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.     

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 microm 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 microm 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 0 degrees phase at resonance using a serial inductor and high impedance coaxial cable. The array's 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 microm at the focus.     

Performance and characterization of new micromachined high-frequency linear arrays

A new approach for fabricating high frequency (> 20 MHz) linear array transducers, based on laser micromachining, has been developed. A 30 MHz, 64-element, 74-microm pitch, linear array design is presented. The performance of the device is demonstrated by comparing electrical and acoustic measurements with analytical, equivalent circuit, and finite-element analysis (FEA) simulations. All FEA results for array performance have been generated using one global set of material parameters. Each fabricated array has been integrated onto a flex circuit for ease of handling, and the flex has been integrated onto a custom printed circuit board test card for ease of testing. For a fully assembled array, with an acoustic lens, the center frequency was 28.7 MHz with a one-way -3 dB and -6 dB bandwidth of 59% and 83%, respectively, and a -20 dB pulse width of -99 ns. The per-element peak acoustic power, for a +/- 30 V single cycle pulse, measured at the 10 mm focal length of the lens was 590 kPa with a -6 dB directivity span of about 30 degrees. The worst-case total cross talk of the combined array and flex assembly is for nearest neighboring elements and was measured to have an average level -40 dB across the -6 dB bandwidth of the device. Any significant deviation from simulation can be explained through limitations in apparatus calibration and in device packaging.     

Microfabrication of electrode patterns for high-frequency ultrasound transducer arrays

High-frequency ultrasound is needed for medical imaging with high spatial resolution. A key issue in the development of ultrasound imaging arrays to operate at high frequencies (?30 MHz) is the need for photolithographic patterning of array electrodes. To achieve this directly on 1-3 piezocomposite, the material requires not only planar, parallel, and smooth surfaces, but also an epoxy composite filler that is resistant to chemicals, heat, and vacuum. This paper reports, first, on the surface finishing of 1-3 piezocomposite materials by lapping and polishing. Excellent surface flatness has been obtained, with an average surface roughness of materials as low as 3 nm and step heights between ceramic/polymer of ～80 nm. Subsequently, high-frequency array elements were patterned directly on top of these surfaces using a photolithography process. A 30-MHz linear array electrode pattern with 50-μm element pitch has been patterned on the lapped and polished surface of a high-frequency 1-3 piezocomposite. Excellent electrode edge definition and electrical contact to the composite were obtained. The composite has been lapped to a final thickness of ～55 μm. Good adhesion of electrodes on the piezocomposite has been achieved and electrical impedance measurements have demonstrated their basic functionality. The array was then packaged, and acoustic pulse-echo measurements were performed. These results demonstrate that direct patterning of electrodes by photolithography on 1-3 piezocomposite is feasible for fabrication of high-frequency ultrasound arrays. Furthermore, this method is more conducive to mass production than other reported array fabrication techniques.     

A high-frequency annular-array transducer using an interdigital bonded 1-3 composite

This paper reports the design, fabrication, and characterization of a 1-3 composite annular-array transducer. An interdigital bonded (IB) 1-3 composite was prepared using two IB operations on a fine-grain piezoelectric ceramic. The final composite had 19-μm-wide posts separated by 6-μm-wide polymer kerfs. A novel method to remove metal electrodes from polymer portions of the 1-3 composite was established to eliminate the need for patterning and aligning the electrode on the composite to the electrodes on a flexible circuit. Unloaded epoxy was used for both the matching and backing layers and a flexible circuit was used for interconnect. A prototype array was successfully fabricated and tested. The results were in reasonable agreement with those predicted by a circuit-analogous model. The average center frequency estimated from the measured pulse-echo responses of array elements was 33.5 MHz and the -6-dB fractional bandwidth was 57%. The average insertion loss recorded was 14.3 dB, and the maximum crosstalk between the nearest-neighbor elements was less than -37 dB. Images of a wire phantom and excised porcine eye were obtained to show the capabilities of the array for high-frequency ultrasound imaging.     

Micro-machined high-frequency (80 MHz) PZT thick film linear arrays

This paper presents the development of a micromachined high-frequency linear array using PZT piezoelectric thick films. The linear array has 32 elements with an element width of 24 μm and an element length of 4 mm. Array elements were fabricated by deep reactive ion etching of PZT thick films, which were prepared from spin-coating of PZT sol-gel composite. Detailed fabrication processes, especially PZT thick film etching conditions and a novel transferring-and-etching method, are presented and discussed. Array designs were evaluated by simulation. Experimental measurements show that the array had a center frequency of 80 MHz and a fractional bandwidth (-6 dB) of 60%. An insertion loss of -41 dB and adjacent element crosstalk of -21 dB were found at the center frequency.     

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 implemented 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 microm 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 microm produced a -6 dB width of 68 microm in the axial and 370 microm 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.     

Ultrasonic imaging using a 5-MHz multilayer/single-layer hybrid array for increased signal-to-noise ratio

Conventional diagnostic ultrasound scanners are bulky and require significant amounts of electrical power during operation. Reducing the size, weight, and consumption of electrical power is made easier through the use of highly integrated compact transmit and receive electronics that may be incorporated in the transducer handle. This necessitates the use of low voltage transmitters and low power receive preamplifiers. Conventional scanners typically use approximately 100-V pulses during transmit; therefore, decreasing the transmit voltage to 15 V decreases the transmit sensitivity. Conventional receive electronics that are located at the scanner degrade the received signal-to-noise ratio (SNR) because the array element cannot efficiently drive the coaxial cable. Transmit sensitivity and received SNR can be radically improved using a multilayer/single-layer hybrid array making integration of electronics into the transducer handle more feasible. In this paper, we discuss the design, fabrication, and testing of a 5-MHz hybrid linear array. The hybrid array included 16 multilayer transmit elements (10 Omega impedance) and 24 single-layer receive elements at a half wavelength element pitch. Low voltage transmitters with an output resistance of 7 Omega and high impedance JFET preamplifiers using 15 V for biasing were located adjacent to the hybrid array in the transducer handle. The transmit sensitivity and received SNR of the hybrid array were compared with a conventional array using 50-Omega transmitters and receive preamplifiers at the scanner. The transmit sensitivity improved by 12.8 dB, and the received SNR improved by 7.8 dB, yielding an overall improvement of 20.6 dB, which compared well with predictions from the KLM model. Images of phantoms and in vivo images of the kidney obtained with the Siemens Model 1200 phased array system showed the increased SNR using the hybrid array. Estimates of penetration in tissue mimicking phantoms (alpha=0.5 dB/(cm MHz)) improved by 7 cm compared with the control.     

Multilayer piezoelectric ceramics for two-dimensional array transducers

In medical ultrasound imaging, 2-D array transducers have become essential to implement dynamic focusing and phase-correction in the elevation dimension as well as real-time volumetric scanning. Unfortunately, the small size of a 2-D array element results in a small clamped capacitance and a large electrical impedance near resonance. These elements have poor sensitivity because their impedance is much higher than the electrical impedance of the transmit and receive circuitry. Sensitivity can be improved by using an N layer structure of PZT ceramic with the layers connected acoustically in series and electrically in parallel. For the multilayer ceramic (MLC), the damped capacitance is multiplied by a factor of N(2) and the electrical impedance by 1/N(2) compared to a single layer element of the same dimensions. A 3x43 phased-array transducer has been fabricated using 3 layer PZT-5H material. Each element had a thickness of 0.66 mm and an area of 0.37x3.5 mm. The MLC was manufactured using thick film technology with plated-through vias to electrically interconnect the electrode layers. The completed transducer was compared to a single layer control array of similar dimensions. With a light epoxy backing and a lambda/4 matching layer, the MLC array elements had an impedance of 100 Omega at series resonance of 2.25 MHz, compared to 800 Omega for the control elements. The lower impedance of the MLC elements resulted in a minimum round-trip insertion loss of 24.0 dB, compared to an 34.1 dB for the control array elements. These results were consistent with KLM modeling. B-scan images were made of cysts in a tissue-mimicking phantom and of the left kidney in vivo. The images clearly showed a higher signal-to-noise ratio for the MLC array compared to the control. As a result, 2-D arrays made of multilayer ceramics can be used to form images at a higher frequency and greater range than single layer arrays.     

Real-time 3-D ultrasound guidance of interventional devices

We have previously developed 2-D array transducers for many real-time volumetric imaging applications. These applications include transducers operating up to 7 MHz for transthoracic imaging, up to 15 MHz for intracardiac echocardiography (ICE), 5 MHz for transesophageal echocardiography (TEE) and intracranial imaging, and 7 MHz for laparoscopic ultrasound imaging (LUS). Now we have developed a new generation of miniature ring-array transducers integrated into the catheter deployment kits of interventional devices to enable real-time 3-D ultrasound scanning for improved guidance of minimally invasive procedures. We have constructed 3 new ring transducers. The first consists of 54 elements operating at 5 MHz. Typical measured transducer element bandwidth was 25%, and the 50 Ohm round trip insertion loss was -65 dB. Average nearest neighbor cross talk was -23.8 dB. The second is a prototype 108-element transducer operating at 5 MHz. The third is a prototype 108-element ring array with a transducer center frequency of 8.9 MHz and a -6 dB bandwidth of 25%. All transducers were integrated with an 8.5 French catheter sheath of a Cook Medical, Inc. vena cava filter deployment device.     

Thermoelastic expansion vs. piezoelectricity for high-frequency, 2-D arrays

Optical generation using the thermoelastic effect has traditionally suffered from low conversion efficiency. We previously demonstrated increased efficiency of nearly 20 dB with an optical absorbing layer consisting of a mixture of polydimethylsiloxane (PDMS) and carbon black spin coated onto a glass microscope slide. In this paper we show that the radiated power from a black PDMS film is comparable to a 20 MHz piezoelectric two-dimensional (2-D) array element. Furthermore, we predict that a thermoelastic array element can produce similar acoustic power levels compared to ideal piezoelectric 2-D array elements at frequencies in the 100 MHz regime. We believe these results show that thermoelastic generation of ultrasound is a promising alternative to piezoelectricity for high-frequency, 2-D arrays.     

Finite element modeling and experimental characterization of crosstalk in 1-D CMUT arrays

Crosstalk is the coupling of energy between the elements of an ultrasonic transducer array. This coupling degrades the performance of transducers in applications such as medical imaging and therapeutics. In this paper, we present an experimental demonstration of guided interface waves in capacitive micromachined ultrasonic transducers (CMUTs). We compare the experimental results to finite element calculations using a commercial package (LS-DYNA) for a 1-D CMUT array operating in the conventional and collapsed modes. An element in the middle of the array was excited with a unipolar voltage pulse, and the displacements were measured using a laser interferometer along the center line of the array elements immersed in soybean oil. We repeated the measurements for an identical CMUT array covered with a 4.5-microm polydimethylsiloxane (PDMS) layer. The main crosstalk mechanism is the dispersive guided modes propagating in the fluid-solid interface. Although the transmitter element had a center frequency of 5.8 MHz with a 130% fractional bandwidth in the conventional operation, the dispersive guided mode was observed with the maximum amplitude at a frequency of 2.1 MHz, and had a cut-off frequency of 4 MHz. In the collapsed operation, the dispersive guided mode was observed with the maximum amplitude at a frequency of 4.0 MHz, and had a cut-off frequency of 10 MHz. Crosstalk level was lower in the collapsed operation (-39 dB) than in the conventional operation (-24.4 dB). The coverage of the PDMS did not significantly affect the crosstalk level, but reduced the phase velocity for both operation modes. Lamb wave modes, A0 and S0, were also observed with crosstalk levels of -40 dB and -65 dB, respectively. We observed excellent agreement between the finite element and the experimental results.     

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.     

Binary code design for high-frequency ultrasound

This paper proposes an approach to designing binary codes suitable for high-frequency applications of coded excitation in medical ultrasound. For a high-frequency ultrasound system, transmitting well-designed binary codes with a low sampling ratio (i.e., the bit rate divided by the transducer center frequency) is a practical way to improve the signal-to-noise ratio (SNR) because the challenge of implementing arbitrary-waveform generators for transmitting nonbinary codes increases with the frequency and the switching speed of square-wave pulsers are limited. One conventional approach designs codes using a base sequence that modulates wideband sequences up to the transducer passband. Because a major portion of codes is excluded as a candidate, codes designed using this approach typically need long compression filters for restoring the axial resolution, and they do not improve the SNR efficiently. In contrast, the approach proposed here searches all the codes that match the transducer passband; hence, the resultant codes exhibit better performance. The technique was tested using a bit rate of 50 MHz and a sampling ratio of 2. For a transducer with an ideal Gaussian frequency response with a center frequency of 25 MHz and a -6 dB bandwidth of 15 MHz, the SNR for the same side-lobe extent was 1 to 6 dB higher for the codes designed using the proposed approach compared with those designed using the conventional approach. When a real transducer response with a center frequency of 26.4 MHz and a one-way -6 dB bandwidth of 20.7 MHz was considered, the codes designed using the proposed approach were superior by 0.5 to 5 dB. Therefore, our approach is better than the conventional approach for designing binary codes for high-frequency ultrasound, with the results indicating that the moderate bit rate of 50 MHz will suffice when the ultrasonic center frequency is 25 MHz.     

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 mum (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 1.9x1.4 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 mum at a distance of 4.0 mm. Grating lobes are suppressed by at least 60 dB at distances >4.0 mm (~f/2). The elevation beam width is 220 mum at 4.0 mm, and a useful depth of field over the axial range from 4 to 10 mm is obtained.     

Characterization of a broadband all-optical ultrasound transducer-from optical and acoustical properties to imaging

A broadband all-optical ultrasound transducer has been designed, fabricated, and evaluated for high- frequency ultrasound imaging. The device consists of a 2-D gold nanostructure imprinted on top of a glass substrate, followed by a 3 microm PDMS layer and a 30 nm gold layer. A laser pulse at the resonance wavelength of the gold nanostructure is focused onto the surface for ultrasound generation, while the gold nanostructure, together with the 30 nm thick gold layer and the PDMS layer in between, forms an etalon for ultrasound detection, which uses a CW laser at a wavelength far from resonance as the probing beam. The center frequency of a pulse-echo signal recorded in the far field of the transducer is 40 MHz with -6 dB bandwidth of 57 MHz. The signal to noise ratio (SNR) from a 70 microm diameter transmit element combined with a 20 microm diameter receive element probing a near perfect reflector positioned 1.5 mm from the transducer surface is more than 10 dB and has the potential to be improved by at least another 40 dB. A high-frequency ultrasound array has been emulated using multiple measurements from the transducer while mechanically scanning an imaging target. Characterization of the device's optical and acoustical properties, as well as preliminary imaging results, strongly suggest that all-optical ultrasound transducers can be used to build high-frequency arrays for real-time high-resolution ultrasound imaging.     

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 μm     

Design and fabrication of a 40-MHz annular array transducer

This paper investigates the feasibility of fabricating a five-ring, focused annular array transducer operating at 40 MHz. The active piezoelectric material of the transducer was a 9-microm thick polyvinylidene fluoride (PVDF) film. One side of the PVDF was metallized with gold and forms the ground plane of the transducer. The array pattern of the transducer and electrical traces to each annulus were formed on a copper-clad polyimide film. The PVDF and polyimide were bonded with a thin layer of epoxy, pressed into a spherically curved shape, then back filled with epoxy. A five-ring transducer with equal area elements and 100-microm kerfs between annuli was fabricated and tested. The transducer had a total aperture of 6 mm and a geometric focus of 12 mm. The pulse/echo response from a quartz plate located at the geometric focus, two-way insertion loss (IL), complex impedance, electrical crosstalk, and lateral beamwidth all were measured for each annulus. The complex impedance data from each element were used to perform electrical matching, and the measurements were repeated. After impedance matching; fc approximately equal to 36 MHz and -6-dB bandwidths ranged from 31 to 39%. The ILs for the matched annuli ranged from -28 to -38 dB.     

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.     

A high-frequency linear ultrasonic array utilizing an interdigitally bonded 2-2 piezo-composite

This paper describes the development of a high-frequency 256-element linear ultrasonic array utilizing an interdigitally bonded (IB) piezo-composite. Several IB composites were fabricated with different commercial and experimental piezoelectric ceramics and evaluated to determine a suitable formulation for use in high-frequency linear arrays. It was found that the fabricated fine-scale 2-2 IB composites outperformed 1-3 IB composites with identical pillar- and kerf-widths. This result was not expected and lead to the conclusion that dicing damage was likely the cause of the discrepancy. Ultimately, a 2-2 composite fabricated using a fine-grain piezoelectric ceramic was chosen for the array. The composite was manufactured using one IB operation in the azimuth direction to produce approximately 19-μm-wide pillars separated by 6-μm-wide kerfs. The array had a 50 μm (one wavelength in water) azimuth pitch, two matching layers, and 2 mm elevation length focused to 7.3 mm using a polymethylpentene (TPX) lens. The measured pulse-echo center frequency for a representative array element was 28 MHz and -6-dB bandwidth was 61%. The measured single-element transmit -6-dB directivity was estimated to be 50°. The measured insertion loss was 19 dB after compensating for the effects of attenuation and diffraction in the water bath. A fine-wire phantom was used to assess the lateral and axial resolution of the array when paired with a prototype system utilizing a 64-channel analog beamformer. The -6-dB lateral and axial resolutions were estimated to be 125 and 68 μm, respectively. An anechoic cyst phantom was also imaged to determine the minimum detectable spherical inclusion, and thus the 3-D resolution of the array and beamformer. The minimum anechoic cyst detected was approximately 300 μm in diameter.