PARAMETRIC ECHOSCANNER FOR MEDICAL DIAGNOSIS

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PARAMETRIC ECHOSCANNER FOR MEDICAL DIAGNOSIS

L. Bjørnø, S. Grinderslev

To cite this version:

L. Bjørnø, S. Grinderslev. PARAMETRIC ECHOSCANNER FOR MEDICAL DIAGNOSIS. Journal

de Physique Colloques, 1979, 40 (C8), pp.C8-111-C8-118. �10.1051/jphyscol:1979820�. �jpa-00219525�

JOURNAL DE PHYSIQUE Colloque C83 supplement au n°ll, tome 40, novembre 1979, page C8-111

PARAMETRIC ECHOSCANNER FOR MEDICAL DIAGNOSIS.

L. BJ0RN0 and S. GRINDERSLEV

Department of Fluid Mechanics Technical University of Denmard Building 404 DK-2800 Lyngby Denmark.

Résumé. - On a étudié l'adaptation d'un réseau paramétrique acoustique pour Véchographie dynamique appliquée au diagnostic médical. L'étude des diagrammes de rayonnement et des amplitudes à diverses fréquences différence (f$ = 0,5, 1, 2 et 4 MHz) obtenus en plaçant ou non une lentille acoustique dans le faisceau paramétrique à la distance de Rayleigh, montre que ce dispositif, en réduisant la

profondeur de champ à l'origine excellente du réseau paramétrique, dans le but d'augmenter forte- ment la résolution latérale et angulaire, est nécessaire â l'obtention de bonnes conditions d'ëcho- graphie. On a obtenu des résultats encourageants concernant la limite à 3 dB (résolution limite) et le gain acoustique dans la zone focale, dans des systèmes de liquides différents.

Abstract. - A study of the adaptation of the parametric acoustic array to medical echoscanning for diagnostic purposes has been performed. Studies of the beam patterns and amplitudes of various dif- ference frequencies (fs = 0.5, 1.0, 2.0 and 4.0 MHz) without and with an acoustic lens introduced into the parametric beam at the Rayleigh distance of the primaries have shown that the trade-off, reducing the original excellent field depth of the parametric array in order to obtain a strongly improved angular and lateral resolution by introduction of the lens, is necessary in order to achieve appropriate scanning conditions. Encouraging results have been obtained concerning 3 dB limits (resolution limits) and acoustic gains in the focal region in various liquid systems.

1. INTRODUCTION. - The importance of ultrasound in medicine is still increasing and as a diagnostic tool ultrasound is now being used on a par with long recognized methods like X-rays and nuclear me- dicine. Being noninvasive, externally applied and nontraumatic ultrasound is in every-day routine clinical use in hospitals around the World for the study of organs like heart, liver, kidney, brain, eyes etc. together with fetal and reproductive sys- tems. This increasing use of ultrasound has also led to a steady increasing research effort aiming at the development of more accurate procedures which for instance can lead to a- strongly improved resolution in order to obtain more detailed and re- liable information on organ defects at stages as early as possible in the development of the sick- ness. A comprehensive review of the-state-of-theart in medical ultrasound may be found in /l/.

The application of acoustic lenses in ultra- sonic scanning devices has contributed to an impro- ved resolution / 2 / , and acoustic lenses now consti- tute a generally accepted focusing part of a scan- ning transducer on a par with focusing geometries of electrostrictive ceramics.

Improved resolution in medical echoscanning operations may also be achieved by the use of the significant inherent advantages of the parametric acoustic array like its narrow beam, its absence of sidelobes and its broad-band capabilities. In

an earlier work Muir et al /3/ obtained some encou- raging results in their attempts to adapt the para- metric array to be used in biomedical diagnostics.

A U.S. - Patent of a "Shrapnel Sonar" became a more concrete result of the investigations performed by Muir et al. Based upon their findings they conclu- ded that parametric arrays are competitive and in many cases superior to un-focused transducers used in diagnosis employing contact scanning, but that better angular resolution may be obtained using conventional focused systems. This paper continues the adaptation of parametric arrays to medical diagnosis originally introduced by Muir et al, and

it extends the investigations also to comprise the influence of acoustic lenses on parametric beam patterns.

2. THEORETICAL CONSIDERATIONS. - The use of a me- dium's nonlinearity to generate intermodulation products when two or more frequencies are mixed has during a long time been recognized as an essential procedure for the establishment of sound beams with a series of advantageous qualities. The absorption

length L of the primary waves in a parametric acoustic array determines the narrow beamwidth of the difference-frequency sound beam and it provides for an exponential shading taper which removes the undesirable minor lobes from the difference-fre- quency directivity pattern / 4 / . If the absorption

Article published online by EDP Sciences and available at

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979820

JOURNAL DE PHYSIQUE

l o s s i n the n e a r f i e l d o f t h e primary waves r a d i a - t e d by a p i s t o n source i s h i g h enough t o ensure t h a t primary wave i n t e r a c t i o n s predominantly take p l a c e w i t h i n t h e Rayleigh distance Rr from t h e primary wave source t h e a b s o r p t i o n l i m i t e d a r r a y thus formed i s comprised b y W e s t e r v e l t ' s s o l u t i o n / 5 / which gives f o r t h e f a r f i e l d d i f f e r e n c e - f r e - quency sound pressure amplitude ps on t h e acous- t i c a x i s a t a source d i s t a n c e R :

where fs i s t h e d i f f e r e n c e frequency, po deno- t e s t h e pressure amplitude o f each o f t h e primary waves and S denotes t h e r a d i a t i n g area of t h e

primary wave source. B = 1

+

B/2A represents t h e i n f l u e n c e o f t h e medium n o n l i n e a r i t y w h i l e p0 and c denote the d e n s i t y and t h e sound v e l o c i t y , r e s p e c t i v e l y , o f t h e f l u i d i n t o which t h e primary waves are radiated. Each p r i m a r y wave i s assumed t o have t h e same absorption c o e f f i c i e n t

a.

.

I n an ampl itude-modulated primary wave f i e l d t h e i n t e r a c t i o n between t h e sidebands and t h e car- r i e r produces a difference-frequency component a t t h e modulation frequency. The f a r f i e l d pressure amplitude o f t h i s component has been shown t o be 2.5 dB h i g h e r than t h e pressure amplitude achieved w i t h a two-component primary f o r t h e same t o t a l power i n the primary s i g n a l /6/, /7/.

The ampl itude-modulation produced difference- frequency s i g n a l pressure amplitude a t t h e source distance R on t h e a c o u s t i c a x i s i s :

2 2 fS.a.po.S.B.6

IP,I

=

2.p . c 4 . ~ . a (2

0 0

where 6 i s a measure o f t h e amount o f modulation (6 = 1 f o r 100 % modulation).

I t may be seen from equations (1) and ( 2 ) t h a t an increase i n t h e v i r t u a l source s t r e n g t h and thus i n ps may be obtained p r i m a r i l y b y t h e f o l l o w i n g procedures. By i n c r e a s i n g t h e amplitude o f t h e c a r r i e r waves, t h e occurence o f a c o u s t i c s a t u r a t i o n w i l l form t h e upper l i m i t , o r b y u s i n g a l i q u i d i n t h e primary wave i n t e r a c t i o n r e g i o n showing a h i g h e r 8-value, a lower P-value o r i n p a r t i c u l a r a lower c-value. Improved conversion e f f i c i e n c i e s along these 1 ines have been s t u d i e d

t h e o r e t i c a l l y and experimentally i n /7/. These ex- periments showed t h a t methanol and ethanol l e d t o considerably increased values of ps

.

For t h e adaptation o f the parametric a r r a y t o be used i n an echoscanner f o r medical diagnosis n o t o n l y t h e difference-frequency source l e v e l , b u t a l s o i t s beanwidth have t o be based on a trade- o f f i n v o l v i n g t h e s e l e c t i o n o f t h e r i g h t m a t e r i a l s f o r t h e primary wave i n t e r a c t i o n region. The a r r a y l e n g t h should n o t be t o o g r e a t i n o r d e r t o reduce t h e f i n a l dimensions o f t h e scanner, b u t t o o s h o r t an a r r a y w i l l increase t h e beanwidth according t o t h e expression f o r t h e half-power beamwidth

o h

i n t h e difference-frequency beam given by :

w i t h ks b e i n g t h e wave nunber i n t h e d i f f e r e n c e - frequency s i g n a l and L i s t h e a r r a y l e n g t h given by : L = (al

+

^a

-

^us)"

.

I n t h i s expression al

0

i s t h e a b s o r p t i o n c o e f f i c i e n t a t t h e lowest side- band frequency i n t h e ampl itude-modul a t e d primary wave spectrum. a s denotes t h e absorption c o e f f i -

c i e n t f o r t h e difference-frequency s i g n a l . An i n - creased beamwidth may l e a d t o a reduced r e s o l u t i o n capabi 1 i t y o f t h e scanner.

Some l i q u i d s o f known p h y s i c a l q u a l i t i e s t o be used i n t h e i n t e r a c t i o n r e g i o n are given i n Table 1. The c a l c u l a t i o n o f t h e a b s o r p t i o n coef- f i c i e n t s i s performed f o r 8 MHz (a1), 10 MHz (ao) and 2 MHz (a,), r e s p e c t i v e l y . It i s moreover assu- med t h a t t h e Rayleigh distance Rr should a r b i - t r a r i l y be s e t t o : Rr = 2L, i n order t o approach an absorption 1 im i t e d array. The values o f Rr thus obtained d i c t a t e t h e necessary transducer diameter a t t h e c a r r i e r frequency fo = 10 MHz.

Table I 1 gives c h a r a c t e r i s t i c features o f t h e pa- r a m e t r i c a r r a y f o r t h e difference-frequencies : fs = 0.5, 1.0, 2.5, 3.0 and 4.0 MHz i n methanol.

It should be emphasized t h a t t h i s l i s t o f f l u i d s t o be used i n t h e i n t e r a c t i o n r e g i o n i s i n no way exhaustive.

The l a r g e depth o f t h e f i e l d t o be a n t i c i p a - t e d by t h e use o f parametric arrays f o r medical echoscanning should a l s o be accompanied by an im- proved l a t e r a l r e s o l u t i o n . This improvement can b e achieved by t h e use o f an a c o u s t i c lens. A broad v a r i e t y o f lens m a t e r i a l s have been suggested and t e s t e d d u r i n g t h e p a s t /2/, /8/, /9/. Based on t h e

L. BJ$RN0 and S. GRINDERSLEV

Table I.

-

Physical q u a l i t i e s of liquids t o be used i n the primary wave interaction region.

Temperature 20' C. The half-power beam-width of the difference-frequency signal i s determined From :

o h

= ~ ( Z L ~ L ) - ~ / ~ and the half-power beanwidth of the c a r r i e r wave (10 MHz) i s deter- mined from : o = Z S ~ ~ ' ~ ( ~ . ~ . C / ( I T . D . ~ ~ ) .

desire of impedance matching between the l i q u i d i n liquids t o be used i n t h e interaction and t i s s u e the interaction region and the liquid simulating regions. Using standard formulas / l o / a planocon- biological t i s s u e and due t o i t s r a t h e r low u l t r a - vex lens having a radius of curvature Of 36

mn

sonic absorption even a t frequencies around 2 MHz was calculated and produced from RTV-118. The fo- a s i l i c o n e rubber, type RTV-118, produced by Gene- cal volume of t h i s lens i s e l l i p s o i d a l showing ral E l e c t r i c was selected as lens material. Moreo- half-power l i m i t s f o r the difference-frequency ver, R'JV-118 will not react chemically with most signal expressed approximately by :

m e t h a - no 1

2 . 2 5

3 . 5 1

0.14

1 1 0 3

7 9 0

8 . 7 1

9 . 6 1

1 1 3 9 3

0 . 1 7 8

0 . 3 5 6

7 . 0 7

3.60

0 . 9 2 a1 (8MHz)

(Neper/m) a(, (l0MHZ)

(FJ e p e r / m ) as (2MHz 1 ( ~ e p e r / m )

C

P

( kg/m3)

z

( 1 0 ~ . k g / s * m ~ )

B/ A

s (m-')

L ( m

Rr ( m

D (mm

eh ( d e g r e e s )

-

0

( d e g r e e s )

c a s t o r o i l

6 . 9 8

1 0 . 9

0 . 4 4

1 5 4 0

9 5 0

1 4 . 6 3

( 9 . 2 )

8 1 6 0

0 . 0 5 7

0 . 1 1 4

4 . 7 3

7.49

1 . 9 1

etha- no 1

4 . 1 2

6 . 4 3

0 . 2 6

1 2 0 7

7 9 1

9 . 5 5

1 0 . 5 2

1 0 4 1 1

0 . 0 9 7

0 . 1 9 4

5 . 4 6

5.10

1 . 3 0 w a t e r

1 . 2 9

2 . 0 2

0 . 0 8

1 4 8 0

998

1 4 . 7 7

5.0

8 4 9 1

0 . 3 1

0 . 6 2

1 0 . 8 0

3.16

0 . 8 0 a l y - c e r o l

1 2 1 . 3

1 8 9 . 6

7 . 5 8

1 9 0 4

1 2 6 0

2 4 . 0 0

8 . 8

6600

0 . 0 0 3 3

0 . 0 0 6 6

1 . 2 6

3 4 . 7 4

8 . 8 8

C8-114

JOURNAL DE PHYSIQUE

Table 11. Parametric a r r a y data i n methanol f o r various d i f f e r e n c e frequencies.

i n t h e l a t e r a l d i r e c t i o n and by :

i n depth. As and F denote t h e d i f f e r e n c e - f r e - quency wavelength i n t h e image-side medium and the f o c a l l e n g t h o f t h e l e n s , r e s p e c t i v e l y , w h i l e A i s t h e 3 dB

-

diameter o f t h e sound beam a t t h e l e n s . b i s a f a c t o r dependent upon the a p e r t u r e ha1 f - a n g l e o f t h e lens

/lo/.

Focusing w i l l o f course l e a d t o t h e c l a s s i - c a l d i l e m a o f t r a d i n g depth o f f i e l d f o r f o c a l spot diameter, i .e. l a t e r a l r e s o l u t i o n . F o r an e f - f e c t i v e lens diameter Ae t h e l e a s t - r e s o l v a b l e s e p a r a t i o n on t h e f o c a l plane due t o Fraunhofer d i f f r a c t i o n i n f l u e n c e i s given by /9/ :

dmin = 1.22.F.Xs/Ae (6 )

G e n e r a t o r

(amln i s t h e r a d i u s o f t h e f i r s t dark r i n g o f the Fraunhofer d i f f r a c t i o n p a t t e r n ) , w h i l e t h e u l t i m a t e r e s o l u t i o n by a given d i f f e r e n c e frequency fS i s expressed b y :

3. EXPERIMENTAL EQUIPMENT AND PROCEDURE.

-

^{The pa-}

r a m e t r i c echoscanner s t u d i e d i n v o l v e d a parametric t r a n s m i t t e r and a l i n e a r r e c e i v e r . A p i s t o n t r a n s - ducer, 7.1 mn i n e f f e c t i v e diameter and w i t h a na- t u r a l frequency o f 10 MHz t r a n s m i t t e d a 100 % am- p l itude-modulated c a r r i e r s i g n a l (10 MHz) w i t h t h e modulation frequency b e i n g equal t o t h e d i f f e r e n c e frequency. A d i f f e r e n c e frequency o f 2 MHz, a f r e - quency normally used i n u l t r a s o n i c medical diagnos- t i c s , was used f o r most i n v e s t i g a t i o n s performed, b u t fs = 0.5, 1.0 and 4.0 MHz were a l s o studied.

A 38/18.3 mm diameter annulus r e c e i v i n g transducer o f n a t u r a l frequency 2 MHz e n c i r c l e d t h e transmit- t e r . The b l o c k diagram i n F i g u r e 1 shows t h e elec- t r o n i c s used f o r t r a n s m i t t i n g and r e c e i v i n g o f t h e parametric s i g n a l s .

Measurements o f t h e d i f f e r e n c e -frequency sound pressure l e v e l s i n t h e parametric beam were performed i n various l i q u i d s

-

see Table I

-

^con-

t a i n e d i n a tank

-

^{60 x 35 x 78 cm}

-

w i t h glass w a l l s a l l o w i n g s c h l i e r e n s t u d i e s o f t h e beam pat-

t e r n s t o be performed. Reduction o f t h e i n f l u e n c e o f r e f l e c t e d and standing waves was achieved through a ' p a r t l y l i n i n g o f the tank w i t h rubber- r i z e d h o r s e h a i r mats.

Sound pressure l e v e l 1 s were measured unsing a m i n i transducer /11/ a u t o m a t i c a l l y b e i n g moved through t h e p a r a m e t r i c beam b y means o f a p o s i t i o - n i n g device governed b y a p r e s e t indexer, thus making p o s s i b l e automatic r e g i s t r a t i o n o f beam

O s c 1 1 1 0 s c o ~ e P o w e r

~ h l l l p s + r n p l l f i e r

Y o d e l 3200 T y p e P* 3231 E N 1 Type 240L PF

I I

I M o t o r i z e d

Oscilloscope

I

H y d r o p h o n e

P h i l i p s T r a n s m l s s r o n

PI1 3 2 3 2

P

XY-P!rlter T y p e I1P 3 1 2 A FNI Type 300L R r

S e r V O q o r

Fig. 1

-

E l e c t r o n i c s used f o r s i g n a l processing on t h e t r a n s m i t t i n g and r e c e i v i n g s i d e o f t h e echoscanner.

L. BJZRNB and S. GRINDRSLEV

p a t t e r n s a t preselected p o s i t i o n s along t h e parame- tri c a r r a y a x i s

.

F r e e - f i e l d measurements'of t h e d i f f e r e n c e - frequency s i g n a l beam p a t t e r n s and sound pressure l e v e l s w i t h o u t the use o f an a c o u s t i c l e n s were performed i n t h e glass-wall ed tank. Some e x p e r i - ments u s i n g t h e same l i q u i d on b o t h sides o f t h e l e n s were a l s o done i n t h i s tank, b u t f o r t h e reso- l u t i o n measurements i n v o l v i n g d i f f e r e n t l i q u i d s on t h e two l e n s sides a s p e c i a l polyethylene tube w i t h an i n n e r diameter o f 68 mm i n t e r n a l l y supplied w i t h a 3 mm t h i c k l a y e r o f an absorbing l i n i n g

(corprene) was used. The tube l e n g t h was 335 mm and t h e t o t a l d i s t a n c e between t h e t r a n s m i t t e r sur- face and t h e a c o u s t i c l e n s forming a window a t t h e end o f t h e tube was equal t o the Rayleigh d i s t a n c e i n ethanol

.

Castor o i l o r glycerine-water m i x t u r e s were p r e l i m i n a r y considered f o r s i m u l a t i n g t h e physical qual i t i e s o f b i o l o g i c a l t i s s u e . Apart from a b s o r p t i o n these l i q u i d s showed a too g r e a t d e v i a t i o n , i n f o r i n s t a n c e s p e c i f i c a c o u s t i c impe- dance, from e s s e n t i a l physical qual i t i e s o f b i o l o - g i c a l t i s s u e , Water was f i n a l l y s e l e c t e d t o simu- l a t e b i o l o g i c a l t i s s u e p r o p e r t i e s among o t h e r t h i n g s due t o i t s s p e c i f i c a c o u s t i c - impedance and i t s transparency t o 1 ig h t .

Measurement o f the beam p a t t e r n s throughout t h e f o c a l r e g i o n o f t h e l e n s was performed u s i n g t h e m i n i transducers. Both CW and pulsed mode ope- r a t i o n s were studied.

I n order t o determine t h e s m a l l e s t area t h e r e f l e c t i o n from which c o u l d be r e c e i v e d by t h e echoscanner, s h o r t s t a i n l e s s s t e e l rods o f diame- t e r s 1, 1.5 and 2 mm were p o s i t i o n e d i n the para- m e t r i c beam w i t h t h e r o d a x i s on and p a r a l l e l w i t h t h e beam a x i s . R e f l e c t i o n measurements w i t h and w i t h o u t t h e use o f t h e a c o u s t i c l e n s were p e r f o r - med i n order t o study t h e s e n s i t i v i t y o f the scan- ning system.

4. EXPERIMENTAL RESULTS AND DISCUSSION.

-

^The

sound pressure l e v e l as a f u n c t i o n o f distances from t h e source measured on the a c o u s t i c a x i s i n t h e 10 MHz c a r r i e r wave i n ethanol i s shown i n F i g u r e 2. This f i g u r e a l s o shows a t t e n u a t i o n cour- ses c a l c u l a t e d f o r t h e i n f l u e n c e o f e i t h e r spheri- c a l divergence o r 1 in e a r a b s o r p t i o n alone. F i g u r e 2, moreover, shows t h e 2 MHz d i f ference-frequency sound pressure l e v e l measured on t h e a c o u s t i c a x i s a t v a r i o u s distances from t h e p i s t o n source. The

:+"? f,= 2 MHz

...."

^(focused)

Fig. 2.

-

Primary and secondary sound pressure l e - v e l s as a f u n c t i o n o f transducer d i s t a n c e measured i n ethanol f o r fo = 10 MHz and fs = 2 MHz.

i n t r o d u c t i o n o f t h e a c o u s t i c l e n s leads t o an i n - crease i n t h e d i fference-frequency sound pressure l e v e l i n the f o c a l p o i n t o f around 15 dB compared t o t h e unfocused a r r a y sound pressure l e v e l . This improves t h e s i g n a l / n o i s e r a t i o o f the r e c e i v e d s i g n a l d u r i n g t h e scannings.

Table I 1 1 shows a comparison between t h e mea- sured and c a l c u l a t e d (using Eq. (3)) half-power beamwidths o f t h e 10 MHz c a r r i e r wave, t h e 2 MHz d i f ference-f requency wave and of a 1 i n e a r l y produ- ced 2 MHz CW s i g n a l generated by the p i s t o n

I

h a l f p o w e r b e a m w i d t h

( d e g r e e s )

1

Table 111.

-

Half-power beamwidths f o r t h e 10 MHz wave ( c a r r i e r ) and f o r t h e 2 MHz p a r a m e t r i c a l l y and l i n e a r l y generated.

2.0

MHz

p r i m a r y 1 0 . 0

MHz p r i m a r y 2 . 0

MHz d i f f e r e n c e

c a l c u -

l a t e d

5 . 0

1 . 0

5.1

mea-

s u r e d

4 . 7

1 . 0

2 . 1

~8-11.6 JOURNAL DE PHYSIQUE

source, a l l measured a t a transducer d i s t a n c e R = 0.51 m. The i n t r o d u c t i o n o f t h e a c o u s t i c lens, strong1 y reduces t h e ha1 f -power beamwi'dth o f t h e 2 MHz d i f f e r e n c e - f r e q u e n c y s i g n a l and thus i n c r e a - ses t h e angular and l a t e r a l r e s o l u t i o n s , which can be seen i n F i g u r e 3. A t a d i s t a n c e R = 0.51 m from

mm

25 20 15 10 5 0 5 10 15 20 25

0 0

- 15 -15

3 2 1 0 1 2 3

degrees

Fig. 3.

-

Unfocused and focused beam p a t t e r n o f t h e 2 MHz d i f f e r e n c e - f r e q u e n c y s i g n a l

.

Table I V .

-

Measured and c a l c u l a t e d 3 d B - l i m i t s ( r e s o l u t i o n l i m i t s ) f o r t h e f o c a l r e g i o n f o r va- r i o u s fs-values i n methanol/water and ethanol/etha- no1 systems. Measured sound pressure l e v e l gains a r e a l s o given.

~neth/water 2.0 MHz

e t h / e t h

t h e transducer, i . e . a t t h e f o c a l d i s t a n c e i n t h e ethanol/ethanol system,

en

i s reduced from 2.1 t o 0.5 degrees which leads t o a s t r o n g l y increased l a t e r a l r e s o l u t i o n from 18.5 t o 4.6 mm.

The difference-frequency f S = 2 MHz was cho- sen from conventional b i o l o g i c a l scanning p r a x i s i n v o l v i n g ultrasound. Other f s - v a l u e s c o u l d j u s t as easy be produced by t h e parametric echoscanner due t o i t s frequency a g i l i t y . Table I V shows a com- p a r i s o n between c a l c u l a t e d and measured l a t e r a l and depth diameters (3 dB l i m i t s ) o f t h e e l l i p s o i d a l f o c a l r e g i o n f o r various fs-values, 0.5, 1.0, 20 and 4.0 MHz i n methanol/water and ethanol/ethanol systems. The c a l c u l a t e d values a r e obtained using t h e Eqs. (4) and ( 5 ) . A good agreement between theo- r e t i c a l and experimental r e s u l t s seemsto be present, and i n s p i t e of t h e r e d u c t i o n i n t h e depth o f t h e f i e l d introduced by t h e lens an a p p r o p r i a t e f i e l d depth i s preserved and a s t r o n g l y improved 1 a t e r a l r e s o l u t i o n i s achieved,in p a r t i c u l a r a t d i f f e r e n c e frequencies of 2 and 4 MHz.Table I V , moreover, com- p r i s e s measured values of t h e g a i n i n sound pressure l e v e l f o r v a r i o u s f s - v a l u e s using t h e a c o u s t i c lens.

fs = 2 MHz here leads t o t h e s t r o n g e s t improvement.

One o f t h e g r e a t advantages by t h e use o f parametric a r r a y s are t h e absence o f s i d e lobes i n t h e difference-frequency beam p a t t e r n . This "clean"

beam p a t t e r n w i l l a f t e r having been focused show a v e r y narrow beam i n t h e f o c a l r e g i o n . The beamwidth w i l l amo-ng o t h e r t h i n g s depend on t h e d i f f e r e n c e - frequency used and thus on t h e o r g i n a l unfocused beam-width and a l s o on t h e systems o f l i q u i d s be- f o r e and a f t e r t h e lens as shown i n F i g u r e 4. This f i g u r e shows measured beam p a t t e r n s a t fs = 0.5, 1.0, 2.0 and 4.0 MHz i n t h e methanol/water system.

The measurements were performed i n t h e f o c a l plane of t h e a c o u s t i c lens. As i t i s t o be expected t h e b e s t angular and l a t e r a l r e s o l u t i o n o f % 2mm (3 dB- beamwidth) i s achieved by t h e h i g h e s t f s - v a l u e i n t h e methanol/water system.

I n o r d e r t o o b t a i n a p r e l i m i n a r y expression f o r t h e s e n s i t i v i t y o f t h e echoscanner, i . e . what i s t h e s m a l l e s t o b j e c t surface o f a g i v e n s p e c i f i c a c o u s t i c impedance which can be c l e a r l y seen i n t h e f o c a l r e g i o n using pulsed mode parametric echoscan- ning, a s e r i e s o f small s t a i n l e s s s t e e l rods were p o s i t i o n e d c o - a x i a l w i t h t h e parametric beam i n t h e methanol/water system. Rod diameters on 2.0, 1.5 and 1.0 mm were used. F i g u r e 5 shows t h e t r a n s m i t - t e d and r e c e i v e d pulses r e f l e c t e d from t h e r o d end

5.9 4 . 6 1 4 3 85 1 5

L. BJpRN0 and S. GRINDRSLEV c8-117

-

0

surfaces f o r f S = 2

MHz.

Displacements o f t h e rods t o o t h e r p o s i t i o n s w i t h i n t h e f o c a l r e g i o n o f t h e parametric beam showed t h e same r e f l e c t i o n f e a t u r e s . Pulsed l i n e a r o p e r a t i o n o f t h e p i s t o n source a t 2 MHz d i d n o t show any d i s t i n g u i s h a b l e r e f l e c t e d s i g n a l n e i t h e r by t h e use o f the acous- t i c l e n s nor w i t h o u t t h e l e n s . Pulsed parametric o p e r a t i o n o f t h e system w i t h o u t t h e i n s e r t i o n o f

--

³ t h e a c o u s t i c l e n s a l s o d i d n o t show any d i s t i n - m u gui shable r e f l e c t e d s i g n a l . These promising focu-

I sed paremetric a r r a y r e s u l t s may be even b e t t e r

W cn

z by a s t r o n g l y improved s i g n a l / n o i s e r a t i o o f t h e a 0 echoscanner. This i s now being studied.

m

diameter equal t o 1 .5 mm F i g . 4.

-

Measured difference-frequency beam p a t -

t e r n s a t v a r i o u s f s - v a l u e s i n methanol/water system.

diameter equal t o 2 mm diameter equal t o 1 mm

F i g . 5 .

-

Transmitted (upper beam) and r e c e i v e d pulses r e f l e c t e d from t h e end s u r f a c e o f s t a i n - l e s s s t e e l rods o f v a r i o u s diameter p o s i t i o n e d on-axis i n the f o c a l r e g i o n o f t h e parametric beam.

JOURNAL DE PHYSIQUE

5. CONCLUSIONS,

-

The i n h e r e n t l a c k o f s i d e lobes by t h e p a r a m e t r i c a l l y generated d i f f e r e n c e - f r e - quency beam p a t t e r n s i s preserved d u r i n g f o c u s i n g through an a c o u s t i c lens. The "clean" s i g n a l thus obtained s t r o n g l y reduces t h e s c a t t e r i n g from bo- d i e s (impedance changes) o u t s i d e t h e f o c u s i n g r e - g i o n compared t o t h e unwanted s c a t t e r i n g normally observed by t h e use o f conventional ( l i n e a r ) u l - t r a s o n i c scanning transducers u s i n g an a c o u s t i c lens o r a changed geometrical shape o f t h e cera- mics f o r the beam forming. The experiments showed t h a t t h e use o f a parametric a r r a y w i t h o u t t h e i n - s e r t i o n o f an a c o u s t i c lens w i l l n o t l e a d t o any

s i g n i f i c a n t improvements i n the angular and l a t e - r a l r e s o l u t i o n irr s p i t e o f t h e excel l e n t depth o f t h e acoustic f i e l d . The t r a d e - o f f , reducing t h e f i e l d depth i n order t o o b t a i n a s t r o n g l y improved angular and l a t e r a l r e s o l u t i o n by i n t r o d u c t i o n o f t h e l e n s i n t o t h e difference-frequency beam, i s necessary. The encouraging r e s u l t s so f a r obtained should i n s p i r e f u r t h e r work t o be done, f o r i n s - tance i n order t o reduce t h e dimensions o f t h e scanner by t h e use o f o t h e r i n t e r a c t i o n r e g i o n li- quids and t o improve t h e s i g n a l / n o i s e r a t i o o f t h e scanning system.

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244, IPC Science & Technology Press Ltd., G u i l d f o r d , England, 1975.

/5/ Westervel t P. J., "Parametric a c o u s t i c a r r a y s " . J . Acoust. Soc. Amer., Vol. - 35, 1963, 535

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35, (2), 1976, 99-106.

/8/ Folds D.L., "Progress r e p o r t on u l t r a s o n i c lens deve- lopment". ~ r o d e e d i n ~ s o f ' t h e Sate1 1 i t e Symposium on Underwater Acoustics, U n i v e r s i t y o f Birmingham, 1974.

/9/ Tannaka Y. & Koshikawa T., " S o l i d - l i q u i d compound hy- droacoustic l e n s o f low aberation'. J. Acoust. Soc.

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/11/ Lewin P.A., "Threshold f o r u l t r a s o n i c exposure o f b i o l o g i c a ! t i s s u e and t h e b i o l o g i c a l e f f e c t s o f u l - trasound". Department o f F l u i d Mechanics, Technical U n i v e r s i t y o f Denmark, I n t e r n a l Report. AFM 78-04, 1978.