Geoffrey R. Lockwood

Optimizing the radiation pattern of sparse periodic linear arrays

We have developed a method for designing sparse periodic arrays. Grating lobes in the two-way radiation pattern are avoided by using different element spacings on transmission and reception. The transmit and receive aperture functions are selected such that the convolution of the aperture functions produces a desired effective aperture. A desired effective aperture is simply an aperture with an appropriate width, element spacing, and shape such that the Fourier transform of this function gives the desired two-way radiation pattern. If a synthetic aperture approach is used, an exact solution to the problem is possible. However, for conventional imaging, often only an approximation of the desired effective aperture can be found. Different strategies for obtaining these approximate solutions are described. The radiation pattern of a sparse array designed using the effective aperture concept is compared experimentally with the radiation patterns of a dense array, and sparse arrays with periodic and random element spacing. We show that the number of elements in a 128-element linear array can be reduced by at least four times with little degradation of the beam forming properties of the array.

Fabrication and Performance of a 40-MHz Linear Array Based on a 1-3 Composite with Geometric Elevation Focusing

The fabrication and performance of a 256-element high-frequency (40-MHz) linear array is described. The array was fabricated using a high-frequency 1-3 PZT-polymer composite material developed in our laboratory. The spacing of the pillars in the composite was chosen to match the 40-mum center-to-center element spacing of the array electrodes. The element electrodes were created using photolithography, and connections to the electrodes were made using ultrasonic wire bonding. The array was focused in the elevation direction by geometrically shaping the composite material using a cylindrical die with a 6-mm radius of curvature. The resulting transducer produced pulses with a -6 dB two-way bandwidth of 50% and a peak-to-peak pressure of 503 kPa when excited with a plusmn30 V monocycle pulse. The measured one-way ( -6 dB) directivity for a single array element was 24 degrees and the -3 dB one-way elevation beamwidth was measured to be 130 mum. The radiation pattern for a focused 64-element subaperture was measured by mechanically translating the aperture above a needle hydrophone. A -3 dB one-way beamwidth of 97 mum was found at a depth of 6 mm. The one-way radiation pattern decreased smoothly to less than -30 dB at a lateral distance of 640 mum.

Optimizing sparse two-dimensional transducer arrays using an effective aperture approach

We have developed a method for reducing the number of elements in a two-dimensional array while minimizing the loss of image resolution and contrast. The method relies on selecting a different arrangement of elements when the array is transmitting and when the array is receiving energy. The transmit and receive aperture functions are chosen to minimize the difference between the effective aperture of the sparse array and the effective aperture of a desired dense array. The effective aperture is defined by the convolution of the transmit and receive aperture functions. Using this method, we show that the number of elements in a 64×64 two-dimensional array can be reduced by more than six times, and the elements in a 128×128 array can be reduced by more than 12 times, with little effect on the beam forming properties of either array

Design of sparse array imaging systems

A method for designing sparse periodic arrays is described. Grating lobes in the two-way radiation pattern are avoided by using different element spacings for transmission and reception. The transmit and receive aperture functions are selected such that the convolution of the aperture functions produces a desired effective aperture. A desired effective aperture is simply an aperture with an appropriate width, element spacing and shape such that the Fourier transform of this function gives the desired two-way radiation pattern. Here, the authors apply the effective aperture method to the design of a sparse array for a very high frame rate (1,000 images/s) imaging system. The high frame rate is achieved by using synthetic aperture beam-forming utilizing only a few transmit pulses for each image. To compensate for the resulting loss in signal, the power delivered to each transmit element is increased and multiple transmit elements are used for each transmit burst. By mechanically rocking the array, in a way similar to what is done with an annular array, a three-dimensional set of images can be collected in the time normally required for a single image.

Fabrication of PZT sol gel composite ultrasonic transducers using batch fabrication micromolding

A new micromolding technique for fabricating high-frequency (>20 MHz) ultrasound transducers has been developed. The technique combines sol gel processing with an epoxy-based, photo-resist Su-8 micromold to form miniature PZT structures. An advantage of this technique as compared to more traditional lithographic galvanforming and abforming (LIGA) processing is that the intermediate step of producing a nickel-plated mold is avoided. Instead, the PZT is formed directly using a photo-resist. The resulting structures can be fabricated with aspect ratios up to 3:1 and thicknesses up to 50 mu. We have successfully fabricated 50-mu thick linear array elements with 23-mu-wide elements separated by 15 kerfs. A 50-mu thick, 2.5-mm diameter, five-element annular array structure with 20-mu kerfs also has been fabricated. The micromolded PZT composite has a density of 5.7-5.8 plusmn 0.4 g/cm3 and a thickness coupling coefficient as high as 0.32

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.

Miniature polymer transducers for high-frequency medical imaging

High frequency polymer transducers have been used in a variety of medical imaging applications since they were first introduced by Sherar and Foster in the late 1980s. The transducers are intrinsically broadband and the flexibility of the polymer material makes fabrication relatively easy. Unfortunately, piezoelectric polymer materials have a low dielectric constant. Unless a large aperture is used, the electrical impedance of the transducer will be high, and the receiver sensitivity will be poor. This problem can be avoided by placing a high impedance pre-amplifier inside the transducer housing. Placing the pre-amplifier close to the transducer is important to avoid standing waves between the high output impedance of the transducer and the high input impedance of the pre-amplifier. We have recently developed a process for fabricating high frequency spherically shaped polymer transducers in which an integrated circuit die is mounted just beneath the surface of the transducer. In this paper we describe a theoretical and experimental analysis of the noise performance of these devices. The signal-to-noise ratio at the output of the pre-amplifier is estimated by combining a simple noise model for the amplifier with a KLM model of the transducer. This analysis provides a useful way of evaluating different transducer/pre-amplifier combinations. Excellent agreement between the model predictions and experimental results proves the validity of this approach.

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.

A sigma-delta-based sparse synthetic aperture beamformer for real-time 3-D ultrasound

Sigma-delta (/spl Sigma//spl Delta/) modulation allows delay resolution in ultrasound beamformers to be achieved by simple clock cycle delays applied to the undecimated bitstream, greatly reducing the complexity of the signal processing and the number of bits in the datapath. The simplifications offered by this technique have the potential for low power and portable operation in advanced systems such as 3-D and color Doppler imagers. In this paper, an architecture for a portable, real-time, 3-D sparse synthetic aperture ultrasound beamformer based on /spl Sigma//spl Delta/ modulation is presented, and its simulated performance is analyzed. Specifically, with a 65-element linear phased array and three transmit events, this architecture is shown to achieve a 1.1/spl deg/ beamwidth, a -54-dB secondary lobe level, and a theoretical frame rate of 1700 frames/s at /spl lambda//64 delay resolution using a second-order low pass /spl Sigma//spl Delta/ modulator. Finally, a technique for modifying the proposed multi-beam architecture to allow improved analog-to-digital (A/D) resolution by premodulating the input signal for bandpass /spl Sigma//spl Delta/ modulation is also presented.

Performance of a 50 MHz annular array based imaging system

The performance of a high frequency annular array based imaging system is described. Radiation patterns generated by the array plus beamformer were evaluated by imaging a point target immersed in a water bath. The lateral image resolution at a depth of 5.5 mm was 96 microns and the secondary lobes were suppressed 60 dB below the main lobe. Preliminary images of CD-1 mice were also generated using the array system. The images were displayed with a dynamic range of up to 60 dB.