Baehyung Kim

24.8 An analog-digital-hybrid single-chip RX beamformer with non-uniform sampling for 2D-CMUT ultrasound imaging to achieve wide dynamic range of delay and small chip area

Ultrasound imaging is widely used for medical diagnosis, because it is harmless to the human body and has real-time processing capability. Usually the focusing (beamforming) operation is performed for both TX and RX. The RX focusing is performed by an RX beamformer [1-5], which consists of delay elements and adders. Nowadays, digital beamformers (DBF) are mostly used for conventional ultrasound imaging because of high SNR. Recently, 2D ultrasound transducers have been introduced for 3D imaging. Since the 2D transducer has a huge number of transducer elements (e.g., 9216 for a 72×128 array), it cannot use DBF because of the huge number of required ADCs and wires inside the probe cable. Therefore, analog beamforming must be performed, at least at the front stage of the 2D transducer.In this work, where a 2D CMUT array is used, the maximum delay difference among transducer elements is 8μs with a maximum steering angle of 45° and a maximum focal depth of 15cm. The target sampling resolution is 6.25ns (λc / 53.3) with a carrier frequency of 3MHz. An analog-digital-hybrid architecture and a non-uniform sampling scheme are used for the RX beamformer of this work to achieve the wide dynamic range of delay time and small chip-area. The RX beamformer consists of 8 analog beamformers (ABF) followed by a single DBF, as shown in Fig. 24.8.1. An ABF performs the focusing operation for the input signals of the adjacent 8 channels to generate an analog output signal. The 8 analog output signals from the 8 ABFs are applied to the DBF. The DBF converts the 8 analog input signals into the 8 digital signals, and then performs the focusing operation on the 8 digital signals to generate a digital output signal for every focal point.

Design and test of a fully controllable 64×128 2-D CMUT array integrated with reconfigurable frontend ASICs for volumetric ultrasound imaging

Up-to-date capacitive micro-machined ultrasonic transducer (CMUT) technologies offer us the chance to shrink the size and cost of ultrasound scanners by integrating the frontend (FE) circuits into CMUT arrays. For true real-time volumetric imaging system using 2-D arrays, CMUT-on-CMOS 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). In this work we present the design and feasibility test of 2-D CMUT arrays integrated with analog front-end electronics for transmitting high voltage pulses and receiving RF echo signals.

Reconfigurable 2D cMUT-ASIC arrays for 3D ultrasound image

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.

An Analog-Digital Hybrid RX Beamformer Chip With Non-Uniform Sampling for Ultrasound Medical Imaging With 2D CMUT Array

To reduce the memory area, a two-stage RX beamformer (BF) chip with 64 channels is proposed for the ultrasound medical imaging with a 2D CMUT array. The chip retrieved successfully two B-mode phantom images with a steering angle from -45° to +45°, the maximum delay range of 8 μs, and the delay resolution of 6.25 ns. An analog-digital hybrid BF (HBF) is chosen for the proposed chip to utilize the easy beamforming operation in the digital domain and also to reduce chip area by minimizing the number of ADCs. The chip consists of eight analog beamformers (ABF) for the 1st-stage and a digital beamformer (DBF) for the 2nd-stage. The two-stage architecture reduces the memory area of both ABF and DBF by around four times. The DBF circuit is divided into three steps to further reduce the digital FIFO memory area by around twice. Coupled with the non-uniform sampling scheme, the proposed two-stage HBF chip reduces the total memory area by around 40 times compared to the uniform-sampling single-stage BF chip. The chip fabricated in a 0.13- μm CMOS process occupies the area of 19.4 mm2, and dissipates 1.14 W with the analog supply of 3.3 V and the digital supply of 1.2 V.

Hybrid volume beamforming for 3-D ultrasound imaging using 2-D CMUT arrays

In real-time 3-D (4-D) ultrasound imaging using 2-D array transducers, manipulating massive ultrasound data acquired from a large number of system channels is a challenge as is interconnecting thousands of elements of 2-D array with the imaging systems front-end electronics. Minimizing the number of transmitting and receiving channels and the firing events without degrading the image quality is one of solutions to reduce the overall system complexity and improve the frame rate. This paper introduces a hybrid volume beam-forming and scanning method for a medical ultrasound imaging system. We also present an efficient volume image-forming architecture using 2-D arrays for miniaturized volumetric ultrasound imaging scanners. To evaluate our works, computer simulations are performed. From the simulation results, we have shown that the presented method provides the comparable image quality to the fully-sampled array (FSA) beam-formation, and is feasible for realtime volumetric ultrasound scanners with miniature devices.

Volumetric ultrasound image-forming using fully controllable 2-D CMUT-on-ASIC arrays

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.

An experimental study on coded excitation in CMUT arrays to utilize Simultaneous Transmission Multiple-zone Focusing method with frequency divided sub-band chirps

The frequency bandwidth of CMUTs (capacitive micromachined ultrasonic transducers) is known as relatively broader than that of other ultrasonic transducers. To utilize the wide bandwidth characteristic of the CMUT arrays, in this paper, we report on coded excitation techniques in the CMUT array. Through simulations, STMF (Simultaneous Transmit Multiple-zone Focusing) based ultrasound imaging techniques using orthogonally frequency-divided chirp signals are investigated. In the simulations, the frequency divided sub-band chirps that have orthogonal property are designed within the frequency bandwidth of the CMUT arrays, and simultaneously fired on multiple ranges, in which each signal is focused at a different range, in one transmission event. This paper also presents ultrasound images through a modulation and demodulation process of orthogonal sub-band coded signals. Experiments on the chirp-coded excitation in CMUT arrays are reported in this paper as a feasibility study of the FDMA (frequency division multiple access) like STMF method. In the experiment, mixed two orthogonal chirp signals are simultaneously fired with the CMUT arrays and the received signals are successfully separated into two compressed signals.

2D capacitive micromachined ultrasound transducer using novel tiling based on silicon frame

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.

Frequency division multiple transmission method to utilize the wide bandwidth property of capacitive micromachined ultrasonic transducer arrays

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.

Hybrid beamformation for volumetric ultrasound imaging scanners using 2-D array transducers

In real-time 3-D ultrasound imaging using 2-D array transducers, manipulating massive ultrasound data acquired from a large number of system channels is a challenge as is interconnecting hundreds or thousands of elements of 2-D array with the imaging systems front-end electronics. Minimizing the number of transmitting and receiving elements and the firing events without degrading the spatial resolution is one of the solutions to reduce the overall system complexity and improve the frame rate. This paper introduces a new hybrid beamformation method for a medical image system. We also present an efficient 2-D array scanning method for miniaturized volumetric ultrasound imaging scanners. To evaluate our works, computer simulations are performed. From the simulation results, we have shown that the proposed hybrid beamforming method can provide comparable image quality to fully-sampled array beamformation, and the proposed method may be feasible for miniaturized real-time volumetric ultrasound scanners.