DOUBLE PULSE TRANSMISSION - DEAD ZONE DECREASING IN

ULTRASOUND IMAGING

Ihor Trots, Andrzej Nowicki and Marcin Lewandowski

Institute of Fundamental Technological Research, Polish Academy of Sciences, Swietokrzyska 21, Warsaw, Poland

Keywords:

Golay complementary sequences, Double pulse transmission, Dead zone.

Abstract:

This study investigates a new composing method of double transmission of short coded sequences based on

well-known Golay complementary codes, which allows to obtain the higher signal-to-noise ratio (SNR) and

decrease dead zone area. The proposed method can potentially ﬁnd application in small parts ultrasonography

and play an important role in examination of superﬁcial structures, e.g. in dermatology, ophthalmology, etc.,

where using longer coded sequences leads to increase of a dead zone and single pulse transmission of short

sequences does not assure sufﬁcient SNR.

This paper discusses the results obtained during the examination of four different length pairs of Golay coded

sequences excited at 3.7 MHz: the single 64-bits pair of Golay sequences and combined sequences consisting

of two 8, 16, and 32-bits Golay codes separated in time. The experimental results have shown that double pulse

transmission allows to suppress considerably the noise level, the SNR increases by 5.7 dB in comparison with

the single pulse transmission of Golay sequences of the same length. The presented results of this work

demonstrate the advantage of double pulse transmission method which enhances SNR while maintaining the

dead zone short.

1 INTRODUCTION

Coded ultrasonography has been intensively devel-

oped and studied in the last decade - from para-

metric imaging of bone in the range 0.5 – 2 MHz,

through imaging in classic ultrasonography (3.5 –

10 MHz) up to imaging in micro ultrasonography

(above 20 MHz). The reasons of such interest are

the properties of the coded transmission: increase

of penetration depth, signal-to-noise ratio improve-

ment, exploration of the signal with lower amplitude

and improvement of the axial resolution moving to

the higher frequency range. Nowadays extensively

explored coded sequences are: linearly frequency

modulated signals (chirp) and phase-modulated sig-

nals like Barker codes and Golay complementary se-

quences (side-lobe cancelling codes). Within the last

few years, the increasing interest in visualization of

tissue surface (Altmeyer P., 1992) as well as vessel

wall research using high frequency ultrasound can be

observed among biologists and clinicians. Develop-

ment of high frequency ultrasound is directed to a

new region of application in dermatology (diagnostics

of skin diseases and lesion treatment) (Kielbasa Z.,

2007), (Hildegard M., 2005). The ultrasound diag-

nostics is applied to examine main skin layers: epi-

dermis, cutis vera and hypodermis. It is very impor-

tant in ophthalmology as it allows to examine and to

diagnose the pathological changes of cornea, iris, etc.

The motivation of this work was to ﬁnd a new

transmission method of coded sequences based on

complementary Golay sequences (CGS) to decrease

a noise level in result RF echo signal and to improve

SNR in ultrasonic imaging. Coded sequences of short

pulses based on CGS separated in time were transmit-

ted in medium and received. This proposed method

can eventually solve the problem connected to dead

zone, because it allows to transmit the shorter codes

which is important in ultrasonography, namely to ex-

amine and diagnose skin layers, cornea, iris, etc. Dou-

ble transmission of shorter codes instead the longer

ones can also allow to obtain the higher SNR.

2 DOUBLE PULSE

TRANSMISSION METHOD

Among the different excitation sequences proposed in

ultrasonography, Golay codes evoke more and more

416

Trots I., Nowicki A. and Lewandowski M. (2009).

DOUBLE PULSE TRANSMISSION - DEAD ZONE DECREASING IN ULTRASOUND IMAGING.

In Proceedings of the International Conference on Bio-inspired Systems and Signal Processing, pages 416-420

DOI: 10.5220/0001777104160420

 SciTePress

Figure 1: Double pulse transmission method using Golay complementary sequences.

interest in comparison to other signals. The reason

of that lies in the fact that Golay codes, like no other

signals, suppress to zero the amplitude of side-lobes

in ideal case. This type of complementary sequences

was introduced by Golay (Golay, 1961). Practical

implementation of complementary Golay sequences

(CGS) is not widely available in the literature, for the

convenience of the reader a step-by-step principle of

construction and properties of the CGS as well as cor-

relation principle are described in (Trots I., 2004).

Figure 1 shows the double pulse transmission

method using the Golay complementary sequences.

As can be easily seen in practice even for coded trans-

mission the different artifacts are present and because

of that the noise elimination and efﬁcient side-lobe

cancellation cannot be obtained. The noise level can

lead to wrong visualization of the examined organs

and range ambiguity. The nice method which can de-

crease the noise level is to use the longer Golay coded

sequences i.e. 128 or even 512 bit lengths. The usage

of longer coded sequences is welcome in radar tech-

nique or hydrolocation - where information located

closely to transducer is not important. In ultrasonog-

raphy the usage of longer coded sequences is rather

limited since it leads to increase of the dead zone that

is not accepted in some diagnostic applications.

Increase of the echo detection using Golay codes

in comparison to short pulse is evident. As reported

experiments show the two echoes received from re-

ﬂectors distanced one from other by 1 cm. SNR im-

provement in compressed signal in comparison to di-

rect echoes is about 15 dB (Nowicki A., 2004).

The idea of double transmission method is based

on an assumption of mutual noise cancellation, where

noise in the resulted RF signal is averaged by sum-

ming two compressed echo signals obtained by single

transmissions. Such solution allows to improve SNR

maintaining the dead zone area short which can po-

tentially increase the application of ultrasound in der-

matology, ophthalmology, oncology etc.

DOUBLE PULSE TRANSMISSION - DEAD ZONE DECREASING IN ULTRASOUND IMAGING

417

3 EXPERIMENTAL SETUP

The block diagram of the measurement arrangement

used is shown in Fig. 2.

Pentium

Fantom

HP 8643A

Signal Generator

Synthesized

ENI 3100LA

RF Power Amp.

EPM7064

Bipolar Coder

HP 54810A

Infinium

Oscilloscope

Figure 2: Diagram of experimental setup.

The Golay sequences with different lengths 8, 16,

32 and 64-bits at frequencies 3.7 MHz were synthe-

sized in the following way. The Signal Generator

HP8643A produced a sine wave at 0 dB level at a

given frequency. This signal was fed to the bipo-

lar modulator driven by the -1,1 sequences from the

custom-designed coder. The coder circuitry based

on the programmed logic EPM7064 allowed gener-

ating switched pair of single 64-bits Golay sequences

and combined sequences of other shorter Golay codes

separated in time as well as single coded sequences

transmitted for later comparison. The coded signals

were then ampliﬁed via the power RF ampliﬁer ENI

3100LA and the transmitted coded burst excited the

ultrasonic transducer which scanned the tissue phan-

tom model 525 Danish Phantom Design. The uncom-

pressed RF echoes data were acquired using a digi-

tal storage 12-bits oscilloscope Inﬁnium HP 54810A

with a sampling rate of 100 MHz. All processing and

display were done on the computer using Matlab rou-

tines. The processing included ampliﬁcation, pulse

compression, sum of Golay sequences, envelope de-

tection and the obtained results were in few seconds

displayed on the monitors.

4 RESULTS AND DISCUSSION

Figure 3 shows the comparison of transmitting 32-

bits Golay coded sequences at nominal frequency

3.7 MHz and time duration 8.64 µs and proposed

method of the double transmission of 16-bits Golay

coded sequences with shorter time duration that is

equal 4.32 µs. The start time of the second sequence

depends on penetration depth that is examined. In the

given case, the plot illustrates the examined environ-

ment on penetration depth up to 8 cm. The starting

time of the second sequence can be calculated from:

t = 2d/c = 100 µs (1)

where d is the depth, and c is the speed of the ul-

trasound wave in examined environment and is equal

to 1540 m/s. In the second case the RF echo sig-

0 20 40 60 80 100 120 140 160 180 200

−1

Amplitude

0 20 40 60 80 100 120 140 160 180 200

−1

Time [µs]

Amplitude

Traditional transmission

Double transmission

Figure 3: Transmission of the 32-bits Golay sequence with

time duration 8.64 µs (top) and double transmission of the

16-bits Golay sequences with time duration 4.32 µs (bot-

tom).

nals split into two sequences, next compressed and

summed. The amplitude of main-lobe in the resulted

compressed signal will be equal to 64 for both cases.

This is because for single 32-bits coded transmission

two RF echo lines are added, whereas in the case of

double 16-bits coded transmission four RF echo lines

need to be summed.

The tissue phantom model 525 Danish Phan-

tom Design with attenuation of background material

0.5 dB×cm×MHz was used in the experiments. The

pair of the Golay sequences of the different lengths

8, 16, 32, and 64-bits at the frequency 3.7 MHz were

used. The centre RF echo lines obtained from the tis-

sue phantom using the CGS of the different lengths

transmitted by the two methods and calculated SNR

are presented in the Figs. 4 – 7. The target reﬂections

are the nylon ﬁlaments, 0.1 mm in diameter spaced

1 cm one from another.

Figs. 4 – 7 show the advantages of double trans-

mission of Golay coded sequences over single trans-

mission used heretofore. In the double transmission

case the SNR increases by about 5.7 dB in compar-

ison to the single transmission of the same length

coded sequences and by about 4 dB in comparison

to the single transmission of the two times longer se-

quences. According to Trots et al. (Trots I., 2004) in

order to obtain the 4 dB SNR improvement the coded

sequence needs to be about 16 times longer in the sin-

gle transmission method. But using longer coded se-

quences results in increasing of the dead zone area

that increases proportionally to the coded sequences

length and inversely to frequency. Theoretically, the

dead zone area is equal to the half burst pulse time

duration. But in practice the time duration of the

burst pulse is calculated from the beginning to mo-

ment when the power drops to the -3 dB level, so the

BIOSIGNALS 2009 - International Conference on Bio-inspired Systems and Signal Processing

418

dead zone area is assumed to be equal to burst pulse

time duration. In case of 8-bits Golay sequences the

dead zone area is equal to 1.66 mm (Fig. 4) and in-

creases up to 13.2 mm for 64-bits Golay coded se-

quences (Fig. 7) for the frequency 3.7 MHz.

0 1 2 3 4 5 6 7 8

0.2

0.4

0.6

0.8

8−bits Golay coded sequences

Norm. Amplitude

0 1 2 3 4 5 6 7 8

0.4

0.8

1.2

1.6

Depth [mm]

Norm. Amplitude

Single transmission

Double transmission

32.3 dB

38.1 dB

Figure 4: The centre RF-lines of the tissue phantom ob-

tained by single transmission (top) and double transmission

(bottom) of 8-bits Golay sequences.

0 1 2 3 4 5 6 7 8

0.2

0.4

0.6

0.8

16−bits Golay coded sequences

Norm. Amplitude

0 1 2 3 4 5 6 7 8

0.4

0.8

1.2

1.6

Depth [mm]

Norm. Amplitude

Single transmission

Double transmission

34.0 dB

39.5 dB

Figure 5: The centre RF-lines of the tissue phantom ob-

tained by single transmission (top) and double transmission

(bottom) of 16-bits Golay sequences.

The comparison of the obtained 2D ultrasonic im-

ages of a tissue phantom using 8-bits, 16-bis, 32-bits

and 64-bits length Golay sequences transmitted by

different methods is shown in Fig. 8. These record-

ings allow to verify axial resolution and the scan ge-

ometry. It consists of several nylon ﬁlaments twists

0.1 mm in diameter positioned every 1 cm ±2% axi-

ally. Additional groups of 11 twisted threads for 6 dB

axial and lateral resolution are placed at the different

depths from top of the phantom. Also some groups of

low contrast cylinders that deviates +3 dB, -3 dB and

-6 dB from the background are placed.

0 1 2 3 4 5 6 7 8

0.2

0.4

0.6

0.8

32−bits Golay coded sequences

Norm. Amplitude

0 1 2 3 4 5 6 7 8

0.4

0.8

1.2

1.6

Depth [mm]

Norm. Amplitude

Single transmission

Double transmission

35.6 dB

41.3 dB

Figure 6: The centre RF-lines of the tissue phantom ob-

tained by single transmission (top) and double transmission

(bottom) of 32-bits Golay sequences.

0 1 2 3 4 5 6 7 8

0.2

0.4

0.6

0.8

64−bits Golay coded sequences

Depth [mm]

Norm. Amplitude

Single transmission

37.1 dB

Figure 7: The centre RF-lines of the tissue phantom ob-

tained by single transmission of 64-bits Golay sequences.

The images in Fig. 8 demonstrate that the ultra-

sound imaging can beneﬁt from double transmission

of Golay sequences yielding a higher SNR and there-

fore a higher contrast resolution, while maintaining

both axial and lateral resolution. The last one de-

pends on transducer acoustic ﬁeld and is discussed

by NOWICKI et al. (Nowicki A., 2007). Also, it

needs to be noted, that in case of double transmission

method in comparison to single transmission one the

code length is two times shorter.

Obtained 2D ultrasonic images clearly demon-

strate the advantage of double pulse transmission. As

was mentioned above, coded length increase leads to

elongated dead zone area and in the case of 8-bits Go-

lay sequences this area is equal to 1.7 mm while in the

case of 64-bits Golay code the dead zone increases

up to 13.4 mm. The dead zone area on the top of

each 2D ultrasound image is created by high inten-

sity echo amplitude, multiple reﬂections and is equal

to the transmitted code length. In this area the real

echoes are masked and using the longer codes lead to

limited application in dermatology, where superﬁcial

structures are needed to be readable.

DOUBLE PULSE TRANSMISSION - DEAD ZONE DECREASING IN ULTRASOUND IMAGING

419

Figure 8: Comparison of 2D ultrasonic images of the tissue phantom obtained using 8-bits, 16-bits, 32-bits and 64-bits length

Golay coded used single transmission and double transmission. Schematic diagram of the examined tissue phantom model

525. The rectangle marks the scanned area.

5 CONCLUSIONS

This paper discusses actual study and development

trend of the coded transmission method in ultrasonog-

raphy. One of the important parameters in ultra-

sound diagnostic is dead zone area that makes the real

echoes lying closely to transducer surface unreadable.

For that reason using of the coded sequences in ultra-

sound imaging is considerably limited.

The proposed work concerns the development

and investigation of a new composing method of

short coded sequences and their transmission based

on well-known Golay complementary codes. This

method allows to increase ﬁeld of ultrasound diagnos-

tic application where dead zone plays important role,

e.g. dermatology, ophthalmology, etc.

The results obtained show the effectiveness of

double transmission and its resistance to the refrac-

tion, attenuation, and reﬂection of ultrasound waves.

The SNR gain is evident when applying double pulse

transmission method in comparison to single pulse

transmission. Also, increasing codes length, the SNR

increases and penetration elongates proportionally.

The proposed coded method of double transmis-

sion can be applied also in standard ultrasonography.

Introduction of double coded transmission method in

medical ultrasound equipment can increase the effec-

tiveness and quality of the ultrasound diagnostic.

ACKNOWLEDGEMENTS

This work was supported by the Polish Min-

istry of Science and Higher Education (Grant

N51804432/3434).

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