Abstrict An ultrasonic pulse Doppler blood flow meter, wherein an amplitude
equalizing circuit is inserted between the high-pass filter provided
in the successive stage of the Doppler detector and the Doppler
analyzer. The amplitude equalizing circuit not only effectively
utilizes the input dynamic range of the Doppler analyzer but also
automatically adjusts the cut-off frequency of the high-pass filter
in accordance with the input signal. Therefore, a blood flow meter
which requires minimal manual adjustment is obtained.
Claims I claim:
1. An ultrasonic pulse Doppler blood flow meter operatively connected
to transmit and receive ultrasonic wave signals, having a specific
repetition frequency, to and from living body tissue, comprising:
an ultrasonic probe for transmitting the ultrasonic wave signals
to the living body tissue and receiving the ultrasonic wave signals
from the living body tissue at the specified repetition frequency;
a receiving amplifier, operatively connected to said ultrasonic
probe, for amplifying the received ultrasonic wave signals from
the ultrasonic probe and outputting an output signal;
a Doppler detector, operatively connected to said receiving amplifier,
for mixing the output signal of said receiving amplifier with a
reference signal and for generating a Doppler signal of a specified
depth having a low frequency element;
a high pass filter, operatively connected to said Doppler detector,
for eliminating the low frequency element of the Doppler signal
and outputting a high pass filtered signal;
an amplitude equalizing circuit, operatively connected to said
high pass filter, for substantially equalizing the amplitude of
the high pass filtered signal to a predetermined signal level and
outputting an equalized signal;
a Doppler analyzer, operatively connected to said amplitude equalizing
circuit, for analyzing the equalized signal of said amplitude equalizing
circuit; and
a display, operatively connected to said Doppler analyzer, for
displaying the result of the analysis of said Doppler analyzer.
2. An ultrasonic pulse Doppler blood flow meter according to claim
1 wherein said receiving amplifier comprises an automatic gain
control receiving amplifier for maintaining a substantially constant
amplitude of the received ultrasonic waves from the specified depth.
3. An ultrasonic pulse Doppler blood flow meter according to claim
1 wherein said amplitude equalizing circuit comprises:
a multiplier operatively connected to said high pass filter;
an integrator circuit operatively connected to said multiplier;
a rectifier, operatively connected between said multiplier and
said integrator circuit, for detecting the amplitude of the output
of said high pass filter; and
a level setter, operatively connected to said integrator circuit,
for designating a desired signal level.
4. An ultrasonic pulse Doppler blood flow meter according to claim
1 wherein said Doppler detector comprises:
a gate circuit, operatively connected to said receiving amplifier
for receiving the output signal and outputting a gate signal, comprising:
a switch operatively connected to receive the gate signal; and
a low pass filter operatively connected to said switch.
5. An ultrasonic pulse Doppler blood flow meter according to claim
1 wherein said receiving amplifier comprises:
a preamplifier, operatively connected to said ultrasonic probe,
for receiving the ultrasonic signals;
amplifiers operatively connected to said preamplifier;
a half-wave rectifier operatively connected to said amplifiers;
a level setter circuit operatively connected to said half-wave
rectifier;
an analog switch circuit operatively connected to said level setter
circuit; and
an integrator circuit operatively connected to said analog switch
circuit and said amplifiers.
6. An ultrasonic pulse Doppler blood flow meter according to claim
1 wherein said Doppler detector comprises an orthogonal detector
for providing real and imaginary components of the Doppler signal;
wherein said high pass filter comprises first and second high pass
filters operatively connected to receive the real and imaginary
Doppler signals, respectively; and
wherein said amplitude equalizing circuit provides gain control
at the same rate for both the real and imaginary components of the
Doppler signal, only one amplitude component of the real and imaginary
Doppler signals being detected and equalized.
7. An ultrasonic pulse Doppler blood flow meter according to claim
1 wherein said Doppler detector comprises an orthogonal detector,
for detecting real and imaginary components of the Doppler signal;
wherein said high pass filter comprises first and second high pass
filters operatively connected to receive the real and imaginary
components of the Doppler signal, respectively; and
wherein said amplitude equalizing circuit equalizes the amplitude
of both the real and imaginary components of the Doppler signal
at the same rate, by detecting both components and maintaining the
relationship R.sup.2 +I.sup.2 =K, where R is the real component,
I is the imaginary component and K is a constant.
8. An ultrasonic pulse Doppler blood flow meter including an ultrasonic
probe for transmitting ultrasonic wave signals to living body tissue
and receiving the ultrasonic wave signals from the living body tissue
at a specific repetition frequency, and a Doppler detector circuit,
operatively connected to the ultrasonic probe, for mixing the ultrasonic
wave signals with a reference signal and generating a Doppler signal,
said ultrasonic pulse Doppler blood flow meter further comprising:
an amplitude equalizing circuit, operatively connected to the Doppler
detector circuit, for substantially equalizing the amplitude of
the Doppler signal to a predetermined signal level and outputting
an equalized signal, said amplitude equalizing circuit comprising:
a multiplier circuit operatively connected to the Doppler detector
circuit;
an integrator circuit operatively connected to said multiplier
circuit;
a rectifier circuit, operatively connected between said integrator
circuit and said first multiplier circuit, for detecting the amplitude
of the Doppler signal; and
a level setter circuit, operatively connected to said integrator
circuit, for designating a desired signal level.
Description BACKGROUND OF THE INVENTION
(1) Field of the Invention
This invention relates to an ultrasonic pulse Doppler blood flow
meter, and more specifically to the characteristic control of a
receiving circuit.
(2) Description of the Prior Art
An ultrasonic pulse Doppler blood flow meter, which measures a
blood flow rate and its distribution by transmitting an ultrasonic
pulse wave to living body tissue and by receiving a reflected wave
from a blood corpuscle, is currently attracting attention. The equipment
currently being used requires many manual adjustments and is not
easy to operate.
FIG. 1 is a block diagram of a conventional ultrasonic pulse Doppler
blood flow meter. In FIG. 1 1 is a master oscillator, 2 is a transmitting
timing generator which generates a transmitting timing signal by
dividing an output of the master oscillator, 3 is a transmitting
amplifier which generates a pulse or burst transmitting signal,
4 is a transducer (ultrasonic wave probe) which generates an ultrasonic
wave to a living body in accordance with the transmitting signal
and receives a reflected wave from the living body, 5 is a receiving
amplifier which amplifies the reflected wave signal received by
the transducer and 6 and 7 are real (R) and imaginary (I) Doppler
element detectors. The detectors 6 and 7 include mixers 61 and 71
which receive the cosine and sine signals, allow a phase difference
of 90.degree. from the master oscillator and carry out orthogonal
detection, low-pass filters (LPFs) 62 and 72 and sample and hold
circuits (S/H) 63 and 73. The detector 6 detects a real element
of the Doppler signal reflected from the specified depth (distance
between the probe and the location generating the reflected wave),
while the detector 7 detects the imaginary element of the Doppler
signal. High-pass filters 8 and 9 (HPFs) eliminate a low frequency
Doppler element, generated by the wall of the heart, at the outputs
of the detectors 6 and 7. A Doppler analyzer 10 is provided with
an A/D converter and a digital processor which analyzes frequency
by a fast Fourier transformation (FFT) at the outputs of the filters
8 and 9 (the Doppler element indicating a blood flow rate). A display
11 is used for indicating the result of the analysis. A sample pulse
generator 12 generates the sample pulse to the sample hold circuits
63 and 73 in accordance with a position designation signal and the
output of the transmitting timing generator 2. As explained above,
since the cosine and sine reference signals, allowing a phase difference
of 90.degree., are input to the mixers 61 and 71 from the master
oscillator and orthogonal detection is carried out by the detectors
6 and 7. A gain of the receiving amplifier 5 can be adjusted by
a variable resistor 13 provided at the operation panel for gain
control.
In the Doppler blood flow meter of this type, since the HPFs 8
and 9 have an upper limit input level (about .+-.10 V), an output
level of the HPF becomes low when the Doppler signal contains a
low frequency element due to movement of the wall of the heart and,
therefore, the Doppler analyzer 10 in the next stage must be highly
accurate. However, there are other problems: (1) brightness of the
display 11 must be adjusted; (2) a cut-off frequency of the HPFs
8 and 9 must be changed in accordance with a blood flow rate; and
(3) a gain of the receiving amplifier must also be changed in accordance
with the level of the receiving signal.
SUMMARY OF THE INVENTION
It is an object of the present invention to alleviate the above-mentioned
problems such as manual adjustment of the amplifier gain and selection
of the cut-off frequency of the filters. The present invention causes
the successive circuits of the receiving amplifier to operate efficiently.
The Doppler analyzer performs an accurate analysis by employing
an AGC type receiving amplifier 5 after an HPF (high pass filter)
for adding the amplitude of a signal from an equalizing circuit
located in a successive stage of the HPFs. AGC type amplifiers are
used before and after a Doppler detector to improve the accuracy
of the device.
The ultrasonic pulse Doppler blood flow meter of this invention
includes an ultrasonic probe which transmits and receives ultrasonic
waves to living body tissue at a specified repetition frequency,
a receiving amplifier which amplifies reflected ultrasonic wave
signals obtained from the ultrasonic probe, a Doppler detector which
mixes an output of the receiving amplifier and the reference signal
and generates a Doppler signal of the specified depth, a high pass
filter which eliminates a low frequency element of the Doppler signal,
an amplitude equalizing circuit which equalizes the amplitude of
an output of the high pass filter, a Doppler analyzer which analyzes
an output of the amplitude equalizing circuit, and a display which
displays the results of the analysis of the Doppler analyzer.
The present invention will be explained in detail by way of the
preferred embodiment of the invention with reference to the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a conventional ultrasonic pulse Doppler
blood flow meter;
FIG. 2 is a block diagram of an amplitude equalizing circuit in
accordance with an embodiment of the present invention;
FIG. 3 is a detailed diagram of FIG. 2;
FIG. 4 is a graph of the frequency characteristic of the amplitude
equalizing circuit of FIG. 2;
FIG. 5 is a schematic diagram of an automatic gain control type
receiving amplifier;
FIG. 6 is a schematic diagram of a gate circuit of a Doppler detector;
and
FIG. 7 is a diagram of an amplification degree selecting circuit
of a multiplier used in the amplitude equalizing circuit.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 2 is a block diagram of the amplitude equalizing circuit 20
to be inserted into the .circle.A -- .circle.A part of FIG. 1 in
accordance with an embodiment of the present invention. A multiplier
21 receives the output R from the HPF 8 as an input and a multiplier
22 receives the output I from the HPF 9 as an input. An integral
circuit 23 multiplies the output amplitudes of the HPFs 8 and 9.
A full-wave or half-wave detector 24 detects the output amplitude
of the HPF 8. A level setter 25 designates the desired signal level
and a difference between the outputs of the detector 24 and the
level setter 25 is integrated by the integrator circuit 23. The
result is input as the gain control signal to the multiplier 21
which is a variable gain amplifier. Therefore, an average value
.sqroot.R.sup.2 +I.sup.2 of the signals R and I applied to the Doppler
analyzer 10 is constant (for example, 5 Vp-p). In this embodiment,
only the output of the multiplier 21 is detected because the amplitude
of the signals R and I are almost equal.
FIG. 3 is a detailed circuit diagram of FIG. 2. The multiplier
21 comprises an integrated circuit chip AD 534 and resistors R.sub.1
and R.sub.2. The multiplier 22 also has the same structure but is
omitted in the figure. A diode D.sub.1 and resistor R.sub.3 form
a half-wave detector 24. The resistors R.sub.4 and R.sub.5 and the
level setting variable resistor VR.sub.1 form an adder circuit and
the difference thereof is input to the integral circuit 23. The
integrator circuit 23 comprises an operational amplifier OP.sub.1
resistors R.sub.6 and R.sub.8 diodes D.sub.2 and D.sub.3 and a
capacitor C.sub.1. Of these elements, a feedback gain and level
adjusting response rate are determined by a time constant circuit
C.sub.1 R.sub.4. An adequate time constant is a single Doppler analysis
time (about 30 msec). The diode D.sub.2 prevents an output of the
operational amplifier OP.sub.1 from becoming negative. The diode
D.sub.3 is connected to the positive (reference) input side of the
operational amplifier OP in order to compensate for a voltage gap
of about 0.7 V by the diode D.sub.2. The resistors R.sub.7 and R.sub.8
divide the maximum output value of the operational amplifier OP
(about 12 V when the power source of +15 V is used) into a value
suitable for the input of the multiplier 21. For example, when an
input limit of the integrated circuit chip AD 534 is 10 V, it is
divided into 10/12 by the resistors R.sub.7 and R.sub.8. The resistors
R.sub.1 and R.sub.2 of multiplier 21 are used for setting the maximum
amplification degree expressed by the following relationship in
the case of the integrated circuit AD 534. The maximum amplification
degree is equal to:
The following two advantages can be obtained by inserting the above-mentioned
amplitude equalizing circuit 20 between .circle.A -- .circle.A .
First, since an input amplitude of the Doppler analyzer is fixed
even if the gain control of the HPFs 8 and 9 is insufficient, the
dynamic range of the A/D converter at the input stage of the analyzer
or digital processor in the successive stage can be used sufficiently.
Thereby, adjustment of the brightness of the display 11 is no longer
necessary. Second, since a cut-off of the filter frequency changes
automatically, it is no longer necessary to adjust the cut-off frequency
selection signal. FIG. 4 is a graph of the frequency characteristics
for explaining these advantages. A transfer characteristic for the
inputs of .+-.10 V of the HPFs 8 and 9 is indicated by a solid line,
and a transfer characteristic of the amplitude equalizing circuit
20 is indicated by a broken line. When an input signal is 200 Hz
or less, the output level drops and, accordingly, the gain of the
amplitude equalizing circuit increases as indicated by the broken
line and becomes constant at the maximum value. Therefore, the overall
frequency characteristic spreads to a lower frequency region as
indicated by the broken line. In this case, a single frequency is
used. When high frequency components generated by blood flow and
low frequency components generated by the wall of the heart co-exist,
the frequency characteristic is as follows. When the blood flow
rate is high and the frequency is 400 Hz in terms of the Doppler
frequency, an output level of HPF is high, the amplitude equalizing
circuit does not operate to increase the amplitude, a frequency
component of about 100 Hz generated by the wall of the heart is
effectively suppressed and, therefore, the overall characteristic
is shown by the solid line in FIG. 4. On the other hand, when the
blood flow rate is low and has a frequency of about 100 Hz, movement
of the wall of the heart is also low and the Doppler frequency can
become as low as 30 Hz. At any rate, since the Doppler frequency
becomes lower than the cut-off frequency of the HPF, an output level
of the HPF is low and, therefore, the amplitude equalizing circuit
starts operating and, accordingly, the overall characteristic, as
shown by the broken line, is obtained. That is, a sufficient gain
is obtained for the frequency of 100 Hz of the blood flow, but almost
no gain is obtained for the 30 Hz frequency of the wall of the heart.
In this case, the blood flow component and heart wall component,
that is, the signal and noise, can be isolated. According to this
circuit, a cut-off frequency of the filter is automatically lowered
when the blood flow rate is low, and automatically increases when
the blood flow rate is high and thereby the low frequency component
of the heart can be eliminated. If the characteristic of the HPF
is not different from the above case when the blood flow rate is
low, only noise is obtained. In the case of conventional methods,
this phenomenon can be prevented by manual adjustment of the cut-off
frequency fc of the HPF. However, according to this invention, the
frequency adjustment is executed automatically and the dynamic range
of successive stages, that is, the Doppler analyzer can eliminate
the low frequency component of the heart and still pick up a low
blood flow rate.
When there is no distortion in the circuits up to the input of
the Doppler detectors 6 and 7 a high precision Doppler analysis
is carried out with the maximum effect of the amplitude equalizing
circuit 20 as explained above. Since the output level of the transducer
fluctuates greatly, gain control of the receiving amplifier 5 is
necessary in order to eliminate any distortion. If the gain control
is adjusted manually, as in the case of FIG. 1 the operations are
very complicated and the effect obtained is also insufficient.
FIG. 5 is a circuit diagram of the receiving amplifier 5. The receiving
amplifier 5 has a series connection of a preamplifier 51 which amplifies
a receiving signal, an AGC amplifier 52 having an approximate gain
of from -10 dB to +30 dB and an amplifier 53 having a gain of +30
dB. The feedback path of the AGC amplifier 52 is provided with a
detector for half-wave rectification comprising a diode D.sub.4
and a resistor R.sub.9 a level setter 56 comprising a variable
resistor VR.sub.2 and a fixed resistor R.sub.11 and an integrator
circuit 57 comprising an operational amplifier OP.sub.2 a diode
D.sub.5 a capacitor C.sub.3 and resistors R.sub.12 R.sub.13 and
R.sub.14. An ordinary AGC amplifier has a decrease in the gain as
the gain control voltage V.sub.A increases as shown by the curve
K. Therefore, unlike FIG. 3 the diode D.sub.4 is connected in the
reverse polarity direction and level setting is carried out by applying
a positive voltage thereto. An analog switch 54 provides a signal
level of only the sample position of the sample gate signal. When
the switch is set to terminal 1 (sample position), feedback is effected
to the AGC amplifier 52 both inputs of the operational amplifier
OP.sub.2 are grounded on the side of the terminal 2 and the gain
of the amplifier 52 is fixed. The level setting is also performed
through the analog switch in order to prevent any change in the
output level due to the sample gate width. In this case, the AGC
amplifier 52 is inserted at an intermediate position because, (1)
it is difficult to use this amplifier at the initial stage due to
a high noise level, and (2) the maximum amplitude changes since
a DC bias changes. In order to make the maximum amplitude constant,
the amplifier 53 is inserted in a successive stage of the receiving
amplifier 5.
This invention is also effective in a case where the gate circuit
shown in FIG. 6 is used in place of the sample and hold circuits
63 and 73 of the Doppler detectors 6 and 7. This gate circuit equivalently
comprises the switch SW and the low-pass filter LPF and changes
a sample volume by changing the widths ta, tb, . . . of the gate
signals A,B, . . . of the switch. When the gate width is changed
as described above, the output amplitude changes, but the change
is absorbed by the amplitude equalizing circuit 20 shown in FIG.
2. At this time, if the gain of the integrated circuit chip AD 534
used in the multipliers 21 and 22 is changed by the gate signals
A, B and C as shown in FIG. 7 the maximum amplification degree
also changes and a more effective result can be obtained. The resistors
R.sub.20 and R.sub.22 correspond to the resistor R.sub.2 of FIG.
3 and the following relationship exists:
In this case, the following relation is obtained between the gate
widths ta, tb, . . . and the gain:
As described above, according to the invention, manual gain adjustment
and cut-off frequency adjustment are no longer necessary, thereby
improving the operability of the device. In addition, an input amplitude
of the Doppler analyzer is always sufficient and the accuracy of
the system is improved. The invention permits manual selection of
the cut-off frequency of the high-pass filter. Moreover, since the
automatic adjusting range is about fc/2 the changeable range of
fc can be widened in combination with the manual adjustment.
|