Shiwei Zhou

Pre-compensated excitation waveform to suppress second harmonic generation in MEMS electrostatic transducers

Microelectromechanical systems (MEMS) electrostatic-based transducers inherently produce harmonics as the electrostatic force generated in the transmit mode is approximately proportional to the square of the applied voltage signal. This characteristic precludes them from being effectively used for harmonic imaging (either with or without the addition of microbubble-based contrast agents). The harmonic signal that is nonlinearly generated by tissue (or contrast agent) cannot be distinguished from the inherent transmitted harmonic signal. We investigated two precompensation methods to cancel this inherent harmonic generation in electrostatic transducers. A combination of finite element analysis (FEA) and experimental results are presented. The first approach relies on a calculation, or measurement, of the transducers linear transfer function, which is valid for small signal levels. Using this transfer function and a measurement of the undesired harmonic signal, a predistorted transmit signal was calculated to cancel the harmonic inherently generated by the transducer. Due to the lack of perfect linearity, the approach does not work completely in a single iteration. However, with subsequent iterations, the problem becomes more linear and converges toward a very satisfactory result (a 18.6 dB harmonic reduction was achieved in FEA simulations and a 20.7 dB reduction was measured in a prototype experiment). The second approach tested involves defining a desired function [including a direct current (DC) offset], then taking the square root of this function to determine the shape of the required input function. A 5.5 dB reduction of transmitted harmonic was obtained in both FEA simulation and experimental prototypes test.

An approach for reducing adjacent element crosstalk in ultrasound arrays

A method is presented for active cancellation of crosstalk effects in ultrasonic arrays. The approach makes use of the programmable transmitter waveform generators that are now being used with growing prevalence in diagnostic ultrasound systems. The arrays transmit mode transfer function is represented by a transfer function matrix. Elements of this matrix are determined by exciting a single, central element with a wideband waveform and determining the resulting pressure output from the central element and adjacent elements. The desired output then is defined (e.g., finite output from a single, central element) and zero output from all other elements. The transfer function matrix equation can be solved to determine the required excitation functions on both the central array element and its neighbors. These excitation functions result in reduced evidence of crosstalk on the output signals. Therefore, the single-element, angular-response function is improved. Using superposition, the approach can be extended to beamformed array excitation. A variety of theoretical and experimental results are shown. The method also can be used in the receive mode but with a less satisfactory solution. A transmitting mode experiment based on a prototype five-element transducer has provided results indicating that sidelobes in the angular response can be reduced using this technique.

Dynamic-transmit focusing using time-dependent focal zone and center frequency

A method for achieving dynamic-transmit focus is presented. Within an initial high bandwidth pulse, successively higher frequencies are focused to successively closer focal zones along a transmitted beam line. During the receive operation, a time varying bandpass filter initially passes the higher frequencies that are focused closer in and successively passes lower center frequencies as time evolves and the focal zone moves out. The dynamically focused receive pulses are digitally sampled and processed by a matched digital filter to minimize phase anomalies. In this way, an improved resolution image is obtained with no loss of frame rate. The method is evaluated using comprehensive simulations that account for realistic levels of phase aberration and tissue attenuation. The method is relatively robust with respect to these perturbations. When the appropriate conditions apply, the new method can achieve an improvement in mean lateral resolution similar to that found in a multiple transmit zone implementation but without the frame rate penalty. A discussion of implementation considerations and limitations is presented.

High element count two dimensional transducer array

Preliminary work on a fully sampled (32/spl times/32 element) 2D array is presented. This forms a component of a planned system capable of C-Scans (and ultimately B-Scans) of shallow blood vessels as a guide for intravenous needle insertion. Amongst the challenges faced when fabricating such an array, the process by which to form 1024 electrical connections in a compact area is probably most burdensome. The approach used here employs a printed circuit board (PCB) with an eight layer substrate through which electrical signal connections to each element were routed. In an early prototype, in which only edge elements were electrically accessible, we have achieved a high yield (approximately 90%) of active elements without open or short circuits. In a later prototype, using a full scale PCB we achieved approximately 80%+ yield of working elements. A discussion of the design and fabrication challenges is presented. Finite element analysis (FEA) was used as part of the design and optimization. Strategies for future improvements to both the design and fabrication processes are presented.

Portable, low-cost medical ultrasound device prototype

The first generation prototype of a portable, low-cost medical ultrasound device is described along with experimental results. The prototype system consists of a fully sampled 2D transducer array, sixteen custom receive circuitry chips multiplexed into two bandpass filter channels, and an onboard programmable logic device. A PC fitted with a commercially available data acquisition card is used for data collection and analysis. Beamforming is performed using the direct-sampled in-phase/quadrature method. Pulse-echo images obtained with the prototype are presented and future work is discussed.

Finite-element analysis of material and parameter effects in laser-based thermoelastic ultrasound generation

Laser-based, thermoelastic transduction methods have potential in very high frequency (>50 MHz), high-density two-dimensional (2-D) arrays for a variety of very high-resolution superficial imaging applications, including in vivo tissue sectioning. Previous studies of these transducers generally have been based on experimental measurements or theoretical analyses using various simplifying assumptions. These theoretical models are mostly 1-D and best matched to simple geometries with a minimum number of component materials. In this work, we use a new thermoelastic solver in a commercially available finite-element analysis (FEA) software package to analyze multidimensional effects in laser-based devices of arbitrary geometry with the potential for use with arbitrary material properties. The FEA approach was verified first against experimental data. Thereafter, we explored the impact of various design variables, including laser spot size and laser penetration depth.

A prototype low-cost handheld ultrasound imaging system

We describe a very low cost handheld ultrasound system that we are currently developing for routine applications such as image guided needle insertion. We provide a system overview and focus discussion on our beamforming strategy, direct sampled I/Q (DSIQ) beamforming. DSIQ beamforming is a low cost approach that relies on phase rotation of in-phase/quadrature (I/Q) data to implement focusing. The I/Q data are generated by directly sampling the received radio frequency (RF) signal, rather than through conventional baseband demodulation. We describe our efficient hardware implementation of the beamformer, which results in significant reductions in beamformer size and cost. We also present the results of experiments and simulations that compare the DSIQ beamformer to more conventional approaches, namely time delay beamforming and traditional complex demodulated I/Q beamforming. Results that show the effect of an error in the direct sampling process, as well as dependence on signal bandwidth and system f number (f#) are presented. These results indicate that the image quality and robustness of the DSIQ beamformer are adequate for routine applications.

A cancellation based approach for reducing acoustic cross-talk in ultrasound arrays

A new cancellation method is described in this paper for controlling the acoustic cross-talk in ultrasonic arrays. The arrays transmit mode transfer function is represented by a transfer function matrix. Elements of this matrix are determined by exciting a single central element with an initial wideband waveform. The desired output is then defined (e.g. finite output from a single central element and zero output from all other elements). The output from the center element and its adjacent element are measured and recorded. The associated electrical excitation is also recorded. These values are used to populate the transfer function matrix. Then, the matrix equation can be solved to determine the required excitation functions on both the central array element and its neighbors. These excitation functions will result in reduced evidence of cross-talk on the output signals. Finite element analysis (FEA) simulation results produced a 9dB cross-talk reduction. Additionally, a transmitting mode experiment based on a prototype five element transducer has provided results indicating that sidelobes in the angular response can be reduced using this technique.

Finite element modeling of laser-based thermoelastic ultrasound generation

Using a scanned laser to generate ultrasound, via the thermoelastic effect, offers an alternative approach for realizing high density, high frequency ultrasound imaging arrays. The approach bypasses the complexity and intricacy required for forming conventional piezoelectric array elements and their associated electrical connections. Thus, it is particularly well suited to 2D arrays. In this paper, the devices considered comprise a carbon black loaded PDMS polymer layer on top of a glass or PDMS substrate. PZFlex Finite Element Analysis (FEA) was used to investigate the impact of a variety of design variables including: laser spot size, substrate material and thermoelastic coupling medium. Predicted single element angular response broadly matched responses obtained by experiment. Specifically, if a low acoustic loss glass substrate is used then measurable sidelobes occur at approximately 40 degrees. However, if the glass substrate is replaced by a PDMS material, then the traveling waves that give rise to sidelobes are no longer supported and a smooth single element angular response is obtained in both experiment and FEA simulation. FEA suggests that there are other modes in addition to the Rayleigh mode observed in the experiment. It is believed that these modes are more quickly damped in the experimental case. Therefore, while FEA provides a very versatile and valuable analysis tool, the accuracy of its predictions are contingent on accurate knowledge of device geometry and relevant material properties.