fabrication and characterization of transducer elements in ......ultrasound beam. recently, several...

364 IEEE TRANSACTIONS ON ULTRASONICS, FERROELECTRICS, AND FREQUENCY CONTROL. VOL. 39, NO. 4, JULY 1992

Fabrication and Characterization of Transducer Elements in Two-Dimensional Arrays

for Medical Ultrasound Imaging Daniel H. Turnbull, Member, IEEE,

Abstruct- Some of the problems of developing a two- dimensional (2-D) transducer array for medical imaging are examined. The fabrication of a 2-D array material consisting of lead zirconate titanate (PZT) elements separated by epoxy is discussed. Ultrasound pulses and transmitted radiation patterns from individual elements in the arrays are measured. A diffraction theory for the continuous wave (CW) pressure field of a 2-D array element is generalized to include both electrical and acoustical cross-coupling between elements. This theory can be fit to model the measured radiation patterns of 2-D array elements, giving an indication of the level of cross-coupling in the array, and the velocity of the acoustic cross-coupling wave. Improvements in bandwidth and cross-coupling resulting from the inclusion of a front acoustic matching layer are demonstrated, and the effects of including a lossy backing material on the array are discussed. A broadband electrical matching network is described, and pulse-echo waveforms and insertion loss from a 2-D array element are measured.

I I. INTRODUCTION

N THE PAST DECADE, many areas of medical ultrasound imaging have been dominated by linear phased-array

transducers that are able to electronically focus and steer the ultrasound beam. Recently, several groups have turned their attention to the problem of developing two-dimensional (2-D) phased-array transducers, capable of symmetrically focusing the ultrasound beam as well as steering the focused beam throughout a three-dimensional (3-D) volume [l], [2]. The development of 2-D arrays is inherently difficult due to the requirement of half wavelength spacing between elements in order to avoid the occurrence of unwanted grating lobes in the array response [2]. This limiting factor implies that 2-D arrays will require a large number of elements, and that these elements will be very small in cross section. As a result, the electrical impedance of each element is much greater than the corresponding impedance of an element in a linear phased-

Manuscript received July 2, 1991; revised December 20, 1991, and January 27, 1992; accepted January 28, 1992. This work was supported in part by the National Cancer Institute of Canada by an Ontario Graduate Scholarship. The work of D. Turnbull was supported in part by a University of Toronto Open Fellowship during the course of this work.

D. H. Turnbull was with the Sunnybrook Health Science Centre Department of Medical Biophysics, University of Toronto. Toronto, ON, Canada and is now with the Department of Medical Physics, Toronto-Bayview Regional Cancer Centre, 2075 Bayview Avenue, Toronto, ON, M4N 3M5 Canada.

F. S. Foster is with the Sunnybrook Health Science Centre and the Department of Medical Biophysics, University of Toronto, Toronto, ON, M4N 3M5 Canada.

IEEE Log Number 9200684.

v u

and F. Stuart Foster, Member, IEEE

array, making impedance matching of 2-D array elements a formidable technical challenge.

In this paper we report the fabrication of 2-D arrays using “dice and fill” techniques that have been previously described for making 1-3 piezoelectric ceramic composite materials [3]: [4]. The arrays fabricated in this way have individual elements that have a cross-sectional size of 100 pm x 100 pm, a center-to-center spacing between elements of 150 pm, and an operating frequency between 4 and 5 MHz. The transducer material in the 2-D array elements is lead zirconate titanate (PZT), and the arrays are supported by epoxy that fills the regions between elements. The problems of connecting each element in the array to the pulsing and receiving electronics are not considered here. Instead electrical connections are made to a single transmitting element, and either one or two receiving elements in the 2-D array. Experiments are described in which the 2-D array element transmits an ultrasound pulse into a water tank, and the resulting radiation pattern is measured.

A number of past investigations have described vibration modes and radiation characteristics of the long strip-shaped transducer elements used in linear phased arrays [5]-[lo]. The main results of these studies are that the individual elements propagate ultrasound into the field as though they were embedded in an acoustically “soft” baffle [6], [7 ] , and that the elements must have a height to width ratio of at least 2 in order to avoid coupling of the desired thickness vibration mode to lateral vibration modes [5], [lo]. Few investigations have been made on piezoelectric, length-expanding bar shaped transducers, but Stephenson [l11 has shown that in barium titanate bars, a length to width ratio of greater than or equal to 2.5 is required to ensure single mode vibration behavior. In linear phased arrays, careful fabrication methods must be employed to avoid cross-coupling between elements, which can produce distortions in the array pulses as well as anomalous radiation patterns [5] , [g], [9]. Unlike a linear-array element, each 2-D array element is surrounded by neighboring elements in both the x and g directions, making element- to-element cross-coupling a more significant problem than in the linear array. Some effects of cross-coupling between elements in broadband 2-D arrays have been discussed in a previous paper [ 2 ] . In this paper, we compare theoretical and measured radiation patterns from a 2-D array element coupled to neighboring elements, and demonstrate array fabrication methods that reduce the effects of cross-coupling. Also described are the effects on element bandwidth, pulse shape

0885-3010/92$03.00 0 1992 IEEE

TURNBULL AXD FOSTER: FABRICATION AND CHARACTERIZATION OF TRANSDUCER ELEMENTS IN 2-D ARRAYS 365

TABLE I MATERIAL PROPERTIES USED IN KLM MODEL OF PZT 2-D ARRAY ELEMENT.

I‘ p ( x 1000 Q ZO ZB ZL - ‘ 3 3

- t t a d x.3 7 (m/s) kg/m3 (dBimm) (MRayl) (MRayl) (MRayl)

1700.0 0.01 0.70 3690.0 7.75 0.2-2.0 28.6 0.0 1.5

and two-way insertion loss resulting from the inclusion of a front acoustic matching layer, a lossy backing material, and a multicomponent electrical matching network.

11. THEORETICAL CONSIDERATIONS

In any phased-array ultrasound transducer, the objectives are twofold: first to produce a short ultrasound pulse (to maintain axial resolution); and second to focus and steer the ultrasound beam over a wide angle. As in linear phased-array transducers [9], deviations of the individual elements from their ideal state (single mode, piston vibrators with no cross-coupling) will have important effects on the performance of the 2-D array transducer. The electromechanical characteristics of an ideal 2-D array element can be computed using a KLM transducer model [12]. In Section 111, measured impedance data from 2-D array materials are compared to the KLM model predictions. In addition, methods are described for computing radiation patterns from an element with cross-coupling to adjacent elements in a 2-D array. In a later section these theoretical radiation patterns are used to characterize the levels of cross-coupling in experimental 2-D arrays.

A. Electromechanical Properties of a 2-D Array Element

The electromechanical properties of a length-expander bar, 2-D array element can be computed using the three-port KLM model shown in Fig. 1 [12]. This circuit model is very similar to the more familiar KLM model that describes thickness- expander plate transducers [13], but there are some significant differences. The bulk capacitance of the transducer material is related to the free dielectric constant &, which is higher than the clamped dielectric constant E&. In addition, the acoustic velocity, and hence the resonance frequency in the 2- D array element is lower than in a plate transducer (3690 m/s versus 4350 m/s for PZTSA). Finally, the electromechanical coupling coefficient of the 2-D array element is k33, which is significantly higher than k~ for the plate transducer (0.7 versus 0.5 for PZTSA). The effect of front and back loads are included as ZL and Z g respectively. Mechanical losses are taken into account by including an attenuation coefficient in the transmission line [14]. For the small lateral dimensions of a typical 2-D array element, one might expect the mechanical losses in the transducer material to be higher than those found in thickness mode plates. Fig. 2 shows the predicted electrical impedance of a PZTSA element, 100 pm x 100 km in cross section and 420 pm long, with a water load on the front and air (i.e., no load) on the back. The material properties used for this computation are listed in Table I. The mechanical loss coefficient N was varied from 0.2 dB/mm (thickness mode value [14]) to 2 dB/mm. From Fig. 2 it is obvious that even with large mechanical losses, a 2-D array element of these

- d

Fig. 1. Three port KLM equivalent circuit describing the electromechanical properties of an ideal 2-D array element. Circuit compo- nents: C, = ((sAg)/c)(l - L.$>), c = d m , 2, - p.Ac,

6 = 1/(2.lf)co~ec((~~~)/(2c)), X L = 2,,~~2sin((, .C)/(2r)) , where

-11 = ( ( x . 3 3 r ) / w o ) ) J - .

dimensions has a very large impedance, which makes matching the element to the transmitting and receiving electronics of the imaging system very challenging indeed!

B. Radiation Pattern of a 2-D Array Element with Cross-Coupling

The farfield acoustic pressure distribution from a continuous wave (CW) radiator can be computed from diffraction theory using the Rayleigh-Sommerfeld conditions for a square piston embedded in an acoustically “soft” baffle [6], [7]:

p(?, t ) = PaX(r)e-JWtH,(Q, 4 ) (1)

where

He(O, 4 ) = sinc(au/A)sinc(av/A) cos 6. (2)

Here n is the element sidelength, U = sinOcos4 and 71 = sinOsinq5 are direction cosines, and sinc(z) = s in (nz ) / (m) . The farfield CW pressure distribution from a 2-D array element can be decomposed into an amplitude that depends on the distance of the field point from the element, multiplied by the element directivity H,(Q; 4), which describes the angular

466 IEEE TRANSACTIONS ON ULTRASONICS, FERROELEmRICS, AND FREQUENCY CONTROL, VOL. 39, NO. 4, JULY 1992

Mechanical Losses 0.2 dB/mm

. . 1 dB/mm 2 dB/mrr i

2 1500 1

2 3 4 5 E Frequency ( M k z )

/ . c .

A. -. -90

- . . - _ 2 3 L 5 6

FveqLency (Mt-r,!

(b) Fig. 2. KLM model predicted impedance for a PZTSA element, 420 p m long and 100 p m x 100 p m in cross section (see Table I for model parameters). (a) impedance magnitude. (b) impedance phase angle.

dependence of the pressure field. The farfield assumptions used to derive (1) are reasonable since, for the 2-D arrays considered in this paper, each element is less than X/2 in lateral dimensions and the field points of interest are all well into the farfield.

Fig. 3 shows the geometry of a square element coupled to a (2Nc + 1) x (2Nc + 1) matrix of square elements. The farfield CW pressure distribution from the coupled element can be expressed as a weighted (in amplitude and phase) sum of the pressure fields of the coupled elements [15], [16]:

N, N ,

(4)

where d is the spacing between elements (Fig. 3) , W is the CW frequency, and IC = 27r/X is the wave number. The farfield assumption is still reasonable since even if the cross-coupling were to increase the effective element aperture to several elements in extent, the field points of interest are still well into the farfield of the coupled elements. In this paper, it is assumed that element-to-element cross-coupling is both electrical and acoustical in nature. In the case of electrical cross-coupling,

‘ l , - Field Point

x/-

Fig. 3. Array geometry used in CW diffraction theory. (Note: (1 = 100 //m and d = 1.N pm for all the results in this paper).

the pressure field of element (71,711) is added to that of (0,O) at a reduced amplitude, Q:,. Acoustical cross-coupling is assumed to be due to a single wave propagating through the 2-D array at velocity V C , so the pressure field contribution from element (71,m) has a reduced amplitude, CY&,, as well as a time (phase) delay At,, = d J ( n 2 + m2)/Vc. Finally, it should be noted that ago = 1 and a&, = 0 to be consistent with the case of an uncoupled 2-D array element.

111. MATERIALS AND METHODS

A. 2-D Array Fabrication

Diced-array material, consisting of PZTSA pillars embedded in an epoxy matrix, have been fabricated by two methods. The first method employs standard techniques developed for making composite transducer materials with 1-3 connectivity [3]. Starting from a block of PZTSA,’ two perpendicular set of grooves are cut using a 45-pm-wide blade on a dicing saw.2 The distance between cuts was 150 pm, and the resulting square ceramic pillars were close to 100 pm x 100 pm in cross section. The PZT was then backfilled with epoxy, after which the 2-D array material was sliced off and lapped to a thickness of 420 pm. All the elements share a common chromium-gold (Cr-Au) electrode that was evaporated onto the bottom side of the array material after lapping. On the top side, each element has a separate Cr-Au electrode as a consequence of the fact that the PZT had an electrode layer evaporated onto it before the cutting procedure started. This diced-array material will be referred to as DAO. In Fig. 4, micrographs of the top surface of the DAO diced array show the periodic 2-D arrangement of PZT5A elements in the epoxy matrix. It should be pointed out that the elements in DAO are PZT5A pillars, and are not composite materials. Previous studies indicate that for 2-D phased-array beam steering, the spacing between elements must be kept at or below Xi2, which is 150 pm for a S-MHz transducer (speed of sound assumed to be 1500 m/s) [2]. It was therefore not feasible to produce composite 2-D array elements using our dicing saw.

EC-65, Ed0 Western Co., Salt Lake City, UT. *Disco Abrasive Systems, Tokyo, Japan.

TURNBULL AND FOSTER: FABRICATION AND CHARACTERIZATION OF TRANSDUCER ELEMENTS IN 2-D ARRAYS 461

(b)

Fig. 4. Micrographs of DAO diced-array sample fabricated from PZTjA and epoxy. The PZTSA elements are 100 pm x 100 I‘m in cross section with center-to-center spacing of 150 {!m, (h) is magnified 3x compared to (a).

A second 2-D array material (DAML) was fabricated using a different method. In this case a 420-pm-thick plate of PZT5A3 with Cr-Au electrode layers on both sides was bonded to a 180-pm-thick conductive matching layer using a conductive epoxy.4 The conductive matching layer has an acoustic impedance close to 6.0 MRayl, intermediate between the PZT5A (29 MRayl) and water (1.5 MRayl). The ultrasound velocity in the matching layer is 3200 m/s, so the 180 pm layer is a quarter wavelength at 4.5 MHz. A diced-array material was

Valpy-Fisher, Hopkinton. MA. 4Ablebond 1 6 1 , Ablestik Laboratories, Gardena, CA

fabricated by cutting through the PZT into, but not through the matching layer. After one set of cuts were made, the PZT was backfilled with epoxy; and after the epoxy cured a second, perpendicular set of cuts were made, followed by a final backfilling step. In this way a diced-array material was produced with an acoustic matching layer that also functioned as the common ground for the array elements. As in DAO, the elements in this array are 100 pm x 100 Irm in cross section and the spacing between elements is 150 pm. In one DAML sample an electrical connection was made to the central element, and a lossy backing material was poured over the array and allowed to cure. The backing material consisted of a loaded mixture of a Si02 filled epoxy (EE 4183 resin + HD 3404 hardener, Hysol Canada Ltd., Scarborough, ON).

B. Water Tank Experimental Setup

Fig. 5 shows the water tank experimental system used to measure 2-D array element radiation patterns. An electrical connection is made to a single transmitting element using a small coaxial probe6 that has a sharp tip 10 to 20 pm in size. This probe is mounted on a three-axis micrometer stage, and can be lowered onto any element in the 2-D array to make the connection to the pulsing electronics of the Aerotech UTA- 3 ultrasonic transducer analyzer.’ In this way the element is excited with a short electrical pulse (100 V, -60 ns) to transmit an ultrasound wave into the water tank. The electrical tuning circuit behind the probe is included to match the electrical impedance of the 2-D array element to the pulsingheceiving circuitry of the UTA-3, and will be discussed later. A second electrical probe is included to (optionally) examine cross- coupling waveforms on neighboring elements in the 2-D array. A 5-MHz Aerotech transducer with a 6-mm aperture is used to detect the transmitted wave as a function of the angle 8. I t should be noted that because of the very small size of the elements in these 2-D arrays, the receiving transducer is well into the farfield of the effective element aperture, even assuming element-to-element cross coupling. For this reason, there will be little or no phase cancellation across the relatively large (6 mm) receiver aperture. The output of the receiver is amplified and sent to a Hewlett Packard 54503A digitizing oscilloscope in order to examine pulse shape and amplitude, and to store the ultrasound waveform for further analysis. The pulse-echo measurements described later were made using the UTA-3 as both the pulser and receiver. Finally, electrical impedance measurements described later were made using a Hewlett Packard 4191A RF impedance analyzer.

Iv. RESULTS AND DISCUSSION

A. Electromechanical Properties

Fig. 2 indicates that the theoretical electrical impedance from an element in DAO should have a maximum amplitude between 500 kR and 2000 kR at the resonant frequency of 4.5 MHz. By carefully calibrating our impedance analyzer at the

Spurrs epoxy, Polysciences Inc., Warrington, PA. ‘Rucker and Kolls, Santa Clara, CA. ’Aerotech Laboratories, Lewistown, PA.

468 IEEE TRANSACTIONS ON ULTRASONICS, FERROELECTRICS, AND FREQUENCY CONTROL, VOL. 39, NO. 4, JULY 1992

Aerotech UTA-3

F% Circuit Coaxial Probe ~ (Optional) Second Probe

Individual CrlAu

electrodes P A 5 A

epoxy Common ground electrode Wor

Transmitting element

conductive matching layer

\

# \ \ \ \ /

Receiving Transducer

Oscilloscope

Fig. 5. Experimental system for measuring 2-D array radiation pattern.

tip of the coaxial probe, we have measured impedance values between 200 to 300 kR at the resonant frequencies of the array elements. At these extremely high impedances, the precision of the impedance analyzer is approximately 2100 kR, which makes the measurements difficult to make. The impedance that is measured from the coaxial probe making contact with a single element in DAO is much lower, having an amplitude of about 8 kfl at the resonant frequency, and is limited by the open circuit impedance of the probe itself (Fig. 6(a)). There is no evidence of mode coupling in the impedance data of this 2-D array element with a length to width ratio of 4.2. In order to improve the efficiency of the transmitting element, the probe was matched to the 50 Q source impedance of the pulser through a transformer with a 1O: l turns ratio. As shown in Fig. 6(b), the resulting impedance from an element in either DAO or DAML is a reactive load with a magnitude of 50 R at 4.5 MHz. The efficiency of the 2-D array elements could probably be increased by using a probe with a shorter line length, and an open circuit impedance closer to the expected impedance of the element, making the electrical tuning of the element more effective.

Pulses transmitted from elements in DAO and DAML are shown in Fig. 7(aj(c), and the spectra of these waveforms are shown in Fig. 7(d). The receiving transducer was placed in the 8=0" position (see Fig. 5 ) to collect the waveforms. The inclusion of the matching layer in DAML has obviously improved the bandwidth of the element, although there are still some residual reverberations in the pulse that would degrade the dynamic range of an imaging system using these elements (Fig. 7(b)). These reverberations were somewhat attenuated

FreqJency (MHz)

(a)

phase - - - - - -

3.0 3.5 4.0 4.5 5 0 5.5 6.0 " _

Frequency (MHz)

Fig. 6 . (a) Measured electrical impedance of DAO element. (b) measured impedance of DAO and DAML elements using matched impedance transformer.

by the inclusion of a lossy backing material (Fig. 7(c)), but the resulting array had significantly higher levels of element- to-element cross-coupling, as will be discussed in the next section. Note that the extra mass loading of the matching layer decreased the center frequency of the pulse to close to 4 MHz in DAML compared to 4.5 MHz in DAO.

B. Cross-Coupling Characteristics

Evidence for significant levels of cross-coupling in the 2- D array materials was found in the waveforms collected at different angles, and in the radiation patterns of elements in DAO and DAML. Fig. 8 shows pulses received at B = 30" from the same elements as Fig. 7. The distortion of the pulse at 30' from DAO, compared to the pulse at 0' is indicative of a high degree of acoustical coupling between elements in the array. This effect is clearly less pronounced in the DAML elements. The directivity of 2-D array elements was measured using the water tank setup shown in Fig. 5 , where the receiving transducer made a radial scan in the z direction of the element. The peak-to-peak amplitude of the pulse, A(B), was measured as a function of 8 at increments of 2 S 0 , and was normalized to the amplitude at B = 0':

Directivity = 20 loglo {A(B)/A(O)} [dB]. ( 5 )

Theoretical element directivity functions were computed from (2) and (4), by fitting the parameters aRm, and V, to


' I ,

, I > 500 - I

255 250 - h ,

r >

b W 1 E v

- Y O D 3 0 d - ._ -

-250 a

-250 - ,

- 500 0

-500 1 2 3 4 5 0 1 2 3 4 5

Time (us) Time (us)

500 I

- 5 0 0 " " " ' " " 0 1 2 3 4 5

Time (us)

(c) (4 Fig. 7. One-way ultrasound pulses from elements in (a) DAO. (b) DAML. (c) DAML with backing. (d) Spectra of the three

waveforms.

the measured data, and normalizing to the value at B = Oo, so that A(B) is replaced by H,(B,O)H,(B,O) in (5). Note that in all the theoretical plots shown in this paper, the directivity is computed in the z direction (i.e., the U direction) of the element, so q5 = v = 0. For both the electrical and acoustical cross-coupling amplitudes, it was assumed that a,, decreased with distance from the excited element according to the relationship:

anm = 10-"d/("*+"2)/20 (6)

where a is specified in dB per element spacing (d), and n and m both range between -10 and 10 (i.e., a 21 x 21 matrix of coupled elements).

Fig. 9 shows the effect of varying the cross-coupling parameters used to fit the theoretical directivity curve for DAO. In this case the element size and the spacing between elements were a = 100 pm and d = 150 pm, and the frequency was taken to be 4.5 MHz, corresponding to a wavelength in water of A = 333 pm. In Fig. 9(a), the acoustical coupling amplitude and velocity were held constant at Q, = -9 dB/d and V, = 1325m/s, and the electrical coupling amplitude a, was varied between -21 and -25 dB/d. Increasing the electrical cross- coupling amplitude has the effect of narrowing the element directivity as well as lowering the sidelobe in the radiation pattern. Fig. 9(b) shows the effect of varying Q, between -7 and -11 dB/d, keeping a, = -23 dB/d and V, = 1325 m/s constant. Increasing the acoustical coupling amplitude increases the amplitude of the sidelobe, and decreases the level

of the null between the mainlobe and sidelobe in the element directivity. Finally, Fig. 9(c) shows the effect of varying the acoustical coupling velocity V, between 1275 and 1375 m/s, keeping a, = -23dBld and a, = -9 dB/d constant. In this case, increasing the coupling velocity narrows the mainlobe of the directivity function, and increases the amplitude of the sidelobe.

Fig. 10 shows the measured and theoretical directivity functions for elements in DAO, DAML, and DAML with backing material. The large sidelobes in the measured directivity of the element in DAO are very similar to anomalous sidelobes found in linear-array elements, which have been shown to be caused by Lamb wave coupling through the array [S]. In Fig. lO(a) (DAO), the frequency was 4.5 MHz (X=0.333 mm), while in Figs. 10(b) and (c) (DAML) the frequency was 4.0 MHz (A=0.375 mm). The curves marked "simple CW diffraction theory" were computed from (5), where A(B) is replaced by He(0,O) computed from (2). The theoretical directivity functions marked "coupled CW diffraction theory" were computed from (2) and (4) as described previously. The parameters used in the coupled directivity functions of Fig. 10 are summarized in Table 11. These parameters were chosen to minimize the error (rms and maximum) between the theoretical and measured element directivities. The amplitude parameters cy, and Q, were fit to the nearest dB/d, while the cross- coupling velocity V, was fit to the nearest 25 m/s. Each set of coupling parameters listed in Table I1 represents at least a local minimum in both error functions.


500 I

- 5 O O I ' ' ' ' ' '

Time (us) 0 1 2 3 4 5

- 500 0 1 2 3 4 5

Time (us)

(b) 500 -

250 - h

2 t l n

-500 ' 0 1 2 3 4 5

Time (us)

(c)

Fig. 8. Ultrasound pulses measured at 8 = 30" from elements in (a) DAO, (b) DAML, and (c) DAML with backing.

The cross-coupling characteristics of the diced-array sam- ples were further investigated by exciting element 0 with the Aerotech UTA-3 pulser and observing the coupled wave at elements 5 , 10 and 15 using a second coaxial probe (Fig. 5). The received signals from the DAO sample (Fig. 11) indicate the presence of at least two coupled waves. The first is a low amplitude, low frequency component with an acoustic velocity close to 2000 m/s, and the second is the 4.5 MHz coupling wave with a velocity close to 1325 m/s. This velocity was further verified by exciting element 0 with a 4.5 MHz toneburst and measuring the arrival time at element 15. In the case of the DAML element directivity, two minima were found that f i t the measured data equally well: one with V, =l250 m/s, and the second with V, =l550 m/s (Fig. 10(b)). There is evidence of two 4-MHz waves with these velocities in the cross-coupling signals shown in Fig. 12. The existence of two

0 oe (dB)

- 2 - $ - 4 v

x .- Y

> - 6

E ._ I

U

- R

-1 0

-1 2 -90 -60 -30 0 30 60 90

Theto (deg)

(a)

(10 (dB)

-90 -60 -30 0 30 60 90 Theta (deg)

(b)

-90 -60 -30 0 30 60 90 Theta (deg)

Fig. 9. Coupled CW diffraction theory for DAO element showing the effect of varying. (a) ac = -2.510 -21 dBid, where cy, = -9 dB/d and VC =l325 m/s. (b) oa = -11 to -7 dBid, where a, = -23dB/d and 1,; =l325 m/s. (c) L> =l275 to 1375 m/s, where a , = -23 dBid and cya = -9dB/d.

separate cross-coupling waves brings into question the validity of (S) and (9), which were derived under the assumption of a single source of acoustical cross-coupling. Without going to a higher order theory and curve fit, it is apparent that the DAML curve fitting results are consistent with an electrical coupling amplitude close to -20-dB per element spacing, and an acoustical coupling amplitude about 8 dB smaller than in the DAO array. Including the backing material on the DAML element significantly increased both the electrical and acoustical cross-coupling, resulting in a severe narrowing in element directivity (Fig. lO(c)), as the -3-dB width decreased to 33" from 55' in the air backed element.

The coupling parameters listed in Table I1 indicate that the main difference between DAO and DAML is a decreased acoustical coupling amplitude, which is consistent with the


- - 0 Experimental Data Simple CW Oiffroction Theory

- Coupled CW Oiffroction Theory

A Coupled Theory:

- 1 2 1 ' ' ' ' . ' ' ' . L Z . I I / o\ \

-90 -60 -30 0 30 60 90 Theta (deg)

(b)

I - 1 2

\

-90 -60 -30 0 30 60 90 Theta (deg)

(c) Fig. 10. Measured radiation patterns from elements in (a) DAO, (b) DAML (where solid rules indicate 1, = 1250 m/s and dotted rules indicate

= 1.550 mis), and (c) DAML with backing. Measured directivities are compared to CW diffraction theory for an uncoupled element and elements with cross-coupling parameters listed in Table 11.

observed pulse shapes shown in Fig. 8. The inclusion of the front matching layer, which increases the efficiency of transmission of ultrasound into the water tank, has effectively decreased acoustical coupling between elements in the 2-D array. One explanation is that the acoustical cross-coupling in DAO is due to Lamb wave propagation through the diced array [g]. The matching layer on the front of DAML reduces this cross-coupling mechanism. This reduction in the acoustical

TABLE I1 CROSS-COUPLING PARAMETERS FIT TO MEASURED DIRECTIVITY DATA

MAX 0, n, 1;. RMS error error

(dB) DAO -23 -9 1325 0.46 1.2 DAML a) -20 -18 1250 0.54 DAML b)

1.4

DAML+Backing -16 - 11 1650 0.45 1.1

(dBid) (dBid) (mis) (dB)

-18 -16 1550 0.53 1.3

- 1 5 ' ' ' ' ' ' ' ' ' 0 1 2 3 4 5 6 7 8

I

15

10

h > v E 5 a Y O .- 1 - E - 5 a

d.

- I 0

- 1 5

CAO: Element 10

- l 0 t - 1 5 ' ' ' .

0 1 2 3 4 5 6 7 8 Time ( u s )

(c) Fig. 11. DAO element 0 is excited with the Aerotech pulser and cross-coupling signals are measured from elements: (a) 5, (b) 10, and (c) 15. Minimum arrival times for acoustic waves travelling at 2000 mis and 1325 m/s are indicated by arrows.

cross-coupling between elements is important in order to ensure a consistent pulse shape over the angular range of the 2-D phased-array transducer, and to eliminate changes in beam

472 IEEE TRANSACTIONS ON ULTRASONICS, FERROELECTRICS, AND FREQUENCY CONTROL, VOL. 39. NO. 4, JULY 1992

DANL: Element 5

- l 0 t -151 . ' ' ' ' ' '

0 1 2 3 4 5 6 7 8

10

- l 0 t - 1 5 1 ' ' ' ' ' ' ' '

0 1 2 3 4 5 6 7 8

15

10 - > v E 5

15

- 1 5 ' ' ' . 0 1 2 3 4 5 6 7 6

Time (us)

( c ) Fig. 12. DAML element 0 is excited with the Aerotech pulser and cross-coupling signals are measured from elements: (a) 5, (b) 10, and (c) 15. Minimum arrival times for acoustic waves travelling at 1250 mis and 1550 m/s are indicated by arrows.

shape as a function of angle [2]. The higher velocity cross- coupling wave in the DAML array most likely propagates through the matching layer. When the lossy backing is added to DAML, the dominant acoustical coupling mechanism changes to a higher velocity wave that probably propagates through the backing material.

C. Pulse-Echo Characteristics In a phased-array imaging system, the individual transducer

elements must be capable of receiving as well as transmitting ultrasound. The pulse-echo characteristics of an element in DAML have been studied by measuring the two-way insertion loss [17]. A quasi-continuous wave input was coupled through a directional coupler to the transducer that transmits

2 3 4 5 6 Frequency (MHz)

(b)

Fig. 13. (a) Broadband tuning circuit used for pulse+cho experiments. (b) Measured electrical impedance from tuned DAML element.

an ultrasound pulse toward a spherically shaped brass mirror with a radius of curvature of 1 cm. The 2-D array element was located at the center of curvature of the mirror, and the returning ultrasound pulse amplitude is compared to the amplitude obtained by coupling the input pulse to an open circuit, ensuring complete reflection. It was found that the matching circuitry of the element had to be altered to measure insertion loss, since the return signal was not detectable in the ringdown of the excitation pulse. The tuning circuit used on the DAML element was designed to maximize the two-way pulse characteristics over the frequency range 3 to 5 MHz (Fig. 13(a)). The measured electrical impedance from the tuned DAML element is shown in Fig. 13(b). The characteristics of the tuning are that when the impedance magnitude is high (compared to 50 $2) the phase angle is close to 0", and when the phase angle is close to * 90" the magnitude is close to 50 R. This ensures that the power transmission coefficient is relatively constant over the entire frequency range, which maximizes the bandwidth of the 2-D array element.

The unamplified two-way return pulse reflected from the brass mirror is shown in Fig. 14(a). The main pulse is about four cycles in length, but as in the one-way waveform there are significant reverberations at the end of the pulse. The two- way insertion loss for DAML is shown in Fig. 15, having a minimum loss of -41 dB at 4.2 MHz. This level of insertion loss may be quite acceptable in a 2-D array element since in most transmit and receive situations a number of elements will be operating simultaneously. Kino and DeSilets reported a


l o o r

-1 00 L 0 1 2 3 4 5

Time (us)

-50 I 0 1 2 3 4 5

Time (us)

0.8 t a

Freauency (MHz)

(c) Fig. 14. Pulse-echo waveforms measured from (a) DAML element. (b) DAML element with backing. (c) Spectrum of pulse from DAML with backing.

two-way insertion loss of -25 dB for 10 linear-array elements operating in parallel [5 ] . It is reasonable to expect that the two-way insertion loss of a full 2-D array made from DAML material would be close to -10 dB, as reported in annular- array transducers made from composite, diced-array transducer materials [18].

The two-way pulse from the DAML element with backing material is shown in Fig. 14(b), and the corresponding spectrum in Fig. 14(c). The broadband characteristics of the pulse are apparent, and there has been a significant reduction in the level of the trailing reverberations. The insertion loss drops to -47 dB at 4.2 MHz as a result of the backing material

0 DAML D DAML + Backing

2.5 3.0 3.5 4.0 4.5 5.0 5.5 Frequency (MHz)

Fig. 15. Two-way insertion loss measured from DAML elements.

(Fig. 15). The main problem with this array configuration is the severe narrowing in element directivity evident in Fig. 9(c). The increased cross-coupling between elements through the backing material was perhaps predictable from past investigations on linear phased arrays. Kino and Desilets [5] found that the transducer elements had to be slotted, with the cuts extending down into the backing material, in order to keep element cross-coupling below the -20-dB level.

V. CONCLUSION

The fabrication of diced 2-D array materials from PZT5A and epoxy have been described, and pulse shapes and radiation patterns were measured from single elements in these arrays. A cross-coupling CW diffraction theory was used to model the radiation patterns of diced-array elements with and without a conductive matching layer. The inclusion of the matching layer involved only a small change to the fabrication method, but produced a significant improvement in the bandwidth of the 2-D array element, as well as decreasing the level of acoustical cross-coupling in the array. A multicomponent tuning circuit was used to match the electrical impedance of the element to the 50-52 pulserireceiver used, and the resulting two-way insertion loss of the 100 pm x 100 pm element was found to be close to -40 dB. The inclusion of a lossy backing material improved the pulse characteristics of the 2-D array element, but narrowed the element directivity as a result of increased cross-coupling through the backing material. As in linear phased arrays, fabrication methods for 2-D medical ultrasound arrays may require cutting slots between elements extending into the backing material.

The simple CW cross-coupling theory presented showed excellent agreement with experimental results but is limited by the underlying assumptions of single mode vibration characteristics of isolated elements, coupled electrically and by a single acoustic wave propagating through the array. The predicted directivities using the CW theory are similar to broadband results given in an earlier paper [2], where electrical and acoustical cross-coupling have been investigated separately. A more rigorous analysis of the cross-coupling behavior in


a 2-D array could be performed using a 3-D finite element approach [ 191.

It is interesting that the air-backed DAML diced-array material produced characteristics most suitable for phased-array applications. Both electrical and acoustical cross-coupling in DAML are close to -20 dB. The 100 pm x 100 pm DAML element has a -3-dB directivity of S o , and the pulse shape is quite constant over more than 60’ in the off-axis angle. The -40-dB insertion loss of an individual DAML element may be acceptable since in a 2-D transducer array there will be a number of elements operating simultaneously on both transmit and receive. The reverberations at the end of the pulse will be suppressed when incoherent waveforms from many elements are summed together. This is the principle employed in a 12-element annular array developed by Foster et al. [20], and preliminary experiments indicate that there is phase incoherency in the trailing edge reverberations of adjacent elements in the DAML array.

The most serious problems facing the development of 2-D arrays for medical imaging involve the number of elements required for focusing and steering the ultrasound beam, and the transducer materials that might be incorporated into such arrays. The integration of the 2-D array material into a hybrid silicon structure holds some promise for simplifying the passive matching network that was required to operate the 2-D array element in a pulse-echo mode. Work currently being done to create integrated polyvinylidene difluoride (PVDF) linear phased arrays [21] could be extended to “diced-array’’ materials in order to decrease element impedance compared to low permittivity materials like PVDF. One interesting transducer material for 2-D array applications is the relaxor ferroelectric Pb(Mg1,3Nb2j3)03 (PMN) and solutions of PMN with PbTi03 (PMN-PT), which are characterized by very large dielectric constants and reasonably large electromechanical coupling coefficients [22].

ACKNOWLEDGMENT The authors thank Henry Yoshida, Paul Lum, and Fleming

Dias of Hewlett Packard Laboratories for assistance in array fabrication, Hans Schwendener of the Ontario Cancer Institute (now at the Sudbury Regional Cancer Centre) for building the water tank experimental equipment, and Geoff Lockwood of the Sunnybrook Health Science Centre for devising the electrical matching network described in this paper.

REFERENCES

[l] S. W. Smith, H. G. Pavy, and 0. T. von Ramm. “High-speed ultrasound volumetric imaging system-Part I: Transducer design and beam steering,” IEEE Trans. Ultrason., Ferroelec., Freq. Contr., vol. 38, pp. 100-108, 1991.

[2] D. H. Turnbull and F. S. Foster, “Beam steering with pulsed two- dimensional transducer arrays,” IEEE Trans. Ultrason.. Ferroelec., Freq. Contr., vol. 38, pp. 320-333, 1991.

131 H. P. Savakas, K. A. Nicker, and R. E. Newnham, “PZT-epoxy . _ piezoelectric transducers: A simplified fabrication procedure,” Mat.*Res. Bull., vol. 16, pp. 677480, 1981.

[4] H. Takeuchi and C. Nakaya, “PZTipolymer composites for medical

151 G. S. Kino and C. S. DeSilets, “Design of slotted transducer arrays with ultrasonic probes,” Ferroelec., vol. 68, pp. 5 3 4 1 , 1986.

[6] B. Delannoy, H. Lasota, C. Bruneel, R. Torguet, and E. Bridoux, “The infinite planar baffles problem in acoustic radiation and its experimental

[7] A. R. Selfridge, G. S. Kino, and B. T. Khuri-Yakub, “A theory for verification,” J . Appl. Phys., vol. 50, pp. 5189-5195, 1979.

the radiation pattern of a narrow-strip acoustic transducer,” Appl. Phys. Lert., vol. 37, pp. 35-36, 1980.

[8] B. Delannoy, C. Bruneel, F. Haine, and R. Torguet, “Anomalous behavior in the radiation pattern of piezoelectric transducers induced by parasitic Lamb wave generation,”J. Appl. Phys.: vol. 51, pp. 3942-3948, 1980.

[9] J. D. Larson, “Non-ideal radiators in phased array transducers,” in Proc. 1981 IEEE Ultrason. Symp., pp. 673483.

[IO] N. De Jong, J. Souquet. G. Faber, and N. Bom, “Transducers in medical ultrasound-Part Two: Vibration modes, matching layers and grating lobes,’’ Ultrasonics, pp. 176182, 1985.

[l11 C. V. Stephenson, “Vibrations in long rods of barium titanate with electric field parallel to length,” J. Acoust. Soc. Amer., vol. 35, pp.

[l21 R. Krimholtz, D. A. Leedom, and G. L. Matthaei, “New equivalent circuits for elementary piezoelectric transducers,’’ Electron. Lett., vol. 6 ,

(131 C. S. Desilets, J. D. Fraser. and G. S. Kino, “The design of efficient broad-band piezoelectric transducers,” IEEE Trans. Sonics Ultrason.,

(141 D. H. Turnbull, M. D. Sherar, and F. S. Foster. “Determination of electromechanical coupling coefficients in transducer materials with high mechanical losses,” in Proc. 1988 IEEE Ultrason. Symp., pp. 631-634.

[IS] K. N. Bates, “Tolerance analysis for phased arrays,” in Acoustical Imaging, vol. 9, K. Y. Wang, Ed. New York: Plenum, 1979, pp. 239-262.

[ 161 B. D. Steinberg, Principles ofAperture andArray System Design. New York: Wiley, 1976.

[l71 F. S. Foster, L. K. Ryan, and D. H. Turnbull, “Characterization of lead zirconate titanate ceramics for use in miniature high frequency (20-80 MHz) transducers,” IEEE Trans. Ultrason., Ferroelec., Freq. Contr., vol. 38, pp. 446453, 1991.

[l81 A. Shaulov, W. A. Smith, J. Zola, and D. Dorman. “Curved annular array transducers made from composite piezoelectric materials,” in Proc. 1989 IEEE Ultrason. Symp., pp. 951-954.

(191 R. Lerch, “Simulation of piezoelectric devices by two- and three- dimensional finite elements,” IEEE Trans. Ultrason., Ferroelec., Freq. Contr., vol. 17, pp. 233-247, 1990.

[20] F. S. Foster, J. D. Larson, M. K. Mason, T. S. Shoup, G . Nelson, and H. Yoshida, “Development of a 12 element annular array transducer for realtime ultrasound imaging.” Ultrasound in Med. and Bioi., vol. 15, pp. 649-659, 1989.

[21] J. H. MO, A. L. Robinson, D. W. Fitting, F. L. Terry, and P. L. Carson. “Micromachining for improvement of integrated ultrasonic transducer

[22] H. Takeuchi, H. Masuzawa, C. Nakaya, and Y. Ito, “Relaxor ferroelectric sensitivity,” IEEE Trans. Electron. Devices, vol. 37, pp. 130-140, 1990.

transducers,” in Proc. 1990 IEEE Ultrason. Symp., 1990. pp. 697-705.

1192-1 194, 1956.

pp. 388-389, 1970.

vol. SU-25, pp. 115-125, 1978.

Daniel H. lhmbull (”92) was born in Niagara Falls, ON, Canada, in 1955. He received the BSc. degree in mathematics in 1978 from Brock Univer- sity, St. Catharines, ON. the M.S. degree in applied mathematics in 1981 from the California Institute of Technology, Pasadena, CA, and the M.A.Sc. degree in mechanical engineering in 1983 and the Ph.D. degree in medical biophysics in 1991 from the University of Toronto, Toronto, ON.

He was a Systems Engineer at Imatron, Inc., a medical imaging company i n South San Francisco,

CA, from 1983 to 1986. He is currently employed in the Department of Medical Physics at the Toronto-Bayview Regional Cancer Centre. His current research interests include transducer materials, ultrasonic phased-array

matched backings,” Ultrasonic Imaging, vol. 1. pp. 189-209, 1979. systems, applications of very high-frequency ultrasonics to medical imaging.

TURNBULL AND FOSTER: FABRICATION AND CHARACTERIZATION OF TRANSDUCER ELEMENTS IN 2-D ARRAYS

F. Stuart Foster ("91) was born in Montreal, PQ, Canada, on July 29, 1951. He received the B.A.Sc. degree in engineering physics in 1974 from the University of British Columbia, Vancouver, BC, Canada, in 1974, and the MSc. and Ph.D. degrees in medical biophysics in 1977 and 1980, respectively, from the University of Toronto, Toronto, ON, Canada.

He is presently a Senior Scientist with the Sun- nybrook Health Science Centre, a Senior Research Scholar of the National Cancer Institute of Canada,

and a Professor of Medical Biophysics at the University of Toronto. He was S Senior Scientist with the Ontario Cancer Institute from 1980 to 1991. He has been involved with the development of conical and annular array transducers, ultrasound backscatter microscopy, and tissue characterization, and more recently, in two-dimensional array technology and intravascular imaging. He has twice won the Ultrasound in Medicine and Biology Prize.

Dr. Foster is on the editorial boards of Ultrasonic Imaging and Ultrusound in Medicine and Biology.

475

fabrication and characterization of transducer elements in ......ultrasound beam. recently, several...

Documents