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The minimum-variance beamformer is used for demonstrating how the low sidelobe levels and narrow beamwidth of adaptive methods can be used, not only to increase resolution, but also to enhance imaging in many ways. The studies have shown that by lowering the frequency of the transmitted beam and beamforming the received data with the minimum variance beamformer, increased depth of penetration is obtained without sacrificing lateral resolution.

A fast method is described for computing the acoustic field of ultrasound transducers with application to rectangular elements that are cylindrically focused. The efficiency is gained by using a separable approximation to the Green's function and the tradeoff between accuracy and spatial sampling rate is analyzed for determining appropriate parameters for a specific transducer.

A scanning head is designed and fabricated that has incorporated a 20-MHz, 128-element linear transducer and the elements of PVDF film with an element. An imaging test has shown that the system has achieved axial and lateral resolutions and the entire scanning head has dissipated at a pulse repetition rate of 750 Hz.

A high-power multifrequency ultrasonic actuator is proposed and characterized. The actuator is used in many applications as a multifrequency ultrasound power generator, but also as a mirror with variable bend radius to deflect laser beams in optical applications.

Results of investigation of a double layer dielectric system applied for pressure distribution determination is discussed in the paper. The system consists of elastic and rigid solid dielectric layers. The trybo-charge is generated on the contact between the rough surface of the elastic and dielectric layers when the system is subjected to an external stress. It was shown that the local effective surface charge density on the rigid dielectric layer depends on the local pressure value. The generated charge produces a proportional voltage across the solid dielectric layer after removing the elastic one (foam). This way, the pressure may be converted into a voltage signal, which is ,memorised' by a solid dielectric layer. Finally, the 'pressure map' converted into ,an electric potential map' can be visualised by potential distribution determination. The potential distributions obtained in the system for different cases of non-uniform pressure distributions and different elastic layers are presented in the paper. Index Terms--Electrostatic processes, electrets, transducers, double layer.

A new type of high-frequency piezo-composite is developed with triangular-pillar geometry instead of the conventional square. A 256-element 30-MHz linear array is prepared as a preliminary examination for using the triangular pillar as the substrate in a high-frequency linear array transducer and in vivo images are generated with both the single-element and linear-array transducers.

3-D images of both 5 pairs of nylon wires embedded in a clear gelatin phantom and an 8 mm diameter cylindrical anechoic cyst phantom acquired from a 256 * 256 2-D array transducer made from a 1-3 composite are described. The cross-section of the cyst is visible in the azimuth B-scan whereas the long axes are observed as a rectangle in the elevation B-scan and C-scans.

The design, fabrication, and evaluation of a high-frequency single-element transducer with annular geometry and the thickness of the piezoelectric material increasing from the center to the outside are presented. The single-element annular transducer (SEAT) could be used for imaging applications that require a wider transducer bandwidth, such as harmonic imaging.

We describe the design, construction, and performance of a waveguide orthomode transducer (OMT) for the 3-mm band (84-116 GHz). The OMT is based on a symmetric backward coupling structure and has a square waveguide input port (2.54 mm x 2.54 mm) and two single-mode waveguide outputs: a standard WR10 rectangular waveguide (2.54 mm x 1.27 mm), and an oval waveguide with full-radius corners. The reverse coupling structure is located in the common square waveguide arm and splits one polarization signal in two opposite rectangular waveguide sidearms using broadband -3-dB E-plane branch-line hybrid couplers. The device was optimized using a commercial 3-D electromagnetic simulator. The OMT consists of two mechanical blocks fabricated in split-block configuration using conventional CNC milling machine. From 84 to 116 GHz, the measured input reflection coefficient was less than -17 dB, the cross polarization was less than -30 dB, the isolation between the outputs was greater than 50 dB, and the insertion loss was less than 0.35 dB at room temperature for both polarization channels. The device is suitable for scaling to higher frequency. Index Terms--Branch-line coupler, hybrid coupler, orthomode transducer (OMT), polarimetry, polarization splitter, power combiner, radio astronomy, reverse coupling, waveguide transitions.

The concentric annular type dual element transducers are developed and fabricated for second harmonic imaging at 20 MHz/40 MHz in order to improve spatial resolution and depth of penetration for ophthalmic imaging applications. The harmonic imaging with 20 MHz and 40 MHz showed superior capability when compared to that of fundamental imaging at 20 MHz to diagnose the retinal disease at the posterior segment of the eye.

A commercial ultrasound imaging system is used for characterizing the capacitive micromachined ultrasonic transducer microlinear (CMUT ML) array integrated with the custom front-end electronics and also for evaluating the imaging results for the array assembly. The fabricated CMUT technology is used for designing even higher frequency arrays with other array geometries for catheter-based intracardiac and intravascular applications.

The origins of the end-element anomalies and corresponding design changes are analyzed. This ultrasonic piezo-composite array is modeled by using the finite element method.

The complex material coefficients of piezoelectric materials and their influence on the energy ratios in coupled systems are analyzed. The studies have shown that the instantaneous ratio of coupled to input energy or the time average of this ratio is the appropriate energy ratio in a dissipating system.

Discontinuous, island-type gold films (typically [less than or equal to] 10 nm nominal thickness) prepared by evaporation of the metal on transparent substrates show a localized surface plasmon resonance (LSPR) extinction in the visible-to-NIR range and can be used as optical transducers for monitoring local refractive index change. Such transducers, operated in the transmission configuration, provide an effective scheme for label-free biological sensing using basic spectrophotometric equipment. Optimization of the sensitivity of LPSR transducers requires consideration of the distance between the metal island surface and the bound analyte, strongly affecting the optical response due to the fast decay of the evanescent field of localized plasmons. In the present work Au island based LSPR transducers were used to monitor antibody--antigen interactions, demonstrating the effect of the biorecognition interface thickness. Evaporated Au island films derivatized with IgG or hCG antigens were used as biological recognition elements for selective sensing of antibody binding, distinguishing between specific and nonspecific interactions. The LSPR results are supported by XPS and ellipsometry data as well as by AFM and HRSEM imaging, the latter providing actual visualization of the two protein binding steps. Increase of the recognition interface thickness leads to a concomitant decrease in the extinction and wavelength sensitivity, generally conforming to a model of an exponentially decaying surface plasmon (SP) evanescent field.

A new generation of miniature ring-array transducers integrated into the catheter deployment kits of interventional devices is developed. The newly constructed ring transducers integrated with an 8.5 French catheter sheath of a vena cava filter deployment device allow real-time 3-D ultrasound scanning for improved guidance of minimally invasive procedures.

A lumped equivalent circuit for the radiation impedance of the circular piston in a rigid baffle is presented. The proposed circuit could be applied in circuit simulators to study the performance of front-end circuits.

The results of an unfocused capacitive micromachined ultrasonic transducer (CMUT) monitored by magnetic resonance (MR)-temperature maps are discussed. Findings reveal the successful heating of a high intensity focused ultrasound (HIFU) phantom by the unfocused CMUT by 14 degree Celsius in 2.5 minutes.

Flexible transducer arrays are desired to wrap around catheter tips for side-looking intravascular ultrasound imaging. We present a technique for constructing flexible capacitive micromachined ultrasonic transducer (CMUT) arrays by forming polymer-filled deep trenches in a silicon substrate. First, we etch deep trenches between the bottom electrodes of CMUT elements on a prime silicon wafer using deep reactive ion etching. Second, we fusion-bond a silicon-on-insulator (SOI) wafer to the prime silicon wafer. Once the silicon handle and buried oxide layers are removed from the back side of the SOI wafer, the remaining thin silicon device layer acts as a movable membrane and top electrode. Third, we fill the deep trenches with polydimethylsiloxane, and thin the wafer down from the back side. The 16 by 16 flexible 2-D arrays presented in this paper have a trench width that varies between 6 and 20 [micro]m; the trench depth is 150 [micro]m; the membrane thickness is 1.83 [micro]m; and the final substrate thickness is 150 [micro]m. We demonstrate the flexibility of the substrate by wrapping it around a needle tip with a radius of 450 [micro]m (less than catheter size of 3 French). Measurements in air validate the functionality of the arrays. The 250-[micro]m by 250-[micro]m transducer elements have a capacitance of 2.29 to 2.67 pF, and a resonant frequency of 5.0 to 4.3 MHz, for dc bias voltages ranging from 70 to 100 V. [2007-0078] Index Terms--Capacitive micromachined ultrasonic transducer (CMUT), fabrication, flexible arrays, intravascular ultrasound (IVUS), polydimethylsiloxane (PDMS), through-wafer interconnects, trench isolation.