High-frequency annular array fabrication using a flex circuit matching layer.

Fabricating arrays for high-frequency image applications such as ophthalmic imaging, intravascular imaging, and small animal imaging is challenging. For example, an array for intravascular imaging must be small enough to fit within the lumen of a catheter and inexpensive enough to be discarded after a single use. This article presents a new method for fabricating high-frequency annular arrays that is simple and inexpensive. The annular array elements are defined by the electrode pattern on a back surface of a polyimide quarter-wavelength matching layer that is glued to the front face of a ceramic transducer substrate (PZT5H). Electrical losses associated with bonding the matching layer to the transducer substrate are reduced by fabricating a second set of electrodes on the transducer substrate and then bonding the substrates using an anisotropic conductive epoxy. The feasibility of this technique was established by fabricating a seven-element, 20-MHz, 5-mm diameter annular array. The prototype array produced a pulse with a -6-dB factional bandwidth of 50%, an insertion loss of 22 dB, and secondary lobes in the radiation pattern at f/2 that decreased to -65 dB with respect to the main lobe with a peak amplitude of -53 dB.

High-frequency ex vivo ultrasound imaging of the auditory system.

A 50MHz array-based imaging system was used to obtain high-resolution images of the ear and auditory system. This previously described custom built imaging system (Brown et al. 2004a, 2004b; Brown and Lockwood 2005) is capable of 50 microm axial resolution, and lateral resolution varying from 80 microm to 130 microm over a 5.12 mm scan depth. The imaging system is based on a 2mm diameter, seven-element equal-area annular array, and a digital beamformer that uses high-speed field programmable gate arrays (FPGAs). The images produced by this system have shown far superior depth of field compared with commercially available single-element systems. Ex vivo, three-dimensional (3-D) images were obtained of human cadaveric tissues including the ossicles (stapes, incus, malleus) and the tympanic membrane. In addition, two-dimensional (2-D) images were obtained of an intact cochlea by imaging through the round window membrane. The basilar membrane inside the cochlea could clearly be visualized. These images demonstrate that high-frequency ultrasound imaging of the middle and inner ear can provide valuable diagnostic information using minimally invasive techniques that could potentially be implemented in vivo.

Real-time volume imaging using a crossed electrode array.

This paper describes a unique crossed electrode array for real-time volume ultrasound imaging. By placing orthogonal linear array electrode patterns on the opposite sides of a hemispherically shaped composite transducer substrate, a 2-D array can be fabricated using a small fraction of the elements required for a traditional 2-D array. The performance of the array is investigated using a computer simulation of the radiation pattern. We show that by using a 288-element crossed electrode pattern it is possible to collect large field of view volume images (60 degrees x 60 degrees sector) at real-time frame rates (>20 volume images/s), with image contrast and resolution comparable to what can be obtained using a conventional 128-element linear phased 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-microm 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 +/-30 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 microm. 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 microm 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 microm.

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 micro. We have successfully fabricated 50-micro-thick linear array elements with 23-micro-wide elements separated by 15 kerfs. A 50-micro thick, 2.5-mm diameter, five-element annular array structure with 20-micro kerfs also has been fabricated. The micromolded PZT composite has a density of 5.7-5.8 micro 0.4 g/cm3 and a thickness coupling coefficient as high as 0.32.

Investigation of cross talk in Kerfless annular arrays for high-frequency imaging.

The effect of electromechanical cross talk in high-frequency (> 30 MHz) kerfless annular arrays is investigated. Finite-element model predictions of the radiation patterns from arrays are compared to predictions from an ideal model without cross talk and with experimental measurements. High cross talk in the array causes element broadening and an increase in the amplitude of secondary lobes in the radiation pattern. However, an increase in the pulse ring-down time was not found. This can be attributed to the absence of lateral modes in the kerfless substrate. The level of the pedestal secondary lobes in the two-way radiation pattern increases linearly with the element path difference. The element broadening increases the effective element path difference, which increases the pedestal level for a kerfless annular array above the level for an ideal array. The broadening limits how close to an array one can image compared to the ideal case by reducing the contrast available in the image at small f-numbers. When the element broadening is taken into account by widening the electrode dimensions, the ideal radiation pattern agrees well with the finite-element model and experimental radiation patterns.

A digital beamformer for high-frequency annular arrays.

Digital transmit and receive beamformers for a 45-MHz, 7-element annular array are described. The transmit beamformer produces 0- to 80-Vpp monocycle pulses with a timing error of less than +/-125 ps. Up to four adjustable transmit focal zones can be selected. The dynamic receive beamformer uses a variable frequency sampling technique in which the frequency of analog-to-digital conversion on each channel is adjusted as the signals are received. The variable frequency clock signals required to trigger analog-to-digital conversion are obtained using a pair of high-frequency field-programmable gate arrays and a precision quartz oscillator. The gate arrays are also used to sum the digitized signals. A maximum receive beamformer timing error of less than +/-900 ps was obtained on each channel. The performance of the combined transmit and receive beamformer was tested by imaging wire phantoms. Images of CD-1 mice were also generated. The system produced images with a dynamic range of 60 dB.

Design and fabrication of annular arrays for high-frequency ultrasound.

The design, fabrication, and performance of miniature high-frequency annular arrays are described. A 50-MHz, 2-mm-diameter, 7-element, equal-area annular array was fabricated and tested. The array elements were defined using photolithography and the electrical contacts were made using ultrasonic wire bonding. The resulting transducer produced pulses with a -6 dB bandwidth of 52% and an insertion loss of -16 dB. A radiation pattern was collected by scanning the transducer array above the tip of a glass fiber. A -6 dB two-way beam width of 75 microns was found at f/2. The radiation pattern decreased smoothly to less than -60 dB at a distance of 550 microns.

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.