Abstrict An ultrasonic Doppler blood flow meter apparatus includes a wall
filter for eliminating clutter from an echo signal of an ultrasonic
pulse signal sent to a living being under test. The wall filter
includes a switch for cutting a part of input data and a control
unit for controlling the switch such that the switch is opened during
a period in which first m data (where m is integer) is inputted
to undergo filtering by the wall filter while the switch is closed
during a period in which data following the (m+1)-th data inclusive
thereof are inputted.
Claims We claim:
1. An ultrasonic Doppler blood flow meter apparatus, comprising:
a wall filter for eliminating clutter from an echo signal of an
ultrasonic pulse signal sent to a living being under test;
said wall filter having a switch for removing a part of input data;
and
control means for controlling opening and closing of said switch
such that said switch is opened during a period in which first m
data (where m is integer) are inputted to undergo filtering by said
wall filter, and said switch is closed during a period in which
data following the (m+1)-th data, inclusive thereof, are inputted
to undergo filtering by said wall filter.
2. An ultrasonic Doppler blood flow meter apparatus according to
claim 1
wherein said wall filter further includes a first filter provided
at the data input side and having a characteristic given by the
Z-transformation (1-Z.sup.-1) and a second filter provided at the
data output side, said first filter being connected to said second
filter by way of said switch, and
wherein said control means controls said switch such that said
switch is opened during a period in which initial data of an input
data train originating for a particular location of the living being
under test are inputted to said first filter while said switch is
closed during a period in which data following the initial data
are inputted to said first filter.
3. An ultrasonic Doppler blood flow meter apparatus according to
claim 1 wherein said wall filter further comprises a filter, operatively
associated with an output of said switch, comprising a data register
for eliminating unwanted signal components of low frequencies, said
data register being cleared to zero in response to said switch being
opened.
4. A filtering system for an ultrasonic Doppler blood flow meter
apparatus, said filtering apparatus comprising:
a wall filter for eliminating clutter from an echo signal of an
ultrasonic pulse signal sent to a living being under test;
said wall filter having a switch for removing a part of input data;
and
control means for controlling opening and closing of said switch
such that said switch is opened during a period in which first m
data (where m is integer) are inputted to undergo filtering by said
wall filter, and said switch is closed during a period in which
data following the (m+1)-th data, inclusive thereof, are inputted
to undergo filtering by said wall filter.
5. A filtering apparatus according to claim 4
wherein said wall filter further includes a first filter provided
at the data input side and having a characteristic given by the
Z-transformation (1-Z.sup.-1) and a second filter provided at the
data output side, said first filter being connected to said second
filter by way of said switch, and
wherein said control means controls said switch such that said
switch is opened during a period in which initial data of an input
data train originating for a particular location of the living being
under test are inputted to said first filter while said switch is
closed during a period in which data following the initial data
are inputted to said first filter.
6. A filtering apparatus according to claim 4 wherein said wall
filter further comprises a filter, operatively associated with an
output of said switch, comprising a data register for eliminating
unwanted signal components of low frequencies, said data register
being cleared to zero in response to said switch being opened.
Description BACKGROUND OF THE INVENTION
The present invention relates to an ultrasonic Doppler blood flow
meter apparatus for measuring blood flow in living beings by utilizing
an ultrasonic Doppler method in the medical field for thereby generating
blood flow images.
There is known a two-dimensional ultrasonic Doppler blood flow
meter apparatus in whcih a blood flow distribution in a living being
is determined by making use of an ultrasonic Doppler's effect and
displayed in superposition on a two-dimensional monochromatic tomograph
with the blood flow distribution being displayed is a color. This
apparatus is also known as a color flow apparatus. The structure
of the two-dimensional Doppler blood flow meter apparatus known
heretofore is shown in FIG. 7.
Referring to FIG. 7 an ultrasonic transmitter 1 emits ultrasonic
pulses through the medium of a probe 2 for irradiation of a portion
of a living being. Echoes generated as the result of the irradiation
with the ultrasonic pulses are converted into an electric signal
through the probe 2 which signal is then amplified by a receiver
3 and undergoes phase shift detection by a phase shift detector
4 to be thereby converted to a Doppler shift signal. The Doppler
shift signal outputted from the phase shift detector 4 is converted
into digital data through an analogue-to-digital (A/D) converter
5 the digital data then being inputted to a wall filter 6 which
is generally constituted by a FIR- or IIR-type filter of degree
several and which serves to eliminate unwanted signal components
of low frequencies (usually referred to as the clutter) which originate
in intravital tissues. FIG. 6 shows, by way of example, a major
portion of the wall filter constituted by an IIR-type filter of
second degree or order. The wall filter 6 of this configuration
is featured in that the characteristic equivalent to that of the
FIR-type filter can be realized by using a smaller number of multipliers
and adders when compared with the FIR-type filter and that the cut-off
characteristic can flexibly be modified by changing the feedback
coefficients K1 and K2. The Doppler shift signal from which the
clutter signals are eliminated by the wall filter 6 is supplied
to a blood flow rate arithmetic unit 7 which is adapted to determine
arithmetically the blood flow rates. The output of the blood flow
rate arithmetic unit 7 is then inputted to a digital scan converter
(hereinafter referred to as the DSC in abbreviation) 9 together
with a B-mode signal outputted from an envelope detector 8. In the
DSC 9 the B-mode signal and the blood flow rate signal are mixed
together, whereby a two-dimensional blood flow image is displayed
on a screen of a monitor display unit 10.
In the ultrasonic Doppler blood flow meter apparatus known heretofore,
it is desirable to impart steep characteristics to the wall filter
6 in order to provide highly accurate blood flow information. In
that case, however, remarkably large transient responses tend to
take place, making it difficult or impossible to obtain the blood
information with high accuracy because the data contains the transient
responses. Under the circumstances, when the blood flow rate is
to be arithmetically determined with reasonable accuracy, much of
the data will have to be discarded, resulting in that the amount
of data supplied to the blood flow rate arithmetic unit will significantly
be decreased, resulting in a problem that S/N ratio is degraded
because the number of samplings decreases. On the other hand, when
the number of data samplings is increased in order to compensate
for the data lost, it is then required to increase the number of
times the ultrasonic pulse is sent out and the echo is received
in the same direction, which will however result in degradation
in the frame rate.
SUMMARY OF THE INVENTION
In the light of the state of the art described above, it is an
object of the present invention to solve the problems of the blood
flow meter apparatus known heretofore and provide an improved two-dimensional
ultrasonic Doppler blood flow meter apparatus which can ensure availability
of the blood flow information with enhanced accuracy.
For achieving the object described above, there is provided, according
to an aspect of the invention, an ultrasonic Doppler blood flow
meter apparatus which comprises a wall filter for eliminating clutters
from an echo signal of an ultrasonic pulse signal sent to a living
being under test, the wall filter having a switch for cutting a
part of the input data, and a control unit for controlling the switch
such that the switch is opened during a period in which first m
data (where m is integer) is inputted to undergo filtering by the
wall filter while the switch is closed during a period in which
data following the (m+1)-th data inclusive thereof are inputted.
In the ultrasonic Doppler blood flow meter apparatus described
above, the switch control unit opens the switch during a period
in which one or more data sets containing the influential transient
response is being inputted for the filtering operation, to thereby
prevent the data having influential transient response from being
transmitted to the filter block provided downstream of the switch.
Thus, the transient response of the wall filter can be reduced with
simple hardware configuration while ensuring availability of blood
flow information with high accuracy.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic block diagram showing generally a structure
of an ultrasonic Doppler blood flow meter apparatus according to
a first embodiment of the invention;
FIG. 2 is a block diagram showing in detail the structure of a
wall filter employed in the apparatus shown in FIG. 1;
FIG. 3 is a timing chart for illustrating operation of a control
unit of the blood flow meter apparatus according to the first embodiment
of the invention;
FIG. 4 is a block diagram showing in detail the structure of the
wall filter according to a second embodiment of the invention;
FIG. 5 is a timing chart for illustrating operation of a control
unit in the apparatus according to the second embodiment of the
invention;
FIG. 6 is a block diagram showing the structure of the wall filter
of an ultrasonic Doppler blood flow meter apparatus hitherto known;
and
FIG. 7 is a block diagram showing the structure of a conventional
blood flow meter apparatus.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows the structure of an ultrasonic Doppler blood flow
meter apparatus according to a first embodiment of the invention.
In the figure, the transmitter 1 the probe 2 the receiver 3 the
phase shift detector 4 the A/D converter 5 the blood flow rate
arithmetic unit 7 the envelope detector 8 the DSC 9 and the monitor
10 are the same as or equivalent to those described hereinbefore
in conjunction with the blood flow meter apparatus known heretofore
in respect to the structure as well as operation. Accordingly, repeated
description thereof is unnecessary.
According to the invention, there is provided a wall filter 20
including a switch 24 and a control unit 30 for controlling the
switch. FIG. 2 is a block diagram showing in detail the structure
of a wall filter employed in the apparatus shown in FIG. 1. As can
be seen in this figure, the wall filter 20 includes a switch 24
controlled by the control unit 30. Now, referring to the input side
of the switch 24 extending from the A/D converter 5 as the preceding
stage while the output side of the switch 24 leading to the blood
flow rate arithmetic unit 7 is referred to as the succeeding stage;
there is implemented in the preceding stage a filter 25 by a register
23a and an adder 21b, which filter 25 has a characteristic of (1-Z.sup.-1)
representing Z-transformation. This filter 25 is an FIR-type filter
of first degree (or order) and exhibits such characteristic that
only the first data of an input data train or sequence contains
transient response with the following data, inclusive of the second,
having no transient response. The output of the filter 25 (i.e.,
output of the adder 21b) of the preceding stage is connected to
the input of the succeeding stage (i.e., input of the adder 21c)
by way of the switch 24 provided according to the teachings of the
invention.
In the ON-state of the switch 24 an input line 24a is connected
to an output line 24b. On the other hand, when the switch 24 is
opened (OFF-state), a constant of zero is connected to the output
line 24b. The "ON" and "OFF" states of the switch
24 are controlled by the control unit 30 via a control line 24c.
FIG. 3 shows, by way of example, the timing at which the control
signal is generated by the control unit 30. In the case of this
example, it is assumed that the transmission/reception of the ultrasonic
pulse signal in the same direction is repeated five times. The data
A1 to A5 and B1 to B5 outputted from the A/D converter 5 represent
trains or sequences of Doppler shift signal data obtained from a
same location of a living being under test. When the first data
(A1 and B1) of each data sequence is inputted to the filter, the
switch 24 is opened (i.e., set to the OFF-state). On the other hand,
upon inputting of second to fifth data (A2-A5 and B2-B5), the switch
24 is closed (ON-state).
In this manner, in the filter circuit of the succeeding stage disposed
downstream of the switch 24 zero is inputted instead of data containing
the transient response due to filtering through the preceding filter
stage 25 when the first data is inputted, while for the succeeding
data inclusive of the second data, data containing no influence
of the transient response due to filtering through the preceding
stage filter 25 are inputted.
As is apparent from the above, through operation of the control
unit 30 and the switch 24 the data exhibiting no transient response
is transmitted from the filter 25 of the preceding stage to the
filter of the succeeding stage, as a result of which the influence
of a transient response contained in the data sent to the blood
flow rate arithmetic unit 7 can be suppressed to a minimum.
FIG. 4 is a block diagram showing in detail the structure of the
wall filter according to a second embodiment of the invention. The
wall filter according to the instant embodiment differs from that
of the first embodiment in that the register 23b shown in FIG. 4
is provided with a zero-clear terminal, whereby the contents of
the register 23b is cleared to zero at the timing when the switch
24 is opened.
FIG. 5 is a timing chart for illustrating operation of the control
unit 30 in the apparatus according to the second embodiment of the
invention. As can be seen in the figure, the content of the register
23b is cleared to zero at the time point when the first data of
each data sequence is inputted to the filter, whereby the outputs
of multipliers 22a and 22b are reset to zero. Thus, influence of
the preceding data sequence is prevented from being fed back.
In this manner, according to the teachings of the invention incarnated
in the second embodiment, the transient response of the filter can
further be reduced by virtue of such arrangement that the content
of the register of the succeeding stage is cleared to zero every
time the first data of the data sequence is inputted in addition
to suppression of the transient response of the filter of the preceding
stage by means of the control unit 30 and the switch 22.
In conjunction with the first and second embodiments, it has been
described that the FIR-type filter of first degree is used as the
filter 25 of the preceding stage with the IIR-type filter being
used in the succeeding stage for implementing the wall filter 20.
It should however be understood the filter of IIR type or FIR type
may be used in both of the preceding and succeeding stages, substantially
to the same effects.
It should further be mentioned that filters of several degrees
may be used as the filters of the preceding and succeeding stages.
In that case, the switch 24 is opened during a period in which the
first m data (m is an integer) to be filtered through the wall filter
is inputted, wherein the number m is determined in the manner, as
mentioned below. When the FIR-type filters are disposed in series
in the preceding stage in a number n (n is an integer) at maximum,
the number of the registers in the preceding stage is n at maximum
in the serial direction. In this case, the data up to the n-th data
in the output data sequence contain the transient response. Thus,
by determining the value of m so that m=n, it is possible to reduce
the influence of transient response contained in the data sent to
the blood flow rate arithmetic unit 7 to a negligible level. When
the preceding stage is constituted by the IIR-type filter, the transient
response in the preceding stage can not completely be nullified.
However, influence of the transient response can previously be estimated
by computation or simulation. Accordingly, the value of m for which
the switch 24 is opened can be selected such that the transient
response occurring in the preceding stage can sufficiently be eliminated
while an amount of data to be discarded can be minimized. The value
of m thus determined can previously be set at the control unit 30.
Besides, the value now of concern can be changed by issuing corresponding
commands to the control unit while observing the picture generated
actually in the course of operation of the ultrasonic Doppler blood
flow meter apparatus.
Although it has been described in conjunction with the first and
second embodiments of the invention that the switch 24 is provided
internally of the wall filter 20 it is of course possible to provide
it externally of the wall filter 20. Besides, it is of no matter
whether or not the switch 24 is implemented integrally with the
wall filer 20. What is required is that a filter including the register
is disposed at the side upstream of the switch 24.
As is apparent from the foregoing description, according to the
teachings of the invention, such control can be realized that transient
response generated by the filter of the preceding stage is essentially
prevented from being transmitted to the filter of the succeeding
stage, whereby the transient response of the wall filter can be
suppressed to a very low level with the addition of a small amount
of hardware, to a great advantage. Thus, even in the case where
steep filter characteristic is set for the wall filter, the blood
flow rate can accurately be estimated with improved S/N ratio at
a high frame rate because the data to be discarded can be suppressed
to a maximum. |