Timothy A. Ritter

A 30-MHz piezo-composite ultrasound array for medical imaging applications

Ultrasound imaging at frequencies above 20 MHz is capable of achieving improved resolution in clinical applications requiring limited penetration depth. High frequency arrays that allow real-time imaging are desired for these applications but are not yet currently available. In this work, a method for fabricating fine-scale 2-2 composites suitable for 30-MHz linear array transducers was successfully demonstrated. High thickness coupling, low mechanical loss, and moderate electrical loss were achieved. This piezo-composite was incorporated into a 30-MHz array that included acoustic matching, an elevation focusing lens, electrical matching, and an air-filled kerf between elements. Bandwidths near 60%, 15-dB insertion loss, and crosstalk less than -30 dB were measured. Images of both a phantom and an ex vivo human eye were acquired using a synthetic aperture reconstruction method, resulting in measured lateral and axial resolutions of approximately 100 /spl mu/m.

Design of efficient, broadband single-element (20-80 MHz) ultrasonic transducers for medical imaging applications

This paper discusses the design, fabrication, and testing of sensitive broadband lithium niobate (LiNbO/sub 3/) single-element ultrasonic transducers in the 20-80 MHz frequency range. Transducers of varying dimensions were built for an f# range of 2.0-3.1. The desired focal depths were achieved by either casting an acoustic lens on the transducer face or press-focusing the piezoelectric into a spherical curvature. For designs that required electrical impedance matching, a low impedance transmission line coaxial cable was used. All transducers were tested in a pulse-echo arrangement, whereby the center frequency, bandwidth, insertion loss, and focal depth were measured. Several transducers were fabricated with center frequencies in the 20-80 MHz range with the measured -6 dB bandwidths and two-way insertion loss values ranging from 57 to 74% and 9.6 to 21.3 dB, respectively. Both transducer focusing techniques proved successful in producing highly sensitive, high-frequency, single-element, ultrasonic-imaging transducers. In vivo and in vitro ultrasonic backscatter microscope (UBM) images of human eyes were obtained with the 50 MHz transducers. The high sensitivity of these devices could possibly allow for an increase in depth of penetration, higher image signal-to-noise ratio (SNR), and improved image contrast at high frequencies when compared to previously reported results.

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 /spl mu/m axially and 100 /spl mu/m laterally.

Design, fabrication, and evaluation of high frequency, single-element transducers incorporating different materials

The performance of high frequency, single-element transducers depends greatly on the mechanical and electrical properties of the piezoelectric materials used. This study compares the design and performance of transducers incorporating different materials. The materials investigated include 1-3 lead zirconate titanate (PZT) fiber composite, lead titanate (PbTiO/sub 3/) ceramic, poly(vinylidene fluoride) (PVDF) film, and lithium niobate (LiNbO/sub 3/) single crystal. All transducers were constructed with a 3-mm aperture size and an f-number between 2 and 3. Backing and matching materials were selected based on design goals and fabrication limitations. A simplified coaxial cable tuning method was employed to match the transducer impedance to 50 /spl Omega/ for the PZT fiber composite and PbTiO/sub 3/ ceramic transducers. Transducers were tested for two-way loss and -6 dB bandwidth using the pulse/echo response from a flat quartz target. Two-way loss varied from 21 to 46 dB, and bandwidths measured were in the range from 47 to 118%. In vitro ultrasonic backscatter microscope (UBM) images of an excised human eye were obtained for each device and used to compare imaging performance. Both press-focusing and application of a lens proved to be useful beam focusing methods for high frequency. Under equal gain schemes, the LiNbO/sub 3/ and PbTiO/sub 3/ transducers provided better image contrast than the other materials.

Passive materials for high-frequency ultrasound transducers

In this paper, the elastic properties of passive materials (matching, backing and lens materials) for ultrasound transducers are explored at room temperature in the frequency range of 25 - 65 MHz using the ultrasonic spectroscopy method. Alumina/EPO-TEK 301 and tungsten/EPO- TEK 301 composites were fabricated and measured. Experimental results display a monotonic rise in acoustic impedance of the composites with the addition of the particle filler. However, there was an attenuation peak occurring at about 8% volume fraction of particle filler. The acoustic impedance of the compositions was modeled. And additional passive materials were fabricated and measured. The measured results showed that materials having high attenuation also had large velocity dispersion, and low attenuation materials displayed low velocity dispersion.

Novel method for producing high frequency 2-2 composites from PZT ceramic

The fabrication of 2-2 PZT/epoxy composites by laminating ceramic tape printed with carbon black was investigated as a way to make very high frequency ultrasound transducers. When the laminates were fired, the tape layers densified to form the PZT beams and the carbon volatilized leaving behind kerf space. The kerf was then filled with epoxy, and resulting composites had properties equivalent to those routinely made by conventional dice and fill technology. Since tape casting and screen printing methods can provide feature sizes less than 5 /spl mu/m, the techniques investigated in this work could potentially be used to fabricate linear and perhaps even phased arrays in the 30 to >50 MHz range.

Design of focused single element (50-100 MHz) transducers using lithium niobate

This paper discusses two fabrication procedures used to build LiNbO/sub 3/ single element ultrasonic transducers with center frequencies in the 50-100 MHz range. Transducers of varying dimensions were built according to an f-number range of 2.5-3.0. A quarter wavelength silver epoxy matching layer (Z/sub a/=7.3 MRayls), and a silver epoxy backing (Z/sub a/=5.9 MRayls), were used in all designs. The desired focal depths were achieved by either casting an acoustic lens on the transducer face or press-focusing the piezoelectric into a spherical curvature. The lens material EPO-TEK 301 (Z/sub a/=3.1 MRayls) was modeled as a second matching layer. Parylene (Z/sub a/=2.6 MRayls) was used as the second matching layer in the press-focused transducer design. For designs that required electrical impedance matching, a low impedance transmission line coaxial cable was used. All transducers were tested in a pulse-echo insertion loss arrangement, whereby the center frequency, bandwidth, insertion loss, and focal depth were measured. Numerous transducers were fabricated with center frequencies in the 50 to 100 MHz range. The measured -6 dB bandwidths and two-way insertion loss values ranged from 50% to 70% and 12.5 dB to 23.0 dB, respectively. Both fabrication procedures proved successful in producing very sensitive, high frequency single element ultrasonic imaging transducers. The press-focused devices displayed significantly lower insertion loss levels than lensed devices. The Parylene matching layer provided a better acoustic match to the load medium water (Z/sub a/=1.5 MRayls) and a reduced attenuation compared to the lens, which may account for the discrepancy insertion in loss levels. Finally, transmission line tuning demonstrated a 3 dB reduction in insertion loss and a 6% increase in device bandwidth.

Innovations in piezoelectric materials for ultrasound transducers

Piezoelectric materials lie at the heart of ultrasonic transducers. For transducers used in medical imaging (3¿7 MHz), PZT-5H ceramics offer high electromechanical coupling (k33 ¿ 75%), resulting in good bandwidth and sensitivity. As transducer arrays become smaller with increasing frequency, the development of high permittivity ( ¿RT > 7,000 vs. 3,400 for PZT-5H), piezoelectrics based on polycrystalline PMN-PT, provide improved electrical impedance matching. Advanced medical diagnostic techniques, including contrast and harmonic imaging, have taken advantage of the recent development in single crystal Relaxor-PTs that offer coupling k33¿s > 90% and subsequently, significant increases in bandwidth. For small animal, ophthalmology and cellular imaging, higher resolution is demanded, thus requiring transducers operational in the range of 20¿100 MHz. Advancements in ceramic processing include pore-free and fine-grain (¿1 micron) piezoelectric ceramics of PT and PZT, being an ¿enabling¿ technology, allowing the fabrication of high frequency single element and annular arrays. Innovations in the fabrication of high frequency arrays (¿ 30 MHz) include tape casting and sol-gel molding techniques. Of particular significance, DRIE (deep reaction ion etching), has demonstrated the ability to mill out ultrafine features, allowing 1¿3 crystal-polymer composites operational at frequencies ¿ 60 MHz, far beyond that achieved by current state-of-the-art dicing.

Electromechanical properties of thin strip piezoelectric vibrators at high frequency

A method was developed and used to determine the electromechanical properties of high frequency (>20 MHz) piezoelectric strip vibrators. A nonlinear regression technique was employed to fit the impedance magnitude and phase as predicted by Mason’s model to measured values. Results from experimental measurements on 30 MHz array elements supported by an attenuative backing indicated degraded performance when compared to values predicted from the electromechanical properties measured at low frequency. This degradation may be attributed to damage incurred during fabrication and grain size effects, with a fine grain sized material providing superior relative performance. This technique may be used in the evaluation and comparison of different fabrication processes and materials for high frequency medical imaging arrays.

30 MHz medical imaging arrays incorporating 2-2 composites

Methods for fabricating and modeling high frequency 2-2 composites and arrays are presented. The composites are suitable for arrays and small aperture single element devices. Coupling coefficients above 0.65 and lateral mode frequencies near 60 MHz have been achieved. Two prototype 4 element 30 MHz linear arrays were designed and built using this composite. Backing and matching layers were fabricated and characterized while coaxial cable was used to electrically tune each element and broaden bandwidth. The measured properties of passive and active components were used to analyze the design in a time-domain finite element analysis program. Agreement between experiment and theory was excellent.