- Email: [email protected]
Ultrasound transducer for medical therapy
Available online at www.sciencedirect.com
Sensors and Actuators A 142 (2008) 124–129
Ultrasound transducer for medical therapy Irinela Chilibon ∗ National Institute of R&D for Optoelectronics, INOE-2000, P.O. Box MG 5, Bucharest 077125, Romania Received 30 September 2006; received in revised form 20 April 2007; accepted 20 April 2007 Available online 3 May 2007
Abstract The paper presents aspects concerning the design and achievement of an ultrasound piezoceramic transducer for medical applications. The structure of piezoceramic transducer vibrates at certain acoustic intensity and work frequency between 800 kHz and 1 MHz and ultrasound vibrations propagate into biologic tissue. The piezoceramic disc converts the electrical energy into a vibratory mechanical one and the ultrasound energy is propagated into the biological tissue. The ultrasound energy effects are: temperature increasing, cavitation, and interstitial liquid dispersion into the tissue. The measurements of the main parameters and characteristics were made by HP 4194 Impedance/Gain-Phase Analyzer. Also, the paper presents the main measurement methods of ultrasound field energy, intensity and derived values generated by ultrasound transducers, distributed in three main groups. The ultrasound piezoceramic transducer 3D-design is useful to present the mechanical elements position inside the transducer case, and necessary to manufacture the transducer. © 2007 Elsevier B.V. All rights reserved. Keywords: PZT; Ultrasound transducer; Medical therapy
1. Introduction The ultrasound therapy is useful in the treatment of diseases of the peripheral nervous system such as neuritis, neuralgia, and inflammation of the sciatic nerve; diseases of the skeletal muscle system, such as arthritis; and diseases of the skin such as varicose veins and scleroderma. The action of ultrasound is not completely understood in treating these ailments, there probably is a dual effect. When applied to the affected area, the acoustic energy produces heat internally, similar to diathermy. At the same time, the vibrations act as a high-frequency massage. The effect of the ultrasound energy is to lessen pain or even eliminate it entirely. Energy is applied from the transducer directly to the skin in the affected area, after the skin is coated with mineral oil. Into a fluid medium, like the water, the attenuation is due to the friction forces which are opposing to the fluid movement at the crossing with the ultrasound wave. The ultrasound intensity depends on ultrasound field as f4 frequency and the resulting absorption is 0.01 dB cm−1 at 1 MHz. Some relaxation mechanisms are responsible for a higher atten-
∗
Tel.: +40 21 4574522; fax: +40 21 4574522. E-mail address: [email protected].
0924-4247/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2007.04.067
uation. Usually, in the biological tissues the attenuation is proportional with the signal frequency [1]. Some examples of the attenuation function of frequency, for main biological classes of tissues are presented in Fig. 1. Most attenuation measurements on biological tissues have been made in vitro. The samples, prepared in the form of homogeneous slice of constant thickness, are placed in a water tank and the attenuation is generally obtained by estimating the loss in signal intensity after transmission through the tissue. For instance, a piezoceramic vibrator transducer of 877 kHz resonance frequency, having omnidirectional directivity pattern can be successful used in doctors’ office, clinics, and hospitals [2,3]. Piezoelectric transducers can serve as electromechanical transceivers in sonar, medical imaging, NDE, and signal processing systems. Despite their relative technical maturity, many opportunities for improvement and innovation remain, too. Also, the ultrasound therapy has been the subject of research for many years [4,5], but it is only recently that this technique has found effective and widespread medical applications. The potential of this technique is extremely promising, but there remains progress to be made, notably in the area of the generation of ultrasonic waves. The large range of potential applications creates a wide range of different objectives. The main parameters are the required action on the biological tissue, the volume and location of the
I. Chilibon / Sensors and Actuators A 142 (2008) 124–129
125
about the application of piezoelectrics and their limitations (see [9]). 2.1. Piezoceramic disc
Fig. 1. Attenuation function of frequency, for main biological classes of tissues: A, lungs; B, skeleton bone; C, ivory bone; D, muscles; E, kidney; F, soft tissues; G, hemoglobin (0.15 g ml−1 , 15 ◦ C); H, water (15 ◦ C).
treatment zone inside the body, the acoustic access to the target area, the limitations on treatment time, the limitations on the acoustic exposure of surrounding tissues, the associated imaging techniques. The development of therapeutic applications of ultrasound depends notably on the availability of high-performance transducers. The measurements of the main characteristics of the ultrasound transducer were made by HP 4194A Impedance/GainPhase Analyzer, into the 800–1100 kHz frequency range. As results, the main experimental characteristics for the ultrasound therapy transducer are presented, such as: electrical impedance versus frequency characteristic measured in air and applied on skin. As result, the resonance frequency of the ultrasound transducer is 990 kHz, and the resonance frequency of the PZT unclamped disc is 800 kHz. 2. Theoretical consideration The proposed ultrasound transducer for medical therapy is an electro-acoustic transducer whose structure is composed by viscoelastic and piezoelectric material. The ultrasound device is able to emit acoustical signals into the human tissues due to the piezoelectric effect of the PZT piezoceramic disc. According to the piezoelectricity phenomenon, the piezoelectric element converts the electrical energy into a mechanical one and the mechanical vibrations into acoustic energy which is propagating to the biological tissue. PZT is the most used piezoceramic, it being widely available in the form of blocks, fibers, sheets, stacks and tubes according to Bernhard in [6]. Piezoelectrics work like sensors both for stress and strain sensing. The modelling of nonlinear ceramics for structural actuation was described by Rogacheva in [7], and [8]. However, the state-of-art research in the application of PZT materials indicates that numerous issues remain unanswered
The piezoceramics are crystallite conglomerate ferroelectrics materials with random orienting and the piezoceramic materials Pb(Zrx Ti1−x )O3 are solid solutions, obtained by the classical technology [10] utilizing oxide powders and impurities as base materials. The properties of piezoceramics are function of the preparation process and the fluctuations may be caused by inhomogeneous chemical composition, mechanical differences in the forming process, varying shrinkage and chemical modification during firing, and by varying response to the poling treatment. A high degree of process control is essential in the manufacture of piezoceramics in order to insure a consistent product with respect to electrical and mechanical properties. During the manufacturing process electrical, mechanical and piezoelectric parameters are accurately controlled. Lead zirconate titanate ceramics (PZT) show extremely strong piezoelectric effects for compositions near the morphotropic phase boundary (MPB). Rhombohedral and tetragonal phases coexist and are related to the presence of a maximum in the dielectric constant, a larger number of orientable polarization directions and a maximum mechanical compliance preventing cracking during domain orientation [11]. Fig. 2 presents a piezoceramic disc by dimensions, electrodes position, polarization orientation and three axes. Also, the up and down surfaces are plated by silver, creating the electrodes [12]. Piezoceramic disc is used in underwater acoustic transducer applications [13], yielding omnidirectional pattern. The experimental and theoretical study of the resonance spectrum of PZT discs and plates is rather presented in literature from 1973 [14]. However, there are also several new attempts of theoretical description of these spectra, as those accomplished in frame of the works [15] and [16]. The vibration movement of the piezoelectric element is described by the differential equation of the radial vibration disc, in [12] and [17], as follows: P c11
∂ 2 ur 1 ∂ur ur ∂ 2 ur + − = ρ ∂r 2 r ∂r r2 ∂t 2
(1)
Fig. 2. Thickness polarized and radial vibrations for a thin piezoceramic disc.
126
I. Chilibon / Sensors and Actuators A 142 (2008) 124–129
where ρ is density, ur is the radial displacement component, and P is described by the relationship: c11 P c11 =
E s11 2
2
E ) − (sE ) (s11 12
(2)
where sijE , i and j = 1 or 2 are the PZT compliance coefficients when electrical field E is constant. Consequently, the vibratory equation solution is ωr (3) ur = AJ1 p ejωt v where ω is the applied field frequency, J1 the Bessel function by first rang and first case, and P /ρ vp = c11 (4) Electrical admittance Ye is jωεP33 πa2 2(kp )2 −1 Ye = h 1 − σp − J¯ 1
(5)
and J¯ 1 is defined by the relationship: J0 (z) J¯ 1 (z) = z J1 (z)
(6)
where J0 is the Bessel function by first rang and zero case, and σ p the planar Poisson rapport: σp = −
E s12 E s11
(7)
The kp coefficient is the planar coupling piezoelectric coefficient for thin discs and it has the relationship: 2
(kp )2 =
(eP31 ) P eP c11 33
1 + σp kP2 2 1 − kp2
(9)
and kP is related by k31 relation: kp2 =
2 2k31 1 − σp
2.2. Ultrasound field measurement methods The mechanical vibrations amplitude of the piezoceramic disc depends on the electrical voltage amplitude applied on its electrodes, and also by the PZT disc thickness, piezoelectric and dielectric characteristics of the piezoceramic material. The acoustical power density on m3 of the piezoceramic transducer is given by the relation: P = 2πfm E2 k2 εT33 QM
(14)
where fm is the resonance frequency, E the voltage amplitude, k the coupling coefficient, εT33 the relative permittivity and QM is the mechanical quality factor. Transducer’s performances must be precise determined, in order to avoid the maximum unfailing values of ultrasonic field emitted into the human body, which could be unhealthy. The measurement methods of ultrasound field energy, intensity and derived values generated by ultrasound transducers are distributed in three main groups: 1. direct measurement methods of total energy (thermal methods); 2. measurement methods of acoustical pressure (optical and electro-acoustical methods); 3. measurement methods of acoustical radiation field (transducers put in evidence the mechanical effects generated by ultrasound propagation, especially the radiating force).
(8)
where eP31 and eP33 are the PZT piezoelectric coefficients. The relation between kp coefficient and the usual kP PZT coupling coefficient is the following: (kp )2 =
and the antiresonance frequencies (Ye = 0) are the solutions of the transcendental equation:
¯J1 ωa = 1 − σp − 2(kp )2 (13) vp
(10)
As result of the above relations combination it can be obtain: 2 J¯ 1 − 1 + σp + kp2 /1 − kP2 P πa 2 (11) (1 − kp ) Ye = jωε33 J¯ 1 − 1 + σp h The resonance frequencies are the solutions of the transcendental Eq. (6):
¯J1 ωa = 1 − σp (12) vp
The main optical methods for the detection and ultrasonic field parameters measurement are: dark field method or Toepler’s method, stroboscopic illumination, turning mirror observation, light beam diffraction, cavitation method, Pohlman’s method (displacement method of illuminated discs). These methods allow visualize and photograph the ultrasonic field, by obtaining pictures which can be utilized for nodes determination, ventral segment positions and propagation velocity, too. The main parameters of the piezoceramic transducers which characterize their emitted ultrasonic field into the medium are as follows: • • • • • • •
radiation effective surface (for receiving); frequency (spectrum into a field point); pulse shape; total transducer output power; total energy flux through the measured surface; temperature changes into a point or many points; presence or absence of cavitation effect and the cavitation threshold.
I. Chilibon / Sensors and Actuators A 142 (2008) 124–129
127
Fig. 3. Cross-section of ultrasound piezoceramic transducer.
3. Experimental and results Fig. 4. Three-dimensional design of ultrasound piezoceramic transducer.
3.1. Transducer manufacture The ultrasound piezoceramic transducer converts the electrical energy into mechanical vibrations, which propagate into the tissue medium. Piezoceramic disc is the active element of the transducer and works in the thickness vibratory mode. The disc has the maximum efficiency at the resonance frequency, when its impedance is lowest. The technological manufacture of piezoceramic transducer requires special mechanical processes to realize the metal case (duralumin), and all component elements for electrical connection. The piezoceramic disc is glued to the end-cap by the rubber resin. All components are fixed together, and the piezoceramic disc electrodes are electrical connected. The duralumin case presents 69 GPa elasticity module suited for obtaining high vibration amplitudes. The soft piezoceramic material of the active piezoceramic disc confers suitable properties to the ultrasound transducer (high ultrasonic power, broad bandwidth and a low Q). The piezoceramic material for the active element of the ultrasound device, obtained by the classic technology [6] in laboratory, is type PZT (lead titanate zirconate) Pb(Zrx Ti1−x )O3 with various additions (Ni, Bi, Mn) and presents piezoelectric and dielectric characteristics like: 7.2 g/cm3 density, 1100 permittivity, 80 quality factor, 0.55 coupling factor and 20 × 10−12 m2 N−1 compliance constant. The piezoceramic transducer for ultrasound therapy has a Ø31 × 2.3 mm piezoceramic disc glued to the bottom of the duralumin shell of 3 mm thickness (Fig. 3). Table 1 presents the main electrical characteristics of 2.3 mm thickness PZT discs, where D is the disc diameter, fm and fn are resonant, respectively, antiresonant frequencies and C is the electrical capacitance.
3.2. Results Fig. 3 shows the cross-section into the experimental ultrasound piezoceramic transducer with piezoceramic disc, realized in the laboratory. Fig. 4 presents the 3D-design of ultrasound piezoceramic transducer, revealing the mechanical elements position inside the transducer case. The piezoelectric element converts the electrical energy into a vibratory mechanical one and the ultrasound energy is propagated into the surrounding medium (biological tissue). The ultrasound energy effects are as follows: temperature increasing, cavitation, interstitial liquid dispersion into the tissue. The measurements of the main characteristics of the ultrasound therapy transducer were made by HP 4194A Impedance/Gain-Phase Analyzer in the 700–1100 kHz frequency range. Fig. 5 presents the frequency characteristic of electrical impedance for the experimental ultrasound therapy transducer. The resonance frequency of the ultrasound transducer is 990 kHz, and the resonance frequency of the PZT unclamped disc is 800 kHz. That means the resonant frequency of the device is 10% higher than those of piezoceramic disc. This information is helpful in the practical design of the transducer. The transducer impedance at antiresonance frequency for the ultrasound piezoceramic transducer measured in air (blue line)
Table 1 PZT discs characteristics PZT disc 1 2 3 4
D (mm) 31.3 31.3 16.1 16.0
fm (kHz)
fn (kHz)
811 806 795 799
865 859 826 813
C (nF) 9.94 9.92 3.34 3.26
Fig. 5. Impedance vs. frequency characteristic for the ultrasound piezoceramic transducer measured in air.
128
I. Chilibon / Sensors and Actuators A 142 (2008) 124–129
4. Conclusion
Fig. 6. Impedance vs. frequency characteristics for the ultrasound piezoceramic transducer measured in air (blue line) and applied on skin (red line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
The advantages of this device are: small dimensions, easiness of handling and the omni directional directivity pattern. The nonlinearities of piezoceramics into the radiating transducers determine the dielectric amplitude dependence, mechanical losses (heat power) and the acoustic signal distortion. The piezoceramic transducer with PZT disc realizes the conversion of electrical energy into a mechanical one at low power (0.05–1.5 W/cm2 ) and ultrasound frequency around1 MHz. The ultrasound vibrations made by the piezoceramic transducer propagate into biologic tissue, which structure vibrates at certain acoustic intensity and work frequency between 800 kHz and 1 MHz. Also, the experimental ultrasound therapy transducer presented 990 kHz resonance frequency and omnidirectional directivity pattern. Suitable electro-acoustic coupling coefficient between the ultrasound transducer and the biological medium (tissue) is realized by vaseline. The resonance frequency of the ultrasound transducer agglutinated on the skin by vaseline is approximately 10% higher than the resonance frequency of the PZT unclamped disc. This practical information should be considered to the transducer design. The tissue acoustical impedance is comparative to the water acoustical impedance that means 1.54 × 106 Rayles. The acoustical properties of PZT, duralumin, tissue and air mediums are quite different. Therefore the ultrasonic transducer yields different resonance frequencies in air and tissue. Accordingly, it should take into account that the ultrasound transducer is manufactured in air, but it works in tissue medium. So, the transducer resonant frequency should be 10% lower than the working resonant frequency.
Fig. 7. Impedance hodograph of ultrasound transducer.
Acknowledgement is higher than those measured with the transducer applied on the skin (red line) (Fig. 6). The impedance hodograph of the device at resonant frequency is represented in Fig. 7. Fig. 8 shows the photo of the ultrasound transducer for medical therapy.
Fig. 8. Ultrasound transducer for medical therapy.
Author has the opportunity to make partially research in the 18/2005 CEEX Romanian project. References [1] J.P. Jones, S. Leeman, Ultrasound tissue characterisation, Rev. Acta Electron. 26-1-2 (1984) 3–31. [2] P.N.T. Wells, N. Jong, N. Bom, Transducers in medical ultrasound. Part 4. Transducer safety, Ultrasounds, 24 (1986) 10. [3] J.Y. Chapelon, D. Cathignol, C. Cain, E. Ebbini, J.U. Kluiwstra, O. Sapozhikov, G. Fleury, R. Berriet, L. Chupin, J.L. Guey, New piezoelectric transducers for therapeutic ultrasound, Ultrasound Med. Biol. (2000) 153–159. [4] G. ter Haar, R.L. Clark, M.G. Vaughan, C.R. Hill, Trackless surgery using focussed ultrasound: technique and case report, Minimal. Invas. Ther. 1 (1991) 13–19. [5] J.Y. Chapelon, P. Faure, M. Plantier, D. Cathignol, R. Souchon, F. Gorry, A. Gelet, Feasibility of tissue ablation using high intensity electronically focused ultrasound, IEEE Ultrason. Symp. (1993) 1211–1214. [6] A.P.F. Bernhard, Smart helicopter rotor with active blade tips, PhD thesis, University of Maryland, College Park, MD, USA, 2000. [7] N.N. Rogacheva, The Theory of Piezoelectric Shells and Plates, CRC Press/London Press, Boca Raton, 1993. [8] H.S. Tzou, Piezoelectric Shells, Kluwer Acad. Publ., 1993.
I. Chilibon / Sensors and Actuators A 142 (2008) 124–129 [9] C. Shakeri, M.N. Noori, Z. Hou, Smart materials and structure. A Review, Report, Mechanical Engineering Department, Worcester Polytechnic Institute, 1996. [10] K. Okazaki, Developments in fabrication of piezoelectric ceramics, Ferroelectrics 41 (1982) 77–96. [11] C. Galassi, E. Ronacari, C. Capiani, A. Costa, Influence of processing parameters on the properties of PZT materials, in: Piezoelectric Materials: Advances in Science, Technology and Applications, NATO Science Series, Kluwer Academic, Netherlands, 3-High Technology, 76 (2000) 149–157. [12] G. Amza, D. Barb, F. Constantinescu, Ultraacoustic Systems. Calculus, Design, Applications in Techniques, “Tehnica” Publisher, Bucharest, Romania, 1986. [13] I. Chilibon, Low frequency underwater piezoceramic transducer, Sens. Actuators A A85 (1–3) (2000) 292–295. [14] B.B. Auld, Acoustic Fields and Waves in Solids, 2, Wiley Interscience, New York, 1973, pp. 63–104. [15] A. Alippi, A. Betucci, F. Craciun, F. Farrelly, A. Petri, G. Shkerdin, Subharmonic wave generation in acoustical resonators, in: P.P. Delsanto, A.W. Saenz (Eds.), In frame of New Perspectives on Problems in Classical and Quantum Physics. Part 2, Gordon and Breach Science Publishers, Amsterdam, 1998, pp. 3–17.
129
[16] V. Chiroiu, et al., Subharmonic generation in piezoelectrics with cantor-like structure, J. Phys. D: Appl. Phys. 34 (2001) 1579–1586. [17] A.H. Meitzeler, H.M O’Bryan Jr., H.F. Tiersten, IEEE Trans. Sonics Ultrasonics SU-20 (1973) 233.
Biography Irinela Chilibon is PhD Senior Researcher of the National Institute of Research and Development for Optoelectronics, INOE-2000, Bucharest, Romania. In 1981 received Electronics Eng. Degree to the Faculty of Electronics and Communications, “Politehnica” University of Bucharest, Romania, and PhD degree in Electronics in 1998. She is member of the International Institute of Acoustics and Vibration (IIAV), International Frequency Sensor Association (IFSA), Balkan Physics Union (BPU), General Association of Romanian Engineers, Romanian Association for Research in Information Technology and Communications (ROMINFOR). Her main research domains are the following: applied electronics, underwater acoustics, vibro-acoustics, sonochemisty, medical devices, ultrasound medical transducers, conventional and non-conventional methods for piezoceramic materials, NDT, solid state physics, new materials for sensors and transducers.
Sensors and Actuators A 142 (2008) 124–129
Ultrasound transducer for medical therapy Irinela Chilibon ∗ National Institute of R&D for Optoelectronics, INOE-2000, P.O. Box MG 5, Bucharest 077125, Romania Received 30 September 2006; received in revised form 20 April 2007; accepted 20 April 2007 Available online 3 May 2007
Abstract The paper presents aspects concerning the design and achievement of an ultrasound piezoceramic transducer for medical applications. The structure of piezoceramic transducer vibrates at certain acoustic intensity and work frequency between 800 kHz and 1 MHz and ultrasound vibrations propagate into biologic tissue. The piezoceramic disc converts the electrical energy into a vibratory mechanical one and the ultrasound energy is propagated into the biological tissue. The ultrasound energy effects are: temperature increasing, cavitation, and interstitial liquid dispersion into the tissue. The measurements of the main parameters and characteristics were made by HP 4194 Impedance/Gain-Phase Analyzer. Also, the paper presents the main measurement methods of ultrasound field energy, intensity and derived values generated by ultrasound transducers, distributed in three main groups. The ultrasound piezoceramic transducer 3D-design is useful to present the mechanical elements position inside the transducer case, and necessary to manufacture the transducer. © 2007 Elsevier B.V. All rights reserved. Keywords: PZT; Ultrasound transducer; Medical therapy
1. Introduction The ultrasound therapy is useful in the treatment of diseases of the peripheral nervous system such as neuritis, neuralgia, and inflammation of the sciatic nerve; diseases of the skeletal muscle system, such as arthritis; and diseases of the skin such as varicose veins and scleroderma. The action of ultrasound is not completely understood in treating these ailments, there probably is a dual effect. When applied to the affected area, the acoustic energy produces heat internally, similar to diathermy. At the same time, the vibrations act as a high-frequency massage. The effect of the ultrasound energy is to lessen pain or even eliminate it entirely. Energy is applied from the transducer directly to the skin in the affected area, after the skin is coated with mineral oil. Into a fluid medium, like the water, the attenuation is due to the friction forces which are opposing to the fluid movement at the crossing with the ultrasound wave. The ultrasound intensity depends on ultrasound field as f4 frequency and the resulting absorption is 0.01 dB cm−1 at 1 MHz. Some relaxation mechanisms are responsible for a higher atten-
∗
Tel.: +40 21 4574522; fax: +40 21 4574522. E-mail address: [email protected].
0924-4247/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2007.04.067
uation. Usually, in the biological tissues the attenuation is proportional with the signal frequency [1]. Some examples of the attenuation function of frequency, for main biological classes of tissues are presented in Fig. 1. Most attenuation measurements on biological tissues have been made in vitro. The samples, prepared in the form of homogeneous slice of constant thickness, are placed in a water tank and the attenuation is generally obtained by estimating the loss in signal intensity after transmission through the tissue. For instance, a piezoceramic vibrator transducer of 877 kHz resonance frequency, having omnidirectional directivity pattern can be successful used in doctors’ office, clinics, and hospitals [2,3]. Piezoelectric transducers can serve as electromechanical transceivers in sonar, medical imaging, NDE, and signal processing systems. Despite their relative technical maturity, many opportunities for improvement and innovation remain, too. Also, the ultrasound therapy has been the subject of research for many years [4,5], but it is only recently that this technique has found effective and widespread medical applications. The potential of this technique is extremely promising, but there remains progress to be made, notably in the area of the generation of ultrasonic waves. The large range of potential applications creates a wide range of different objectives. The main parameters are the required action on the biological tissue, the volume and location of the
I. Chilibon / Sensors and Actuators A 142 (2008) 124–129
125
about the application of piezoelectrics and their limitations (see [9]). 2.1. Piezoceramic disc
Fig. 1. Attenuation function of frequency, for main biological classes of tissues: A, lungs; B, skeleton bone; C, ivory bone; D, muscles; E, kidney; F, soft tissues; G, hemoglobin (0.15 g ml−1 , 15 ◦ C); H, water (15 ◦ C).
treatment zone inside the body, the acoustic access to the target area, the limitations on treatment time, the limitations on the acoustic exposure of surrounding tissues, the associated imaging techniques. The development of therapeutic applications of ultrasound depends notably on the availability of high-performance transducers. The measurements of the main characteristics of the ultrasound transducer were made by HP 4194A Impedance/GainPhase Analyzer, into the 800–1100 kHz frequency range. As results, the main experimental characteristics for the ultrasound therapy transducer are presented, such as: electrical impedance versus frequency characteristic measured in air and applied on skin. As result, the resonance frequency of the ultrasound transducer is 990 kHz, and the resonance frequency of the PZT unclamped disc is 800 kHz. 2. Theoretical consideration The proposed ultrasound transducer for medical therapy is an electro-acoustic transducer whose structure is composed by viscoelastic and piezoelectric material. The ultrasound device is able to emit acoustical signals into the human tissues due to the piezoelectric effect of the PZT piezoceramic disc. According to the piezoelectricity phenomenon, the piezoelectric element converts the electrical energy into a mechanical one and the mechanical vibrations into acoustic energy which is propagating to the biological tissue. PZT is the most used piezoceramic, it being widely available in the form of blocks, fibers, sheets, stacks and tubes according to Bernhard in [6]. Piezoelectrics work like sensors both for stress and strain sensing. The modelling of nonlinear ceramics for structural actuation was described by Rogacheva in [7], and [8]. However, the state-of-art research in the application of PZT materials indicates that numerous issues remain unanswered
The piezoceramics are crystallite conglomerate ferroelectrics materials with random orienting and the piezoceramic materials Pb(Zrx Ti1−x )O3 are solid solutions, obtained by the classical technology [10] utilizing oxide powders and impurities as base materials. The properties of piezoceramics are function of the preparation process and the fluctuations may be caused by inhomogeneous chemical composition, mechanical differences in the forming process, varying shrinkage and chemical modification during firing, and by varying response to the poling treatment. A high degree of process control is essential in the manufacture of piezoceramics in order to insure a consistent product with respect to electrical and mechanical properties. During the manufacturing process electrical, mechanical and piezoelectric parameters are accurately controlled. Lead zirconate titanate ceramics (PZT) show extremely strong piezoelectric effects for compositions near the morphotropic phase boundary (MPB). Rhombohedral and tetragonal phases coexist and are related to the presence of a maximum in the dielectric constant, a larger number of orientable polarization directions and a maximum mechanical compliance preventing cracking during domain orientation [11]. Fig. 2 presents a piezoceramic disc by dimensions, electrodes position, polarization orientation and three axes. Also, the up and down surfaces are plated by silver, creating the electrodes [12]. Piezoceramic disc is used in underwater acoustic transducer applications [13], yielding omnidirectional pattern. The experimental and theoretical study of the resonance spectrum of PZT discs and plates is rather presented in literature from 1973 [14]. However, there are also several new attempts of theoretical description of these spectra, as those accomplished in frame of the works [15] and [16]. The vibration movement of the piezoelectric element is described by the differential equation of the radial vibration disc, in [12] and [17], as follows: P c11
∂ 2 ur 1 ∂ur ur ∂ 2 ur + − = ρ ∂r 2 r ∂r r2 ∂t 2
(1)
Fig. 2. Thickness polarized and radial vibrations for a thin piezoceramic disc.
126
I. Chilibon / Sensors and Actuators A 142 (2008) 124–129
where ρ is density, ur is the radial displacement component, and P is described by the relationship: c11 P c11 =
E s11 2
2
E ) − (sE ) (s11 12
(2)
where sijE , i and j = 1 or 2 are the PZT compliance coefficients when electrical field E is constant. Consequently, the vibratory equation solution is ωr (3) ur = AJ1 p ejωt v where ω is the applied field frequency, J1 the Bessel function by first rang and first case, and P /ρ vp = c11 (4) Electrical admittance Ye is jωεP33 πa2 2(kp )2 −1 Ye = h 1 − σp − J¯ 1
(5)
and J¯ 1 is defined by the relationship: J0 (z) J¯ 1 (z) = z J1 (z)
(6)
where J0 is the Bessel function by first rang and zero case, and σ p the planar Poisson rapport: σp = −
E s12 E s11
(7)
The kp coefficient is the planar coupling piezoelectric coefficient for thin discs and it has the relationship: 2
(kp )2 =
(eP31 ) P eP c11 33
1 + σp kP2 2 1 − kp2
(9)
and kP is related by k31 relation: kp2 =
2 2k31 1 − σp
2.2. Ultrasound field measurement methods The mechanical vibrations amplitude of the piezoceramic disc depends on the electrical voltage amplitude applied on its electrodes, and also by the PZT disc thickness, piezoelectric and dielectric characteristics of the piezoceramic material. The acoustical power density on m3 of the piezoceramic transducer is given by the relation: P = 2πfm E2 k2 εT33 QM
(14)
where fm is the resonance frequency, E the voltage amplitude, k the coupling coefficient, εT33 the relative permittivity and QM is the mechanical quality factor. Transducer’s performances must be precise determined, in order to avoid the maximum unfailing values of ultrasonic field emitted into the human body, which could be unhealthy. The measurement methods of ultrasound field energy, intensity and derived values generated by ultrasound transducers are distributed in three main groups: 1. direct measurement methods of total energy (thermal methods); 2. measurement methods of acoustical pressure (optical and electro-acoustical methods); 3. measurement methods of acoustical radiation field (transducers put in evidence the mechanical effects generated by ultrasound propagation, especially the radiating force).
(8)
where eP31 and eP33 are the PZT piezoelectric coefficients. The relation between kp coefficient and the usual kP PZT coupling coefficient is the following: (kp )2 =
and the antiresonance frequencies (Ye = 0) are the solutions of the transcendental equation:
¯J1 ωa = 1 − σp − 2(kp )2 (13) vp
(10)
As result of the above relations combination it can be obtain: 2 J¯ 1 − 1 + σp + kp2 /1 − kP2 P πa 2 (11) (1 − kp ) Ye = jωε33 J¯ 1 − 1 + σp h The resonance frequencies are the solutions of the transcendental Eq. (6):
¯J1 ωa = 1 − σp (12) vp
The main optical methods for the detection and ultrasonic field parameters measurement are: dark field method or Toepler’s method, stroboscopic illumination, turning mirror observation, light beam diffraction, cavitation method, Pohlman’s method (displacement method of illuminated discs). These methods allow visualize and photograph the ultrasonic field, by obtaining pictures which can be utilized for nodes determination, ventral segment positions and propagation velocity, too. The main parameters of the piezoceramic transducers which characterize their emitted ultrasonic field into the medium are as follows: • • • • • • •
radiation effective surface (for receiving); frequency (spectrum into a field point); pulse shape; total transducer output power; total energy flux through the measured surface; temperature changes into a point or many points; presence or absence of cavitation effect and the cavitation threshold.
I. Chilibon / Sensors and Actuators A 142 (2008) 124–129
127
Fig. 3. Cross-section of ultrasound piezoceramic transducer.
3. Experimental and results Fig. 4. Three-dimensional design of ultrasound piezoceramic transducer.
3.1. Transducer manufacture The ultrasound piezoceramic transducer converts the electrical energy into mechanical vibrations, which propagate into the tissue medium. Piezoceramic disc is the active element of the transducer and works in the thickness vibratory mode. The disc has the maximum efficiency at the resonance frequency, when its impedance is lowest. The technological manufacture of piezoceramic transducer requires special mechanical processes to realize the metal case (duralumin), and all component elements for electrical connection. The piezoceramic disc is glued to the end-cap by the rubber resin. All components are fixed together, and the piezoceramic disc electrodes are electrical connected. The duralumin case presents 69 GPa elasticity module suited for obtaining high vibration amplitudes. The soft piezoceramic material of the active piezoceramic disc confers suitable properties to the ultrasound transducer (high ultrasonic power, broad bandwidth and a low Q). The piezoceramic material for the active element of the ultrasound device, obtained by the classic technology [6] in laboratory, is type PZT (lead titanate zirconate) Pb(Zrx Ti1−x )O3 with various additions (Ni, Bi, Mn) and presents piezoelectric and dielectric characteristics like: 7.2 g/cm3 density, 1100 permittivity, 80 quality factor, 0.55 coupling factor and 20 × 10−12 m2 N−1 compliance constant. The piezoceramic transducer for ultrasound therapy has a Ø31 × 2.3 mm piezoceramic disc glued to the bottom of the duralumin shell of 3 mm thickness (Fig. 3). Table 1 presents the main electrical characteristics of 2.3 mm thickness PZT discs, where D is the disc diameter, fm and fn are resonant, respectively, antiresonant frequencies and C is the electrical capacitance.
3.2. Results Fig. 3 shows the cross-section into the experimental ultrasound piezoceramic transducer with piezoceramic disc, realized in the laboratory. Fig. 4 presents the 3D-design of ultrasound piezoceramic transducer, revealing the mechanical elements position inside the transducer case. The piezoelectric element converts the electrical energy into a vibratory mechanical one and the ultrasound energy is propagated into the surrounding medium (biological tissue). The ultrasound energy effects are as follows: temperature increasing, cavitation, interstitial liquid dispersion into the tissue. The measurements of the main characteristics of the ultrasound therapy transducer were made by HP 4194A Impedance/Gain-Phase Analyzer in the 700–1100 kHz frequency range. Fig. 5 presents the frequency characteristic of electrical impedance for the experimental ultrasound therapy transducer. The resonance frequency of the ultrasound transducer is 990 kHz, and the resonance frequency of the PZT unclamped disc is 800 kHz. That means the resonant frequency of the device is 10% higher than those of piezoceramic disc. This information is helpful in the practical design of the transducer. The transducer impedance at antiresonance frequency for the ultrasound piezoceramic transducer measured in air (blue line)
Table 1 PZT discs characteristics PZT disc 1 2 3 4
D (mm) 31.3 31.3 16.1 16.0
fm (kHz)
fn (kHz)
811 806 795 799
865 859 826 813
C (nF) 9.94 9.92 3.34 3.26
Fig. 5. Impedance vs. frequency characteristic for the ultrasound piezoceramic transducer measured in air.
128
I. Chilibon / Sensors and Actuators A 142 (2008) 124–129
4. Conclusion
Fig. 6. Impedance vs. frequency characteristics for the ultrasound piezoceramic transducer measured in air (blue line) and applied on skin (red line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
The advantages of this device are: small dimensions, easiness of handling and the omni directional directivity pattern. The nonlinearities of piezoceramics into the radiating transducers determine the dielectric amplitude dependence, mechanical losses (heat power) and the acoustic signal distortion. The piezoceramic transducer with PZT disc realizes the conversion of electrical energy into a mechanical one at low power (0.05–1.5 W/cm2 ) and ultrasound frequency around1 MHz. The ultrasound vibrations made by the piezoceramic transducer propagate into biologic tissue, which structure vibrates at certain acoustic intensity and work frequency between 800 kHz and 1 MHz. Also, the experimental ultrasound therapy transducer presented 990 kHz resonance frequency and omnidirectional directivity pattern. Suitable electro-acoustic coupling coefficient between the ultrasound transducer and the biological medium (tissue) is realized by vaseline. The resonance frequency of the ultrasound transducer agglutinated on the skin by vaseline is approximately 10% higher than the resonance frequency of the PZT unclamped disc. This practical information should be considered to the transducer design. The tissue acoustical impedance is comparative to the water acoustical impedance that means 1.54 × 106 Rayles. The acoustical properties of PZT, duralumin, tissue and air mediums are quite different. Therefore the ultrasonic transducer yields different resonance frequencies in air and tissue. Accordingly, it should take into account that the ultrasound transducer is manufactured in air, but it works in tissue medium. So, the transducer resonant frequency should be 10% lower than the working resonant frequency.
Fig. 7. Impedance hodograph of ultrasound transducer.
Acknowledgement is higher than those measured with the transducer applied on the skin (red line) (Fig. 6). The impedance hodograph of the device at resonant frequency is represented in Fig. 7. Fig. 8 shows the photo of the ultrasound transducer for medical therapy.
Fig. 8. Ultrasound transducer for medical therapy.
Author has the opportunity to make partially research in the 18/2005 CEEX Romanian project. References [1] J.P. Jones, S. Leeman, Ultrasound tissue characterisation, Rev. Acta Electron. 26-1-2 (1984) 3–31. [2] P.N.T. Wells, N. Jong, N. Bom, Transducers in medical ultrasound. Part 4. Transducer safety, Ultrasounds, 24 (1986) 10. [3] J.Y. Chapelon, D. Cathignol, C. Cain, E. Ebbini, J.U. Kluiwstra, O. Sapozhikov, G. Fleury, R. Berriet, L. Chupin, J.L. Guey, New piezoelectric transducers for therapeutic ultrasound, Ultrasound Med. Biol. (2000) 153–159. [4] G. ter Haar, R.L. Clark, M.G. Vaughan, C.R. Hill, Trackless surgery using focussed ultrasound: technique and case report, Minimal. Invas. Ther. 1 (1991) 13–19. [5] J.Y. Chapelon, P. Faure, M. Plantier, D. Cathignol, R. Souchon, F. Gorry, A. Gelet, Feasibility of tissue ablation using high intensity electronically focused ultrasound, IEEE Ultrason. Symp. (1993) 1211–1214. [6] A.P.F. Bernhard, Smart helicopter rotor with active blade tips, PhD thesis, University of Maryland, College Park, MD, USA, 2000. [7] N.N. Rogacheva, The Theory of Piezoelectric Shells and Plates, CRC Press/London Press, Boca Raton, 1993. [8] H.S. Tzou, Piezoelectric Shells, Kluwer Acad. Publ., 1993.
I. Chilibon / Sensors and Actuators A 142 (2008) 124–129 [9] C. Shakeri, M.N. Noori, Z. Hou, Smart materials and structure. A Review, Report, Mechanical Engineering Department, Worcester Polytechnic Institute, 1996. [10] K. Okazaki, Developments in fabrication of piezoelectric ceramics, Ferroelectrics 41 (1982) 77–96. [11] C. Galassi, E. Ronacari, C. Capiani, A. Costa, Influence of processing parameters on the properties of PZT materials, in: Piezoelectric Materials: Advances in Science, Technology and Applications, NATO Science Series, Kluwer Academic, Netherlands, 3-High Technology, 76 (2000) 149–157. [12] G. Amza, D. Barb, F. Constantinescu, Ultraacoustic Systems. Calculus, Design, Applications in Techniques, “Tehnica” Publisher, Bucharest, Romania, 1986. [13] I. Chilibon, Low frequency underwater piezoceramic transducer, Sens. Actuators A A85 (1–3) (2000) 292–295. [14] B.B. Auld, Acoustic Fields and Waves in Solids, 2, Wiley Interscience, New York, 1973, pp. 63–104. [15] A. Alippi, A. Betucci, F. Craciun, F. Farrelly, A. Petri, G. Shkerdin, Subharmonic wave generation in acoustical resonators, in: P.P. Delsanto, A.W. Saenz (Eds.), In frame of New Perspectives on Problems in Classical and Quantum Physics. Part 2, Gordon and Breach Science Publishers, Amsterdam, 1998, pp. 3–17.
129
[16] V. Chiroiu, et al., Subharmonic generation in piezoelectrics with cantor-like structure, J. Phys. D: Appl. Phys. 34 (2001) 1579–1586. [17] A.H. Meitzeler, H.M O’Bryan Jr., H.F. Tiersten, IEEE Trans. Sonics Ultrasonics SU-20 (1973) 233.
Biography Irinela Chilibon is PhD Senior Researcher of the National Institute of Research and Development for Optoelectronics, INOE-2000, Bucharest, Romania. In 1981 received Electronics Eng. Degree to the Faculty of Electronics and Communications, “Politehnica” University of Bucharest, Romania, and PhD degree in Electronics in 1998. She is member of the International Institute of Acoustics and Vibration (IIAV), International Frequency Sensor Association (IFSA), Balkan Physics Union (BPU), General Association of Romanian Engineers, Romanian Association for Research in Information Technology and Communications (ROMINFOR). Her main research domains are the following: applied electronics, underwater acoustics, vibro-acoustics, sonochemisty, medical devices, ultrasound medical transducers, conventional and non-conventional methods for piezoceramic materials, NDT, solid state physics, new materials for sensors and transducers.