RAPID COMMUNICATION

Experimental verification of color flow imaging basedon wideband Doppler method

Naohiko Tanaka

Received: 11 February 2013 / Accepted: 22 April 2013

� The Japan Society of Ultrasonics in Medicine 2013

Abstract The purpose of this study is to eliminate the

aliasing in color flow imaging. The wideband Doppler

method is applied to generate a color flow image, and the

validity of the method is experimentally confirmed. The

single beam experiment is carried out to confirm the

velocity estimation based on the wideband Doppler

method. The echo data for the conventional pulsed Doppler

method and the wideband Doppler method are obtained

using a flow model, and the estimated velocity for each

method is compared. The color flow images for each

method are also generated using several types of flow

model. The generated images are compared, and the

characteristics of the imaging based on the wideband

Doppler method are discussed. The high velocity beyond

the Nyquist limit is successfully estimated by the wideband

Doppler method, and the availability in low velocity esti-

mation is also confirmed. The aliasing in color flow images

is eliminated, and the generated images show the signifi-

cance of the elimination of the aliasing in the flow imaging.

The aliasing in color flow imaging can be eliminated by the

wideband Doppler method. This technique is useful for the

exact understanding of blood flow dynamics.

Keywords Color flow imaging � Pulsed Doppler �Aliasing � Nyquist limit

Introduction

Aliasing is one of the major problems in color flow

imaging for the visualization of blood flow. The blood

flow, the velocity of which exceeds the Nyquist limit,

will be observed as a flow of opposite direction because

of the aliasing. This phenomenon has been explained by

the sampling theorem. The target velocity is estimated

by the phase changes of echoes in the conventional

pulsed Doppler method applied to the medical color

flow imaging system. The phase information can be

obtained as a discrete time signal sampled at pulse

repetition frequency, because one phase data is detected

by one pulse transmission. Thus, the velocity, which can

be correctly estimated, is limited by the sampling

theorem.

In order to overcome the aliasing problem, various

methods have been investigated since the basic principle of

color flow imaging was proposed by Kasai et al. [1]. The

use of a transmission pulse with two different frequencies

has been studied [2, 3]. The phase changes are detected for

each frequency component of echoes, and the target

velocity is determined by the difference of each phase

change. The resulting phase difference will correspond to

that which is obtained by the pulse transmission with the

difference frequency. Since the difference frequency can be

made much smaller than each frequency, the velocity range

can be expanded. A similar idea was applied to a non-

equally-spaced pulse transmission method [4]. However,

the use of a second-order phase difference causes a rise in

the influence of noise.

A time-domain correlation technique has also been

studied [5–8]. This scheme is based on estimation of the

time delay of echoes using a cross-correlation function.

The cross-correlation-based methods have a weakness in

N. Tanaka (&)

Department of Electronic Information Systems,

Shibaura Institute of Technology, 307 Fukasaku,

Minuma-ku, Saitama 337-8570, Japan

e-mail: tanaka@shibaura-it.ac.jp

123

J Med Ultrasonics

DOI 10.1007/s10396-013-0462-3

that the influence of the velocity variance of targets is

relatively large.

Elimination of the aliasing in the estimation of blood

velocity could possibly be realized by the wideband

Doppler method [9], which utilizes all the information of

the phase spectrum of echoes obtained by the wideband

pulse transmission. The phase correction in this method

enables estimations of high velocity beyond the Nyquist

limit. Its robustness with respect to noise and the velocity

variance was confirmed by computer simulation. The

principle of the wideband Doppler method and the simu-

lation results were described in a previous paper [9]. Use of

the wideband echo information was also studied by Kondo

et al. [10]. The target velocity is estimated by using only

the gradient of the phase spectrum in this method, resulting

in insufficient robustness.

The purpose of this study is to experimentally val-

idate the performance of the wideband Doppler

method. Several types of flow model are employed for

the acquisition of echo data. The color flow images are

generated by the conventional pulsed Doppler method

and the wideband Doppler method. The resulting ima-

ges yielded by the two methods are compared and

discussed.

Materials and methods

Basic stance of the experimental study

In general, a new method should be verified by using

more reliable data obtained by another method. In the

case of this study, it is ideal to measure the velocity

profile in the flow model by using a non-ultrasound-based

method with higher accuracy. However, it is difficult to

measure the velocity with the same scan format and with

the same spatial resolution at the same time as the

ultrasonic measurement. To avoid this difficulty, the

estimated velocity yielded by the wideband Doppler

method is compared with the result yielded by the con-

ventional pulsed Doppler method.

Experimental setup for the confirmation of velocity

estimation using a single beam transducer

A simple flow phantom and data acquisition system was

developed for confirmation of the velocity estimation of

the wideband Doppler method. The schematic diagram of

this system is shown in Fig. 1. Two fluid bags are con-

nected by a rubber tube with an internal diameter of

4 mm, and are filled with the Doppler test fluid (ATS

Model 707). Bidirectional flow is easily realized by

pressing these bags alternately. The pulser generates the

transmission pulse. A four-cycle burst pulse and one-

cycle burst pulse are used for the conventional pulsed

Doppler and the wideband Doppler measurement,

respectively. A sequence of eight pulses for the conven-

tional Doppler and a sequence of eight pulses for the

wideband Doppler are alternately transmitted to obtain

the echo signal for each Doppler method at almost the

same time. The transmission pulse for the wideband

Doppler measurement has the same amplitude as that in

the conventional Doppler measurement. Because the burst

cycle is reduced, the signal-to-noise ratio (SNR) of the

echo signal for the wideband Doppler measurement is

lower than that in the conventional pulsed Doppler

measurement. A concave PZT (13/, 75R) transducer with

a 75 % fractional band width is used in this experiment.

The parameters of this experiment are shown in Table 1.

The received echo signal is amplified and then A/D

converted with 14-bit resolution at a 50-MHz sampling

rate. The A/D converter used here contains a large-sized

buffer memory, and the echo data obtained during a

10-second period can be stored in the buffer memory.

The digitized echo data are then sent to a PC, and all the

signal processing is carried out offline on the PC. The

Fig. 1 Schematic diagram of the single beam experiment for the

confirmation of the velocity estimation

Table 1 Parameters for the single beam experiment

Parameter Value

Center frequency of transmission pulse 2 MHz

Burst cycle of transmission pulse

Conventional pulsed Doppler 4

Wideband Doppler 1

Number of pulse transmissions (packet size) 8

Pulse repetition time 256 ls

Range gate size 5.12 ls

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audible Doppler signal is also generated for setting of the

transducer position.

Experimental setup for color flow imaging

In order to generate the color flow image of the flow

model, a linear array transducer and a multi-channel data

acquisition system are used as shown in Fig. 2. The array

consists of a 128-element transducer with a center fre-

quency of 7.5 MHz. Table 2 shows the parameters for

the imaging. The 32-ch pulser and the 32-ch A/D con-

verter with 12-bit resolution are connected to the array

transducer through a cross-point switch. The transmission

pulse for the wideband Doppler measurement has the

same amplitude as that in the conventional Doppler

measurement. The beam scanning and the pulser control

in this system are carried out by specified hardware. On

the other hand, receiver beam forming, Doppler velocity

estimation, scan conversion, and all other processing for

generating the color flow image are carried out by soft-

ware on an offline PC. The buffer memory to store echo

data has the capacity for 8-pulse transmission, i.e., the

packet size is 8. Because of some limitations in the

circuit design, a sufficient amount of buffer memory

could not be realized. Consequently, the data acquisition

for the conventional Doppler method and that for the

wideband Doppler method are carried out individually

within a few minutes. During the data acquisition for the

two methods, the flow phantom is carefully controlled to

maintain a uniform condition, and thus the flow models

in the two cases have almost the same velocity profile,

but are not exactly equal. Thus, the images yielded by

the two methods can be compared qualitatively.

Results

Single beam experiment

The results of the velocity estimation obtained by the fixed

ultrasound beam are shown in Figs. 3 and 4. In these cases,

the fluid bags are pressed every few seconds alternately to

generate bidirectional flow. Figure 3 shows the estimation

result of the conventional pulsed Doppler method. The

Nyquist limit in this case is ±0.73 m/s, and is shown by the

dotted line in Fig. 3. The bidirectional velocity can be

identified in Fig. 3, but the estimated velocity aliases at the

Nyquist limit. Figure 4 shows the estimation result of the

wideband Doppler method. The estimation range of

velocity is set to ± 2.05 m/s, and a 35 pattern is employed

for the phase correction. The velocity beyond the Nyquist

limit is also estimated correctly. For the comparison of

these results, the estimated velocity of the wideband

Fig. 2 Schematic diagram of

the color flow imaging

Table 2 Parameters for color flow imaging

Parameter Value

Center frequency of transmission pulse 7.5 MHz

Sampling frequency 50 MHz

Burst cycle of transmission pulse

Conventional pulsed Doppler 4

Wideband Doppler 1

Number of pulse transmissions (packet size) 8

Pulse repetition time 250 ls

Nyquist limit (conventional pulsed Doppler) 0.2 m/s

Ultrasound beam 128 line

Array pitch 0.4 mm

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Doppler method is forcibly aliased and superimposed upon

the result of the conventional Doppler method, as shown in

Fig. 5. The good agreement of these results can be con-

firmed by Fig. 5.

Experiment of color flow imaging

The color flow imaging using the single tube flow model

is carried out for two flow velocities. In all cases, no

image processing is applied, and the color of a pixel is

determined by the estimated velocity of its position.

Figure 6 is generated by using the conventional pulsed

Doppler method. The fluid is moving from the lower left

to the upper right of this figure, so the area in the tube

should be filled by a red color. The blue region of Fig. 6

is caused by aliasing. The result of the wideband Doppler

method is shown in Fig. 7. Although a small number of

error pixels exist, almost the entire region in the tube is

filled by the red color, and aliasing is not observed. The

image pattern of the non-aliased region in Fig. 6 is the

same as that in Fig. 7. This means that the flow is suf-

ficiently stable.

Faster flow is also tested using the same flow model.

The color flow image for a faster flow generated by the

wideband Doppler method is shown in Fig. 8. In this case,

the flow velocity is about twice that in Fig. 7, and the

velocity range in the wideband Doppler method is three

times that of the conventional pulsed Doppler method. A

considerable number of error pixels (blue) are observed in

Fig. 8. The color flow image with reduced spatial resolu-

tion is shown in Fig. 9. This image is generated using the

same data as used in Fig. 8, but the adjacent two scan lines

are combined and the range gate size is twice that of Fig. 8.

The number of the error pixels is decreased by reducing

spatial resolution.

Fig. 3 Estimated velocity by using the conventional pulsed Doppler

method. The dotted lines show the Nyquist limit

Fig. 4 Estimated velocity by using the wideband Doppler method

Fig. 5 Comparison of the estimation result. The red plot is the result

of the wideband Doppler method, and is forcibly aliased. The black

plot is the result of the conventional pulsed Doppler method

Fig. 6 A color flow image of the single tube model. The velocity is

estimated by the conventional pulsed Doppler method

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The advantage of the wideband Doppler method will be

clear in the experiment using the jet flow model. The color

flow images generated by the conventional pulsed Doppler

method and the wideband Doppler method are shown in

Figs. 10 and 11, respectively. The nozzle is placed at the

lower right, and the fluid spurts from there to the upper left

Fig. 7 A color flow image of the single tube model. The velocity is

estimated by the wideband Doppler method

Fig. 8 A color flow image for the faster flow of the single tube

model. The velocity is estimated by the wideband Doppler method

Fig. 9 A color flow image for the faster flow of the single tube

model. The velocity is estimated by the wideband Doppler method.

An enlarged sample volume is used

Fig. 10 A color flow image of the jet flow model. The velocity is

estimated by the conventional pulsed Doppler method

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of the figure. The velocity of the fluid has a maximum

value in the nozzle. Obviously, aliasing is observed in the

image yielded by the conventional pulsed Doppler method

shown in Fig. 10. In contrast, the flow velocity is estimated

successfully in the case of the wideband Doppler method,

as shown in Fig. 11. The slow flow around the jet is also

imaged properly in Fig. 11.

The double tube flow model is employed to confirm the

estimation of low speed flow. Figure 12 shows the image

generated by the conventional pulsed Doppler method. The

fluid in the upper tube is moving from left to right, and the

fluid in the lower tube is moving in the direction opposite

that in the upper tube. The lower flow is faster than the

upper flow. Therefore, the upper and lower tube should be

filled by dark red and light blue, respectively. The red color

in the lower tube in Fig. 12 is caused by the aliasing. The

result of the wideband Doppler method is shown in Fig. 13.

In this case, the flow in each tube is correctly imaged, but it

may be hard to see the dark red in the upper tube. The color

coding on a logarithmic scale is useful for the expression of

wide range velocity. Figure 14 shows the color flow image

on a logarithmic scale. Both the fast flow and the slow flow

are clearly imaged without aliasing.

Discussion

The validity of the velocity estimation based on the

wideband Doppler method is experimentally confirmed by

using several flow models. In all cases, the result generated

Fig. 11 A color flow image of the jet flow model. The velocity is

estimated by the wideband Doppler method

Fig. 12 A color flow image of the double tube model. The velocity is

estimated by the conventional pulsed Doppler method

Fig. 13 A color flow image of the double tube model. The velocity is

estimated by the wideband Doppler method. The upper tube is filled

by dark red because of low velocity

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by the wideband Doppler method is free from aliasing. In

particular, the results of the jet flow model express the

significance of the aliasing elimination in color flow

imaging.

The results of the single beam experiment show that the

estimated velocity in the wideband Doppler method is in

agreement with that in the conventional pulsed Doppler

method. In addition, it is found that good stability in the

estimation of low velocity is achieved by the wideband

Doppler method. The line fitting process in the method will

contribute to the stability in velocity estimation. The

advantage in low velocity estimation can also be confirmed

in Figs. 11 and 14.

As mentioned above, the electric power of the trans-

mission pulse in the wideband Doppler method is lower

than that in the conventional method, so the SNR of the

echo signal for the wideband Doppler method is also lower

than that in the conventional method. The results of the

wideband Doppler method are obtained under the lower

SNR condition. Though the wideband system is, in general,

considered to be sensitive to noise, the proposed method

has good robustness for noise. The line fitting process

contributes to the robustness for noise.

The results of the fast flow imaging shown in Figs. 8 and

9 suggest a relation between the spatial resolution and the

maximum velocity in the estimation. The difference

between Figs. 8 and 9 is in the size of the sample volume

for the velocity estimation. The flow velocity is estimated

by the echoes from the target within the sample volume.

Because the targets are moving, a part of the targets will

enter the sample volume, and a part of the targets in the

sample volume will exit. This causes changes in the echo

waveform and a decline in the correlation with subsequent

echoes. The ratio of replaced targets is determined by the

target velocity and the size of the sample volume. Conse-

quently, the sample volume must be enlarged for faster

flow estimation.

A small number of error pixels exist in the results of the

wideband Doppler method. This error is caused by acci-

dental aliasing [11], which originates from the abnormal

narrowing of the echo spectrum resulting from the inter-

ference of echoes. The wideband Doppler method is based

on the scheme that the target velocity is estimated by using

all the phase information of echoes in many frequencies.

When the bandwidth of the echo spectrum becomes nar-

row, the estimation of the wideband Doppler method will

degenerate into that of the conventional pulsed Doppler

method. There are two ways to suppress the error rate

caused by accidental aliasing. The first way is to make the

bandwidth of the transmission pulse wider, and the other is

to expand the range gate. Image processing, such as using a

median filter, is also effective in making the error pixels

invisible.

Conclusion

The wideband Doppler method for the elimination of ali-

asing in color flow imaging is investigated in this study.

High velocities beyond the Nyquist limit are successfully

estimated by the wideband Doppler method, and the

availability in low velocity estimation is also confirmed.

The color flow images of the flow model show the sig-

nificance of the elimination of aliasing.

The hard limit of the velocity range in the wideband

Doppler method is determined by the number of the phase

correction pattern. In addition, the error rate in the velocity

estimation is determined by the size of the sample volume,

the bandwidth, and the SNR of the echo signal. In blood

flow imaging, the error rate is the dominant factor that

determines the velocity range. Further study, including

statistical analysis of the error rate, will be required.

Quantitative evaluation of the velocity estimation will also

be required in a future study.

The recognition of aliasing in clinical situations is now

dependent upon the skill of the ultrasonographer or medical

doctor. The proposed method will release these profes-

sionals from the burden of recognition of aliasing, and will

contribute to the exact understanding of blood flow

dynamics.

Fig. 14 A color flow image of the double tube model. The velocity is

estimated by the wideband Doppler method. Color coding on a

logarithmic scale is used

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Conflict of interest The author has no conflicts of interest or rela-

tionships to disclose.

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