U.S. patent application number 11/917787 was filed with the patent office on 2009-12-10 for ultrasound imaging apparatus.
Invention is credited to Kunio Hashiba, Hiroshi Masuzawa, Satoshi Tamano, Shinichiro Umemura.
Application Number | 20090306510 11/917787 |
Document ID | / |
Family ID | 37532057 |
Filed Date | 2009-12-10 |
United States Patent
Application |
20090306510 |
Kind Code |
A1 |
Hashiba; Kunio ; et
al. |
December 10, 2009 |
Ultrasound Imaging Apparatus
Abstract
An ultrasound imaging apparatus comprising a plurality of
transducer elements images an object by making use of the plurality
of transducer elements whose received signals are given delays,
transmitting a pulse ultrasonic wave to the object and receiving
its reflected wave. The transducer elements are divided into a
plurality of blocks and the transducer elements in each of the
blocks are selected by a selecting means so that the delays given
to the received signals for the transducer elements in each of the
blocks are identical.
Inventors: |
Hashiba; Kunio; (Tokyo,
JP) ; Umemura; Shinichiro; (Sendai, JP) ;
Masuzawa; Hiroshi; (Machida, JP) ; Tamano;
Satoshi; (Kashiwa, JP) |
Correspondence
Address: |
ANTONELLI, TERRY, STOUT & KRAUS, LLP
1300 NORTH SEVENTEENTH STREET, SUITE 1800
ARLINGTON
VA
22209-3873
US
|
Family ID: |
37532057 |
Appl. No.: |
11/917787 |
Filed: |
January 30, 2006 |
PCT Filed: |
January 30, 2006 |
PCT NO: |
PCT/JP2006/301404 |
371 Date: |
August 12, 2009 |
Current U.S.
Class: |
600/447 |
Current CPC
Class: |
G01S 15/8927 20130101;
G01S 15/8922 20130101; G01S 7/52095 20130101; G01S 15/8925
20130101; G01S 15/8993 20130101; G10K 11/346 20130101 |
Class at
Publication: |
600/447 |
International
Class: |
A61B 8/14 20060101
A61B008/14 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 17, 2005 |
JP |
2005-177131 |
Jun 17, 2005 |
JP |
2005-177133 |
Claims
1. An ultrasound imaging apparatus comprising, a two-dimensional
ultrasound transducer array which consists of a plurality of
transducer elements distributed two-dimensionally and transmits a
pulse ultrasonic wave to an object, each of the transducer elements
receives a reflected wave of the pulse ultrasonic wave, the
received signal being given a delay corresponding to an elapsed
time from a transmit time when the pulse ultrasonic wave is
transmitted to a receive time when each of the transducer elements
receives the reflected wave of the pulse ultrasonic wave for the
object to be imaged, and a selecting means for selecting the
transducer elements, wherein the transducer elements are divided
into a plurality of blocks and the transducer elements in each of
the blocks are selected by the selecting means so that the delays
given to the received signals for the transducer elements in each
of the blocks are identical.
2. The ultrasound imaging apparatus according to claim 1, wherein a
plurality of beams o be arranged at different positions on a focal
plane are formed within a time of the delay, and wherein the
selecting means is a multiplexer switch that connects mutually the
transducer elements in an identical phase area with respect to
forming a focal point at the center position of the plurality of
beams.
3. The ultrasound imaging apparatus according to claim 2, wherein a
boundary between the blocks is a divisional line corresponding to a
reciprocal lattice for the different positions.
4. The ultrasound imaging apparatus according to claim 2, wherein
an aperture of the two-dimensional ultrasound transducer array is
shaped in a rectangle, and the boundaries of the blocks are
parallel to sides or diagonals of the rectangle.
5. The ultrasound imaging apparatus according to claim 4, wherein
the boundaries of the blocks divide the rectangle equally into four
rows and four columns parallel to sides of the rectangle, and a
number of the beams formed is four.
6. The ultrasound imaging apparatus according to any of claims 1 to
4, wherein the two-dimensional ultrasound transducer array is
divided into n number of blocks of M1, M2, . . . , or Mn transducer
elements, each block having Nn, less than Mn, leading lines, and
the selecting means comprises Mn multiplexer switches having one
input and Nn outputs, respectively connected to the transducer
elements of an n-th block of the blocks, where n, M, N are natural
numbers.
7. An ultrasound imaging apparatus comprising a two-dimensional
array which consists of a plurality of transducer elements
distributed two-dimensionally and transmits and receives ultrasonic
waves while scanning an area to be imaged to create an ultrasound
three-dimensional image, wherein the transducer elements are
divided into a plurality of element blocks including a first
element block of which a size in a second direction of an
arrangement surface of the two-dimensional array is larger than a
size in a first direction of the surface, and a second element
block of which a size in the first direction is larger than a size
in the second direction, and each of the element blocks is divided
into a predetermined number of groups so as to form a transmit beam
and a plurality of receive beams in the area to be imaged, the
ultrasound imaging apparatus further comprising a selecting means
for making transmit/receive channels of the transducer elements
grouped to be one channel in each of the groups.
8. An ultrasound imaging apparatus comprising a two-dimensional
array which consists of a plurality of transducer elements
distributed two-dimensionally and transmits and receives ultrasonic
waves while scanning an area to be imaged to create an ultrasound
three-dimensional image, wherein the transducer elements are
divided into a plurality of element blocks including a differently
shaped element block, and each of the element blocks is divided
into a predetermined number of groups so as to form a transmit beam
and a plurality of receive beams in the area to be imaged, the
ultrasound imaging apparatus further comprising selecting means for
making transmit/receive channels of the transducer elements grouped
to be one channel in each of the groups.
9. The ultrasound imaging apparatus according to claim 7 or 8,
wherein the element blocks are rectangular.
10. The ultrasound imaging apparatus according to claim 7, wherein
the element blocks are arranged symmetric with respect to the first
direction and the second direction.
11. The ultrasound imaging apparatus according to claim 7 or 8,
wherein the selecting means changes a pattern of the groups between
when forming the transmit beam and when forming the receive
beams.
12. The ultrasound imaging apparatus according to claim 7 or 8,
wherein the size in the first direction of the first element block
is equal to or smaller than the product of the number of channels
per element block and an element pitch of the two-dimensional
array, and the size in the second direction of the second element
block is equal to or smaller than the product of the number of
channels per element block and the element pitch of the
two-dimensional array.
13. The ultrasound imaging apparatus according to claim 7 or 8,
wherein the selecting means divides each of the element blocks into
the predetermined number of the groups such that the integral of
occurrence frequency of a delay to be given to the transducer
element over a range of the delay for each of the groups is the
same.
14. An ultrasound imaging apparatus which has a two-dimensional
array having a plurality of transducer elements arranged
two-dimensionally and, by the two-dimensional array, transmits and
receives ultrasonic waves scanning an area to be imaged to produce
an ultrasound three-dimensional image, wherein the transducer
elements are divided into a plurality of element blocks including
an element block different in width in a elevational axis direction
or a lateral axis direction of an arrangement surface of the
two-dimensional array, and each of the element blocks is divided
into a predetermined number of groups so as to form a transmit beam
and a plurality of receive beams in the area to be imaged, the
ultrasound imaging apparatus further comprising selecting means for
making transmit/receive channels of the transducer elements grouped
to be one channel in each of the groups.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application incorporates the disclosure of U.S.
patent application Ser. No. 11/719,770 filed on May 21, 2007,
herein by reference in its entity.
TECHNICAL FIELD
[0002] The present invention relates to an ultrasound imaging
apparatus and particularly to an ultrasound imaging apparatus
having a two-dimensional array of electro-acoustic transducer
elements (transducers) arranged two-dimensionally that, by the
two-dimensional array, transmits and receives ultrasonic waves
scanning an area to be imaged and produces an ultrasound
three-dimensional image.
BACKGROUND ART
[0003] Ultrasonic diagnostic apparatuses utilizing a pulse-echo
method that transmit pulsed ultrasonic waves to a living body and
receive the reflected waves thus imaging the inside of the living
body are widely used for medical diagnosis, as well as X-ray and
MRI.
[0004] In order to achieve three-dimensional imaging for medical
diagnosis with use of a two-dimensional array of ultrasound
transducers, the number of signal lines led out from the
transducers poses a problem. That is, because the two-dimensional
array needs about 10.sup.3 to 10.sup.4 transducers in total, if a
signal line is individually led out from every transducer, the
number of the signal lines will be so great that the connection
cable becomes too thick to handle.
[0005] In order to solve this problem, a method is disclosed in
Japanese Patent Application Laid-Open Publication No. 2001-286467
(hereinafter called a reference 1), where switch circuitry is
mounted on a two-dimensional array of ultrasound transducers, and
elements forming the array are connected together as needed via the
switch circuitry connecting to a cable, thereby reducing the number
of cable cores led out by the order of one or two digits. The phase
distribution at the plane reception surface of ultrasonic waves
emitted from the focal position takes the form of concentric
circles. Hence, in the reference 1, elements on the same circle of
the concentric circles are connected together to the same cable
core so as to lead out a signal. Further, because the pattern for
connecting elements together needs to vary according to the beam
formation direction, the connection pattern is changed using the
switch circuitry.
[0006] In achieving three-dimensional imaging with use of a
two-dimensional array of ultrasound transducers, another problem is
with forming a plurality of beams simultaneously. High
information-acquisition throughput is needed to acquire a large
amount of image information necessary to form a three-dimensional
image with utilizing high time resolution characteristic of the
ultrasound imaging. Thus, the use of multiple beams for the
simultaneous transmit/receive beam is indispensable. However, when
using the element connection patterns of the reference 1 as they
are, only one transmit/receive beam is formed corresponding to one
pattern. Thus, this method is not suitable for high speed
imaging.
[0007] Accordingly, a first object of the present invention is to
provide an ultrasound imaging apparatus capable of simultaneously
forming multiple beams suitable for high speed imaging at a low
cost.
[0008] In order to realize the dynamic state of a dynamic part of
an object three-dimensionally in real time by, for example, the
observation of blood flow through a coronary artery of the heart
and the measurement of systolic output, it was considered to obtain
three-dimensional images in real time using an ultrasound probe
comprising a two-dimensional array having electro-acoustic
transducer elements arranged in a plane. However, because there was
a conflicting relationship between the breadth of the field of view
(the depth and viewing angle), the height of resolution, and the
height of a frame rate (real-time capability), in order to improve
an element, another element had to be sacrificed.
[0009] For example, assuming that the viewing angle is 60 degrees
in both the lateral axis direction and elevational axis direction
of the two-dimensional array and that the scan line interval is 1.5
degrees, then the number of scan lines per frame is 1,600. In order
to obtain images of an object up to a depth of, e.g., 20 cm (the
both-way distance for ultrasonic waves being 40 cm), scan time per
scan line is at least about 260 .mu.s because the speed of sound in
usual parts of a living body is about 1,530 m/s. Thus, in this
case, the frame cycle is about 0.4 sec and the frame rate is about
2.5 Hz, so that a frame rate of 20 to 30 Hz or greater, which is
necessary for the observation of the cardiac dynamic state, could
not be achieved.
[0010] Accordingly, in, e.g., Japanese Patent No. 2961903
(paragraphs 0008-0009, FIG. 3), an ultrasound three-dimensional
imaging apparatus has been proposed which has a phase adjusting
circuit that adjusts the phases of received signals output from a
two-dimensional array of oscillators, which are divided into
groups, to simultaneously form, e.g., four receive beams deflected
at different small angles relative to a transmit direction.
[0011] Moreover, in Japanese Patent Application Laid-Open
Publication No. 2000-33087 (paragraph 0090, FIG. 11), a phased
array acoustic apparatus with in-group processors has been proposed
where an array of 3,000 transducers is divided into 120 groups, or
sub-arrays, each comprising 25 transducers and where an in-group
processor delays and sums individual transducer signals and
supplies the summed signal to one channel of a receive beam
former.
[0012] Furthermore, in Japanese Patent Application Laid-Open
Publication No. 2001-286467 (paragraph 0021, FIG. 3), an ultrasound
diagnostic apparatus has been proposed where a delay corresponding
to the distance to the focal point is given to each group of
oscillators in a concentric annular area of a two-dimensional
oscillator array such that ultrasonic waves emitted from each
ring-like group of oscillators are converged on the focal point and
that the ultrasonic waves reflected from the focal point are
directed to the ring-like group of oscillators.
[0013] With a conventional ultrasound three-dimensional imaging
apparatus, if four receive beams are formed simultaneously for one
transmit beam, with the same breadth of the area to be imaged and
the same resolution, a frame rate will quadruple. Hence, in the
above example, in order to achieve a frame rate of 20 Hz or
greater, eight or more ultrasound receive beams need to be formed
for one ultrasound transmit beam.
[0014] However, in order to obtain images of sufficient resolution,
a two-dimensional array of several thousand oscillators needs to be
used. Accordingly, the conventional ultrasound imaging apparatus
requires several thousand delay means and summing means, so that
the size of a delay-and-sum circuit becomes huge. Thus, there is
the problem that it is difficult to realize the apparatus as well
as production costs being high. Further, if it is produced, the
number of connection lines from the two-dimensional array of
oscillators will be several thousand, resulting in imaging
operation being actually impossible.
[0015] Generally, in order to obtain sufficient resolution, the
aperture length of the two-dimensional array of oscillators needs
to be made as large as possible to use a large number of
electro-acoustic transducer elements. However, a receive beam
former having several thousand input channels is unrealistic in
terms of circuit size. Hence, it has been considered to reduce
several thousand channels of electro-acoustic transducer elements
to about 100 to 200 channels.
[0016] With the conventional phased array acoustic apparatus with
in-group processors, because the number of channels is reduced, the
circuit size is reduced and thus the improvement in operability can
be expected. However, a grating lobe may occur depending on the
shape of the sub-arrays of transducer elements. Thus, sufficient
resolution and contrast may not be obtained, or noise or a false
image may occur, so that desired image quality may not be obtained.
Further, if more finely grouped, the number of channels increases
and the circuit size increases, so that a desired frame rate may
not be achieved.
[0017] Moreover, with the conventional ultrasound diagnostic
apparatus, there are the following problems. Because the ring width
of each ring-like group of oscillators is constant (a pitch of two
elements), the intervals between the groups may be almost equal to
the wavelength (the pitch of two elements) depending on the
direction in which the ultrasound beam is directed, and thus a
large grating lobe may occur and degrade image quality. If the
intervals between the groups are decreased to suppress the
occurrence of a grating lobe, the circuit size increases. Further,
because the number of oscillators is extremely different between
the inner ring and the outer ring, electrical characteristics such
as impedance are greatly different for each ring. Thus, the size of
circuitry for correction becomes large, or image quality is
reduced. Or, if thinning the oscillators out so as to make the
electrical characteristics the same for each ring, resolution will
be reduced.
[0018] As such, in the case of reducing the number of channels of
electro-acoustic transducer elements by grouping them, there is the
problem that, because a conflicting relationship exists between
reducing the number of channels and suppressing a grating lobe, as
the number of channels is reduced, image quality is degraded.
[0019] Accordingly, a second object of the present invention is to
solve the above problems and provide an ultrasound imaging
apparatus that can produce ultrasound three-dimensional images with
a broad field of view, high resolution, and a high frame rate at
low cost.
DISCLOSURE OF THE INVENTION
[0020] To achieve the first objective, according to the present
invention there is provided an ultrasound imaging apparatus in
which a two-dimensional ultrasound transducer array having a
plurality of transducer elements arranged two-dimensionally
transmits pulse ultrasonic waves to an object, and each of the
transducer elements receives a reflected wave of the pulse
ultrasonic wave and which gives the received signal a delay
corresponding to elapsed time from a time that each of the
transducer elements transmitted to a time that the transducer
element received so as to image the object. The plurality of
transducer elements are divided into a plurality of blocks, and the
delay is given to the received signal that has passed through
selecting means for selecting from the transducer elements in each
of the blocks.
[0021] The plurality of transducer elements arranged
two-dimensionally are divided along concentric circles, and by
giving the same delay to the elements in each divided concentric
annular area, ultrasonic waves to converge on a focal point are
generated. Each transducer element receives a reflected wave of the
ultrasonic waves irradiated onto an object at the focal point.
Then, by giving the received signal a delay corresponding to
elapsed time from a time that each transducer element transmitted
to a time that the transducer element received, the object is
imaged. The plurality of transducer elements divided along
concentric circles are divided into a plurality of blocks, and the
selecting means connects the transducer elements in each concentric
annular area part in each block so that delays are given to
received signals on a per block basis. By this means, using a
plurality of receive beams simultaneously formed corresponding to
the divided blocks, the object can be imaged.
[0022] According to the present invention, there is provided an
ultrasound imaging apparatus which can simultaneously form a
plurality of beams suitable for high speed imaging at a low cost.
In particular, because of connecting together transducer elements
in each of equal phase areas in forming the plurality of beams, the
number of cable cores leading out from the transducer elements is
reduced.
[0023] To achieve the second object, according to the present
invention there is provided an ultrasound imaging apparatus which
has a two-dimensional array having a plurality of transducer
elements arranged two-dimensionally and, by the two-dimensional
array, transmits and receives ultrasonic waves scanning an area to
be imaged to produce an ultrasonic three-dimensional image. The
transducer elements are divided into a plurality of element blocks
including a first element block of which a size in a second
direction of an arrangement surface of the two-dimensional array is
larger than a size in a first direction of the surface, and a
second element block of which a size in the first direction is
larger than a size in the second direction, and each of the element
blocks is divided into a predetermined number of groups so as to
form a transmit beam and a plurality of receive beams in the area
to be imaged, the ultrasound imaging apparatus further comprising
selecting means for making transmit/receive channels of the
transducer elements in each of the groups converge so as to be
reduced to one channel.
[0024] With the ultrasound imaging apparatus of the present
invention, real-time ultrasound three-dimensional images with a
broad field of view, high resolution, and a high frame rate can be
produced at low cost.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1 shows a relationship between the position of a focal
point relative to a transducer transmit/receive surface and a phase
distribution on the transmit/receive surface;
[0026] FIG. 2 shows a relationship between a distribution of
multiple beams on a focal plane and aperture divisional lines;
[0027] FIG. 3 shows an array element connection pattern to form
focal points of multiple beams on a line perpendicular to the
transducer reception surface;
[0028] FIG. 4 shows an array element connection pattern to form
focal points of multiple beams in an oblique direction relative to
the line perpendicular to the transducer reception surface;
[0029] FIG. 5 shows an array element connection pattern to form
focal points of multiple beams in an oblique direction relative to
the line perpendicular to the transducer reception surface;
[0030] FIG. 6 is a block diagram showing the configuration of an
ultrasound diagnostic apparatus of an embodiment;
[0031] FIG. 7 shows distribution of a first receive beam on a focal
plane when the focal point is shifted in azimuth;
[0032] FIG. 8 shows distribution of a second receive beam on the
focal plane when the focal point is shifted in azimuth;
[0033] FIG. 9 shows distribution of a third receive beam on the
focal plane when the focal point is shifted in azimuth;
[0034] FIG. 10 shows distribution of a fourth receive beam on the
focal plane when the focal point is shifted in azimuth;
[0035] FIG. 11 shows the configuration of a two-dimensional array
configured such that aperture divisional lines are at an angle
greater than 0.degree. and less than 90.degree. to element
divisional lines;
[0036] FIG. 12 shows distribution of receive beams in azimuth
orthogonal or parallel to aperture divisional lines;
[0037] FIG. 13 shows distribution of receive beams in azimuth
orthogonal or parallel to element divisional lines;
[0038] FIG. 14 is a block diagram showing the configuration of an
ultrasound imaging apparatus of the present invention;
[0039] FIG. 15 illustrates the concept of the block division of the
two-dimensional array;
[0040] FIG. 16 illustrates the way to group electro-acoustic
transducer elements;
[0041] FIG. 17 is a block diagram showing in detail the
configuration of a selector unit;
[0042] FIG. 18 is a block diagram showing in detail the
configuration of the selector unit through a receive beam former
for processing received signals from selectors;
[0043] FIG. 19 is a block diagram showing the configuration of
another ultrasound imaging apparatus of the present invention;
[0044] FIG. 20 is a pattern diagram showing the block division of a
two-dimensional array as a comparative example;
[0045] FIG. 21 is a pattern diagram showing grouping as a
comparative example;
[0046] FIG. 22 illustrates a relationship between a Fresnel zone
plate and the two-dimensional array;
[0047] FIG. 23 is a pattern diagram showing an example of the block
division of the two-dimensional array according to the present
invention;
[0048] FIG. 24 is pattern diagrams showing the grouping according
to the present invention;
[0049] FIG. 25 is pattern diagrams showing other examples of the
block division of the two-dimensional array according to the
present invention;
[0050] FIG. 26 is a conceptual diagram showing a geometric
positional relationship between an element block of the blocks into
which the two-dimensional array is divided and a focal point for
determining the grouping pattern;
[0051] FIG. 27 is a histogram showing a frequency distribution of
delays to be given to the electro-acoustic transducer elements in
an element block, to illustrate the grouping method for the
comparative example;
[0052] FIG. 28 is a histogram showing a frequency distribution of
delays to be given to the electro-acoustic transducer elements in
an element block, to illustrate the grouping method of the present
embodiment;
[0053] FIG. 29 is a pattern diagram showing a comparative example
of grouping;
[0054] FIG. 30 is a pattern diagram showing a first example of
grouping;
[0055] FIG. 31 is a pattern diagram showing a second example of
grouping;
[0056] FIG. 32 is a pattern diagram showing a third example of
grouping;
[0057] FIG. 33 illustrates a display coordinate system for a beam
profile;
[0058] FIG. 34 is a contour line diagram showing the beam profile
in a (u, v) coordinate system for the grouping pattern of the
comparative example;
[0059] FIG. 35 is a contour line diagram showing the beam profile
in the (u, v) coordinate system for the grouping pattern of the
first example;
[0060] FIG. 36 is a contour line diagram showing the beam profile
in the (u, v) coordinate system for the grouping pattern of the
second example; and
[0061] FIG. 37 is a contour line diagram showing the beam profile
in the (u, v) coordinate system for the grouping pattern of the
third example.
BEST MODE FOR CARRYING OUT THE INVENTION
First Embodiment
[0062] A first embodiment of the present invention will be
described below in detail with reference to FIGS. 1 to 13.
(Basic Principle)
[0063] The greatest advantage of an ultrasound diagnostic apparatus
that other image diagnostic modalities such as X-ray and MRI do not
have is that its imaging speed is so high as to enable real-time
image display. That is, this imaging speed is such that the
ultrasound diagnostic apparatus can update an image every about 30
ms, which is the time resolution of the human visual sense.
Further, it is possible to even achieve time resolution with which
to obtain an image every 15 ms for the diagnosis of the motion of a
cardiac valve by slow speed reproduction.
[0064] Meanwhile, depth-wise (z direction) distance resolution from
among spatial resolutions of a pulse echo method is determined by a
time resolution required for an ultrasound pulse to travel to and
back from a reflector. Because the propagation speed of ultrasonic
waves in a living body is 1,500 m/s. This propagation speed is
almost the same as in water, if the ultrasonic frequency is at or
above several MHz, distance resolution of about 1 mm can be easily
obtained from a time resolution of about 1 .mu.s.
[0065] Spatial resolutions in directions (x, y directions)
orthogonal thereto, i.e., azimuth resolutions are increased by
focusing transmit waves or receive waves. In order to obtain
azimuth resolutions of no greater than several times the ultrasonic
wavelength, the focus needs to be so strong that the F number,
which is the ratio of aperture to focal distance, becomes close to
one. Thus, the depth of focus field corresponding to the depth of
field for cameras becomes as small as several times the wavelength,
which corresponds to about 1 .mu.s in which an ultrasonic wave
propagates both ways. Because of recent years' remarkable advance
in high speed electronic circuit technology, reception focal
distance can be changed while an ultrasonic wave propagates over
this distance. This real-time reception dynamic focus technique
enables always-focused imaging when receiving.
[0066] A living body is an existent occupying a three-dimensional
space, and in observing the state of a disease developed therein
through its images, essentially, three-dimensional observation
should be performed if possible. In order to perform
three-dimensional imaging while utilizing its high time resolution,
which is an advantage of ultrasound imaging, and maintaining the
high spatial resolution that has been achieved recently as
described above, a two-dimensional ultrasound transducer array
needs to be used to electronically scan along all the dimensions,
because with a currently widely used method which mechanically
scans a one-dimensional ultrasound transducer array
(one-dimensional probe) along another dimension, it is difficult to
move manually or mechanically the probe being in contact with the
body surface of an object having irregular surfaces for
three-dimensional measurement.
[0067] In achieving three-dimensional imaging for medical diagnosis
using a two-dimensional ultrasound transducer array, there are at
least two major problems. One is the problem with the number of
signal lines led out from the ultrasound transducer array.
[0068] In order to freely form transmit and receive beams by the
transducer array, the size in an arrangement direction of the
elements forming the array needs to be equal to or smaller than
about half the wavelength. This size is about 0.25 to 0.37 mm when
an ultrasonic frequency of 2 to 3 MHz, usual in medical diagnosis,
is used. Meanwhile, even a small-size transmit/receive aperture
whose azimuth resolution is sacrificed somewhat to obtain an image
of the heart between ribs is about 12 to 20 mm in width.
[0069] Accordingly, the two-dimensional array needs about 10.sup.3
to 10.sup.4 elements in total, and if all signal lines connected
thereto are individually led out from the ultrasound transducers,
the number of the signal lines will be great, resulting in the
cable being thick and difficult to handle. In order to solve this
problem by dealing with the cable, the cable smaller in outside
diameter is needed. Accordingly, the cable cores whose thickness is
already close to the limit of production technology need to be made
even thinner, which is difficult to achieve. Further, the problem
occurs that a large number of delay circuits need to be provided,
which causes the capacitance of the ultrasound transducer to be
smaller than the capacitance of the cable connected thereto, so
that a received signal voltage is lowered.
[0070] Next, attention is paid to the phase distribution when
ultrasonic waves emitted from a point reflector at the focal
position reach a substantially planar transducer reception surface.
The equal phase areas thereof form a concentric circular Fresnel
distribution with the foot of the perpendicular line from the point
reflector to the transducer reception surface as the center as
shown in FIG. 1. Conversely, by transmitting with giving the same
delay time to the elements in each concentric circular area,
ultrasonic waves to converge on a focal point are generated. The
pattern of connecting elements together changes according to the
beam formation direction: 1001a when the focal point is right in
front of the transducer reception surface, and 1001b or 1001c when
in an oblique direction. Where multiple receive beams are formed,
the equal phase areas that are formed by ultrasonic waves emitted
from the point reflectors respectively at multiple focal points
when reaching the transducer reception surface are slightly shifted
from each other according to the shift amount in azimuth between
the multiple focal points.
[0071] Thus, in order to form the multiple receive beams
simultaneously, the transducer reception surface is divided into a
number of blocks, and it is effective to lead out signal lines from
each block using an equal phase area connection pattern that is
average for the multiple focal points.
[0072] FIG. 2 shows a relationship between a distribution of
multiple beams on a focal plane and aperture divisional lines. In
the upper side of FIG. 2, aperture divisional lines dividing the
aperture of the transducer reception surface into parts are
represented by broken lines. The lower side of FIG. 2 shows the
distribution of four receive beams on the focal plane, and the
center position (indicated by "x"). In other words, the signal
lines of the elements in each equal phase area are connected such
that a focal point is formed at the center position of the multiple
beams. Here, intervals between the four receive beams are very
short compared with the size of the transducer reception
surface.
[0073] Where forming four receive beams simultaneously, with use of
FIG. 2, the relationship between their distribution on the focal
plane and a reception surface division pattern optimal in forming
them simultaneously can be examined. It is well known that there is
a relationship between the sound field around the focal point on
the focal plane and the sound field on the transmit/receive
surface, where one is Fourier transformed into the other as with
the relationship between an atom arrangement in a crystal and its
X-ray diffraction pattern. Therefore, it is considered optimal to
divide the reception aperture using the reciprocal lattice of the
arrangement of multiple receive beams on the focal plane as
divisional lines. Although in FIG. 2 four receive beams are located
at the four vertexes of a square, if they form a rectangle with the
length in the x direction longer than in the y direction,
considering its reciprocal lattice, an optimum aperture division
pattern can be obtained by making divisional line intervals in the
y direction longer than in the x direction.
[0074] FIGS. 3 to 5 show schematically array element connection
patterns obtained in this way for forming four receive beams
simultaneously. In these examples, the entire reception aperture is
divided into 4.times.4 blocks, and the elements in each block are
connected according to the equal phase area connection pattern of a
Fresnel distribution for forming the focal point at the center of
the four receive beams (indicated by "x" in FIG. 2). In other
words, the elements of each equal phase area are connected together
such that the focal point is formed at the center position of
multiple receive beams located at different positions on the focal
plane. In the figures, each block is colored black or white such
that adjacent blocks are opposite in color to each other. FIG. 3
shows the pattern for 1001a of FIG. 1 where the focal point is
right in front of the transducer reception surface, and FIG. 4
shows the pattern for 1001b of FIG. 1 where the focal point is in
an oblique direction relative to the transducer reception surface,
in which pattern, stripes parallel to the y direction follow one
after another in each block. FIG. 5 shows the pattern for 1001c of
FIG. 1, in which stripes at an angle of 45 degrees to the x and y
axes follow one after another in each block.
[0075] An ultrasound diagnostic apparatus (ultrasound imaging
apparatus) according to an embodiment of the present invention will
be described below with use of the figures.
[0076] FIG. 6 is a block diagram showing a typical configuration of
an ultrasound diagnostic apparatus according to a pulse echo
method. A transmit/receive sequence control unit 1012 controls a
transmit beam former 1013, a receive beam former 1020, a selector
1011, and a grouping switch control unit 1010. The selector 1011
and the grouping switch control unit 1010 apply signals to control
the connection pattern to grouping switch blocks 1002 that connect
respectively to blocks of transducer array elements 1001 forming an
ultrasound probe.
[0077] When transmitting ultrasonic waves, a single ultrasonic beam
is formed instead of multiple beams. Accordingly, the grouping
switch blocks 1002 as selector means, group the transducer array
elements 1001 to form Fresnel rings so as to form an ultrasonic
beam having a predetermined focal distance. That is, transducer
array elements to be driven simultaneously are selected. The
transmit beam former 1013 drives each of the transducer array
elements 1001 with use of a delayed waveform according to the
Fresnel distribution.
[0078] Meanwhile, when receiving ultrasonic waves, waves reflected
at a place where variation in sound impedance expressed as the
product of speed of sound and density in material is large are
received. At this time, by receiving multiple beams, time
resolution is increased. In the receive beam former 1020, the input
signals received from transducer array elements 1001 via the
grouping switch blocks 1002 and the selector 1011 are amplified by
preamplifiers and then sampled and A/D converted to be temporarily
stored in memory.
[0079] To be more specific, usually, immediately after the
preamplifiers, the signals pass through TGC (Time Gain Control)
amplifiers, controlled such that their gains gradually increases
according to the increase in elapsed time from transmission and
then are A/D converted. This is for compensating for the decrease
in the amplitude of the received signals so as to keep the
amplitude at the inputs of the A/D converter within a constant
range, because ultrasonic waves propagating in a living body
attenuates almost proportionally to their propagation distance and
correspondingly the amplitude of the received signals decreases
almost proportionally to the increase in elapsed time from
transmission. By this means, the signal dynamic range can be
prevented from decreasing due to amplitude quantization by the A/D
conversion. In addition, it is well known that by making the
signals pass through a band limiting filter before the A/D
conversion, aliasing due to time axis quantization by the A/D
conversion can be prevented.
[0080] In order to obtain receive wave directivity, after giving
the received signal from each element temporarily stored in memory
a delay corresponding to the position of the element, the received
signals need to be summed to obtain a convergence effect. An
optimum value of the delay to be given to the received signal from
each element varies depending on the receive wave focal distance.
Further, an optimum value of the receive wave focal distance in
obtaining a good pulse echo image becomes greater proportionally to
the increase in elapsed time from transmission and speed of sound.
It is desirable to use a dynamic focus reception scheme which
changes the delay to be given to the signal from each element
according to elapsed time from transmission. With the configuration
where the received signal from each element is temporarily stored
in memory and read out to sum the received signals, this scheme can
be relatively easily realized by control when reading out or
storing.
[0081] The output signals of the receive beam former 1020 are
stored into receive memories 1021 on a per receive beam basis. In
the present embodiment, because four receive beams are formed, four
receive memories 1021 are provided. The signals stored in these
memories are sequentially selected and read by the selective switch
1022. The read signals pass through a filter 1023 and are sampled
and held in an envelope detector 1024 to detect envelope signals.
Then, the envelope signals are logarithmically compressed into
display signals. A scan converter 1025 converts the signals into a
two-dimensional image or a three-dimensional image, which is
displayed on a display 1026 constituted by a CRT or a liquid
crystal display.
Simulation Example
[0082] Next, an example of a receive sound field formed using the
ultrasound diagnostic apparatus of the present embodiment will be
shown below.
[0083] FIGS. 7 to 10 show distributions of first to fourth receive
beams on a focal plane when the focal point is shifted in azimuth.
In these figures, the receive gain is normalized for up to 10 mm in
the x and y directions (azimuth). A 16 mm.times.16 mm reception
aperture formed of 64.times.64 elements with an ultrasonic
frequency of 3 MHz was equally divided into 4 rows.times.4 columns,
and the receive beams were simultaneously formed respectively on
the vertexes of a square of 4 mm in size on the focal plane 60 mm
away from the reception surface. Although any receive beam is seen
to have a side lobe, the gain of the side lobe relative to the main
beam is about 0.2, which practically does not pose a problem.
[0084] The -6 dB beam width of these receive beams is about 5 mm.
It is appropriate to set the distance between the centers of
adjacent ones of the simultaneously formed receive beams at a beam
width of about -3 to -6 dB as in this example. If the interval is
narrower than this, the independency of information as an echo
signal obtained from each receive beam becomes less, thus reducing
the value of parallel reception. Conversely, if the interval
between adjacent receive beams is set to be broader than this, the
possibility that an echo from a reflector in the middle between
adjacent receive beams may be missed in detection will increase.
When a reception aperture is equally divided into 4 rows.times.4
columns, as in this example, multiple receive beams that are a beam
width of about -3 to -6 dB apart can be simultaneously formed.
[0085] If the number of divisions is smaller than this, it is
difficult to suppress the intensity of side lobes occurring when
simultaneously forming multiple receive beams that are a beam width
of about -3 to -6 dB apart to within an allowable range.
Conversely, if the number of divisions becomes greater than this,
the number of signal lines to be led out increases, while it
becomes easy to simultaneously form multiple receive beams that are
a beam width of about -3 to -6 dB apart.
[0086] The 64.times.64 transducer elements forming a
two-dimensional ultrasound transducer array are equally divided
into blocks, arranged in 4 rows.times.4 columns, each having
16.times.16 transducer elements, and each block has 15, less than
16, leading cable cores, and the transducer elements of each block
are connected to the leading cable cores via 16 multiplexer
switches having 16 inputs and 15 outputs. Generalizing this, a
two-dimensional ultrasound transducer array is divided into n
number of blocks of M1, M2, . . . , or Mn transducer elements, each
block having Nn, less than Mn, leading lines, and the elements of
an nth block are connected to Mn multiplexer switches having one
input and Nn outputs, where n, M, N are natural numbers.
[0087] Where the scheme is applied to a rectangular reception
aperture where, after the reception aperture is divided into
multiple blocks, the elements of each block are connected by
switches according to a connection pattern corresponding to the
equal phase areas, the practical problem occurs that, because the
element interval in a direction of a diagonal, longer than a side
of a block, is finer than the element interval in a direction of
the side, the number of leading lines necessary to make the receive
beams deflect in the diagonal direction of the reception aperture
is about twice the number of leading lines necessary to make the
receive beams deflect in the side direction of the reception
aperture.
[0088] This problem can be solved by configuring the transducer
array such that divisional lines for transducer elements are
parallel to diagonals of the reception aperture, or blocks into
which it is divided, as shown in FIG. 11. FIGS. 12 and 13 show
receive beam distributions with such a transducer array when the
receive beams deflect at 45 degrees respectively in an aperture
divisional line direction and in an element divisional line
direction. In this example, a 5 mm.times.5 mm reception aperture
formed of 48.times.48 elements with an ultrasonic frequency of 2
MHz was equally divided into 4 rows.times.4 columns, 16 blocks, and
receive beams with a focal distance of 57.3 mm were formed. In the
figures, the azimuth distance in units of mm is the same in
numerical value as the angle in units of deg. When the main beam is
deflected in the element divisional line direction, a grating lobe
occurs at 90 degrees on the opposite side to the deflection
direction, but it is perceived to be outside the practical field of
view, thus posing no substantial problem.
[0089] As described above, according to the present embodiment,
multiple receive beams can be simultaneously formed with
suppressing the number of leading cable cores. Hence, using an
ultrasound probe connected by a not too thick cable, real-time
three-dimensional imaging suitable for imaging the heart or the
like can be achieved, which achievement can be significant in
medical care and industry.
(Variants)
[0090] The present invention is not limited to the above
embodiment, but various variants thereof as shown below are
possible.
(1) Although in the above embodiment a single beam is formed and
made to converge on a predetermined focal position when
transmitting, multiple beams may be formed and transmitted. (2)
Although in the above embodiment a reception aperture is divided
into 4.times.4 rectangles, 16 parts, it may be divided into parts
of an equal angle of concentric rings.
Second Embodiment
[0091] A second embodiment of the present invention will be
described below in detail with reference to FIGS. 14 to 37.
[0092] FIG. 14 is a block diagram showing the configuration of an
ultrasound imaging apparatus 100A.
[0093] The ultrasound imaging apparatus 100A scans ultrasonic waves
over an object to obtain real-time ultrasound three-dimensional
images of an area to be imaged, and comprises a two-dimensional
array 1, a selector unit 2, a transmit/receive separation switch 3,
a transmit beam former 4, an amplifier 5, a receive beam former 6,
a signal process unit 7, a three-dimensional memory 8, a display 9,
and a control unit 10.
[0094] The two-dimensional array 1 has multiple electro-acoustic
transducer elements (transducers) arranged in a plane or a curved
surface, and each individual electro-acoustic transducer element is
driven by the transmit beam former 4 to transmit ultrasonic waves
and receives and converts ultrasonic waves reflected from an object
into an electrical signal.
[0095] The selector unit 2 makes multiple input/output channels
converge, as described later, which correspond to the
electro-acoustic transducer elements of the two-dimensional array 1
so as to reduce the number of input/output channels of the
two-dimensional array 1 to connect to the transmit/receive
separation switch 3.
[0096] The transmit/receive separation switch 3, according to the
control of the control unit 10, connects the transmit beam former 4
and the selector unit 2 when transmitting ultrasonic waves to an
object, and connects the selector unit 2 and the amplifier 5 when
receiving an echo from the object, thereby separating the transmit
system (the transmit beam former 4) and the receive system (the
amplifier 5 through the display 9).
[0097] The transmit beam former 4 electrically drives the
two-dimensional array 1 to form a transmit beam T and scans the
entire area to be imaged with shifting the direction of the
transmit beam T according to the control of the control unit
10.
[0098] The amplifier 5 amplifies the received signals from the
two-dimensional array 1 and outputs to the receive beam former
6.
[0099] The receive beam former 6 performs delaying and summation on
each signal output from the selector unit 2 to simultaneously
generate echo signals corresponding to multiple, e.g. four, receive
beams R1 to R4 for the transmit beam T.
[0100] The signal process unit 7 performs preprocessing (logarithm
conversion, filtering, gamma correction, etc.) on the echo signals
from the receive beam former 6.
[0101] The three-dimensional memory 8 functions as a digital scan
converter (DSC) and image memory, that is, converts the echo
signals from the signal process unit 7 to digital form to be
stored, produces three-dimensional image data to be stored, and
outputs it in the form matching the display format of the display
9.
[0102] The display 9 reads three-dimensional image data from the
three-dimensional memory 8 and displays a three-dimensional image
or a tomogram of the object.
[0103] The control unit 10 controls the selector unit 2, the
transmit/receive separation switch 3, the transmit beam former 4,
the amplifier 5, the receive beam former 6, the signal process unit
7, the three-dimensional memory 8, and the display 9.
[0104] An ultrasound probe (not shown) includes the two-dimensional
array 1 and the selector unit 2, and the transmit/receive
separation switch 3 and the subsequent other components are
provided on the main body (not shown). The ultrasound probe and the
main body are connected by a cable, but because the number of
input/output channels of the two-dimensional array 1 is reduced by
the selector unit 2 as described later, the number of cores of this
cable is also reduced. Therefore, the diameter of the cable can be
made thin, thus improving the operability of the ultrasound imaging
apparatus 100A.
[0105] Next, the concept of an example will be described where the
two-dimensional array 1 is divided into four element blocks and
where by assigning each element block four channels, the total
number of channels of the two-dimensional array 1 is reduced to 16.
The element block division according to the present invention is
performed, e.g., as described later with reference to FIG. 23, but
here, for convenience of description of the concept of element
block division and group division, the case of dividing into
element blocks (the first element blocks in claim 7) having its
size in the lateral axis direction (the transverse direction in the
figure; the second direction in claim 7) longer than its size in
the elevational axis direction (the longitudinal direction in the
figure; the first direction in claim 7) will be described.
[0106] FIG. 15 illustrates the concept of the block division of the
two-dimensional array 1.
[0107] The two-dimensional array 1 has 4n electro-acoustic
transducer elements arranged in a matrix. Description will be made
of the case where the arrangement surface is a plane, but the
arrangement surface may be curved. This two-dimensional array 1 is
divided into two parts in the elevational axis direction (the
longitudinal direction in the figure; the first direction in claim
7) and into two parts in the lateral axis direction (the transverse
direction in the figure; the second direction in claim 7) to form
four element blocks 11 to 14 of n electro-acoustic transducer
elements each. As shown in the figure, the element block 11
comprises electro-acoustic transducer elements 111 to 11n; the
element block 12 comprises electro-acoustic transducer elements 121
to 12n; the element block 13 comprises electro-acoustic transducer
elements 131 to 13n; and the element block 14 comprises
electro-acoustic transducer elements 141 to 14n.
[0108] An example where rectangular electro-acoustic transducer
elements are arranged in a matrix is described, but instead of the
rectangle, electro-acoustic transducer elements of another shape
such as a triangle or a hexagon may be used, or instead of the
matrix, the elements may be arranged in a honeycomb or
randomly.
[0109] FIG. 16 illustrates the way to group the electro-acoustic
transducer elements 111 to 11n, 121 to 12n, 131 to 13n, and 141 to
14n. In the figure, the areas of adjacent groups are distinguished
from each other by being made shaded or hollow.
[0110] The electro-acoustic transducer elements 111 to 11n, 121 to
12n, 131 to 13n, and 141 to 14n in the element blocks 11 to 14 are
divided according to three concentric circles 151 to 153 into
groups.
[0111] That is, the element block 11 is grouped into four groups a1
to d1, and one channel is assigned to each group a1 to d1.
Likewise, the element block 12 is grouped into four groups a2 to
d2, and one channel is assigned to each group a2 to d2; the element
block 13 is grouped into four groups a3 to d3, and one channel is
assigned to each group a3 to d3; and the element block 14 is
grouped into four groups a4 to d4, and one channel is assigned to
each group a4 to d4.
[0112] Since the electro-acoustic transducer elements of each of
the four element blocks 11 to 14 are grouped such that the transmit
beam T is formed in the direction that the two-dimensional array 1
faces and that the receive beams R1 to R4 are formed around the
transmit beam T, the center of the concentric circles 151 to 153
coincides with the center of the two-dimensional array 1. Thus,
when the beams are deflected in the elevational axis direction, the
center of the concentric circles 151 to 153 deviates in the
elevational axis direction, and when the beams are deflected in the
lateral axis direction, the center of the concentric circles 151 to
153 deviates in the lateral axis direction.
[0113] That is, in scanning an object, in order to change the beam
direction, the group division is changed, but the block division
need not be changed.
[0114] FIG. 17 is a block diagram showing in detail the
configuration of the selector unit 2.
[0115] The selector unit 2 functions to realize the grouping of the
electro-acoustic transducer elements, and comprises a selector 21
connected to the element block 11, a selector 22 connected to the
element block 12, a selector 23 connected to the element block 13,
and a selector 24 connected to the element block 14.
[0116] As shown in FIG. 17A, the selector 21 comprises switches 211
to 21n each for connecting one of the electro-acoustic transducer
elements 111 to 11n forming the element block 11 to any of the four
channels for the groups a1 to d1. The switches 211 to 21n,
according to the control of the control unit 10, connect each of
the electro-acoustic transducer elements 111 to 11n to one of the
four channels for the groups a1 to d1 according to the scan
direction, thereby performing the grouping.
[0117] As shown in FIG. 17B, the selector 22 comprises switches 221
to 22n each for connecting one of the electro-acoustic transducer
elements 121 to 12n forming the element block 12 to any of the four
channels for the groups a2 to d2. The switches 221 to 22n,
according to the control of the control unit 10, connect each of
the electro-acoustic transducer elements 121 to 12n to one of the
four channels for the groups a2 to d2 according to the scan
direction, thereby performing the grouping.
[0118] As shown in FIG. 17C, the selector 23 comprises switches 231
to 23n each for connecting one of the electro-acoustic transducer
elements 131 to 13n forming the element block 13 to any of the four
channels for the groups a3 to d3. The switches 231 to 23n,
according to the control of the control unit 10, connect each of
the electro-acoustic transducer elements 131 to 13n to one of the
four channels for the groups a3 to d3 according to the scan
direction, thereby performing the grouping.
[0119] As shown in FIG. 17D, the selector 24 comprises switches 241
to 24n each for connecting one of the electro-acoustic transducer
elements 141 to 14n forming the element block 14 to any of the four
channels for the groups a4 to d4. The switches 241 to 24n,
according to the control of the control unit 10, connect each of
the electro-acoustic transducer elements 141 to 14n to one of the
four channels for the groups a4 to d4 according to the scan
direction, thereby performing the grouping.
[0120] FIG. 18 is a block diagram showing in detail the
configuration of the selector unit 2 through the receive beam
former 6 for processing the received signals from the selectors 21
to 24.
[0121] When receiving waves reflected from the object, the
transmit/receive separation switch 3 connects the channels for the
groups a1 to d1, a2 to d2, a3 to d3, and a4 to d4 of the selectors
21, 22, 23, 24 to the amplifier 5 according to the control of the
control unit 10.
[0122] The amplifier 5 amplifies the received signal transmitted
over each of the channels with a predetermined gain for the channel
according to the control of the control unit 10 and outputs to the
receive beam former 6.
[0123] The receive beam former 6 functions to form the four receive
beams R1 to R4 and comprises a bus 61, delay units 621 to 624, and
adders 631 to 634.
[0124] In the bus 61, each of the channels for the groups a1 to d1,
a2 to d2, a3 to d3, and a4 to d4 branches into four channels, which
are respectively connected to the delay units 621 to 624. Thereby,
the same signal is inputted to all the delay units 621 to 624.
[0125] The delay units 621 to 624 delay the signal by a different
delay for each channel so as to form the receive beams R1 to R4
deflected relative to the transmit beam and to perform dynamic
focusing reception in the depth direction. To be specific, the
delay units 621 to 624 give the continuously received signal the
combined delay of a delay for the focal point and a delay for the
deflections of the receive beams R1 to R4, and by repeating the
above process, performs dynamic focusing reception for a different
focal point as well.
[0126] The adders 631 to 634 add the outputs of the delay units 621
to 624 respectively for the receive beams R1 to R4 and output the
added signals to the signal process unit 7 (see FIG. 14).
[0127] Referring back to FIG. 14, the signal process unit 7
performs signal processing such as filtering, interpolation,
detection, etc., on the signals associated with the receive beams
R1 to R4 from the adders 631 to 634 and outputs to the
three-dimensional memory 8.
[0128] The three-dimensional memory 8 produces and stores
three-dimensional image data. Using this three-dimensional image
data, the display 9 performs three-dimensional display or the
display of a sectional view.
[0129] The example where the number (here 4n) of channels of the
two-dimensional array 1 is reduced to 16 has been described. In
practice, considering required imaging performance such as image
quality, the scale of the apparatus, costs, and convenience of
handling, the number of electro-acoustic transducer elements and
the reduced number of channels are determined. For example, using
the two-dimensional array 1 having several thousand
electro-acoustic transducer elements (several thousand channels),
the number of channels is reduced to about 100 to 200. From the
view point of suppressing a grating lobe, the larger number of
channels after the reduction is more preferable, but from the view
point of reducing circuit size and the number of cores as
connection lines thereby improving operability, the smaller number
of channels after the reduction is more preferable.
[0130] FIG. 19 is a block diagram showing the configuration of
another ultrasound imaging apparatus 100B of the present
invention.
[0131] The ultrasound imaging apparatus 100B is substantially the
same in configuration as the ultrasound imaging apparatus 100A
except that it comprises a transmission selector unit 2T and a
receiving selector unit 2R instead of the selector unit 2.
[0132] The transmit/receive separation switch 3, when transmitting
ultrasonic waves, connects the transmission selector unit 2T and
the two-dimensional array 1 and, when receiving ultrasonic waves,
the two-dimensional array 1 and the receiving selector unit 2R.
[0133] The transmission selector unit 2T is substantially the same
in configuration as the selector unit 2, and makes multiple input
channels to the transmit/receive separation switch 3 converge which
correspond to the electro-acoustic transducer elements of the
two-dimensional array 1, thus grouping electro-acoustic transducer
elements of each of the element blocks 11 to 14. The input
terminals of the transmission selector unit 2T are connected to the
output terminals of the transmit beam former 4.
[0134] The receiving selector unit 2R is substantially the same in
configuration as the selector unit 2, and makes multiple output
channels from the transmit/receive separation switch 3 converge
which correspond to the electro-acoustic transducer elements of the
two-dimensional array 1, thus grouping electro-acoustic transducer
elements of each of the element blocks 11 to 14 into a grouping
pattern different from that of the transmission selector unit 2T.
The output terminals of the receiving selector unit 2R are
connected to the input terminals of the amplifier 5.
[0135] According to the ultrasound imaging apparatus 100B, it is
possible to group into a different pattern between when
transmitting and when receiving. Hence, the place where a grating
lobe occurs can be changed between when transmitting and when
receiving, thus improving image quality.
[0136] FIG. 20 is a pattern diagram showing the block division of
the two-dimensional array 1 as a comparative example.
[0137] In this comparative example, the two-dimensional array 1 is
divided into four parts in the elevational axis direction (the
longitudinal direction in the figure) and into four parts in the
lateral axis direction (the transverse direction in the figure),
and thus divided into 16 element blocks. Here, in each element
block, 12 electro-acoustic transducer elements are arranged in the
elevational axis direction and 16 elements are arranged in the
lateral axis direction. Thus, the number of electro-acoustic
transducer elements per element block is 192. The two-dimensional
array 1 has a total of 3,072 elements. The input/output channels
(3,072 channels) of the two-dimensional array 1 are grouped so as
to be reduced to 128 channels, and thus the number of channels per
element block is 8.
[0138] FIG. 21 is a pattern diagram showing the grouping as a
comparative example. FIG. 21A shows the case of forming the
transmit beam in the direction that the two-dimensional array 1
faces, and FIG. 21B shows the case of deflecting the beam in the
lateral axis direction (the transverse direction in the
figure).
[0139] The element pitch of the two-dimensional array 1 of FIGS.
21A and 21B is preferably no greater than half the wavelength of
ultrasonic waves transmitted and received, in order to suppress a
grating lobe. For example, if the center frequency of ultrasonic
waves is 2.5 MHz, the element pitch may be 0.3 mm. In this case,
the size of the two-dimensional array 1 is 19.2 mm in the lateral
axis direction and 14.4 mm in the elevational axis direction. When
obtaining an image of the heart inside the body, because of picking
up through between ribs, the representative size in the elevational
axis direction of the two-dimensional array 1 needs to be about 20
plus several mm in maximum.
[0140] FIG. 21A shows a pattern with the focal distance F of 50 mm
in the case where the transmit beam is directed in a direction
perpendicular to the two-dimensional array 1, and FIG. 21B shows a
pattern with the focal distance F of 50 mm in the case where the
beam is deflected at 30.degree. in the lateral axis direction
relative to the direction perpendicular to the two-dimensional
array 1. In either case, the pattern is a concentric circular or
arc-shaped pattern with the beam axis as the center.
[0141] FIG. 22 illustrates a relationship between a Fresnel zone
plate 30 and the two-dimensional array 1.
[0142] The Fresnel zone plate 30 has annular areas defined by
concentric circles having radiuses proportional to the square roots
of 1, 2, 3, . . . and having as their center the center axis A of
the transmit beam when transmitted in the direction that the
two-dimensional array 1 faces, every second one of the annular
areas being opaque to ultrasonic waves. Thus, the width of the
annular area further away from the center axis A is narrower. In
FIG. 22, the hatched areas of the Fresnel zone plate 30 are opaque
to ultrasonic waves with the hollow areas transparent, but the
hatched areas may be transparent with the open areas opaque.
[0143] The size of the concentric circles of the Fresnel zone plate
30, that is, a grouping pattern for the two-dimensional array 1 is
determined by the focal distance. FIG. 22 shows the case where,
with the focal distance F of 50 mm, the size of the two-dimensional
array 1 is 19.2 mm in the lateral axis direction and 14.4 mm in the
elevational axis direction.
[0144] In order to prevent a grating lobe from occurring, the
grouping pattern for the two-dimensional array 1 needs to be finer
than the pattern of the annular areas of the Fresnel zone plate 30.
As the relative position of the two-dimensional array 1 becomes
closer to the center axis A of the Fresnel zone plate 30, the
grouping pattern for the two-dimensional array 1 can be coarser.
Conversely, as it becomes further away from the center axis A of
the Fresnel zone plate 30, the grouping pattern for the
two-dimensional array 1 needs to be finer.
[0145] As shown in FIG. 22, for example, when the beam is formed in
the front direction, the relative position of the two-dimensional
array 1 in the Fresnel zone plate 30 is as indicated by 1a. Thus,
because the widths of the annular areas of the Fresnel zone plate
30 are wide, the number of groups of the two-dimensional array 1
may be small. However, when the beam deflected relative to the
front direction is formed, the relative position of the
two-dimensional array 1 in the Fresnel zone plate 30 is as
indicated by 1b. Thus, because the widths of the annular areas of
the Fresnel zone plate 30 are narrow, the two-dimensional array 1
has to be divided finely into annular groups corresponding to these
annular areas, or otherwise a grating lobe would occur.
[0146] However, there is a limit to making the element pitch of the
two-dimensional array 1 finer, and there is a restriction on the
number of channels assigned to each element block of the
two-dimensional array 1. Therefore, when the beam is deflected to a
great degree, image quality is object to a grating lobe because the
grouping pattern of electro-acoustic transducer elements cannot be
made finer than the pattern of the annular areas of the Fresnel
zone plate 30.
[0147] From the view point of suppressing a grating lobe, when the
beam is deflected in the lateral axis direction, the interval
between groups adjacent in the lateral axis direction is preferably
at a pitch of one element. However, in the example shown, e.g., in
FIG. 21B, since 8 channels are assigned to each element block, the
interval between adjacent groups is almost at a pitch of two
elements. Hence, a large grating lobe occurs, thus degrading image
quality.
[0148] As described above, where electro-acoustic transducer
elements are divided into blocks and further grouped to reduce the
number of channels, and multipoint simultaneous reception is
performed, the reduction in the number of channels and suppressing
a grating lobe are in a conflicting relationship.
[0149] Next, the concept of suppressing a grating lobe by setting
the block division and grouping of the two-dimensional array 1 to
obtain a real-time three-dimensional ultrasonic image of higher
quality under the condition of such a conflicting relationship will
be described in detail.
[0150] FIG. 23 is a pattern diagram showing an example of the block
division of the two-dimensional array 1 according to the present
invention.
[0151] In this example, in the two-dimensional array 1, there are
mixed (laterally long) element blocks 11a, 11b of which the size in
the elevational axis direction (the longitudinal direction in the
figure) is smaller than the size in the lateral axis direction (the
transverse direction in the figure) and element blocks 12a to 12f
of which the size in the lateral axis direction is smaller than the
size in the elevational axis direction, these blocks being arranged
line-symmetric with respect to either of the lateral axis and the
elevational axis.
[0152] Here, let k be the number of channels assigned to one
element block, m1 be the number of elements of element block 11a,
11b along its edge in the lateral axis direction (the transverse
direction in the figure), m2 be the number of elements along its
edge in the elevational axis direction (the longitudinal direction
in the figure), m3 be the number of elements of element block 12a
to 12f along its edge in the lateral axis direction (the transverse
direction in the figure), and m4 be the number of elements along
its edge in the elevational axis direction (the longitudinal
direction in the figure).
[0153] In this case, it is desirable that m2.ltoreq.k. This is
because, when the beam is deflected in the lateral axis direction,
the interval between respective groups for the channels almost
equals the element pitch in the elevational axis-wise long element
blocks 12a to 12f, thus suppressing a grating lobe.
[0154] Further, in this case, it is desirable that m3.ltoreq.k.
This is because, when the beam is deflected in the elevational axis
direction, the interval between respective groups for the channels
almost equals the element pitch in the elevational axis-wise short
element blocks 11a, 11b, thus suppressing a grating lobe.
[0155] FIG. 24 is pattern diagrams showing the grouping according
to the present invention. FIG. 24A shows the case of forming the
beam in the direction that the two-dimensional array 1 faces, and
FIG. 24B shows the case of deflecting the beam in the lateral axis
direction (the transverse direction in the figure).
[0156] The conditions for the grouping is the same as the
comparative example (see FIGS. 21A, 21B) except that the block
division is different.
[0157] Comparing the patterns of the comparative example and of the
present invention, it is seen that according to the block division
method of the present invention, the intervals between concentric
circular groups are finer with the same number of channels, and
thus delays can be controlled more appropriately in forming the
beams.
[0158] FIG. 25 is pattern diagrams showing other examples of the
block division of the two-dimensional array 1 according to the
present invention.
[0159] In two-dimensional arrays 1 shown in FIGS. 25A to 25E, there
are mixed (longitudinally long) element blocks of which the size in
the elevational axis direction (the longitudinal direction in the
figure) is larger than the size in the lateral axis direction (the
transverse direction in the figure) and (laterally long) element
blocks of which the size in the lateral axis direction is larger
than the size in the elevational axis direction, these blocks being
arranged symmetric with respect to both the lateral axis and the
elevational axis. The element blocks may be rectangular, or
non-rectangular, e.g., L-shaped element blocks may be combined as
shown in FIG. 25E. Although cases of the rectangular
two-dimensional array 1 have been illustrated, the two-dimensional
array 1 may be of another shape such as an ellipse.
[0160] Next, a method of determining the grouping pattern for each
element block according to the present invention will be described
in detail with reference to FIGS. 26 to 28.
[0161] FIG. 26 is a conceptual diagram showing a geometric
positional relationship between an element block 11 of the blocks
into which the two-dimensional array 1 is divided and a focal point
R for determining the grouping pattern.
[0162] Let .tau.max be a delay to be given to the electro-acoustic
transducer element furthest away from the focal point R from among
the electro-acoustic transducer elements in the element block 11
and .tau.min be a delay to be given to the electro-acoustic
transducer element closest to the focal point R.
[0163] FIG. 27 is a histogram showing a frequency distribution of
delays to be given to the electro-acoustic transducer elements in
the element block 11, to illustrate the grouping method for the
comparative example.
[0164] In this grouping method, groups are formed in the element
block 11 such that delay intervals (steps) between channels become
the same regardless of the occurrence frequency of each delay. That
is, letting .tau.max be the maximum delay for the element block 11
and .tau.min be the minimum delay, the electro-acoustic transducer
elements in the element block 11 are grouped into four groups a1 to
d1 such that a delay interval .DELTA..tau. between adjacent
channels equals (.tau.max-.tau.min)/4. The grouping pattern shown
in FIGS. 21A, 21B, 24A, 24B were obtained by making delay intervals
between channels in each element block be the same according to
this method.
[0165] In this grouping, when forming the same beams, the same
weight in summation is given to the channels associated with the
groups b1, c1 for which the frequency (the number of elements) of
giving delays is high, so that their influence on beam formation is
relatively large, and to the channels associated with the groups
a1, d1 for which the frequency (the number of elements) of giving
delays is low, so that their influence on beam formation is
relatively small.
[0166] That is, large weight should be applied to the channels
associated with the groups b1, c1 whose number of elements is
large, and small weight should be applied to the channels
associated with the groups a1, d1 whose number of elements is
small, but due to the above-mentioned factor, the influence of the
channels associated with the groups b1, c1 becomes relatively
smaller, and the influence of the channels associated with the
groups a1, d1 becomes relatively larger. Therefore, the effect of
the grouping may not be sufficient.
[0167] Further, because electrical characteristics of each channel
such as impedance are different between the group c1 whose number
of elements is very large and the group a1 whose number of elements
is very small, conditions for driving electro-acoustic transducer
elements for each channel vary, thus complicating a correction
circuit or degrading image quality.
[0168] FIG. 28 is a histogram showing a frequency distribution of
delays to be given to the electro-acoustic transducer elements in
the element block 11, to illustrate the grouping method of the
present embodiment.
[0169] In this grouping, the electro-acoustic transducer elements
in the element block 11 are grouped such that the integral S of the
occurrence frequency of the delay over the delay range of each
group is equal for each of the channels associated with the groups
a1 to d1, that is, the area in the histogram of FIG. 28 is equal
for each channel.
[0170] To be specific, if the electro-acoustic transducer elements
are the same in size and shape, they are grouped in ascending (or
descending) order of the magnitude of the delay to be given to them
such that the number of electro-acoustic transducer elements
belonging to each group is the same.
[0171] With this grouping, because the groups b1, c1 high in the
frequency of having large influence on beam formation are
controlled finely in terms of delay, the occurrence of a grating
lobe can be suppressed. Further, because the number of
electro-acoustic transducer elements grouped for each channel is
substantially the same, electrical characteristics of each channel
such as impedance become the same, thus reducing circuit size and
improving image quality.
[0172] According to the ultrasound imaging apparatuses 100A and
100B of the present embodiment, the number of signal channels from
electro-acoustic transducer elements of the two-dimensional array
can be reduced, and by appropriate block division of the
two-dimensional array as shown in FIG. 23 and appropriate grouping
in each element block as shown in FIGS. 30, 32 according to the
method shown in FIG. 28, simultaneous reception for imaging
three-dimensional images at high speed with suppressing a grating
lobe becomes possible. Therefore, real-time three-dimensional
images of high quality can be obtained at low cost.
Examples
[0173] The grating lobe suppressing effect of the ultrasound
imaging apparatus according to the present invention was confirmed
by beam simulation.
[0174] A rectangular two-dimensional array having 3,072 square
electro-acoustic transducer elements arranged in a matrix with 64
elements in the lateral axis direction and 48 elements in the
elevational axis direction was used. The element pitch was 0.3 mm,
and the size of the two-dimensional array was 19.2 mm in the
lateral axis direction and 14.4 mm in the elevational axis
direction.
[0175] As to the number of channels, 3,072 channels were reduced to
128 channels, and the scan-line direction was obliquely at 45
degrees relative to the center axis of the two-dimensional array
(.theta..sub.j=.phi..sub.j=45.degree. in FIG. 33 described later),
and the focal distance Fd for determining the grouping pattern was
50 mm. Transmit waves were pulse waves having a center frequency of
2.5 MHz, a band width of 1 MHz, and a pulse width of 2 .mu.s.
[0176] FIG. 29 is a pattern diagram showing a comparative example
of grouping.
[0177] In this comparative example, first, the two-dimensional
array 1 was divided into four parts in the elevational axis
direction and into four parts in the lateral axis direction, and
thus divided into 16 element blocks in a matrix, and eight channels
were assigned to each element block. Then, each element block was
divided into eight groups such that the differences in delay
between them were the same as shown in FIG. 27, and the channels of
the electro-acoustic transducer elements in each group were made to
converge so as to assign one channel to the group so that the
number of channels of the two-dimensional array 1 became 128.
[0178] FIG. 30 is a pattern diagram showing a first example of
grouping.
[0179] In the first example, first, the two-dimensional array 1 was
divided into 16 element blocks as in the comparative example, and
eight channels were assigned to each element block. Then, each
element block was divided into eight groups such that the integral
of the occurrence frequency of the delay was the same for them as
shown in FIG. 28, and the channels of the electro-acoustic
transducer elements in each group were made to converge so as to
assign one channel to the group so that the number of channels of
the two-dimensional array 1 became 128.
[0180] FIG. 31 is a pattern diagram showing a second example of
grouping.
[0181] In the second example, first, the two-dimensional array 1
was divided into eight element blocks such that, as shown in FIG.
23, there are mixed element blocks of which the size in the
elevational axis direction is larger than the size in the lateral
axis direction and element blocks of which the size in the lateral
axis direction is larger than the size in the elevational axis
direction, and 16 channels were assigned to each element block.
Then, each element block was divided into 16 groups such that the
differences in delay between them were the same as shown in FIG.
27, and the channels of the electro-acoustic transducer elements in
each group were made to converge so as to assign one channel to the
group so that the number of channels of the two-dimensional array 1
became 128.
[0182] FIG. 32 is a pattern diagram showing a third example of
grouping.
[0183] In the third example, first, the two-dimensional array 1 was
divided into eight element blocks like in the second example. Then,
each element block was divided into 16 groups such that the
integral of the occurrence frequency of the delay was the same for
them as shown in FIG. 28, and the channels of the electro-acoustic
transducer elements in each group were made to converge so as to
assign one channel to the group so that the number of channels of
the two-dimensional array 1 became 128.
[0184] For each of the grouping patterns of the comparative example
and the first to third examples, a hemispheric receive beam profile
with the focal distance of 50 mm as the radius was calculated. The
effective value of sound pressure per unit pulse width was obtained
as the level of the beam.
[0185] FIG. 33 illustrates a display coordinate system for the beam
profile.
[0186] The two-dimensional array 1 is placed in the xy plane of an
orthogonal coordinate system (x, y, z) where the z axis is the
center axis, and the beam pattern on a hemisphere Q with the radius
equal to the focal distance F and the point where x=y=z=0 being the
center is projected onto a coordinate system (u, v) into which the
(x, y) coordinate system is normalizing with the focal distance F.
The coordinates (x.sub.j, y.sub.j, z.sub.j) of the focal point R on
the hemisphere Q can be expressed by a coordinate system (F,
.theta..sub.j, .phi..sub.j) where F is the focal distance,
.theta..sub.j is the rotation angle relative to the z axis, and
.phi..sub.j is the rotation angle relative to the x axis. The
conversion to the (u, v) coordinate system of the focal point R can
be expressed as u.sub.j=sin .theta..sub.j sin .phi..sub.j,
v.sub.j=sin .theta..sub.j cos .phi..sub.j. The position in the (u,
v) coordinate system corresponding to the scan-line direction
deflected obliquely at 45.degree.
(.theta..sub.j=.phi..sub.j=45.degree.) relative to the front
direction previously mentioned is expressed as u=v=0.5.
[0187] FIG. 34 is a contour line diagram showing the beam profile
in the (u, v) coordinate system for the grouping pattern of the
comparative example.
[0188] It is seen that when using the grouping pattern of the
comparative example, a large grating lobe of the same level as at
the focal point R occurs in the front direction.
[0189] FIG. 35 is a contour line diagram showing the beam profile
in the (u, v) coordinate system for the grouping pattern of the
first example.
[0190] It is seen that when using the grouping pattern of the first
example, a grating lobe is suppressed as compared with the
comparative example, because of grouping such that the integral of
the occurrence frequency of the delay in the histogram is the
same.
[0191] FIG. 36 is a contour line diagram showing the beam profile
in the (u, v) coordinate system for the grouping pattern of the
second example.
[0192] It proves that when using the grouping pattern of the second
example, a grating lobe is suppressed as compared with the
comparative example, because of grouping such that there are mixed
element blocks of which the size in the elevational axis direction
is larger than the size in the lateral axis direction and element
blocks of which the size in the lateral axis direction is larger
than the size in the elevational axis direction.
[0193] FIG. 37 is a contour line diagram showing the beam profile
in the (u, v) coordinate system for the grouping pattern of the
third example.
[0194] It proves that when using the grouping pattern of the third
example, a grating lobe is further suppressed as compared with the
first and second examples, because of grouping such that the
integral of the occurrence frequency of the delay in the histogram
is the same, as well as that there are mixed element blocks of
which the size in the elevational axis direction is larger than the
size in the lateral axis direction and element blocks of which the
size in the lateral axis direction is larger than the size in the
elevational axis direction.
[0195] According to these simulation results, the following effects
were confirmed.
(1) By grouping such that there are mixed element blocks of which
the size in the elevational axis direction is larger than the size
in the lateral axis direction and element blocks of which the size
in the lateral axis direction is larger than the size in the
elevational axis direction, a grating lobe can be suppressed. (2)
By grouping such that the integral of the occurrence frequency of
the delay in the histogram is the same, a grating lobe can be
suppressed. (3) By using both the (1) and (2), the grating lobe
suppressing effect can be further increased.
* * * * *