Capsule Ultrasound Device: Further Developments

Farah Memon*, Gerard Touma*, Junyi Wang*,

Spyridon Baltsavias, Azadeh Moini, Chienliu Chang, Morten Fischer Rasmussen,

Amin Nikoozadeh, Jung Woo Choe, Eric Olcott, R. Brooke Jeffrey, Amin Arbabian, and Butrus T. Khuri-Yakub

*Equal contribution

Stanford University, USA

Corresponding email: farahm@stanford.edup

Abstract— We are developing a capsule ultrasound (CUS)

device – a pill with the capability to scan the gastrointestinal (GI)

tract through ultrasound. In this paper, we discuss the design

and fabrication of the main components of the CUS device

including the CMUT array, front-end electronics, and the

wireless transmitter. We demonstrate a successfully fabricated

128-element CMUT array with polydimethylsiloxane (PDMS)-

filled trenches and show their input impedance in air. The front-

end electronics, measuring 6 mm by 6 mm and the high-data rate

wireless transmitter, measuring 1 mm by 1.76 mm, have been

fabricated. Our preliminary power analysis indicates that our

total power consumption is less than 20 mW for the CUS device.

Our future work involves integrating these core components for

imaging experiments.

Keywords— CMUT; capsule endoscope; front-end electronics;

wireless; medical devices; medical imaging; ultrasound

I. INTRODUCTION

Currently, one of the main ultrasound screening tools for the digestive system is endoscopic ultrasound (EUS) [1][2]. To increase the convenience and prevalence of ultrasound-based gastrointestinal (GI) tract screening, we are developing a capsule ultrasound (CUS) device. This ingestible pill has a transducer array wrapped around its central body, as depicted in Fig. 1. This device will have the capability to acquire and wirelessly transmit B-mode images of the entire GI tract, thus introducing ultrasound to the realm of capsule-based devices.

Fig. 1. The CUS device and the 128-element cylindrical ultrasound array with

16 active elements [3].

A 128-element capacitive micromachined ultrasonic transducer (CMUT) array, with 16 active elements and linear array imaging, will be used for the CUS device to acquire 360o field-of-view (FOV) B-mode images. Additionally, the CUS device will also include front-end electronics for interfacing with the CMUT array and a wireless transmitter for sending the beamformed data to an external receiver [3]. In this paper, we discuss our further progress towards the development of the CUS device.

II. CMUT ARRAY

A. CMUT Array Fabrication

CMUT arrays have been fabricated using the wafer-bonding process and incorporated with polydimethylsiloxane (PDMS)-filled trenches between elements [3]. PDMS-filled trenches provide flexibility to the array and insulation between elements. The CMUT arrays have under-bump metallization (UBM) electrodes on the back-side to facilitate its flip-chip bonding onto a flexible printed circuit board (PCB) and have aluminum pads for contact on the front-side. A cross-section of the CMUT array is depicted in Fig. 2 and a picture showing a fabricated device is displayed in Fig. 3.

The front-side trenches have a width of approximately 4 μm to preserve most of the area for CMUT cells and the back-side trenches have a width of 50 μm to allow PDMS deposition in the trenches. At the end of the fabrication process, all elements are connected via a bridge to a common frame, which is adhered to the top and bottom of the devices. Prior to bending the CMUT arrays, the CMUT arrays will be flip chip bonded to a flexible PCB and we will then break off the frame, leaving the elements connected to each other via PDMS on the flexible PCB. We will then wrap the CMUT array with the flexible PCB around the CUS device. [3].

Fig. 2. Cross-section of the CMUT array. Picture not drawn to scale [3].

Fig. 1. The CUS device and the 128-element cylindrical ultrasound array

with 16 active elements.

978-1-4673-9897-8/16/$31.00 ©2016 IEEE 2016 IEEE International Ultrasonics Symposium Proceedings

Fig. 3. A fabricated 128-element CMUT array with PDMS-filled trenches.

The bridges, connecting the elements to the common frame, measure approximately 50 μm in width, 60 μm in length, and 100-120 μm in height. The bridge dimensions were chosen to allow the CMUT arrays to remain intact at the end of the fabrication process and also enable the breaking of the bridge to separate the CMUT elements on the flexible PCB. To demonstrate this phenomenon, we broke off the frame of a test device and its side-view is displayed in Fig. 4. In the figure, the front-side and back-side trenches, along with the bridges and the PDMS, are shown. The device is also curved, corroborating the flexible nature of our devices. In addition, a cross-section of the test device with PDMS-filled trenches is displayed in Fig. 5. In our process, the spin speed used for PDMS deposition results in the trenches being partially-filled with PDMS. Nonetheless, the PDMS provides sufficient mechanical support and flexibility for our arrays.

To test our idea of bending the CMUT array with a flexible PCB around the pill, we wrapped a test device with a flexible PCB around a cylindrical rod. This test device has PDMS-filled trenches and can easily conform to the shape of the rod, as shown in Fig. 6.

Fig. 4. Side-view of test device after breaking the frame. PDMS-filled

trenches and the bridges can be seen.

Fig. 5. Cross-section of test device showing the trenches filled with PDMS.

Fig. 6. A test device with PDMS filled trenches and a flexible PCB wrapped

around a 1 cm cylindrical rod.

Our CMUT arrays, measure approximately 2.88 cm in length and 4 mm in height, excluding the frame. We are currently integrating the CMUT arrays on a flexible PCB and breaking the frame off the CMUT arrays using the same strategy as used with the test devices

B. CMUT Array Characterization

The CMUT arrays are designed for low-voltage operation. To characterize the fabricated devices, we biased a CMUT element at different DC voltages and measured its input impedance in air using an impedance analyzer (Agilent Technologies, Model 4294A). The results of the impedance analysis indicate the functionality of the CMUT arrays at the end of the fabrication process and demonstrate healthy performance in the low-voltage regime.

Fig. 7. Input Impedance of an element of the fabricated CMUT array

Bridge

Front-side trench

PDMS

Back-side trench

III. FRONT-END ELECTRONICS

In order to activate and receive signals from the transducer array elements, we designed a front-end electronics chip. The block diagram of front-end electronics chip is depicted in Fig. 8. It comprises 128 channels, each including transmitting and receiving circuitries that interface with the corresponding 128 CMUT elements.

In the transmitting circuitry, every channel is loaded by a 4-bit delay code that corresponds to the required phase delay for that channel. Non-active channels are loaded with a ‘zero’ code. When transmit operation starts, a 4-bit counter increments, and when its output matches the delay code of a channel, the pulser for that channel excites the CMUT element by a unipolar pulse with a controllable range of 15-18 V. We chose a 4-bit delay code and a counter frequency of 20 MHz to provide the required delay resolution (50 ns), while having a wide enough range of phase delays. This offers more flexibility in changing the focal distance of the output ultrasound beam.

In the receiving circuitry, a common source trans-impedance amplifier (TIA) amplifies the received RF signals. The TIA provides a gain of 90 dB, and a bandwidth of 10 MHz. Next, a time-gain compensation (TGC) amplifier is designed to compensate for the attenuation of the ultrasound signal inside the human body. The TGC amplifier provides a controllable gain range of 0-40 dB, with steps of 10 dB. Next, the amplified signal is digitized by a 10-bit, 20 MS/s analog-to-digital converter (ADC), with a signal-to-noise-and-distortion ratio (SNDR) of 57.5 dB. The ADC stage is followed by a digital delay-and-sum circuitry that reconstructs the beamformed data using signals from active channels [3]. As a result, the output beamformed signal is 14 bits with a transmission rate of 20 MS/s (280 Mbits/s).

We have taped-out the chip using the TSMC 0.18-μm high voltage process, and we are currently testing it. The chip, shown in Fig. 9, has an area of 6 mm by 6 mm, and fits inside the capsule, along with all the other required components. This first version chip is powered and controlled externally, and it implements all the aforementioned functionalities. The chip also has a simulated power of approximately 7.3 mW. Fig. 10(a) shows the simulated pulse waveform of an active channel, and Fig. 10(b) shows the simulated frequency response of the ADC.

Fig. 8. Block diagram of the front-end electronics showing the transmitting

and receiving circuitries.

Fig. 9. Picture of the fabricated front-end electronics chip.

(a) (b)

Fig. 10. (a) Simulated waveform of pulser without (blue) and with (red)

parasitic effects and (b) simulated ADC performance of the front-end

electronics chip.

IV. WIRELESS TRANSMITTER

We have also taped-out a wireless transmitter for the CUS device. To wirelessly transmit the ultrasound beamformed data with a frame rate of up to 4 frames/sec, we designed the data rate to range from 5 to 10 Mbits/s. The transmitter chip, displayed in Fig. 11, measures approximately 1.76 mm by 1 mm and has an expected average power consumption of 6 mW with an output power of > 3 dBm. For wireless transmission, it uses a 915 MHz ISM frequency band to accommodate the signal bandwidth while minimizing attenuation through tissue. Due to the difference in data acquisition rate of the front-end electronics (280 Mbits/s) and data transmission rate of the wireless transmitter, we plan to use an off-the-shelf memory to buffer the data.

Fig. 11. Picture of the fabricated wireless transmitter for the CUS device.

V. POWER ANALYSIS

We have performed a power analysis of the entire device including the CMUT array, front-end electronics, wireless transmitter, and memory [5]. The analysis is based on the activation time of the electronics and their data rates, as shown in Table I. Our power consumption breakdown, shown in Fig. 12, gives an estimated total average power consumption of 19.27 mW. This number is the typical power budget of capsule-based devices, as they are powered by two 1.5 V batteries that provide 20 mW for up to 8 hours, which is the approximate travel duration time inside the GI tract [4].

VI. CONCLUSION AND FUTURE WORK

In this paper, we report our progress towards the development of the CUS device. The CMUT array, front-end electronics, and the wireless transmitter have been designed and fabricated. On the CMUT array side, our future work involves integrating the transducer arrays with the flexible PCB and characterizing the image quality of the CMUT array-flexible PCB assembly.

In addition, we plan to test the front-end electronics chip with the CMUT array to assess its ability to capture beamformed data. We will also test the front-end electronics with the wireless transmitter to evaluate the effect of wireless transmission on image quality. In the first stage, the operation of the electronic components will be coordinated by an external field-programmable gate array (FPGA). Eventually, we will integrate the complete control unit on chip. The control unit is responsible for loading the delay values for focusing, signaling the start of the transmit and receive modes, and controlling the data transfer outside the pill.

Ultimately, the CUS device will use two 1.5 V batteries to power itself. Hence, in the future, we plan incorporate a power management unit in the electronics to step-up the DC voltages for CMUT biasing.

Fig. 12. Power consumption breakdown of the CUS device.

TABLE I. DATA RATES AND ACTIVATION TIMES OF ELECTRONIC

COMPONENTS OF THE CUS DEVICE

Parameter Value

Penetration Depth 5 cm

Frames/sec 4

Data Acquisition Rate 280 Mbits/s

Data Transmission Rate 5-10 Mbits/s

Time for Data Acquisition ~8.6 ms

Time for Data Transmission ~240 ms

ACKNOWLEDGMENT

Funding for this project was provided by the General Electric Company. We would like to thank the National Science Foundation (NSF) for the Graduate Research Fellowship. The CMUT array fabrication was performed at Stanford Nanofabrication Facility (Stanford, CA), part of the NSF-funded NNCI and NNIN programs. Part of the work was performed at the Stanford Nano Shared Facilities (SNSF).

REFERENCES

[1] A. Chak, “EUS in submucosal tumors,” Gastrointestinal Endoscopy, vol. 56, 2002.

[2] F. G. Gress, R.H. Hawes, T.J. Savides, S.O. Ikenberry, O. Cummings, K. Kopecky, S. Sherman, M. Wiersema, and G.A. Lehman, “Role of EUS in the preoperative staging of pancreatic cancer: a large single-center experience,” Gastrointestinal Endoscopy, vol. 50, no. 6, pp. 786-791, 1999.

[3] F. Memon, G. Touma, J. Wang, S. Baltsavias, A. Moini, C. Chang, M. F. Rasmussen, A. Nikoozadeh, J. W. Choe, A. Arbabian, R. B. Jeffrey, E. Olcott, and B. T. Khuri-Yakub, "Capsule ultrasound device," Ultrasonics Symposium (IUS), 2015 IEEE International, Taipei, 2015, pp. 1-4.

[4] G. Ciuti, A. Menciassi and P. Dario, "Capsule Endoscopy: From Current Achievements to Open Challenges," in IEEE Reviews in Biomedical Engineering, vol. 4, no. , pp. 59-72, 2011.

[5] Datasheet: http://media.digikey.com/pdf/Data%20Sheets/Cypress%20PDFs/CY621472EV30LL-45ZSXI.pdf