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Description  |
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This invention relates to the technique of 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
effective 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 MHz frequency range, along a line
called the beam axis into the examined object where any acoustic impedance
discontinuities in the object reflect and return some of the energy along
the same beam axis 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 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 which is the beam axis.
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 Proceeding 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. Ultrasonic 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.
This known system suffers from a disadvantage due to the time required to
obtain a cross-sectional picture. The cross-sectional picture is made up
of a multiplicity of lines of information corresponding to each beam axis
position at which a pulse was transmitted and echoes received. The time
required to obtain each line of information is fixed by the depth of the
tissues of interest and the velocity of propagation of sound in the
tissues to be examined. For a particular area of interest neither of these
parameters is under the control of the operator and they form a basic
limitation on the time required to obtain an echogram.
In U.S. Pat. No. 3,789,833 to Bom, there is disclosed the formation of an
array of transducer elements arranged in a line, each providing a separate
ultrasonic line of sight. Each array element is pulsed in turn, returned
echoes are received and displayed on a screen and then another array
element is pulsed. The time required to form each displayed image is
2.times.D.times.N/v where D is the required depth of penetration, v is the
velocity of propagation and N is the number of lines required. Thus, for a
64 line cross-sectional display with 20 cm penetration in tissue having a
velocity of propagation of 1540 m/s, 16.7 msec are required to obtain a
complete picture. Thus the rate of obtaining pictures is approximately
sixty times per second which may be insufficient for some diagnostic
situations such as the visualisation of the heart. Of course, increasing
the number of lines of the depth of penetration increases the time
required to form a complete picture and decreases the number of pictures
per second which can be obtained. One prime limitation of the system
disclosed by Bom is that the line spacing fixes the size of the transducer
elements and the resulting ultrasonic resolution. U.S. Pat. No. 3,881,466
to Wilcox discloses the use of a plurality of transducer elements to form
each ultrasonic beam, thus breaking the link between line spacing and
resolution, and it is also known to apply time delays to signals
associated with the various elements of a transducer element array in
forming the ultrasonic beam, to cause a focusing action within the plane
of scan. In all cases, however, the minimum time requirement in formation
of a complete picture is limited as has been described above.
It is a primary object of the present invention to provide an improved
apparatus and method for the ultrasonic echoscopic examination whereby the
time required to obtain each cross-sectional picture is reduced allowing
the examination of moving structures with greater clarity while retaining
all the above described improvements in resolution and accuracy.
According to this invention, there is provided apparatus for the ultrasonic
examination of an object comprising a linear array of discrete transducer
elements;
means to energize at least one element of said array to transmit pulses of
ultrasonic energy into the object in the form of a diverging beam of
transmitted energy which ensonifies a region within the object; and
means for receiving echoes of said pulses of ultrasonic energy reflected by
acoustic impedance discontinuities within said region of the object, said
means for receiving echoes comprising means to activate said transducer
elements to receive echoes of each transmitted pulse along a plurality of
received beams.
Preferably, the means for receiving echoes comprises a plurality of said
transducer elements arranged to provide a plurality of received beams with
beam axes corresponding to each position required for a line of
information on a resulting cross-sectional visualisation of the object;
and
said means for transmitting pulses comprises a number of said transducer
elements less in number than the number of received beams and arranged to
provide said diverging beam of transmitted energy to ensonify the region
within the object covered by the plurality of received beams.
In another aspect, said means for transmitting pulses comprises a single
element of said array and said means for receiving echoes comprises a
plurality of groups of elements of said array providing said plurality of
received beams, and further including means for repeatedly sampling the
echo information of each of said received beams during the period while
echoes are being received.
In another aspect, this invention provides a method of ultrasonic
examination of an object comprising the steps of
transmitting pulses of ultrasonic energy into the object by energizing at
least one element of a linear array of discrete transducer elements to
form a diverging beam of transmitted energy which ensonifies a region
within the object; and
receiving echoes of said pulses of ultrasonic energy reflected by acoustic
impedance discontinuities within the object by activating a plurality of
said transducer elements to receive echoes of each transmitted pulse along
a plurality of received beams.
In order to display the echo information from each received beam axis, the
beam selector switch switches rapidly from one beam forming circuit to the
next many times during the period in which echoes are returning and the
deflected spot on the display is caused to move likewise. In this way a
plurality of lines of echo data are received for each pulse transmitted
and hence the absolute minimum time required for a cross-sectional picture
is reduced, with a proportionally greater reduction as the number of beam
axis positions used for each transmitted pulse is increased. By this
technique, the time required for formation of a complete picture is
2.times.D.times.N/(v.times.M), where D,N and v are as previously defined,
and M is the number of beams received and displayed from a single
transmitted pulse. Typically, M could be 6 and the rate of obtaining
complete pictures with the same number of lines per picture would thus be
increased by a factor of 6 without adversely affecting display quality.
The apparatus of the present invention thus also includes display means
including switch means for operating the display means to display echo
information from the received beams.
The electrical and mechanical structure necessary to implement the
invention are well known in the art, and it should be understood that
applicant's invention lies in the novel concept of arranging such known
parts to produce a new result. In this respect, reference is made to U.S.
Pat. Nos. 3,166,731 and 3,086,195 as showing electronic scanning and
focusing arrangements. In view of the detailed disclosure of the
electronics and structure of these prior arrangements, no detailed
discussion of the various electronic and mechanical structures need be
made herein.
One embodiment of the invention is illustrated in the accompanying drawings
in which:
FIG. 1 illustrates an arrangement of transducer elements which may be used
in accordance with this invention;
FIG. 2 shows the transmitted and received beams and the processing system
utilising the principles of the present invention;
FIG. 3 illustrates a method of echo display in accordance with this
invention; and
FIG. 4 illustrates the incorporation of the arrangement of elements
according to the present invention into an ultrasonic examination system.
The transducer array depicted in FIG. 1 consists of a plurality of active
transducer elements, and by way of example thirty rectangular elements
1-30 are shown mounted on the flat rectangular strip 31. The width of each
rectangular element is made equal to the actual spacing required between
received beams. This will normally be sufficiently small with respect to
the wavelength that the transmitted pulse beam diverges to ensonify the
region above a number of receiver elements. For example, referring to FIG.
2, assume that element 5 is used to transmit as shown in FIG. 2 and its
beam 32 ensonifies the region above elements 1 to 10. To obtain a narrow
received beam a plurality of transducer elements may be used together with
appropriate time delays being utilised to obtain focusing, for example
using elements 1-5 beam 33 may be formed with its axis above element 3;
similarly using elements 2-6 a beam 34 may be formed above element 4 and
so on.
The display of FIG. 3 is generated by knowledge of the values x, y,
.delta.x shown on the Figure. The value x is given by the distance along
the transducer to the centre of the transmitting transducer element. The
value y is given by a constant multiplied by time and represents the depth
into the examined object. The value .delta.x is the offset from the centre
of the transmitting transducer element to the centre of the formed
received beam. This display is more complex than displays obtained by the
prior art methods because it comprises a display of a plurality of
received beams concurrently. Thus the displayed point must traverse a
curve or locus across all the received beams at a similar time delay of
received echo before going onto greater time delays and thus greater
ranges. The curve 73-75-80 represents the position of reflectors which
give rise to echoes at constant time delay. Thus the path lengths
65-73-63, 65-75-65, 65-80-70 are all equal. The direct path length
65-75-65 given by 2y must equal any inclined path length such as 65-78-68
which is given by
.sqroot.(.delta.x).sup.2 +(y-.delta.y).sup.2 +(y-.delta.y); i.e.,
2y=.sqroot.(.delta.x).sup.2 +(y-.delta.y).sup.2 +y-.delta.y.
From this relationship the required value for .delta.y can be found to be
.delta.y=.delta.x.sup.2 /4y.
Also shown in FIG. 2 is a scheme for processing the returned echo data in
which beam forming circuits 43, 44 45 produce signals representing echoes
received along beams in beamaxis positions 33, 34, 35 etc. respectively
and these are fed to the beam selector switch 51. During the time the
echoes are returning, the beam selector 51 and the position of the dot on
the display screen are switched rapidly to obtain and display all the
data. For instance FIG. 3 shows a diagram of a number of positions 73-80
on a number of beams from which echoes return at the same delay time, the
said positions lying on a parabolic curve. Each of the said beams must be
sampled and the results displayed during the time available until echoes
are returning from the next set of sample points 83-90. Therefore in this
case a complete set of information on eight beam axis positions is
obtained for one transmitted pulse. The procedure is then repeated using
another transmitting element, such as 7.
An ultrasonic examination system incorporating this invention is shown in
FIG. 4. In this system, timing is derived from the main clock 46 which is
the most rapid timing interval in the system. A sample is taken from one
of the received beams for each pulse of the main clock. The received beam
to be used is selected by the beam address counter 47 whose output goes to
the beam selector switch 51. The output from the beam address counter 47
also provides information on the value of x for use in the x scan
generator 48 and y scan generator 49. The master clock signal 46 is
divided down by transmitter clock divider 50 and used to trigger the
transmitter address counter 52 and the y time base generator 53. The
transmitter address counter 52 output is fed to the transducer switch 60
which selects the appropriate transducer elements from transducer 30 for
transmitting and receiving. The transmitting element is pulsed by
transmitter 54 and receive elements are connected to beam forming circuits
43, 44, 45. The transmitter address counter 52 is also fed to the x
deflection generator 55, the output x of which is added to the output
.delta.x of X scan generator 48 in adder 56 and fed to the X deflection of
the display 58. The output y of the Y time base 53 is combined with the
output of the beam address counter 47 in Y scan generator 49 according to
the formula
.delta.y=(.delta.x.sup.2 /4y)
and the output .delta.y of Y scan generator 49 combined with the output y
of Y time base generator 53 in subtractor 57 and fed into the Y deflection
of the display 58. is as well known in the current art and processes the
signals from the beam selector switch 51 for the Z axis input of display
58.
The method may be modified in detail to optimise its performance. For
instance a plurality of transducer elements may be employed on
transmission to control the amount of divergence of the transmitted beam.
The beam forming circuits 43,44,45 etc., may include adding circuits and
delay circuits to shape the received beam patterns. Such circuits are well
known and reference is made to U.S. Pat. No. 3,166,731 to Joy and No.
3,086,195 to Halliday which disclose electronic steering, focusing and
reception of ultrasonics beams.
The method may also be extended in a straight forward fashion to operate on
a three-dimensional basis, rather than two-dimensionally as herein
described. In this extension it is necessary to have a two-dimensional
array of transducer elements and to receive along many lines of sight both
within the plane of section before described and also in adjacent planes.
In this case means are required to store the information from adjacent
planes for subsequent display.
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