U.S. patent number RE35,371 [Application Number 08/019,654] was granted by the patent office on 1996-11-05 for method and system for controlling ultrasound scanning sequence.
This patent grant is currently assigned to Kabushiki Kaisha Toshiba. Invention is credited to Yasutsugu Seo.
United States Patent |
RE35,371 |
Seo |
November 5, 1996 |
Method and system for controlling ultrasound scanning sequence
Abstract
The sequence for a case of scanning by an ultrasound prove
having a plurality of transducers, for example, m numbers, is as
follows: first scan line.fwdarw.(m-l)th scan line.fwdarw.mth scan
line.fwdarw.first scan line.fwdarw.second scan line.fwdarw.mth scan
line.fwdarw.first scan line.fwdarw.second scan line.fwdarw.third
scan line.fwdarw.first scan line.fwdarw.. . . The ultrasound probe
transmits ultrasound beams to a subject and receives echo signals
reflected from the subject in accordance with the above sequence.
Doppler signals obtained from the echo signals are stored in a
memory. When the predetermined number of Doppler signals in each
line are stored in the memory, the Doppler signals in each line are
read out from the memory. By such scanning sequence, a repeat
frequency, i.e., a sampling frequency of Doppler signals of the
ultrasound beams transmitted in the same direction, is a shorter.
As a result, a low limit of measurable flow velocity is a lower,
and a timing for reading out (outputting) the Doppler signals from
the memory can be a constant.
Inventors: |
Seo; Yasutsugu (Ootawara,
JP) |
Assignee: |
Kabushiki Kaisha Toshiba
(Kawasaki, JP)
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Family
ID: |
16437726 |
Appl.
No.: |
08/019,654 |
Filed: |
February 19, 1993 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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228590 |
Aug 5, 1988 |
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Reissue of: |
423713 |
Oct 18, 1989 |
04993417 |
Feb 19, 1991 |
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Foreign Application Priority Data
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Aug 12, 1987 [JP] |
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62-201244 |
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Current U.S.
Class: |
600/455 |
Current CPC
Class: |
A61B
8/06 (20130101); A61B 8/13 (20130101); G01S
7/52071 (20130101); G01S 7/52085 (20130101); G01S
15/8979 (20130101); G01S 7/52034 (20130101); G01S
7/5206 (20130101); G01S 15/52 (20130101) |
Current International
Class: |
G01S
15/89 (20060101); G01S 15/00 (20060101); G01S
7/52 (20060101); G01S 15/52 (20060101); A61B
008/06 () |
Field of
Search: |
;128/660.07,660.04-660.05,661.08-661.10 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0190979 |
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Aug 1986 |
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EP |
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3417418 |
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Nov 1985 |
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DE |
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3605163 |
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Aug 1986 |
|
DE |
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3614688 |
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Oct 1986 |
|
DE |
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Primary Examiner: Jaworski; Francis
Attorney, Agent or Firm: Cushman, Darby & Cushman
Parent Case Text
This is a continuation of application Ser. No. 07/228,590, filed
Aug. 5, 1988, and now abandoned.
Claims
What is claimed is:
1. A method for controlling an ultrasound scanning sequence, the
method comprising the steps of:
setting a number of ultrasound beams transmitted to a subject in a
same direction;
setting a detection ratio used for detecting a flow velocity of the
subject; and
controlling the ultrasound scanning sequence in accordance with the
number of ultrasound beams and the detection ratio, so as to
increase a transmission interval of the ultrasound beams in the
same direction without changing a generation interval of an
ultrasound pulse.
2. A method according to claim 1, wherein the detection ratio P is
obtained by the following equation:
where fr is a frequency corresponding to a transmission interval of
the ultrasound beams in the same direction and fr' is the frequency
corresponding to the generation interval of ultrasound pulse.
3. A method as in claim 1, wherein said controlling step includes
the steps of, in sequence:
(a) commanding transmitting an ultrasound beam to the subject in a
first direction;
(b) commanding transmitting ultrasound beams to the subject in
.Iadd.a number of .Iaddend.additional directions, .[.including a
second direction through a direction having a number.].
.Iadd.whereby a total number of said number of additional
directions and said first direction is .Iaddend.equal to said
detection ratio; and
(c) repeating said steps (a) and (b) a number of times equal to
.[.said generation interval.]. .Iadd.a number of times a sample is
taken in each direction.Iaddend..
4. A system for controlling an ultrasound scanning sequence, the
system comprising:
transmitting means for transmitting ultrasound beams to a
subject;
receiving means for receiving echo signals from the subject;
processing means for processing echo data representing the received
echo signals to obtain a flow velocity;
storing means for storing the echo data; and
control means for controlling the transmitting means, the receiving
means and the storing means in accordance with the ultrasound
scanning sequence, so as to increase a transmission interval of the
ultrasound beams in the same direction without changing a
generation interval of an ultrasound pulse.
5. A system according to claim .[.4.]. .Iadd.6.Iaddend., wherein
the detection ratio P is obtained by the following equation:
where fr is the frequency corresponding to the transmission
interval of the ultrasound beams .Iadd.in the same direction
.Iaddend.and fr' is the frequency corresponding to the general
interval of .Iadd.the .Iaddend.ultrasound pulse.
6. A system according to claim .[.5.]. .Iadd.4.Iaddend., wherein
the control means includes:
first setting means for setting a number or ultrasound beams
transmitted to a subject in the same direction:
second setting means for setting a detection ratio used for
detecting a flow velocity or the subject; and
generating means for generating control signals for controlling the
ultrasound scanning sequence by the number set by the first setting
means and the detection ratio set by the second setting means.
7. A system as in claim .[.3.]. .Iadd.6.Iaddend., wherein said
control means includes means for, in sequence:
(a) controlling the transmitting means to transmit an ultrasound
beam to the subject in a first direction;
(b) controlling the transmitting means to transmit ultrasound beams
to the subject in a .Iadd.number of .Iaddend.additional
.[.direction, including a second direction through a direction.].
.Iadd.directions, whereby a total number of said number of
additional directions and said first direction is .Iaddend.equal to
said detection ratio; and
(c) repeating said steps (a) and (b) a number of times equal to
.[.said generation interval.]. .Iadd.a number of times a sample is
taken in each direction.Iaddend.. .Iadd.
8. A method according to claim 3, further comprising the steps
of:
transmitting the ultrasound beams so as to cause echo signals to be
echoed from the subject;
receiving the echo signals;
generating echo data corresponding to the received echo
signals;
storing the echo data; and
reading out the stored data, corresponding to the received echo
signals from each direction, at non-equal
intervals..Iaddend..Iadd.
9. A method according to claim 1, wherein said controlling step
comprises in sequence the steps of:
(a) commanding transmitting ultrasound beams to the subject in
several directions of a predetermined number of directions in an
interleaved fashion, until a predetermined number of samples are
taken in each direction; and
(b) repeating said step (a) until the ultrasound beams are
transmitted to all predetermined directions. .Iaddend..Iadd.
10. A system according to claim 7, wherein the control means
controls the ultrasound scanning sequence so that the stored echo
data, corresponding to the received echo signals from each
direction, is read out from the storing means at non-equal
intervals. .Iaddend..Iadd.
11. A system according to claim 4, wherein said control means
comprises means for changing direction of the transmitted
ultrasound beams in a fashion interleaved in each of several
directions. .Iaddend..Iadd.
12. A method for controlling an ultrasound scanning sequence, the
method comprising the steps of:
setting a number of ultrasound beams to be transmitted to a subject
in a same direction;
setting a detection ratio used for detecting a flow velocity of the
subject;
controlling the ultrasound scanning sequence in accordance with the
number of ultrasound beams and the detection ratio, so as to
increase a transmission interval of the ultrasound beams in the
same direction without changing a generation interval of an
ultrasound pulse by commanding the transmission of ultrasound beams
to the subject in a number of additional directions;
transmitting the ultrasound beams so as to cause echo signals to be
echoed from the subject;
receiving the echo signals;
generating output data corresponding to the received echo signals
from each direction at equal intervals. .Iaddend..Iadd.
13. A method according to claim 12, wherein said output data
corresponding to the received echo signals from each direction is
generated at equal intervals which are larger than 1/fr', where fr'
is a frequency corresponding to the generation interval of the
ultrasound pulse. .Iaddend..Iadd.
14. A method according to claim 13, wherein said intervals are
equal to n/fr', where n is a number of received echo signals from
each direction. .Iaddend..Iadd.15. A method for controlling an
ultrasound scanning sequence, the method comprising the steps
of:
setting a number of ultrasound beams to be transmitted to a subject
in a same direction;
setting a detection ratio used for detecting a flow velocity of the
subject;
controlling the ultrasound scanning sequence in accordance with the
number of ultrasound beams and the detection ratio, so as to
increase a transmission interval of the ultrasound beams in the
same direction without changing a generation period of an
ultrasound pulse by commanding the transmission of ultrasound beams
to the subject in a number of additional directions;
transmitting the ultrasound beams so as to cause echo signals to be
echoed from the subject;
receiving the echo signals;
generating output data corresponding to the received echo signals
from each
direction at non-equal intervals. .Iaddend..Iadd.16. A method
according to claim 15, wherein said output data corresponding to
the received echo signals from each direction is generated
repeatedly at a first interval (n-1)/fr' and a second interval
(n+1)/fr', where fr' is a frequency corresponding to the generation
period of the ultrasound pulse and n is a number of the received
echo signals from each direction.
.Iaddend..Iadd. A system for controlling an ultrasound scanning
sequence, the system comprising:
transmitting means for transmitting ultrasound beams to a subject
in a number of directions in an interleaved fashion, so as to cause
Doppler-shifted echo signals to be echoed from the subject;
receiving means for receiving the Doppler-shifted echo signals
until a predetermined number of Doppler-shifted echo signals are
received from each of the several directions;
generating means for generating output data corresponding to the
received Doppler-shifted echo signals; and
control means for controlling the transmitting means and the
receiving means in accordance with the ultrasound scanning
sequence, so as to generate the output data corresponding to the
received Doppler-shifted
echo signals from each directions at equal intervals.
.Iaddend..Iadd.18. A system according to claim 17, wherein said
output data corresponding to the received echo signals from each
direction is generated at equal intervals which are larger than
1/fr', where fr' is a frequency corresponding to the generation
interval of the ultrasound pulse. .Iaddend..Iadd.19. A system
according to claim 18, wherein said intervals are equal to n/fr',
where n is a number of the received echo signals from each
direction. .Iaddend..Iadd.20. A system for controlling an
ultrasound scanning sequence, the system comprising:
transmitting means for transmitting ultrasound beams to a subject
in a number of directions in an interleaved fashion, so as to cause
Doppler-shifted echo signals to be echoed from the subject;
receiving means for receiving the Doppler-shifted echo signals
until a predetermined number of Doppler-shifted echo signals are
received from each of the several directions;
generating means for generating output data corresponding to the
received Doppler-shifted echo signals; and
control means for controlling the transmitting means and the
receiving means in accordance with the ultrasound scanning
sequence, so as to generate the output data corresponding to the
received Doppler-shifted echo signals from each direction at
non-equal intervals. .Iaddend..Iadd.21. A system according to claim
20, wherein said output data corresponding to the received echo
signals from each direction is generated repeatedly at a first
interval (n-1)/fr' and a second interval (n+1)/fr', where fr' is a
frequency corresponding to a generation period of an ultrasound
pulse and n is a number of the received echo signals from
each direction. .Iaddend..Iadd.22. A method for controlling an
ultrasound scanning sequence, the method comprising the steps
of:
(a) transmitting ultrasound beams to a subject in several
directions in an interleaved fashion and receiving Doppler-shifted
echo signals until a predetermined number of Doppler-shifted echo
signals are received from each of the several directions; and
(b) switching directions of transmission of the ultrasound beams to
other several directions. .Iaddend..Iadd.23. A system for
controlling an ultrasound scanning sequence, the system
comprising:
transmitting means for transmitting ultrasound beams to a subject
in several directions in an interleaved fashion and receiving
Doppler-shifted echo signals until a predetermined number of
Doppler-shifted echo signals are received from each of the several
directions; and
switching means for switching directions of transmission of the
ultrasound beams to other several directions. .Iaddend..Iadd.24. A
dual mode ultrasound imaging system having an array of acoustic
transducer elements comprising:
B-mode imaging means to produce an electronically scanned acoustic
image of an organism under examination, said B-mode image
substantially representing an intensity of echoes returned from
said organism along multiple B-mode scan lines;
Doppler-shifted-mode imaging means to produce an electronically
scanned Doppler-shifted-mode image of said organism, said
Doppler-shifted-mode image representing estimates of one of average
velocity, variance, and power of moving scatters derived from
Doppler-shifted echoes from said moving scatters in said organism
from multiple sample volumes acquired along a direction of multiple
independently propagated Doppler-shifted scan lines; and
a color display monitor displaying the B-mode image as a two
dimensional image with echo intensities as encoded using a first
mapping function and simultaneously displaying the Doppler-shifted
image as a two dimensional image using a second and distinct
mapping of red, green and blue components that is spatially
coordinated with said B-mode image,
wherein the multiple lines of Doppler-shifted information are
acquired in interleaved fashion in the Doppler-shifted-mode.
.Iaddend.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and system for
controlling ultrasound scanning sequence so as to permit detection
of a low flow velocity.
2. Description of the Related Art
Conventionally, an apparatus has been used which combines the
ultrasonic Doppler method and the pulse echo method to acquire a
blood-flow image and a tomogram image (B-mode image) of a subject
under examination and displays the acquired images in colors in
real time. The principle of measurement of blood-flow velocity used
in the apparatus will be described hereinafter.
When ultrasound beams are transmitted to blood flowing within the
living body of a subject, the beams are scattered by moving blood
cells so that the center frequency fc of the beams is
Doppler-shifted by a Doppler frequency fd. As a result, the
received ultrasound frequency f
becomes.function.=.function.c+.function.d. In this case, the
Doppler frequency fd is represented by
where v is a blood flow velocity, .theta. is an angle made by the
ultrasonic beam with the blood vessel, and c is the acoustic
velocity. It will thus be understood that the Doppler frequency fd
is used to detect the blood flow velocity v in this way.
The blood flow velocity v is displayed in a two-dimensional form as
follows. First, as shown in FIGS. 1 and 2, ultrasound probe 1
transmits ultrasound beams sequentially to the subject in
directions of A, B, C, . . . by pulse signals provided from
transmitting circuit 7 under the sector-scan control. In place of
the sector-scan control the linear-scan control may be
performed.
For example, echo signals of the ultrasound beams transmitted in
the direction of A, which has been Doppler-shifted by the blood
flow, are received by ultrasonic probe 1 and applied to receiving
circuit 2 after conversion of an electric signal. Phase detecting
circuit 3 detects Doppler signals from the received echo signals.
The Doppler signals at, for example, 256 sampling points along a
scan line (in the direction A) of the ultrasound beam transmitted
are detected. The Doppler signals detected at each sampling point
are frequency-analyzed in frequency analyzer 4 and then provided to
display 6 via digital scan converter (DSC) 5. As a result, a
blood-flow-velocity distribution image in the direction of A is
displayed in real time.
Subsequently, the same operations are repeated for each of the scan
directions of B, C, . . . , and blood-flow-velocity distribution
images for scan directions are displayed as a two-dimensional
image.
It is to be noted that the detectability of a blood flow velocity
depends upon the data length of a Doppler signal. That is to say,
if the sampling frequency of the Doppler signal is fr and the
number of samples is n, then the data length T of the Doppler
signal will be given by
In this case, the frequency resolution .DELTA.fd will become
Therefore, the Doppler frequency fd min corresponding to the
measurable lower limit of the flow velocity will be represented
by
Therefore, it will be understood that either the sampling frequency
fr of the Doppler signal has to be lowered, or the sampling number
n has only to be increased (see FIGS. 3 and 4, FIG. 4 is obtained
by fourier-transforming the waveform shown in FIG. 3), in order to
detect the blood flow of a low velocity.
In the two-dimensional Doppler blood-flow imaging, the next
relationship holds.
where Fn is the number of frames displayed in one second, m is the
number of scan lines and fr' is the repetition frequency of the
ultrasound pulses. The frame number Fn is associated with the
real-time display of a two-dimensional blood flow image and usually
takes a value between 8.about.30 so that 8.about.30 frames will be
displayed in one second. For example, in the sector scan as shown
in FIG. 5, when the scan line number m=32, the repetition frequency
fr' of ultrasound pulses=4 KHz, and the sampling number n=8, the
frame number Fn will become about 16. That is, if the scan line
number m increases, the frame number Fn decreases, causing flicker.
If the scan line number decreases, the density of scan lines would
become coarse and hence the quality of image would be degraded.
Therefore, an apparatus has been desired which is capable of
detecting a low flow velocity without decreasing the number of
frames and degrading the image quality.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method and
system for controlling the ultrasonic scanning sequence to permit
the detection of a low flow velocity.
According to one aspect of the present invention, there is provided
a method for controlling ultrasound scanning sequence, the method
comprising the steps of:
setting a sampling number of Doppler signals in a same
direction;
setting an improvement ratio used for detecting a low flow velocity
of the subject; and
controlling the ultrasound scanning sequence by generating
transducer selection signals in accordance with the sampling number
of the Doppler signals and the improvement ratio, so as to decrease
a sampling frequency of the Doppler signals without changing an
ultrasound pulse repetition frequency.
According to another aspect of the present invention, there is
provided a system for controlling ultrasound scanning sequence, the
system comprising:
transmitting means for transmitting ultrasound beams to a
subject;
receiving means for receiving echo signals reflected from the
subject by the ultrasound beams transmitted by the transmitting
means;
detecting means for detecting Doppler signals from the echo signals
received by the receiving means;
storing means for storing the Doppler signals detected by the
detecting means; and
control means for controlling the transmitting means, the receiving
means and the storing means in accordance with the ultrasound
scanning sequence, so as to decrease a sampling frequency of
Doppler signals without changing an ultrasound pulse repetition
frequency.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a conventional scanning pattern;
FIG. 2 is a block diagram of a conventional ultrasonic diagnostic
apparatus;
FIGS. 3 and 4 are diagrams for explaining the data length and
frequency resolution of a Doppler signal;
FIG. 5 is a diagram for explaining a sector scan;
FIG. 6 is a system block diagram of an embodiment of the present
invention;
FIGS. 7A through 7C show signal waveforms in principal part of the
embodiment system;
FIG. 8 shows an arrangement of an MTI filter used with the
embodiment system;
FIG. 9 shows an arrangement of a control circuit used with the
embodiment system;
FIG. 10 is a flowchart of operations of the control circuit;
FIG. 11 shows a first type of scanning sequence used with the
control circuit;
FIG. 12 shows a second type of scanning sequence used with the
control circuit;
FIG. 13 is a timing diagram of the second type of scanning
sequence;
FIG. 14 shows a third type of scanning sequence used with the
control circuit; and
FIG. 15 is a timing diagram of the third type of scanning
sequence.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to FIG. 6, sector-scan analog section 12 comprises
preamplifier 13, pulser 14, oscillator 15, delay line 16, adder 17
and detector 18, and ultrasonic probe 11 transmits and receives
ultrasound beams to and from a subject under examination.
Control circuit 35 controls ultrasonic scanning sequence in
sector-scan analog section 12 so as to lower a sampling frequency
of a Doppler signal without changing a repetition frequency of
ultrasound pulses. Control circuit 35 comprises a central
processing unit (CPU), and a programmable read only memory (PROM)
and the like.
A signal provided from adder 17 is applied, via detector 18, DSC
19, color processor 20, digital/analog converter 21, to color
monitor 23 and video tape recorder (VTR) 22. An ultrasound tomogram
image (B-mode image) is displayed on color monitor 23.
The signal provided from adder 17 is also used for displaying an
ultrasound blood flow image and hence applied to mixers 24a and
24b. In mixer 24a, the output signal of adder 17 is multiplied by a
reference signal of a reference frequency fo provided by oscillator
15, while, in mixer 24b, the output signal of adder 17 is
multiplied by a 90-degree phase-shifted reference signal from
90-degree phase shifter 25 connected to oscillator 15. As a result,
the frequency component of fd of a Doppler signal and a high
frequency component of (2fo+fd) are obtained. The frequency
components are applied to low pass filters (LPFs) 26a and 26b so as
to remove the high frequency component (2fo+fd), and the frequency
component fd of the Doppler signal only is obtained in LPFs 26a and
26b. The Doppler signal is used as a phase detect output signal for
ultrasonic blood flow imaging.
FIGS. 7A to 7C, FIG. 7A shows an ultrasound transmitting pulse
signal transmitted to the subject from ultrasound probe 11, FIG. 7B
shows a receiving echo signal reflected from the subject, and FIG.
7C shows a phase detect output signal.
The phase detect output signal contains not only components
associated with blood flow but also unwanted components resulting
from reflection from slowing moving objects (clutters) such as
heart walls. To remove the unwanted components, the phase detect
output signal is applied to moving target indicator (MTI)
processing unit 27.
MTI processing unit 27 comprises A/D converters 28a and 28b,
memories 34a and 34b, MTI filters 29a and 29b, auto correlation
processor 30, average velocity processor 31, variance processor 32
and power processor 33.
A/D converters 28a and 28b convert output signals of LPFs 26a and
26b to digital signals, respectively. The digital signals are
applied to memories 34a and 34b. respectively. Memories 34a and 34b
each store Doppler signals for a plurality of scanning lines in
accordance with the ultrasound scanning sequence under the control
of control circuit 35.
FIG. 8 shows an arrangement of the MTI filter, in which 1/Z is for
a delay of one rate, .SIGMA. is a summation, and k1 and k2 are each
a coefficient.
Auto correlator processor 30 is used for performing, in real time,
frequency analysis for multipoints of two dimensions.
Average velocity processor 31 calculates an averaged Doppler
frequency fdon the basis of the following equation.
where S(f) is a power spectrum.
Variance processor 32 calculates variance .sigma..sup.2 on the
basis of the following equation.
Power processor 33 calculates a power TP on the basis of the
following equation.
The power TP is proportional to the intensity of echoes scattered
from blood cells. Echoes from moving objects corresponding to
frequencies below the cut-off frequency of the MTI filter are
eliminated.
Values of the average velocity, variance and power calculated at
each sample point are applied to DSC 19 and, after the
interpolation processing, converted to color data in color
processor 20. In the case of velocity-variance (v-.sigma..sup.2)
display, the flow in the direction approaching to ultrasound probe
11 is converted to, for example, a red color, while the flow in the
direction leaving from ultrasound probe 11 is converted to a blue
color. The average velocity is represented by differences in
brightness, and the velocity variance is indicated by a hue (mixed
with green).
Next, the operation of the system of the present invention will be
described.
In the present system, the control of the ultrasound scanning
sequence is performed by control circuit 35. As shown in FIG. 9,
control circuit 35 comprises sampling number setting circuit 35a
for setting the sampling number n of the same scanning line,
improvement ratio setting circuit 35b for setting an improvement
ratio P to improve the detectability of a low flow velocity, and
selection pulse generating circuit 35c for generating a pulse
signal to select a scanning sequence. Selection pulse generating
circuit 35c determines a scan line and the timing of a transmitting
pulse signal in accordance with the sample number n and the
improvement number P set by setting circuits 35a and 35b, and
outputs the pulse signal at the determined time to sector scan
analog section 12, DSC 19 and MTI processing unit 27.
Control circuit 35 is comprised of CPU, PROM and the like as
described above and has functions by circuits shown in FIG. 9. The
operations of the control circuit will be described with reference
to a flowchart shown in FIG. 10.
As shown in FIG. 10, in step S1, the sampling number n of a Doppler
signal is set in sampling number setting circuit 35a. In step S2,
the improvement ratio P used for detecting a low flow velocity is
set in improvement ratio setting circuit 35b. In step S3, the
scanning sequence selection pulse signal is generated in selection
pulse generating circuit 35c in accordance with the sampling number
n and the improvement ratio P set in steps S1 and S2.
In practice, the control of the scanning sequence is performed as
follows.
As shown if FIG. 11, in the sequence where an ultrasound beam is
first transmitted from the right end of ultrasound probe 11, the
scanning is performed in sequence of the first scan line at the
right end, the second scan line, the third scan line, and the first
scan line at the right end, and so on. In this case (P=3), the
repetition frequency fr of the ultrasound beam (the sampling
frequency of a Doppler signal) transmitted in the same direction
becomes
As can be seen from equation (4), the frequency fd min
corresponding to the lower limit of the measurable flow velocity
decreases to one third compared with a conventional method where an
ultrasound beam is transmitted n times along a scan line, and
similarly an ultrasound beam is transmitted n times along the next
adjacent scan line.
In the case of FIG. 11, the time of transmission of the ultrasound
beam in the same direction, i.e., the sampling number n of the
Doppler signal, is four. Thus, the ultrasound beam is transmitted
and received in accordance with the ultrasound scanning sequence,
and the Doppler signal is stored in memories 34a and 34b. When the
fourth data (n=4) is stored in memories 34a and 34b for each of
scan lines, the four pieces of data for each of the scan lines are
read out from memories 34a and 34b. In the case of FIG. 11, the
readout of the four pieces of data for each of scan lines is not
performed at equal intervals, complicating the timing control for
data readout.
To perform the readout of data for each of scan lines at nearly
equal intervals, such scanning sequence as shown in FIGS. 12 and 13
may be utilized. That is, in the case of the scan starting from the
right end of ultrasound probe 11 the scan may be performed in
sequence of the first scan line, the (m-1)-th scan line, the m-th
scan line, the first scan line, the second scan line, the m-th scan
line, the first scan line, the second scan line, the third scan
line, the first scan line, and so on. According to the scanning
sequence, the repetition frequency (the sampling frequency of
Doppler signal) fr of ultrasound beam transmitted in the same
direction decreases to 1/3, and the data stored in memories 34a and
34b can be read out at equal time intervals.
In general, by the repetition frequency fr of an ultrasound beam
transmitted in the same direction and the repetition frequency fr'
of an ultrasound transmitting pulse, the improvement ratio P for
the detectability of a low flow velocity is represented by
FIGS. 11 and 12 show a case where P=3.
It is to be noted that, when P is an integral multiple of n, the
data stored in memories 34a and 34b cannot be read out at equal
time intervals. FIG. 14 shows scanning sequence and its timing
diagram for data readout in the case of n=4 and P=2, for example.
As can be seen from the drawings, the data stored in memories 34a
and 34b is read out at different time intervals of 3/fr', 5/fr',
3/fr', 5/fr', . . .
The blood flow information obtained by the scanning sequence as
described above and B-mode image is applied to color monitor 23
through DSC 19, color processor 20 and D/A converter 21. The blood
flow information and the B-mode image may be recorded by VTR 22 as
needed.
As described above, by controlling the ultrasound scanning sequence
in the embodiment of the present invention, the sampling frequency
fr of the Doppler signal can be decreased without changing the
repetition frequency fr' of the ultrasound pulse. Therefore, the
blood flow of a low velocity also can be detected without
decreasing the frame number Fn and degrading the image quality.
Although the preferred embodiment of the present invention has been
described and disclosed, it is apparent that other embodiments and
modifications of the invention are possible.
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