James R. Talman

Real-time 3-D ultrasound imaging using sparse synthetic aperture beamforming

A method for real-time three-dimensional (3-D) ultrasound imaging using a mechanically scanned linear phased array is proposed. The high frame rate necessary for real-time volumetric imaging is achieved using a sparse synthetic aperture beamforming technique utilizing only a few transmit pulses for each image. Grating lobes in the two-way radiation pattern are avoided by adjusting the transmit element spacing and the receive aperture functions to account for the missing transmit elements. The signal loss associated with fewer transmit pulses is minimized by increasing the power delivered to each transmit element and by using multiple transmit elements for each transmit pulse. By mechanically rocking the array, in a way similar to what is done with an annular array, a 3-D set of images can be collected in the time normally required for a single image.

Orthogonal-coil RF probe for implantable passive sensors

A versatile orthogonal-coil radio frequency (RF) probe suitable for detecting the resonant frequency of miniature implantable passive sensors has been designed and tested. The probe sensitivity has been tested using printed-circuit spiral inductors of various sizes (3-15 mm) in series with discrete surface-mount capacitors designed to resonate over a range of frequencies (50-200 MHz). Close agreement between theoretical calculations and experimental results has been obtained. An equation is derived for transmit/receive (T/R) isolation that agrees with experimental measurements over the frequency range 1-500 MHz. The probe includes an additional coil to compensate for the effect of eddy currents in the human body on the probe. T/R isolation of at least 90 dB over the frequency range 1-100 MHz can be achieved when the probe is placed in close proximity to the human body.

Integrated circuit for high-frequency ultrasound annular array

An integrated circuit capable of focusing a high-frequency ultrasound annular array is presented. It uses a novel unit-delay architecture to accomplish focusing of the array with a single control voltage. System measurements for a 5-element array indicate excellent pulse fidelity with a dynamic amplitude range of 60 dB at 50 MHz. This is the highest frequency single-chip ultrasound beamformer that has been demonstrated to date.

Unit-delay focusing architecture and integrated-circuit implementation for high-frequency ultrasound

High-frequency ultrasound (above 10 MHz) has been used successfully in many medical applications, including eye, skin, gastrointestinal, intravascular, and Doppler flow imaging. Most of these applications use single-element transducers, thereby imposing a tradeoff between resolution and depth of field. Fabrication difficulties and the need for high-speed electronic beamformers have prevented widespread use of arrays at high frequencies. In this paper, a unit-delay focusing architecture suitable for use with high-frequency ultrasound annular arrays is described. It uses a collection of identical, active delay cells that may be simultaneously varied to accomplish focusing. Results are presented for an analog integrated circuit intended for use with a five-element, 50-MHz planar annular array. Focusing is possible over an axial range for which the ratio of maximum to minimum f-number is 2.1. Unit-delay architectures also are described for curved annular arrays and linear arrays.

Design and Analysis of MEMS Based PVDF Ultrasonic Transducers for Vascular Imaging

Polyvinilidene fluoride (PVDF) single-element transducers for high-frequency (>30 MHz) ultrasound imaging applications have been developed using MEMS (Micro-electro-Mechanical Systems) compatible techniques. Performance of these transducers has been investigated by analyzing the sources and effects of on-chip parasitic capacitances on the insertion-loss of the transducers. Modeling and experimental studies showed that on-chip parasitic capacitances degraded the performance of the transducers and an improved method of fabrication was suggested and new devices were built. New devices developed with minimal parasitic effects were shown to improve the performance significantly. A 1-mm aperture PVDF device developed with minimal parasitic effects has resulted in a reduction of insertion loss of 21 dB compared with devices fabricated using a previous method.

Evaluation of the radiation pattern of a split aperture linear phased array for high frequency imaging

Developing transducer arrays for high frequency medical imaging is complicated because of the extremely small size and spacing of the array elements. For example, a 50 MHz linear phased array requires a center-to-center spacing of only 15 /spl mu/m (one-half wavelength in water) to avoid the formation of grating lobes in the radiation pattern of the array. Fabricating an array with these dimensions is difficult using conventional technology. A split aperture design that permits much larger element spacing (3 to 4 times) while avoiding the formation of grating lobes is described. The 3-D radiation pattern of a 1.9/spl times/1.4 mm, 50-MHz split aperture linear phased array with 33 transmit elements and 33 receive elements has been evaluated theoretically. The azimuthal beam width is 90 /spl mu/m at a distance of 4.0 mm. Grating lobes are suppressed by at least 60 dB at distances >4.0 mm (/spl sim/f/2). The elevation beam width is 220 /spl mu/m at 4.0 mm, and a useful depth of field over the axial range from 4 to 10 mm is obtained.