## Abstrict An ultrasonic Doppler blood flow meter comprises a transmit-receive
transducer transmitting ultrasonic wave toward and into a living
body and receiving an echo signal, quadrature detectors detecting
a real (R) component and an imaginary (I) component of the receiving
echo signal respectively, A/D converters converting analog output
signals of the quadrature detectors into digital signals respectively,
a converter subjecting the R and I components to quadrature transformation
to generate output signals representing the absolute value and phase
angle (.theta.) respectively of the received echo signal, and a
signal processing circuit calculating the mean value of the differences
(.DELTA..theta..sub.i =.theta..sub.i -.theta..sub.i-1) between the
phase angles (.theta..sub.i) and their preceding ones (.theta..sub.i-1)
detected when the ultrasonic wave is transmitted and the echo signal
is received a predetermined number of times. In the signal processing
circuit, the phase angle differences (.DELTA..theta.) are each resolved
into an X-axis component (cos .DELTA..theta.) and a Y-axis component
(sin.DELTA..theta.), and, after calculation of the mean values X
and Y of n consecutive X-axis and Y-axis components respectively,
the mean Doppler shift phase angle .DELTA..theta.=tan.sup.-1 (Y/X)
is calculated on the basis of the calculated mean values X and Y.
## Claims We claim:
1. An ultrasonic Doppler blood flow meter comprising:
ultrasonic wave transmitting and receiving means for sequentially
transmitting ultrasonic waves toward and into a living body at a
predetermined time period and receiving their echo signals;
means to detect a real component and an imaginary component of
each successive received echo signals in quadrature;
converting means for sequentially subjecting the real component
and the imaginary component to circuitry for calculating a phase
angle (.THETA..sub.i) of a received echo signal relative to the
transmitted ultrasonic wave;
two-axial-component resolving means for resolving the hase angle
(.THETA..sub.i) calculated by said converting means and its preceding
phase angle (.THETA..sub.i-1) directly into an X-axis component
(cos .DELTA..THETA..sub.i) and a Y-axis component (sin .DELTA..THETA..sub.i)
of the difference (.DELTA..THETA..sub.i) between the phase angle
(.THETA..sub.i) and its preceding phase angle (.THETA..sub.i-1)
on an orthogonal coordinate system;
means for calculating mean values (X,Y) of n consecutive X-axis
components and n consecutive Y-axis components, n being a predetermined
integer number, respectively detected by said two-axial-component
resolving means when the ultrasonic wave is transmitting n consecutive
times from said ultrasonic wave transmitted and receiving means;
mean Doppler shift phase-angle calculating means for obtaining
as an output a mean Doppler shift phase-angle (.DELTA..THETA.),
by distinguishing to which quadrant among four quadrants the obtained
angle (.DELTA..THETA.) belongs, utilizing signals corresponding
to said mean values (X,Y) of the X-axis and Y-axis components by
taking into consideration polarities of each of the mean values
(X,Y) according to the equation .DELTA..THETA.=tan.sup.-1 (Y/X);
and
display means operatively connected to the output of said mean
Doppler shift phase-angle calculating means for displaying velocity
of blood flow.
2. An ultrasonic Doppler blood flow meter according to claim 1
wherein said mean Doppler shift phase-angle calculating means includes
a read-only memory having a two-dimensional map which stores values
of tan.sup.-1 (X/Y) with respect to positive and negative values
of the X-axis and Y-axis components.
3. An ultrasonic Doppler blood flow meter according to claim 1
wherein data compressing means for compressing said real component
and said imaginary component respectively are provided on an input
side of said converting means.
4. An ultrasonic Doppler blood flow meter according to claim 1
wherein said converting means sequentially calculates amplitude
of the echo signal from said real and imaginary components, and
said ultrasonic Doppler blood flow meter further comprises means
for calculating a mean value of n consecutive amplitude values calculated
by said converting means when the ultrasonic wave is transmitted
n consecutive times from said ultrasonic wave transmitting and receiving
means, and means multiplying said mean Doppler shift phase angle
by the mean value of the n consecutive amplitude values for displaying
the product representing the velocity of blood flow of said display
unit.
5. An ultrasonic Doppler blood flow meter according to claim 1
wherein said converting means sequentially calculates the amplitude
of the echo signal from said real and imaginary components, and
said ultrasonic Doppler blood flow meter further comprises weighting
means for weighting the X-axis and Y-axis components generated from
said two-axial-component resolving means by a weighting coefficient
relating to the amplitude calculated by said converting means.
6. In an ultrasonic Doppler blood flow meter including ultrasonic
wave transmitting and receiving means for sequentially transmitting
an ultrasonic wave toward and into a living body at a predetermined
period and receiving its echo signal;
means for sequentially subjecting the echo signal to quadrature
detection to detect a real component (R) and an imaginary component
(I) of the received echo signal;
converting means for sequentially subjecting the real component
and the imaginary component to calculate a phase angle (.THETA.i)
of the received echo signal relative to the transmitted ultrasonic
wave;
two-axial-component resolving means for resolving the phase angle
(.THETA.i) calculated by said converting means and its preceding
one (.THETA.i-1) into a real component (R) and an imaginary component
(I) of the difference (.DELTA..THETA.i) between the phase angle
(.THETA.i) and its preceding phase angle (.THETA.i -1); and
means for calculating mean values (R,I) of the real and imaginary
components,
the improvement comprising:
mean Doppler shift phase-angle calculating means for obtaining
the mean Doppler phase shift-angle (.DELTA..THETA.), by distinguishing
to which quadrant among four quadrants the obtained angle (.DELTA..THETA.)
belongs, on the basis of the mean values (R,I) of the real and imaginary
components by taking into consideration polarities of each of the
mean values (R,I) according to an equation .DELTA..THETA.=tan.sup.-1
(I,R); and
display mans operatively connected to the output of said mean Doppler
shift phase-angle calculating means for displaying the velocity
of blood flow.
## Description BACKGROUND OF THE INVENTION
This invention relates to an ultrasonic Doppler blood flow meter,
and more particularly to a blood flow meter of the type described
above in which the manner of measurement of the frequency of echoes
of ultrasonic wave reflected from a living body is improved.
Prior art, ultrasonic Doppler blood flow meters are disclosed in,
for example, C. Kasai et al, "Real-Time Two-Dimensional Blood
Flow Imaging Using an Autocorrelation Technique" IEEE Transactions
on Sonics and Ultrasonics, Vol. SU-32 No. 3 May 1985 pp. 458-464
and D. W. Baker, "Pulsed Ultrasonic Doppler Blood-Flow Sensing"
IEEE Transactions on Sonics and Ultrasonics, Vol. SU-17 No. 3
July 1970 pp. 170-185. Each of the disclosed devices is essentially
composed of a driver transmitting an ultrasonic wave signal toward
and into a living body, a receiver receiving an echo signal of the
transmitted ultrasonic wave signal, an oscillator generating an
oscillation output signal having a pulse repetition frequency n
times (n: an integer) as high as the repetition frequency of ultrasonic
wave transmission, and a signal processing circuit processing the
received echo signal. The method employed in the disclosed devices
comprises transmitting the ultrasonic wave signal at the predetermined
period toward a blood vessel in a living body, receiving an echo
signal of the transmitted ultrasonic wave signal reflected by blood
flow in the blood vessel, and measuring the Doppler shift frequency
of the echo signal to measure the velocity and direction of blood
flow in the blood vessel. By the above mentioned, the value of v.multidot.cos
.beta. can be measured, where .beta. is the angle defined between
the direction of blood flow and the direction of transmission of
the ultrasonic wave signal, and v is the velocity of blood flow.
For the purpose of blood flow measurement in the manner described
above, methods such as a zero-cross method and a fast Furrier transform
(FET) method are commonly used. However, the latter method requires
many hardware parts. with a view to decrease the number of hardware
parts while taking into consideration the factors such as the accuracy
of blood flow measurement, devices similar to a blood flow measuring
device as shown in FIG. 1 are now proposed, as disclosed in, for
example, JP-A-58-188433 JP-A-62-41645 JP-A-60-119929 and JP-A-61-25527.
Referring to FIG. 1 a received input signal 7a is applied to quadrature
detectors 11a and 11b. In the detector 11a, the input signal 7a
is multiplied by a cosine wave signal 3b (described later) to provide
an analog output signal 30a representing a real component (R=.alpha.
cos .theta.), while in the detector 11b, the input signal 7a is
multiplied by a sine wave signal 3a (described later) to provide
an analog output signal 31a representing an imaginary component
(I=.alpha. sin .theta.). These analog signals 30a and 31a are then
A/D converted by A/D converters 12a and 12b respectively, and these
digital signals representing the R and I components respectively
are used to measure the Doppler shift phase angle thereby displaying
the velocity v of blood flow on a display unit 20.
In the blood flow measuring device shown in FIG. 1 an oscillator
2 generates a stable high-frequency oscillation output signal which
is applied to a frequency-dividing and synchronizing circuit 3.
In response to the application of the high frequency signal, the
circuit 3 generates a digital pulse signal 3d for ultrasonic pulse
beam transmission, a sine wave signal 3a and a cosine wave signal
3b for quadrature detection, and a reset pulse signal 3c having
a period n times (n: an integer) as large as that of the pulse signal
3d.
In response to the application of the digital pulse signal 3d,
a driver circuit 4 applies an analog pulse signal 4a having, for
example, a 1/2 cycle pulse to a probe 6 through a transmit/receive
change-over circuit 5. The probe 6 is excited to transmit an ultrasonic
pulse beam toward a blood vessel 9 of a living body 10 to be examined.
The signal reflected from the blood vessel 9 of the living body
10 is converted by the probe 6 into an electrical signal, and this
electrical signal is applied through the transmit/receive change-over
circuit 5 to a high frequency amplifier 7 to be amplified and appears
as a receive input signal 7a which is applied to the quadrature
detectors 11a and 11b. FIG. 2 shows waveforms of the signals 3a,
3b, 3c, 3d, 4a and 7a shown in FIG. 1. The received input signal
7a having a waveform as shown in (f) of FIG. 2 is applied to the
qudrature detectors 11a and 11b which are in the form of multipliers.
In the detectors 11a and 11b, the receive input signal 7a is multiplied
by the cosine and sine wave signals 3b and 3a having waveforms as
shown in (c) and (b) of FIG. 2 to appear as the analog output signals
30a and 31a representing the R and I components respectively.
These analog signals 30a and 31a are then converted into digital
signals 30b and 31b by the A/D converters 12a and 12b respectively,
and these digital signals 30b and 31b are passed through MTI (moving
target indication) filters 13a and 13b to appear as signals 30c
and 31c respectively. These signals 30c and 31c are then applied
to an amplitude and phase angle calculator 14 and an output signal
32 representing the amplitude .alpha..sub.i and an output signal
34 representing the phase angle .theta..sub.i are generated from
the calculator 14. On the basis of the phase angle .theta..sub.i
shown in (f) of FIG. 2 the Doppler shift phase angle .DELTA..theta..sub.i
(.DELTA..theta..sub.i =.theta..sub.i -.theta..sub.i-1) is calculated,
and, in order to improve the S/N ratio, the mean value .DELTA..theta.
of a plurality of such Doppler shift phase angles is calculated
by a mean Doppler shift phase-angle calculator 18. That is, the
signal 34 representing the phase angle .theta..sub.i is applied,
on one hand, directly and, on the other hand, through a delay element
16 to a subtractor 17. The delay element 16 has a delay time corresponding
to one period T of the pulse signal 4a. Therefore, the subtactor
17 generates an output signal 35 representing the difference .DELTA..theta..sub.i
(.DELTA..theta..sub.i =.theta..sub.i -.theta..sub.i-1) between the
present phase angle .theta..sub.i and the preceding phase angle
.theta..sub.i-1 and such a signal 35 is applied to the mean Doppler
shift phase-angle calculator 18. The calculator 18 calculates the
mean value .DELTA..theta.(.DELTA..theta.=(.SIGMA.(.theta..sub.i
-.theta..sub.i-1)/n) of n consecutive Doppler shift phase angles
.DELTA..theta..sub.i, and its output signal 38 representing .DELTA..theta.
is applied to the display 20. Thus, when the value of n is, for
example, four, the reset pulse signal 3c having a waveform as shown
in (e) of FIG. 2 has a period which is five times as large as that
of the pulse signal 4a having a waveform as shown in (d) of FIG.
2. On the other hand, the signal 32 representing the amplitude .alpha..sub.i
is applied to a mean amplitude calculator 15. The calculator 15
calculates the mean value .alpha.(.alpha.=.SIGMA..alpha..sub.i /n)
of n consecutive amplitude values .alpha..sub.i, and its output
signal 33 representing .alpha. is applied to the display 20.
In the display 20 the mean value of Doppler shift phase angle
.DELTA..theta. or the mean value of amplitude .alpha. is displayed
independently. The former relates the velocity, so does the latter
the power of blood flow.
The prior art, blood flow measuring device shown in FIG. 1 has
had such a problem that, in the calculation of the mean Doppler
shift frequency, that is, the mean Doppler shift phase angle, the
direction of the mean Doppler shift phase angle is not always the
same as the direction of the Doppler shift phase angle as shown
in FIG. 3A. This is because the Doppler shift phase angle that can
be displayed is limited to within the range of -.pi. to +.pi. due
to the structural limitation of the circuit. Therefore, the mean
Doppler shift phase angle cannot be determined by merely simply
calculating the numerical values of the Doppler shift phase angles.
When, for example, the input signal 35 that is sequentially applied
to the mean Doppler shift phase angle calculator 18 is successively
representative of 170.degree., 175+, -175.degree. and -170.degree.
as shown by the arrows a, b, c and d in FIG. 3A, mere addition of
these numerical values and division of the sum by the factor of
four does not provide a mean Doppler shift phase angle of 180.degree.
as shown by the arrow e in FIG. 3A, but provides a mean Doppler
phase shift angle of 0.degree. as shown by the arrow e' in FIG.
3A. FIG. 3B illustrates that the mean value shown by e' is 0 degrees.
The above problem is attributable to the fact that the displayable
range of the Doppler shift phase angle is limited so as to simplify
the hardware design for the purpose of minimizing the scale of the
circuit.
SUMMARY OF THE INVENTION
With a view to obviate the defects of the prior art ultrasonic
Doppler blood flow meter, it is an object of the present invention
to provide an ultrasonic Doppler blood flow meter according to which
the proper direction of Doppler shift phase angles can be measured
when their mean value shows primarily an anlge close to 180.degree.
even if the displayable range of the phse angles is limited due
to a structural limitation of hardware parts.
In the present invention which attains the above object, Doppler
shift phase angles .DELTA..theta..sub.i (=.theta..sub.i -.theta..sub.i-1)
are each resolved into two axial components on an orthogonal coordinate
system, that is, an X-axis component (cos .DELTA..theta..sub.i)
and a Y-axis component (sin .DELTA..theta..sub.i), and, after calculation
of the mean value X=(.SIGMA. cos .DELTA..theta..sub.i)/n of n consecutive
X-axis components and the mean value Y=(.SIGMA. sin .DELTA..theta..sub.i)/n
of n consecutive Y-axis components, the mean Doppler shift phase
angle .DELTA..theta.=tan.sup.-1 (Y/X) is calculated on the basis
of these mean values X and Y.
According to the ultrasonic Doppler blood flow meter of the present
invention, the accuracy of calculation can be greatly improved without
appreciately increasing the circuit scale as compared to that of
the prior art blood flow meter.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a prior art ultrasonic Doppler blood
flow meter.
FIG. 2 shows signal waveforms appearing at various parts of FIG.
1.
FIGS. 3A and 3B illustrate how a mean Doppler shift phase angle
is calculated in the prior art Doppler blood flow meter shown in
FIG. 1.
FIG. 3C illustrates how a mean Doppler shift phase angle is calculated
according to the present invention.
FIG. 4 is a block diagram of a preferred embodiment of the ultrasonic
Doppler blood flow meter of the present invention.
FIG. 5 shows in detail the arrangement of principal parts of the
blood flow meter shown in FIG. 4.
FIG. 6 is a block diagram of another preferred embodiment of the
ultrasonic Doppler blood flow meter of the present invention.
FIG. 7 is a circuit diagram showing the structure of one form of
the weighting coefficient calculator shown in FIG. 6.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Preferred embodiments of the present invention will now be described
in detail with reference to the accompanying drawings.
In the present invention, Doppler shift phase angles .DELTA..theta..sub.i
=(.theta..sub.i -.theta..sub.i-1) are each resolved into two axial
components on an orthogonal coordinate system, that is, an X-axis
component (cos .DELTA..theta..sub.i) and a Y-axis component (sin
.DELTA..theta..sub.i) as shown in FIG. 3C, and the mean value X=(.SIGMA.
cos .DELTA..theta..sub.i)/n of n consecutive X axis components and
the mean value Y=(.SIGMA. sin .DELTA..theta..sub.i)/n of n consecutive
Y-axis components are calculated to calculate the mean Doppler shift
phase angle .DELTA..theta.=tan.sup.-1 (Y/X) on the basis of these
mean values X and Y. Therefore, the mean Doppler shift phase angle
.DELTA..theta. can be detected with high accuracy irrespective of
the values of the Doppler shift phase angles.
FIG. 4 is a block diagram of a preferred embodiment of the ultrasonic
Doppler blood flow meter according to the present invention, and,
in FIG. 4 the same reference numerals are used to designate the
same or equivalent functional parts appearing in FIG. 1.
The amplitude and phase angle calculator 14 in the embodiment of
the present invention preferably includes a read-only memory (ROM),
and, in response to the application of the signals 30c and 31c representing
the real component (R=.alpha. cos .theta.) and imaginary component
(I=.alpha. sin .theta.) from the MTI filters 13a and 13b respectively,
the calculator 14 generates the signals 32 and 34 representing the
amplitude .alpha. and the phase angle .theta. ##EQU1## respectively.
Therefore, the ROM included in the calculator 14 preferably stores
a two-dimensional map of values of tan.sup.-1 I/R corresponding
to the values of the two input signals 30c and 31c representing
the R and I components respectively.
The value of the phase angle .theta..sub.i represented by the output
signal 34 of the calculator 14 lies generally in the range of 0
to 2.pi. or -.pi. to +.pi..The output signal 34 of the calculator
14 representing the phase angle .theta..sub.i and the output signal
35 of the delay element 16 representing the preceding phase angle
.theta..sub.i-1 are applied to a pair of trigonometric function
calculator 48a and 48b. Each of these trigonometric function calculators
48a and 48b acts to resolve the Doppler shift phase angle .DELTA..theta..sub.i
into its X-axis component and Y-axis component and preferably includes
a read-only memory (ROM) such as that of Type 27512 made by the
INTEL Corporation. The ROM in the calculator 48a preferably stores
a two-dimensional map of values of cos (.theta..sub.i -.theta..sub.i-1)
corresponding to the values of the two input signals 34 and 35 representing
the phase angles .theta..sub.i and .theta..sub.i-1 respectively,
as shown in FIG. 5. Therefore, the calculator 48a generates an output
signal 36a representing the value of cos (.theta..sub.i -.theta..sub.i-1)(=cos
.DELTA..theta.i) corresponding to the values of the phase angles
.theta..sub.i and .theta..sub.i-1 applied as the inputs.
Similarly, the calculator 48b preferably includes a ROM which stores
a two-dimensional map of values of sin (.theta..sub.i -.theta..sub.i-1)
corresponding to the values of the two input signals 34 and 35 representing
the phase angles .theta..sub.i and .theta..sub.i-1 respectively
and generates an output signal 37a representing the value of sin
(.theta..sub.i -.theta..sub.i-1)(=sin .DELTA..theta..sub.i) corresponding
to the values of the phase angles .theta..sub.i and .theta..sub.i-1
applied as the inputs. The output signals 36a and 37a of the calculators
48a and 48b are applied to mean value calculators 49a and 49b respectively.
The pulse signal 3c, whose period is n times (for example, five
times) as large as that of the pulse signal 4a, is applied to these
calculators 49a and 49b. Therefore, the calculator 49a calculates
the mean value ##EQU2## of X-axis components cos .DELTA..theta..sub.i
of five consecutive Doppler shift phase angles .DELTA..theta..sub.i
and generates an output signal 36b representing the mean value X.
Similarly, the calculator 49b calculates the mean value ##EQU3##
of Y-axis components sin .DELTA..theta..sub.i of five consecutive
Doppler shift phase angles .DELTA..theta..sub.i and generates an
output signal 37b representing the mean value Y. Thereafter, the
calculators 49a and 49b are reset by the reset pulse signal 3c to
start to calculate the mean values of succeeding five consecutive
X-axis components and five consecutive Y-axis components respectively.
The output signals 36b and 37b of the respective calculators 49a
and 49b are applied to a trigonometric function calculator 50 which
calculates the mean Doppler shift phase angle .DELTA..theta.(=tan.sup.-1
(Y/X)) on the basis of the mean values X and Y.
The calculator 50 preferably includes a read-only memory (ROM)
such as that of Type 27512 made by the INTEL Corporation. This ROM
stores a two-dimensional map of values of tan.sup.-1 (Y/X) corresponding
to the values of the mean phase angles X and Y applied as the inputs.
Therefore, the calculator 50 generates an output signal 38a representing
the value of tan.sup.-1 (Y/X) corresponding to the input values
of X and Y. Because the calculation of the value of tan.sup.-1 (Y/X)
is based on a two-dimensional map stored in a ROM, the values of
.DELTA..THETA. are determined on the basis of the polarities of
X and Y as follows:
where:
By use of a two dimensional map, discrimination between values
in the first and third quadrants and in the second and fourth quadrants
is easily ascertained.
On the other hand, the output signal 32 of the amplitude and phase
angle calculator 14 representing the amplitude .alpha..sub.i is
applied to the mean amplitude calculator 15. The pulse signal 3c
is also applied to this calculator 15. The calculator 15 calculates
the mean value ##EQU4## of five consecutive amplitude values .alpha..sub.i
and generates the output signal 33 representing the mean amplitude
.alpha..
The signal 38a representing .DELTA..theta. is multiplied by the
signal 33 representing .alpha. by the multiplier 19 so as to weight
the mean Doppler shift phase angle .DELTA..theta. by the mean amplitude
.alpha., and an output signal 39a representing the result of multiplication
.alpha..multidot..DELTA..theta. is applied to the display unit 20
to display the multiplication value of the velocity v and the amplitude
.alpha. of blood flow.
It is possible to display the mean value of Doppler shift phase
angle .DELTA..theta. or the mean value of amplitude .alpha. independently
as the same as the prior art as shown in FIG. 1.
Therefore, in the first embodiment of the present invention, phase
angles as shown by the arrows a to d in FIG. 3A are resolved into
two axial components as shown in FIG. 3C, and their mean values
X and Y are calculated respectively, so that the correct mean Doppler
shift phase angle .DELTA..theta. can be detected with high accuracy.
It is commonly done to limit the displayable range of data, that
is, to decrease the displayed number of bits of data in order to
reduce the circuit scale (the number of circuit elements). In the
case of the embodiment shown in FIG. 4 the input signal 7a is multiplied
by the cosine wave signal and the sine wave signal, and the resultant
analog signals are applied, after A/D conversion, to the MTI filters
13a and 13b respectively. Output data of these MTI filters 13a and
13b may possibly require a number of bits larger than that of output
data of the other parts of the circuit from the aspect of accuracy.
When the number of bits of the input signal 7a is, for example,
7 the output signals 30c and 31c of the respective MTI filters
13a and 13b may require 12 bits. In this case, when the 12-bit output
data of the MTI filters 13a and 13b are applied intact to the amplitude
and phase angle calculator 14 the calculator 14 will require a
plurality of 8-bit ROM's, resulting in complexity of the circuit
structure. To avoid such a structural complexity, data compressors
46a and 46b for compressing the 12-bit inputs into, for example,
8-bit inputs are preferably provided on the input side of the calculator
14. These data compressors 46a and 46b may be those which generate
output signals representing logarithmic values log R and log I or
n-th roots R.sup.1/n and I.sup.1/n of the input signals representing
the R and I components respectively.
The 8-bit data output signals of the data compressors 46a and 46b
are applied to the amplitude and phase angle calculator 14 and
the phase-angle data output signal 34 of, for example, 12 bits is
generated from the calculator 14.
In such a case, the ROM included in the calculator 14 stores preferably
a two-dimensional map of output values of .theta.=tan.sup.-1 I/R
corresponding to the two input values of log R and log I (or R.sup.1/n
and I.sup.1/n) as shown in FIG. 5 and the resultant data output
signal 34 of the calculator 14 represents .theta..sub.i of, for
example, 12 bits.
The calculator data output signal 34 representing the phase angle
.theta..sub.i of 12 bits is applied to the trigonometric function
calculators 48a and 48b. In the calculators 48a and 48b, more significant
8 bits of the 12-bit data input signal representing .theta..sub.i
are used for calculation. In this case, the trigonometric function
calculator 50 generates a data output signal of 7 bits.
Therefore, provision of a single ROM in each of the amplitude and
phase angle calculator 14 and trigonometric function calculators
48a, 48b, 50 is only required so that the circuit structure can
be simplified.
The numbers of bits of data outputs of the individual circuit elements
are referred to for illustrative purposes only, and the present
invention is in no way limited to such specific numbers of bits
of data outputs.
It can be seen from the above description that the embodiment is
advantageous in that the hardware can be simplified, and the circuit
scale can be reduced to a minimum.
FIG. 6 is a block diagram of a second embodiment of the ultrasonic
Doppler blood flow meter according to the present invention, and,
in FIG. 6 the same reference numerals are used to designate the
same or equivalent functional parts included in the first embodiment.
In this second embodiment, a weighting coefficient corresponding
to the amplitude .alpha..sub.i of the input signal 7a is used in
the calculation of the mean Doppler shift phase angle .DELTA..theta..
By the use of such a weighting coefficient, the mean Doppler shift
phase angle .DELTA..theta. is determined in dependence on an input
signal having a large amplitude and without dependence on an input
signal having a small amplitude, so as to improve the accuracy of
detection of the mean Doppler shift phase angle .DELTA..theta..
In the second embodiment shown in FIG. 6 a weighting coefficient
A.sub.i corresponding to the amplitude .alpha..sub.i of the input
signal 7a is used for weighting the X-axis component cos .DELTA..theta..sub.i
and Y-axis component sin .DELTA..theta..sub.i of the Doppler shift
phase angle .DELTA..theta..sub.i. For this purpose, a weighting
coefficient calculator 23 is provided which receives the output
signal 32 of the amplitude and phase angle calculator 14 as its
input and generates an output signal 40 representing the value of
A.sub.i. This output signal 40 is applied to each of trigonometric
function calculators 58a and 58b.
The weighting coefficient A.sub.i provided by the output signal
40 of the weighting coefficient calculator 23 is as follows:
In the above expressions, .alpha..sub.i and .alpha..sub.i-1 designate
a first and a second amplitude output respectively of the amplitude
and phase angle calculator 14. FIG. 7 is a circuit diagram showing
the structure of one form of the weighting coefficient calculator
23 from which the signal 40 representing, for example, the weighting
coefficient A.sub.i =.alpha..sub.i .multidot..alpha..sub.1-1 is
generated. Referring to FIG. 7 a memory 60 acts as a delay element,
and its output is .alpha..sub.i-1 when its input is .alpha..sub.i.
A ROM 62 preferably stores a two-dimensional map of power values
of .alpha..sub.i .multidot..alpha..sub.i-1 which is the product
of its two amplitude inputs .alpha..sub.i and .alpha..sub.i-1. The
trigonometric function calculators 58a and 58b preferably include
ROM's for generating A.sub.i .multidot.cos (.theta..sub.i -.theta..sub.i-1)
and A.sub.i .multidot.sin (.theta..sub.i -.theta..sub.i-1) as their
outputs respectively in response to the application of the three
inputs representing A.sub.i, .theta..sub.i and .theta..sub.i-1.
More precisely, the calculator 58a includes a ROM storing a three-dimensional
map of values of A.sub.i .multidot. cos (.theta..sub.i -.theta..sub.i-1)
corresponding to its three inputs A.sub.i, .theta..sub.i and .theta..sub.i-1
and, similarly, the calculator 58b includes a ROM storing a three-dimensional
map of values of A.sub.i .multidot.sin (.theta..sub.i -.theta..sub.i-1)
corresponding to its three inputs A.sub.i, .theta..sub.i and .theta..sub.i-1.
The output signals 36a and 37a of the trigonometric function calculators
58a and 58b representing A.sub.i .multidot.cos (.theta..sub.i -.theta..sub.i-1)
and A.sub.i .multidot.sin (.theta..sub.i -.theta..sub.i-1) are applied
to the mean value calculators 49a and 49b respectively. The calculators
49a and 49b calculate the mean values X and Y of n continuous outputs
A.sub.i .multidot.cos .DELTA..theta..sub.i and A.sub.i .multidot.sin
.DELTA..theta..sub.i of the trigonometric function calculators 58a
and 58b respectively. Thus, when n is, for example, five, the calculator
49a calculates the mean value ##EQU5## and generates its output
signal 36b representing the mean value X, and the calculator 49b
calculates the mean value ##EQU6## and generates its output signal
37b representing the mean value Y. The output signals 36b and 37b
of the calculators 49a and 49b representing the mean values X and
Y respectively are applied to the trigonometric function calculator
50 and, in the calculator 50 the weighted mean Dopper shift phase
angle .DELTA..theta.(.DELTA..theta.=tan.sup.-1 Y/X) is calculated
on the basis of the mean values X and Y. The output signal 38b of
the calculator 50 is applied to the display unit 20 as the signal
representing the velocity v of blood flow.
Thus, according to the present invention described above, Doppler
shift phase angles .DELTA..theta..sub.i =(.theta..sub.i -.theta..sub.i-1)
are resolved into two axial components on an orthogonal coordinate
system, that is, X-axis components (cos .DELTA..theta..sub.i) and
Y-axis components (sin .DELTA..theta..sub.i), and the mean value
X=(.SIGMA. cos .DELTA..theta..sub.i)/n of n consecutive X-axis components
and the mean value Y=(.SIGMA. sin .DELTA..theta..sub.i) of n consecutive
Y-axis components are calculated to calculate the mean Doppler shift
phase angle .DELTA..theta.=tan.sup.-1 (Y/X). Therefore, the mean
Doppler shift phase angle can be detected with high accuracy without
appreciably increasing the circuit scale, even when the displayable
range is limited to within -.pi. to +.pi. due to the structural
limitation of the hardware and also even when the mean Doppler shift
phase angle has a value close to .pi. |