In real-time 3-D ultrasound imaging using 2-D array transducers, a large number of the 2-D array elements pose challenges in fabricating and transferring signals from/into the system. This fabrication problem has been solved by using a silicon micromachining process for capacitive micromachined ultrasonic transducer (CMUT) arrays. For realtime 3-D ultrasound imaging, manipulating massive ultrasound data acquired from a large number of system channels is a challenge as is fabricating and interconnecting hundreds or thousands of elements of 2-D array with the imaging system’s front-end (FE) electronics. Minimizing the number of transmitting and receiving elements and the firing events without degrading the image quality is one of the solutions to reduce the overall system complexity and improve the frame rate. We have been developing a real-time 3-D volumetric ultrasound imaging system using 2-D CMUT arrays by integrating FE electronics with a large number of 2-D array elements. Here, we explore a configuration method to design a scalable 2-D CMUT array and a new volumetric image-formation method to provide higher information rate of a volume image. In this paper, we present the 2-D CMUT-on-ASIC arrays designed to reduce the overall system complexity, and a new volume scanning and image-forming method for real-time 3-D volumetric ultrasonic imaging using 2-D CMUT-on-ASIC arrays. To evaluate our works, we performed from theoretical studies for point spread functions of the array configuration to phantom experiments with off-the-line images.
CMUT-on-ASIC integration techniques are promising for the development of lower cost smaller volume scanners with higher performance in terms of features and image qualities because it minimizes parasitic capacitances and ultimately improves signal-to-noise ratio (SNR). Moreover, a frequency bandwidth of CMUT array is known as relatively broader than that of other ultrasonic transducer arrays. To utilize the wide bandwidth characteristic of the CMUT arrays, in this paper, we introduce a FDMA (frequency division multiple access) based ultrasound imaging technique using orthogonally band-divided coded signals to provide dynamic transmit focused imaging without sacrificing the frame rate. In the presented method, the orthogonal sub-band coded signals are simultaneously fired on multiple ranges, in which each signal is focused at a different range, in one transmission event. This paper also presents an ultrasound imageformation method and a modulation and demodulation process of orthogonal sub-band coded signals designed within the frequency bandwidth of the CMUT arrays. The presented method is verified by computer simulations using Field II and experiments. The simulation results using a computer generated tissue mimicking phantom show that the presented method can be achieved with both increased image quality and frame rate. The experimental results to verify the feasibility of the presented method using orthogonal sub-band coded signals show that the reflected signals from targets are successfully separated into two compressed signals. Currently, we are extending the presented approach to ultrasound imaging technique for volumetric ultrasound scanners using 2-D CMUT-on-ASIC arrays.
Up-to-date capacitive micromachined ultrasonic transducer (CMUT) technologies provide us unique opportunities to minimize the size and cost of ultrasound scanners by integrating front-end circuits into CMUT arrays. We describe a design prototype of a portable ultrasound scan-head probe using 2-D phased CMUT-on-ASIC arrays of 3-MHz 250 micrometer-pitch by fabricating and integrating front-end electronics with 2-D CMUT array elements. One of the objectives of our work is to design a receive beamformer architecture for the smart probe with compact size and comparable performance. In this work, a phase-rotation based receive beamformer using the sampling frequency of 4 times the center frequency and a hybrid beamforming to reduce the channel counts of the system-side are introduced. Parallel beamforming is considered for the purpose of saving power consumption of battery (by firing fewer times per image frame). This architecture has the advantage of directly obtaining I and Q components. By using the architecture, the interleaved I/Q data from the storage is acquired and I/Q demodulation for baseband processing is directly achieved without demodulators including sin and cosine lookup tables and mixers. Currently, we are extending the presented architecture to develop a true smart probe by including lower power devices and cooling systems, and bringing wireless data transmission into consideration.
In this study, we showed the new transducer and probe integration of 2D ultrasound probe using cMUT. cMUT ultrasound probe having 8192 elements is assembled with tiling frame. Flip chip bonded cMUT-ASIC tiles were arrayed along 2×8 directions to enlarge lateral aperture. Tiling gap between two tiles was under 100μm. RTV layer that has 1mm thick is used in 2-D probe system as a lens and protection layer. Thermal module is also analyzed by using the thermal network analysis, which is realized with the air fans and the fins. Designed PCB circuit for tiling module which is considered with cooling spread concept is 5cm × 5cm dimension. Uniformity and performance of tiled ultrasound transducer were tested under soybean oil at 3MHz frequency successfully. The measured 256 elements distribution has only 4.45% deviation. If we can remove the side edge error, the deviation will be under 3%. The performance after RTV lensing showed 35% attenuation in Tx and 35~45% attenuation in Rx.
KEYWORDS: Ultrasonography, Analog electronics, Transducers, 3D image processing, Signal to noise ratio, Electronics, Amplifiers, Switches, Signal processing, Image quality
This paper describes the design and implementations of the complete 2D capacitive micromachined ultrasound
transducer electronics and its analog front-end module for transmitting high voltage ultrasound pulses and receiving its
echo signals to realize 3D ultrasound image. In order to minimize parasitic capacitances and ultimately improve signal-to-
noise ratio (SNR), cMUT has to be integrate with Tx/Rx electronics. Additionally, in order to integrate 2D cMUT
array module, significant optimized high voltage pulser circuitry, low voltage analog/digital circuit design and packaging
challenges are required due to high density of elements and small pitch of each element. We designed 256(16x16)-
element cMUT and reconfigurable driving ASIC composed of 120V high voltage pulser, T/R switch, low noise
preamplifier and digital control block to set Tx frequency of ultrasound and pulse train in each element. Designed high
voltage analog ASIC was successfully bonded with 2D cMUT array by flip-chip bonding process and it connected with
analog front-end board to transmit pulse-echo signals. This implementation of reconfigurable cMUT-ASIC-AFE board
enables us to produce large aperture 2D transducer array and acquire high quality of 3D ultrasound image.
KEYWORDS: Signal to noise ratio, Ultrasonography, Modulation, Signal processing, Optical correlators, Imaging systems, Fermium, Frequency modulation, Transducers, Fourier transforms
In this paper, a novel method for simultaneous transmit multi-zone (STMZ) focusing along multiple scan lines using modulated orthogonal codes is presented. In this method, M mutually orthogonal Golay codes are transmitted along M respective directions to obtain M scan lines simultaneously, where each Golay code consists of M complementary sequences. This implies that along each scan direction M complementary sequences comprising one of the Golay code are to be fired sequentially. Therefore, total M transmit and receive (T/R) events are required to obtain M scan lines. In the proposed method, however, each complementary code is convolved with the sum of L orthogonal chirps that are focused at L different depths. Consequently, the proposed method requires M T/R events to obtain M scan lines with L transmit foci each, whereas ML T/R events are required in conventional pulse echo imaging. After M firings, first the M orthogonal Golay codes are separately compressed, secondly each compressed Golay signal is correlated with L respective orthogonal chirps, and finally the L compressed chirp signals are independently focused and combined to provide the dynamically focused beams along M scan lines, each with multi-zone transmit foci. Experimental results with a 7.5MHz linear array, for M=L=2, showed that the proposed method can provide ultrasound images with higher frame rates and significantly improved SNR compared to conventional multi-zone focusing methods using short pulses.
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