10.1109/ULTSYM.2013.0506
Wide Aperture Convex Array Transducer with PMNPT Piezoelectric Single Crystals Heewon Kim, Jongkil Kim, Susung Lee, Sangwoong Lee, Boyeon Cho, Wonho Noh
Nelson H. Oliver Three Umbrellas Services Beaverton, Oregon, USA
Probe Development Dept Alpinion medical systems Seoul, Korea
[email protected]
[email protected]
Abstract—The piezoelectric single crystal PMN-PT has lower acoustic impedance and a higher electromechanical coupling coefficient than conventional piezoelectric materials. For these reasons, single crystal PMN-PT can be a good way to enhance the sensitivity and increase the bandwidth of ultrasonic transducers. However the single crystal has certain limitations in size, manufacturability, and a higher electrical impedance. Thus transducer manufacturers commonly use single crystal material only for a small aperture and lower frequency transducer designs such as a phased array transducer. This paper presents the application of single crystal PMN-PT to a convex transducer with a large aperture, including adaptive design and manufacturability considerations. Keywords-component; single crystal, wide aperture, convex array
aperture size (≥65mm azimuth length, 60mm radius of curvature, 192 channels) and a higher center frequency (≥4MHz) than the typical phased array transducer. II.
A. Target Requirements The target applications are abdominal and OB/GYN, with 192channels and a 60mm radius of curvature (ROC) selected for the transducer architecture. An integrated imaging system using a maximum of 128 channels for beam-forming is targeted. Table I lists the specific acoustic performance targets. TABLE I.
I.
INTRODUCTION
Many kinds of PZT (Pb(ZrTi)O3) may be used for the piezoelectric material in a medical diagnostic ultrasound transducer. The PZT ceramic has a high acoustic impedance compare to human tissue, which limits its electro-mechanical coupling and therefore a transducer’s performance. Many types of piezo-composites have been developed to overcome this impedance-mismatch limitation by combining PZT and a polymer to provide an active material with an acoustic impedance closer to that of tissue [1]. More recently, acoustic designers have begun to incorporate single-crystal PMN-PT material into their designs, which offers both a lower acoustic impedance and a higher electromechanical coupling coefficient than conventional PZT. The attractive properties of PMN-PT allow for the design of transducers that are higher sensitivity and broader bandwidth [2],[3]. However commercial single crystal has certain problematic limitations, including high cost, small available sizes, fragility that makes it difficult to grind and dice, and finally a capacitance that is lower than PZT. Thus many transducer manufacturers commonly use single crystal material only for a small aperture and lower frequency transducer designs such as a phased array transducer [4].[5]. In this paper, we describe how single-crystal PMN-PT is applied to a wide aperture convex transducer design that has a larger
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TRANSDUCER DESIGN
Target value
TARGET PERFROMANCE
2way Sensitivity*
-6dB center frequency
-6dB fractional bandwidth
-20dB pulse length
≥ -55 dB
4 MHz
≥ 80 %
≤ 4 cycles**
ⅹ
ⅹ
* 20 log10 (Rx voltage / Tx voltage) ** at -6dB center frequency
Because the actual application is not in the pure water of a test tank, but in the absorptive tissue environment, a Gaussian spectrum is needed. In addition, there must be no beam divergence to the maximum effective imaging depth of 150mm. B. Designing The proper pitch is calculated based on the requirements of a maximum 128 system channels for beam forming, a 4MHz center frequency, and a 60mm ROC. Figure 1 Illustrates how the proper pitch varies with changes in channel count and ROC, and how 340μm is selected as the pitch. The optimum thickness of the single-crystal material is calculated based on the target center frequency and frequency down-shift factors. A design incorporating two sub-elements per acoustic element is selected to avoid spurious resonances [6].
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Figure 1. Proper pitch calculation at 4MHz.
Two matching layers are developed by an acoustic impedance calculation using transmission line theory [7]. FEM simulation with PZflex (Weidlinger Associates Inc.) and DOE (design of experiment) with a reduced gradient algorithm are performed to find the optimal design thickness of each layer. A thin substrate that is acoustically similar to the backing block is used for manufacturing, component lamination, dicing, and forming to the final curved shape. Table II illustrates design outputs and Figure 2 shows the simulation results. TABLE II. Name of material
1st matching layer 2nd matching layer Substrate Backing block Lens
DESIGN OUTPUTS Impedance [Mrayls]
Thickness [microns]
7.9 3.1 3.2 3.1 1.2
90 94 1520 ≥5000 913
Figure 3. Elevation beam profile simulation.
III.
PROTOTYPE
Because single crystal material has a low Curie temperature, any materials and processes requiring high temperatures are avoided. The component lamination process is divided into three steps. First, two single crystals are aligned and bonded to the flexible printed circuit board, so that the junction of the two crystals falls within a kerf at dicing. Fiducial marks on the flexible printed circuit board, together with a customized alignment tool, facilitate the critically important crystaljunction alignment. In the second step, the flex-crystal assembly is bonded to the two matching layers and the substrate. After dicing, the array is bonded to a convex backing block in the third and final lamination step. Kerf filling and lens casting are performed in separate steps, completing the fabrication process. Figure 4 illustrates whole process flow chart. Figure 5 shows a photo of the aligned flex-crystal assembly with its two short single crystals, and a photo of the completed prototype transducer.
Figure 2. PZflex result
A silicone elastomer that has a lower longitudinal velocity than human tissue is used as a lens material. A beam-field simulation is performed using the Field II program to find the proper mechanical focusing to meet the depth requirement [8]. Figure 3 shows beam-field simulation versus mechanical focus, along with some measured benchmarking data. A mechanical focus of 95mm and an elevation radius of curvature of 51.3mm are chosen based on the longitudinal velocity of the silicone lens material [9].
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Figure 4. Process flow chart
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domain responses, which are an average of all the channels. The second row in Figure 7 shows the response of a single channel in the center of the array. These two pairs of graphs are almost identical, demonstrating the acoustic uniformity of the channels. The last two graphs in Figure 7 shows the sensitivity variation and phase uniformity across all the channels. The key finding of these last two plots is there is no discontinuity between the 96th channel and the 97th channel, which is where the two single crystals are joined. Therefore a manufacturing process is successfully demonstrated for synthesizing a wide aperture transducer from two shorter single crystals.
Figure 5. Upper:aligned assembly with two single crystals and flexible circuit; Lower:completed prototype transducer
IV.
MEASUREMENT
Low-capacitance coaxial cable with series inductor tuning is used to match the high electrical impedance of single crystal array. A custom water tank and test system are used for the 2way response characterization. Figure 6 shows the test system that consists of a pulser/receiver, an oscilloscope and a multiplexer. The alignment between the transducer and a curved steel target is performed automatically with a LabVIEW program. The test medium is 37.5°C deionized water.
Figure 7. 2-way response test report.
For the beam field measurement, an AIMS system (Onda Corporation) and needle-type hydrophone are used. Figure 8 shows the beam-field measurement result, which closely matches the simulation. Therefore our targeted ROI (region of interest) extending 150mm into the image is covered without any noticeable beam divergence.
Figure 6. Customized 2-way test system.
All measured data are post-processed with MatLab programs. Figure 7 shows test results that include time- and frequency-domain responses along with the sensitivity uniformity and TOF (time of flight). The top row of plots in Figure 7 show the composite time-domain and frequency-
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Figure 8. Beam field measurement result.
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V.
[6]
IMAGE EVALUATION
After integrating the prototype transducer into an imaging system (E-cube 15, Alpinion Medical Systems), a live scan is performed. Figure 9 shows images captured with the transducer, on the left the abdominal aorta, and on the right an image of a fetus. The signal intensity and resolution are sufficient across the whole ROI (region of interest) to use this wide-aperture, convex array, single-crystal transducer in both abdominal and OB/GYN applications.
[7]
[8] [9]
Ronald E McKeighen, “Design Guidelines for Medical Ultrasonic Arrays,” SPIE Vol. 3341, 1998. Lawrence E. Kinsler, Austin R. Frey, Alan B. Coppens, James V. Sanders, “Fundamentals of Acoustics,” third edition, John Wiley & Sons, pp.124-140, 1982. J. A. Jensen, “Field: A program for simulating ultrasound systems,” Med. Biol. Eng. Comput., vol.34, 1996. Ching-Hua Chou, Butrus T, Khuri-Yakub, Gordon S. Kino, “Lens Design for Acoustic Microscopy,” IEEE Transactions on Ultrasonics, Ferroelectrics, And Frequency Control, Vol 35 no 4 July 1988.
Figure 9. Live scan image (left : liver, abdominal application; right : fetus, OB/GYN application)
VI.
CONCLUSION
A wide-aperture convex transducer using two PMN-PT single crystals has been designed, fabricated and characterized. The single crystal material PMN-PT has attractive piezoelectric properties that can give better transducer performance. The challenges arising from the small available sizes and fragility of single-crystal material can be overcome through an adaptive design and process method. The 2-way response uniformity across all channels in our prototype transducer shows the viability of our approach. The broad Gaussian frequency response and high sensitivity of the single crystal transducer make it useful for abdominal and OB/GYN applications. We hope that this trial and its excellent results encourage wider adoption of single crystal transducers for medical sonography. ACKNOWLEDGMENT This work was supported by the R&D program of MOTIE/KEIT(10042581) REFERENCES [1] [2]
[3]
[4]
[5]
Wallace Arden Smith, “The Role of Piezocomposites in Ultrasonic Transducers,” IEEE Ultrasonics Symposium, pp.755-766, 1989. P. D. Lopath, S. E. Park, K. K. Shung, and T. R. Shrout, “Ultrasonic Transducer Using Piezoelectric Single Crystal Perovskites,” Proc, 10th IEEE Int. Symp. Appl. Ferroelectrics, pp.543-546, 1996. Ping Sun, Qifa Zhou, Benpeng Zhu, Dawei Wu, Changhoon Hu, Jonathan M. Cannata, Jin Tian, Pengdi Han, Gaofeng Wang, and K. Kirk Shung, “Design and Fabrication of PIN-PMN-PT Single-Crystal HighFrequency Ultrasound Transducers,” IEEE Trans Ultrasonic Ferroelectric Frequency Control, 56(12): 2760-2763, December, 2009. X. Ming Lu, T. L. Proulx, “Single Crystals vs PZT Ceramics for Medical Ultrasound Applications,” IEEE Ultrasonics Symposium, pp.227-230, September, 2005. Jie Chen and Rajesh Panda, “Review: Commercialization of Piezoelectric Single Crystals for Medical Imaging Applications,” IEEE Ultrasonics Symposium, 2005.
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2013 Joint UFFC, EFTF and PFM Symposium
