Jonathan M. Cannata

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.

PMN-PT single crystal, high-frequency ultrasonic needle transducers for pulsed-wave Doppler application

High-frequency needle ultrasound transducers with an aperture size of 0.4 mm were fabricated using lead magnesium niobate-lead titanate (PMN-33%PT) as the active piezoelectric material. The active element was bonded to a conductive silver particle matching layer and a conductive epoxy backing through direct contact curing. An outer matching layer of parylene was formed by vapor deposition. The active element was housed within a polyimide tube and a 20-gauge needle housing. The magnitude and phase of the electrical impedance of the transducer were 47 Omega and -38deg, respectively. The measured center frequency and -6 dB fractional bandwidth of the PMN-PT needle transducer were 44 MHz and 45%, respectively. The two-way insertion loss was approximately 15 dB. In vivo high-frequency, pulsed-wave Doppler patterns of blood flow in the posterior portion and in vitro ultrasonic backscatter microscope (UBM) images of the rabbit eye were obtained with the 44-MHz needle transducer

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.

Development of a real-time, high-frequency ultrasound digital beamformer for high-frequency linear array transducers

A real-time digital beamformer for high-frequency ( >20 MHz) linear ultrasonic arrays has been developed. The system can handle up to 64-element linear array transducers and excite 16 channels and receive simultaneously at 100 MHz sampling frequency with 8-bit precision. Radio frequency (RF) signals are digitized, delayed, and summed through a real-time digital beamformer, which is implanted using a field programmable gate array (FPGA). Using fractional delay filters, fine delays as small as 2 ns can be implemented. A frame rate of 30 frames per second is achieved. Wire phantom (20 /spl mu/m tungsten) images were obtained and -6 dB axial and lateral widths were measured. The results showed that, using a 30 MHz, 48-element array with a pitch of 100 /spl mu/m produced a -6 dB width of 68 /spl mu/m in the axial and 370 /spl mu/m in the lateral direction at 6.4 mm range. Images from an excised rabbit eye sample also were acquired, and fine anatomical structures, such as the cornea and lens, were resolved.

Self-focused high frequency ultrasonic transducers based on ZnO piezoelectric films

A micromachined self-focusing high frequency ultrasonic transducer was fabricated with a 13μm thick ZnO film deposited on a silicon substrate by sputtering. X-ray diffraction shows that the film has a high (002) orientation. The element aperture size of the transducer was 2.5mm, and the fundamental resonant frequency was designed to be over 200MHz with approximately 28% bandwidth through only one matching layer. Experimental results show that this type of focused high frequency ultrasound device may have potential for cellular microstructure imaging and skin cancer detection.

Fabrication of focused poly(vinylidene fluoride-trifluoroethylene) P(VDF-TrFE) copolymer 40-50 MHz ultrasound transducers on curved surfaces

Copolymer films such as poly(vinylidene fluoride-trifluoroethylene) P(VDF-TrFE) have lower acoustic impedance compared to their ceramic counterparts, allowing for a better acoustic match to tissues in the human body. Because of this, copolymerultrasonic transducers are capable of yielding the desirable characteristics of broad bandwidth and short pulse duration that allow better image resolution to be achieved. In the past, such transducers in the frequency range from 40 to 80 MHz have frequently been fabricated by spin coating the copolymer film onto a flat substrate and then applying the film to a curved backing using an adhesive layer. The adhesive layer may cause spurious signals at these frequencies, in addition to the film damage that may occur as a result of such processing. In order to avoid these problems, a copolymer film can be directly spin coated onto a curved substrate. The resulting devices had an operating frequency of over 40 MHz and approximately a 75% bandwidth. The potential of several approaches that could be further explored to increase the level of performance of such devices is also discussed.

A High-Frame Rate High-Frequency Ultrasonic System for Cardiac Imaging in Mice

We report the development of a high-frequency (30-50 MHz), real-time ultrasonic imaging system for cardiac imaging in mice. This system is capable of producing images at 130 frames per second (fps) with a spatial resolution of less than 50 mum. A novel mechanical sector probe was developed that utilizes a magnetic drive mechanism and custom-built servo controller for high speed and accuracy. Additionally, a very light-weight (< 0.28 g), single-element transducer was constructed and used to reduce the mass load on the motor. The imaging electronics were triggered according to the angular position of the transducer in order to compensate for the varying speed of the sector motor. This strategy ensured the production of equally spaced scan lines with minimal jitter. Wire phantom testing showed that the system axial and lateral resolutions wore 48 mum and 72 mum, respectively. In vivo experiments showed that high-frequency ultrasonic imaging at 130 fps is capable of showing a detailed depiction of a beating mouse heart.

Development of a high-frequency (> 50 MHz) copolymer annular-array, ultrasound transducer

The development of a high frequency (> 50 MHz) annular array ultrasonic transducer is presented. The array was constructed by bonding a 9 mum P(VDF-TrFE) film to a two-sided polyimide flexible circuit with annuli electrodes on the top layer. Each annulus was separated by a 30 mum kerf and had several electroplated microvias that connected to electrode traces on the bottom side of the flex circuit. In order to improve device sensitivity, each element was electrically matched to an impedance magnitude of 50 Omega and 0deg phase at resonance using a serial inductor and high impedance coaxial cable. The arrays performance was evaluated by measuring the electrical impedance, pulse echo response, and cross talk between elements. The average round trip insertion loss was -33.5 dB after compensating for diffractive and attenuative losses. The measured average center frequency and bandwidth for an element was 55 MHz and 47%, respectively. The measured cross talk between adjacent elements remained below -29 dB at the center frequency in water. A vertical wire phantom was imaged using a single focus transmit beamformer and dynamic focusing receive beamformer. This image showed a significant improvement in lateral resolution over a range of 9 mm after the dynamic focusing receive algorithm was applied. These results correlated well with predictions from a field II simulation. After beamforming, the minimum lateral resolution achieved by the array (-6 dB) was 108 mum at the focus

Design and fabrication of PIN-PMN-PT single-crystal high-frequency ultrasound transducers

High-frequency PIN-PMN-PT single crystal ultrasound transducers at center frequencies of 35 MHz and 60 MHz were successfully fabricated using lead indium niobate-lead magnesium niobate-lead titanate (0.23PIN- 0.5PMN-0.27PT) single crystal. The new PIN-PMN-PT single crystal has higher coercivity (6.0 kV/cm) and higher Curie temperature (160°C) than PMN-PT crystal. Experimental results showed that the PIN-PMN-PT transducers have similar performance but better thermal stability compared with the PMN-PT transducers.