Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2002-01-25
2004-04-20
Jaworski, Francis J. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C600S437000, C367S089000
Reexamination Certificate
active
06725076
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to an apparatus and a method for determining the velocity vector of a remotely sensed object using either sound, in particular ultrasound, or electromagnetic radiation. The movement of the object is determined by emitting and receiving a pulsed field focused in the direction of the velocity vector. By using a number of pulse emissions, the inter pulse movement can be estimated and the velocity found from the estimated movement and the time between pulses. The invention is based on the principle of using a directionally focused field for making the received signal influenced by motion along the direction of the movement.
BACKGROUND OF THE INVENTION
Medical ultrasound is extensively used for studying flow dynamics in the human body by using color flow mapping. The technique displays a color image of the flow superimposed on the normal anatomic B-mode image. Traditionally, the velocity component along the ultrasound beam direction is measured, and a flow transverse to the beam is not displayed, since it is not measured or estimated. An example of this is shown in
FIG. 1
, where the flows in the carotid artery and the jugular vein are displayed. The image is acquired with a convex array transducer, and the angles between flow direction and the ultrasound beam change over the image. Notice the change of estimated flow direction around the dashed line in both vessels due to the change of angle between the flow and the ultrasound beam. This is one of the main limitations of current ultrasound flow systems, since most vessels are parallel to the skin surface, and therefore it is a problem to get a sufficiently small angle between the flow and the beam. Also the flow is often not parallel to the vessel surface, and it is therefore difficult, if not impossible, to estimate the correct angle and compensate for it [1]. In European patent application EP 616 231 [2] the velocity is found through a cross-sectional area using a 2D matrix transducer that can focus on the individual areas in the cross-section. The volume flow through the cross-section is then found, but still only the velocity in direction of the ultrasound beam is estimated. Several authors have attempted to remedy this artifact. Fox [3] suggested using two beams to find the transverse component. The system works well for large transducers and investigations close to the transducer, but the variance of the transverse component increases for situations with large depths and smaller transducers as used in cardiac scanning through the ribs. Trahey and co-workers [4] have suggested using speckle tracking in which a small search region in one image is correlated or compared to a subsequent image. This approach has problems in terms of frame rate, since images are compared, and the resolution of the velocity estimates can be low. Newhouse et al. [5] developed a method in which the total bandwidth of the received signal is affected by the transverse velocity. It is, however, often difficult to find this bandwidth due to the inherent noise in the signal.
Of special interest is the working by Bonnefous [6], which uses a number of parallel beams to find the transverse velocity. The approach does, however, not work for a velocity that is not orthogonal to the ultrasound beam direction.
In this invention a new technique using a focused signal in the direction of the velocity is used. The velocity is then found by acquiring two or more of these focused signals and cross correlating them to find the displacement between pulse emission, whereby the velocity can be determined.
PRIOR ART APPROACH
This section summarizes the article written in 1988 by Bonnefous [6], where a method for estimating the transverse velocity was suggested.
Transverse velocity estimation must perform a signal processing, where the effect of axial motion is negligible compared to the transverse one. The idea presented by Bonnefous requires a broad beam in emission (a plane ultrasound wave front), and a number of parallel identical beams, separated by a pitch w in the transverse direction, are generated in reception, see FIG.
2
.
For a given depth z, the signal received S
n
(x,t) from the beam centered in x=0 is
S
n
(0
,t
)=∫
p
(
x,t
)
D
n
(
x
)
dx
(1)
where p(x,t) is the response of a scatterer located at x in the ultrasound beam centered at x=0, and D
n
(x) is the scatterer distribution at the instant nT
prf
, where T
prf
is the pulse repetition period. In the same way, the signal received from the beam centered at x=w is
S
n
(
w,t
)=∫
p
(
x−w,t
)
D
n
(
x
)
dx
(2)
If only a transverse uniform motion of the scatterers in considered, the displacement between two consecutive pulses nT
prf
and (n+1)T
prf
will give the relation for the distribution of the scatterers
D
n+1
(
x
)=
D
n
(
x−v
x
T
prf
) (3)
where v
x
is the velocity of the scatterers. Combining equations (2) and (3) gives
S
n+1
(0
,t
)=∫
p
(
x,t
)
D
n+1
(
x
)
dx=∫p
(
x,t
)
D
n
(
x−v
x
T
prf
)
dx=∫p
(
x+v
x
T
prf
,t
)
D
n
(
x
)
dxS
n+1
(0
,t
)=
S
n
(−
v
x
T
prf
,t
) (4)
For a beam centered in x=w the relation between the consecutive signals is
S
n+1
(
w,t
)=
S
n
(
w−v
x
T
prf
,t
) (5)
The signal received in x=w at the pulse time (n+1)T
prf
is, thus, the same as the signal received in a beam centered in x=w−v
x
T
prf
at the pulse time nT
prf
.
The correlation between the two signals p(x,t) and p(x−w,t) is an averaging of the received signals over the random scatterer distributions. The cross-correlation of the received signals from two adjacent signals is
C
1
(
w
)
=
∑
n
∫
t
0
t
0
+
Δ
t
S
n
(
0
,
t
)
S
n
+
1
(
w
,
t
)
ⅆ
t
=
∑
n
∫
t
0
t
0
+
Δ
t
S
n
(
0
,
t
)
S
n
(
w
-
v
x
T
prf
,
t
)
ⅆ
t
C
1
(
w
)
=
C
0
(
w
-
v
x
T
prf
,
t
)
(
6
)
where
C
0
(
w
)
=
∑
∫
t
0
t
0
+
Δ
t
S
n
(
0
,
t
)
S
n
(
w
,
t
)
ⅆ
t
,
is the autocorrelation function averaged over a number of received lines, where the line number is denoted by n. The interval (t
0
,t
0
+&Dgr;t) is the range gate selected for the received signals. Equation (6) show that the shift of C
1
compared with C
0
is the transverse displacement between the instants nT
prf
and (n+1)T
prf
. Therefore, the maximum of C
1
(w) is C
1
(v
x
T
prf
)=C
0
(0).
For the general case, in which both axial and transverse motion takes place, the equation that relates the received signals from two successive pulses will be
S
n
+
1
(
w
,
t
)
=
S
n
(
w
-
v
x
T
prf
,
t
-
2
v
z
T
prf
c
)
(
7
)
Here t
s
=2v
z
T
prf
/c, is the time shift for the axial motion.
The cross-correlation and autocorrelation functions are generalized to two-dimensional functions and their expressions are
C
1
(
w
,
u
)
=
∑
n
∫
t
0
t
0
+
Δ
t
S
n
(
0
,
t
)
S
n
+
1
(
w
,
t
+
u
)
ⅆ
t
(
8
)
=
∑
n
∫
t
0
t
0
+
Δ
t
S
n
(
0
,
t
)
S
n
(
w
-
v
x
T
prf
,
t
-
2
v
z
T
prf
c
+
u
)
ⅆ
t
(
9
)
C
0
(
w
,
u
)
=
∑
n
∫
t
0
t
0
+
Δ
t
S
n
(
0
,
t
)
S
n
(
w
,
t
+
u
)
ⅆ
t
(
10
)
The relation between the cross-correlation and the autocorrelation is
C
1
(
w
,
u
)
=
C
0
(
w
-
v
x
T
prf
,
u
-
2
v
z
T
prf
c
)
(
11
)
The two-dimensional determination
B-K Medical A/S
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Jaworski Francis J.
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