Biomedical subjects

Qifa Zhou

Publications and source records attributed to Qifa Zhou.

6 recordsLinked to original sources

PubMed2006

Coded excitation using biphase-coded pulse with mismatched filters for high-frequency ultrasound imaging.

A scheme of using phase-coded excitation and mismatched filter compression for high-frequency ultrasound imaging is presented in this paper. Biphase-coded pulses were constructed to excite the transducer. Received signals were compressed with mismatched filters optimized by minimizing peak-sidelobe-level (PSL). Both simulation and experiments were carried out to demonstrate the advantage of this technique. The simulation results demonstrated a possible sidelobe reduction (<-90 dB) with a slightly decrease of the signal-to-noise ratio of less than 1 dB compared with the compression using matched filters alone. The experimental results showed about 14 dB SNR improvement as well as -40 dB sidelobe level when the Barker-13 code excitation with 3-cycle sinusoidal wave carrier was used.

Acoustics↗

PubMed2006

Development of a 35-MHz piezo-composite ultrasound array for medical imaging.

This paper discusses the development of a 64-element 35-MHz composite ultrasonic array. This array was designed primarily for ocular imaging applications, and features 2-2 composite elements mechanically diced out of a fine-grain high-density Navy Type VI ceramic. Array elements were spaced at a 50-micron pitch, interconnected via a custom flexible circuit and matched to the 50-ohm system electronics via a 75-ohm transmission line coaxial cable. Elevation focusing was achieved using a cylindrically shaped epoxy lens. One functional 64-element array was fabricated and tested. Bandwidths averaging 55%, 23-dB insertion loss, and crosstalk less than -24 dB were measured. An image of a tungsten wire target phantom was acquired using a synthetic aperture reconstruction algorithm. The results from this imaging test demonstrate resolution exceeding 50 microm axially and 100 microm laterally.

Ceramics↗

PubMed2005

Half-thickness inversion layer high-frequency ultrasonic transducers using LiNbO3 single crystal.

Half-thickness inversion layer high-frequency ultrasonic transducers were fabricated using lithium niobate (LiNbO3) single crystal plate. The transducers developed for this study used a 36 degrees rotated Y-cut LiNbO3 thin plate with an active element thickness of 115 microm. The designed center frequency was in the range of 30 to 60 MHz. Half-thickness inversion layer was formed after the sample was annealed at a high temperature, and it is shown that the inversion layer thickness can be controlled by the temperature. Silver powder/epoxy composite and parylene were used as acoustic matching layers. A lossy silver epoxy was used as the backing material. Using an analytical method, the electrical impedance for different inversion layer ratios was determined. The measured resonant frequency was consistent with the modeled data. Even-order higher frequency broadband ultrasonic transducers with a center frequency at 60 MHz was obtained using half-thickness inversion layer of LiNbO3 single crystal.

Journal Article↗

PubMed2005

Fabrication and characterization of micromachined high-frequency tonpilz transducers derived by PZT thick films.

Miniaturized tonpilz transducers are potentially useful for ultrasonic imaging in the 10 to 100 MHz frequency range due to their higher efficiency and output capabilities. In this work, 4 to 10-microm thick piezoelectric thin films were used as the active element in the construction of miniaturized tonpilz structures. The tonpilz stack consisted of silver/lead zirconate titanate (PZT)/lanthanum nickelate (LaNiO3)/silicon on insulator (SOI) substrates. First, conductive LaNiO3 thin films, approximately 300 nm in thickness, were grown on SOI substrates by a metalorganic decomposition (MOD) method. The room temperature resistivity of the LaNiO3 was 6.5 x 10(-6) omega x m. Randomly oriented PZT (52/48) films up to 7-microm thick were then deposited using a sol-gel process on the LaNiO3-coated SOI substrates. The PZT films with LaNiO3 bottom electrodes showed good dielectric and ferroelectric properties. The relative dielectric permittivity (at 1 kHz) was about 1030. The remanent polarization of PZT films was larger than 26 microC/cm2. The effective transverse piezoelectric e31,f coefficient of PZT thick films was about -6.5 C/m2 when poled at -75 kV/cm for 15 minutes at room temperature. Enhanced piezoelectric properties were obtained on poling the PZT films at higher temperatures. A silver layer about 40-microm thick was prepared by silver powder dispersed in epoxy and deposited onto the PZT film to form the tail mass of the tonpilz structure. The top layers of this wafer were subsequently diced with a saw, and the structure was bonded to a second wafer. The original silicon carrier wafer was polished and etched using a Xenon difluoride (XeF2) etching system. The resulting structures showed good piezoelectric activity. This process flow should enable integration of the piezoelectric elements with drive/receive electronics.

Journal Article↗

PubMed2005

An analytical model of multilayer ultrasonic transducers with an inversion layer.

Inversion layer ultrasonic transducers have been investigated recently as an interesting approach in wideband transducer design. In this paper we present an analytical model of multilayer ultrasonic transducers with an inversion layer. Our analysis of the wave propagation problem of an inversion layer transducer includes a functional decomposition of the electrical input impedance. It becomes clear from this decomposition that an inversion layer transducer can be modeled as three elements in series connection, i.e., a clamped capacitance, a classical motional impedance, and a coupled motional impedance. The first two elements make up the classical model of a single element transducer. The coupled motional impedance describes the coupled interaction between the regular and the inverted piezoelectric sublayers, and thus reflects the effect of an inversion layer. We present examples which show that inversion layer transducers are advantageous in achieving such useful features as dual-frequency operation mode as used in harmonic imaging or broadband performance desired in most ultrasonic applications.

Journal Article↗

PubMed2005

Design and fabrication of broadband graded ultrasonic transducers with rectangular kerfs.

Broadband ultrasound imaging is capable of achieving superior resolution in clinical applications. An effective and easy way of manufacturing broadband transducers is desired for these applications. In this work, a graded material in which the piezoelectric plate is mechanically graded with rectangular grooves is introduced. Finite element analysis (FEA) demonstrated that the graded piezoelectric material could achieve a broadband, time-domain response resulting from multiple resonant modes. Experimental tests were carried out to validate these theoretical results. Based upon the FEA designs, several single-element transducers were fabricated using either a nondiced ceramic or a diced, graded ceramics. A superior bandwidth of 92% was achieved by the graded transducer when compared to a bandwidth of 56% produced by the nondiced ceramic transducer at the expense of a reduced sensitivity.

Ceramics↗