microscopía acústica tomografica de scanning_review

8/3/2019 Microscopa Acstica Tomografica de Scanning_REVIEW

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168 IEEE TRANSACTIONS ONSONICS AND ULTRASONICS, VOL. SU-32, NO. 2 , MARCH 198

Scanning Tomographic Acoustic Microscope:A Review

Abstmct-Tomography can be used o nhance the capabilityofacoustic microscopy. An approach foremploying these principles, calledscanning tomographic acoustii microscopy (STAM), makes use of to-mographicreconstruction echniques to process data acquired by amodified scanning laser acousticmicroscope (SLAM). Using plane-waveinsonification, SLAM can be modified to perform the data acquisition.Digital processing of the acquired data carries out back-and -forthpropagation nd ccomplishes he mage econstruction.This p-proach to acoustic microscopy has advantage s over conventional ap-proaches,especially orviewing he ubsurfaceof complex objects,where the structure of interest lies in various well-defined planes.The principles of back-and-forth propagation associated withTAMimage reconstruction are summarized. The required m echanical andelectronic m odifications of SLAMn order to facilitate proper data ac-quisition of STAM are discussed. The formula s fo r d eterminin ghe the-oretical resolution limits ofTAM are introduced. Sim ulations are pre-sented to demonstrate the depth resolving capability of TAM.

I. INTRODUCTIONA COUSTIC MICROSCOPY, whichmploys ultra-sound in the ange of hundreds and thousands ofmegahertz, is capable of producing micrographs with ahigh degree of resolution. As contrasted with the conven-tional optical microscopy, ultrasound can image the inter-nal structure of opaque specimens. Such microscopes canbe used to study solid-state surfaces, the layers beneaththese surfaces, and he nterior structure of opaque (tolight) microscopic life [l]-[4]. The use of a rapid scan-ning technique can provide imaging in real time [ ], [3].Both transmission and reflection modes have been em-ployed in acoustic microscopy. The scanning laser acous-tic microscope (SLAM) represents a typical example ofthe application of ultrasound to transmission-mode mi-croscopy. Its operation is based on transmitting the ultra-sound through the specimen and obtaining a dynamic rip-ple caused by the sound on a solid mirror-like free surface(coverslip) surrounding the specimen. The resulting im-age depicts the sound absorption properties of the insoni-fied specimen and s obtained by means of a scanning laserbeam reflected from the mirror-like surface. The knife-edge technique s used to detect the esired acoustic signal

Manuscript received May 1984; revised September 1984.This work was jointly supp orted by the Santa Barbara Research C ente rand the MICRO program of the University of California.Z-C. Lln and G . Wade are with the Department cf Electrical and Com-puterEngin eering, University of Californ ia, Santa Barb ara, CA 93106,USA.H. ee is with the Departm ent of Electrical and Compu ter Engin eering,University of Illinois at Urbana, IL 61801, USA.

USER BEAM

SCAN DIfbZCTION+WATER COUPLING

Fig. 1 . Schematic diagram of SLAM[ 5 ] . Water is the fluid medium in which the specimen icommonly submerged. The schematic diagram of SLAMis shown in Fig. 1 . This system is capable of providinhigh-quality acoustic micrographs of test samples.Because these micrographs are essentially two-dimensional views of three-dimensional objects, certain limitations exist in imaging specimens of substantial thicknessTo obtain anunambiguous magewithoutaconfusingoverlapping of the internal structure, the specimens muhave little or no structural variation in the depth directioA partial solution to avoid such a constraint has been attempted by using a double-view stereo optic technique iwhich the depth information is extracted manually. Thiprocedure, however, is found to be useful only for imaginsimple objects. Thus, in general, practical application ilimited to test samples that are structurally uncomplicatand thin in the direction of sound propagation. This, iturn, frequently requires thin sectioning of the test samples in much the sameway as is normally done in standaroptical microscopy.The principles of tomography can be used to enhancthe capability of acoustic microscopy.A echnique foapplying these principles, called scanning omographicacoustic microscopy (STAM) [6], utilizes digital signaprocessing of information obtained with modifieSLAM. STAM s capable of producing microscopic tomograms (cross-sectional images) to overcome the imaging constraints. The technique is holographic in characteand is reminiscent of conventional X-ray focal-plane tomography. It is most suitable for subsurface imaging of ob

oo18-9537/85/0300-0168$01.00 0 985 IEEE


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LIN et al.: SCANNING TOMOGRAPHICCOUSTICICROSCOPE 169jects with planar structure. Using plane-wave insonifica-tion, LAManemodified to performhe dataacquisition. Digital processing of the acquired data ac-complishes the image reconstruction.The image reconstruction algorithm that we have pro-posed for STAM is called back-and-forth propagation[7]. This algorithm has appreciable advantages over con-ventional algorithms for tomographic imaging of complexobjects, where the structureof interest lies in various well-defined planes. The modifications and revisions of this al-gorithm for digital image reconstruction with plane-waveinsonification are described in this paper. A properly mod-ified SLAM system is required to perform the data acqui-sition for STAM.Rotatingeither he ransducer or hespecimen is necessary to generate tomographic projec-tions in sequence. Both schemes f rotation are consideredto achieve this purpose.Because our technique for STAM is holographic in na-ture, both amplitude and phase of the resultant wavefieldare required. Modification of the electronic circuits ofSLAM is necessary in order to measure both the phaseand amplitude [8]. The information hat is contained inthe data acquired by a conventional SLAM and he re-quired modification of SLAM detection devices to facili-tate proper data acquisition for STAM are also noted here.The paper summarizes he required mechanical modifi-cations and he mage reconstruction procedures corre-sponding to the wo rotation schemes. The advantages nddisadvantages of these two kinds of mechanical rotationwhen acquiring the data are discussed.The resolving capability of back-and-forth propagationhas been studied recently [9], [101. Ambiguity functionshave been presented for the two kinds of rotation schemesto investigate he theoretical resolution limits. We de-scribe the formulas or determining the resolving capabil-ities both in depth and ateral directions. Somesimulationresults are presented to show the depthesolving capabil-ity of STAM. We point out the inherent deficiency of theconventional knife-edge demodulation technique; that is,artifactscaused by the generation of spurious surfacewaves on the coverslip and other sources of inaccuraciesthat may degrade the reconstructed tomograms.

11. IMAGEECONSTRUCTION ALGORITHMOR STAMThe three-dimensional structure of an object can be de-termined from holographic ata [ l ] . In general many ho-lograms,which orrespond to different directions of

illumination of the object, are needed to reconstruct thethree-dimensional structure. Back-and-forthpropagationis a holographic approach for tomographic image recon-struction [7]. We have presented the principle of this al-gorithmwith plane-wave insonification and showed thesimilarity between it andotherapproach of diffractiontomography in an earlier paper [121. The system operatesin he ransmission mode and is depicted inFig. 2. Asshown, the object region lies between z = 0 and z = z r .The incident angle of the insonifying plane wave insidethe homogeneous portion of the object region is denoted

ULTRASOUNDFig. 2. A n object of planar structure insonified by ultrasound coming frombelow.by p. For each angle p, the received wavefield u , ( x , y;z) is back-propagated to obtain the wavefield U, ( x , y; zat z = z , and the input source wavefield U, (x , y; 0) isforward-propagated to find U, (x , y; z) Assuming line-arity we model the relationship between these quantitiesas follows:

Y ; z = V@ y; z,)t(x, y; z + n , ( x , y ; z,) (1)where t ( x , y ; z represents the transmission function forthe distribution of interest at z = z,, and n,(x, y; z is anundesired signal caused by scattering due to ali the un-known wave interactions within the object. As the anglep varies, n, (x , y ; z changesboth in magnitudeandphase. We assume that n , ( x , y ; z has little correlationwith respect to p. Then a least-squares estimate [l31 ofthe distribution t ( x , y ; z can be found from he followingequation:

I9S-, U,(& y; 2 3 ; (x, y ; 2,) &JL , ( x , Y ; z,)ui (x , y; z,) dvh ; z,) = (2)where the angle between -19 and t9 represents the avail-able angular view.In practice, signal processing of the data is performedin the discrete form. The detected ignal is digitized, andonly a finite number of projections are available. Equation(2) becomes

where the summation is carried out over all different an-gles of view. Since plane waves are used, uuj (x, y ; z v i j(x, y ; z remains constant for different incident angles pj .Equation (3 ) can be further simplified; i.e.

?(x, Y ; z,) = K C Upjuij (X , Y ; Z J , (4)Iwhere K is a scaling factor which depends on the numberof projections used for image reconstruction.For each angle ( p j , the received wavefield uV j x, y; z)is back-propagated to obtain uV j ( x ,y; z , ) , and the inputsource field uVj(x,y; 0) is forward-propagated to find uVj(x,Y ; z,). Finally, t ( x , y; z,) is computed as the least-squaresestimate of t ( x , y ; z,). In this way a tomogram at plane z ,


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170 IEEE TRANSACTIONS ON SONICS AN D ULTRASONICS, VOL. SW-32, NO. , " C H 19PHOTODlMlE

BAND-PASS SYNCHRONOUSFILTER DETECTOR

S r ( t )Fig. 3. Block diagram of the electronic processingof the detected signal inSLAM. , ( r ) is a reference signal which can be added to produce holo-graphic operation.can be obtained. The data set acquired when the acousticplane wave propagates at a specific incident angle istermed a projection. As described previously, projectionscorresponding to various incident angles are required toreconstruct a tomogram. A number of projections can begenerated sequentially by rotating the acoustic transduceror the specimen through an angular incrementt each step.The principle of back-and-forth propagation is simple.It is a convenient and feasible approach in both data ac-quisition and computation for imaging objects of planarstructure. The application of this algorithm to obtain mi-croscopic tomograms is promising with modifications ofthe data acquisition of SLAM.

111. DATAACQUISITIONF STAMThe conventional SLAMoperates in either a holo-graphic mode or a direct-imaging mode. In the direct-im-aging mode an amplitude-only image is obtained. In theholographic mode a reference signal is added in order toretain phase information as well as amplitude information.The simplified block diagram of the electronics is shown

in Fig. 3. Because of the scanning of the laser beam, thefrequency of the desired signal detected by the photodiodeis Doppler-shifted [S] by a frequency proportional to thescanning speed. The bandpass filter in Fig. 3 is designedto pass the single sideband of the signal which has beenDoppler-shifted [S]. In the direct-imaging mode, the band-passed signal is fed into a synchronous detector that canextract the amplitude of its input signal. At the output ofthe low-pass filter we obtain the amplitude nformation [81.In the holographic mode, a reference signal is added atthe input of the synchronous detector. Thus phase infor-mation is obtained in the output of the low-pass filter. Asdescribed in the earlier paper [S], although phase infor-mation is retained in the holographic mode of operation,it is very difficult to make use of it. The output consistsof many signal components that are circuit-dependent andhard to utilize [8].Although it may be possible to extract thedesired phaseinformationwith he conventional SLAM, anotherap-proach, which involves changing the electronic circuits, isrelatively convenient. The modified circuit uses a quad-rature receiver to obtain both real and imaginary parts oftheoutputsignal. Two reference signals are employed.One is the 90-degree phase-shifted version of the other.

PHOTOOIOOE

Fig. 4. Block diagram of the electron ic circuit forobtaining both amplituand phase information.

/ 0.4t/ \\'

-0.'4 ' 1- ' ' ' '0.0

0.4 1.0orr0 fM

\ I t

1 - l . OttFig. 5. Transfer function M& f,) of the knife-edge detector orientedp

pendicular to the scan directionof the laser beam that is assumed t o haa Gaussian ntensity profile. The plots are normalized to the maximuresponse M,,,.The maximum response occurs at ref, = 0 for spatial fquencies perpendicularto the scan direction nd at rofx = 0.29 for spatfrequencies in the scan direction.The changes are shown in Fig. 4. With this modificatioboth mplitude and phasenformation of the desireacoustic field can readily be obtained.In SLAM the knife-edge demodulation techniques employed. The transfer function for the knife-edge detectiois antisymmetric in its response to spatial-frequency varations in the scan direction. For negative spatial frequecies (corresponding to sine waves on the coverslip surfatraveling in the opposite direction), the recovered signis opposite in phase to that for positive frequencies. Thresponse to spatial frequencies along and perpendicularthe scan direction for a laser beam witha Gaussian intesity profile is shown in Fig. 5 [141. The plots are functionof the dimensionless productof the nominal spot radiusof the laserbeam and the spatial frequencies f, and f y .In SLAM, the spatial frequency spectrum of an objeis usually biased by a carefully chosen nonzero spatial frquency at or near the frequency for maximum responsThis is accomplished by employing obliquely incident i


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LIN et d.: SCANNING TOMOGRAPHIC ACOUSTIC MICROSCOPE 171

COVERSLIPU EAMSPECIMEN

% AXISTRANSDUCER

\ fU S E R BEAM

COVERSUP-AXIS SPECIMEN

TRANSDUCER

(3)Fig. 6. Two different schemes of rotation to generate projections for STAM.(a) Rotation of the transducer. (b) Rotation of the specimen.

sonification. In order to avoid the asymmetrical responseabout the spatial frequency corresponding to the bias, onesideband of he object spectrum is filtered out. This issuitable for real objects that have symmetrical sidebands.Single sideband detection with a synchronous detector, asdescribed before, can thus be used to reconstruct the im-ages of such objects. In STAM, however, the acquired sig-nal is simply the data to be processed for image recon-struction. Hence, frequency spectrumormalization of theacquired data using inverse-filtering or other signal-pro-cessing techniques has to be employed to compensate forthe nonuniform response of the knife-edge technique inSTAM.A number of projections have o be generated in se-quence by rotating the acoustic transducer or the specimento reconstruct a tomogram. Two different schemes of ro-tation are currently being considered. One is shown in Fig.6(a). The transducer is rotated around an axis that is thecenter line of the top surfaceof the transducer andpointsperpendicularly out of the figure. The distance betweenthe object and the rotation axis must be kept constant toavoid phase errors caused by the rotation. With this rota-tional scheme, healgorithm described in the previoussection can readily be applied.Fig. 6(b) shows the ther method of rotation. Theobjectis circularly rotated around avertical axis passing throughthe origin of the coordinate system. The transducer re-mains stationary. This is equivalent to rotating the trans-ducer so that the constant amplitude wave vectors of thegenerated plane waves are rotating circularly. Because ofthe rotation of the object, we have to perform coordinate

transformation on the received and incident wavefields inorder to obtain the data acquired by rotating the trans-ducer. The back-and-forth propagation algorithm can thenbe applied to reconstruct the images. One mportant pointis that the coordinate transformation can be performedafter he wave propagation.This saves onecoordinatetransformation for each angle of view and also allows theforward-propagated incident wave to be used for all dif-ferent angles of view. Simplifications in the algorithmgreatly reduce he computation-time required for imagereconstruction.We have simulated two different rotation schemes foracquiring the data in the ideal situation [ 6 ] .Although bothschemes show that STAM can clearly differentiate a struc-ture that corresponds to various layers in a three-dimen-sional object, one scheme s distinctly more practical thanthe other. The first scheme requires rotating the rans-ducer. Detrimental phase errors will be introduced if therotation axis is not kept at the specified position to withina fraction of awavelength. t is extremely difficult toachieve this stability. Another difficulty is that the transferfunction for the knife-edgedetection is antisymmetric andnonuniform as described previously. Rotation of the trans-ducer may result in operating at angles where the acquiredsignal is extremely weak and the signal-to-noise ratio verylow. Compensation for the nonuniformity of the transferfunction is very hard to accomplish.The second scheme does not have these problems be-cause the transducer is held stationary during the rotatingprocess. Every projection is acquired with the same con-dition. Although we have to use an inverse-filtering tech-nique to compensate for the fact that the transfer unctionis not flat, the incident angle can be chosen to have opti-mum response and avoid low signal-to-noise ratios. Never-theless, to avoid phase errors, the specimen still has to bekept flat and at the same horizontal level during the rota-tion. From practical experience, we have noted that it iseasier to stabilize the object than the transducer.As shown in Fig. 1, the data acquired in the SLAM sys-tem are measurements of the acoustic wavefields along areceiving plane. Since it is not practical to have an infi-nitely wide receiving aperture, the measured wave field istherefore truncated by a limited-aperture window. We havepointed out previously that even with an infinitely widereceiving aperture, the process would have to be repeatedwith the object turned 90 degrees in order to acquire allthe necessary data for optimal image reconstruction [121.This results from the fact that a range of view angle equalto 180degrees is not enough for perfect image reconstruc-tion whenever diffraction is present [151. Because of align-ment problems and geometrical constraint,urning the ob-ject 90 degrees is not practical. In an actual systemgeometrical constraints limit both the range of the viewangle and the size of the receiving aperture. This cansesa limitation in the resolution of the reconstructed tomo-grams. We can characterize STAM as being a limited-ap-erture, limited-angle tomography which takes diffractioninto account.


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172 IEEE TRANSACTIONS ON SONICS A ND ULTRASONICS, VOL. SU-32, NO. 2 . MARCH 19IV. RESOLVING APABILITY

An important parameter of an imaging system is its re-solving capability. There are two kinds of resolution ofinterest. One is the depth resolution; the other is the lat-eral resolution. The theoretical resolution limitsof STAMcan be investigated by studying the ambiguity functionsof the system [lo]. In this section the ambiguity functionsof back-and-forth propagation corresponding to the twoschemes of rotation are illustrated. The analysis hasbeenpresented in detail [9], 101. In order to study the reso-lution of this imaging system, it is assumed that in theobject region only a oint scatterer exists and is located atposition (x, , yo ; z ,) . An infinitely wide receiving apertureis assumed. Consider the first schemef rotation, the scat-terer possesses a different transmittance U from its trans-parent surroundings as shown in Fig. 7. n order to studythe point spread function, we obtain for the reconstructedimage at (x , + Ax , y o + A y ; zo + Az) [101

?(x, + A x , y o + Ay; zo+ Az)" PU , - j @ n I A ) ( A x sin ' p + A z cos 'p)

R. 1 2 r ( h A * + h A Y ) df dx Y' ( 5 )Theambiguityfunction g(Ax, Ay,Az, 9) of the m-aging system is defined as follows:

g w , AY , Az,1')) . r 8E K - , - j ( 2 r / A ) ( A x sin ' p + A z co s 'p)

where K s such thatg@, 0, ,8)= 1 (7)

and the evanescent waves are neglected so that the inte-gration is performed within the region R defined by

1f: +f; < 2.In addition . r 8

g l ( A x , A z , 9) e - , - j ( 2 A ) ( A x sin ' p + A z cos c ) d2 9 -8 P

Z

Fig. 7. Geometry of the imaging system employing the first scheme of rotation. A single point scatte rer is ocated at position ( x , , y , ; z , ) in thobject region. The scatterer has a transmittance U and transparent sur-rounding. The acoustic transducer is rotated such that the angle p is vaied from -8 to 8.

z

Fig. 8. Geometry of the imaging system employing the second scheme orotation. A point scatte rer is located at (xo, yo ; ,) in the object region.The ge ometry shown assumes that the incident wave vector rather thanthe object is rotated circularly. The angle t9 is kept constant and the anglp is varied from 0 to 360 egrees.

and" "

J JR

Similarly, the ambiguity function corresponding to thsecond scheme of rotation can also be studied. For convenience, the analysis was made by assuming that the in-cidentangleswerechanging ather han heobject srotated [lo]. Fig. 8 shows hegeometry of the mag-ing system. We can obtain for the reconstructed image a( x , + Ax, o + A y ; zo + A z ) (101


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LIN et al. : SCANNING TOMOGRAPHIC ACOUSTIC MICROSCOPE 173

n n

R

where\k = ( A x sin cp + A y cos p) sin 19 + A z cos d . (12)

The ambiguity function of the imaging system employingthe second scheme of rotation is now defined to beg,(&, AY, A 2 9 8) = K c- dcpS e - j ( 2 ~ / X ) ,27r 0

R. 1 2 * ( f r ~ + f + * Y ) d l dfX Y

= Kc&(& AY, A 2 9 8)- g2(& AY, W , (13)where the additional subscript c stands for the circular ro-tation scheme and K c is such that

g m , 0 , 0 , 8)= 1. (14)In additiong,c(Ax, AY,AZ, 8) = -n S

e - j ( 2 ~ / h ) [ ( A x in + A y cos v) in 0 + A z cos 191 dP (15)and g 2 ( A x , A y , A z ) has been defined in (10).By substituting values of A x , A y , and A z in (6) or (13)the lateral resolution and depth resolution can be investi-gated. Thus, (6) or (13) constitutes a formula for deter-mining the theoretical resolution limit for STAM. From(6), (9), (lo), (13), and (15) we observe that both the depthand ateral resolutions depend on the available angularview 8 and the wavelength X. When there is only one in-cident angle available, the resolution of the ystem isequivalent to that of the conventional holographic imagereconstruction using backpropagation.The g 2 ( A x r A y,Az ) is the effect on resolution due to holographic recon-struction and is independent of the angular view d . g , ( Ax ,Az, 8) nd glc(Ax,Ay, Az, 8) re resolution factors dueto tomographic reconstruction and are dependent on d .We find that the two schemes of rotation have identicalholographic effects but differnt tomographic effects on res-olution. This is intuitively true since the holographic effectis due to the wave propagation.Figs. (9) and (10) show the amplitude distributions ofthe ambiguity functions corresponding to the two schemesof rotation. From the amplitude distributions,e note thatthe first scheme will result in different resolution in x andy directions. In the direction of the scan of the ultrasonic

source, i.e., x direction, the resolution will be improveddue to a wider observation angle. The second schemewillhave identical resolving capability in both x and y direc-tions because the path of scan provides the same obser-vation aperture. The lateral resolution is better than thatof the first scheme of rotation. However, the depth reso-lution of the second scheme is worse than that of the firstscheme of rotation, because during the rotation the anglet9 is kept constant. Therefore, the rotation does not pro-vide more information in the z direction.

VI. SIMULATIONESULTSPrevious simulations have demonstrated that the tomo-graphic imaging technique of STAM using back-and-forthpropagation can significantly reduce theblurring and over-lapping of various layers in an object and give well-differ-entiated omograms [6].In order to show the resolvingcapability of this algorithm, we present some simulationresults for the case of rotating the specimen. We assumethat the transfer function of the knife-edge demodulationis well compensated, and hence flat response can be as-sumed, and that the data are acquired under a noise-freecondition. Evanescent waves are assumed to be negligible.A three-dimensional object with planar structure is usedas the simulated specimen. The simulated insonificationis by plane waves. The object is assumed to be attenua-tion-free with respect to the acoustic waves except for twohorizontal thin layers ten wavelengths apart. The top ayeris eight wavelengths from the receiving plane. Differentpatterns involving a structure that is assumed to be 50percent transparent to the acoustic waves are contained inthe two layers. The geometry of the object is shown nFig. 11. Fig. 12 shows the patterns for these two layers.The transducer is assumed to be stationary and tilted so

that the incident angle of the acoustic wave inside thespecimen is 45 degrees. The object is assumed to be ro-tated circularly with a constant angular increment through360 degrees. Nine projections are generated by rotatingthe specimen with a 40 degree ncrement at each step.Fig. 13 shows the amplitude (SLAM) images correspond-ing to two of the preceding nine projections. The imagesare ambiguous due to overlapping of the structure fromthe two layers and diffraction of the sound waves as theypropagate through the object. The shaded region near theleft edge is caused by the limited-input aperture.The projections are then processed by the proceduresdescribed in Section 11. The reconstructed images corre-sponding to the top and bottom layers are shown in Fig.14. It is also assumed that the positions within the speci-men of these two layers are unknown, and thewhole struc-ture of this object must be reconstructed to obtain a three-dimensional mage.Figs. 15-17 show thereconstructedimages that are obtained at the depths near the layers ofinterest but not at the correct depths. In other words theyare tomograms not in focus. Part (a) in each figure illus-trates he tomogram several wavelengthsbelow the toplayer of interest. Part (b) illustrates the tomogram everalwavelengths above the bottom layer of interest. From those


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174 IEEE TRANSACTIONS ON SONICS ANDULTRASONICS, VOL. SU-32, N O . 2, MARCH 198

Fig. 9. Amplitude distributions of the ambiguity function of the magingsystem employing the first scheme o f rotation. (a) g(Ax, Ay, 0,8) s afunction of A x and A y with 19 = 45" . (b ) g(Ax, 0, z , 8) s a functionof A x and Az with 8 = 4 5 " .


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LIN et al.: SCANNING TOMOGRAPHICACOUSTIC MICROSCOPE 175

Fig. 9. (Continued.) (c) g(0, Ay, A.?, 9 ) as a function of A y and A z withl9 = 45".

Fig. IO . Amp litude distributions of th e ambiguity function of the imagingsystem employing the second scheme of rotation. (a) g , . (A x , A y , 0, 9) asa function of A x and A y with 9 = 4 5 " .


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176 IEEE TRANSACTIONS ONSONICS AND ULTRASONICS, V O L . SU-32, N O . 2 , MARCH 198

\_SJo.0

Fig. IO . ( Conr inued . ) b) g , ( A x , 0, A z , 19) (org,(O, A y , Az , 19 ) as a func-tion of A x an d A z (or A y and A z ) with ~9 = 45 " .

Fig. 11. Three-dimensional geometry of the simulated object.

images we can obtain a qualitative evaluation of the re-solving capability of STAM in the depth direction.Simulationswerelso erformedssuming similarspecimens but with the two layers of interest closer to eachother. Figs. 18-20 show the reconstructed tomograms ofthe top and bottom layers when the distances between thetwo layers within thespecimen are six, four, and twowavelengths, respectively. All of the omograms are re-constructed at the correct depths, and therefore they arewell in focus. We note that when the two layers are veryclose, the tomogram of one layer contains leakage fromthe other layer.

VII. CONCLUSIONN D DISCUSSIONSTAM is inherently capable of providing unambiguoustornograms. We have reviewed the principle of back-and-forth propagation for the case of plane-wave insonification

and digital image econstruction.The oftware reconstructionrocedureorresponding to two differenschemes of rotation has been discussed. Signal detectioand electronic processing of conventional SLAM have beedescribed.Thenecessary modification of SLAM'S mechanical elements and electronic circuits for proper STAMdata acquisition have been addressed. We conclude tharotating the specimen in acquiring the data is more favorable than rotating the transducer.In SLAM the ultrasonic field beingmeasured passethrough the specimen and impinges upon a coverslip. Inthis way the acoustic information is impressed on a scanning laser beam via deflection modulation.There arproblems in acquiring data in this fashion. First, the normal surface displacement of the reflector is, in generalnot simply proportional to the impinging sound field, buit also is dependent on the angle of incidence [16]. Second, several types of surface waves can be generated onthe modulating reflector, and waves of the Rayleigh typcan become a pernicious cause of artifacts in SLAM images [171. These surface waves constitute a sourceof spurious disturbancespropagating on themirror urface.Third, conversion into shear waves of the impinging compressional wave on the object of interest also needs to beconsidered [ l ] . In contrast to SLAM, the data obtainedfrom a single scan does not produce the final STAM image: it produces only a single projection. These data mus


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LIN er al . : SCANNING TOMOGRAPHIC ACOUSTIC MICROSCOPE 177

(a) @)Fig. 12. Simulated patterns for the two layers of interest within the object.

(a) (b)Fig. 13. Tw o of the simulated amplitude images (i .e ., SL AM images) fromprojections with illuminating wave of incident angle 9 = 4 5 " .

(a) (b)Fig. 14. To mo gram reconstructed from nine projections generated by ro-tating the specim en. (a) Top layer. (b) Bottom layer.

be processed digitally by computer, along with the data have been obtained from an experimental STAM n thefor a number of other projections, to give a STAM image. past, but the reconstructed omograms are not yetpub-The spurious disturbances on he coverslip give rise to lished because of these problems. Their solution is cur-errors in the acquired data. To obtain good tomograms, rently under investigation.these problems must be overcome. Some experimental data Aswehave discussedbefore, it is more favorable to


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178 IEEE TRANSACTIONS ON SONICS AND ULTRASONICS, VOL. S U - 3 2 , N O . 2, MARCH

(a) (b)Fig. 15. Tomogram s recon structe d for the planes that are one wavelengthaway from the layers of interest; i .e., A z = A . (a ) One wavelength belowthe top layer of interest. (b) One wavelength above the bottom layer ofinterest.

(a ) (b)Fig. 16. Tornograms reconstructed for the planes that are two wavelengthsaway from he layers of interest; i . e . A z = 2 A . (a) Twowavelengthsbelow the top layer of interes t. (b) Two wavelengths above the bottomlayer of interest.

(a ) (b)Fig. 17. Tomograma reco nstruc ted for the planes which are hree wave-lengths away from the layers of interest; i .e. A z = 3 A . (a) Three wave-lengths below the top layer of interest. (b) Three wavelengths above thebottom layer of interest.

rotate the specimen for data acquisition from the stand- function. The bandwidth of thespatial frequency sppoint of the SLAM system. If the specimen s rotated, trum of the detected acoustic wavefield has to be limiobliquely incident insonification at the optimum angle of so that it does not extend beyond the positive part of incidence can be employed as in the conventional S LA M. transfer unction in Fig. 5 in order to avoid the nDigital signal processing could then be applied to com- response earheeropatial frequency. Therefopensate for nonuniformities in theknife-edge ransfer resolution of the reconstructed image will be limited.


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LIN er al.: SCANNING TOMOGRAPHIC ACOUSTIC MICROSCOPE 179

Fig. 18. Tomograms re constructed for the object in which the two layers ofintere st are six wavelengths apa rt. (a ) Top layer. (b) Bottom layer.

(a) CO)Fig. 19. Tomograms reconstructed for the object in which the two layers ofinterest are four wavelengths apa rt. (a) Top layer. (b) Bottom layer.

(a) (b)Fig. 20. Tornograms reconstructed for the object i n which the two layersof interes t are two wavelengths apart. (a ) Top layer. (b ) Bottom layer.

improve the resolution, a demodulation scheme using a TDID than with the knife edge. Hence, higher resolutiontime-delay interferometric detector (TDID) can be em- of the reconstructed image should be possible.ployed [14]. Since the transfer function for the TDID issymmetric and has the maximum response for zero fre-quency, the detected acoustic wavefield can have a greater The authors are greatly indebted to Carl F. Schueler ofbandwidth of the spatial frequency spectrum with the SBRC for his constant assistance; to Lawrence W. KesslerACKNOWL E DGM E NT S


13/13

18 0 IEEE TRANSACTIONS ON SONICS AND ULTRASONICS,OL. SU-32, NO. 2, MARCH 19and Michael G.Oravecz of Sonoscan for their valuable [ l51 X. .Pa nand A. C. K ak, A computationalstudy of reconstructiodiscussions Of the associated with backropagation, IEEE Trans. Acoust.,peech,ignalrocessing,algorithms for diffraction omograph y: Interpolation versus filtered-SLAM; and to Robert Adler of Zenith for his uggestion ASSp-31, no. 5 , pp. 1262-1275, Oct . 1983.of object rotation instead of transducer rotation. In addi- [l61 R. L. Whitm an, M. Ahm ed, and A. Korpel, A progress eportontion, the would like to thank Rolf K. ~ ~ l l ~ ~f th easercannedcousticamera,nAcousricalHolography,ol . 4the University of Minnesota for his vahable comments on [l71 R. K. Muelleran dR. L. Rylander,Coverslip nducedartifacts nsurface-w ave problems nd arious inaccuracies inherent high-resolutioncanningasercousticmicroscopemages,nAcousric maging, E. A.As hand C.R .Hi ll. New York: Plenum

G. Wade, Ed. NewYork: Plenum, 1972, pp. 11-32.

in SLAM systems. 1982, vol. 12, pp . 37-45.REFERENCES[ l ] L. W. K essler, Imaging with dynamic-ripp le diffraction, in Acous-tic maging, G . Wade,Ed. NewYork: Plenum, 1976, chap. 10, pp .229-239.[2 ] C . F. Quate, Imaging Using Lenses, in Acoustic Imaging, G. Wade,Ed. New York: Plenum , 1976, chap. 11 , pp . 241-305.[3 ] L . W. esslerand D. E.Yuhas,AcousticMicroscopy - 1979,Proc. IEEE, vol. 67 , pp. 526-536, Apr. 1979.[4 ] C. F. Quate, A. Atalar, and H. K. W ickramasinghe, Acoustic mi-croscopywithmechanical scanning-A Review, Proc. IEEE. vol.[5 ] R. L. Whitman and A . Korpel, Probing of acoustic surface pertur-bations by coherent ight, Applied Optics, vol. 8, no. 8, pp. 1567-1576, Aug. 1969.16) Z . Lin, H. ee, G. Wade,and C. F. Schueler,Computer-assisted

tomographic acoustic microscopy for subsurface imaging, in Acous-tical Imuging. vol. 13, R.K. Mueller, M. aveh, and J. F . Greenleaf,Eds. NewYork: Plenum, 1984, pp . 91-105.[7 ] H. Lee, C. F . Schueler, G. Flesher, and G. Wade, Ultrasound planarscanned omography, n Acoustical Imaging , vol. 11, John Powers,Ed. New York: Plenum, 1982, pp. 309-323.[S] Z. Lin, H. Lee, G. Wade, M. G . Oravecz, and L. W. Kessler, Dataacquisition in tomographic acoustic microscopy, in Proc. IEEE UI-trason. Symp., 1983, pp. 627-631.[9 ] 2. Lin and G . Wade, On the resolution of planar ultrasonic tomog-raphy, J. Acoust . Soc. Am., vol. 77 , no. 1 , pp , 139-143, Ian. 1985.[IO] Z . Lin, A planarultrasonic omographic maging system , Ph.D .dissertation,Depa rtmen t of Electricalan dComputerEngineering,University of California, Santa Barbara, CA , 1984.[l l ] E. Wolf, Three-dimensiona l structure determination of semi -trans-parent objects from holographic data, Optics Comm unications, vol.1, no. 4, pp. 153-156, Sep./Oct. 1969.[l21 Z . Lin, H. Lee, and G . Wade, Back-an d-forth propagation for dif-fraction tomography, IEEE Trans. Sonics Ultrason., vol. SU-31, no .6, pp . 000, Nov. 1984.

67 , pp . 1092-1114, Aug. 1979.

Zse-Cherng Lin (S83-85) was born inTaiwan on April 16, 1955. He received the B.S. andM.S. degrees in electrical engineering from Na-tional Taiwan University, Taiwan, in 1977 an d 19respectively, and the Ph.D . degr ee in electrical acomputer engineering from the University of Caifornia, Santa Barbara, CA, in 1984.Since 1985 heha sbeen PostdoctoralRe-search Engineer in the Departm ent of Electrican dComputerEngineeringat heUniversity California.His esearch nterests ncludesignalprocessing, magingsystemanalysis, mage econstructon,an dacousticmicroscopy.

Hua Lee (S78-MSO-SM83)was born in TaiTaiwan, on September 30, 1952. He received thB.S.degree nelectricalengineering romNa-tional Taiwan University in 1974, and the M.S. anPh.D.degrees inelectricalan dcomputerengi-neering from he University of California, SantBarbara, in 1978 an d 1980, respectively.In 1980 he joined the faculty f the DepartmeI University of California,SantaBarbara,asa Viof Electricalan dComputerEngineering at thiting Assistant Professor. Since 1982 he has beewith the Department of Electrical and Computer Engineering, and the Coordinated Science Laboratory f the University of Illinois at U rbana-Chapaign w here now he is an A ssistant Profe ssor. His current resea rch interinclude imaging systems, signal analysis, and processing.Dr. Lee is a member of Acoustical Society of America, Eta Kappa Nuand Tau Beta Pi.

[ l31 A.-D. Whalen, Detection of Signal n Noise. New York: Academic[ 141 R. L. Rylander, A laser-scanned ultrasonic microscope incorporating1971, pp , 335-336.a time-delay interferometric detector, Ph.D. dissertation, University Glen Wade (S51-A55-SM57-F62), fo raphotographan dbiographyof Minnesota, Minneapolis, MN , Dec. 1982. please see page 16 of the January 1982 issue of this TRANSACTIONS.

microscopía acústica tomografica de scanning_review

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