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Fabrication and performance of a 64 × 5 element 1.5D active matrix array transducer for medical imaging
Sensors and Actuators A 214 (2014) 81–87
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Fabrication and performance of a 64 × 5 element 1.5D active matrix array transducer for medical imaging Jue Peng ∗ , Wenjuan Wang, Hu Tang, Tianfu Wang, Siping Chen National-Regional Key Engineering Laboratory for Medical Ultrasound, Guangdong Key Laboratory for Biomedical Measurements and Ultrasound Imaging, Department of Biomedical Engineering, School of Medicine, Shenzhen University, Shenzhen 518060, China
a r t i c l e
i n f o
Article history: Received 18 January 2014 Received in revised form 16 April 2014 Accepted 19 April 2014 Available online 26 April 2014 Keywords: Ultrasonic transducer Active matrix array Slice thickness control
a b s t r a c t Compared to 1D phased array probes with a fixed focus in elevation, multi-row arrays can significantly improved the slice thickness throughout image by expanding aperture and dynamic focusing in elevation. This paper describes the design and measurement of a 64 × 5 element 1.5D ultrasonic transducer that enables dynamic focusing and apodization in the elevation direction. We manufactured a transducer with an aperture size of 24 mm × 16 mm using a widely used piezoelectric ceramic (PZT-5H) as the piezoelectric vibrator. The measured center frequency and −6 dB fractional bandwidth of the 1.5D transducer were 3 MHz and 79%, respectively. A two-way insertion loss of −58 dB was obtained at the average center frequency. We carried out a sound field simulation and measured the actual transmitting (one-way) sound field data by using a hydrophone. In this way the sound beam profile in elevation direction was obtained, showing the −6 dB slice thickness measured was about 2 mm throughout the whole range of interest (from near to far field). This provided a much greater focal depth and much better imaging resolution in the elevation direction than can be achieved by using 1D probe. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Ultrasonic transducers play a fundamental role in determining the quality of diagnostic ultrasound imaging equipment. Current cardiac applications use linear phased arrays (1D) which can only be steered and dynamically focused in the azimuth direction, while the beam width in the plane perpendicular to the imaging plane, which is often referred to as the elevation beam width or slice thickness, has received relatively little attention. The acoustic noise generated by a relatively thick slice degrades the contrast resolution in small anechoic structures such as blood vessels or cysts [1]. Multidimensional transducers have received considerable attention in recent years as a way to control scan slice thickness. According to Wildes, the evolution of multi-dimensional transducers has been from 1.25D to 1.5D to 1.75D to 2D [2], which are defined as follows:
1D: The elevation aperture is fixed and focused at a fixed range. 1.25D: The elevation aperture is variable, but focusing remains static.
∗ Corresponding author. Tel.: +86-75586671915. E-mail address: [email protected] (J. Peng). http://dx.doi.org/10.1016/j.sna.2014.04.028 0924-4247/© 2014 Elsevier B.V. All rights reserved.
1.5D: The elevation aperture, shading, and focusing are dynamically variable, but symmetric about the centerline of the array. 1.75D: A 1.5D array without the symmetry constraint. The elements are large (several wavelengths) in elevation, so very little steering is possible. 2D: The elevation geometry and performance are comparable to those of the azimuth, with full electronic apodization, focusing, and steering. Three major challenges to the design of 2D matrix transducers have been identified: the requirement for thousands of channels, the difficulty of making such a large number of electrical connections and controlling the resulting array elements independently, and the extremely low sensitivity of the tiny elements [3]. Because of those difficulties, 1.5D fractional dimensional transducers have attracted particularly great interest among scholars worldwide. In this type of probe, the single row of 1D elements is replaced by three to seven rows of smaller elements. As the elements in the elevation direction are fired symmetrically about the central element, dynamic focusing other than steering can be realized, thereby reducing scan slice thickness. 1.5D probes were proposed and manufactured in some groups between 1994 and 1999 [4–7], but the difficulties associated with 1.5D included the increased number of electrical connections and channels compared with a 1D probe. In 2003 Sato designed a transducer that had a lens
82
J. Peng et al. / Sensors and Actuators A 214 (2014) 81–87
Fig. 1. The geometric arrangement of the 1.5D transducer in (a). The five elements (A, B, C, D, and E) in the elevation direction fire symmetrically about the central element C in (b).
that used electrorheological fluid as another method for obtaining elevation focusing [8]. In 2003–2006 Daft et al. introduced cMUTs, which can provide slice thickness control through varying the polarity of the bias voltage [9–11]. However, these two methods of slice thickness control required expensive equipment and were complex processes, factors which led to a high cost. In addition, in 2008 Zhang et al. simulated the focusing acoustic fields of fractional dimensional ultrasound phased arrays and showed that the 1.5D array had better convergent characteristics in elevation than the 1D phased array [12]. In summary, 1.5D ultrasonic probes are needed in order to improve imaging quality. However, current solutions for 1.5D probes cannot provide low cost, simple but solid technologies for the purpose. This report introduces the design and fabrication of a 3 MHz 64 × 5 element 1.5D active matrix array. The fabrication difficulties associated with making electrical connections to 320 elements were solved by using a stack of multi-layer flexible printed circuit board (FPCB). The experimental results show that the 1.5D array can effectively reduce the scan slice thickness in the elevation direction and thus can improve the imaging resolution. Major aspects of the electrical and acoustic performance were analyzed and are presented below. 2. Design and fabrication The geometric arrangement of equal-area rows of the 1.5D array is schematically shown in Fig. 1. In Fig. 1(a) the x-axis indicates the azimuth direction and the y-axis indicates the elevation direction, in which dynamic focusing was realized through electronic delays attached to five elements (A + E, C, B + D) of one column. Compared with the traditional fabrication process, this 1.5D probe improved the connection of the signal lines. We used multiple layers of welding wire combined with a poured backing layer to form isolated electrical connections. The fabrication processes for bonding the different layers of the FPCB are illustrated in Fig. 2, which shows a cross section of the 1.5D array. We used welding copper wire with a
Fig. 2. Cross section of the 1.5D schematic representation of the assembly of the different layers.
diameter of 0.045 mm for element electrical interconnects through the backing layer, whose acoustic impedance was measured as 3.02 Mrayls. The presence of the wire does not significantly affect the impedance or attenuation because the wire and the solder point only occupy a small portion of area (<5%) for each element. The inner and outer matching layers with 9.81 Mrayls and 3.01 Mrayls were cast. And then we diced the acoustic components (two matching layers PZT-5H and backing layer) in the azimuth and elevation directions and filled the kerfs with epoxy to produce a transducer with 5 rows and 64 columns of 0.3 mm wide elements that filled a total aperture of 16 mm × 24 mm. Finally, we cast the lens on the top of transducer. Fig. 3 shows the 1.5D phased array we constructed to achieve dynamic focusing in the elevation direction. It operated at 3 MHz and was not a sparse array in that the 64 × 5 elements filled the entire 24 mm × 16 mm aperture. 3. Results and discussion 3.1. Performance evaluation Several parameters were used to evaluate the performance of the ultrasound transducer. The electromechanical factor (Kt ) was obtained with an impedance/phase analyzer using the measured resonant frequency (fr ) and anti-resonant frequency (fa ). Utilizing the pulse echo experimental results, the central frequency (fc ), −6 dB bandwidth (BW), two-way insertion loss (IL), and axial resolution (AR) were calculated using the following formulas:
Kt = fc =
fr fr · tan · · 1− 2 fa 2 fa
fL + fH 2
(1)
(2)
Fig. 3. Outline of the 64 × 5 element 1.5D array, which was 24 mm × 16 mm.
J. Peng et al. / Sensors and Actuators A 214 (2014) 81–87
83
300
Frequency(MHz)
275
-66
2
0.41
3
4
Voltage Spectrum
0.3
-68
6 0
5
Normalized Amplitude indB (dB)
Impedance Phase
225
fa = 3.9 MHz
200
-74
Voltage (V)
-72
Phase (Degree)
Impedance (Ohms)
-6
BW-6 dB = 79%
0.1
-12
0.0 -18
-0.1
-76
fr = 3.6 MHz
-0.2 -24
175
-78
150 2.5
3.0
3.5
4.0
-80 4.5
-0.3
64
fH − fL × 100% fc
IL = 20 × log
V R
VT
c = 2 · BW 2 · fc · BW
66
68
70
72
Time (μs)
Fig. 4. The measured electrical impedance magnitude (black line) and phase degree (red line) of a single column in which the parallel elements were simultaneously excited. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
BW =
-30 74
-0.4
Frequency (MHz)
AR =
fc = 3 MHz
0.2
-70
250
(3) (4) (5)
where fL and fH represent the lower and upper −6 dB frequencies, respectively, where the magnitude of the amplitude in the spectrum drops to 50% (−6 dB) of the maximum; VR and VT refer to the echo voltage and excitation voltage, respectively; and c is the speed of sound in the medium. 3.2. Impedance and phase testing After manufacturing the 1.5D phased array, 5 of the 64 columns were randomly selected for impedance measurements. One complete column was excited in parallel (A + B + C + D + E) by an impedance analyzer (Agilent 4294A). The impedance and phase spectra experimental results showed that fr was 3.6 MHz when the impedance value was 161 and fa was 3.9 MHz when the impedance value was 175 . The electromechanical factor Kt that could be calculated by fr and fa equaled 0.42. The detailed impedance and phase degree plots are shown in Fig. 4.
Fig. 5. The measured echo waveform (black line) and spectrum (red line) of a single column in which all the parallel elements were simultaneously excited. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
with the 50 coupling. The received echo was measured by the oscilloscope with the 10 M coupling. Three randomly selected columns were used to characterize the sensitivity of the 1.5D transducer. The insertion loss close to 58 dB was obtained at the average central frequency, as in Fig. 6. In a future study, we plan to fabricate the probe using new piezoelectric materials, such as a piezoelectric single crystal. 3.5. Cross talk measurement The level of electrical and acoustical separation between columns was determined by crosstalk measurement. The probe was placed opposite to an absorptive piece of rubber in a tank filled with deionized water. A Tektronix model AFG-3102 function generator was used to apply a 20-cycle sine wave with 1 Vpp on column 3 as a reference. Voltages cross to columns 1–5 were measured by an Agilent Mso7034B oscilloscope. The cross talk was expressed in decibels as the ratio of the voltage on columns 1, 2, 4, 5 to the voltage of the reference on column 3. The measured data is shown in Table 1. A cross talk of less than −28 dB was achieved for the adjacent columns at center frequency.
68
column 1 column 2 column 3
3.3. Pulse echo testing
Insertion Loss (dB)
66
The pulse echo response was characterized by using a stainless steel reflector positioned 50 mm away from the transducer, which was tested in de-ionized water. A Panametrics 5900PR pulser/receiver was used to excite the transducer and to receive the first echo from the reflector. The echo waveform was obtained using a digital oscilloscope. After a Fourier transformation, the fc , −6 dB BW, and AR, were 3 MHz, 79%, and 0.32 mm, respectively, according to formulas (2), (3) and (5). More detailed information is shown in Fig. 5.
64
62 58.5 dB @ 3.1 MHz 60
3.4. Two-way insertion loss
58.3 dB @ 3.2 MHz 58
To measure the two-way insertion loss, we used a Tektronix model AFG-3102 function generator and a Tektronix model DPO4104 oscilloscope with a 50 and 10 M coupling. A tone burst of a 20-cycle sine wave with an amplitude of 1 V at the central frequency of the 1.5D transducer, as measured by the oscilloscope
58 dB @ 3.1 MHz 1.5
2.0
2.5
3.0
3.5
4.0
4.5
Frequency (MHz) Fig. 6. The insertion loss from three randomly selected columns.
5.0
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J. Peng et al. / Sensors and Actuators A 214 (2014) 81–87
Fig. 7. Sound field simulation of one column in the elevation direction (a), −6 dB slice thickness resolution in (b).
3.6. Sound field measurement Before performing the sound field measurement experiments, we used Field II to simulate the sound field in the elevation direction. Fig. 7(a) shows the sound field simulation of one column, and Fig. 7(b) shows that we obtained a −6 dB slice thickness resolution of about 1.6 mm. Then the Optison Ultrasound Beam Analyzer and the Acoustic Intensity Measurement System (AIMS) with membrane hydrophones (Onda Corp., Sunnyvale, CA, USA) were used to measure the acoustic fields of the 1.5D transducer. Because of the low sensitivity, the emitted sound field of the 1.5D array was too weak to be visualized by human eyes. Finally, we decided to quantitatively observe the acoustic fields in the azimuth and elevation directions using an AIMS with a hydrophones model HMB-0500 system. The commercial PCIAD850 system (Ultratek, Inc., USA) which supports 8 channels was used to excite one column, as in Fig. 8. According to the Pythagorean Theorem, three electronic time delays, corresponding to channels 1–3, were calculated. The hydrophone started to scan when the vertical distance was 10 mm from the transducer surface in order to avoid damage to the hydrophone. The focused sound field at 41 mm was achieved when excitation pulses were delayed for 128 ns, 83 ns, and 0 ns, respectively, for channels 1, 2 and 3 (see Fig. 9(a)). Then when the vertical distance between the transducer and the hydrophone was 41 mm, the hydrophone was used to obtain the focal planes (xoy plane). This hydrophone experiment yielded a −6 dB slice
Fig. 8. Dynamic focusing in the elevation direction (yoz plane) of one column through three electronic delays attached to channels 1, 2, and 3.
approximately 2 mm thick, as shown in Fig. 9(b). The focal depth ranged from 21 mm to more than 46 mm with almost uniform slice thickness, which means that a high elevational resolution was able to be achieved over a wide range. This experimental result was quite consistent with the simulation results presented in Fig. 7. The −6 dB lateral resolution was poor because only the elements of one column were excited, which caused the focal spot to look like an ellipse, rather than forming a round shape. In order to achieve a further extension of the focal depth with a maximal resolution, the 1.5D array requires dynamic focusing along the axial (Z) direction by varying the delay times for each element. We carried out dynamic focusing for the fabricated 1.5D probe by using an 8-channel pulse generation module. The delay times were well controlled so that the focal point swept from 26 mm to 85 mm along the axial direction. Then we measured the half beam width (1/2 slice thickness) in the elevation direction for each focal point. The result is shown in Fig. 10. We found that the half slice thickness was within 1.5 mm, which means that the beam width was well controlled in the elevation direction so that it was within 3 mm across the entire dynamic focusing range. In order to compare the performance of 1.5D probe with that of conventional 1D probe, we also operate the 1.5D probe as a “1D” array by applying no delays to the three channels in elevation. The resulted sound field was measured and the elevation beam profile was drawn in the dash line as shown in Fig. 10. The −6 dB slice thickness of 1.5D array is much smaller for the entire focusing range. Especially, the 1.5D array shows great advantage over the “1D” array in the near field. This result is consistent with literature [13]. In the reported case of normal 1D probe, only within a very limited focal depth (∼10 mm) can the slice thickness be down to 3–5 mm. Beyond the focal zone, the slice thickness increases up to 10 mm in the near and far fields. This relatively large beam width can create partial volume artifacts when imaging small lesions. The contours may be blurred resulting in a detected signal level that is a combination of the lesion and the surrounding tissue. Therefore, our experimental results showed the overwhelming advantage of 1.5D probes over conventional 1D probes in that it controlled the slice thickness, allowing us to obtain accurate elevation focusing (Table 1). Furthermore, azimuth directivity of a single column of elements was measured. For this test, we excited the five elements in parallel and used a hydrophone to sweep along the azimuth direction. A full width acceptance angle of 25 degrees (±12.5 degree) at −6 dB was obtained from the data shown in Fig. 11. In addition,
J. Peng et al. / Sensors and Actuators A 214 (2014) 81–87
85
Fig. 9. Focused sound field in the elevation direction in (a) and focal planes in (b).
8
-6dB Sclice thickness of 1.5D -6dB Sclice thickness of 1D
7
Column 1 Column 2 Column 3 Column 4 Column 5
6
Elevation (mm)
Table 1 Crosstalk of the 1.5D arrays when Column 3 was excited.
5
1.8 MHz (dB)
3 MHz (dB)
4.2 MHz (dB)
−28 −23 – −24 −27
−30 −28 – −29 −30
−35 −29 – −29 −36
4 3 2 1 0 10
20
30
40
50
60
70
80
Range (mm) Fig. 10. Dynamic focusing of the 1.5D in elevation and elevation beam profile of “1D” when both exciting one column.
we also tested the beam steering performance of our 1.5D probe by firing two adjacent columns, as illustrated in Fig. 12. The ultrasound field distribution was measured by using a hydrophone with mechanical scanning. The results are depicted in Fig. 13(b), in which we can clearly see the beam steering 10◦ from normal in the azimuth direction when the excitation of the left column was delayed for 60 ns, in contrast to the result from simultaneously exciting two columns, as shown in Fig. 13(a). This result shows that by exerting different delays to each column of elements, we can operate the 1.5D probe like a normal phased array probe to achieve beam steering for imaging across a wider view angle. Again, the major advantages of our 1.5D probe over normal phased array probes are the much improved resolution in the elevation direction and the much extended focal depth along the axial direction.
Fig. 12. Beam steering along the azimuth direction by firing two adjacent columns Fig. 11. Azimuth directivity of 1.5D when exciting one column in parallel.
when electronic delays were attached to channels
.
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J. Peng et al. / Sensors and Actuators A 214 (2014) 81–87
Fig. 13. (a) Simultaneous excitation. (b) Left element excitation delayed 60 ns. Compared with (a), beaming steering in the azimuth direction can clearly be observed in (b).
4. Conclusion We manufactured a prototype of a 3 MHz 64 × 5 1.5D phased array and showed that the average central frequency was 3 MHz and that −6 dB bandwidths as great as 79% were obtained. This is greater than the 71.54% that was reported by Barthe’s group [7]. The insertion loss of −58 dB that we measured outweighs the relatively subtle advantages in Zhang’s report [12], in which the sensitivity was −61.4 dB. In addition, cross talk less than −28 dB between adjacent column was achieved at center frequency. The actual transmitting (one-way) sound field data were measured by using a hydrophone. A full width acceptance angle of 25 degrees (±12.5 degree) at −6 dB were measured for single column. We achieved dynamic focusing in the elevation direction and found that the −6 dB slice thickness was close to 2 mm, which is much smaller than normal 1D probes (3–5 mm at the focal point). Most importantly, by using dynamic focusing, we achieved a close-to2 mm slice thickness over a wide range (from 26 mm to 85 mm) along the axial direction, which is impossible to achieve with normal 1D probe. Once the 1.5D probe is applied to an ultrasonic imaging system that supports such a 1.5D control, we can expect to achieve significant image quality improvements such as better resolution and wider field of view.
Acknowledgments This work was supported by theNational Natural Science Foundation of China (Grant Nos. 10904093 and 61031003), the Natural Science Foundation of Guangdong Province (No. S2011010000447) and the Science and Technology Grant Scheme funds from the Shenzhen Government (No. JC201104210034A). The authors also appreciate the extensive English and content editing provided by Rhoda E. and Edmund F. Perozzi.
References [1] D.G. Wildes, L.S. Smith, Advanced ultrasound probes for medical imaging, AIP Conf. Proc. 1430 (2012) 801–808. [2] D.G. Wildes, R.Y. Chiao, C.M.W. Daft, et al., Elevation performance of 1.25 D and 1.5 D transducer arrays, IEEE Trans. Ultrason. Ferroelectr. Freq. Control. 44 (1997) 1027–1037. [3] S.W. Smith, W. Lee, E.D. Light, et al., Two dimensional arrays for 3-D ultrasound imaging, Proc. IEEE Ultrason. Symp. 1 (2002) 1545–1553. [4] T.J. Teo, 1.5 D transducer for medical imaging, Int. Soc. Opt. Photon. 3037 (1997) 55–61.
[5] J. Zhang, Q. Xue, S. Ogawa, 1.5 dimensional arrays for effective dynamic focusing and receiving, Proc. IEEE Ultrason. Symp. 2 (1996) 1531–1534. [6] P. Tournois, S. Calisti, Y. Doisy, et al., A 128× 4 channels 1.5 D curved linear array for medical imaging, Proc. IEEE Ultrason. Symp. 2 (1995) 1331–1335. [7] P. Barthe, M. Slayton, Characterization of 1.5-D ultrasound transducer arrays, Proc. IEEE. Ultrason. Symp. 2 (1996) 1489–1493. [8] S. Sato, H. Katsura, K. Kobayashi, et al., Elevation focusing of a phased array using variable ultrasound propagation speed in inter-mediate layer, Proc. IEEE. Ultrason. Symp. 1 (2003) 1052–1055. [9] C. Daft, P. Wagner, B. Bymaster, et al., cMUTs and electronics for 2D and 3D imaging: monolithic integration, in-handle chip sets and system implications, Proc. IEEE. Ultrason. Symp. 1 (2005) 463–474. [10] C. Daft, P. Wagner, S. Panda, et al., Elevation beam profile control with bias polarity patterns applied to microfabricated ultrasound transducers, Proc. IEEE. Ultrason. Symp. 2 (2003) 1578–1581. [11] C.M. Daft, P.A. Wagner, I. Ladabaum, Microfabricated ultrasonic transducers with bias polarity beam profile control and method of operating the same, U.S. Patent (2006). [12] B. Zhang, C. Zhang, F. Deng, Non-integral dimensions ultrasonic phased arrays in a borehole, Acta Acustica 28 (2009) 221–230. [13] W.R. Hedrick, D.L. Hykes, D.E. Starchman, Ultrasound Physics and Instrumentation [M], Elsevier Science Health Science, 2005.
Biographies Jue Peng received her B.S. degree in gemology and materials engineering from China University of Geosciences at Wuhan, China, in 2000, and her Ph.D. degree in materials physics from Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, China, in 2005. From 2006 to 2008, she served as the research associate for the Department of Applied Physics on Medical Ultrasonic Transducer Technology at the Hong Kong Polytechnic University. Then, she joined the Department of Biomedical Engineering at Shenzhen University. From 2008 to 2010, she held the title of Associate Professor of Biomedical Engineering at Shenzhen University. Her research interests include the design, modeling, and fabrication of multi-dimensional ultrasonic transducers, miniaturized high frequency ultrasonic transducers, piezoelectric MUT, echo-endoscope Probe for medical applications. Wenjuan Wang received his B.S. degree in electronic information science and technology from Taishan Medical College, Shandong, China. Now, she is a postgraduate student at the department of biomedical engineering in Shenzhen University, Guangdong, China. Her research interests include Field II simulation and ultrasonic transducer measurements. Hu Tang received his B.S. degree in electronic and information engineering in 2007 from Hubei University of Technology, Wuhan, China. He received his M.S. degrees in biomedical engineering from Shenzhen University in 2011. He is currently a technician in the Department of Biomedical Engineering, Shenzhen University. His research interests include ultrasonic measurements and finite element analysis of ultrasonic transducer. Tianfu Wang received his B.S. and M.S. degrees in electrical engineering from East China Normal University, Shanghai, China, in 1989 and 1992, respectively, and received his Ph.D. degree in biomedical engineering from Sichuan University in 1997. He is currently a professor in the Department of Biomedical Engineering, and a vice-president of School of Medicine at Shenzhen University. His research interests include medical image processing, neural networks, and pattern recognition.
J. Peng et al. / Sensors and Actuators A 214 (2014) 81–87 Siping Chen received his Ph.D. degree in biomedical engineering from Xi’an Jiaotong University, Shanxi, China, in 1987. After a postdoctoral fellowship in Zhejiang University, Zhejiang, China, he joined Shenzhen Anke High-Tech Co. Ltd. as chief technology officer in 1989. He has developed the first digital color Doppler ultrasound system in China and has received more than 10 National and Ministry awards for his outstanding research and distinguished contribution to convert
87
academic research outcomes into healthcare products. Having served in Anke Company for 16 years, he joined Shenzhen University as Vice President and chair professor of Department of Biomedical Engineering in 2005. He has published more than 300 papers and book chapters. Dr. Chen’s research interest is in Code Ultrasound and beamforming, Shear wave elasticity imaging, new imaging systems.
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Fabrication and performance of a 64 × 5 element 1.5D active matrix array transducer for medical imaging Jue Peng ∗ , Wenjuan Wang, Hu Tang, Tianfu Wang, Siping Chen National-Regional Key Engineering Laboratory for Medical Ultrasound, Guangdong Key Laboratory for Biomedical Measurements and Ultrasound Imaging, Department of Biomedical Engineering, School of Medicine, Shenzhen University, Shenzhen 518060, China
a r t i c l e
i n f o
Article history: Received 18 January 2014 Received in revised form 16 April 2014 Accepted 19 April 2014 Available online 26 April 2014 Keywords: Ultrasonic transducer Active matrix array Slice thickness control
a b s t r a c t Compared to 1D phased array probes with a fixed focus in elevation, multi-row arrays can significantly improved the slice thickness throughout image by expanding aperture and dynamic focusing in elevation. This paper describes the design and measurement of a 64 × 5 element 1.5D ultrasonic transducer that enables dynamic focusing and apodization in the elevation direction. We manufactured a transducer with an aperture size of 24 mm × 16 mm using a widely used piezoelectric ceramic (PZT-5H) as the piezoelectric vibrator. The measured center frequency and −6 dB fractional bandwidth of the 1.5D transducer were 3 MHz and 79%, respectively. A two-way insertion loss of −58 dB was obtained at the average center frequency. We carried out a sound field simulation and measured the actual transmitting (one-way) sound field data by using a hydrophone. In this way the sound beam profile in elevation direction was obtained, showing the −6 dB slice thickness measured was about 2 mm throughout the whole range of interest (from near to far field). This provided a much greater focal depth and much better imaging resolution in the elevation direction than can be achieved by using 1D probe. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Ultrasonic transducers play a fundamental role in determining the quality of diagnostic ultrasound imaging equipment. Current cardiac applications use linear phased arrays (1D) which can only be steered and dynamically focused in the azimuth direction, while the beam width in the plane perpendicular to the imaging plane, which is often referred to as the elevation beam width or slice thickness, has received relatively little attention. The acoustic noise generated by a relatively thick slice degrades the contrast resolution in small anechoic structures such as blood vessels or cysts [1]. Multidimensional transducers have received considerable attention in recent years as a way to control scan slice thickness. According to Wildes, the evolution of multi-dimensional transducers has been from 1.25D to 1.5D to 1.75D to 2D [2], which are defined as follows:
1D: The elevation aperture is fixed and focused at a fixed range. 1.25D: The elevation aperture is variable, but focusing remains static.
∗ Corresponding author. Tel.: +86-75586671915. E-mail address: [email protected] (J. Peng). http://dx.doi.org/10.1016/j.sna.2014.04.028 0924-4247/© 2014 Elsevier B.V. All rights reserved.
1.5D: The elevation aperture, shading, and focusing are dynamically variable, but symmetric about the centerline of the array. 1.75D: A 1.5D array without the symmetry constraint. The elements are large (several wavelengths) in elevation, so very little steering is possible. 2D: The elevation geometry and performance are comparable to those of the azimuth, with full electronic apodization, focusing, and steering. Three major challenges to the design of 2D matrix transducers have been identified: the requirement for thousands of channels, the difficulty of making such a large number of electrical connections and controlling the resulting array elements independently, and the extremely low sensitivity of the tiny elements [3]. Because of those difficulties, 1.5D fractional dimensional transducers have attracted particularly great interest among scholars worldwide. In this type of probe, the single row of 1D elements is replaced by three to seven rows of smaller elements. As the elements in the elevation direction are fired symmetrically about the central element, dynamic focusing other than steering can be realized, thereby reducing scan slice thickness. 1.5D probes were proposed and manufactured in some groups between 1994 and 1999 [4–7], but the difficulties associated with 1.5D included the increased number of electrical connections and channels compared with a 1D probe. In 2003 Sato designed a transducer that had a lens
82
J. Peng et al. / Sensors and Actuators A 214 (2014) 81–87
Fig. 1. The geometric arrangement of the 1.5D transducer in (a). The five elements (A, B, C, D, and E) in the elevation direction fire symmetrically about the central element C in (b).
that used electrorheological fluid as another method for obtaining elevation focusing [8]. In 2003–2006 Daft et al. introduced cMUTs, which can provide slice thickness control through varying the polarity of the bias voltage [9–11]. However, these two methods of slice thickness control required expensive equipment and were complex processes, factors which led to a high cost. In addition, in 2008 Zhang et al. simulated the focusing acoustic fields of fractional dimensional ultrasound phased arrays and showed that the 1.5D array had better convergent characteristics in elevation than the 1D phased array [12]. In summary, 1.5D ultrasonic probes are needed in order to improve imaging quality. However, current solutions for 1.5D probes cannot provide low cost, simple but solid technologies for the purpose. This report introduces the design and fabrication of a 3 MHz 64 × 5 element 1.5D active matrix array. The fabrication difficulties associated with making electrical connections to 320 elements were solved by using a stack of multi-layer flexible printed circuit board (FPCB). The experimental results show that the 1.5D array can effectively reduce the scan slice thickness in the elevation direction and thus can improve the imaging resolution. Major aspects of the electrical and acoustic performance were analyzed and are presented below. 2. Design and fabrication The geometric arrangement of equal-area rows of the 1.5D array is schematically shown in Fig. 1. In Fig. 1(a) the x-axis indicates the azimuth direction and the y-axis indicates the elevation direction, in which dynamic focusing was realized through electronic delays attached to five elements (A + E, C, B + D) of one column. Compared with the traditional fabrication process, this 1.5D probe improved the connection of the signal lines. We used multiple layers of welding wire combined with a poured backing layer to form isolated electrical connections. The fabrication processes for bonding the different layers of the FPCB are illustrated in Fig. 2, which shows a cross section of the 1.5D array. We used welding copper wire with a
Fig. 2. Cross section of the 1.5D schematic representation of the assembly of the different layers.
diameter of 0.045 mm for element electrical interconnects through the backing layer, whose acoustic impedance was measured as 3.02 Mrayls. The presence of the wire does not significantly affect the impedance or attenuation because the wire and the solder point only occupy a small portion of area (<5%) for each element. The inner and outer matching layers with 9.81 Mrayls and 3.01 Mrayls were cast. And then we diced the acoustic components (two matching layers PZT-5H and backing layer) in the azimuth and elevation directions and filled the kerfs with epoxy to produce a transducer with 5 rows and 64 columns of 0.3 mm wide elements that filled a total aperture of 16 mm × 24 mm. Finally, we cast the lens on the top of transducer. Fig. 3 shows the 1.5D phased array we constructed to achieve dynamic focusing in the elevation direction. It operated at 3 MHz and was not a sparse array in that the 64 × 5 elements filled the entire 24 mm × 16 mm aperture. 3. Results and discussion 3.1. Performance evaluation Several parameters were used to evaluate the performance of the ultrasound transducer. The electromechanical factor (Kt ) was obtained with an impedance/phase analyzer using the measured resonant frequency (fr ) and anti-resonant frequency (fa ). Utilizing the pulse echo experimental results, the central frequency (fc ), −6 dB bandwidth (BW), two-way insertion loss (IL), and axial resolution (AR) were calculated using the following formulas:
Kt = fc =
fr fr · tan · · 1− 2 fa 2 fa
fL + fH 2
(1)
(2)
Fig. 3. Outline of the 64 × 5 element 1.5D array, which was 24 mm × 16 mm.
J. Peng et al. / Sensors and Actuators A 214 (2014) 81–87
83
300
Frequency(MHz)
275
-66
2
0.41
3
4
Voltage Spectrum
0.3
-68
6 0
5
Normalized Amplitude indB (dB)
Impedance Phase
225
fa = 3.9 MHz
200
-74
Voltage (V)
-72
Phase (Degree)
Impedance (Ohms)
-6
BW-6 dB = 79%
0.1
-12
0.0 -18
-0.1
-76
fr = 3.6 MHz
-0.2 -24
175
-78
150 2.5
3.0
3.5
4.0
-80 4.5
-0.3
64
fH − fL × 100% fc
IL = 20 × log
V R
VT
c = 2 · BW 2 · fc · BW
66
68
70
72
Time (μs)
Fig. 4. The measured electrical impedance magnitude (black line) and phase degree (red line) of a single column in which the parallel elements were simultaneously excited. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
BW =
-30 74
-0.4
Frequency (MHz)
AR =
fc = 3 MHz
0.2
-70
250
(3) (4) (5)
where fL and fH represent the lower and upper −6 dB frequencies, respectively, where the magnitude of the amplitude in the spectrum drops to 50% (−6 dB) of the maximum; VR and VT refer to the echo voltage and excitation voltage, respectively; and c is the speed of sound in the medium. 3.2. Impedance and phase testing After manufacturing the 1.5D phased array, 5 of the 64 columns were randomly selected for impedance measurements. One complete column was excited in parallel (A + B + C + D + E) by an impedance analyzer (Agilent 4294A). The impedance and phase spectra experimental results showed that fr was 3.6 MHz when the impedance value was 161 and fa was 3.9 MHz when the impedance value was 175 . The electromechanical factor Kt that could be calculated by fr and fa equaled 0.42. The detailed impedance and phase degree plots are shown in Fig. 4.
Fig. 5. The measured echo waveform (black line) and spectrum (red line) of a single column in which all the parallel elements were simultaneously excited. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
with the 50 coupling. The received echo was measured by the oscilloscope with the 10 M coupling. Three randomly selected columns were used to characterize the sensitivity of the 1.5D transducer. The insertion loss close to 58 dB was obtained at the average central frequency, as in Fig. 6. In a future study, we plan to fabricate the probe using new piezoelectric materials, such as a piezoelectric single crystal. 3.5. Cross talk measurement The level of electrical and acoustical separation between columns was determined by crosstalk measurement. The probe was placed opposite to an absorptive piece of rubber in a tank filled with deionized water. A Tektronix model AFG-3102 function generator was used to apply a 20-cycle sine wave with 1 Vpp on column 3 as a reference. Voltages cross to columns 1–5 were measured by an Agilent Mso7034B oscilloscope. The cross talk was expressed in decibels as the ratio of the voltage on columns 1, 2, 4, 5 to the voltage of the reference on column 3. The measured data is shown in Table 1. A cross talk of less than −28 dB was achieved for the adjacent columns at center frequency.
68
column 1 column 2 column 3
3.3. Pulse echo testing
Insertion Loss (dB)
66
The pulse echo response was characterized by using a stainless steel reflector positioned 50 mm away from the transducer, which was tested in de-ionized water. A Panametrics 5900PR pulser/receiver was used to excite the transducer and to receive the first echo from the reflector. The echo waveform was obtained using a digital oscilloscope. After a Fourier transformation, the fc , −6 dB BW, and AR, were 3 MHz, 79%, and 0.32 mm, respectively, according to formulas (2), (3) and (5). More detailed information is shown in Fig. 5.
64
62 58.5 dB @ 3.1 MHz 60
3.4. Two-way insertion loss
58.3 dB @ 3.2 MHz 58
To measure the two-way insertion loss, we used a Tektronix model AFG-3102 function generator and a Tektronix model DPO4104 oscilloscope with a 50 and 10 M coupling. A tone burst of a 20-cycle sine wave with an amplitude of 1 V at the central frequency of the 1.5D transducer, as measured by the oscilloscope
58 dB @ 3.1 MHz 1.5
2.0
2.5
3.0
3.5
4.0
4.5
Frequency (MHz) Fig. 6. The insertion loss from three randomly selected columns.
5.0
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Fig. 7. Sound field simulation of one column in the elevation direction (a), −6 dB slice thickness resolution in (b).
3.6. Sound field measurement Before performing the sound field measurement experiments, we used Field II to simulate the sound field in the elevation direction. Fig. 7(a) shows the sound field simulation of one column, and Fig. 7(b) shows that we obtained a −6 dB slice thickness resolution of about 1.6 mm. Then the Optison Ultrasound Beam Analyzer and the Acoustic Intensity Measurement System (AIMS) with membrane hydrophones (Onda Corp., Sunnyvale, CA, USA) were used to measure the acoustic fields of the 1.5D transducer. Because of the low sensitivity, the emitted sound field of the 1.5D array was too weak to be visualized by human eyes. Finally, we decided to quantitatively observe the acoustic fields in the azimuth and elevation directions using an AIMS with a hydrophones model HMB-0500 system. The commercial PCIAD850 system (Ultratek, Inc., USA) which supports 8 channels was used to excite one column, as in Fig. 8. According to the Pythagorean Theorem, three electronic time delays, corresponding to channels 1–3, were calculated. The hydrophone started to scan when the vertical distance was 10 mm from the transducer surface in order to avoid damage to the hydrophone. The focused sound field at 41 mm was achieved when excitation pulses were delayed for 128 ns, 83 ns, and 0 ns, respectively, for channels 1, 2 and 3 (see Fig. 9(a)). Then when the vertical distance between the transducer and the hydrophone was 41 mm, the hydrophone was used to obtain the focal planes (xoy plane). This hydrophone experiment yielded a −6 dB slice
Fig. 8. Dynamic focusing in the elevation direction (yoz plane) of one column through three electronic delays attached to channels 1, 2, and 3.
approximately 2 mm thick, as shown in Fig. 9(b). The focal depth ranged from 21 mm to more than 46 mm with almost uniform slice thickness, which means that a high elevational resolution was able to be achieved over a wide range. This experimental result was quite consistent with the simulation results presented in Fig. 7. The −6 dB lateral resolution was poor because only the elements of one column were excited, which caused the focal spot to look like an ellipse, rather than forming a round shape. In order to achieve a further extension of the focal depth with a maximal resolution, the 1.5D array requires dynamic focusing along the axial (Z) direction by varying the delay times for each element. We carried out dynamic focusing for the fabricated 1.5D probe by using an 8-channel pulse generation module. The delay times were well controlled so that the focal point swept from 26 mm to 85 mm along the axial direction. Then we measured the half beam width (1/2 slice thickness) in the elevation direction for each focal point. The result is shown in Fig. 10. We found that the half slice thickness was within 1.5 mm, which means that the beam width was well controlled in the elevation direction so that it was within 3 mm across the entire dynamic focusing range. In order to compare the performance of 1.5D probe with that of conventional 1D probe, we also operate the 1.5D probe as a “1D” array by applying no delays to the three channels in elevation. The resulted sound field was measured and the elevation beam profile was drawn in the dash line as shown in Fig. 10. The −6 dB slice thickness of 1.5D array is much smaller for the entire focusing range. Especially, the 1.5D array shows great advantage over the “1D” array in the near field. This result is consistent with literature [13]. In the reported case of normal 1D probe, only within a very limited focal depth (∼10 mm) can the slice thickness be down to 3–5 mm. Beyond the focal zone, the slice thickness increases up to 10 mm in the near and far fields. This relatively large beam width can create partial volume artifacts when imaging small lesions. The contours may be blurred resulting in a detected signal level that is a combination of the lesion and the surrounding tissue. Therefore, our experimental results showed the overwhelming advantage of 1.5D probes over conventional 1D probes in that it controlled the slice thickness, allowing us to obtain accurate elevation focusing (Table 1). Furthermore, azimuth directivity of a single column of elements was measured. For this test, we excited the five elements in parallel and used a hydrophone to sweep along the azimuth direction. A full width acceptance angle of 25 degrees (±12.5 degree) at −6 dB was obtained from the data shown in Fig. 11. In addition,
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Fig. 9. Focused sound field in the elevation direction in (a) and focal planes in (b).
8
-6dB Sclice thickness of 1.5D -6dB Sclice thickness of 1D
7
Column 1 Column 2 Column 3 Column 4 Column 5
6
Elevation (mm)
Table 1 Crosstalk of the 1.5D arrays when Column 3 was excited.
5
1.8 MHz (dB)
3 MHz (dB)
4.2 MHz (dB)
−28 −23 – −24 −27
−30 −28 – −29 −30
−35 −29 – −29 −36
4 3 2 1 0 10
20
30
40
50
60
70
80
Range (mm) Fig. 10. Dynamic focusing of the 1.5D in elevation and elevation beam profile of “1D” when both exciting one column.
we also tested the beam steering performance of our 1.5D probe by firing two adjacent columns, as illustrated in Fig. 12. The ultrasound field distribution was measured by using a hydrophone with mechanical scanning. The results are depicted in Fig. 13(b), in which we can clearly see the beam steering 10◦ from normal in the azimuth direction when the excitation of the left column was delayed for 60 ns, in contrast to the result from simultaneously exciting two columns, as shown in Fig. 13(a). This result shows that by exerting different delays to each column of elements, we can operate the 1.5D probe like a normal phased array probe to achieve beam steering for imaging across a wider view angle. Again, the major advantages of our 1.5D probe over normal phased array probes are the much improved resolution in the elevation direction and the much extended focal depth along the axial direction.
Fig. 12. Beam steering along the azimuth direction by firing two adjacent columns Fig. 11. Azimuth directivity of 1.5D when exciting one column in parallel.
when electronic delays were attached to channels
.
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Fig. 13. (a) Simultaneous excitation. (b) Left element excitation delayed 60 ns. Compared with (a), beaming steering in the azimuth direction can clearly be observed in (b).
4. Conclusion We manufactured a prototype of a 3 MHz 64 × 5 1.5D phased array and showed that the average central frequency was 3 MHz and that −6 dB bandwidths as great as 79% were obtained. This is greater than the 71.54% that was reported by Barthe’s group [7]. The insertion loss of −58 dB that we measured outweighs the relatively subtle advantages in Zhang’s report [12], in which the sensitivity was −61.4 dB. In addition, cross talk less than −28 dB between adjacent column was achieved at center frequency. The actual transmitting (one-way) sound field data were measured by using a hydrophone. A full width acceptance angle of 25 degrees (±12.5 degree) at −6 dB were measured for single column. We achieved dynamic focusing in the elevation direction and found that the −6 dB slice thickness was close to 2 mm, which is much smaller than normal 1D probes (3–5 mm at the focal point). Most importantly, by using dynamic focusing, we achieved a close-to2 mm slice thickness over a wide range (from 26 mm to 85 mm) along the axial direction, which is impossible to achieve with normal 1D probe. Once the 1.5D probe is applied to an ultrasonic imaging system that supports such a 1.5D control, we can expect to achieve significant image quality improvements such as better resolution and wider field of view.
Acknowledgments This work was supported by theNational Natural Science Foundation of China (Grant Nos. 10904093 and 61031003), the Natural Science Foundation of Guangdong Province (No. S2011010000447) and the Science and Technology Grant Scheme funds from the Shenzhen Government (No. JC201104210034A). The authors also appreciate the extensive English and content editing provided by Rhoda E. and Edmund F. Perozzi.
References [1] D.G. Wildes, L.S. Smith, Advanced ultrasound probes for medical imaging, AIP Conf. Proc. 1430 (2012) 801–808. [2] D.G. Wildes, R.Y. Chiao, C.M.W. Daft, et al., Elevation performance of 1.25 D and 1.5 D transducer arrays, IEEE Trans. Ultrason. Ferroelectr. Freq. Control. 44 (1997) 1027–1037. [3] S.W. Smith, W. Lee, E.D. Light, et al., Two dimensional arrays for 3-D ultrasound imaging, Proc. IEEE Ultrason. Symp. 1 (2002) 1545–1553. [4] T.J. Teo, 1.5 D transducer for medical imaging, Int. Soc. Opt. Photon. 3037 (1997) 55–61.
[5] J. Zhang, Q. Xue, S. Ogawa, 1.5 dimensional arrays for effective dynamic focusing and receiving, Proc. IEEE Ultrason. Symp. 2 (1996) 1531–1534. [6] P. Tournois, S. Calisti, Y. Doisy, et al., A 128× 4 channels 1.5 D curved linear array for medical imaging, Proc. IEEE Ultrason. Symp. 2 (1995) 1331–1335. [7] P. Barthe, M. Slayton, Characterization of 1.5-D ultrasound transducer arrays, Proc. IEEE. Ultrason. Symp. 2 (1996) 1489–1493. [8] S. Sato, H. Katsura, K. Kobayashi, et al., Elevation focusing of a phased array using variable ultrasound propagation speed in inter-mediate layer, Proc. IEEE. Ultrason. Symp. 1 (2003) 1052–1055. [9] C. Daft, P. Wagner, B. Bymaster, et al., cMUTs and electronics for 2D and 3D imaging: monolithic integration, in-handle chip sets and system implications, Proc. IEEE. Ultrason. Symp. 1 (2005) 463–474. [10] C. Daft, P. Wagner, S. Panda, et al., Elevation beam profile control with bias polarity patterns applied to microfabricated ultrasound transducers, Proc. IEEE. Ultrason. Symp. 2 (2003) 1578–1581. [11] C.M. Daft, P.A. Wagner, I. Ladabaum, Microfabricated ultrasonic transducers with bias polarity beam profile control and method of operating the same, U.S. Patent (2006). [12] B. Zhang, C. Zhang, F. Deng, Non-integral dimensions ultrasonic phased arrays in a borehole, Acta Acustica 28 (2009) 221–230. [13] W.R. Hedrick, D.L. Hykes, D.E. Starchman, Ultrasound Physics and Instrumentation [M], Elsevier Science Health Science, 2005.
Biographies Jue Peng received her B.S. degree in gemology and materials engineering from China University of Geosciences at Wuhan, China, in 2000, and her Ph.D. degree in materials physics from Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, China, in 2005. From 2006 to 2008, she served as the research associate for the Department of Applied Physics on Medical Ultrasonic Transducer Technology at the Hong Kong Polytechnic University. Then, she joined the Department of Biomedical Engineering at Shenzhen University. From 2008 to 2010, she held the title of Associate Professor of Biomedical Engineering at Shenzhen University. Her research interests include the design, modeling, and fabrication of multi-dimensional ultrasonic transducers, miniaturized high frequency ultrasonic transducers, piezoelectric MUT, echo-endoscope Probe for medical applications. Wenjuan Wang received his B.S. degree in electronic information science and technology from Taishan Medical College, Shandong, China. Now, she is a postgraduate student at the department of biomedical engineering in Shenzhen University, Guangdong, China. Her research interests include Field II simulation and ultrasonic transducer measurements. Hu Tang received his B.S. degree in electronic and information engineering in 2007 from Hubei University of Technology, Wuhan, China. He received his M.S. degrees in biomedical engineering from Shenzhen University in 2011. He is currently a technician in the Department of Biomedical Engineering, Shenzhen University. His research interests include ultrasonic measurements and finite element analysis of ultrasonic transducer. Tianfu Wang received his B.S. and M.S. degrees in electrical engineering from East China Normal University, Shanghai, China, in 1989 and 1992, respectively, and received his Ph.D. degree in biomedical engineering from Sichuan University in 1997. He is currently a professor in the Department of Biomedical Engineering, and a vice-president of School of Medicine at Shenzhen University. His research interests include medical image processing, neural networks, and pattern recognition.
J. Peng et al. / Sensors and Actuators A 214 (2014) 81–87 Siping Chen received his Ph.D. degree in biomedical engineering from Xi’an Jiaotong University, Shanxi, China, in 1987. After a postdoctoral fellowship in Zhejiang University, Zhejiang, China, he joined Shenzhen Anke High-Tech Co. Ltd. as chief technology officer in 1989. He has developed the first digital color Doppler ultrasound system in China and has received more than 10 National and Ministry awards for his outstanding research and distinguished contribution to convert
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academic research outcomes into healthcare products. Having served in Anke Company for 16 years, he joined Shenzhen University as Vice President and chair professor of Department of Biomedical Engineering in 2005. He has published more than 300 papers and book chapters. Dr. Chen’s research interest is in Code Ultrasound and beamforming, Shear wave elasticity imaging, new imaging systems.