REAL-TIME IN-VIVO IMAGING WITH CAPACITIVE MICROMACHINED ULTRASOUND TRANSDUCER (CMUT) LINEAR ARRAYS David M. Mills and L. Scott Smith General Electric Global Research Center, Niskayuna, NY 12309
Abstract - Capacitive micromachined ultrasound transducers (cMUTs) have emerged as a leading area of research because these devices are nonresonant and can be integrated along with signal processing electronics. However, few publications discuss cMUT arrays packaged in a handheld probe capable of generating real-time ultrasound images. This paper presents real-time in-vivo images from a cMUT linear array and compares them to PZT array images that were made with a GE LOGIQ 9 ultrasound imager. CMUT images show equivalence in most cases to PZT images. In some cases, anatomical borders and texture are more clearly resolved in the cMUT array images. Pulse-echo data showing a fractional bandwidth of 110% will be presented and compared to PZT arrays. Improvement in cMUT sensitivity is still needed to compete fully with PZT arrays, as these cMUT arrays show -10 dB worse sensitivity than that from comparable PZT arrays. Images of a cMUT probe will also be shown to demonstrate the progress of packaging such an array and the accompanying electronics into a clinically relevant size. I. INTRODUCTION Medical ultrasound transducer technology has remained largely unchanged for the last 50 years. Late in the 1940’s, ceramic resonators based on barium titanate were developed as high-permittivity ceramics that exhibited a piezoelectric effect. Then, in the 1950’s lead based ceramics began to replace barium titanate in ultrasonic wave generation and reception applications [1]. Inherent to both piezoceramics was the need to attach front matching and backing layers to increase the frequency range of operation for these transducers. Otherwise, they
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would have a very high Q response and instead of having a compact impulse response, would ring for many cycles after electrical excitation. Compact impulse response is essential for medical ultrasound imaging in order to obtain good axial resolution, which is necessary to resolve the targets of interest in tissue. Further shortening the impulse response of transducers by broadening the frequency response, will lead to both improved resolution in images and a broader operating range for a particular device. Recent research in the areas of composites, single crystal transducers, and capacitive micromachined ultrasound transducers (cMUTs) have focused on exactly this feature. In 1996, Ladabaum and KhuriYakub, et. al. introduced cMUT technology for immersion applications based on silicon nitride membranes as a revolutionary method of fabricating efficient ultrasound transducers with a broad frequency response [2]. More recently, Ergun and Khuri-Yakub et. al. introduced cMUT technology based on a simpler wafer bonding approach with silicon membranes [3]. This paper presents the application of cMUT technology to medical ultrasound imaging and shows preliminary results and images made using this technology. The hypothesis of this paper is that cMUTs can be used to simplify transducer array fabrication while yielding improved performance over existing medical ultrasound transducer technology. Existing challenges for cMUT technology include the reliability, cross-talk between independent elements, packaging, and spectral notches. II. METHODS In contrast to ceramic and single crystal piezoelectrics, immersion cMUTs are inherently non-
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resonant devices, as the resonance of the membrane is damped by the mechanical loading. Fabrication of a cMUT is described in detail in the literature [2,3]. A cMUT consists of a silicon substrate with a membrane, typically silicon or silicon nitride, over a vacuum gap. Varying the electrical signal applied to electrodes above and below the gap, causes the membrane to deflect due to electrostatic forces, which in turn generate acoustic waves. Conversely, incident acoustic waves deflect the membrane varying the capacitance between the electrodes, which can be detected electrically. Since electrostatic forces are only attractive, a DC bias, that is larger than half of the peak-to-peak amplitude of the AC signal, must be applied to the cMUT. As discussed in the literature, typical dimensions for cMUT cells used for immersion devices are between 10-100 microns in width with a gap between the electrodes of less than a micron [4]. In order to demonstrate the use of cMUTs for medical imaging, the array geometry of a high frequency linear array ultrasound probe (GE 12L) was selected. Sensant Corporation fabricated cMUT die based on these specifications and sent them to the GE Global Research Center for interconnect, assembly into a finished probe, and acoustic testing. To connect to each element of the cMUT array, metal traces were patterned onto a sheet of polyimide film to create a flexible circuit, or high density interconnect (HDI) flex. This flex was attached to the cMUT array, as shown in Fig. 1, and to a coaxial cable bundle via biasing electronics, which could then be connected to a test rack or ultrasound imaging system. Prior to testing the cMUT array in a water tank, a film (3-10 microns) of parylene was applied to the face of the array to electrically isolate it from the water tank. An acoustic lens and matched backing were also applied to the array for improved patient isolation and imaging performance. Probe assembly was critical so that live human subjects could be scanned to demonstrate the ability to form anatomical images using this transducer technology. Considerations were given to patient safety, ease of scanning by the operator of the probe, and access to the patient, since the 12L is predominantly a small parts, high frequency probe. Once the probe was assembled, Fig. 2, the integrity of the electrical isolation was tested using two electrical methods. First, AC line voltage (120 Vrms) was
Figure 1: cMUT array with flexible interconnect circuit.
applied to all conductors inside the array handle and the sealed face was submerged in 5% saline solution which was grounded. The leakage current was measured and could not be greater than 50 microamps (AC rms). Next, a high-potential (hi-pot) test was performed in a similar fashion, where the peak applied voltage was 1500 VACrms and the current could not exceed 100 micro-amps (AC rms). At this high voltage, any potential conduction path, such as an air bubble in the dielectric coating, would arc and cause a test failure.
Figure 2: Assembled cMUT probe showing the ergonomic handle, cable, and system connector.
Acoustic testing of these devices included pulseecho testing in a water tank to demonstrate the hypothesized broadband response. The water tank contained de-ionized water and a flat plate reflector (glass) that was placed beneath the transducer face. As shown in Fig. 3, a 5-axis positioning system was used to position the cMUT array 12 mm above the
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target. A DC power supply biased all of the cMUT elements to 100-200 volts, which was typically between 75 and 90% of membrane collapse voltage. To measure the pulse-echo response, a pulser/receiver was connected to the individual elements of the transducer array. After amplification, a LeCroy LC334A oscilloscope digitized the returned echo, that was subsequently stored on a computer.
Figure 4: Experimental pulse-echo response in water of a representative cMUT element.
Figure 3: Acoustic testing setup showing the water tank, 5axis positioning system, and cMUT array with biasing electronics.
III. RESULTS Pulse echo results, including the transient response, spectrum, and pulse-shape, are shown in Fig. 4. As measured in the water tank, the cMUT array had a -6 dB bandwidth of 110%, which is much greater than piezoelectric ceramic based probes that typically have bandwidths between 70-80%. The loop gain of the cMUT array was about –10 dB relative to the PZT array in the configuration measured in our watertank. Note also the short pulse length shown in Fig. 4. The sensitivity variation across a cMUT array is shown in Fig. 5. Figure 5: Sensitivity variation across a cMUT array.
Real-time, in-vivo images of a carotid artery were made using the PZT and cMUT arrays as shown in Figures 6 (short-axis) and 7 (long-axis). Improved axial resolution yielded slightly finer texture in the thyroid gland as seen in Fig. 6, while slightly better defined intimal lining is visible in the cMUT image
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of Fig. 7. Shallower penetration is also evident from these images as seen in both figures, which shows the lower sensitivity for the cMUT array.
micro-calcifications in the breast, small abnormalities in the thyroid, or better delineation of the intimal linings of blood vessels to search for plaque build-up. Once the sensitivity of cMUT arrays has been improved the required depth of penetration for a wide array of applications should be possible. Also, more dramatic improvement in resolution must be visible in the images to prove that this technology is a viable replacement for lead-based piezoelectric ceramics and single crystals. V. ACKNOWLEDGEMENTS The authors thank the Sensant Corporation employees who contributed to the design and fabrication of the cMUT arrays used in this work. Also, the authors thank our colleagues at GEMS and GEGR for their help with circuit design, packaging, assembly, and imaging.
Figure 6: Short-axis view of the carotid artery and thyroid gland from a PZT array (left) and a cMUT array (right).
VI. REFERENCES [1] J. Moulson and J. M. Herbert, Electroceramics, New York: Chapman & Hall, 1990, pp. 3, 276, & 285. [2] I. Ladabaum, X. Jin, H. T. Soh, F. Pierre, A. Atalar, and B. T. Khuri-Yakub, “Microfabricated ultrasonic transducers: towards robust models and immersion devices,” in Proceedings of the 1996 IEEE Ultrasonics Symposium, pp. 335 338.
Figure 7: Long-axis view of the carotid artery from a PZT array (left) and a cMUT array (right).
IV. DISCUSSION This paper has shown some of the first side-by-side comparison in medical images between PZT and cMUT medical imaging arrays. CMUTs are showing real potential in terms of improved axial resolution and extremely broad bandwidth operation. Improved axial resolution will allow for smaller targets to be resolved in tissue for such applications as imaging
[3] A. S. Ergun, Y. Huang; C. H. Cheng; O. Oralkan, J. Johnson, H. Jagannathan, U. Dernirci, G. G. Yaralioglu, M. Karaman, B. T. Khuri-Yakub, “Broadband capacitive micromachined ultrasonic transducers ranging from 10 kHz to 60 MHz for imaging arrays and more,” in Proceedings of the 2002 IEEE Ultrasonics Symposium, pp. 1039 – 1043. [4] I. Ladabaum, X. Jin, H. T. Soh, A. Atalar, and B. T. Khuri-Yakub, “Surface micromachined capacitive ultrasonic transducers,” IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, vol. 45, no. 3, pp. 678-690, May 1998.
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