Ultrasonics 56 (2015) 227–231
Contents lists available at ScienceDirect
Ultrasonics journal homepage: www.elsevier.com/locate/ultras
Focused intravascular ultrasonic probe using dimpled transducer elements Y. Chen a,b,1, W.B. Qiu c,1, K.H. Lam d, B.Q. Liu c, X.P. Jiang e, H.R. Zheng c, H.S. Luo f, H.L.W. Chan a,b, J.Y. Dai a,b,⇑ a
The Hong Kong Polytechnic University Shenzhen Research Institute, Shenzhen, China Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong, China Paul C. Lauterbur Research Center for Biomedical Imaging, Institute of Biomedical and Health Engineering, Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen, China d Department of Electrical Engineering, The Hong Kong Polytechnic University, Hong Kong, China e Department of Materials Science and Engineering, Jingdezhen Ceramic Institute, Jingdezhen, China f Information Materials and Devices Research Center, Shanghai Institute of Ceramics, Chinese Academy of Science, Shanghai, China b c
a r t i c l e
i n f o
Article history: Received 17 June 2014 Received in revised form 9 July 2014 Accepted 19 July 2014 Available online 27 July 2014 Keywords: Intravascular ultrasound Dimple technique PMN–PT single crystal Lead-free ceramic
a b s t r a c t High-frequency focused intravascular ultrasonic probes were fabricated in this study using dimple technique based on PMN–PT single crystal and lead-free KNN–KBT–Mn ceramic. The center frequency, bandwidth, and insertion loss of the PMN–PT transducer were 34 MHz, 75%, and 22.9 dB, respectively. For the lead-free probe, the center frequency, bandwidth, and insertion loss were found to be 40 MHz, 72%, and 28.8 dB, respectively. The ultrasonic images of wire phantom and vessels with good resolution were obtained to evaluate the transducer performance. The 6 dB axial and lateral resolutions of the PMN–PT probe were determined to be 58 lm and 131 lm, respectively. For the lead-free probe, the axial and lateral resolutions were found to be 44 lm and 125 lm, respectively. These results suggest that the mechanical dimpling technique has good potential in preparing focused transducers for intravascular ultrasound applications. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Cardiovascular disease is the leading cause of morbidity and mortality [1]. Although angiography is currently the gold standard for the assessment of cardiovascular disease such as the degree of atherosclerotic vessel stenosis, the structural information of stenotic vessel wall as well as the compositions of atherosclerotic plaque cannot be acquired which are very important for accurate assessment of atherosclerotic disease burden [2]. Catheter-based intravascular imaging modalities including intravascular ultrasound (IVUS) [3,4], intravascular optical coherence tomography (IVOCT) [5], and intravascular photoacoustics (IVPA) [6] have been developed to greatly enrich the knowledge of atherosclerosis. IVOCT is famous with high resolution but suffers very limited penetration for intravascular applications. Alternative method by combining IVUS and IVOCT together was also proposed for better
⇑ Corresponding author at: Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong, China. E-mail address: [email protected] (J.Y. Dai). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.ultras.2014.07.011 0041-624X/Ó 2014 Elsevier B.V. All rights reserved.
visualization of plaque [7]. IVPA is a newly developed technique which is claimed to be capable to acquire anatomic, functional, and molecular information about biological tissue, but the receive part is still based on ultrasonic transducer. So to our best knowledge, IVUS is still the key catheter-based technology for the assessment and treatment of cardiovascular disease. In the clinical IVUS system, the diameter of the catheter is fairly small (1 mm) so that the catheter can reach the appropriate position inside the blood vessels. It is a challenge to design and fabricate such miniaturized transducer with high frequency (20–50 MHz) ability. Plane shape single element transducer is still the dominant technology for IVUS applications although focused transducer was already demonstrated to exhibit better performance for high resolution ultrasound imaging [8]. Therefore, focused IVUS transducers should have high potential to enhance the imaging performance. Nevertheless, there are few studies reported for the focused IVUS transducer because of the difficult fabrication technique. Among transducer preparation techniques, a mechanical dimpling method was reported to be able to prepare both low frequency (5 MHz) and high frequency (30 MHz and 80 MHz) PMN–PT focused transducers with significant
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Y. Chen et al. / Ultrasonics 56 (2015) 227–231
Table 1 Properties of the PMN–PT single crystal, lead free KNN–KBT–Mn ceramic and traditional PZT ceramic. Material
PMN–0.28PT S 33
Dielectric constant e (e0) Electromechanical coupling coefficient kt Piezoelectric constant d33 (pC/N) Acoustic impedance Za (MRayl) Curie temperature(°C)
KNN–KBT–Mn
PZT–5H [18,19]
1030
730
1470
0.6 1600 37 131
0.5 189 25 358
0.51 593 34 200
enhancement of transducer performance [9,10]. Therefore, based on the experience of our previous wok, the dimpled PMN–PT single crystal was firstly employed for side-view IVUS probe fabrication to further improve the IVUS transducer performance. Recently, lead-free piezoelectric materials with relatively good piezoelectric properties especially for the perovskite structure have been attracted significant attention owning to environmental conservation. Thus, many single-element ultrasonic transducers were designed and developed based on lead-free materials [11– 15]. Previously, an 0.97K0.5Na0.5NbO3–0.03(Bi0.5K0.5)TiO3 lead-free ceramic with 0.4 wt% MnO doping (KNN–KBT–Mn) was reported to have a high piezoelectric constant d33 of 200 and low dielectric loss of 2.5% [16]. In this paper, the focused KNN–KBT–Mn ceramic probe was also fabricated for intravascular ultrasound application. Meanwhile, a PMN–PT single crystal plane IVUS probe was also fabricated for performance comparison with the dimpled ones.
Fig. 1. The pulse-echo waveform and frequency spectra of the (a) dimpled PMN–PT, (b) plane PMN–PT and (c) lead-free transducers.
Fig. 2. Wire phantom evaluation of the dimpled probes. (a) Image acquired by PMN–PT probe and (b) image acquired by lead-free probe.
Y. Chen et al. / Ultrasonics 56 (2015) 227–231
2. Experiments The PMN–0.28PT single crystal was grown at the Shanghai Institute of Ceramics using the modified Bridgman method [17]. The 0.97KNN–0.03KBT+0.4 wt% Mn lead free ceramics was prepared by a solid-state reaction [16]. Before poling, Cr/Au electrodes were coated on both top and bottom surfaces of the samples. The single crystal was poled in silicone oil under an electric field of 1 kV/mm at room temperature for 15 min. The lead free ceramic was poled under an electric field of 4 kV/mm in silicone oil at 80 °C for 15 min. Table 1 lists the properties of the PMN–PT single crystal and KNN–KBT–Mn lead free ceramic. The performance of traditional PZT–5H ceramic was also added for comparison [18,19]. A silver epoxy (E-solder 3022, supplied by Von Roll Isola, New Haven, CT) as the conductive backing layer was casted on the back-face of the piezoelectric materials. According to the specific characteristics, the transducer element with the backing layer (acoustic stack) was lapped to its designated thickness. The
229
front-face of the transducer element was then dimpled and polished using a dimple grinder (Gatan, Model656). Before dimpling, the acoustic stack was attached on the base of the dimple grinder using wax. The transducer element was rotated with a uniform speed. At the same time, the grinding wheel rotated and moved downward with a controlled force. The dimpling speed can be controlled by the downward force of the wheel. The focal length of the dimpled element depends on the radius of the grinding wheel [9,10]. Consequently, a dimple with round and smooth appearance was formed. Depended mainly on the dielectric properties, the dimpled transducer elements with the corresponding proper size were diced using a DAD 321 dicing saw (Disco Corp., Japan). The designated size of the PMN–PT single crystal and lead-free KNN–KBT–Mn ceramic was 0.7 0.7 mm2 and 1.0 1.0 mm2, respectively. After dicing, the dimpled acoustic stacks were packaged using a side-view metal housing. Transducer probes were connected to a flexible metal catheter with a SMA (SubMiniature version A) connector. The transducer performance
Fig. 3. Lateral and axial envelopes of echo signals from the wire acquired by (a and b) the dimpled PMN–PT probe and (c and d) dimpled lead-free probe.
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was characterized by a conventional pulse-echo response method. The parameters of the transducers, such as the center frequency, bandwidth, and insertion loss, were determined [12,20]. An open system was employed to evaluate the performance of the proposed probes [21–23]. A mono-cycle bipolar pulse with 96 Vp–p amplitude was used to excite the transducer. The center frequency of the excitation pulse was 40 MHz, and 6 dB bandwidth was 9–75 MHz when applying 50 X matching resistor. The gain of the receiver is 47 dB with a dynamic range of 51 dB. A 12 bit, 250 MSPS analog-to-digital converter (ADC) was employed to digitize the ultrasound echo signal. A programmable field programmable gate array (FPGA) component was used for image processing algorithms including the band-pass filter, Hilbert transform, envelope extraction, digital scan conversion, and logarithmic compression. Afterwards, the ultrasound data was transferred to a computer through a high speed universal serial bus (USB) interface.
3. Results and discussions The pulse-echo waveform and frequency spectra of the dimpled PMN–PT, plane PMN–PT and lead-free IVUS transducer probes are shown in Fig. 1(a–c). The center frequency, 6 dB bandwidth, and insertion loss of the dimpled PMN–PT probe is 34 MHz, 75%, and 22.9 dB, respectively. For the plane PMN–PT probe, the center frequency, 6 dB bandwidth, and insertion loss was found to be 29 MHz, 30%, and 21.8 dB, respectively. It can be clearly seen that the dimpled focused IVUS probe exhibits much broader bandwidth compared to the plane one. For the dimpled lead-free probe, the center frequency, 6 dB bandwidth, and the insertion loss was found to be 40 MHz, 72%, and 28.8 dB, respectively. Both dimpled IVUS probes show broad bandwidth because of the multi-resonances from a continuous change of thickness along the curve surface of the dimpled samples [9,10]. On the other hand, due to the outstanding piezoelectric properties of the PMN–PT single crystal, the insertion loss of the PMN–PT probe is much better than that of the lead-free one. Imaging was performed with of a customized tungsten wire phantom and healthy swine aorta to evaluate the performance of the proposed probes for intravascular imaging applications. The tip of the probe was placed inside the object to be imaged, which was immersed in a water tank during the experiment. Circumferential scanning was achieved by rotating the water tank using a step motor while the probe remained still. The wire phantom consisted of four 12-lm-diameter tungsten wires located at different depths. The ultrasonic images of the wire phantom acquired by PMN–PT and lead-free probes are shown in Fig. 2, which demonstrate good imaging capability of the probes. The images were displayed with a dynamic range of 42 dB. Each image was composed by 1000 A-lines ultrasound data. The imaging resolution was kept uniformly throughout the depth of view. The envelopes, or point spread functions of echo signals are displayed in Fig. 3. The axial and lateral resolutions were determined from the 6 dB envelope widths, which were 58 and 131 lm, respectively for the PMN–PT probe. The axial and lateral resolutions were 44 and 125 lm, respectively for the lead-free probe. The sensitivity is higher for PMN–PT probe that supports higher signal-to-noise ratio (SNR). In contrast, the lead-free probe shows lower envelope widths but with decreased SNR. Besides the wire, an in vitro swine aorta specimen was also used to evaluate the imaging performance. During the experiment, the probes were inserted into the specimen for cross-sectional imaging. The dynamic range was set to 42 dB. Fig. 4 shows the ultrasound images of the swine aorta. It should be noted that two images are not perfectly identical as they were acquired in
Fig. 4. In vitro imaging of swine aorta acquired by (a) a dimpled PMN–PT probe and (b) a dimpled lead-free probe.
different time. The detailed pattern of the artery can be clearly identified in the ultrasound images in which the artery wall and the surrounding fatty tissues could be clearly visualized. Due to higher sensitivity (lower insertion loss) of PMN–PT probe, the intensity of image acquired is much higher. 4. Conclusions The dimpled transducer elements of PMN–PT single crystal and lead-free KNN–KBT–Mn ceramic were used for fabricating highfrequency focused IVUS probe, respectively. The probes with dimpled elements were found to exhibit enhanced performance using the conventional pulse-echo measurement method. Besides, to evaluate the imaging capability of the dimpled IVUS probes, ultrasonic images of the wire phantom and swine aorta were performed. With reasonably good performance of the dimpled elements, the artery wall and fatty tissues could be well distinguishable. The results indicate that the mechanical dimpling technique can also be employed to prepare focused transducer elements especially for the application of broad bandwidth IVUS probes. Acknowledgements This research was supported by the National key Basic Research Program of China (973 Program, Grant No. 2013CB632900), financial support from The Hong Kong Polytechnic University strategic plan (Nos: 1-ZVCG & 1-ZV9B). National Science Foundation Grants
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(NSFC Grant Nos. 61302038 and 11325420), Guangdong Innovative and Entrepreneurial Research Team Program, and Shenzhen Peacock Plan. References [1] A.S. Go, D. Mozaffarian, V.L. Roger, E.J. Benjamin, J.D. Berry, et al., Heart disease and stroke statistics-2013 update: a report from the American Heart Association, Circulation 127 (2013) e6–e245. [2] P.J. de Feyter, P.W. Serruys, M.J. Davies, P. Richardson, J. Lubsen, M.F. Oliver, Quantitative coronary angiography to measure progression and regression of coronary atherosclerosis. Value, limitations, and implications for clinical trials, Circulation 84 (1991) 412–423. [3] R.A. Nishimura, W.D. Edwards, C.A. Warnes, G.S. Reeder, D.R. Holmes, A.J. Tajik, P.G. Yock, Intravascular ultrasound imaging: in vitro validation and pathologic correlation, J. Am. Coll. Cardiol. 16 (1990) 145–154. [4] S.E. Nissen, P. Yock, Intravascular ultrasound: novel pathophysiological insights and current clinical applications, Circulation 103 (2001) 604–616. [5] I.K. Jang, B.E. Bouma, D.H. Kang, S.J. Park, S.W. Park, K.B. Seung, K.B. Choi, M. Shishkov, K. Schlendorf, E. Pomerantsev, S.L. Houser, H.T. Aretz, G.J. Tearney, Visualization of coronary atherosclerotic plaques in patients using optical coherence tomography: comparison with intravascular ultrasound, J. Am. Coll. Cardiol. 39 (2002) 604–609. [6] X. Bai, X. Gong, W. Hau, J. Zheng, C. Liu, R. Lin, X. Zou, C. Zeng, H. Zheng, L. Song, Intravascular optical-resolution photoacoustic tomography with a 1.1 mm diameter catheter, PLoS One 9 (2014) e92463. [7] X. Li, J. Yin, C. Hu, Q. Zhou, K.K. Shung, Z. Chen, High resolution coregistered intravascular imaging with integrated ultrasound and optical coherence tomography probe, Appl. Phys. Lett. 97 (2010) 133702. [8] F.S. Foster, C.J. Pavlin, K.A. Harasiewicz, D.A. Christopher, D.H. Turnbull, Advances in ultrasound biomicroscopy, Ultrasound Med. Biol. 26 (2000) 1–27. [9] K.H. Lam, Y. Chen, K.F. Cheung, J.Y. Dai, PMN–PT single crystal focusing transducer fabricated using a mechanical dimpling technique, Ultrasonics 52 (2012) 20–24. [10] Y. Chen, K.H. Lam, D. Zhou, W.F. Cheng, J.Y. Dai, H.S. Luo, H.L.W. Chan, High frequency PMN–PT single crystal focusing transducer fabricated by a mechanical dimpling technique, Ultrasonics 53 (2013) 345–349.
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[11] D.W. Wu, R.M. Chen, Q.F. Zhou, K.K. Shung, D.M. Lin, H.L.W. Chan, Lead-free KNLNT piezoelectric ceramics for high-frequency ultrasonic transducer application, Ultrasonics 49 (2009) 395–398. [12] Y. Chen, X.P. Jiang, H.S. Luo, J.Y. Dai, H.L.W. Chan, High-frequency ultrasonic transducer fabricated with lead-free piezoelectric single crystal, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 57 (2010) 2601–2604. [13] X. Yan, K.H. Lam, X. Li, R. Chen, W. Ren, X. Ren, Q. Zhou, K.K. Shung, Lead-free intravascular ultrasound transducer using BZT–50BCT ceramics, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 60 (2013) 1272–1276. [14] K.H. Lam, H.F. Ji, F. Zheng, W. Ren, Q.F. Zhou, K.K. Shung, Development of leadfree single-element ultrahigh frequency (170–320 MHz) ultrasonic transducers, Ultrasonics 53 (2013) 1033–1038. [15] X. Yan, H. Ji, K.H. Lam, R. Chen, F. Zheng, W. Ren, Q. Zhou, K.K. Shung, Lead-free BNT composite film for high-frequency broadband ultrasonic transducer applications, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 60 (2013) 1533– 1537. [16] X.P. Jiang, Y. Chen, K.H. Lam, S.H. Choy, J. Wang, Effects of MnO Doping on properties of 0.97K0.5Na0.5NbO3–0.03(Bi0.5K0.5)TiO3 piezoelectric ceramics, J. Alloys Compd. 506 (2010) 323–326. [17] H. Luo, G. Xu, P. Wang, H. Xu, Z. Yin, Compositional homogeneity and electrical properties of lead magnesium niobate titanate single crystals grown by a modified Bridgman technique, Jpn. J. Appl. Phys. 39 (2000) 5581–5585. [18] L.F. Brown, Design considerations for piezoelectric polymer ultrasound transducers, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 47 (2000) 1377– 1396. [19] Measurement Specialties Inc., . [20] K.R. Erikson, R.A. Banjavic, P.L. Carson, Standard methods for testing single element pulse-echo ultrasonic transducers – AIUM interim standard, J. Ultrasound Med. 1 (S) (1981) 1–18. [21] W.B. Qiu, Y. Yu, F.K. Tsang, L. Sun, A multi-functional, reconfigurable pulse generator for high frequency ultrasound imaging, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 59 (2012) 1432–1442. [22] W.B. Qiu, Y. Yu, F.K. Tsang, L. Sun, An FPGA based open platform for ultrasound biomicroscopy, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 59 (2012) 1558–1567. [23] W.B. Qiu, Y. Chen, X. Li, Y. Yu, W.F. Cheng, F.K. Tsang, Q. Zhou, K.K. Shung, J. Dai, L. Sun, An open system for intravascular ultrasound imaging, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 59 (2012) 2201–2209.
Contents lists available at ScienceDirect
Ultrasonics journal homepage: www.elsevier.com/locate/ultras
Focused intravascular ultrasonic probe using dimpled transducer elements Y. Chen a,b,1, W.B. Qiu c,1, K.H. Lam d, B.Q. Liu c, X.P. Jiang e, H.R. Zheng c, H.S. Luo f, H.L.W. Chan a,b, J.Y. Dai a,b,⇑ a
The Hong Kong Polytechnic University Shenzhen Research Institute, Shenzhen, China Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong, China Paul C. Lauterbur Research Center for Biomedical Imaging, Institute of Biomedical and Health Engineering, Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen, China d Department of Electrical Engineering, The Hong Kong Polytechnic University, Hong Kong, China e Department of Materials Science and Engineering, Jingdezhen Ceramic Institute, Jingdezhen, China f Information Materials and Devices Research Center, Shanghai Institute of Ceramics, Chinese Academy of Science, Shanghai, China b c
a r t i c l e
i n f o
Article history: Received 17 June 2014 Received in revised form 9 July 2014 Accepted 19 July 2014 Available online 27 July 2014 Keywords: Intravascular ultrasound Dimple technique PMN–PT single crystal Lead-free ceramic
a b s t r a c t High-frequency focused intravascular ultrasonic probes were fabricated in this study using dimple technique based on PMN–PT single crystal and lead-free KNN–KBT–Mn ceramic. The center frequency, bandwidth, and insertion loss of the PMN–PT transducer were 34 MHz, 75%, and 22.9 dB, respectively. For the lead-free probe, the center frequency, bandwidth, and insertion loss were found to be 40 MHz, 72%, and 28.8 dB, respectively. The ultrasonic images of wire phantom and vessels with good resolution were obtained to evaluate the transducer performance. The 6 dB axial and lateral resolutions of the PMN–PT probe were determined to be 58 lm and 131 lm, respectively. For the lead-free probe, the axial and lateral resolutions were found to be 44 lm and 125 lm, respectively. These results suggest that the mechanical dimpling technique has good potential in preparing focused transducers for intravascular ultrasound applications. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Cardiovascular disease is the leading cause of morbidity and mortality [1]. Although angiography is currently the gold standard for the assessment of cardiovascular disease such as the degree of atherosclerotic vessel stenosis, the structural information of stenotic vessel wall as well as the compositions of atherosclerotic plaque cannot be acquired which are very important for accurate assessment of atherosclerotic disease burden [2]. Catheter-based intravascular imaging modalities including intravascular ultrasound (IVUS) [3,4], intravascular optical coherence tomography (IVOCT) [5], and intravascular photoacoustics (IVPA) [6] have been developed to greatly enrich the knowledge of atherosclerosis. IVOCT is famous with high resolution but suffers very limited penetration for intravascular applications. Alternative method by combining IVUS and IVOCT together was also proposed for better
⇑ Corresponding author at: Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong, China. E-mail address: [email protected] (J.Y. Dai). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.ultras.2014.07.011 0041-624X/Ó 2014 Elsevier B.V. All rights reserved.
visualization of plaque [7]. IVPA is a newly developed technique which is claimed to be capable to acquire anatomic, functional, and molecular information about biological tissue, but the receive part is still based on ultrasonic transducer. So to our best knowledge, IVUS is still the key catheter-based technology for the assessment and treatment of cardiovascular disease. In the clinical IVUS system, the diameter of the catheter is fairly small (1 mm) so that the catheter can reach the appropriate position inside the blood vessels. It is a challenge to design and fabricate such miniaturized transducer with high frequency (20–50 MHz) ability. Plane shape single element transducer is still the dominant technology for IVUS applications although focused transducer was already demonstrated to exhibit better performance for high resolution ultrasound imaging [8]. Therefore, focused IVUS transducers should have high potential to enhance the imaging performance. Nevertheless, there are few studies reported for the focused IVUS transducer because of the difficult fabrication technique. Among transducer preparation techniques, a mechanical dimpling method was reported to be able to prepare both low frequency (5 MHz) and high frequency (30 MHz and 80 MHz) PMN–PT focused transducers with significant
228
Y. Chen et al. / Ultrasonics 56 (2015) 227–231
Table 1 Properties of the PMN–PT single crystal, lead free KNN–KBT–Mn ceramic and traditional PZT ceramic. Material
PMN–0.28PT S 33
Dielectric constant e (e0) Electromechanical coupling coefficient kt Piezoelectric constant d33 (pC/N) Acoustic impedance Za (MRayl) Curie temperature(°C)
KNN–KBT–Mn
PZT–5H [18,19]
1030
730
1470
0.6 1600 37 131
0.5 189 25 358
0.51 593 34 200
enhancement of transducer performance [9,10]. Therefore, based on the experience of our previous wok, the dimpled PMN–PT single crystal was firstly employed for side-view IVUS probe fabrication to further improve the IVUS transducer performance. Recently, lead-free piezoelectric materials with relatively good piezoelectric properties especially for the perovskite structure have been attracted significant attention owning to environmental conservation. Thus, many single-element ultrasonic transducers were designed and developed based on lead-free materials [11– 15]. Previously, an 0.97K0.5Na0.5NbO3–0.03(Bi0.5K0.5)TiO3 lead-free ceramic with 0.4 wt% MnO doping (KNN–KBT–Mn) was reported to have a high piezoelectric constant d33 of 200 and low dielectric loss of 2.5% [16]. In this paper, the focused KNN–KBT–Mn ceramic probe was also fabricated for intravascular ultrasound application. Meanwhile, a PMN–PT single crystal plane IVUS probe was also fabricated for performance comparison with the dimpled ones.
Fig. 1. The pulse-echo waveform and frequency spectra of the (a) dimpled PMN–PT, (b) plane PMN–PT and (c) lead-free transducers.
Fig. 2. Wire phantom evaluation of the dimpled probes. (a) Image acquired by PMN–PT probe and (b) image acquired by lead-free probe.
Y. Chen et al. / Ultrasonics 56 (2015) 227–231
2. Experiments The PMN–0.28PT single crystal was grown at the Shanghai Institute of Ceramics using the modified Bridgman method [17]. The 0.97KNN–0.03KBT+0.4 wt% Mn lead free ceramics was prepared by a solid-state reaction [16]. Before poling, Cr/Au electrodes were coated on both top and bottom surfaces of the samples. The single crystal was poled in silicone oil under an electric field of 1 kV/mm at room temperature for 15 min. The lead free ceramic was poled under an electric field of 4 kV/mm in silicone oil at 80 °C for 15 min. Table 1 lists the properties of the PMN–PT single crystal and KNN–KBT–Mn lead free ceramic. The performance of traditional PZT–5H ceramic was also added for comparison [18,19]. A silver epoxy (E-solder 3022, supplied by Von Roll Isola, New Haven, CT) as the conductive backing layer was casted on the back-face of the piezoelectric materials. According to the specific characteristics, the transducer element with the backing layer (acoustic stack) was lapped to its designated thickness. The
229
front-face of the transducer element was then dimpled and polished using a dimple grinder (Gatan, Model656). Before dimpling, the acoustic stack was attached on the base of the dimple grinder using wax. The transducer element was rotated with a uniform speed. At the same time, the grinding wheel rotated and moved downward with a controlled force. The dimpling speed can be controlled by the downward force of the wheel. The focal length of the dimpled element depends on the radius of the grinding wheel [9,10]. Consequently, a dimple with round and smooth appearance was formed. Depended mainly on the dielectric properties, the dimpled transducer elements with the corresponding proper size were diced using a DAD 321 dicing saw (Disco Corp., Japan). The designated size of the PMN–PT single crystal and lead-free KNN–KBT–Mn ceramic was 0.7 0.7 mm2 and 1.0 1.0 mm2, respectively. After dicing, the dimpled acoustic stacks were packaged using a side-view metal housing. Transducer probes were connected to a flexible metal catheter with a SMA (SubMiniature version A) connector. The transducer performance
Fig. 3. Lateral and axial envelopes of echo signals from the wire acquired by (a and b) the dimpled PMN–PT probe and (c and d) dimpled lead-free probe.
230
Y. Chen et al. / Ultrasonics 56 (2015) 227–231
was characterized by a conventional pulse-echo response method. The parameters of the transducers, such as the center frequency, bandwidth, and insertion loss, were determined [12,20]. An open system was employed to evaluate the performance of the proposed probes [21–23]. A mono-cycle bipolar pulse with 96 Vp–p amplitude was used to excite the transducer. The center frequency of the excitation pulse was 40 MHz, and 6 dB bandwidth was 9–75 MHz when applying 50 X matching resistor. The gain of the receiver is 47 dB with a dynamic range of 51 dB. A 12 bit, 250 MSPS analog-to-digital converter (ADC) was employed to digitize the ultrasound echo signal. A programmable field programmable gate array (FPGA) component was used for image processing algorithms including the band-pass filter, Hilbert transform, envelope extraction, digital scan conversion, and logarithmic compression. Afterwards, the ultrasound data was transferred to a computer through a high speed universal serial bus (USB) interface.
3. Results and discussions The pulse-echo waveform and frequency spectra of the dimpled PMN–PT, plane PMN–PT and lead-free IVUS transducer probes are shown in Fig. 1(a–c). The center frequency, 6 dB bandwidth, and insertion loss of the dimpled PMN–PT probe is 34 MHz, 75%, and 22.9 dB, respectively. For the plane PMN–PT probe, the center frequency, 6 dB bandwidth, and insertion loss was found to be 29 MHz, 30%, and 21.8 dB, respectively. It can be clearly seen that the dimpled focused IVUS probe exhibits much broader bandwidth compared to the plane one. For the dimpled lead-free probe, the center frequency, 6 dB bandwidth, and the insertion loss was found to be 40 MHz, 72%, and 28.8 dB, respectively. Both dimpled IVUS probes show broad bandwidth because of the multi-resonances from a continuous change of thickness along the curve surface of the dimpled samples [9,10]. On the other hand, due to the outstanding piezoelectric properties of the PMN–PT single crystal, the insertion loss of the PMN–PT probe is much better than that of the lead-free one. Imaging was performed with of a customized tungsten wire phantom and healthy swine aorta to evaluate the performance of the proposed probes for intravascular imaging applications. The tip of the probe was placed inside the object to be imaged, which was immersed in a water tank during the experiment. Circumferential scanning was achieved by rotating the water tank using a step motor while the probe remained still. The wire phantom consisted of four 12-lm-diameter tungsten wires located at different depths. The ultrasonic images of the wire phantom acquired by PMN–PT and lead-free probes are shown in Fig. 2, which demonstrate good imaging capability of the probes. The images were displayed with a dynamic range of 42 dB. Each image was composed by 1000 A-lines ultrasound data. The imaging resolution was kept uniformly throughout the depth of view. The envelopes, or point spread functions of echo signals are displayed in Fig. 3. The axial and lateral resolutions were determined from the 6 dB envelope widths, which were 58 and 131 lm, respectively for the PMN–PT probe. The axial and lateral resolutions were 44 and 125 lm, respectively for the lead-free probe. The sensitivity is higher for PMN–PT probe that supports higher signal-to-noise ratio (SNR). In contrast, the lead-free probe shows lower envelope widths but with decreased SNR. Besides the wire, an in vitro swine aorta specimen was also used to evaluate the imaging performance. During the experiment, the probes were inserted into the specimen for cross-sectional imaging. The dynamic range was set to 42 dB. Fig. 4 shows the ultrasound images of the swine aorta. It should be noted that two images are not perfectly identical as they were acquired in
Fig. 4. In vitro imaging of swine aorta acquired by (a) a dimpled PMN–PT probe and (b) a dimpled lead-free probe.
different time. The detailed pattern of the artery can be clearly identified in the ultrasound images in which the artery wall and the surrounding fatty tissues could be clearly visualized. Due to higher sensitivity (lower insertion loss) of PMN–PT probe, the intensity of image acquired is much higher. 4. Conclusions The dimpled transducer elements of PMN–PT single crystal and lead-free KNN–KBT–Mn ceramic were used for fabricating highfrequency focused IVUS probe, respectively. The probes with dimpled elements were found to exhibit enhanced performance using the conventional pulse-echo measurement method. Besides, to evaluate the imaging capability of the dimpled IVUS probes, ultrasonic images of the wire phantom and swine aorta were performed. With reasonably good performance of the dimpled elements, the artery wall and fatty tissues could be well distinguishable. The results indicate that the mechanical dimpling technique can also be employed to prepare focused transducer elements especially for the application of broad bandwidth IVUS probes. Acknowledgements This research was supported by the National key Basic Research Program of China (973 Program, Grant No. 2013CB632900), financial support from The Hong Kong Polytechnic University strategic plan (Nos: 1-ZVCG & 1-ZV9B). National Science Foundation Grants
Y. Chen et al. / Ultrasonics 56 (2015) 227–231
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