 |
Description  |
|
|
BACKGROUND OF THE INVENTION
1. Field of the Invention:
This invention relates to an ultrasonic Doppler blood flow velocity
detection apparatus and to a method for detecting blood flow velocity.
2. Description of the Prior Art:
An ultrasonic Doppler blood flow velocity detection apparatus is used for
detecting blood flow velocity which can be used in diagnoses. There are
many types of Doppler blood flow detection apparatus utilizing the Doppler
effect through reflection of ultrasonic waves. The ultrasonic Doppler
blood flow velocity detector can select a portion where blood flow is to
be detected with respect to distance and direction.
The most popular Doppler blood flow velocity detection apparatus detects
blood flow velocity as follows:
The Doppler blood flow velocity detection apparatus transmits an
ultrasonic-wave pulse whose center frequency is "f", at a predetermined
interval into the human body by a transducer; then it receives a reflected
signal, i.e., an echo signal, from a moving reflective object, such as a
blood corpuscle; and detects the amount of phase shift of the echo signal,
i.e., Doppler shift. An output signal of the phase shift amount, i.e.
Doppler signal, indicates blood flow velocity.
In the Doppler blood flow velocity detection apparatus, the relation
between a shift frequency fd of a Doppler signal and blood flow velocity V
is given by:
fd=(2V/c).multidot.f cos .theta. (1)
where "c" is a sound velocity in the human body; .theta. is an angle made
by the ultrasonic transmitting direction with the direction of blood flow,
wherein the shift frequency fd is subjected to a limitation given by:
.vertline.fd.vertline..ltoreq.fp/2 (2)
where fp is a repetition frequency of ultrasonic-wave pulse (also referred
to as a rate frequency). The Doppler shift frequency fd should not exceed
a half of the frequency fp because of the sampling theory. If blood
velocity V exceeds a velocity corresponding to ultrasonic-wave pulse
repetition frequency fp, the ultrasonic Doppler blood flow velocity
detection apparatus outputs incorrect velocity and direction.
Particularly, if a deep portion is measured, the period of time from
transmission of an ultrasonic-wave pulse to reception of the reflected
ultrasonic waves by a sensor of the apparatus becomes long. Then, the
frequency fp of the ultrasonic-wave pulse should be set to a low value.
Therefore, it is difficult to detect a high velocity of blood flow.
An ultrasonic Doppler blood velocity detecting method is disclosed in the
technical report of the Institute of Electronics, Information and
Communication Engineers, Vol. 87, No. 294, U.S. 87-51, 1987, which is
provided to moderate the limitation of measurable blood flow velocity.
FIG. 6 shows a waveform of transmitted ultrasonic-wave pulses according to
the above-mentioned prior art. In FIG. 6, ultrasonic-wave pulses are
outputted repeatedly at intervals T and T+Ts alternately. The echo signal
has phase shift .DELTA..theta. when an ultrasonic-wave pulse is
transmitted which has the interval T to the subsequent pulse and phase
shift .DELTA..theta.' when an ultrasonic-wave pulse is transmitted which
has the interval Ts to the subsequent pulse. A velocity of blood flow is
given by a phase shift .DELTA..DELTA..theta. which is obtained by
.DELTA..theta.'-.DELTA..theta.. The measurable range is given by
.vertline.f.sub.d .vertline..ltoreq.(1/2).multidot.(1/Ts):
Therefore, decrease in Ts extends measurable range of blood flow velocity
in consideration of Equation (1).
However, there is a drawback that the ultrasonic Doppler blood velocity
detection apparatus according to the above-mentioned method is complicated
because ultrasonic-wave pulses should be transmitted at two different
intervals and a phase shift calculation is required to obtain the phase
shift .DELTA..DELTA..theta.. The detection of the phase shift
.DELTA..DELTA..theta. should be measured repeatedly to obtain a means
value because it is not accurate. Therefore, there is also a drawback that
it is impossible to obtain an instantaneous value of the blood flow
velocity.
SUMMARY OF THE INVENTION
The present invention has been developed in order to remove the
above-described drawbacks inherent to the conventional ultrasonic Doppler
blood flow velocity detection apparatus.
According to the present invention there is provided an ultrasonic Doppler
blood flow velocity detection apparatus comprising: a pulse generation
circuit for generating pulses at a predetermined interval; a transducer
for transmitting ultrasonic waves in response to each of said pulses and
for receiving reflected ultrasonic waves from an ultrasonic-wave
reflective object in the blood of a human body and converting the received
ultrasonic waves into an electric echo signal; a first signal extraction
circuit for extracting signal component of a first frequency f1 from said
echo signal; a second signal extraction circuit for extracting a signal
component of a second frequency f2 different from said first frequency f1
from said echo signal; a frequency difference signal producing circuit for
producing a signal having a frequency of .vertline.f1-f2.vertline. using
output signals of said first and second signal extracting circuit; a
detection circuit for detecting amplitude of an output signal of said
frequency difference signal producing circuit and for phase-comparing said
output signal with a reference signal; and a reference signal generation
circuit for generating said reference signal of a predetermined frequency
fr in response to each of said pulses, said predetermined frequency fr
being approximately equal to the frequency difference
.vertline.f1-f2.vertline..
According to the present invention there is also provided a method for an
ultrasonic Doppler blood flow velocity detection comprising the steps of:
generating pulses at a predetermined interval; transmitting ultrasonic
waves in response to each of said pulses and receiving the reflected
ultrasonic waves from an ultrasonic-wave reflective object in the blood of
a human body and converting the received ultrasonic waves into an electric
echo signal; extracting a signal component of a first frequency f1 from
said echo signal; extracting a signal component of a second frequency f2
different from said first frequency f1 from said echo signal; producing a
signal having a frequency .vertline.f1-f2.vertline. using output signals
of said first and second signal extracting steps; detecting amplitude of
an output signal of said producing step and phase-comparing said output
signal with said reference signal; and generating a reference signal of a
predetermined frequency fr in response to each of the pulses, said
predetermined frequency fr being approximately equal to the frequency
difference .vertline.f1-f2.vertline..
BRIEF DESCRIPTION OF THE DRAWINGS
The object and features of the present invention will become more readily
apparent from the following detailed description taken in conjunction with
the accompanying drawings in which:
FIG. 1 is a block diagram of an ultrasonic Doppler blood flow velocity
detection apparatus according to the invention;
FIG. 2 shows waveforms for illustrating the operation of the ultrasonic
Doppler blood flow velocity detection apparatus of FIG. 1;
FIG. 3 is a block diagram of direction detector of FIG. 1;
FIG. 4 is a block diagram of frequency equalizer of FIG. 1;
FIG. 5 is a chart for illustrating the operation of the frequency equalizer
of FIG. 4; and
FIG. 6 shows a waveform of a prior art ultrasonic Doppler blood velocity
detection apparatus and a method.
The same or corresponding elements or parts are designated at like
references throughout the drawings.
DETAILED DESCRIPTION OF THE INVENTION
Referring now to the drawings, FIG. 1 is a block diagram of an ultrasonic
Doppler blood flow velocity detection apparatus of the invention.
In FIG. 1, a pulse generator 2 produces a pulse signal P at a predetermined
interval Tp shown in FIG. 2. A driver 3 produces an ultrasonic drive
signal in response to an output signal of the pulse generator 2. The
transducer 1 outputs an ultrasonic-wave signal in response to the
ultrasonic drive signal. The ultrasonic-wave signal transmitted from the
transducer 1 is reflected at an object, such as a blood corpuscle in the
blood. The reflected ultrasonic-wave signal is received by the transducer
1. The transducer 1 converts the reflected and received ultrasonic-wave
signal into an electric signal which will be referred to as an echo signal
"e". The echo signal "e" is sent to an amplifier 6 which amplifies the
echo signal "e" at a predetermined gain in order to obtain a desired
output level. An output signal of the amplifier 6 is sent to a first
bandpass filter 7 whose center frequency is f.sub.1 and to a bandpass
filter 8 whose center frequency is f.sub.2 which is different from
f.sub.1. The first and second bandpass filters 7 and 8 extract frequency
components of f.sub. 1 and f.sub.2 respectively. A multiplier 9 multiplies
an echo signal e.sub.1 of an output signal from the first bandpass filter
7 by an echo signal e.sub.2 of an output signal from the second bandpass
filter 8 and outputs a product echo signal e'.sub.0. A third bandpass
filter 10 extracts a component of a frequency f.sub.0 from the produce
echo signal e'.sub.0 to product and output an echo signal e.sub.0. The
echo signal e.sub.0 or the output signal of the third bandpass filter 10
is sent to a detector 11 (also referred to as a quadrature detector) which
has two multipliers 11a and 11b. The multiplier 11a multiplies the echo
signal e.sub.0 by a reference signal r.sub.x which is produced by a
reference signal generator 5, and whose frequency substantially equals the
frequency f.sub.0 of bandpass center frequency of the bandpass filter 10.
The multiplier 11b multiplies the echo signal e.sub.0 by a reference
signal r.sub.y which is produced by a reference signal generator 5, and
whose frequency substantially equals the frequency f.sub.0 of the center
frequency of the bandpass filter 10. The reference signal generator 5
produces the reference signals r.sub.x and r.sub.y (also referred to as
quadrature reference signals), in response to the output signal of the
pulse generator 2, which have a phase difference of 90.degree. to each
other. The output signals of multipliers 11a and 11b are used for
detecting direction of the movement of the object. Output signals of the
multipliers 11a and 11b are sent to integrators 12a and 12b respectively
which integrate the output signals in response to an output signal of a
gate circuit 4. The gate circuit 4 produces a gate signal G in response to
the pulse signal P with delay time Td shown in FIG. 2. The delay time Td
is determined by a first control signal applied to the gate circuit 4. The
gate signal G has a duration Tw which is determined by a second control
signal applied to the gate circuit 4. The delay time Td determines the
center of the duration of the gate signal Td, as shown in FIG. 2. The
integrators 12a and 12b integrate output signals from the multipliers 11a
and 11b respectively in response to the gate signal G for a duration Tw to
output Doppler signals V.sub.xn and V.sub.yn respectively.
The detector 11 detects a blood flow velocity from the echo signal e.sub.0
in response to the reference signals r.sub.x and r.sub.y. The integrators
12a and 12b respectively integrate the output signals of the quadrature
detector 11 in response to the gate signal with the delay time Td
indicative of the depth of a portion where blood flow velocity is
measured, for the duration of time Tw indicative of a region to be
measured.
The Doppler signals Vxn and Vyn are sent to a frequency analyzer 13 which
analyzes the Doppler signal V.sub.xn and V.sub.yn to output a spectrum
signal indicative of level of plural frequency components. A correction
circuit 14 corrects levels of plural frequency components of the spectrum
signal. A display 15 displays blood flow velocity information repeatedly
according to the output signal of the correction circuit.
The Doppler signals V.sub.xn and V.sub.yn may be processed by the following
circuits. A direction detector 16 detects the direction of blood flow in
response to the Doppler signals V.sub.xn and V.sub.yn as to whether the
object moves "toward" or "away" from the transducer 1. In this
specification, the term "toward" is used to indicate the direction of the
object moving toward the transducer 1; the term "away", to indicate the
direction of the object moving away from the transducer 1. The direction
detector 16 outputs either of Doppler signals V.sub.xn and V.sub.yn.
Frequency equalizers 17a and 17b equalize output signals of the direction
detector 16 for compensating for frequency characteristic of the Doppler
signals V.sub.xn and V.sub.yn respectively. Amplifiers 18a and 18b amplify
output signals of the frequency equalizers 17a and 17b to drive speakers
19a and 19b respectively. Speakers 19a and 19b emit sounds in response to
the output signals of the amplifiers 18a and 18b respectively to inform an
operator of the Doppler signals indicative of blood flow velocity to an
operator, such as stereo reproduction.
Hereinbelow will be described operation of the ultrasonic Doppler blood
flow velocity detection apparatus.
The received ultrasonic-wave signal, i.e., echo signal "e" is applied to
the bandpass filters 7 and 8. The bandpass filters 7 and 8 have center
frequencies f.sub.1 and f.sub.2 respectively. The frequency difference
between the center frequencies f.sub.1 and f.sub.2 is set to f.sub.0. The
echo signal "e" is subjected to phase shift by the movement of the object
which reflects transmitted ultrasonic-wave signal, the Doppler effect.
Therefore, the echo signal "e" has a spectrum distribution according to
the phase shift. The echo signal "e" is processed as it passes through
bandpass filters 7 and 8 having different pass bands. In this way two echo
signals e.sub.1 and e.sub.2 having different center frequencies f.sub.1
and f.sub.2 are obtained. The multiplier 9 produces the product echo
signal e'.sub.0 of a beat signal between echo signals e.sub.1 and e.sub.2
by multiplying the echo signal e.sub.1 by the echo signal e.sub.2. The
bandpass filter 10 is set to have a center frequency f.sub.0 which is
equal to the frequency difference between the center frequencies of the
bandpass filters 7 and 8, and which is approximately equal to the
frequency of the reference signals r.sub.x and r.sub.y. Thus, a signal
component of frequency f.sub.0 is extracted from the product echo signal
e'.sub.0 by the bandpass filter 10. The multipliers 11a and 11b multiply
the echo signal e.sub.0 by the reference signals r.sub.x and r.sub.y to
provide detection signals indicative of amplitude and phase relation
between the echo signal e.sub.0 and the reference signals r.sub.x and
r.sub.y respectively. If the echo signal e.sub.0 is in phase with the
reference signal r.sub.x the output level of the multiplier 11a is large.
If the echo signal e.sub.0 has a phase displacement of 90.degree. with the
reference signal r.sub.x the output level of the multiplier is small.
Similarly, if the echo signal e.sub.0 is in phase with the reference
signal r.sub.x, the output level of the multiplier 11a is large. If the
echo signal e.sub.0 has a phase displacement of 90.degree. with the
reference signal r.sub.x the output level of the multiplier is small.
Hereinbelow will be described operation of the ultrasonic Doppler blood
flow detection apparatus more specifically.
The received echo signal e.sub.1 has a spectrum distribution because of the
Doppler effect. The echo signals e.sub.1 and e.sub.2 are given by:
e.sub.1 =A.sub.1 .multidot.sin {.omega..sub.1 (t+.DELTA.t.sub.n)}
e.sub.2 =A.sub.2 .multidot.sin {.omega..sub.2 (t+.DELTA.t.sub.n)}
(.omega..sub.1 =2.pi.f.sub.1, .omega..sub.2 =2.pi.f.sub.2) (5)
where A1 and A2 are amplitudes; .omega..sub.1 and .omega..sub.2 are angular
frequencies corresponding to the center frequencies f.sub.1 and f.sub.2 of
the bandpass filters 7 and 8 respectively; and .DELTA.t.sub.n is a
difference between periods of time required for receiving the reflected
echo signal at (n-1)'.sup.th detection and that at n'.sup.th detection.
This difference corresponds to a distance of movement of the reflective
object. If the reflective object moves, .DELTA.t varies because a period
of time for transmission from the reflective object to the transducer 1
changes.
The product echo signal e'.sub.0 which is outputted at the multiplier 9 is
given by
e'.sub.0 =(1/2)A1A2[cos {(.omega..sub.1 -.omega..sub.2)
(t-.DELTA.t.sub.n)}-cos {(.omega..sub.1 +.omega..sub.2)
(t+.DELTA.t.sub.n)}] (6)
which has frequency components of sum of .omega..sub.1 and .omega..sub.2
and difference between .omega..sub.1 and .omega..sub.2. Therefore, the
echo signal e.sub.0 is given by:
e.sub.0 =(1/2).multidot.A.sub.1 A.sub.2 cos {.omega..sub.0
(t-.DELTA.t.sub.n)} (7)
This is done by passing the product echo signal e'.sub.0 through the
bandpass filter 10. It is assumed that A.sub.1 =A.sub.2 (=A) because the
echo signals e.sub.1 and e.sub.2 are detected at the same point. Thus, the
echo signal e.sub.0 is given by:
e.sub.0 =(1/2)A.sup.2 cos {.omega..sub.0 (t-.DELTA.t.sub.n)}(8)
The reference signals r.sub.x and r.sub.y are:
r.sub.x =1.multidot.cos .omega..sub.r t
r.sub.y =1.multidot.sin .omega..sub.r t (9)
where "1" is an amplitude; and .omega..sub.r =2.pi.f.sub.r.
The Doppler signals V.sub.xn and V.sub.yn are obtained by multiplying the
echo signal e.sub.0 by the reference signals r.sub.x and r.sub.y and by
integration by the integrators 12a and 12b. The frequencies of the
reference signals r.sub.x and r.sub.y, the difference frequency between
center frequencies of the bandpass filters 7 and 8, and the center
frequency of the bandpass filter 10 are set to be the same value, i.e.,
f.sub.0. Therefore, the Doppler signals V.sub.xn and V.sub.yn are given
by:
##EQU1##
The Doppler signals V.sub.xn and V.sub.yn which are obtained at n'.sup.th
transmission and receiving are discrete signals and indicate a phase of
the echo signal e.sub.0 with the reference signals r.sub.x and r.sub.y. It
is assumed that .DELTA.t.sub.n is a variation of time period
.DELTA.t'.sub.n per interval Tp for transmission and receiving the
ultrasonic-wave signal. The Doppler signals V.sub.xn and V.sub.yn are
given by:
V.sub.xn =(1/4)A.sup.2 cos (.omega..sub.0 .DELTA.t'.sub.n /Tp)
V.sub.yn =(1/4)A.sup.2 sin (.omega..sub.0 .DELTA.t'.sub.n /Tp)(11)
Eq. (11) is given at a discrete time T by:
V.sub.xn =(1/4)A.sup.2 cos (.omega..sub.d .multidot.T)
V.sub.yn =(1/4)A.sup.2 sin (.omega..sub.d .multidot.T) (12)
The relation between .omega..sub.d and .omega..sub.0 is given by:
.omega..sub.d =.omega..sub.0 (.DELTA.t'.sub.n /Tp) (13)
The variation of time period is given by:
.DELTA.t'.sub.n =(2/c) (.DELTA.1.sub.n /Tp)=(2/c)V (14)
where .DELTA.l.sub.n /Tp is a distance over which the reflective object
moves, i.e., velocity V. From Eqs. (13) and (14), a Doppler shift
frequency f.sub.d with respect to a velocity V is given by:
f.sub.d =(2V/c)f.sub.o .multidot.cos .theta. (15)
The Doppler shift frequency f.sub.d is subjected to the same limitation as
Eq. (2).
The frequency analyzer 13 provides the Doppler shift frequency f.sub.d by
frequency analyzing of the Doppler signals V.sub.xn and V.sub.yn. However,
amplitudes of the Doppler signals V.sub.xn and V.sub.yn are not
proportional to amplitude of the echo signal "e" because amplitudes of the
Doppler signals V.sub.xn and V.sub.yn are given by a term A.sup.2. The
correction circuit 14 corrects the Doppler signals V.sub.xn and V.sub.yn
so that the output signal is directly proportional to the amplitude of the
receive echo signal.
The frequency f.sub.o determining the frequency of the Doppler shift signal
f.sub.d has no relation to a frequency of the transmitted ultrasonic-wave
signal but corresponds to the difference frequency between the center
frequencies f.sub.1 and f.sub.2 of the bandpass filters 7 and 8 and to the
reference frequency f.sub.r. Therefore, the measurable range can be
extended by selecting the value of the frequency f.sub.o. For example, if
the center frequency of the ultrasonic-wave signal is 5 MHz and the
repetition frequency f.sub.p is 3 KHz, the ultrasonic Doppler blood flow
velocity of up to about 230 cm/sec is measurable where the center
frequency f.sub.1 of the first bandpass filter is set to be 4.75 MHz; the
center frequency of the second bandpass filter f.sub.2, to be 5.25 MHz;
and thus, the reference frequency f.sub.r is to be the frequency f.sub.0
(=f.sub.2 -f.sub.1 =500 KHz). On the other hand, in the above-mentioned
prior art, the measurable velocity range is about 23 cm/sec from Eqs. (1)
and (2) where C=1540 m/sec, .theta.=0. Therefore, the measurable range of
blood flow velocity of ultrasonic Doppler blood flow velocity detection
according to the invention is ten times that of the prior art ultrasonic
Doppler blood flow velocity detector.
Each of the Doppler signals V.sub.xn and V.sub.yn shows the blood flow
velocity and it is determined by a relation between the Doppler signals
V.sub.xn and V.sub.yn whether the object moves in the direction of
"toward" or "away". FIG. 3 is a block diagram of the direction detector 16
which detects the direction of movement of the object in accordance with
the relation between the Doppler signals V.sub.xn and V.sub.yn. In FIG. 3,
the Doppler signal V.sub.xn is applied to a phase-shift circuit 16a for
phase-shifting it by 90.degree. of the reference signals r.sub.x. The
Doppler signal V.sub.yn is added to the output signal of the phase-shift
circuit 16a, shown by V'.sub.xn in the drawing, by an adder 16b. The
signal V'.sub.xn is subtracted from the Doppler signal V.sub.yn by a
subtractor 16c. Output signals of the adder 16b and subtractor 16c are
sent to the equalizers 17a and 17b respectively.
It is assumed that a Doppler shift by "toward" flow of blood is +d.omega.;
another Doppler shift by "away" flow of blood, -d.omega.. In the case of
"toward" flow of blood, Eq. (12) is:
V.sub.xn =(1/4)A.sup.2 cos (+.omega..sub.d .multidot.T)=(1/4)A.sup.2 cos
(.omega..sub.d .multidot.T)
V.sub.yn =(1/4)A.sup.2 sin (.omega..sub.d .multidot.T)=(1/4)A.sup.2 sin
(.omega..sub.d .multidot.T) (12a)
in the case of "away" flow of blood, Eq. (12) is:
V.sub.xn =(1/4)A.sup.2 cos (-.omega..sub.d .multidot.T)=(1/4)A.sup.2 cos
(.omega..sub.d .multidot.T)
V.sub.yn =(1/4)A.sup.2 sin (-.omega..sub.d .multidot.T)=(1/4)A.sup.2 sin
(.omega..sub.d .multidot.T) (12b)
The phase-shift circuit 16a shifts the Doppler signal V.sub.xn by
90.degree. to output the signal V'.sub.xn. Therefore, at either outputs of
the adder 16b and subtractor 16c, a Doppler signal is selectively
outputted. The signal V'.sub.xn is:
V.sub.xn =(1/4)A.sup.2 sin (.omega..sub.d +T) (12c)
The output signal V.sub.OUT1 of the adder 16a and the output signal
V.sub.OUT2 of the subtractor 16c are:
V.sub.OUT1 =V.sub.xn '+V.sub.yn
V.sub.OUT2 =V.sub.xn '-V.sub.yn (12d)
In the case of "toward" flow of blood, V.sub.OUT1 and V.sub.OUT2 are:
V.sub.OUT1 =(1/4)A.sup.2 sin (.omega..sub.d .multidot.T)
V.sub.OUT2 =0 (12e)
In the case of "away" flow of blood,
V.sub.OUT1 =0
V.sub.OUT2 =(1/4)A.sup.2 sin (.omega..sub.d .multidot.T) (12f)
Therefore, in the case of "toward" flow of blood, the Doppler signal is
outputted from the output OUT1; in the case of "away" flow of blood, the
Doppler signal is outputted from the output OUT2. Thus, either sound
signals from the speakers 19a and 19b are outputted according to movement
direction of the reflective object. The sound signals indicative of
Doppler signals show the blood flow velocity well because the direction of
blood flow is clearly indicated and sound signals are corrected to show
linear characteristics.
FIG. 4 shows a block diagram of frequency equalizer 17. In FIG. 4, the
operator can judge the blood flow velocity by listening to sound signals
indicative of the Doppler signals V.sub.xn and V.sub.yn and by recognizing
its tone. However, a level of the sound signal is not directly
proportional to that of received echo signal because the Doppler signals
V.sub.xn and V.sub.yn have the term A.sup.2, as shown in Eq. (12). The
frequency equalizer 17 is provided for correcting this relation. The
frequency equalizer 17 comprises four bandpass filters F1, F2, F3, and F4
having different bandpass characteristics, and whose inputs are connected,
in common, to the output of the direction detector 16. The frequency
equalizer 17 further includes amplitude detectors E1, E2, E3, and E4 for
detecting amplitudes of outputs of the bandpass filters F1, F2, F3, and F4
respectively, Electronic volume control circuits V1, V2, V3, and V4 are
included in equalizer 17 for amplifying output signals of bandpass filters
F1, F2, F3, and F4 at gains determined by levels of signals from the
amplitude detectors E1, E2, E3, and E4 respectively so that each gain is
proportional to the square root of input level thereof. Equalizer 17
further includes adder for adding output signals of the electronic volume
control circuits V1, V2, V3, and V4. The output signal of the detection
circuit 16 is equalized with respect to the level over a frequency range
by the frequency equalizer 17. Therefore, the sound signals from the
speakers 19a and 19b have sound levels proportional to the level of the
echo signal. FIG. 5 shows frequency characteristics for illustrating the
equalizer 17. In FIG. 17, a curve "Si" shows frequency spectrum of the
input signal of the equalizer 17; curves S.sub.1 to S.sub.2 show frequency
spectrums of output signals of the electronic volume circuits V1, V2, V3,
and V4; and a curve S.sub.0 shows a frequency spectrum of the output
signal of the equalizer 17.
As mentioned above, the ultrasonic Doppler blood flow velocity detection
apparatus according to the invention can detect a high speed blood flow
velocity by extracting two components of different frequencies from the
echo signal; extracting a component of that difference frequency from
these components; and by detecting a Doppler signal by multiplying the
component by the quadrature reference signals of that difference
frequency.
* * * * *
|
|
 |
|
 |
Description |
|
