elements. Thus lf the outer
rings are energised first and the inner ones consecutively
with an appropriate time delay a concave wave front is
produced which converges on a point in the object under
ex~m;n~tion. Thus, consecutive transmit pulses can be
focused at different ranges to achieve a good focusing
action over the entire range of operation. On reception,
electronic delay means in the receiver allow waves from
a point at a given distance along ~he axis of the array
to be aligned and combined to form an effective focus,
the ~;ru~ delay being applied to the inner ring or
disc and delay being applied sequentially up to a
minimum or zero delay being applied to the outer ring.
By varying the delay during the reception period, it is
possible to move the position of the ocus so that as
signals are picked up from deeper in the object under
examination, they always remain in focus. This system
is known in the art and described in "Vaxiable ~ocus
Transducer", O.H. Schuck, U,S. Patent 3,090,030, May 14,
1963. The ~ nsions of the central disc and surrounding
ring elements are fixed by physical requirements. It is
~5 necessary that the distance from any point on ~he axis
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to all parts of the surface of a single element is
within one half wave length of the sound wave. This
condition is most difficult to satisfy for a point
near the transducer and therefore the radius of the
central ring is set so that the distance from the
nearest point at which echoes are required to the
periphery of the disc is exactly one half wave length
greater than the distance from the central point of the
disc to the field point. Similarly the width of the
first ring is fixed so that the distance to the outer
edge of the ring exceeds by-one half wave length the
distance from the inner edge of ~he ring. This
condition leads to a relatively large central disc
and successively thinner ring elements. As the
transducer size and therefore the ultimate diffraction
limited resolution is increased the number of
transducer ring elements needed and the amount of
electronic delay required to focus at a given distance
increases at a rapid rate. This leads to a big
increase in cost both of the transducer and the associated
electronic circuitry as the number of rings increases.
It is the primary object of the present
invention to provide the same performance as a planar
electronically focused transducer array ~ut wi~h a
reduc~ion in the number of rings and the cost and
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complexity of the associated electronic circuitry.
According to the present invention, there
is provided apparatus for the ultrasonic examination
of an object CQmpriSing means for transmitting pulses
of ultrasonic energy into the object and for receiving
echoes of the pulses reflected by acoustic impedance
discontinuities within the object, the means compris-
ing a mechanically prefocused transducer array com-
prising a central transducer element, the beam axis
of the array lying along a line through the centre of
the central element at right angles thereto, and a
plurality of annular transducer elements positioned
concentrically of the beam axis, and electronic means
for varyiny the focus of the transducer array during
transmission of the pulses into the object and receipt
of the echoes, the transducer array having a spherically
curved surface.
The transducer array, having a fixed focus,
is arranged to focus the ultrasonic ~eam at some
point within the depth range to be used by the ap-
paratus. The annular elements can then be made wider,
since fewer are required for a given transducer aper-
ture, and thus the complexity and cost of the
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transducer array and associated electronic components
can be markedly reduced.
An embodiment of the invention is illustrated
in the accompanying drawing. In the drawing the trans-
ducer 1 is spherical with its focus at point 5 and has
applied to it a central circular electrode 2 and a
plurality of concentric sets of annular or ring elements
illustrated by two rings 3 and 4. One cycle of the
electronic operation is initiated by a pulse from the
clock 6 which is applied to the time delay elements 7,
8, 9. To achieve the focus point at a distance closer
than the geometrically focused point of the transducer
the time delay element 7 has zero delay, time delay 8
a small delay and time delay 9 a large delay. The
delayed pulses are applied to the transmitter circuits
10, 11 and 12 and thence a transmitter pulse is applied
to the electrodes 4, 3 and 2. The emergent wave
front 13 is more sharply curved than the transducer
face and is focused on the point 14 which is nearer
to the transducer than the geometrical focus 5b If
the relative time delays 7, 8, 9 are reversed then the
emergent wave front 15 would be less sharply curved than
the face of the transducer 1, and would focus at a point
16 which is further distant than the geometric focus 5.
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Thus a transmitted wave can be focused at any
required distance from the transducer and the distance
can be changed for consecutive pulses. On reception a
sim1lar action occurs in reverse, echoes from the near
point 14 give rise to a sharply curved wave front 13
and echoes from the far point 16 give rise to a less
curved wave front 15. By using time delay elements
17, 18, 19 which may be varied electronically during the
time the echoes are returning it is possible to effectively
focus at each distance as echoes from tha~ distance are
being received. This means that at the com~ncement of
the receive period when echoes axe arriving from near
echo point 14 time delay element 17 has a large time
delay, time delay element 18 has a smaller time delay an-d~
time delay element 19 has no time delay and echoes from
wavefront 13 all arrive simultaneously àt the adder 20.
As echoes from more distant points are received the
relative time delays must be re~ersed to focus for
flatter wavefronts such as wave~ront 15 originating at
far point 16. Electronic circuitry to achieve the
functions o~ the clock 6, pulsed time delay elements
7, 8, 9, transmitters 10, 11, 12, analogue time delay
elements 17, 18, 19, and adder 20 are well known in the
art and detailed description thereo is not considered
necessary in the present specification.
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It will be appreciated that the schematic
representation included herein is included by way of
example only, and not by way of limitation of the present
invention. Many variations and modifications may be
made to the embodiment described herein without departing
from the concept of the present invention, and all such
variations and modifications are included within the
scope of the invention.
