HIGH-RESOLUTION, HIGH-CONTRAST ACOUSTIC IMAGING

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HIGH-RESOLUTION, HIGH-CONTRAST ACOUSTIC IMAGING

J. Cunningham, C. Quate

To cite this version:

J. Cunningham, C. Quate. HIGH-RESOLUTION, HIGH-CONTRAST ACOUSTIC IMAGING. Jour-

nal de Physique Colloques, 1972, 33 (C6), pp.C6-42-C6-47. �10.1051/jphyscol:1972609�. �jpa-00215127�

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MGH-RESOLUTION, HIGH-CONTRAST ACOUSTIC IMAGING (*)

J. A. CUNNINGHAM and C. F. QUATE

Stanford University, Stanford, California, U. S. A.

R6sum6. ^-Nous presentons, dans cet article, deux dispositifs qui donnent une imagerje acoustique de haute rtsolution et de fort contraste. Dans le premier, l'objet est irardie par deux faisceaux acoustiques croises, les champs propages formant un systeme d'ondes stationnaires modulees par les propri6tks de transmissions de l'objet. Dans le second, un faisceau acoustique unique tombe normalement sur l'objet qui, 18 encore, modifie les champs transmis. Dans les deux dispositifs, une couche mince d'une emulsion de petites spheres de latex dans un liquide detecte non lineairement ces ondes transmises dans le champ, tout pres de l'objet, pour former une image.

Le mtcanisme primaire de la detection est la pression de radiation exercee sur les sphkres.

Nous presentons des resolutions supMeures a 10 microns pour des densites de puissance modeste de 10-3 d 10-2 wattJcm2. Nous pouvons obtenir une image de specimens biologiques avec un assez bon contraste, ce qui donne une application pratique de ce dispositif d'imagerie.

Abstract.

-

In this paper we present two systems capable of high-resolution, high-contrast acoustic imaging. In the first system, the object is illuminated by two intersecting acoustic beams, the transmitted fields forming a standing-wave pattern modulated by the object's transmission properties. In the second, a single acoustic beam is normally incident upon the object which again modifies the transmitted fields. In both systems a thin film emulsion of small (1 micron) latex spheres in liquid nonlinearly detects these transmitted waves in the very near field of the object to form an image. The primary detection mechanism is radiation pressure exerted on the spheres.

We demonstrate system resolutions better than 10 microns at moderate power levels of 10-3 watt/cmz to 10-2 watt/cm2. We are also able to image biological specimens with rather good contrast, one practical application of such an imaging system.

1. Introduction.

-

We wish to discuss our work on high frequency acoustic imaging where the resolution is comparable with that obtained with an optical system. There are two primary advantages of an acoustic imaging system when it is compared to its optical counterpart. First, optically opaque objects can be probed rather easily with acoustic waves. Second, and perhaps more important, higher contrasts can be achieved for those objects which are nearly transparent to optical waves. This comes about since the change in elastic properties over the cross section of the object is much greater than the corresponding change in the index of refraction. The research that has been carried out on acoustic imaging and the realization of the above goals have been reviewed recently by Mueller [l].

The work has traditionally been divided between acoustic holography and direct acoustic imaging.

In either system the method used to convert the acoustic patterns to a visible display is the central problem.

We have previously demonstrated [2] a system for high-resolution acoustic holography which consists of a reference beam and a beam as scattered by the object. These two acoustic beams intersect at

(*) This work was supported by a grant from the John A. Hartford Foundation, Inc.

a solid surface which contains the imaging film. The two intersecting beams are quite analogous to the optical system for holography [3], but the imaging film is quite different from photographic film. Our imaging film is an emulsion of small latex particles suspended in liquid. The forces exerted on these spheres, or particles, by the two acoustic beams condense the particles into a pattern that reproduces the acoustic hologram. The holograms as recorded in this way, are then reconstructed by conventional optical methods. We believe that this application of

ct acoustic radiation pressure ⁾⁾ is most appropriate for a colloquium in honor of Paul Langevin, who himself made original contributions to this particular branch of physical acoustics [4].

In this report we want to present a continuation of our earlier work [2]. We use the same technique for imaging (as described below) but we replace the large crystals used in the holography experiment with a thin mylar film. The mylar film supports the emulsion of suspended particles and, since the imaging region is very near the object, it is reminiscent of the techniques used for ⁽⁽contact printing ^>)in photo- graphy. In this new system we are not able to introduce a reference beam that is separate from the beam as scattered by the object and, as a result, we do not form holograms. However, the images are interesting

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1972609

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HIGH-RESOLUTION, HIGH-CONTRAST ACOUSTIC IMAGING C6-43 in themselves and the resolution is higher than that and

reported previously. The determining factor for resolution is the wavelength of the sound waves in the liquid object cell and for a fixed frequency the wavelength in liquid is much smaller than that in a solid.

In our research we have developed two rather simple systems with the required acoustic imaging capabilities of high resolution and high contrast.

In the first system, the object is illuminated by two intersecting acoustic beams, the transmitted fields forming a standing-wave pattern modulated by the object's transmission properties. In the second system a single acoustic beam is normally incident upon the object, the transmitted fields again being modified by the object's transmission properties. Both imaging systems rely on the nonlinear detection, in the near field, of these transmitted fields. The detector again is a thin liquid film in which small (1 micron) polystyrene spheres are suspended. We believe the main detection mechanism to be the radiation pressure on these spheres with a contributing effect of acoustic streaming created in the liquid. Utilizing these systems we demonstrate resolutions better than 10 microns at sensitivities on the order of watt/cm2 to

watt/cm2.

2. Theoretical considerations.

-

The general theoretical basis for both our imaging systems is illustrated in figure 1. In our system the particles respond to the

FIG. 1.

-

General acoustic imaging geometry.

acoustic intensity within the film. We will proceed with the calculation of this parameter for the situation of figure 1. In the two-beam system the object, charac- terized by its complex transmission function s(x, y, 0), is illuminated by a plane acoustic beam U,(x, z) with wavevector k, at an angle 8, and another beam U2(x, z) with wavevector k2 at an angle 8,. Considering the time factor e-'Or to be understood for all field quantities we have

U,(x,z) = U,exp[jk,(- sin8,x

+

cos8, z)], (1) U2(x, Z) = U2 exp[jk2(sin 82 x

+

cos 82 x)] , (2)

Since the two beams are coherent, we have for the total incident field,

In our cases we have chosen k, = k2 = k and 8, = 82 = 8. If we now make the reasonable assump- tion that U, = U2 = U, then we have for the incident field

Ui(x, z) = U (exp[jk(- sin Ox

+

cos dz)]

+

exp[jk(sin Ox

+

^{cos 8z)l}

^} .

(5) Since we are going to detect in the very near field of the object, the transmitted acoustic field may be written to a good approximation as

Ut(x, y, Az) s U

{

exp[jk cos Az]

)

x

(

exp[jk sin 8x1

+

exp[- jk sin Ox]

)

s(x, y, 0) (7) where s(x, y, 0) is defined in eq. (3). Hence,

U,(x, y, Az) E 2 U

(

exp[jk cos Az]

)

x

x cos (k sin e x ) s(x, y, 0) exp[- jq(x, y, 0)]

.

(8) The acoustic intensity detected is then given by I(x, y, Az) r

I

Ut(x, y, Az)

1'

^z

2 U' cos2 (k sin Ox) s2(x, y, 0) (9) where the factor of

4

accounts for averaging over time. Using the identity cos2 a = $(I

+

^{cos 2}^a)

we finally obtain

I(x, y, Az) r U2[1

+

cos (2 k sin Ox)] s2(x, y, 0)

.

(10) The main feature of the intensity distribution of eq. (10) is the standing wave along the x-axis with a fringe spacing of

Ax = n/2 k sin 8 = 112 sin 8

.

(1 1) This, of course, is superimposed upon the image intensity distribution s2(x, y, 0).

The forces on the immersed particles are related to this intensity. In a classical paper 151, King has calculated the force on a rigid sphere in a standing acoustic wave. For our case k, a = 0.583, where a is the radius of the spheres, k, = k sin 8, and k is the wavenumber in the solid. The appropriate radiation pressure equation is then

where

(4)

and c is the speed of sound in the fluid, p, and p, being the respective densities of the spheres and liquid.

King has shown that for F(p,/p,)

>

0 the spheres will move toward a velocity loop of the standing wave and for F(p,/p,) < 0 toward a velocity node.

This theory has been extended to include the compres- sibility of the sphere, o = c,/c, = k,/k,, by Yosioka and Kawasima [6] to yield

where now

The same conditions apply to F(pl/po, o) as did to F(p,/p,). For the polystyrene spheres in solution used in our experiments, both theories yield a particle distribution corresponding to that expected from eq. (10).

The situation for single-beam imaging is similar to that of figure 1, but now we have

For the transmitted field this yields

and hence an intensity distribution given by

I(x, y, Az) r

3 u2

s2(x, y, 0)

.

(1 8) In this case we must consider the radiation pressure on a sphere due to a plane progressive wave, provided that we neglect the diffraction that takes place in the distance Az. At the frequency of 1.1 GHz used for single-beam imaging ka = 2.58. Fox [7] has extended King's theory for radiation pressure due to a plane progressive wave to include this range yielding

for pl/p,

--

1. The above analysis will be considered again in a later section when we discuss system sensitivity.

3. The water cell and image detector.

-

The water cell and acoustic detection film are shown in the inserts of figures 2 and 5. In both imaging systems the object to be viewed is immersed in the thin water cell. The acoustic beam or beams are passed into this cell by means of acoustic anti-reflection coatings.

On top of this cell is stretched a 5 micron mylar film which is acoustically matched to the liquid surroundings. It has an optical anti-reflection coating to avoid direct viewing of the object. This film serves

1

l p m P O L Y S T Y R E N E B E A D S

7) \

I N S O L U T I O N

WATER C E L L

C O A T I N G S

FIG. 2. - Experimental arrangement for two-beam imaging.

the purpose of separating the object space from the detector space. On top of this film a lucite disk uni- formly compresses an emulsion of 5

%

by volume 1 micron polystyrene spheres in a 90 %-I0

%

water- glycerine solution. The acoustic image is recorded as a redistribution of these spheres on a 1 : 1 basis.

Magnification is then obtained optically by viewing this distribution with a microscope.

4. Experimental results.

-

The experimental arrangement used in our two-beam imaging system is shown in figure 2. Two plane acoustic beams at 260 MHz

(A

= 23 microns) are generated in the fused quartz prism by matched Y/350 LiNbO, plate transducers and intersect at the surface at an angle 6 = 450.

This determines a linear fringe spacing in the intensity pattern of 16.2 microns as predicted by eq. (11).

The complex transmission function, s(x, y, 0), of the object determines the amplitude of the fringes.

It should also be noted here that since the acoustic fields are passed into the detecting film by means of a mylar membrane, a standing wave pattern is also set up in this membrane. Acoustic streaming effects of the type noted by Jackson and Nyborg [8] could therefore be a contributing "factor in this imaging system. A resultant limiting flow velocity similar to that of their eq. (13) would assist in moving particles toward velocity loops of the standing wave pattern.

One of the objects imaged in this system is a nickel mesh with rather ragged circular holes as shown optically in figure 3a. The acoustic image is shown in figure 3b with the superimposed 16.2 pm fringe pattern. Here the object is totally transmissive.

The raggedness of the holes is evident in the acoustic image. We also used a square 75 mesh (E 3 linejmm) in this system, the optical image of which is shown in figure 4a and its acoustical counterpart in figure 4b.

The image quality is good and the fringe pattern stands out clearly. This image demonstrates a resolution of at least 60 microns. An attempt to image a 500 mesh (- 20 lineslmm) showed that ambiguities result in the image when the object has the same periodicity as the standing wave pattern. No attempt has been made to improve image quality by removing the fringes, but this can be easily accomplished by optical spatial filtering [9].

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HIGH-RESOLUTION, HIGH-CONTRAST ACOUSTIC IMAGING C6-45

FIG. 3. - Circular nickel mesh (300 ym holes onl420 pm cen- ₍₁₎ ters).

a) Optical image : b) Acoustical image. FIG. 4.

-

Square 75 mesh (280 ym ho!es and 60 pm bars).

a) Optical image : b) Acoustical image.

Our single-beam experimental set-up is shown in figure 5. Here a thin film ZnO transducer excites a 1.1 GHz plane acoustic beam into the buffer rod.

This buffer rod can be almost any solid material, but we have chosen to use YAG because of its low acoustic attenuation and high isotropy factor. It should be noted that this system is capable of an extremely large field of view, being limited only by the size of the transducer. Again, as in the case of the two-beam system, the mylar film becomes a source of acoustic streaming as well as radiation pressure [lo].

Due to velocity gradients created by the differential transmission properties of the object, streaming again has an effect on the acoustic imaging [I I].

As an estimate of the resolution capability of this

system we have imaged a portion of-the 1951 USAF Test Resolution Chart shown optically in figure 6a.

As can be seen in the acoustic image of figure 6b, Group ^$. 5 subgroup (5) is clearly distinguished.

This demonstrates a resolution of better than 51 lineslmm or slightly less than 9 microns. The wavelength in the emulsion is 1.35 micron. The resolution is lower than we would expect for two reasons.

First, due to the minuteness of the elements of the resolution chart they may not be filling with water and hence not passing acoustic energy. Second, the diffrac- tive spreading of the beam within the thickness of the mylar film imposes a lower limit on resolution.

If we assume the mylar thickness to be Az (= 5 microns)

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BUFFER ROD

VIEW

4 .~

^IpmPOLYSTYRENE '\SPHERES IN WATEE

71

\

LUCITE DISK

J -

n,

-

^-

y\ $3

/ACOUSTIC ANTI REFLECTION COATINGS

and two object points to be separated by a distance d, then in order to be resolved we must satisfy the inequality

d - 2 A z t a n a

5

0 , (20) where

sin a = /Z,,,,,/d. (21)

L *

Au COUNTER ELECTRODE

t

^ZnOTRANSDUCER MICROWAVE

POWER INPUT

FIG. 5. ^-Experimental arrangement for single-beam imaging.

FIG. 6.

-

1951 USAF Resolution Test Chart ; Groups and

+

^5.

a) Group

+

⁴ ^b)Group

+

⁵

(1) 16.00 Li/mm (1) 32.00 Li/mm

-+I I+-

^{100 ,urn}

(2) 17.96 - (2)35.92

-

(3)20.32 - (3) 40.64 - (b)

(4) 22.80 - (4) 45.60 -

(5) 25.56 - (5) 51.12 - FIG. 7. - Microtomed section of human lung tissue.

(6) 28.51 - (6) 57.02 -- U ) Optical image : b) Acoustical image.

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HIGH-RESOLUTION, HIGH-CONTRAST ACOUSTIC IMAGING C6-47

For the frequency at which we are operating this would place a lower limit on our resolution capability of approximately 4 microns. Hence, we are only a factor of two above the diffraction-limited resolution.

One primary purpose of our research has been the high-resolution acoustic imaging of biological specimens with good contrast. Shown optically in figure 7a is a microtomed section of human lung tissue which is embedded in epoxy. As can be seen from the acoustic image in figure 7b, the resolution achieved is as good as that obtained optically and the contrast is very high. There also seems to be struc- ture in the acoustical image which does not appear in the optical image. We see this ability to image biological specimens as one practical application of such an imaging system.

5. Sensitivity.

-

The downward pressure on the spheres due to gravity counteracted by buoyancy is given by

and is on the order of 4 x dyne/cm2 for our experimental conditions. In the two-beam imaging scheme the theories of King or Yosioka and Kawasima predict radiation pressures on the order of 4 (dyne/cm2)/(watt/cm2). Since we must only push the spheres sideways and not lift them against gravity, we may safely assume a force of 0.1 PD/na2 needed to form our fringe pattern. This would predict a sensitivity of approximately watt/cm2. Our experimentally determined sensitivity for this configu-

ration, however, lies in the range of watt/cm2.

For the single-beam imaging system, which is operating at a much higher frequency and thus higher losses, the theory of Fox predicts a sensitivity of approximately watt/cm2. Again, our experimentally determined sensitivity is an order of magnitude higher at approximately 5 x l o u 2 watt/cm2.

This order-of-magnitude discrepancy in both systems can be accounted for in terms of the increased attenuation in the detecting emulsion over that of pure water.

This attenuation has been found to increase linearly with particle concentration for all conditions. At high frequencies or for large radius spheres, scattering becomes the dominant loss factor [12], 1131.

6. Summary and conclusions. - In this paper we have presented two systems capable of high- resolution acoustic imaging by the nonlinear detection of acoustic fields in a thin emulsion film. We have demonstrated a resolution capability of 9 microns at a sensitivity on the order of 5 x watt/cm2 with our latter system. We have also been able to image biological specimens with rather good contrast, as expected. By using thinner mylar films and operating at higher frequencies we should be able to increase our resolution capabilities without degrading contrast sensitivity.

Acknowledgments. - The authors would like to acknowledge contributions to this work by Bob Griffin for orienting and polishing the crystals ; Poul Galle for depositing the acoustic films and transducers and Forrest Futtere for preparation of the mylar films.

References MUELLER R. K., << Acoustic Holography >>, Pvoc.

ZEEE 59 (1971) 1319.

CUNNINGHAM J. A. and QUATE C. F., << Acoustic, Interference in Solids and Holographic Imaging >>

in Acoustical Holography, vol. 4, G. Wade, Ed. (Plenum Press, New York, 1972), 667.

COLLIER R. J., BURCKHARDT C. B. and LIN L. H., Optical Holography (Academic Press, New York, 1971).

LANGEVIN P., Lectures given at the Ecole Supkrieure de Physique et de Chimie, Paris, France (1923).

KING L. V., << On the Acoustic Radiation Pressure on Spheres >>, Proc. R. Soc. London A 147 (1934) 212.

YOSIOKA K. and KAWASIMA Y., << Acoustic Radiation Pressure on a Compressible Sphere >>, Acustica 5 (1955) 167.

FOX F. E., << Sound Pressure on Spheres >>, J. Acoust.

Soc. Am. 12 (1940) 147.

[8] JACKSON F. J. and NYBORG W. L., << Small Scale Acoustic Streaming near a Locally Excited Membrane >>, J. Acowt. Soc. Am. 30 (1958) 614.

[9] GOODMAN J. W., Introduction to Fourier Optics (McGraw-Hill, New York, 1968), chap. ^7.

[lo] CADY W. G. and GITTINGS C. E., << On the Measu- rement of Power Radiated from an Acoustic Source >>, J. Acoust. Soc. Am. 25 (1953) 892.

[ll] KOLB J. and NYBORG W. L., << Small-Scale Acoustlc Streaming in Liquids >>, J. Acoust. Soc. Am.

28 (1956) 1237.

[12] EPSTEIN P. J. and CARHART R. R., << The Absorption of Sound in Suspensions and Emulsions I.

Water Fon in Air. J. Acoust. Soc. Am. 25 (1953) 553:

[13] ALLEGRA J. R. and HAWLEY S. A., << Attenuation of Sound in Suspensions and Emulsions : Theory and Experiments ^P,J. Acoust. Soc. Am. 51 (1972) 1545.

DISCUSSION

P. DAS. - 1) Could ^YOUtell us something about as make of ZnO films (sputtered). The insertion the transducers, like bawdwidth, insertion loss, etc

...

? loss is 5 dB and the bandwith is 100 MHz.

2, How much power did you use for the pictures 2) For the holograms which were at first you showed in the slides ? we use approximately l o u 3 watt/cm2. For the others

C. E. QUATE.

-

1) The transducers are standard we use about l o v 2 watt/cm2.