A Doppler-type ultrasonic diagnostic apparatus is provided for smoothing a Doppler spectrum. In the apparatus, a tomographic image of a diagnostic portion of an object being examined is obtained, the diagnostic portion including a fluid in motion, and a range gate is placed at a position of the fluid on the tomographic image. The apparatus comprises an element for transmitting an ultrasonic beam signal for the diagnostic portion and receiving an ultrasonic echo signal reflected by the fluid, an element for converting the ultrasonic echo signal into a corresponding electrical echo signal, an element for extracting a Doppler signal from the electrical echo signal, the the Doppler signal being formed by the fluid flowing at the position of the range gate, an element for calculating data of a Doppler spectrum being composed of a plurality of instantaneous spectra each including a plurality of Doppler frequency components on the basis of the extracted Doppler signal, an element for smoothing the data of the Doppler spectrum, and an element for displaying the smoothed data of the Doppler spectrum.

BACKGROUND OF THE INVENTION 
The present invention relates to a Doppler-type diagnostic apparatus, and 
more particularly to the apparatus that is in use for diagnosing motion of 
fluids such as blood within an object being examined by utilizing the 
Doppler effect of ultrasonic signals. 
At present, there has already been provided a Doppler-type ultrasonic 
diagnostic apparatus in which ultrasonic pulse Doppler and ultrasonic 
pulse reflection methods are used together to obtain a tomographic image 
(black/white B-mode image) and a real-time blood flow image through a 
single ultrasonic probe. 
FIG. 1 exemplifies such a Doppler-type ultrasonic diagnostic apparatus by 
which the speed of a blood flow is measured as blood flow information. In 
the apparatus shown in the figure, connected to an ultrasonic probe 201 
are a transmitting pulser 202 and receiving pre-amplifier 203. The output 
of the pre-amplifier 203 is connected, by way of a mixer 204, lowpass 
filter 205, sample and hold circuit 206, bandpass filter 207, and 
frequency analyzer 208, in turn, with a display unit 209. 
Further, the diagnostic apparatus is also provided with a pulse generator 
210 for transmitting/receiving control and a range gate circuit 211 for 
range gate control. The pulse generator 210 incorporates frequency 
dividers and gate circuits, thus supplying a clock pulse S.sub.a of a 
specific frequency (refer to FIG. 2) to the range gate circuit 211 and 
mixer 204, creating a rate pulse S.sub.b of ultrasonic repetition 
frequency (refer to FIG. 2) on the basis of the clock pulse S.sub.a and 
supplying it to the pulser 202 and range gate circuit 211. 
The pulser 202 creates a high-voltage driving pulse using the supplied rate 
pulse S.sub.b in order to drive the ultrasonic probe 201. This driving 
allows the probe 201 to transmit an ultrasonic pulse signal into an object 
P. Part of the transmitted ultrasonic signal makes an ultrasonic echo 
signal by being reflected at the wall BW and blood flow B (mainly, red 
corpuscle) of a blood vessel BV. The ultrasonic echo signal will then be 
received by the probe 201, where a corresponding electrical echo signal 
S.sub.d (refer to FIG. 2) is yielded. 
The electrical echo signal S.sub.d reflects the Doppler effect, that is, 
the Doppler shift in frequency caused by scattering of the ultrasonic 
signal by corpuscles in motion. According to this effect, the central 
frequency f.sub.c of a received ultrasonic echo signal changes by a 
Doppler shift frequency f.sub.d, thus making its receiving frequency 
f=f.sub.c +f.sub.d. The Doppler shift frequency f.sub.d is approximately 
expressed as follows by assuming a blood flow speed is v, an angle between 
an ultrasonic beam and a blood vessel is .theta., and a sound speed is c. 
EQU f.sub.d ={(2.multidot.v.multidot.cos .theta.19 f.sub.c)/c}.multidot.f.sub.c 
Detecting the Doppler shift frequency fd from the received electrical echo 
signal Sd provides the blood flow speed v, which gives a foundation to the 
receiving system of the present Doppler-type ultrasonic diagnostic 
apparatus. In detail, the electrical echo signal S.sub.d is amplified by 
the pre-amplifier 203 and then sent to the mixer 204, where the amplified 
echo signal S.sub.d is mixed with the clock pulse S.sub.a to be supplied 
to the next lowpass filter 205. The mixed signal is lowpassed by the 
lowpass filter 205, so that higher-frequency components such as an 
ultrasonic carrier are removed from the mixed signal; only low frequency 
components centered on the Doppler shift frequency f.sub.d are sent to the 
sample and hold circuit 206. 
By using a sampling pulse S.sub.c (refer to FIG. 2) that corresponds to the 
distance from the surface of an object to the position O of a range gate 
(i.e., sampling point or sampling volume) placed on a blood flow B of a 
tomographic image, the filtered signal is then sampled and held by the 
sample and hold circuit 206, the held signal being sent to the bandpass 
filter 207 by which excessive higher and lower frequency components are 
removed to extract only the Doppler shift frequency component of the blood 
flow B. The extracted signal is then frequency-analyzed with fast Fourier 
transformation, for example, for obtaining a frequency spectrum pattern of 
Doppler shift frequencies (i.e., Doppler spectrum). This Doppler spectrum, 
which is displayed on the display unit 209 as shown in FIG. 3, represents 
changes in Doppler shift frequency in a two-dimensional coordinate system 
whose vertical axis is assigned to the frequency and its horizontal axis 
to time, where strength of each frequency component is depicted by 
altering pixel brightness. 
For a further examination of blood flows, it is sometimes required to 
observe changes in time of maximum speeds of blood flows. In such 
observation, the maximum frequencies of a real-time Doppler spectrum are 
automatically traced on its image or the maximum frequencies of a frozen 
Doppler spectrum are traced by hand, as shown by a bold line MF in FIG. 4, 
so that the traced values (maximum frequencies) are extracted. 
Another analysis technique is to use a histogram of flow speeds, which is a 
speed component distribution where its horizontal axis is assigned to 
frequencies (i.e., Doppler shift frequencies corresponding to blood flow 
speeds) and its vertical axis is assigned to powers (strength) of 
respective frequencies. To obtain the histogram of blood flows, a Doppler 
spectrum image is first frozen and a desired time position in the 
horizontal axis of the spectrum image is then specified with a cursor 
marker (for instance, refer to a marker MT in FIG. 5). In response to 
this, a calculator (not shown) works to calculate a distribution of speed 
components at the specified time position. The calculated distribution 
data is normally displayed as shown in FIG. 6. 
However, there are a wide variety of drawbacks in the above image 
processing. All of those drawbacks are resulted from the fact that the 
ultrasonic echo signal received by the probe 201 includes known speckle 
components caused by phase interferences of reflected echoes of corpuscles 
in a blood flow. 
Concretely, the speckle components makes a Doppler spectrum change little 
by little, but rough, in the vertical frequency (blood flow) direction, as 
shown in FIG. 3, and a histogram of flow speeds changes largely and 
narrowly in the vertical power axis, as shown in FIG. 6, thus 
deteriorating accuracy in displaying a blood flow. 
As a result, when a Doppler spectrum is observed, it is difficult to 
recognize a steady frequency range at a glance, because there are large 
changes in the frequency and density. 
In addition, automatic or manual tracing the maximum frequencies on a 
Doppler spectrum results in small and frequent ups and downs of a traced 
curve, which makes the tracing difficult and inefficient and imposes 
comparatively heavy burden on an operator. 
Further, when such a histogram of flow speeds shown in FIG. 6 is observed, 
it is hard to recognize how the entire histogram image spreads. In this 
case, it is necessary to repeat the specification of another time position 
on a Doppler histogram for displaying revised histogram images. This 
operation necessarily involves frequently-repeated judgement of whether or 
not a histogram image now on is an observable distribution images. 
Therefore, in case of obtaining a histogram of flow speeds by the 
conventional technique, much operation load will be put on an operator and 
a diagnostic time will be longer. 
SUMMARY OF THE INVENTION 
Accordingly, it is a primary object of the present invention to provide a 
Doppler-type ultrasonic diagnostic apparatus that is able to display a 
Doppler spectrum having a superior visibility by suppressing changes in 
its frequency-axis direction which are caused due to speckle components. 
Further, it is another primary object to provide a Doppler-type ultrasonic 
diagnostic apparatus by which maximum frequencies on a Doppler spectrum 
can be traced easily, and efficiently, other than accurately. 
Still further, it is another primary object to provide a Doppler-type 
ultrasonic diagnostic apparatus that is able to not only display a 
histogram of flow speeds whose entire spread is easily recognizable by 
suppressing changes in its power-axis direction caused due to speckle 
components but reduce operational load and a required diagnostic time. 
These and other objects can be achieved according to the present invention, 
in one aspect by providing a Doppler-type ultrasonic diagnostic apparatus, 
in which a tomographic image of a diagnostic portion of an object being 
examined is obtained, the diagnostic portion including a fluid in motion, 
and a range gate is placed at a position of the fluid on the tomographic 
image, the apparatus comprising: an element for transmitting an ultrasonic 
beam signal for the diagnostic portion and receiving an ultrasonic echo 
signal reflected by the fluid; an element for converting the ultrasonic 
echo signal into a corresponding electrical echo signal; an element for 
extracting a Doppler signal from the electrical echo signal, said the 
Doppler signal being formed by the fluid flowing at the position of the 
range gate; an element for calculating data of a Doppler spectrum being 
composed of a plurality of instantaneous spectra each including a 
plurality of Doppler frequency components on the basis of the extracted 
Doppler signal, said Doppler spectrum having a two-dimension coordinate 
consisting of a time axis and a Doppler shift frequency axis corresponding 
to a flow speed of the fluid; an element for smoothing the data of the 
Doppler spectrum; and an element for displaying the smoothed data of the 
Doppler spectrum. 
As another aspect of the present invention, there is provided a 
Doppler-type ultrasonic diagnostic apparatus comprises, in addition to the 
above construction, an element for tracing maximum frequencies of the 
smoothed data of the Doppler spectrum. 
Further, as another aspect of the present invention, there is provided a 
Doppler-type ultrasonic diagnostic apparatus, in which a tomographic image 
of a diagnostic portion of an object being examined is obtained, the 
diagnostic portion including a fluid in motion, and a range gate is placed 
at a position of the fluid on the tomographic image, the apparatus 
comprising: an element for transmitting an ultrasonic beam signal for the 
diagnostic portion and receiving an ultrasonic echo signal reflected by 
the fluid; an element for converting the ultrasonic echo signal into a 
corresponding electrical echo signal; an element for extracting a Doppler 
signal from the electrical echo signal, said the Doppler signal being 
formed by the fluid flowing at the position of the range gate; a first 
element for calculating data of a Doppler spectrum being composed of a 
plurality of instantaneous spectra each including a plurality of Doppler 
frequency components on the basis of the extracted Doppler signal, the 
Doppler spectrum having a two-dimension coordinate consisting of a time 
axis and a Doppler shift frequency axis corresponding to a flow speed of 
the fluid; an element for smoothing the data of the Doppler spectrum; a 
first element for displaying the smoothed data of the Doppler spectrum; an 
element for freezing the Doppler spectrum displayed by the first 
displaying element; an element for specifying an arbitrary time position 
on the Doppler spectrum frozen-displayed by the first displaying element; 
a second element for calculating data of a histogram of flow speeds of the 
fluid from the data of the Doppler spectrum in accordance with the time 
position specified by the specifying element; and a second element for 
displaying the data of the histogram of flow speeds calculated by the 
second calculating element.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An embodiment of the present invention will now be described with reference 
to FIGS. 7 to 13. 
A Doppler-type ultrasonic diagnostic apparatus according to the present 
embodiment, as shown in FIG. 7, is provided with an 
electronic-scanning-type ultrasonic probe (hereafter, referred to as 
"probe") 11 and an electronic scanning section 12. 
The electronic scanning section 12 is further provided with a reference 
oscillator 20 for generating a reference clock, a delay line 21 for 
creating a delay driving signal based on the reference signal, and a 
pulser 22 for driving a group of phased array transducers arranged in the 
probe 1 1 by means of receiving the delay driving signal from the delay 
line 21. The delay line 21 is also used in receiving an ultrasonic echo 
signal. The electronic scanning section 12 also incorporates a receiving 
system; that is, it is provided with a pre-amplifier 23 connected with the 
probe 11, the delay line 21 delaying a signal outputted from the 
pre-amplifier 23, an adder 24 adding a delayed signal outputted from the 
delay line 21, and a detector 25 performing logarithmic amplification and 
envelope detection on an output signal of the adder 24. The delay line 21 
and adder 24 permit a received ultrasonic echo signal to be beam- formed, 
thus an electronic scanning being carried out. 
The output signal of the detector 25 will be supplied, as an image signal 
composing a B-mode tomographic image, to a DSC (Digital Scan Converter) 
30, where the signal thus-supplied is converted into a signal in standard 
TV scanning. The converted signal by the DSC 30 is then given, via a D/A 
converter 31, to a display unit 32 (CRT in this embodiment). 
The output of the adder 24 in the electronic scanning section 12 is also 
connected, through a mixer 40 for use in phase detection, to a lowpass 
filter 42. Although not shown in detail in the figure, two channels for 
sine and cosine function signals are given to the mixer 40 and lowpass 
filter 42, respectively. The output of the reference oscillator 20 in the 
electronic scanning section 12 is directly connected to one channel input 
of the mixer 40, while it is connected, via a 90.degree. phase shifter 41, 
to the other channel input of the mixer 40. Such construction makes it 
possible to not only provide the mixer 40 a beam-formed echo signal in the 
electronic scanning section 12 but provide the two channel inputs of mixer 
40 reference signals f.sub.o and f.sub.o having a difference of 90.degree. 
in phase to each other. The mixer 40 will thus send a Doppler shift signal 
"f.sub.d" and a signal of "2f.sub.o +f.sub.d" to the lowpass filter 42, 
where only high-frequency components among the mixed signal are removed to 
obtain the Doppler shift signal f.sub.d. The Doppler shift signal f.sub.d 
corresponds to a phase detection output for calculating blood flow 
information and is then supplied to a next Doppler spectrum calculation 
section 50. 
In the Doppler spectrum calculation section 50, provided are a range gate 
circuit 60 outputting a sampling pulse, a sample and hold circuit 61 
receiving the sampling pulse, a bandpass filter 62 filtering an output 
signal of the sample and hold circuit 61, and A/D converter 63 digitizing 
an output signal of the filter 62, a frequency analyzer 64 analyzing in 
frequency an output signal of the A/D converter. There are two channels of 
signal processing in the sample and hold circuit 61, bandpass filter 62, 
and A/D converter 63 (not shown), respectively. The output of the 
frequency analyzer 64 is connected to the DSC 30 through a spectrum 
averaging circuit 70. 
The sample and hold circuit 61 is provided for extracting a Doppler signal 
of a blood flow at only a specified depth within an object, whose target 
signal is therefore a phase-detected output signal S.sub.d supplied from 
the lowpass filter 42. 
The range gate circuit 60 is constructed in a manner that it is capable of 
setting arbitrarily a delay time on the basis of a range gate position 
signal RG given from an operation panel 82 which will be later described. 
Hence, supplied from the range gate circuit 60 to the sample and hold 
circuit 61 is a sampling pulse S.sub.c, having a specified pulse width, 
delayed from a rate pulse S.sub.b by an interval corresponding to a period 
during which an ultrasonic signal travels back and forth between the 
positions of the probe 11 and a given range gate (sometimes, called as 
"sampling point" or "sampling volume"). Hence, the sample and hold circuit 
61 will sample and hold the phase-detected signal outputted from the 
lowpass filter 42 using the thus-created sampling pulse S.sub.c. The 
phase-detected signal thus-sampled and held then passes the bandpass 
filter 62, in which removed are harmonic components caused by sampling in 
the sample and hold circuit 61, permanent echo components from the wall of 
a blood vessel etc., and a Doppler shift component generated due to a 
comparatively slower motion of a blood flow, thus only a Doppler shift 
signal according to the true movement of a blood flow being extracted. 
The frequency analyzer 64 has a fast Fourier transformer which performs 
frequency analysis of a Doppler shift signal sent from the A/D converter 
63, the analysis results, that is, a Doppler spectrum (frequency spectrum 
pattern), being then sent, via the spectrum averaging circuit 70, to the 
DSC 30. This allows the display unit 32 to display the Doppler spectrum 
image and B-mode tomographic image in a divided form thereon. 
As shown in FIG. 8, the spectrum averaging circuit 70 has a single 
coefficient determining circuit 71 and a plurality of weighting 
delay/addition circuits 72-1 to 72-m. 
In the coefficient determining circuit 71, a speckle duration will be 
calculated on the basis of an average speed of a blood flow during a 
certain time and a plurality "n+1" of coefficients reflecting the speckle 
duration are determined to be supplied to the weighting delay/addition 
circuits 72-1 to 72-m, respectively. For example, assuming that the 
repetition frequency of a rate pulse is 3[kHz], a data number used in 
one-time FFT (Fast Fourier Transform) processing is 64 (in FIG. 8, m=64), 
a data number transferred between two-times of FFT processing is 8, the 
width of an ultrasonic beam is 1 [mm], and an average speed of a blood 
flow is 10[cm/sec], a speckle duration corresponding to the ultrasonic 
beam width is therefore 
EQU 1[mm]/10[cm/sec]=0.01[sec]. 
This speckle duration=0.01[sec] is then converted into 4 times of 
FFT processing by 0.01/{(1/3000).times.8}. 
Thus, "n+1" pieces of coefficients K.sub.0 to K.sub.n are designated such 
as, for example, 
EQU K.sub.0 =K.sub.1 =K.sub.2 =K.sub.3 =1.0 and 
EQU K.sub.4 =K.sub.n =0.0, 
corresponding coefficient setting signals to those coefficients K.sub.0 to 
K.sub.n being supplied to "m" pieces of the weighting delay/addition 
circuits 72-1 to 72-m, respectively. 
Further, each of the weighting delay/addition circuits 72-1 to 72-m has a 
digital FIR (Finite Impulse Response) filter; it is provided with a delay 
element group 73 consisting of "n" pieces of delay elements, a multiplier 
group 74 consisting of "n+1" pieces of multipliers for each multiplying 
delayed outputs from the delay element group 73 by the coefficients, and 
an adder 75 adding the outputs of the multipliers. 
The delay element group 73 in each of the delay/addition circuits 72-1 to 
72-m will receive output signals from the frequency analyzer 64, the 
output signals consisting of the analysis results of individually-assigned 
frequency components to its delay element group 73, where the received 
signals will be delayed every sampling timing to be sent to higher-order 
elements. Each of the coefficient setting signals determined by the 
coefficient determining circuit 71 is supplied to the multiplier group 74 
of the individual weighting delay/addition circuit 72-1 (to 72-m). The 
multipliers of n+1 pieces in the multiplier group 74 are capable of 
updating, in real time, their own coefficients K.sub.0 to K.sub.n into 
specified values in response to the coefficient setting signals. In 
consequence, in each of the weighting delay/addition circuits 72-1 to 
72-m, the analyzed data of each of the frequency components is weightedly 
delayed and added using the coefficients K.sub.0 to K.sub.n determined on 
the basis of a speckle duration corresponding to an ultrasonic beam width. 
In other words, the results from the frequency analyzer 64, each frequency 
component, are averaged over time with moving average technique, each 
plurality of instantaneous spectra determined by a given period, by the 
digital filter (FIR filter), the averaged value being supplied to the DSC 
30 for every frequency component. 
In the present embodiment, although the time length of the moving average 
is assigned to a single speckle duration, the present invention is not 
necessarily limited to such value; it is possible to assign the time 
length several times or several fractions of a speckle duration. As to the 
coefficients, where their values are selected to be not zero, it is 
acceptable to select arbitrary values between "0" to "1" for them, not 
limited to "1" as explained above. Further, at the output side of the 
spectrum averaging circuit 70, as shown in FIG. 7, there is provided a 
freezing memory 80 and a histogram calculator 81. The freezing memory 80 
is capable of not only updating and memorizing, every several frames, the 
averaged results supplied from the spectrum averaging circuit 70 at all 
times but providing the DSC 30 and histogram calculator 81 the averaged 
Doppler spectrum data at a specific frame that has been memorized so far, 
when a freezing signal FR is given from an operation panel 82. Hence, when 
an operator instructs the freezing of a Doppler spectrum through the 
operation panel 82, the image of a Doppler spectrum (i.e., averaged 
spectrum) that has been displayed in real time so far will be frozen. The 
histogram calculator 81 includes a computer unit and, in response to a 
freezing signal FR, reads in freezed Doppler spectrum data from the 
freezing memory 80. Moreover, the histogram calculator 81 is to calculate 
a distribution of fluid speed components, that is, a histogram of flow 
speeds at a specified timing, according to a later-described processing 
procedure in FIG. 9, for supplying the resultant calculation data to the 
DSC 30. Therefore, the histogram will also be imaged on the display unit 
32. 
The operation panel 82 is equipped with a track ball and/or key board, and 
using the operation panel 82, an operator is able to give the apparatus 
the range gate position signal RG and freezing signal FR and to give the 
histogram calculator 81 a calculation position (i.e., time position) 
signal CP at a specified time position on an averaged Doppler spectrum 
image on the display unit 82. In addition, corresponding to initiation of 
the range gate position signal RG and calculation position signal CP, data 
of specific markers are sent from a graphic memory (not shown) to the 
display unit 32 for their indication thereon. 
The above range gate position signal RG is also sent to the delay line 21 
of the electronic scanning section 12 for focusing a receiving ultrasonic 
beam along a scanning line including a specified range gate position. 
In the next place, the processing of the histogram calculator 81 will be 
explained according to FIG. 9. 
At Step 90 in the figure, by the histogram calculator 81, a freezing signal 
FR is examined to judge whether or not freezing of a Doppler spectrum is 
ordered. If the judgement of NO is determined thereat (the freezing has 
not been ordered yet), the process at Step 90 will be repeated for 
waiting. In contrast, the judgement is YES (the freezing is ordered), 
Steps 91 and later will then be processed. 
At Step 91, data of a Doppler spectrum (averaged data by the spectrum 
averaging circuit 70) in accordance with the freezing timing are taken 
from the freezing memory 80. Then at Step 92, a calculation position 
signal CP supplied from the operation panel 82, corresponding to a certain 
single point on the time axis (horizontal axis) of a Doppler spectrum now 
displayed, is taken in, the single point being specified by moving the 
position of a marker MT (refer to FIG. 12). 
Then at Step 93, data of a histogram of flow speeds are formed as data of a 
distribution between frequency components (fluid speed components) and 
their powers (strengths) at a specified timing by the marker MT. 
Then at Step 94, the data of the histogram calculated at Step 93 are 
displayed. As a result, displayed on the display unit 32 is a histogram 
image of flow speeds shown in FIG. 13, for instance, together with its 
tomographic and Doppler spectrum images. 
As explained above, in this embodiment, the mixer 40, lowpass filter 42, 
range gate circuit 60, sample and hold circuit 61, and bandpass filter 62 
composes Doppler signal extracting means. The A/D converter 63 and 
frequency analyzer 64 composes the Doppler spectrum calculating means. The 
spectrum averaging circuit 70 corresponds to the Doppler spectrum data 
smoothing means. 
Now, the entire operation will be explained. When the Doppler-type 
ultrasonic diagnostic apparatus begins to work, in response to a rate 
pulse S.sub.b supplied from the reference oscillator 20 and a scanning 
signal (for example, a signal according to a sector scanning) specifying a 
sampling mode and a raster address, which is supplied from a control 
circuit (not shown), the electronic scanning section 12 drives the probe 
11 to transmit an ultrasonic beam signal into an object being examined, 
The ultrasonic beam signal is then reflected in the object to be received 
by the probe 11. From the probe 11, the converted electrical echo signal 
is sent to the electronic scanning section 12, where the echo signal is 
formed to have its receiving focus by beam forming process and is detected 
and converted into image signals at a specified raster address. Such image 
signals forming a B-mode tomographic image are then supplied to the DSC 
30. 
On one hand, the echo signal at a specified raster address, which is 
focused in the electronic scanning section 12, is phase-detected by the 
mixer 40 and lowpass filter 42, being supplied to the Doppler spectrum 
calculation section 50. In this calculation section 50, a Doppler signal 
at a specified range gate position is sampled from the phase-detected echo 
signal and its signal is processed by a real-time frequency analysis. The 
resultant data of Doppler spectrum is forwarded to the spectrum averaging 
circuit 70. 
In the spectrum averaging circuit 70, the Doppler spectrum data (a 
plurality of instantaneous spectra), each frequency component, are then 
averaged over a period corresponding to the speckle duration, the averaged 
data being sent to the DSC 30. 
In the DSC 30, the B-mode image data and averaged Doppler spectrum data are 
combined and converted into image data of the standard TV scanning, and 
sent to the display unit 32 via the D/A converter 31. In consequence, 
displayed in a divided form on the display unit 32 are a B-mode 
tomographic image and a Doppler spectrum of a diagnostic portion. 
The averaged Doppler spectrum is shown in real time as in FIG. 10, for 
instance, where the horizontal axis is assigned to time and the vertical 
axis to Doppler shift frequencies (i.e., fluid speeds). Comparing with the 
aforementioned conventional technique illustrated in FIG. 3, the Doppler 
spectrum in FIG. 10 is depicted as a entirely denser picture whose upper 
and lower edges are smoothed and simplified profiles to form a clear band 
shape. In short, the Doppler spectrum excludes little by little changes in 
its vertical direction and large changes in degrees of density, both of 
which had been seen frequently before due to the speckle components. 
Therefore, for an operator, the visibility of the Doppler spectrum will be 
greatly improved to provide a more distinguishable frequency band width. 
On one hand, in case where the maximum frequencies are traced on a Doppler 
spectrum by hand, its image will be frozen first. Then such a device as a 
track ball is operated to trace the maximum frequencies (i.e., maximum 
blood flow speeds) on the frozen Doppler spectrum image, as illustrated in 
FIG. 11 (refer to a bold line MF therein). Since the upper edge profile 
has been simplified and smoothed by means of averaging, having reduced its 
meaningless ups and downs in the vertical direction compared with the 
conventional non-averaged Doppler spectrum (refer to FIG. 4), it is easy 
to recognize a profile where tracing should be done. Thus the manual 
tracing operation can be done efficiently and accurately in a shorter 
operation time. 
In case of automatic tracing, the maximum frequencies of a real-time or 
frozen Doppler spectrum are extracted by the DSC 30 and displayed by the 
display unit 32, as shown in FIG. 11. The objected Doppler spectrum has 
been averaged also in this case, in consequence the traced maximum 
frequency curve is also smoothed, without meaningless ups and downs, and 
easy to observe. 
Furthermore, in case of displaying a histogram of flow speeds, an operator 
handles the operation panel 82 to output a freezing signal FR. In response 
to this, the processing of Steps 91 to 95 in FIG. 9 is carried out by the 
histogram calculator 81; through the processing, a position (time) being 
calculated is specified by a marker MT, as shown in FIG. 12, and a 
histogram of flow speeds at the specified timing is displayed as shown in 
FIG. 13 on the display unit 32, together with other produced images. 
As can be understood from FIG. 13, the averaging over time enables the flow 
speed histogram thus-displayed to substantially suppress little by little 
changes in its vertical direction representing power, as compared with the 
conventional one in FIG. 6. In other words, little by little changes in 
power resulted from speckle components are eliminated to be smooth over 
its entire range of flow distribution. 
In this way, the histogram is automatically and quickly displayed that 
reflects an honest blood flow and is less changes in the vertical 
direction. This improvement helps the easier recognition of the whole 
histogram image. Further, there is almost no need for repeatedly setting a 
time position on the Doppler histogram image and/or repeatedly judging 
whether an observable histogram image is obtained or not, which noticeably 
reduces operation load to the operator and greatly shortens a diagnostic 
time accompanying analysis of the histogram. 
Other variations according to the present inventions are possible. In the 
above-mentioned embodiment, by the coefficient determining circuit 71, the 
speckle duration has been calculated on an averaged blood flow speed 
during a fixed time and the coefficients have been determined on the basis 
of the calculated speckle duration, while variations for this technique 
are as follows; (i) as to the speckle duration, the speckle duration may 
be calculated on the basis of an averaged or maximum momentary speed of 
blood flows with the flow speed of blood adjusted by its averaged or 
maximum momentary speed, (ii) as to the coefficients, there are two ways: 
one is that change rates (acceleration) in averaged or maximum momentary 
speeds of blood flows are calculated to work out a time length (e.g., the 
larger acceleration, the smaller time length) when characteristics of 
flows are regarded as almost the same, the regarded time length being 
involved in determining the coefficients, the other is that an arbitrary 
time length is set by an operator through the operation panel 82 and the 
arbitrary time length is used in determining the coefficients. In this 
manual specification, an initial Doppler spectrum image is once displayed 
according to an arbitrary time length. If the displayed spectrum image is 
yet unsatisfied, the time length will be changed by hand in a try and 
error manner. Such a construction of the apparatus has an advantage of 
less processing amount in CPUs. Further, an operator may select and 
specify one of fixed values, preset as the time length, according to 
different typical diagnostic portions. 
Moreover, in the multiplier group 74 of the spectrum averaging circuit 70 
in the above embodiment, every frequency data, the results from four 
multipliers, individually using four coefficients K.sub.0 to K.sub.3 (=1), 
are added (averaged), while a hold means for holding the maximum among the 
results from the four multipliers may be provided for outputting the 
maximum of each frequency data, respectively. The number of multipliers, 
which are used in averaging or maximum detecting, is not limited to four; 
any number is acceptable. 
Still moreover, although the above spectrum averaging circuit 70 has 
adopted a way by which each frequency component is averaged over time, the 
way is not so limited, possible are averaging frequency data over their 
frequency direction by using a known averaging circuit such as a FIR 
filter and/or maximum-detecting frequency data over their frequency 
direction by using a known maximum detecting circuit, those circuits being 
placed instead of the spectrum averaging circuit 70 in the figure. 
Obviously, further modifications and variations of the present invention 
are possible in light of the above teachings. It is therefore to be 
understood that within the scope of the appended claims, the invention may 
be practiced otherwise than as specifically described therein.