Method and apparatus for ultrasound imaging

A method and apparatus for ultrasonically producing 3-dimensional images in real time, wherein a device transmits an ultrasonic wave toward a solid bearing angle obtained by dividing into multiple portions a solid bearing angle subtended by a sonic field to be examined, and receives echoes therefrom, with the transmitted waves being sequential and the received echoes being concurrent multiple waves from each small solid angle, and a device for producing the 3-dimensional images by range gating the received echo signals in the depth direction of the sonic field being examined and then producing direct vision images in the direction of the transmitted ultrasonic waves based on the echo signals, wherein the range gated signals are quadrature detected in one embodiment and/or are subjected to 2-dimensional Fourier transformation in another embodiment.

FIELD OF THE INVENTION 
The present invention relates to an ultrasound imaging method and apparatus 
for producing 3-dimensional images in real-time. 
BACKGROUND OF THE INVENTION 
An ultrasound imaging system transmits ultrasound to a subject, receives 
the reflected waves (echoes), and produces images of the interior of the 
subject based on the echo data. Ultrasound is transmitted and received in 
directional beam (sound ray). A desired region to be imaged inside a 
subject is scanned by the sound ray-sequential transmittion and reception 
of ultrasound, and the received signals necessary to produce images are 
acquired. 
When the 3-dimensional images are to be produced, a 3-dimensional region in 
the sound field is scanned, and the acquired echo data therefrom is used 
to produce the images. 
In order to display the real-time 3-dimensional images, the image display 
speed (frame rate) on the order of 20--30 FPS (frame per second) is 
required. Thus, the echo data sufficient to produce one frame per 30-50 ms 
must be acquired. 
However, since the sound velocity inside a subject is limited, when the 
sound ray-sequential scanning is employed no more than about 100 sound 
rays can be transmitted/received in such a period of time. If the scanning 
is performed using such a few sound rays, to attain practical spatial 
resolution would result in the limited 3-dimensional region to be scanned, 
while to extend the 3-dimensional region up to the practical size would 
result in the reduction in the sound ray density and the spatial 
resolution of the image, neither of which are suitable for practical use. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide an ultrasonic 
imaging method and apparatus for producing 3-dimensional images in 
real-time. 
In the first aspect, the present invention provides a method for ultrasound 
imaging, comprising: transmitting ultrasound to the interior of a subject 
for each of small solid bearing angles, said small solid bearing angles 
being obtained by dividing a solid bearing angle corresponding to an 
object to be imaged into multiple portions; receiving simultaneously 
echoes from said subject in a plurality of sound rays from each of said 
small solid bearing angles; range gating the received signals of said 
sound rays in the depth direction of said subject; and producing images of 
said subject at a predetermined depth and thickness based on said range 
gated signals. 
In order to obtain images with high spatial resolution, the above-described 
method preferably further comprises: quadrature detecting said range gated 
received signals respectively; and 2-dimensional Fourier transforming said 
quadrature detected received signals of the plurality of sound rays in the 
bearing angle direction. 
In the second aspect, the present invention provides an apparatus for 
ultrasound imaging, comprising: means for transmitting/receiving 
ultrasound, said means transmitting ultrasound to the interior of a 
subject for each of small solid bearing angles, said small solid bearing 
angles being obtained by dividing a solid bearing angle corresponding to 
an object to be imaged into multiple portions, and receiving 
simultaneously echoes from said subject as a plurality of sound rays from 
each of said small solid bearing angles; means for range gating the 
received signals of said sound rays in the depth direction of said 
subject; and means for producing images of said subject at a predetermined 
depth and thickness based on said range gated signals. 
In the third aspect, the present invention provides an apparatus for 
ultrasound imaging, wherein said means for producing images in the 
above-constructed ultrasonic imaging apparatus further comprises: means 
for quadrature detecting said range gated received signals respectively; 
and means for 2-dimensional Fourier transforming said quadrature detected 
received signals of the plurality of sound rays in the bearing angle 
direction. 
In accordance with the present invention, the 3-dimensional images of an 
object can be produced in real-time by scanning the sound field to be 
examined sequentially for each small solid bearing angle; receiving echoes 
in multi-beam; and producing direct vision images in the sound ray 
direction based on the echoes. 
Further objects and advantages of the present invention will be apparent 
from the following description of the preferred embodiments of the 
invention as illustrated in accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, there is shown a block diagram of an ultrasonic 
imaging apparatus, which is one embodiment of the present invention. The 
arrangement of the apparatus represents one embodiment of an apparatus in 
accordance with the present invention, and the operation thereof 
represents one embodiment of a method in accordance with the present 
invention. 
In FIG. 1, an ultrasonic probe 1 transmits an ultrasonic beam to the sound 
field to be examined (not shown) and receives echoes therefrom. The 
ultrasonic probe 1 comprises, for example, a 2-dimensional array of a 
plurality of transducer elements. 
Referring to FIG. 2, there is shown an example of the transducer array in 
the ultrasonic probe 1. As shown in the drawing, the 2-dimensional array 
is comprised of, for example, 64 ultrasonic transducer elements 11 in an 
8.times.8 matrix. The ultrasonic transducer elements 11 are constructed of 
piezoelectric materials such as PZT (lead zirconate titanate) and 
supported by backing materials (not shown). 
A transmitting/receiving section 2 activates the ultrasonic probe 1 to 
transmit ultrasonic beams and receives echo signals detected by the 
ultrasonic probe 1. 
FIG. 3 is a schematic view of the ultrasound transmission/reception, 
wherein (a) is a plan view, and (b) and (c) are side views. 
As shown in (a), for example, a front view FR from the ultrasonic probe 1 
is determined in the sound field to be examined. The front view FR is a 
2-dimensional view of the 3-dimensional region to be imaged from the 
front. The term "front" refers to a plane in the sound field to be 
examined, viewed straightforward in the sound ray direction. The solid 
angle which the front view FR subtends at the ultrasonic probe 1 provides 
a solid bearing angle of the front view. 
A small front view fr is determined in the front view FR. The small front 
view fr is geometrically similar to the front view FR and, for example, 
reduced to a scale of 1/8 of the front view FR. The solid angle which the 
small front view fr subtends at the ultrasonic probe 1 provides a small 
solid bearing angle of the small front view. 
In transmission, as shown in (b), an ultrasonic beam SBM having the 
thickness to contain the entire small front view fr is transmitted or sent 
to the small front view fr. Such a transmitted or sent beam SBM is formed 
by the transmission beam forming at the transmitting/receiving section 2 
in the form of a cone beam or a fan beam, for example. 
The corresponding echoes are received in a plurality of receive beams RBM 
as shown in (c). The plurality of receive beams RBM are formed by the 
reception beam forming at the transmitting/receiving section 2. Thus, the 
received echo signals along the sound rays (e.g., 64 rays) which are 
distributed uniformly in the small front view fr can be acquired in 
parallel. 
When the small front view fr is represented as an 8 .times.8 matrix as 
shown in (a), the above-described received echo signals correspond to the 
echo signals from the respective matrix elements. This means that the 
received signals which represent the small front view fr in an 8.times.8 
matrix can be acquired at once in one transmission/reception. 
The ultrasonic beam transmission/reception is repeated shifting 
sequentially the small front view fr to the adjacent region. This 
sequential shifting of the small front view fr is accomplished by the 
sequential switching of the transmission/reception beam forming at the 
transmitting/ receiving section 2. 
The front view FR is thus scanned sequentially for each small front view fr 
in 2 dimensions, and when the scale of the small front view fr is 1/8 of 
the front view FR, the entire front view FR is scanned by 64 
transmission/receptions. 
By scanning as above, the 3-dimensional sound field to be examined which 
has, for example, the 10cm.times.10cm front view FR and 10cm in depth can 
be scanned at the frame rate of, for example, 30FPS. In this case, the 
received echo signals of 64.times.64=4096 sound rays per frame are 
acquired. 
That is, the received echo signals which correspond to, for example, the 
10cm.times.10cm front view scanned with 64.times.64=4096 sound rays can be 
acquired at the frame rate of 30 FPS. 
The echo signals received by the transmitting/receiving section 2 are 
supplied into a range gating section 3. The range gating section 3 
performs the range gating of the supplied received echo signals for the 
respective sound rays. 
The range gating cuts out from the received echo signal a signal fraction 
SF over the desired depth as shown in FIG. 4. 
The signal fraction SF is, for example, the echo reflected from the surface 
of an object to be imaged OB in the sound field to be examined as shown in 
FIG. 5, and the timing and range of the range gate are controlled so that 
the desired signal fraction is cut out. As an example, the signal fraction 
SF is the echo from the body surface on a fetus in amniotic fluid inside a 
uterus. 
Since the acoustic impedance of a fetus and that of amniotic fluid are 
considerably different, the range gating is performed based on the 
skin-trigger technique in the same manner as the conventional ultrasonic 
imaging technique using a water bag. The range gating by the skin-trigger 
technique is desirable with regard to its convenience. 
The range gated received echo signals are quadrature detected and separated 
into a in-phase component i and a quadrature component q at a quadrature 
detecting section 4. The quadrature detecting section 4 is one embodiment 
of means for quadrature detecting in accordance with the present 
invention. The received echo signals of the respective sound rays which 
have been separated into the in-phase component i and quadrature component 
q are supplied into a 2-dimensional Fourier transforming section 5. 
The 2-dimensional Fourier transforming section 5 performs the 2-dimensional 
Fourier transformation of the supplied received echo signals for all of 
the sound rays in the bearing angle direction. The 2-dimensional Fourier 
transforming section 5 is one embodiment of means for 2-dimensional 
Fourier transforming in accordance with the present invention. The results 
of the 2-dimensional Fourier transformation are supplied into a imaging 
section 6. 
Instead of the 2-dimensional Fourier transformation, other mathematically 
equivalent techniques such as the fractional Fourier transformation or the 
Gabor transformation can be used to provide the same results. 
The imaging section 6 calculates powers for the respective data 
(2-dimensional data) obtained from the 2-dimensional Fourier 
transformation, and produces an image using the powers as pixel values. 
This provides the direct vision image in the sound ray direction, i.e., 
the 3-dimensional image, representing the surface of the object to be 
imaged (e.g., a fetus). 
Such a 3-dimensional image is superior in spatial resolution, because it is 
produced based on the 2-dimensional Fourier transformed data. 
The quadrature detecting section 4, the 2-dimensional Fourier transforming 
section 5 and the imaging section 6 constitute means for producing images 
10. 
Also, the means for producing images 10 may be constructed to produce 
images based on the intensities of the received echo signals for the 
respective range gated sound rays without using the 2-dimensional Fourier 
transformation. Such means for producing images is desirable with regard 
to the simplification of the system. 
The output image from the means for producing images 10 is supplied to a 
display section 7 and displayed as a visual image. 
The range gating section 3, the quadrature detecting section 4, the 
2-dimensional Fourier transforming section 5 and the imaging section 6 
which are associated with the received echo signal processing have the 
operation speed sufficient to process the received echo signals in 
real-time for 4096 sound rays at the frame rate of 30 FPS, for example. 
Therefore, the image on the display section 7 is the real-time 
3-dimensional image. 
A control section 8 executes ultrasonic imaging according to the 
predetermined sequence by controlling the aforementioned components. The 
control section 8 is comprised of, for example, a computer. 
An operating section 9 is connected to the control section 8. The operating 
section 9 is manipulated by an operator, and receives commands, imaging 
parameters and the like for the control section 8. 
In above-described embodiments, the transducer array in the ultrasonic 
probe 1 is arranged in an 8.times.8 matrix. However, the matrix size is 
not limited thereto but any size may be selected, e.g., a 16.times.16 or 
32.times.32 matrix. 
As far as the operation speed at the range gating section 3, the quadrature 
detecting section 4, the 2-dimensional Fourier transforming section 5 and 
the imaging section 6 which are associated with the received echo signal 
processing permits, the matrix size is preferably selected as large as 
possible in order to obtain the wide imaging range or high resolution of 
the image. 
When the large matrix size such as a 64.times.64 matrix is selected, it is 
preferred that a multiplexer is attached at the output of the transducer 
array, and the received echo signal processing, such as the beam forming 
and the like, is performed in time-sharing in order to avoid the large 
scale system. This technique is referred to as a "synthetic aperture" 
technique. 
Further, the arrangement of the transducer array is not necessarily limited 
to an exact rectangular matrix but simpler matrices can be used, an 
example of which is shown in FIG. 6. FIG. 6 illustrates one example of the 
array in which the ultrasonic transducer elements 11 are arranged in 2 
concentric circles. Each row is comprised of, for example, 32 ultrasonic 
transducer elements. 
Even if such a transducer array is used, a plurality of the receive beams 
which divides the small front view fr in the front view FR into, for 
example, an 8.times.8 matrix can be formed by the beam forming at the 
transmitting/receiving section 2. 
However, the receive beam which is to be formed will have a large side 
lobes SL as shown in FIG. 7. But it is the acuteness of a main lobe ML 
that is important with respect to obtaining direct vision images in the 
sound ray direction, and the acuteness of the main lobe ML is almost the 
same as formed by the ideal beam forming (the dot-dashed lines), causing 
no problem. 
Furthermore, the array may be shaped into a rectangular ring. The 
transducer elements on the peripherary of the rectangular matrix array may 
be used to arrange the rectangular ring-shaped array as shown in FIG. 8 as 
a hatched portion. 
This technique is desirable, because the shape and size of the rectangular 
ring-shaped array can be unrestrictedly determined within the matrix 
according to the selection of the transducer elements. 
In addition, as shown in FIG. 9 as a hatched portion, the beam forming can 
be performed using the transducer arrangement in which the transducer 
elements associated with the beam forming are sparsely distributed in the 
matrix. Such an array is referred to as a "sparse array". The sparse array 
is desirable, because the degree of freedom of the array arrangement can 
be increased, benefited by the selection of the transducer elements. 
When the rectangular ring-shaped array or sparse array is used, the receive 
beam to be formed by the beam forming will contain considerable side 
lobes, causing no problem for the reason described above. 
Many widely different embodiments of the invention may be constructed 
without departing from the spirit and the scope of the present invention. 
It should be understood that the present invention is not limited to the 
specific embodiments described in the specification, except as defined in 
the appended claims.