Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2001-11-09
2004-02-10
Imam, Ali M. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
Reexamination Certificate
active
06689063
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to an apparatus for fast imaging of eg the human body using ultrasound.
The image is made using a multi-element probe in which all or selected ones of the elements are used as transmitters. The reflected signal from the object is then measured by all elements and ultrasound beams are simultaneously focused throughout the imaging region. The image is updated with the new information every time a new element or a group of elements are used as a transmitting aperture. Hereby the image is continuously updated and can be used for probing moving structures and enhance the imaging of blood velocities.
BACKGROUND OF THE INVENTION
The technological development has constantly led to improvements in ultrasound beam formers as described in [1]. The introduction of digital beam formers made the dynamic receive focusing possible. Unfortunately it is not possible to dynamically control the transmitted acoustic energy, and only a fixed focus is used in transmit.
The basic concept of focusing is to constructively add energy in the combined transmit-receive signal. Ideally, by means of the receive processing it is possible to compensate for the phase changes in the transmit. This corresponds to a dynamically focused transmit and receive imaging system. A composite image (obtained by multiple emissions, focused at different depths in transmit) is the ideal reference image.
One way to obtain a dynamic transmit focus is to use synthetic array imaging. There are three variations of the synthetic aperture imaging. 1: using a synthetic transmit aperture, 2: using a synthetic receive aperture, and 3: using a combination of synthetic transmit-receive aperture. All of these combinations have been studied, and have shown different advantages and drawbacks. The time necessary to acquire a single image T
acquire image
is proportional to the number of emissions N
emission
, the time necessary to record the reflected ultrasound wave from a single emission T
record emission
, the number of scan lines N
1
, and it is inversely proportional to the number of the parallel receive beam formers N
parallel
:
T
aquireimage
=T
acquirescanline
·N
emission
·N
1
/N
parallel
(1)
The acquisition time for an image, with a typical depth of 15 cm assuming that the speed of sound is 1540 m/s, is 200 &mgr;s. If N
emission
=64 and N
1
=N
parallel
then T
acquire image
=12.8 ms, which results in 78 frames/sec. For every new frame in the imaging process, the data acquired for the previous frame is discarded.
The recursive imaging method and apparatus according to the invention uses the beam formed lines from the previous frame to create a new frame after every emission. This results in N
emission
=1, T
acquire image
=200 &mgr;s, and a frame rate of 5000 frames/sec. The invention uses a synthetic transmit aperture and preferably receives with the full aperture. The prior art synthetic transmit aperture focusing [2], [3] is presented below. Based on it, the new recursive ultrasound imaging technique according to the invention is mathematically derived and discussed below.
One of the problems in emitting with only one element is the signal-to-noise ratio, since there are physical limitations to what power can be sent with a single transducer array element. The problem was previously studied [2], [4], and a solution using multiple elements in transmit was suggested in [2]. This problem is discussed below.
The other problem is the presence of motion artifacts due to the time of acquisition for a single image. This is avoided with the invention by decreasing the number of emissions necessary to create one frame. This can be done with a sparse synthetic aperture, and the results of the decreased number of emissions is presented.
Prior Art Synthetic Array Imaging
Phased linear arrays are used for creating sector B-mode images as shown on FIG.
2
. The image consists of a number of scan lines N
1
. The scan lines l=(1 . . . N
1
) have a common origin. Each of them has a different angle &thgr;
1
with the normal vector to the transducer's surface. For simplicity all considerations are made only in the z-x plane, and it is assumed that a single transducer element emits a cylindrical wave propagating at a constant speed c in a linear medium, as shown in FIG.
3
.
The measurement situation is shown in FIG.
4
. Element i with center coordinates (x
i
,z
i
) emits a spherical wave. The wave front reaches point P(x
P
,z
P
) after time
t
iP
=
(
x
P
-
x
i
)
2
+
(
z
P
-
z
i
)
2
c
,
(
2
)
where c is the speed of sound. If all the elements are excited, they will form a pressure field in front of the transducer, which is a sum of the fields created by each of the elements. The emitted waves reach point P at different time instances depending on the element positions.
In order to align the wave fronts and thereby focus the acoustic energy in point P, the signals from the individual array elements must be appropriately delayed. In the calculation of the delays, one point from the transducer is selected as a reference point. All the delays are calculated relative to the time necessary for a sound wave to travel the distance between the reference point and the focal point. For a sector image, the center of the transducer array is the usual choice. The reference point is the center of element C(x
c
,z
c
) for the case depicted on FIG.
4
. The delay for element i for focusing the energy at point P(x
p
,y
p
) is calculated by:
d
i
=
t
CP
-
t
iP
d
i
=
(
x
C
-
x
P
)
2
+
(
z
C
-
z
P
)
2
-
(
x
i
-
x
P
)
2
+
(
z
i
-
z
P
)
2
c
(
3
)
FIG. 5
shows the geometry used for the simulations. The origin of the coordinate system O(0,0) lies in the middle of the physical linear array transducer. All scan-lines (l=1 . . . N
1
) start from the origin O and have angle &thgr;
1
with the z-axis. The focal points lie on these scan lines and have coordinates:
x
f1
=r
f
·sin&THgr;
1
z
f1
=r
f
·cos&THgr;
1
(4)
where r
f
is the axial distance to the focal point. Dynamic focusing is obtained if r
f
changes in time as
r
f
=ck/f
s
(5)
where k is the sample number and f
s
is the sampling frequency. In conventional imaging systems the transmission uses a fixed r
f
, whereas the receive can be dynamically focused along the current scan line.
One way to create a synthetic aperture image is to emit with all the elements from the transducer array one at a time (see FIG.
6
). During reception data is recorded and beam formed. After a number of emissions N
xmt
all the beam formed RF lines from the separate emissions are summed to create the scan-lines that are envelope-detected and displayed.
The beam formed signal from emission n with element i for line l is s
li
(n)
(t). The number for the emission n (0<n<∞) is relative to the beginning of the continuous imaging process. This number is relevant only for the recursive imaging and in this section a simplified notation is used: s
li
(t)=s
li
(n)
(t). The time t is relative time from the emission of the pulse. The RF signal that is envelope-detected and displayed is:
s
l
(
t
)
=
∑
i
=
1
N
xmt
s
li
(
t
)
(
6
)
s
li
(
t
)
=
∑
j
=
1
N
rcv
a
lij
·
s
ij
(
t
-
d
lij
)
(
7
)
where N
xmt
is the number of transmit elements, N
rev
is the number of receive elements, s
ij
(t) is the recorded data, a
lij
is a weighting coefficient (apodization), and d
lij
is the delay for image line l, when emitting with element i and receiving with element j.
The delays d
lij
is the sum of the delays for elements i and j calculated with formula (3) for the points in line l
d
lij
=d
li
+d
lj
For the case in
FIG. 5
, the delays are calculated by:
d
lij
=
x
fl
2
+
z
fl
2
-
(
x
t
-
x
fl
)
2
+
z
fl
2
c
+
x
fl
2
+
z
fl
2
-
(
x
j
-
x
fl
)
2
+
z
fl
2
c
.
(
8
)
This is the
Jensen Jørgen Arendt
Nikolov Svetoslav
B-K Medical A/S
Imam Ali M.
LandOfFree
Method and apparatus for acquiring images by recursive... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for acquiring images by recursive..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for acquiring images by recursive... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3329326