## Abstrict A flow meter for detecting the average speed of an object in such
a manner that an ultrasonic pulsed continuous wave is repeatedly
transmitted toward the object to obtain a phase vector from the
reception signal of the wave reflected from the object, the phase
difference between consecutive ones of phase vectors obtained at
an interval T equal to the period of transmitted wave is detected,
a plurality of phase difference values are added and averaged to
obtain an average phase difference value, and a Doppler frequency
is calculated from the average phase difference value to obtain
the average speed of the object, in which in order to remove an
error caused by aliasing of an angle in the course of the arithmetic
operation for obtaining the average phase difference value, a plurality
of presumed average phase difference values having the error are
calculated from the average phase difference value, the difference
between the change of a phase caused by adding the phase difference
values successively and the change of phase based upon each of the
presumed average phase difference values is estimated to select
the most appropriate one of the presumed average phase difference
values, and the selected average phase difference value is used
as a corrected average phase difference value.
## Claims We claim:
1. A method of correcting an average phase difference obtained
by an ultrasonic Doppler flow meter, comprising the steps of:
transmitting an ultrasonic pulsed continuous wave repeatedly toward
an object at predetermined intervals to receive a wave reflected
from the object and to detect the phase of the reflected wave, thereby
generating a phase vector each time the reflected wave is received;
detecting the phase difference between a current phase vector and
a preceding phase vector each time the phase vector is obtained;
adding and averaging a predetermined number of phase difference
values to obtain a primary average phase difference value;
calculating a plurality of presumed average phase difference values
having an error due to the aliasing of angle from the primary average
phase difference value;
adding the phase difference values successively, and detecting
the change of a phase in the adding process;
calculating a principal value of the difference between the change
of a phase based upon each of the presumed average phase difference
values and the change of a phase in the adding process at a plurality
of time points, and adding the principal values calculated at the
time points to obtain a total error with respect to each presumed
average phase difference value; and
selecting that one of the presumed average phase difference values
which produces the smallest total error to use the selected presumed
average phase difference value as a corrected average phase difference
value for indicating the average speed of the object.
2. A method according to claim 1 wherein one of the presumed average
phase difference values is equal to the primary average phase difference
value, and remaining ones of the presumed average phase difference
values differ from the primary average phase difference value by
.+-.m.multidot.360.degree., N where m=1 2 3 and so on, and N
indicates the number of phase difference values to be averaged.
3. A method according to claim 1 wherein the principal value of
the difference between the change of a phase based upon each presumed
average phase difference value and the change of a phase in the
adding process is calculated each time a phase difference value
is added to a preceding sum of phase difference values.
4. A pulse Doppler flow meter comprising:
transmitter-receiver means for transmitting an ultrasonic pulsed
continuous wave repeatedly toward an object at predetermined intervals
and receiving a wave reflected from the object to obtain a reception
signal;
phase detecting means for generating a phase vector indicative
of the phase of the reception signal each time the reception signal
is obtained;
phase difference detecting means for detecting the phase difference
between a current phase vector and a preceding phase vector each
time the phase vector is obtained;
average phase difference calculating means for adding and averaging
a plurality of phase difference values outputted from the phase
difference detecting means to obtain a primary average phase difference
value;
means for calculating a plurality of presumed average phase difference
values from the primary average phase difference value and for calculating
total errors indicative of the degree of coincidence between the
change of a phase caused by adding the phase difference values successively
and the change of a phase based upon each of the presumed average
phase difference values, to select that one of the presumed average
phase difference values which produces the smallest total error
as a corrected average phase difference value; and
display means for displaying the corrected average phase difference
value as the average speed of the object.
5. An ultrasonic Doppler flow meter comprising:
transmitter-receiver means for transmitting an ultrasonic pulsed
continuous wave repeatedly toward an object at predetermined intervals
and receiving a wave reflected from the object to obtain a reception
signal;
phase detecting means for generating a phase vector indicative
of the phase of the reception signal each time the reception signal
is obtained;
first average phase difference calculating means for calculating
a phase difference vector by autocorrelation processing between
a current phase vector and a preceding phase vector, to calculate
a first average phase difference value from the argument of the
sum vector of a plurality of phase difference vectors;
second average phase difference calculating means for detecting
the phase difference between the current phase vector and the preceding
phase vector and for adding and averaging a plurality of phase difference
values to obtain a second average phase difference value;
phase difference correcting means for calculating a plurality of
presumed average phase difference values from the second average
phase difference value and for calculating total errors indicative
of the degree of coincidence between the change of a phase caused
by adding the phase difference values successively and the change
of a phase based upon each of the presumed average phase difference
values to select that one of the presumed average phase difference
values which produces the smallest total error as a corrected second
average phase difference value; and
selection means for selecting one of the first average phase difference
value and the corrected second average phase difference value as
the average speed of the object.
## Description CROSS-REFERENCE TO RELATED APPLICATIONS
The present application relates to the subject matter described
in application Ser. No. 609657 filed on Nov. 6 1990 (claiming
priority based on Japanese Patent Application No. 01-288908 filed
on Nov. 8 1989) entitled "Ultrasonic Doppler Flow Meter,"
by the same inventors and assigned to the same assignee of the present
application.
BACKGROUND OF THE INVENTION
The present invention relates to a pulse Doppler measuring apparatus,
and more particularly to an apparatus for detecting the velocity
of an object by using an ultrasonic wave, for example, a pulse Doppler
measuring apparatus capable of measuring the speed of blood flow
with a high signal-to-noise ratio, in a case where the blood flow
speed in a living body is measured in real time.
Various kinds of apparatuses have hitherto been known which detect
the flow speed of an object by utilizing the Doppler effect of an
acoustic wave. Specifically, in an apparatus using the pulse Doppler
method which is described in, for example, an article entitled "Pulsed
Ultrasonic Doppler Blood Flow Sensing" by D.W. Baker (IEEE
Trans. SU-17 No. 3 pages 170 to 185 1970), as is known, it is
possible to specify a measured part by transmitting an ultrasonic
pulsed continuous wave repeatedly and by setting a time gate corresponding
to the distance to the measured part on a reception signal.
As conventional ultrasonic Doppler blood flow measuring apparatuses,
as disclosed in, for example, JP-A-58-188433 JP-A-60-119929 and
JP-A-61-25527 there are known apparatuses for measuring a blood
flow by transmitting an ultrasonic wave toward a blood vessel and
by measuring the Doppler shift frequency of the ultrasonic wave
reflected from the blood in the blood vessel to measure vcos.theta.,
where .theta. indicates the angle between the direction of the blood
flow and the transmission direction of the ultrasonic wave, and
indicates a blood flow speed.
Further, a technique called "color flow mapping" for
measuring the distribution of blood flow speed at a certain cross
section of a living body to display the distribution on a tomographic
image, is described in an article entitled "Real-Time Two-Dimensional
Blood Flow Imaging Using an Autocorrelation Technique" by C.
KASAI et al., IEEE Trans. SU-32 No. 3 pages 458 to 464 1985.
In order to achieve a desired image frame rate in the color flow
mapping, the blood flow speed at each of pixels is determined by
averaging the measured values of a plurality of Doppler shift obtained
by a relatively small number of measurements. In the example described
above, the autocorrelation method is used, in which a difference
vector between a vector indicated by a Doppler signal detected currently
and a vector indicated by a Doppler signal detected by the preceding
measurement is detected by an autocorrelator each time the measurement
is repeated, and an average speed is calculated from the argument
of a vector given by the sum of a plurality of difference vectors.
Meanwhile, U.S. Pat. No. 4809703 discloses the so-called "two
axial component method", in which the phase difference .DELTA..theta.
of a Doppler signal is detected each time the measurement is repeated,
to be decomposed into a cosine component and a sine component, a
plurality of values obtained for each of these components are added
and averaged, and the phase difference indicated by the average
cosine component and the average sine component is transformed into
a velocity.
Further, on pages 348 to 352 of 1978 Ultrasonic Symposium Proceedings
is described a method, in which the phase difference of a Doppler
signal is obtained each time the measurement is repeated, an average
phase difference is calculated by adding a plurality of the phase
difference values directly, and the average phase difference thus
obtained is transformed into a velocity. This method will hereinafter
be referred to as "phase difference averaging method".
Meanwhile, it is pointed out by U.S. Pat. No. 4905206 that when
a true average phase difference is close to +.pi. or -.pi., that
is, a high-speed region is measured, the phase difference averaging
method produces a large calculation error, and that when the true
average phase difference is close to zero, that is, a low-speed
region is measured, the autocorrelation method and the two axial
component method produce a large calculation error. Further, in
this U.S. Patent are described a circuit configuration for changing
one of the two methods over to the other so that the above difficulties
are eliminated, and a circuit configuration for transforming phase
difference values obtained by repeated measurement into phase difference
values in a new polar coordinate system having a reference axis
which is indicated by the angle of the average phase difference
calculated by the autocorrelation method, and for adding and averaging
the phase difference values in the new polar coodinate system.
SUMMARY OF THE INVENTION
It has been known by the further investigation conducted by the
present inventors that even when one of the autocorrelation method
and the phase difference averaging method is changed over to the
other as disclosed in U.S. Pat. No. 4905206 it is impossible
to completely avoid the effect of an error which is introduced by
a process for adding angles in accordance with the phase difference
averaging method.
It is accordingly an object of the present invention to provide
a Doppler flow meter which can calculate a correct average phase
difference value by correcting an error and thus can obtain a correct
velocity, and to provide an error correcting method used in the
above Doppler flow meter.
According to the phase difference averaging method, in which an
angle obtained by adding and averaging a plurality of phase difference
angles is used as an average value, when a true phase difference
angle exceeds 180.degree. or becomes less than -180.degree., an
error is produced by the aliasing of the phase difference angle.
Hence, in the error correcting method according to the present invention,
a plurality of presumed averaged values having the above error are
first calculated from the average value obtained by adding and averaging
a plurality of phase difference angles. Next, for each of the presumed
average values, the difference between each of those ones of integrated
values obtained by adding the phase difference angles successively
which are detected at a plurality of time points, and a corresponding
one of integrated values of the presumed average value is calculated.
The sum of the absolute values of principal values of a plurality
of values of the above difference is calculated to be used as an
index for indicating the appropriateness of each presumed average
value. The indexes thus obtained are compared to select the most
appropriate one of the presumed average values. The selected average
value is used as the corrected average value of the phase difference
angles.
When the above-mentioned aliasing of angle occurs once, the error
in the sum of angles is equal to +360.degree. or -360.degree.. Accordingly,
when the number of sampled phase difference values to be averaged
and an average value to be corrected are expressed by N and .DELTA..theta.,
respectively, a plurality of presumed average values which are to
be estimated on the basis of the indexes, are indicated by .DELTA..theta.
and .DELTA..theta..+-.m..multidot.360.degree./N where m=1 2 3
and so on. For example, in a case where five presumed average values
are estimated, the presumed average values are given by the following
equation: ##EQU1## where k=1 2 3 4 and 5.
Further, an apparatus according to the present invention is characterized
by comprising first average phase difference calculating means for
calculating autocorrelation between two consecutive ones of successively
obtained phase vector signals and for adding a plurality of phase
difference vectors obtained by the autocorrelation to calculate
the argument of a vector given by the sum of the phase difference
vectors, second average phase difference calculating means for detecting
the phase difference between two consecutive ones of successively
obtained phase vector signals to add and average a plurality of
phase difference angles, error correcting means for correcting the
average phase difference obtained from the second average phase
difference calculating means by the above-mentioned method, and
selection means for selecting one of the output of the first average
phase difference calculating means and the corrected output of the
second average phase difference calculating means. The selection
means selects the corrected output of the second average phase difference
calculating means to use the selected output as a blood flow speed,
when the output of the first average phase difference calculating
means is less than a predetermined threshold value. Further, when
the output of the first average phase difference calculating means
exceeds the predetermined threshold value, this output is selected
to be used as a blood flow speed.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram showing an embodiment of the present
invention.
FIG. 2 is a diagram showing phase change characteristics which
are used for explaining the principle of phase error correction
in the embodiment of FIG. 1.
FIG. 3 is a problem analysis diagram showing the operation for,
the above phase error correction.
FIG. 4 is a problem analysis diagram showing the average value
selection algorithm used in the embodiment of FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 is a block diagram showing the whole construction of an
embodiment of an ultrasonic pulse Doppler measuring apparatus according
to the present invention.
The present embodiment is characterized in that the phase difference
averaging method capable of improving a signal-to-noise ratio is
used as a fundamental method, an output according to the phase difference
averaging method is selected for a low Doppler velocity, and an
output according to the autocorrelation method is selected for a
medium or high Doppler velocity. The outline of the operation of
the present embodiment is as follows.
A transmitting circuit 2 outputs a pulsed continuous wave to a
transducer 1 at predetermined intervals T. Thus, the transducer
1 emits an ultrasonic pulsed continuous wave toward a reflecting
body 11 at the predetermined intervals. A reflected acoustic wave
is produced and returns to the transducer 1. Reflection signals
from the transducer 1 are successively detected by a receiving circuit
3. In a Phase comparator 4 detected received signals are mixed
with two kinds of reference signals .alpha.=A cos .omega.t and .alpha.'=A
sin .omega.t, respectively. Thus Doppler signals V.sub.R and V.sub.I
having the phase information of the reflection signals are obtained.
An A/D converter 5 samples the signals V.sub.R and V.sub.I of the
reflected wave from the reflecting object 11 located at a specified
depth, at an interval of T, to convert the sampled signals into
digital signals. When the number of the repetition of wave transmission
is expressed by n (where n=1 2 3 and so on), the digital signals
V'.sub.Rn and V'.sub.In are given by the following equations:
An MTI filter 6 produces a first order difference of the output
of the A/D converter 5 to remove the unvariable reflected wave
signal coming from a fixed substance. For the sake of simplicity,
let us rewrite the equations (1) and (2) in a single equation as
follows:
Then, the output of the MTI filter is given by the following equation:
Hereinafter V.sub.n will be referred to as a "phase vector".
The successively obtained phase vectors V.sub.n are applied to
a first average phase difference calculating circuit 7 for calculating
an average phase difference by the autocorrelation method and to
a second average phase difference calculating circuit 8 for calculating
an average phase difference by the phase difference averaging method.
The phase vectors are applied to an autocorrelator (that is, phase
difference detector) 701. The autocorrelator 701 carries out the
complex multiplication of the phase vector V.sub.n and the complex
conjugate vector V*.sub.n-1 of the phase vector V.sub.n-1 preceding
by one period. When the output obtained as the result of complex
multiplication is expressed b Y.sub.n, the output Y.sub.n is given
by the following equation:
A complex adder 702 adds the outputs R.sub.n +jI.sub.n of the autocorrelator
701 N times. When the sum thus obtained is expressed by R+jI, the
sum R+jI is given by the following equation: ##EQU2##
An ATAN memory 703 stores therein a table for obtaining the argument
of a vector from the real and imaginary parts thereof. When the
real part R and imaginary part I of the vector obtained as the result
of addition made by the complex adder 702 are applied to the memory
703 the argument of the above vector is read out. The read-out
argument is used as the output .DELTA..theta..sub.A of the first
average phase difference calculating circuit 7. The output .DELTA..theta..sub.A
can be simply expressed by the following equation:
In the second average phase difference calculating circuit 8 another
average phase difference .DELTA..theta..sub.T is calculated, each
time N phase vectors are supplied to the circuit 8. When the real
part V.sub.Rn and imaginary part V.sub.In of each phase vector V.sub.n
are applied to an ATAN memory 801 having the same construction as
that of memory 703 the argument .theta..sub.n of the phase vector
is read out. In a phase difference detector 802 the difference
between the current argument .theta..sub.n and argument preceding
by one period is detected, to deliver a phase difference .DELTA..theta..sub.n.
The phase difference .DELTA..theta. is delivered in the form of
the principal value of a difference angle. Accordingly, the phase
difference .DELTA..theta..sub.n is given by the following equation:
##EQU3##
Each time N phase difference values .DELTA..theta..sub.n are delivered,
the phase difference values are added and averaged to obtain an
average phase difference. In the present embodiment, however, not
only the simple arithmetical mean of the Phase difference values
but also the weighted average value with a weight corresponding
to the amplitude of each phase vector can be calculated. In an amplitude
operator 17 the amplitude A.sub.n of each phase vector V.sub.n
is calculated from the real part V.sub.Rn and imaginary part V.sub.In
thereof by the following equation:
Further, in the amplitude operator 17 the sum A.sub.S of the amplitude
A.sub.n from n=1 to n=N is calculated as follows: ##EQU4## In a
weight center operator 803 the average phase difference .DELTA..theta..sub.T
is calculated by using the amplitude A.sub.n, as indicated by the
following equation: ##EQU5##
In a case where the simple arithmetical mean is selected, the amplitude
A.sub.n applied to the operator 803 is made equal to 1. In this
case, the average phase difference is given by the following equation:
##EQU6##
Further, the amplitude operator 17 may be replaced by a circuit
for calculating the power P.sub.n of the phase vector by the following
equation, to obtain the following power-weighed average phase difference:
##EQU7##
This value .DELTA..theta..sub.T is used as the output of the second
average phase difference calculating circuit 8. According to the
calculation of average value based upon the addition of phase difference
values, when a true phase difference angle as put outside of a range
from -180.degree. to 180.degree. in the adding process, the phase
difference angle is folded back as indicated by the equation (8),
and thus an error is introduced into the result of addition. Now,
let us consider a case where the true phase .theta. of the phase
vector V.sub.n changes as indicated by a broken line in FIG. 2.
When the true phase difference between the present phase and the
preceding phase exceeds 180.degree. at a time point i, the phase
difference is expressed by an angle within a range from -180.degree.
to 180.degree.. At this case, the sum of phase difference values
changes as indicated by a solid line. Thus, an error of -360.degree.
is introduced. Accordingly, the average value of N phase difference
values has an error of -360.degree./N. Similarly, when two true
phase difference angles exceed 180.degree., an error of -720.degree./N
is introduced.
An error corrector 9 shown in FIG. 1 checks the introduction of
such errors into the value .DELTA..theta..sub.T and performs a correcting
operation. That is, the error corrector 9 forms a plurality of presumed
average values having the above errors, and estimates the presumed
average values to select the most appropriate one, thereby performing
the correcting operation. The error corrector 9 is formed of a programmable
data processor.
FIG. 3 shows an example of the phase difference correcting program
executed by the error corrector 9 in the form of a PAD (Problem
Analysis Diagram). In this example, five kinds of presumed average
values are estimated. After the program has been started in step
801 the repetition index k of repeated processing is set, in step
802 to a numeral of 1 to M (in the present example, M=5). The repeated
processing includes steps 803 to 805. In the steps 803 and 804
the initial value of an error S and the initial value .theta..sub.o
of an angle are set to zero. Next, the repetition index n of repeated
processing is set, in the step 805 to a numeral of 1 to N (where
N is the number of sampled angles to be averaged). The contents
of the repeated processing are as follows. In step 806 the phase
difference values .DELTA..theta..sub.1 to .DELTA..theta..sub.n of
phase difference values .DELTA..theta..sub.1 .DELTA..theta..sub.2
. . . , .DELTA..theta..sub.N which are successively delivered from
a block 732 of the average phase difference calculating circuit
7 are added to obtain a phase .theta..sub.n. FIG. 2 shows the change
of .theta..sub.n with the variable n (where n=1 2 3 . . . N).
Meanwhile, by using the value .DELTA..theta..sub.T calculated by
the average phase difference calculating circuit 8 five presumed
average values which are to be estimated are given by the following
equation: ##EQU8## where k =1 2 3 4 and 5.
Let us suppose that the phase difference values are not dispersed
at all, and each of the phase difference values is equal to the
presumed average phase difference .DELTA..theta..sub.k. Then, an
integrated value of the phase difference values will travel on each
of five dot-dash lines shown in FIG. 2. Accordingly, in step 807
the difference D between the sum .theta..sub.n of actual phase difference
values .DELTA..theta..sub.1 to .DELTA..theta..sub.n and a corresponding
value on each dot-dash line is calculated from the following equation:
##EQU9## Next, in steps 811 to 822 the difference D is transformed
into the principal value thereof (that is, a value within a range
from -180.degree. to +180.degree.). Next, in step 823 the absolute
value of the principal value of the difference D is added to an
error S which has been obtained by the preceding processing, to
obtain a new value of the error S. The above processing is repeatedly
carried out from n =1 to n=N. The error S thus obtained is an index
for indicating the degree of coincidence between one of the dot-dash
lines of FIG. 2 and the solid line. Accordingly, in step 826 a
total error A(k) is set to this value of S. The above process is
repeated from k=1 to k=5 as indicated by the step 802. In other
words, five total errors A(k) are obtained which indicate the degree
of coincidence between each of five dot-dash lines shown in FIG.
4 and the solid line. Strictly speaking, since the program includes
the process shown in the steps 811 to 822 for transforming the difference
D into the principal value thereof, the value of A(k) does not indicate
the total sum of difference values between the solid line of FIG.
4 and each dot-dash line, but indicates the total sum of absolute
values of principal values of the above difference values. When
the repetitive process for each of k=1 to k=M is completed, five
total errors A(k) thus obtained (where k=1 2 3 4 5) are compared
with one another, to select the smallest one of five total errors,
and the variable k of the smallest total error A(k) is expressed
by k.sub.min (step 827). Thus, it is known which of five dot-dash
lines indicates a correct average value most accurately. In steps
828 and 829 the most appropriate average value .DELTA..theta..sub.c
is calculated from the following equation: ##EQU10## Specifically,
as shown in the step 829 the .DELTA..theta..sub.c for k.sub.min
=3 is given as follows:
In a case where the aliasing of angle occurs only once at the time
point n =i as indicated by a broken line in FIG. 2 an average value
for k=4 that is, .DELTA..theta..sub.T +360.degree./N is the correct
average value. In the above-mentioned program, the difference D
is transformed into the principal value thereof by the process of
the steps 811 to 822. Thus, after the time point n=i, the difference
values between the shifted data on the broken line and data on the
dot-dash line for k=4 are added. Accordingly, it is judged by the
program that the total error A(4) is the smallest one of the total
errors A(k), where k=1 2 3 4 and 5. Thus, the correct average
value is rightly selected.
Referring back to FIG. 1 the above explanation will be continued.
One of the output .DELTA..theta..sub.A of the first average phase
difference calculating circuit 7 and the correct output .DELTA..theta..sub.c
of the second average phase difference calculating circuit 8 is
selected and transformed into a Doppler shift frequency .omega.'.sub.d
by a selector 14. The frequency .omega.'.sub.d is delivered from
the selector. The variance .sigma..sub.s of angles which is necessary
for the above selection is calculated on the basis of the average
phase difference .DELTA..theta..sub.A. Each time a phase difference
vector Y.sub.n from the autocorrelator 701 is applied to an ATAN
memory 15 the argument of the vector Y.sub.n, that is, the phase
difference angle .DELTA..theta..sub.n, is read out from the memory
15. In a variance operator 16 the variance .sigma..sub.s of phase
difference angles .DELTA..theta..sub.n for the average value .DELTA..theta..sub.T
(where n=1 . . . N) is calculated from the following equation:
##EQU11##
In the selector 14 an operation for determining which of .DELTA..theta..sub.T
and .DELTA..theta..sub.A is selected as the blood flow speed .omega..sub.d
displayed by a display device 15 is performed while using the argument
(namely, average phase difference) .DELTA..theta..sub.A obtained
by the autocorrelation method, the variance .sigma..sub.s, and the
amplitude sum A.sub.s obtained by the amplitude calculator 17 as
parameters.
FIG. 4 shows the PAD of the selection algorithm. In FIG. 4 .sigma..sub.c
indicates a threshold value for the variance of angles. First, it
is checked whether or not .sigma..sub.s is less than .sigma..sub.c.
When the result of check is "YES", it is checked whether
or not the absolute value of .DELTA..theta..sub.A is less than a
threshold value .theta..sub.d for an angle. When the result of check
is "YES", .DELTA..theta..sub.T is selected as a blood
flow speed. When the result of check is "NO", .DELTA..theta..sub.A
is selected as a blood flow speed. Further, when .sigma..sub.c is
greater than or equal to .sigma..sub.c, it is checked whether or
not the amplitude sum A.sub.S is greater than a threshold value
A.sub.d for the intensity of reflected wave. When the result of
check is "YES", .DELTA..theta..sub.A is selected as the
blood flow speed. When the result of the check is "NO",
displayed data is determined, depending upon the value of a switch
SW set by an operator. In the case of SW =0 .DELTA..theta..sub.A
is selected, independently of the magnitude of A.sub.S. In the case
of SW=1 it is judged that noise is measured because of A.sub.s
<A.sub.d, and thus a display screen is made blank, or a speed
of zero is displayed. By adjusting the gain of an amplification
system and utilizing the switch SW, a displayed image can be changed
in accordance with various purposes.
Further, one of .DELTA..theta..sub.T and .DELTA..theta..sub.A can
be selected by a switch on an console 12 independently of the judgement
due to the selection algorithm. Further, the average value .DELTA..theta..sub.T
or .DELTA..theta..sub.A itself may be applied, as a signal indicating
a blood flow speed .omega..sub.d, to the display device 15. In order
to calculate .omega..sub.d accurately, it is necessary to use a
divider for dividing the average value .DELTA..theta..sub.T or .DELTA..theta..sub.A
by the period T of transmitted wave. That is, an apparatus, in which
the period T is selected from a plurality of values, cannot dispense
with such a divider. |