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Claims  |
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What is claimed is:
1. In apparatus for forming images from energy contained in pulses of
cycles of ultrasonic pressure waves that are transmitted into a body to be
examined, the combination of
an array of transducers, each transducer translating pressure waves
impinging on it into corresponding electrical waves,
a plurality of phase changing means, each having an input and an output,
means respectively coupling the inputs of said phase changing means to
receive the electrical waves provided by different transducers,
delay means providing different discrete amounts of delay between each of a
plurality of inputs and a summing point,
means for respectively coupling the outputs of said phase changing means
during a predetermined period to selected inputs of said delay means, said
selected inputs having the discrete delays closest to the ideal
compensating delays required for respectively focussing the transducers of
the array at a given focal point, the differences between the said closest
discrete delays and the ideal compensating delays being such that the
pulses of electrical waves resulting from pressure waves reflected to the
transducers from the given point arrive at said summing point at different
times, and
phase control means coupled to said phase changing means for causing the
latter to set the phase of the waves at the outputs of each of said phase
changing means so that the cycles of electrical waves within the portion
of said pulses that arrive at said summing point in overlapped
relationship have sufficient phase coherence at at least one time during
said predetermined period to produce a signal having greater amplitude
than any of the individual electrical waves.
2. The combination set forth in claim 1 wherein said means for controlling
the phase of the waves at the output of each of said phase changing means
sets the phases of the cycles of electrical waves at the outputs of said
phase changing means so that they arrive at said summing point with
precise phase coherence a plurality of times during said predetermined
period.
3. The combination as set forth in claim 1 wherein each of said phase
changing means is a phase shifter responsive to said phase control means.
4. The combination as set forth in claim 1 wherein each of said phase
changing means includes
at least one mixer coupled between the input and output of said phase
changing means,
a source of waves of a given frequency coupled to said mixer so as to cause
it to produce electrical waves of upper and lower bands of intermediate
frequency, and
means for setting the phase of the waves coupled from said source to said
mixer in response to said phase contriol means.
5. The combination as set forth in claim 1 wherein each of said phase
changing means includes
at least one mixer,
a phase shifter coupled in series with said mixer between the input of said
phase changing means and its output,
a source of waves of a given frequency coupled to said mixer so as to cause
it to produce waves of an intermediate frequency, and
means for setting the phase shift provided by said phase shifter in
response to said phase control means.
6. In apparatus for forming real time images of portions of a body from
signals derived from energy contained in pulses of ultrasonic pressure
carrier frequency waves that are transmitted into the body, the
combination of
an array of transducers, each of said transducers translating the pulses of
ultrasonic carrier pressure waves impinging on them into corresponding
pulses of electrical waves,
delay means having a plurality of input taps and a summing point,
a plurality of phase changing means, each having an input coupled to a
different transducer and an output coupled to a tap on said delay means,
each of said phase changing means producing electrical waves of a given
frequency at its output,
the input taps on said delay means being spaced from said output so that
the smallest difference in the delays between adjacent taps and said
output is more than an eighth of a period of the electrical waves of the
carrier frequency supplied to said phase changing means from the outputs
of said transducers, and
means for causing each said phase changing means to set the phase of the
waves of the given frequency that it produces in response to carrier waves
reflected from a given focal point so that the waves of the given
frequency arrive at said summing point of said delay means with a phase
sufficiently close to the phases of waves of the given frequency arriving
at said output from other phase changing means to produce a signal of
relatively greater amplitude than the amplitude of the waves from each
phase changing means.
7. In apparatus for deriving signals that can be used to form images of a
body in response to energy contained in pulses of pressure waves, the
combination of
means for repeatedly transmitting pulses of pressure waves of a frequency
w.sub.c into a body,
an array of transducers,
a plurality of phase changing means, each having an input and an output,
and including means for heterodyning signals applied to its input with an
effective frequency w.sub.o so as to produce pulses of intermediate
frequencies and also having means for controlling the phase .OMEGA..sub.h
of the intermediate frequencies,
means respectively coupling the inputs of said phase changing means to
different transducers,
delay means providing different discrete amounts of delay between each of a
plurality of inputs and a summing point,
means coupling the outputs of said phase changing means during a
predetermined period after the transmission of each pulse of pressure
waves to selected inputs of said delay means, said selected inputs having
delays T.sub.h ** closest to the ideal compensating delays T.sub.h (t)
required for focussing the array at a given focal point, the difference
between the discrete delays T.sub.h ** and the ideal delays T.sub.h being
such that the pulses of intermediate frequency waves provided by a
plurality of said phase changing means to the inputs of said delay means
arrive at said summing point at different times, and
control means coupled to the phase control means in each phase changing
means for causing the intermediate frequency waves at the output of the
phase changing means to have at at least one time t during said period to
a phase .OMEGA..sub.h comprised of a fixed component w.sub.o T.sub.h **
combined with a variable component w.sub.c [T.sub.h (t + T.sub.h **) -
T.sub.h **] wherein T.sub.h (t) is the ideal compensating delay at time t.
8. In apparatus for deriving signals that can be used to form images of a
body in response to energy contained in pulses of pressure waves, the
combination of
means for repeatedly transmitting pulses of pressure waves of a frequency
w.sub.c into a body,
an array of transducers for translating pressure waves that impinge on them
into corresponding electrical waves,
a plurality of mixers, each having first and second inputs and an output,
the first inputs of said plurality of mixers being respectively coupled to
a plurality of said transducers,
means for respectively applying to said second inputs of said mixers
selected phases .OMEGA..sub.h of a wave having a frequency w.sub.o so as
to produce pulses of upper and lower sidebands of intermediate frequencies
at the outputs of said mixers,
a summing point,
means respectively providing during a given period after the transmission
of each pulse predetermined discrete delays between each of the outputs of
said mixers and said summing point, the discrete delays being those
closest to the ideal compensating delays required for focussing the
transducers at a given focal point, the respective differences between the
discrete delays and the ideal compensating delays being such that the
pulses of intermediate frequencies at the outputs of the mixers arrive at
said summing pointr at different times, and
means for making the selected phases of the waves of the frequency w.sub.o
that are applied to said second inputs of said mixers respectively have at
at least one time during said given period a fixed component equal to
w.sub.o times the delay between the output of the mixer and said summing
point and a variable component equal to the frequency w.sub.c times the
difference between the discrete delay between the output of the mixer and
said summing point and the ideal compensating delay required to focus the
transducers.
9. In apparatus for deriving signals that can be used to form images of a
body in response to energy contained in pulses of pressure waves, the
combination of
means for repeatedly transmitting pulses of pressure waves of a frequency
w.sub.c into a body,
an array of transducers for translating pressure waves that impinge on them
into corresponding electrical waves,
a plurality of mixers, each having first and second inputs and an output,
the first inputs of said plurality of mixers being respectively coupled to
a plurality of said transducers,
means for respectively applying to said second inputs of said mixers
selected phases .OMEGA..sub.h of a wave having a frequency w.sub.o so as
to produce pulses of upper and lower sidebands of intermediate frequencies
at the outputs of said mixers,
a summing point,
means respectively providing during a given period after the transmission
of each pressure pulse discrete delays T.sub.h ** between each of the
outputs of said mixers and said summing point that are respectively
closest to the ideal compensating delays T.sub.h (t) required for
focussing the array at a given focal point, the differences between
T.sub.h ** and T.sub.h (t) being such that the pulses of intermediate
frequency waves at the outputs of said mixers that are related to pulses
of pressure waves reflected from said given focal point arrive at said
summing point at different times,
said means for applying selected phases of a frequency w.sub.o being such
as to cause the phase .OMEGA..sub.h of intermediate frequency waves at the
outputs of said mixers at at least one time t during said period to have a
fixed component within a predetermined number of radians of w.sub.o
T.sub.h ** and a variable component within a predetermined number of
radians of w.sub.c [T.sub.h (t + T.sub.h **) - T.sub.h **] wherein T.sub.h
(t) is the expression for the ideal compensating delay as a function of
time.
10. The combination as set forth in claim 9 wherein
means are coupled to said summing point for forming images from the signals
appearing thereat, and
filter means for passing only one of said sidebands of intermediate
frequencies coupled between the outputs of said mixers and said latter
means.
11. The combination as set forth in claim 10 wherein said fixed and
variable components of the phase .OMEGA..sub.h are both positive and
wherein the sideband of intermediate frequencies passed by said filter
means is the lower sideband.
12. The combination as set forth in claim 10 wherein said fixed component
of said phase .OMEGA..sub.h is positive and said variable component is
negative and wherein the sideband of intermediate frequencies passed by
said filter means is the upper sideband.
13. In apparatus for forming real time images of portions of a body from
signals derived from energy contained in pulses of ultrasonic pressure
carrier frequency waves of a given duration that are transmitted into the
body, the combination of
an array of transducers, each of said transducers translating the pulses of
ultrasonic carrier pressure waves impinging on them into corresponding
pulses of electrical waves,
delay means having a plurality of input taps and an output,
a plurality of phase changing means, each having an input coupled to a
different transducer and responsive to a pulse of electrical waves
supplied by that transducer to provide at an output thereof a pulse of
electrical waves having a selected phase,
tap selecting means for respectively connecting the outputs of said phase
changing means to selected taps on said delay means, and
means for focussing the array at a point along a given direction comprising
control means coupled to each of said phase changing means for causing it
to set the selected phase of the electrical waves at its output that are
related to reflected pressure waves received at the transducer from the
given point at such an angle that they arrive at the output of said delay
line means with sufficient phase coherence to provide a greater signal
than would result from electrical waves having random phase relationships.
14. Apparatus as described in claim 13 wherein means are provided for
making said control means cause the phase changing means to set the
selected phase of the electrical waves at successively different locations
so that the waves at its output that are related to pressure waves
reflected from a series of points and received at the transducer arrive at
the output of said delay line with precise phase coherence.
15. Apparatus as described in claim 13 wherein each of said phase changing
means is comprised of a phase shifter.
16. Apparatus as set forth in claim 13 wherein said phase changing means
includes a mixer having first and second inputs and an output, a phase
shifter connected between the transducer and said first input, the angle
by which said phase shifter shifts the signals applied to it being
determined by connection to said control means, a source of electrical
waves, said source being coupled to said second input of said mixer, and
said output of said mixer being connected to said tap selecting means.
17. Apparatus as set forth in claim 13 wherein said phase changing means
for changing the phase of the alternating current electrical waves
includes a mixer having first and second inputs and an output, a phase
shifter connected between the output of said mixer and said tap selecting
means, the angle by which said phase shifter shifts the signals applied to
it being determined by connection to said control means, and a source of
waves having a frequency different from said carrier frequency, said
source being coupled to said second input of said mixer.
18. Apparatus as set forth in claim 13 wherein each means for changing the
phase of the alternating current electrical waves includes a mixer having
first and second inputs and an output, said first input of said mixer
being coupled to the transducer, a source for respectively supplying at
outputs thereof different phases of waves having a given frequency, means
for selectively coupling one of said outputs to said second input of said
mixer, the output selected being determined by connection to said control
means, whereby pulses of intermediate frequency waves of the selected
phase appear at the outputs of said mixers.
19. Apparatus as described in claim 18 wherein the means for supplying
different phases of a frequency different from that of said carrier waves
includes a digital clock and digital means for selecting one of several
output phases thereof and applying it to said second input of said mixer.
20. Apparatus as described in claim 18 wherein the frequency supplied by
said source is such that the frequency of said intermediate frequency
waves is lower than the frequency of the carrier waves.
21. Apparatus as described in claim 18 wherein the frequency supplied by
said source is such that the frequency of said intermediate frequency
waves is higher than the frequency of said carrier waves.
22. Apparatus as described in claim 18 wherein the frequency supplied by
said source is such that the frequency of said intermediate frequency is
the same as the frequency of the carrier waves.
23. Apparatus as described in claim 21 wherein said delay line means is an
acoustic delay line.
24. Apparatus as set forth in claim 23 wherein means are provided for
changing the phase of the intermediate frequency waves applied to each tap
by an amount such as to compensate for any error in the delay provided by
the tap.
25. In an ultrasonic scanner for producing real time images of the human
body from carrier presusre waves comprising,
an array of transducers for converting pulses of alternating carrier
pressure waves into corresponding pulses of alternating current electrical
waves, the difference in compensating delay required by at least some of
said transducers when they are focussed over a given range along a given
direction being greater than one-eighth a period of the carrier pressure
waves,
phase changing means coupled to each transducer,
delay line means having a plurality of input taps and an output, the delays
between said taps and said output differing by more than one-eighth of the
period of the carrier pressure waves,
means for connecting the output of each of said phase changing means to a
tap on said delay lines means during the time reflections are received
from targets within said given range, and
means for focussing said array between minimum and maximum points of said
range comprising means for causing at least some of said phase changing
means to change the phase of the electrical waves they provide in such
manner that they arrive at the output of said delay line means within less
than 180.degree. of each other.
26. In a system for producing real time images of a body from pulses of
ultrasonic carrier pressure waves, the combination of
a transducer,
first and second heterodyning means having inputs coupled to the output of
said transducer, said heterodyning means providing at their respective
outputs bands of intermediate frequencies having phase quadrature
relationships,
a first delay line means having a plurality of input taps and an output,
means for connecting the output of said first heterodyning means to an
input tap on said first delay line means,
a second delay line means having a plurality of input taps and an output,
means for connecting the output of said second heterodyning means to an
input tap on said second delay line means, and
a summing point coupled to said outputs of said delay lines.
27. The combination as set forth in claim 26 wherein there is also included
a first mixer having two inputs and an output, one of said inputs being
coupled to the output of said first delay line means, said output of said
mixer being connected to said summing point, and
a second mixer having two inputs and an output, one of said inputs being
coupled to the output of said second delay line means, said output of said
mixer being connected to said summing point, and
means for respectively applying waves having phase quadrature relationship
to the other inputs of said first and second mixers.
28. In apparatus for producing real time images of a portion of a body from
pulses of ultrasonic carrier waves, E.sub.h (t) cos (w.sub.c t +
.phi..sub.h), the combination of
a transducer,
a source of wave cos w.sub.1 t,
a first mixer having first and second inputs and an output, said first
input being connected to said transducer, said second input being
connected to said source,
a second mixer having first and second inputs and an output,
a band pass filter passing frequencies (w.sub.1 - w.sub.c) connected
between said output of said first mixer and said first input of said
second mixer,
delay line means having a plurality of input taps,
means for connecting the output of said second mixer to one of said taps,
and
a source of waves cos [w.sub.2 + .OMEGA..sub.h (t)] coupled to said second
input of said second mixer, wherein
.OMEGA..sub.h (t) = (w.sub.1 + w.sub.2)T.sub.h ** + w.sub.c [T.sub.h (t +
T.sub.h **) - T.sub.h **],
modulo 2.pi. radians, in which T.sub.h ** is the delay at the tap to which
the transducer is connected and T.sub.h (t) is the ideal delay for that
transducer.
29. In apparatus for producing real time images of a portion of a body from
pulses of ultrasonic carrier waves, E.sub.h (t) cos (w.sub.c t +
.phi..sub.h), the combination of
a transducer,
a source of wave cos w.sub.1 t,
a first mixer having first and second inputs and an output, said first
input being connected to said transducer, said second input being
connected to said source,
a second mixer having first and second inputs and an output,
a band pass filter passing frequencies (w.sub.1 - w.sub.c) connected
between said output of said first mixer and said first input of said
second mixer,
delay line means having a plurality of input taps,
means for connecting the output of said second mixer to one of said taps,
and
a source of waves cos [w.sub.2 + .OMEGA..sub.h (t)] coupled to said second
input of said second mixer, wherein
.OMEGA..sub.h (t) = (w.sub.2 - w.sub.1)T.sub.h ** - w.sub.c [T.sub.h (t +
T.sub.h **) - T.sub.h **],
modulo 2.pi. radians, in which T.sub.h ** is the delay at the tap to which
the transducer is connected and T.sub.h (t) is the ideal delay for that
transducer.
30. In apparatus for producing real time images of a portion of a body from
pulses of ultrasonic carrier waves, E.sub.h (t) cos (w.sub.c t +
.phi..sub.h), the combination of
a transducer,
a source of wave cos w.sub.1 t,
a first mixer having first and second inputs and an output, said first
input being connected to said transducer, said second input being
connected to said source,
a second mixer having first and second inputs and an output,
a band pass filter passing frequencies (w.sub.1 + w.sub.c) connected
between said output of said first mixer and said first input of said
second mixer,
delay line means having a plurality of input taps,
means for connecting the output of said second mixer to one of said taps,
and
a source of waves cos [w.sub.2 + .OMEGA..sub.h (t)] coupled to said second
input of said second mixer, wherein
.OMEGA..sub.h (t) = (w.sub.2 - w.sub.1)T.sub.h ** + w.sub.c [T.sub.h (t +
T.sub.h **) - T.sub.h **],
modulo 2.pi. radians, in which T.sub.h ** is the delay at the tap to which
the transducer is connected and T.sub.h (t) is the ideal delay for that
transducer.
31. In apparatus for producing real time images of a portion of a body from
pulses of ultrasonic carrier waves, E.sub.h (t) cos (w.sub.c t +
.phi..sub.h), the combination of
a transducer,
a source of wave cos w.sub.1 t,
a first mixer having first and second inputs and an output, said first
input being connected to said transducer, said second input being
connected to said source,
a second mixer having first and second inputs and an output,
a band pass filter passing frequencies (w.sub.1 + w.sub.c) connected
between said output of said first mixer and said first input of said
second mixer,
delay line means having a plurality of input taps,
means for connecting the output of said second mixer to one of said taps,
and
a source of waves cos [w.sub.2 + .OMEGA..sub.h (t)] coupled to said second
input of said second mixer, wherein
.OMEGA..sub.h (t) = (w.sub.2 + w.sub.1)T.sub.h ** - w.sub.c [T.sub.h (t +
T.sub.h **) - T.sub.h **],
modulo 2.pi. radians, in which T.sub.h ** is the delay at the tap to which
the transducer is connected and T.sub.h (t) is the ideal delay for that
transducer.
32. In apparatus for producing real time images of a portion of the body
from pulses of ultrasonic carrier waves, E.sub.h (t) cos (w.sub.c t +
.phi..sub.h), the combination of
a transducer,
a source of waves cos w.sub.1 t,
a first mixer having first and second inputs and an output, said first
input being connected to said transducer and said second input being
connected to said source,
a second mixer having first and second inputs and an output,
a band pass filter passing the frequencies (w.sub.1 - w.sub.c) and an
incremental delay line connected in series between the output of said
first mixer and said first input of said second mixer,
means for selecting the delay supplied by said incremental delay line,
a master delay line having a plurality of input taps,
means for connecting the output of said second mixer to one of said input
taps, and
a source of waves cos (w.sub.2 t + .OMEGA..sub.h (t)) coupled to said
second input of said second mixer, wherein
.OMEGA..sub.h (t) = w.sub.1 T.sub.h *(t + T.sub.MD) + w.sub.2 T.sub.MD +
w.sub.c [T.sub.h (t + T.sub.MD) - T.sub.h *(t + T.sub.MD)],
and in which T.sub.h * = the actual delay from the tap of the incremental
delay line to the output of the master delay line, and T.sub.h (t) is the
ideal compensating delay for the transducer.
33. In apparatus for forming real time images of portions of a body from
signals derived from pulses of ultrasonic pressure carrier frequency waves
w.sub.c transmitted into the body comprising
an array of transducers,
a plurality of mixers, each mixer having first and second inputs and an
output,
circuits respectively coupling said transducers to said first inputs of
said mixers,
a source of different phases of alternating current waves having a
frequency w.sub.o,
phase selectors for respectively applying at selected times a phase
.OMEGA..sub.h (t) of the alternating current waves from said source to
said second inputs of said mixers so as to produce upper and lower
sidebands at the outputs thereof, the phase .OMEGA..sub.h (t) being varied
with time so as to be within a predetermined number of degrees of the
angle specified by the expression
w.sub.o T.sub.h ** + w.sub.c [T.sub.h (t + T.sub.h **) - T.sub.h **]
wherein h is the distance from the transducer to the center line of the
array, T.sub.h ** is the actual delay between the tap to which a
transducer is coupled via its associated mixer and the output of the
master delay line and T.sub.h (t) is the ideal compensating delay that
would produce precise phase coherence,
a master delay line having a plurality of input taps and an output,
coupling circuits for respectively applying the outputs of said mixers to
selected input taps on said master delay line,
an output terminal coupled to said output of said delay line, and
filtering means for premitting only the lower sideband to pass to said
output terminal.
34. In apparatus for forming real time images of portions of a body from
signals derived from pulses of ultrasonic pressure carrier frequency waves
w.sub.c transmitted into the body comprising
an array of transducers,
a plurality of mixers, each mixer having first and second inputs and an
output,
circuits respectively coupling said transducers to said first inputs of
said mixers,
a source of different phases of alternating current waves having a
frequency w.sub.o,
phase selectors for respectively applying at selected times a phase
.OMEGA..sub.h (t) of the alternating current waves from said source to
said second inputs of said mixers so as to produce upper and lower
sidebands at the outputs thereof, the phase .OMEGA..sub.h (t) being varied
with time so as to be within a predetermined number of degrees of the
angle specified by the expression
w.sub.o T.sub.h ** - w.sub.c [T.sub.h (t + T.sub.h **) - T.sub.h **]
wherein h is the distance from the transducer to the center line of the
array, T.sub.h ** is the actual delay between the tap to which a
transducer is coupled via its associated mixer and the output of the
master delay line, and T.sub.h (t) is the ideal compensating delay that
would produce precise phase coherence,
a master delay line having a plurality of input taps and an output,
coupling circuits for respectively applying the outputs of said mixers to
selected input taps on said master delay line,
an output terminal coupled to said output of said delay line, and
filtering means for permitting only the upper sideband to pass to said
output terminal.
35. In an ultrasonic scanner for producing images from carrier pressure
waves
a plurality of transducers for converting pressure waves into corresponding
electrical waves,
first phase changing means coupled to each of said transducers,
delay line means having a plurality of taps and an output, said taps having
errors in their respective spacing from said output,
second phase changing means respectively coupled between the outputs of
said first phase changing means and each of said taps, said second phase
changing means adjusting the phase of signals applied to it so as to
compensate for errors in the spacing of said taps, and
means for applying the output of each phase changing means to a desired
input circuit.
36. An ultrasonic scanner as set forth in claim 35 wherein said second
phase changing means includes
a plurality of mixers, each having first and second inputs and an output,
one input of each mixer being respectively coupled to the output of one of
said first phase changing means, and
means for providing a frequency w.sub.1 of a given phase to the other input
of said mixer, the frequency w.sub.1 being such as to produce bands of
frequencies being within the pass band of said delay line, the given phase
having a first component compensating for errors in the spacing of the tap
to which the output of the mixer is connected and a second component equal
to w.sub.1 T.sub.h **, wherein T.sub.h ** is the delay provided by the tap
to which the output of the mixer is connected. |
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Description |
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BACKGROUND OF THE INVENTION
Much attention has been given in recent years to the development of
ultrasonic systems for producing real time images of internal portions of
the human body. In one system, an array of transducers for converting
short pulses of electrical alternating current carrier waves into
corresponding pressure waves is placed in contact with the body. By
choosing the relative times of application of the pulses of electrical
carrier waves to the transducers, the pressure waves for each pulse can be
formed into a beam extending in any desired direction, and the beam
direction can be changed so as to effectively scan a sector. As the pulses
of pressure waves pass through the body, a portion of their energy is
reflected whenever they encounter tissue having a different acoustic
characteristic. An array of receiving transducers is provided for
converting the reflected pressure waves into corresponding electrical
waves.
Precise focussing of the array of receiving elements at a given point
requires that all of the few cycles of the alternating current waves
derived by each of the transducers from a pulse of pressure waves
reflected from that point be superimposed in time synchrony at a summing
point so that the pulses are perfectly aligned. This produces a strong
signal whereas reflections of pressure waves from other points produce
weak signals because the corresponding electrical waves arrive at the
summing point with random phase relationships. The distances between any
desired focal point and the various receiving transducers being different,
the reflections arrive at the transducers at different times. It is
therefore necessary in order to achieve precise focussing to introduce
compensating delays between each transducer and the summing point so that
the total time between reflections of a pressure wave at the focal point
and the arrival of the corresponding electrical wave at the summing point
is the same regardless of which transducer is involved. The compensating
delays may be varied so that the focal point is dynamically scanned from
minimum to maximum range along each direction of the transmitted pulses.
In some present equipment, the variations in compensating delays are
achieved by changing taps on delay lines. The taps cannot be more than a
small fraction of the period of a carrier wave apart if the cycles of the
carrier wave are to arrive at the summing point nearly in phase. Inasmuch
as the total change in compensating delay for some transducers as they are
focussed from minimum to maximum range and from minimum to maximum sector
angle is equal to many periods of the carrier wave, the number of taps
required is large. At the carrier frequencies employed, only the
relatively expensive electrical delay line can be used because of
bandwidth considerations, and the provision of a large number of taps on
this type of line is a significant portion of the cost of the entire
instrument.
Of equal significance is the fact that, unless expensive tap changing
switches are used, the switching transients cause a significant amount of
noise in the signals arriving at the summing point and therefore in the
image produced from them.
The problems of cost and transient noise just referred to increase in
severity when the carrier frequency is increased to obtain better
definition, the aperture of the array is increased to obtain better
focussing, or the minimum range is decreased so as to permit the
examination of infants.
BRIEF DESCRIPTION OF THE INVENTION
This invention is especially advantageous when focussing within the near
field. Instead of dynamically focussing an array of transducers from
minimum to maximum range along each of a plurality of directions by
varying their respective compensating delays with changes in connection
from one closely spaced delay line tap to another, dynamic focussing is
effected in accordance with this invention by changing the phase of AC
waves derived from each transducer in such manner that the waves related
to each focal point arrive at the summing point with reasonable phase
coherence. Inasmuch as the delay line taps are not relied on for producing
phase coherence, it is only necessary that they be close enough to provide
reasonable overlap at the summing point of the pulses of AC waves derived
from the various transducers. Thus, instead of being spaced by a small
fraction of the period of an AC wave, the taps can be spaced much farther
apart. A spacing corresponding to a delay approximately equal to half the
duration of a pulse has been found advantageous. Furthermore, only one or
at the most a few tap settings will be required for any transducer while
scanning from minimum to maximum range so that the occurrence of tap
switching transients are either of no consequence or have very little
effect.
The phase changing may be accomplished by phase shifters inserted at some
point between each transducer and the summing point, but in accordance
with a preferred aspect of the invention, the phase changing is more
simply and less expensively effected by respectively heterodyning the
carrier waves from each transducer with different phases of an oscillator
output that are selected so as to focus the array at one point and
changing the phases selected in order to focus the array at successive
points along a given direction. The intermediate frequency waves thus
derived from each transducer are applied to one of the coarsely spaced
taps on a delay line system that have delays nearest to that required for
precisely focussing that transducer. With a heterodyning system, the
oscillator and the phase selecting means can be digital and therefore less
expensive and less noisy than analog oscillators and phase selectors.
By selection of the frequency of the local oscillator, one of the
intermediate frequency sidebands of the mixer outputs can be low enough in
frequency to permit the use of cheaper electrical delay lines, or it can
be made high enough in frequency to permit the use of surface acoustic
delay lines that cost must less than an electrical delay line. With
respect to cost mush less than an electrical delay line. With respect to
the surface acoustic wave delay line, the use of more widely spaced taps
in accordance with this invention simplifies design problems because there
are fewer taps to cause troublesome reflections. In either case, the phase
angle of each local oscillator must include, for reasons that will be
explained, a component that compensates for the fact that the delay
provided by the coarsely tapped delay line is not that required for
precise focussing as well as a component that takes into account the fact
that the heterodyning is done ahead of the delay lines.
Another advantage of heterodyning, aside from making it possible to use
different types of delay lines, is that if a different carrier frequency
is to be used it is only necessary to change the oscillator frequency.
Many systems for utilizing heterodyning in accordance with the invention
are possible, each with its own advantages. In some, double heterodyning
and/or incremental delay lines may be used. In others, the output of each
transducer may be heterodyned with quadrature phases of the oscillator
output before being applied to separate delay lines. Whatever the
arrangement, the preferred utilization of the invention involves
heterodyning the outputs of each transducer prior to the delay lines,
controlling the phase of the intermediate frequency waves produced by the
heterodyning, and applying the intermediate frequency waves derived from
each transducer to only one or at the most a few coarsely spaced taps on a
delay line during the focal scansion along each direction.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates an ultrasonic system of the prior art;
FIG. 2 illustrates dynamic focussing of the prior art;
FIG. 3 illustrates dynamic focussing by use of phase shifters;
FIG. 4 illustrates dynamic focussing by heterodyning;
FIG. 5A shows a transducer channel of the prior art;
FIG. 5B shows a channel of the invention having an ideal delay;
FIGS. 5C and 5D show channels of the invention having imprecise delays;
FIG. 6 includes graphs showing the change in delay with time of various
transducers as they are focussed along a direction of +45.degree. with the
center line of an array;
FIG. 7 illustrates a portion of an ultrasonic system using heterodyning for
changing the carrier frequency and attaining phase coherence;
FIG. 7A shows an arrangement for using an acoustic delay line in FIG. 7;
FIG. 8 illustrates a portion of an ultrasound system employing heterodyning
and wherein the phase of the intermediate frequency wave is changed by
phase shifters;
FIG. 9 illustrates an ultrasonic system in which phase quadrature
techniques are used;
FIG. 10 illustrates a double heterodyning system; and
FIG. 11 illustrates a double heterodyning system incorporating an
incremental delay line.
GENERAL CONSIDERATIONS
For purposes of discussion of the general factors involved, consider the
ultrasonic system of the prior art shown in FIG. 1. It is comprised of a
planar array 2 of transducers that can be used for transmitting acoustic
pulses as well as receiving them. A group 4 of delay lines is provided,
each having one end connected to a different transducer and a tap
connected to a summing point S. The array 2 is shown in contact with a
body 6 containing an internal organ O that is to be examined. By means not
shown, a few cycles of electrical waves having a carrier frequency w.sub.c
are applied to the transducers so as to cause them to transmit pulses of
ultrasonic waves of the carrier frequency w.sub.c into the body 6. The
relative timing with which the transducers of the array 2 are energized
determines the direction and the shape of the wavefront of the beam of
acoustic energy thus radiated into the body. If, for example, they are
successively energized beginning with the bottom transducer of the array
2, the beam may take a direction at an angle .theta. with respect to the
center line of the array so that most of the acoustic energy would be in
the form of a wave with a planar front moving away from the array 2
between dotted lines 10 and 12. Instead of forming a beam of acoustic
energy, it would be possible to energize the transducers so as to cause
the wave of acoustic energy to have a curved front, but as far as this
invention is concerned, the particular manner of transmitting acoustical
energy into the body 6 is unimportant.
In the case illustrated, where a pulse of acoustic energy is transmitted
into the body 6 at an angle .theta., the receiving array 2 is focussed
successively at points f.sub.1, f.sub.2, f.sub.3 and f.sub.4, etc., until
a maximum range is reached. Then another pulse is transmitted in a
slightly different direction and the array is progressively focussed along
this new direction. The process is repeated until a desired sector is
scanned. When the reflected pulses of acoustic carrier waves reach the
transducers, they produce corresponding pulses of electrical carrier waves
that are added together at the summing point S after each has been
suitably delayed by one of the delay lines 4. The voltage at the summing
point S is used to modulate the intensity of the electron beam of a
cathode ray tube 14. The beam of the cathode ray tube is deflected so as
to follow radial paths corresponding to the different directions scanned
by the focal point of the array 2.
Perfect focussing requires that all the carrier wave cycles of the pulses
of electrical waves from all transducers arrive at the summing point
precisely in phase. The delay, T.sub.h (t), required for perfect focussing
of each transducer of a planar array may be defined as that delay which
produces an output X(t-T.sub.h (t)) when a signal X(t) is applied to its
input and may be expressed as follows:
##EQU1##
wherein t is the time following the transmission of a pulse from the
center of the array, h is the distance of the transducer from the center
line of the array, c is the average velocity of the acoustic wave in the
body being examined, and .theta. is the angle between the center line of
the array and the radial line from the center of the array to the focal
point. T.sub.o is a fixed delay that is included in each transducer
channel so as to prevent the ideal delay, T.sub.h (t), as determined by
equation (1) from ever becoming negative.
In prior art arrangements, such as illustrated in FIG. 1, the ideal delay,
T.sub.h (t), can be reasonably approximated if the delay line taps are
close enough together. For example, if it is decided to keep the cycles of
a carrier frequency of 2.5 Mhz within +22.5.degree. of the phase required
for perfect focussing, the taps would have to be spaced 50 ns apart, a
spacing that would be very expensive.
CONCEPT OF THE INVENTION
Reference is made to FIGS. 2, 3 and 4 for an illustration of some of the
advantages derived from the use of certain aspects of the invention.
FIG. 2 shows a system of the prior art, such as already discussed in
connection with FIG. 1, wherein transducers TR.sub.1 and TR.sub.2 are
respectively connected to the ends of delay lines D.sub.1 and D.sub.2. The
taps on the delay lines are a fraction of a carrier wavelength apart. If
the tap switches s.sub.1 and s.sub.2 for the delay lines D.sub.1 and
D.sub.2 happen to be connec | | |
