Hojong Choi

Development of integrated preamplifier for high-frequency ultrasonic transducers and low-power handheld receiver

This paper describes the design of a front-end circuit consisting of an integrated preamplifier with a Sallen-Key Butterworth filter for very-high-frequency ultrasonic transducers and a low-power handheld receiver. This preamplifier was fabricated using a 0.18-μm 7WL SiGe bi-polar complementary metal oxide semiconductor (BiCMOS) process. The Sallen-Key filter is used to increase the voltage gain of the front-end circuit for high-frequency transducers which are generally low in sensitivity. The measured peak voltage gain of the frontend circuits for the BiCMOS preamplifier with the Sallen-Key filter was 41.28 dB at 100 MHz with a-6-dB bandwidth of 91%, and the dc power consumption of the BiCMOS preamplifier was 49.53 mW. The peak voltage gain of the front-end circuits for the CMOS preamplifier with the Sallen-Key filter was 39.52 dB at 100 MHz with a-6-dB bandwidth of 108%, and the dc power consumption of the CMOS preamplifier was 43.57 mW. Pulse-echo responses and wire phantom images with a single-element ultrasonic transducer have been acquired to demonstrate the performance of the front-end circuit.

Investigating contactless high frequency ultrasound microbeam stimulation for determination of invasion potential of breast cancer cells.

In this article, we investigate the application of contactless high frequency ultrasound microbeam stimulation (HFUMS) for determining the invasion potential of breast cancer cells. In breast cancer patients, the finding of tumor metastasis significantly worsens the clinical prognosis. Thus, early determination of the potential of a tumor for invasion and metastasis would significantly impact decisions about aggressiveness of cancer treatment. Recent work suggests that invasive breast cancer cells (MDA‐MB‐231), but not weakly invasive breast cancer cells (MCF‐7, SKBR3, and BT‐474), display a number of neuronal characteristics, including expression of voltage‐gated sodium channels. Since sodium channels are often co‐expressed with calcium channels, this prompted us to test whether single‐cell stimulation by a highly focused ultrasound microbeam would trigger Ca2+ elevation, especially in highly invasive breast cancer cells. To calibrate the diameter of the microbeam ultrasound produced by a 200‐MHz single element LiNbO3 transducer, we focused the beam on a wire target and performed a pulse‐echo test. The width of the beam was ∼17 µm, appropriate for single cell stimulation. Membrane‐permeant fluorescent Ca2+ indicators were utilized to monitor Ca2+ changes in the cells due to HFUMS. The cell response index (CRI), which is a composite parameter reflecting both Ca2+ elevation and the fraction of responding cells elicited by HFUMS, was much greater in highly invasive breast cancer cells than in the weakly invasive breast cancer cells. The CRI of MDA‐MB‐231 cells depended on peak‐to‐peak amplitude of the voltage driving the transducer. These results suggest that HFUMS may serve as a novel tool to determine the invasion potential of breast cancer cells, and with further refinement may offer a rapid test for invasiveness of tumor biopsies in situ. Biotechnol. Bioeng. 2013;110: 2697–2705.

A configurable dual-frequency transmit/receive system for acoustic angiography imaging

Acoustic angiography is a high-resolution imaging modality for small vascular structure. It utilizes the nonlinear backscatter of microbubble contrast agents (MCAs) to delineate blood vessels. In acoustic angiography, where MCAs are insonified with high rarefractional pressures at resonance frequency (6.5MHz), high-order harmonics (30 MHz) become more evident and can be utilized to produce high-resolution images for detecting small vascular structures. We developed a configurable dual-frequency system platform dedicated to acoustic angiography. The system consists of pulse generation, data acquisition and signal processing blocks. It is controlled by a field programmable gate array (FPGA), which enables flexible programming, and many on-board processing and stimulation modes. The system was shown to be capable of acoustic angiography as well as traditional B-mode imaging.

New MOSFET-based expander for high frequency ultrasound systems

Diode-based expanders (expanders with single-crossed diodes) are widely used for pulse-echo measurements and in imaging systems. The function of the diode-based expander is to obstruct the noise from the transmitted output signals of the pulser and block the returned signals from the ultrasonic transducers to the pulser. It may also degrade the performance of the transducer and electronic components on the imaging side of the system because of non-linearity and attenuation at higher voltages. A new type of expander using power MOSFETs for high frequency ultrasound systems is proposed and studied. Compared to diode-based expander, this device is shown to yield lower insertion loss and total harmonic distortion, and faster reverse recovery time.

Novel power MOSFET-based expander for high frequency ultrasound systems.

The function of an expander is to obstruct the noise signal transmitted by the pulser so that it does not pass into the transducer or receive electronics, where it can produce undesirable ring-down in an ultrasound imaging application. The most common type is a diode-based expander, which is essentially a simple diode-pair, is widely used in pulse-echo measurements and imaging applications because of its simple architecture. However, diode-based expanders may degrade the performance of ultrasonic transducers and electronic components on the receiving and transmitting sides of the ultrasound systems, respectively. Since they are non-linear devices, they cause excessive signal attenuation and noise at higher frequencies and voltages. In this paper, a new type of expander that utilizes power MOSFET components, which we call a power MOSFET-based expander, is introduced and evaluated for use in high frequency ultrasound imaging systems. The performance of a power MOSFET-based expander was evaluated relative to a diode-based expander by comparing the noise figure (NF), insertion loss (IL), total harmonic distortion (THD), response time (RT), electrical impedance (EI) and dynamic power consumption (DPC). The results showed that the power MOSFET-based expander provided better NF (0.76 dB), IL (-0.3 dB) and THD (-62.9 dB), and faster RT (82 ns) than did the diode-based expander (NF (2.6 dB), IL (-1.4 dB), THD (-56.0 dB) and RT (119 ns)) at 70 MHz. The -6 dB bandwidth and the peak-to-peak voltage of the echo signal received by the transducer using the power MOSFET-based expander improved by 17.4% and 240% compared to the diode-based expander, respectively. The new power MOSFET-based expander was shown to yield lower NF, IL and THD, faster RT and lower ring down than the diode-based expander at the expense of higher dynamic power consumption.

Bipolar-power-transistor-based limiter for high frequency ultrasound imaging systems

High performance limiters are described in this paper for applications in high frequency ultrasound imaging systems. Limiters protect the ultrasound receiver from the high voltage (HV) spikes produced by the transmitter. We present a new bipolar power transistor (BPT) configuration and compare its design and performance to a diode limiter used in traditional ultrasound research and one commercially available limiter. Limiter performance depends greatly on the insertion loss (IL), total harmonic distortion (THD) and response time (RT), each of which will be evaluated in all the limiters. The results indicated that, compared with commercial limiter, BPT-based limiter had less IL (-7.7 dB), THD (-74.6 dB) and lower RT (43 ns) at 100 MHz. To evaluate the capability of these limiters, they were connected to a 100 MHz single element transducer and a two-way pulse-echo test was performed. It was found that the -6 dB bandwidth and sensitivity of the transducer using BPT-based limiter were better than those of the commercial limiter by 22% and 140%, respectively. Compared to the commercial limiter, BPT-based limiter is shown to be capable of minimizing signal attenuation, RT and THD at high frequencies and is thus suited for high frequency ultrasound applications.

New modified Butterworth Van-Dyke model for high frequency ultrasonic imaging

For ultrasonic transducers, a number of equivalent circuit models including KLM, Mason and Butterworth Van Dyke (BVD) have been developed. To allow them to be incorporated into design tools for complex integrated circuits, KLM or Mason models are not practical because the discrete components of the equivalent circuits have negative values. Therefore, BVD model appears to be more appropriate but it needs to be improved because the resolution of a high frequency ultrasound system may be severely affected by impedance mismatching between the transducer and the system, as well as attenuation due to parasitic impedances of the systems. A new modified BVD model has been developed and the results demonstrate its usefulness in modeling high frequency ultrasonic transducer and its imaging systems.

Novel limiter using biploar power transistors for high frequency ultrasonic transducer applications

Novel limiter circuit using bipolar power transistors for high frequency ultrasonic transducers is presented. The performances of the limiter such as magnitude response, noise figure and total harmonic distortion were measured. Furthermore, pulse-echo responses were obtained to compare the performances between novel limiter and commercial diode limiter with a 100MHz single element ultrasonic transducer. The wider -6dB bandwidth and higher amplitude of echo signals would be achieved with novel limiter.

Harmonic distortion reduction technique of the power amplifier for very high frequency ultrasonic transducer applications

Power amplifiers are used to trigger the transducers in order to generate the acoustic waves into the desired target. The resolution of the ultrasound imaging systems is related with the sensitivity of the transducers so the high voltage output signals of the power amplifiers are required to be used to drive the transducers. Therefore, a linear power amplifier is more attractive choice because of its higher dynamic range of the output signals. The novel pre-distortion circuit using the passive resistor, capacitor and inductor components with a power MOSFET device was designed to enhance the dynamic range of the power amplifiers. The dynamic range of the power amplifiers is defined as the characteristic to amplify an input signal with less distortion. This characteristic can be improved by reducing the harmonic distortion components of the high voltage signals produced by the power amplifiers.

Low cross-talk kerfless annular array ultrasound transducers using 1–3 piezocomposites with pseudo-random pillars

This paper describes the design and fabrication of a high-frequency kerfless annular array transducer utilizing a novel 1-3 piezocomposite which was designed to reduce inter-element cross-talk. 1-3 piezocomposites have significant advantages over bulk piezoelectric materials and other types of piezocomposites; however, their benefits come at the expense of introducing more undesired interpillar resonances. At high frequencies, this is especially detrimental to kerfless annular array transducers. We have previously shown that this unwanted coupling effect (high inter-element crosstalk), can be further reduced by employing a pseudo-random pillar geometry. Utilizing the 1-3 composites with the pseudo-random pillars, a 40 MHz annular array transducer was fabricated. Each annular array was designed to have six equal-area elements and a center frequency of 40 MHz. The average center frequency estimated from the measured pulse-echo responses of array elements was 38.7 MHz and the -6 dB bandwidth was 51 %. The average insertion loss recorded was 23.1 dB, and the maximum combined crosstalk between the adjacent elements was less than -31 dB. These results demonstrate that the 1-3 piezocomposite with the pseudo-random pillars may be a better choice for fabricating enhanced high frequency linear array ultrasound transducers; especially when mechanical dicing is used.