Piezoelectric Materials for Imaging

CME Introducing With this issue, the Journal of Ultrasound in Medicine (JUM) is pleased to introduce a new series of occasional articles under the banner Reverberations, intended for our clinical readership. These articles are the result of efforts by the American Institute of Ultrasound in Medicine (AIUM) Technical Standards Committee to help deepen physician and sonographer knowledge of ultrasound science and technology. This series of articles will cover various aspects of ultrasound technology, at a relatively basic tutorial level, with continuing medical education (CME) credit, in part to entice ultrasound professionals to read them and to give them credit for enhancing their knowledge of ultrasound technology. Each article is designed to address one specific aspect of ultrasound technology and will provide appropriate CME questions. This issue contains the first article in the series. The article, by Szabo and Lewin, discusses applications of piezoelectric materials to imaging. Future articles will discuss harmonic imaging, bioeffects, and other topics of interest and importance to ultrasound professionals. Because the significant progress that has occurred in medical ultrasound has been the result of important scientific and technical advances, the Reverberations articles are one way in which the JUM and the Technical Standards Committee are working to enhance educational opportunities for the AIUM membership by discussing these advances in an easily accessible format. As there is a great need for physicians and sonographers to understand the basic science and technology of their daily profession, it is our goal to encourage our membership to read these articles and add to their foundation of knowledge to enhance the performance of their profession. We at the JUM and the Technical Standards Committee welcome requests and suggestions for topics to be included. Please address comments and suggestions to Thomas R. Nelson, PhD, Deputy Editor, at jumaium@aol.com (please include Reverberations in the subject line of e-mails), or Journal of Ultrasound in Medicine, Reverberations, 333 Longwood Ave, Suite 400, Boston, MA 02115 USA. Thomas R. Nelson, PhD Piezoelectric Materials for Imaging Thomas L. Szabo, PhD, Peter A. Lewin, PhD The purpose of this article is to present the reader with a brief description of those characteristics of piezoelectric materials that directly influence imaging. Key words: array; imaging; piezoelectric; transducer; ultrasound. Abbreviations BW, bandwidth; LiNbO 3, lithium niobate; MR, megarayls; PMN-PT, lead magnesium niobate-lead titanate; PSMNZT, lead scandium niobate-lead magnesium niobatepoly(2,6)-naphthalene naphthalate-lead strontium zirconium titanate; PVDF, polyvinylidene difluoride; PZT, lead zirconate titanate; TF, transmission factor Received August 3, 2006, from the Department of Biomedical Engineering, Boston University, Boston, Massachusetts USA (T.L.S.); and School of Biomedical Engineering, Science, and Health Systems, Drexel University, Philadelphia, Pennsylvania USA (P.A.L.). Revision requested September 5, 2006. Revised manuscript accepted for publication November 10, 2006. Address correspondence to Thomas L. Szabo, PhD, Department of Biomedical Engineering, Boston University, 44 Cummington St, Boston, MA 02215 USA. E-mail: tlszabo@bu.edu CME Article includes CME test U ltrasound transducers work because of piezoelectricity. Piezoelectricity was discovered by the Curie brothers in the 1880s. 1 They found that an electric charge appeared on electrodes placed on a compressed quartz crystal: the direct piezoelectric effect shown in Figure 1A. They also verified the reverse piezoelectric effect illustrated in Figure 1B by measuring a displacement caused by a voltage applied to a quartz crystal. These are the key properties that enable a transducer to transmit ultrasound and, reciprocally, to generate electrical signals from received ultrasound waves. Until the 1950s, piezoelectricity was exploited in naturally occurring crystals such as quartz, Rochelle salt, and tourmaline. Since then, several new piezoelectric materials have been created, and these are compared in Table 1. In a nutshell, all piezoelectric devices contain material that converts mechanical into electrical energy when a stress is applied and, conversely, electrical energy into mechanical when a voltage is applied to electrodes that are appropriately deposited on the piezoelectric material. 2007 by the American Institute of Ultrasound in Medicine J Ultrasound Med 2007; 26:283 288 0278-4297/07/$3.50

Piezoelectric Materials for Imaging A B Figure 1. A, The direct piezoelectric effect is a buildup of (electric) charge and voltage caused by a change in thickness of the piezoelectric material by an applied mechanical force (eg, returning ultrasound echoes). B, Reverse piezoelectric effect shown as a change in the thickness of the piezoelectric material caused by a voltage applied to electroded side surfaces marked A. How Do Piezoelectric Transducers Work? The heart of the transducer is a block of piezoelectric material with electrodes on two sides marked A in Figure 2. Basically, this block is a capacitor with dielectric material between the electrodes. The capacitance is found from (1) C 0 = εa/d, where ε is the electrical permittivity, A is the electrode area, and d is the thickness. This material also contains electrical dipoles that can align with an electric field. An analogous situation can be found in magnetic materials. Iron filings align with the field lines in a magnetic field. When a solid piece of paramagnetic material is placed in a field, the material is magnetized when the magnetic dipoles align with the field to create north and south poles. Such materials that contain aligned magnetic moments are referred to as ferromagnetics. In a similar way, the piezoelectric material is more than a mere dielectric; it is ferroelectric because it consists of electric dipoles that are or can be aligned to each other. Piezoelectric materials differ in their ability to align dipoles, as discussed in the next section. Other ferroelectrics include electrostrictive (square law responders) piezoelectric materials and ferroelectrets. Electroded piezoelectric blocks as shown in Figure 1 are also acoustic resonators. Because of the usual acoustic mismatch between the block and its softer surrounding material (air, water, or tissue), the block resonates at a frequency f 0 = c/2d, where c is the sound speed in the piezoelectric, as illustrated by Figure 2. In fact, it resonates at odd harmonics too (3f 0, 5f 0, etc) because these waves encounter softer materials at their electroded surfaces and bounce back into the piezoelectric a number of times. Therefore, the piezoelectric block is not just a capacitor; it is a singing capacitor. A measure of this acoustic mismatch and how well acoustic energy is transferred from a piezoelectric crystal to adjacent material (such as tissue) is the transmission factor (TF). Piezoelectric material typically is hard and has an acoustic impedance, Z c, on the order of 30 megarayls (MR), that is, 30 10 6 Rayls. From acoustic theory, the energy that is transferred from one acoustic material, Z c, to an adjacent material of impedance Z can be quantified as (2) TF = 4ZZ c /(Z c + Z) 2. 284 J Ultrasound Med 2007; 26:283 288

Szabo and Lewin Table 1. Key Piezoelectric Transducer Design Parameters for Several Piezoelectric Materials and Composites 2 10 C TL, Z L, Material ε S /ε 33 0 k T km/s MR TF 100*BW Quartz 4.5 0.093 5.0 13.3 0.3720 1.10 LiNbO 3 39 0.49 7.36 34.2 0.1649 30.56 PZT-5A* 830 0.66 3.227 25.01 0.2185 55.45 PZT-5H* 1470 0.70 3.80 29.0 0.1915 62.38 PMN-PT* 680 0.9066 3.057 24.64 0.2214 104.6 Comp A 376 0.80 3.0 18.03 0.2899 81.47 PVDF 12 0.11 2.2 3.92 0.8099 150.0 PSMNZT* 2700 0.69 3.80 29.0 0.1915 60.62 Y. Hosono, MS, written communication, 2005. *For these materials, 33 constants are tabulated instead, such as Z 33 and c 33. This subscript 33 applies to the parameters used in an array geometry. Permittivity is normalized to a free space value. PZT is a registered trademark of the Vernitron Piezoelectric Division (Bedford, OH). The BW for PVDF is a round-trip value estimated for a receiver configuration. Later, this factor can be used to compare different materials. Because a typical crystal has two electroded surfaces, energy is transferred out through both surfaces depending on loading. On the surface facing away from tissue, a backing material is used to shape the pulse, enhance sensitivity, and broaden bandwidth, BW, important for subharmonic and harmonic imaging. On the side facing tissue, one or more matching layers are used to minimize the mismatch between the piezoelectric material and tissue, so TF can be made approximately equal to 1 (100% energy transfer). A theory of matching layers will be explained in more detail in the next article on transducers. The most important property of a piezoelectric is how it can convert electric energy to acoustic energy (and vice versa, which is the same by reciprocity). The accepted measure of this property is the electromechanical coupling factor, k. The ratio of the acoustic to electromagnetic energies is proportional to k 2. In the following discussion, it is assumed that the piezoelectric materials operate in the linear region, that is, that the resulting pressure or echo voltage is proportional to the driving voltage or incoming pressure. Note that the resulting effect depends on the polarity of the incoming signal. On the electrical side, how well the transducer electrical impedance is matched to the electronics of the imaging system controls the transfer of electrical energy in and out of the transducer. This impedance is called the radiation impedance. When the real part of the radiation impedance, the radiation resistance, is close in value to the electrical source impedance, energy transfer is efficient. The radiation resistance is proportional to the coupling constant squared and inversely proportional to the capacitance. This relationship, along with equation 1, means that the radiation impedance is inversely proportional to the electrical permittivity. Large values of the electrical impedance of a smallarray element can cause a significant electrical mismatch; therefore, this mismatch can be reduced by using as large a permittivity as possible. The mismatch can also be reduced (but not eliminated) by using an inductor or electrical matching network, which tunes out the nonreal impedance of the capacitance. Finally, the fractional BW achievable with a piezoelectric material can be estimated by the electrical quality factor Q e : (3) BW = 1/Q e = 4k 2 /π. Figure 2. Basic construction of a piezoelectric imaging transducer, showing the piezoelectric material with deposited electrodes, attached terminals, backing, and matching layers. J Ultrasound Med 2007; 26:283 288 285

Piezoelectric Materials for Imaging How Do Piezoelectric Materials Compare? The parameters just introduced can be used as a basis for the comparison of different piezoelectric materials. The parameters, listed in order by column in Table 1 2 10 (Y. Hosono, MS, written communication, 2005), are relative electrical permittivity, electromechanical coupling constant, sound speed, acoustic impedance, acoustic TF, and BW (percent). Graphs of these parameters are given in Figures 3 6. As already mentioned, the Curie brothers discovered piezoelectricity on quartz crystals. 1 This material is still in use today for precise timing and resonator applications. Quartz is a naturally occurring single-crystal material. Because of its symmetric structure and chemical composition, it is weakly piezoelectric with a low coupling constant of 0.093, implying extremely narrow BWs (Figure 5). The dielectric constant is extremely low (4.5), indicating difficult electrical matching (Figure 3). Lithium niobate (LiNbO 3 ) is a synthetically made hard single crystal and was designed to have a moderate coupling constant (0.49) but has a relatively low dielectric constant (39). 1 The moderate acoustic impedance (34.2 MR) results in poor energy transfer to tissue. It is a glasslike material unsuitable for fabrication of modern ultrasound transducers such as imaging arrays. The discovery of lead zirconate titanate (PZT) 2 ceramics in 1954 led to a family of high-coupling (0.7) synthetic materials suitable for many applications. 2 The combination of high coupling and Figure 4. Acoustic TF of piezoelectric materials. a large dielectric constant (830 1470 in Table 1) makes these materials the most popular choice for ultrasound imaging transducers and arrays. These ceramics contain electric dipoles that align to a large degree when the material is poled. Poling is the application of a strong electric field across the material analogous to the magnetization of a paramagnetic material. As noted earlier, the moderate BWs and matching to tissue can be improved through the use of acoustic matching layers. Recently, domain-engineered single crystals such as lead magnesium niobate-lead titanate (PMN-PT) have been developed. 4 6 Even though these crystals take a longer time to manufacture, they offer exceptional performance. Once poled, nearly all the electric dipole domains are aligned with the poling field. This high degree of alignment produces coupling constants greater than 0.9 and fractional BWs approaching 100% with Figure 3. Relative (normalized) electrical permittivity of piezoelectric materials. Asterisks in Figures 3 6 refer to Table 1. Figure 5. Bandwidth in percent for piezoelectric materials. 286 J Ultrasound Med 2007; 26:283 288

Szabo and Lewin comparable electrical matching to conventional PZT materials (see Figure 5). A 2-octave BW is very desirable because it would allow one imaging probe to be used for several imaging frequencies or for both subharmonic and harmonic imaging. The permittivity of PMN-PT, however, is less than that of PZT materials. Composites are a marriage of high-coupling piezoelectrics with soft materials. 3,7 One of the most popular configurations consists of crystal posts embedded in an epoxy matrix. This arrangement preserves high electromechanical coupling and, because the overall combined density is low, creates a piezoelectric composite with a lower acoustic impedance. The example in Table 1, Comp A, combines a wide BW with improved acoustic matching (18 MR) and low permittivity. 7 Piezoelectric organic polymers such as polyvinylidene difluoride (PVDF) are softer, conformable piezoelectrics that are often used in sheet form. 8 Their low acoustic impedance (4 MR) provides good matching to water and tissue. Because of their low coupling (0.11) and high internal mechanical losses, they make poor transmitting transducers; however, these materials function well as wideband receivers and are often used as hydrophones. Hydrophones are indispensable to determine the thermal index and mechanical index that are widely used as safety indicators in ultrasound diagnostics. A new class of complex system piezoelectrics such as lead scandium niobate-lead magnesium niobate-poly(2,6)-naphthalene naphthalate-lead Figure 6. Overall factor equal to the product of normalized e33 and BW for piezoelectric materials. strontium zirconium titanate (PSMNZT) shares similar properties with PZT in terms of BW and acoustic properties 9 (Y. Hosono, MS written communication, 2005). Their advantage of much higher permittivity provides significantly improved electrical matching for small elements (see Figure 3). Conclusions Two major aspects of piezoelectric materials that directly affect imaging are sensitivity and BW. Sensitivity is influenced by several factors: electrical permittivity, electromechanical coupling, and the acoustic TF. Of these, the TF is less important because acoustic mismatch can be eliminated with acoustic matching layers. Sensitivity and, therefore, the achievable penetration depth are related to the square of the electromechanical coupling constant and to permittivity. For small-array elements, a larger permittivity reduces electrical mismatch of the element to the imaging system electronics. Single-crystal piezoelectrics and composites offer significant improvements in BW (Figure 5), the most important factor. Bandwidth and normalized permittivity are multiplied to form an overall figure of merit that is plotted in Figure 6. The near doubling of permittivity in PSMNZT materials can provide substantial increases in sensitivity. For more details about transducers, see the references by Szabo, 1 Sonic Concepts, 10 and Reid and Lewin. 11 More information about transducer design, types, and applications will be covered in a forthcoming publication. References 1. Szabo TL. Diagnostic Ultrasound Imaging: Inside Out. Burlington, MA: Elsevier; 2004. 2. Berlincourt D. Piezoelectric crystals and ceramics. In: Mattia OE (ed). Ultrasonic Transducer Materials. New York, NY: Plenum Press; 1971:chap 2. 3. Zipparo M, Oakley C, Hackenberger W, Hackenberger L. Single crystal composites, transducers and arrays. Proc IEEE Ultrason Symp 1999; 965 968. 4. Sato S, Kobayashi T, Takeuchi T, Harada K, Shimanuki S, Yamashida Y. A 3.7 MHz phased array probe using PZN- 9% PT single crystal. IEEE Trans Ultrason Ferroelectr Freq Control 1999; 46:424 421. J Ultrasound Med 2007; 26:283 288 287

Piezoelectric Materials for Imaging 5. Hackenberger WC, Rehrig PW, Ritter TA, Shrout TR. Advanced piezoelectric materials for medical ultrasound transducers. Proc IEEE Ultrason Symp 2001; 1101 1104. 6. Gururaja TR, Schulze WA, Cross LE, Newnham RE. Piezoelectric composite materials for ultrasonic transducer applications, part 11: evaluation of ultrasonic medical applications. IEEE Trans Sonics Ultrasonics 1985; SU-32: 499 513. 7. Ritter T, Geng X, Shung K, Lopath P, Park SC, Shrout T. Single crystal PZN/PT polymer composites for ultrasound transducer applications. IEEE Trans Ultrason Ferroelectr Freq Control 2000; 47:792 800. 8. Ziskin M, Lewin PA (eds). Ultrasonic Exposimetry. Boca Raton, FL: CRC Press; 1993. 9. Hosono Y, Yamashita Y. Piezoelectric ceramics with high dielectric constants for ultrasonic medical transducers. IEEE Trans Ultrason Ferroelectr Freq Control 2005; 52:1823 1828. 10. Sonic Concepts. Online PiezoCAD information. Bothell, WA: Sonic Concepts; 2005. Available at: http://www.sonicconcepts.com/. 11. Reid JM, Lewin PA. Ultrasound imaging transducers. In: Webster JG (ed). Encyclopedia of Electrical and Electronics Engineering. Vol 22. New York, NY: John Wiley & Sons; 1999:664 672. 288 J Ultrasound Med 2007; 26:283 288