Noise reduction and aliasing compensation in a doppler mode image
View Patent ↗The present invention relates to an ultrasound system and a method of forming a Doppler mode image. The ultrasound system comprises a transmission/reception unit operable to transmit/receive ultrasound signals at a pulse repetition frequency (PRF) to form an n number of Doppler signals. The ultrasound system further comprises a signal processing unit. The signal processing unit computes a first mean frequency based on an n th Doppler signal and an (n−1) th Doppler signal and a second mean frequency based on an n th Doppler signal and an (n−k) th Doppler signal, wherein k is a positive integer equal to or greater than 2. The signal processing unit estimates a Doppler frequency and compensate for aliasing of the Doppler signal based on the first and second mean frequencies to produce Doppler signal with the aliasing compensated. An image processing unit operable to form a Doppler mode image based on the Doppler signals produced by the signal processing unit.
1. An ultrasound system, comprising:
a transmission/reception unit operable to transmit/receive ultrasound signals in a pulse repetition frequency (PRF) to form an n number of Doppler signals;
a signal processing unit operable to compute a first mean frequency based on an n th Doppler signal and an (n−1) th Doppler signal and a second mean frequency based on an n th Doppler signal and an (n−k) th Doppler signal, respectively, wherein k is a positive integer equal to or greater than 2, the signal processing unit being further operable to check whether the first and second mean frequencies exist within predetermined frequency ranges associated with the PRF, respectively, to determine occurrence of aliasing in the Doppler signals and compensate for the aliasing of the Doppler signals to thereby produce Doppler signals with the aliasing compensated;
wherein the signal processing unit includes a quadrature demodulating unit operable to demodulate the Doppler signals into baseband signals consisting of an I signal of an in-phase component and a Q signal of a quadrature component, and
wherein the signal processing unit is operable to compute the first and mean frequencies through following equations,
First
mean
frequency
=
P
R
F
2
π
tan
-
1
∑
n
=
1
N
i
(
n
-
1
)
q
(
n
)
-
i
(
n
)
q
(
n
-
1
)
∑
n
=
1
N
i
(
n
)
i
(
n
-
1
)
+
q
(
n
)
q
(
n
-
1
)
,
and
Second
mean
frequency
=
P
R
F
2
k
π
tan
-
1
∑
n
=
1
N
i
(
n
)
q
(
n
-
k
)
-
i
(
n
-
k
)
q
(
n
)
∑
n
=
1
N
i
(
n
)
i
(
n
-
k
)
+
q
(
n
)
q
(
n
-
k
)
wherein i(n) represents and nth I signal outputted from the quantrature demodulating unit, q(n) represents an nth Q signal outputted from the quadrature demodulating unit, i(n−1) represents an (n−1)I signal, q(n−1) represents an (n−1) th Q signal, n represents an ensemble length, i(n−k) represents an (n−k) th I signal outputted from the quadrature demodulating unit, and q(n−k) represents an (n−k) th Q signal outputted from the quadrature demodulating unit;
and
an image processing unit operable to form a Doppler mode image based on the Doppler signals produced by the signal processing unit.
2. The ultrasound system of claim 1 , wherein the signal processing unit includes:
a mean frequency computing unit operable to compute the first and second mean frequencies based on the baseband signals; and
a Doppler signal processing unit operable to perform the first and second signal processing based on the first and second mean frequencies.
3. The ultrasound system of claim 2 , wherein the mean frequency computing unit include:
a first mean frequency computing unit operable to compute the first mean frequency based on an n th IQ signal and an (n−1) th IQ signal outputted from the quadrature demodulating unit; and
a second mean frequency computing unit operable to compute the second mean frequency based on n th IQ signals and (n−k) th IQ signals outputted from the quadrature demodulating unit, wherein k is an integer equal to or greater than 2.
4. The ultrasound system of claim 2 , wherein the Doppler signal processing unit includes:
a mean frequency analyzing unit operable to analyze the first and second mean frequencies to output analysis result signals;
a first signal processing unit operable to perform a first signal processing upon the Doppler signals in response to the analysis result signals; and
a second signal processing unit operable to perform a second signal processing upon the Doppler signals in response to the analysis result signals.
5. The ultrasound system of claim 4 , wherein the mean frequency analyzing unit is operable to:
if the aliasing does not occur in the first and second mean frequencies, output a first analysis result signal,
if the aliasing does not occur in the first mean frequency at a negative frequency region and the aliasing occurs in the second mean frequency, output a second analyzing result,
if the aliasing does not occur in the first mean frequency at a positive frequency region and the aliasing occurs in the second mean frequency, output a third analyzing result,
if the first mean frequency varies multiple times at ±PRF/2 and the second mean frequency increases, output a fourth analysis result signal, and
if the first mean frequency varies multiple times at ±PRF/2 and the second mean frequency decreases, output a fifth analysis result signal.
6. The ultrasound system of claim 5 , wherein the first signal processing unit is operable to perform the first signal processing through the following equation in response to the first analysis result signal
f
out
=
1
st
mean
frequency
+
2
nd
mean
frequency
2
.
7. The ultrasound system of claim 5 , wherein the first signal processing unit is operable to perform the first signal processing through the following equation in response to the second analysis result signal
f
out
=
1
st
mean
frequency
+
2
nd
mean
frequency
-
P
R
F
2
2
.
8. The ultrasound system of claim 5 , wherein the first signal processing unit is operable to perform the first signal processing through the following equation in response to the third analysis result signal
f
out
=
1
st
mean
frequency
+
2
nd
mean
frequency
+
P
R
F
2
2
.
9. The ultrasound system of claim 5 , wherein the second signal processing unit performs the second signal processing by adding PRF to the first mean frequency aliased in a −PRF/2 direction in response to the fourth analysis result signal.
10. The ultrasound system of claim 5 , wherein the second signal processing unit performs the second signal processing by adding −PRF to the first mean frequency aliased in a PRF/2 direction in response to the fifth analysis result signal.
11. A method of forming a Doppler mode image in an ultrasound system having a transmission/reception unit, a signal processing unit and an image processing unit, comprising:
a) at the transmission/reception unit, transmitting/receiving ultrasound signals in a pulse repetition frequency (PRF) to form an n number of Doppler signals;
b) at the signal processing unit, demodulating the Doppler signals into baseband signals consisting of I signal of an in-phase component and a Q signal of a quadrature component, and computing a first mean frequency based on an n th Doppler signal and an (n−1) th Doppler signal and a second mean frequency based on an n th Doppler signal and an (n−k) th Doppler signal outputted from the transmission/reception unit, respectively, wherein k is a positive integer equal to or greater than 2, wherein the first and mean frequencies are calculated through the following equations,
First
mean
frequency
=
P
R
F
2
π
tan
-
1
∑
n
=
1
N
i
(
n
-
1
)
q
(
n
)
-
i
(
n
)
q
(
n
-
1
)
∑
n
=
1
N
i
(
n
)
i
(
n
-
1
)
+
q
(
n
)
q
(
n
-
1
)
Second
mean
frequency
=
P
R
F
2
π
tan
-
1
∑
n
=
1
N
i
(
n
)
q
(
n
-
k
)
-
i
(
n
-
k
)
q
(
n
)
∑
n
=
1
N
i
(
n
)
i
(
n
-
k
)
+
q
(
n
)
q
(
n
-
k
)
wherein i(n) represents an n th I signal outputted from the quadrature demodulating unit, q(n) represents an n th Q signal outputted from the quadrature demodulating unit, i(n−1) represents an (n−1) th I signal, q(n−1) represents an (n−1) th Q signal, N represents an ensemble length, i(n−k) represents an (n−k) th I signal outputted from the quadratue demodulating unit, and q(n−k) represents an (n−k) th Q signal outputted from the quadrature demodulating unit;
c) at the signal processing unit, checking whether the first and second mean frequencies exist within predetermined frequency ranges associated with the PRF, respectively, to determine occurrence of aliasing in the Doppler signals and compensating for the aliasing of the Doppler signals to produce Doppler signals with the aliasing compensated; and
d) at the image processing unit, forming a Doppler mode image based on the Doppler signals produced by the signal processing unit.
12. The method of claim 11 , wherein the step c) includes:
c1) analyzing the first and second mean frequencies to output analysis result signals; and
c2) performing first and second signal processing in response to the analysis result signals.
13. The method of claim 12 , wherein the step c1) includes:
if the aliasing does not occur in the first and second mean frequencies, outputting a first analysis result signal;
if the aliasing does not occur in the first mean frequency at a negative frequency region and the aliasing occurs in the second mean frequency, outputting a second analyzing result;
if the aliasing does not occur in the first mean frequency at a positive frequency region and the aliasing occurs in the second mean frequency, outputting a third analyzing result;
if the first mean frequency varies multiple times at +PRF/2 and the second mean frequency increases, outputting a fourth analysis result signal; and
if the first mean frequency varies multiple times at ±PRF/2 and the second mean frequency decreases, outputting a fifth analysis result signal.
14. The method of claim 13 , wherein the c2) includes performing the first signal processing through the following equation in response to the first analysis result signal.
f
out
=
1
st
mean
frequency
+
2
nd
mean
frequency
2
.
15. The method of claim 13 , wherein the c2) includes performing the first signal processing through the following equation in response to the second analysis result signal
f
out
=
1
st
mean
frequency
+
2
nd
mean
frequency
-
P
R
F
2
2
.
16. The method of claim 13 , wherein the c2) includes performing the first signal processing through the following equation in response to the third analysis result signal.
f
out
=
1
st
mean
frequency
+
2
nd
mean
frequency
+
P
R
F
2
2
.
17. The method of claim 13 , wherein the c2) includes performing the second signal processing by adding PRF to the first mean frequency aliased in a −PRF/2 direction in response to the fourth analysis result signal.
18. The method of claim 13 , wherein the c2) includes performing the second signal processing by adding −PRF to the first mean frequency aliased in a PRF/2 direction in response to the fifth analysis result signal.