Lequan Zhang

A high-frequency linear ultrasonic array utilizing an interdigitally bonded 2-2 piezo-composite

This paper describes the development of a high-frequency 256-element linear ultrasonic array utilizing an interdigitally bonded (IB) piezo-composite. Several IB composites were fabricated with different commercial and experimental piezoelectric ceramics and evaluated to determine a suitable formulation for use in high-frequency linear arrays. It was found that the fabricated fine-scale 2-2 IB composites outperformed 1-3 IB composites with identical pillar- and kerf-widths. This result was not expected and lead to the conclusion that dicing damage was likely the cause of the discrepancy. Ultimately, a 2-2 composite fabricated using a fine-grain piezoelectric ceramic was chosen for the array. The composite was manufactured using one IB operation in the azimuth direction to produce approximately 19-μm-wide pillars separated by 6-μm-wide kerfs. The array had a 50 μm (one wavelength in water) azimuth pitch, two matching layers, and 2 mm elevation length focused to 7.3 mm using a polymethylpentene (TPX) lens. The measured pulse-echo center frequency for a representative array element was 28 MHz and -6-dB bandwidth was 61%. The measured single-element transmit -6-dB directivity was estimated to be 50°. The measured insertion loss was 19 dB after compensating for the effects of attenuation and diffraction in the water bath. A fine-wire phantom was used to assess the lateral and axial resolution of the array when paired with a prototype system utilizing a 64-channel analog beamformer. The -6-dB lateral and axial resolutions were estimated to be 125 and 68 μm, respectively. An anechoic cyst phantom was also imaged to determine the minimum detectable spherical inclusion, and thus the 3-D resolution of the array and beamformer. The minimum anechoic cyst detected was approximately 300 μm in diameter.

A high-frequency, high frame rate duplex ultrasound linear array imaging system for small animal imaging

High-frequency (HF) ultrasound imaging has been shown to be useful for non-invasively imaging anatomical structures of the eye and small animals in biological and pharmaceutical research, achieving superior spatial resolution. Cardiovascular research utilizing mice requires not only realtime B-scan imaging, but also ultrasound Doppler to evaluate both anatomy and blood flow of the mouse heart. This paper reports the development of an HF ultrasound duplex imaging system capable of both B-mode imaging and Doppler flow measurements, using a 64-element linear array. The system included an HF pulsed-wave Doppler module, a 32-channel HF B-mode imaging module, a PC with a 200 MS/s 14-bit A/D card, and real-time Lab View software. A 50 dB SNR and a depth of penetration of larger than 12 mm were achieved using a 35-MHz linear array with 50 μm pitch. The two-way beam widths were determined to be 165 to 260 μm and the clutter-energy-to-total-energy ratio (CTR) were 9.1 to 12 dB when the array was electronically focused at different focal points at depths from 4.8 to 9.6 mm. The system is capable of acquiring real-time B-mode images at a rate greater than 400 frames per second (fps) for a 4.8 × 13 mm field of view, using a 30 MHz 64-element linear array with 100 μm pitch. Sample in vivo cardiac high frame rate images and duplex images of mouse hearts are shown to assess its current imaging capability and performance for small animals.

Real-time assessment of flow reversal in an eccentric arterial stenotic model

Plaque rupture is the leading cause of acute coronary syndromes and stroke. Plaque formation, otherwise known as stenosis, preferentially occurs in the regions of arterial bifurcation or curvatures. To date, real-time assessment of stenosis-induced flow reversal remains a clinical challenge. By interfacing microelectromechanical system (MEMS) thermal sensors with the high frequency pulsed wave (PW) Doppler ultrasound, we proposed to assess flow reversal in the presence of an eccentric stenosis. We developed a 3-D stenotic model (inner diameter of 6mm, an eccentric stenosis with a height of 2.75 mm, and width of 21 mm) simulating a superficial arterial vessel. We demonstrated that heat transfer from the sensing element (2 x 80 μm²) to the flow field peaked as a function of flow rates at the throat of the stenosis along the center/midline of arterial model, and dropped downstream from the stenosis, where flow reversal was detected by the high frequency ultrasound device at 45 MHz. Computational fluid dynamics (CFD) codes are in agreement with the ultrasound-acquired flow profiles upstream, downstream, and at the throat of the stenosis. Hence, we characterized regions of eccentric stenosis in terms of changes in heat transfer along the midline of vessel and identified points of flow reversal with high spatial and temporal resolution.

Development of a 64 channel ultrasonic high frequency linear array imaging system.

In order to improve the lateral resolution and extend the field of view of a previously reported 48 element 30 MHz ultrasound linear array and 16-channel digital imaging system, the development of a 256 element 30 MHz linear array and an ultrasound imaging system with increased channel count has been undertaken. This paper reports the design and testing of a 64 channel digital imaging system which consists of an analog front-end pulser/receiver, 64 channels of Time-Gain Compensation (TGC), 64 channels of high-speed digitizer as well as a beamformer. A Personal Computer (PC) is used as the user interface to display real-time images. This system is designed as a platform for the purpose of testing the performance of high frequency linear arrays that have been developed in house. Therefore conventional approaches were taken it its implementation. Flexibility and ease of use are of primary concern whereas consideration of cost-effectiveness and novelty in design are only secondary. Even so, there are many issues at higher frequencies but do not exist at lower frequencies need to be solved. The system provides 64 channels of excitation pulsers while receiving simultaneously at a 20-120 MHz sampling rate to 12-bits. The digitized data from all channels are first fed through Field Programmable Gate Arrays (FPGAs), and then stored in memories. These raw data are accessed by the beamforming processor to re-build the image or to be downloaded to the PC for further processing. The beamformer that applies delays to the echoes of each channel is implemented with the strategy that combines coarse (8.3 ns) and fine delays (2 ns). The coarse delays are integer multiples of the sampling clock rate and are achieved by controlling the write enable pin of the First-In-First-Out (FIFO) memory to obtain valid beamforming data. The fine delays are accomplished with interpolation filters. This system is capable of achieving a maximum frame rate of 50 frames per second. Wire phantom images acquired with this system show a spatial resolution of 146 μm (lateral) and 54 μm (axial). Images with excised rabbit and pig eyeball as well as mouse embryo were also acquired to demonstrate its imaging capability.

Design of a 64 channel analog receive beamformer for high frequency linear arrays

A new 64 channel analog receive beamformer was developed to support a recently built 30 MHz 256-element linear array with 50 μm pitch. Each channel of the proposed beamformer consisted of a coarse delay and a fine delay circuit block. The delay blocks were implemented using wide bandwidth tapped delay lines. Multiplexers were used to select the output taps of the fine delay and coarse delay. Field Programmable Gate Arrays (FPGA) devices which stored the delay profile of each channel were used to update the control signals for the multiplexers at each scan line and focal zone switching. Due to the transient effect when turning a switch on or off, glitch noise could be introduced during each focal zone transition. A differential circuit, as well as switch offsetting, was used to suppress such an effect. The design was shown to be capable of providing a total delay of 157 ns with a resolution of 1ns. The rise time of the cascading delay lines was 8.66 ns, which set the −3dB bandwidth to about 40MHz. The test on two channels of the beamformer showed a 20 dB improvement in SNR by the two glitch suppressing methods. Phantoms images shows that the system can achieve lateral resolution of 120 μm, and can resolve anechoic spheres as small as 300 μm in diameter.

Development of a C-Scan phased array ultrasonic imaging system using a 64-element 35MHz transducer

Phased array imaging systems provide the features of electronic beam steering and dynamic depth focusing that cannot be obtained with conventional linear array systems. This paper presents a system design of a digital ultrasonic imaging system, which is capable of handling a 64-element 35MHz center frequency phased array transducer. The system consists of 5 parts: an analog front-end, a data digitizer, a DSP based beamformer, a computer controlled motorized linear stage, and a computer for post image processing and visualization. Using a motorized linear stage, C-scan images, parallel to the surface of scanned objects may be generated. This digital ultrasonic imaging system in combination a 35 MHz phased array appears to be a promising tool for NDT applications with high spatial resolution. It may also serve as an excellent research platform for high frequency phased array design and testing as well as ultrasonic array signal algorithm developing using systems raw RF data acquisition function.

Development of a digital high frequency ultrasound array imaging system

In order to improve the lateral resolution and extend the image view of the previously developed 16-channel digital beamformer for a 30MHz ultrasound linear array, the design of an ultrasound imaging system with a higher frame rate and increased channel count is reported in this paper. The system is composed of 256 channels of analog front-end pulser/receiver, 64 channels of Time-Gain Compensation (TGC), 64 channels of high-speed digitizer as well as a beamformer. A PC is used as user interface to display the real time images. This system is designed for imaging using up to 256 elements linear arrays. The system provides 64 channels of excitation pulsers while receiving simultaneously at a 20 MHz-500 MHz sampling rate with 12-bit resolution. The digitized data of all channels are first fed through FPGAs, and then stored in memories. Those raw data are accessed by the beamforming processor to rebuild the image or download to the PC for further processing. The beamformer that applies delays to the echoes of each channel is implemented with the strategy that combines coarse and fine delays. The coarse delays are integer multiple the sampling clock rate. They are achieved by controlling write enable pin of First-In-First-Out memory to obtain valid beamforming data. The fine delays are accomplished with interpolation filters. The design allows a maximum frame rate of 50 frames per second to be achieved. The wire phantom images show the spatial resolution of 146 μm (lateral) and 54 μm (axial). Images with excised rabbit and pig eyeball as well as mouse embryo were acquired.

Improved high-frequency high frame rate duplex ultrasound linear array imaging system

In this paper, we report recent progress that has been made in the development of a high frame rate duplex HF ultrasound system with both B-scan imaging and Doppler flow measurements. A 32-channel HF analog beamformer module with transmitting focusing and dynamic focusing on reception was implemented. High-speed timing circuits were used to achieve high imaging frame rate by reducing the acquisition overhead. Therefore, the frame rate of the system relies only on the field of view. The system also included a 64-channel analog front-end pulser/receiver, a HF pulsed-wave (PW) Doppler module, a PC with a 200 MS/s 14-bit PCI A/D card and real-time Labview software for data acquisition and image display. A wire phantom used to evaluate the resolution of the system. High frame rate B-scan images of mouse hearts have been obtained, as well as the PW Doppler blood flow velocity profiles at the specified location. The system is able to acquire real-time B-mode images at a rate greater than 400 frames per second in a 4.8 times 13 mm field of view. In vivo mouse experiment results show a promising future of this system in small animal research. The system will be expanded to support future arrays with more elements.

Adaptive clutter filter design for micro-ultrasound color flow imaging of small blood vessels

In micro-ultrasound, which uses imaging frequencies above 20 MHz, obtaining color flow images (CFI) of small blood vessels using is not a trivial task because it is more challenging to suppress tissue clutter properly given the stronger blood signal power at high imaging frequencies and the slow blood velocity inside the microcirculation. To improve clutter suppression in micro-ultrasound CFI, this paper presents an adaptive clutter filtering approach that is based on a two-stage eigen-analysis of slow-time ensemble characteristics. The approach first identifies tissue pixels in the imaging view by examining whether high-frequency contents are absent in the principal slow-time eigen-components for each pixel as computed from single-ensemble eigen-decomposition. It then computes the filtered slow-time ensemble for each pixel by finding the least-squares projection residual between the pixels slow-time ensemble and the clutter eigen-components estimated from a multi-ensemble eigen-decomposition of tissue slow-time ensembles within a spatial window. In this filtering approach, the clutter eigen-components are chosen based on whether their mean frequency lies within a spectral band. To analyze the efficacy of the proposed adaptive filter, both in-vitro experiments and Field II simulations were carried out. For the experiments, raw CFI data were acquired using a 64-element, 33 MHz linear array prototype (pulse duration: 2 cycles, PRF: 1 kHz, transmit focus: 8mm, F-number: 5). Their imaging view corresponded to the cross-section of a 0.9mm-diameter tube that was placed on top of an unsuspended table where ambient vibrations may appear; flow velocity (5, 7, 10, 15 mm/s) within the tube was controlled using a syringe pump. For the simulations, raw CFI data was computed for both plug and parabolic flow profiles, and tissue motion was modeled as 0.5 mm/s sinusoidal vibrations. For all flow velocities tested in our in-vitro study, the proposed adaptive filter improved the flow detection sensitivity as compared to existing ones. In the slow-flow case (5 mm/s), we observed over 70% increase in flow detection sensitivity (assuming a 5% false alarm rate). This effectively reduced flashing artifacts in the resulting CFIs and gave a more consistent visualization of the flow tube.

5B-5 High-Frequency Duplex Ultrasound Imaging System for Biomedical Applications Using 30 MHz Linear Arrays

High-frequency (HF) ultrasound imaging has been shown to be useful for imaging anatomical structures of the eye and small animals in biological and pharmaceutical research, achieving good spatial resolution at an affordable price. Cardiovascular research utilizing mice requires not only real-time B-scan imaging, but also ultrasound Doppler to evaluate both movements and blood flow of the mouse heart. In this paper, we report the development of the first real-time duplex HF ultrasound system with both B-scan imaging and Doppler imaging, using a 30 MHz 64-element linear array. The system included a HF pulsed-wave Doppler module, a 16-channel HF analog beamformer module, a PC with a 200 MS/s 14-bit PCI A/D card, and real-time Labview software. Both a wire phantom and a micro flow phantom were used to evaluate system performance. The system has a lateral resolution better than 160 urn and is capable of measuring motion velocity as low as 0.1 mm/s and as high as 1 m/s. B-scan images of excised rabbit eyes have been achieved, as well as clear blood flow velocity profiles in mouse superficial vessels with diameters of 200 mum and major aortas. The system is able to acquire real-time B-mode and Doppler images. The system will also have the capability of acquiring 400 B-mode images per second. In vivo zebrafish and mouse experiment results show a promising future of this system in small animal research.