25 MHz ultrasonic transducers with lead- free piezoceramic, 1-3 PZT fiber-epoxy composite, and PVDF polymer active elements

This paper presents the fabrication and characterization of single-element ultrasonic transducers whose active elements are made of lead-free piezoceramic, 1-3 PZT/polymer composite and PVDF film. The lead free piezoelectric KNNLT- LS(K_0.44Na_0.52Li_0.04)(Nb_0.84Ta_0.10S_0.06b)O₃ powders and ceramics were prepared under controlled humidity and oxygen flow rate during sintering. Due to its moderate longitudinal piezoelectric charge coefficient (175 pC/N) and k_t of 0.50, the KNN-LT-LS composition may be a good candidate for highfrequency transducer applications. PZT fibers with 25 μm diameter formed by the viscose suspension spinning process were incorporated into epoxy to fabricate 1-3 composites with the averaged k_t = 0.64 and d₃₃ = 400 pC/N. Using KNN-LS-LT ceramic, 1-3 PZT fiber composite, and PVDF film, 3 different unfocused single element transducers with center frequencies of 25 MHz were fabricated. The acoustic characterization of the transducers demonstrated that wideband and low insertion loss could be obtained employing KNN-LS-LT ceramic. The ?6 dB bandwidth and insertion loss were 70% and ?21 dB, respectively. In comparison, the insertion loss of the ceramic transducer was much smaller than those made with 1-3 composite and PVDF film. This was attributed to closer electrical impedance match to 50 Ω and higher thickness coupling coefficient of the ceramic transducer.     

A single-element transducer with nonuniform thickness for high-frequency broadband applications

The design, fabrication, and evaluation of a high-frequency single-element transducer are described. The transducer has an annular geometry, with the thickness of the piezoelectric material increasing from the center to the outside. This single-element annular transducer (SEAT) can provide a broader frequency range than a conventional single-element transducer with a uniform thickness (single-element uniform transducer, or SEUT). We compared the characteristics of a SEAT and a SEUT. Both transducers used 36deg-rotated, Y-cut lithium niobate (LiNbO₃) material. The SEAT had a diameter of 6 mm and comprised 6 subelements of equal area (electrically connected by a single electrode on each side) whose thickness ranged from 60 mum (center) to 110 mum (outside), which resulted in the center frequency of the subelements varying from 59.8 MHz to 25 MHz. The overall center frequency was 42.4 MHz. The annular pattern was constructed using an ultrasonic sculpturing machine that reduced the root-mean-square value of the surface roughness to 454.47 nm. The bandwidth of the SEAT was 19% larger than that of the SEUT. However, compared with the SEUT, the 2-way insertion loss of the SEAT was increased by 3.1 dB. The acoustic beam pattern of the SEAT was also evaluated numerically by finite-element simulations and experimentally by an ultrasound beam analyzer. At the focus (10.5 mm from the transducer surface), the -6 dB beam width was 108 mum. There was reasonable agreement between the data from simulations and experiments. The SEAT can be used for imaging applications that require a wider transducer bandwidth, such as harmonic imaging, and can be manufactured using the same techniques used to produce transducers with multiple frequency bands.     

High-frequency ultrasonic transducer fabricated with lead-free piezoelectric single crystal

High-frequency (25 MHz) ultrasonic transducers with Na(0.5)Bi(0.5)TiO(3)-BaTiO(3) (NBT-BT) lead-free piezoelectric single crystal as the active elements are fabricated and characterized. The impedance measurement reveals that the poled [001]-oriented NBT-BT single crystal exhibits a high thickness electromechanical coefficient k(t) of 0.52 and a low clamped dielectric constant of 80. The -6-dB bandwidth of the transducer is 46.16% and the insertion loss at the center frequency is -31.89 dB. The good performance of the transducer indicates that the NBT-BT single crystal would be a promising lead-free material for ultrasonic transducer applications.     

Disposable PVDF ultrasonic transducers for nondestructive testingapplications

Disposable ultrasonic contact transducers have been constructed with inexpensive PVDF films for nondestructive testing (NDT) applications. This paper reports the temperature-dependent ultrasonic performance of commercial polyvinylidene fluoride (PVDF) films and PVDF sensors. PVDF film was evaluated for its material properties of interest for ultrasonic transducer performance including the relative dielectric constant ε_r, dielectric loss tangent tan δ_e , electromechanical coupling constant k_t, mechanical quality factor Q_m, and acoustic impedance Z. Disposable PVDF transducers were then coupled to both low and high acoustic impedance test panels and their ultrasonic insertion loss/sensitivity and frequency bandwidth measured as a function of temperature in both the pulse-echo and pitch-catch modes. The results yielded center frequencies in the 3.5-16 MHz range, with -6-dB fractional bandwidths for some of the transducers exceeding 100%. The temperature dependence of the transducers' performance is reported for the -40 to 80°C range and shows an approximate linear decrease in center frequency and increase in fractional bandwidth with increasing temperature. These measured results are compared with predictions from computer simulations based on Mason's model. These tests showed that the same disposable transducers can be used for both ultrasonic and acoustic emission sensing NDT applications. This paper summarizes the quantitative ultrasonic and vibration sensing performance for all of the disposable PVDF film transducers which were tested     

Fabrication of high frequency spherically shaped ceramictransducers

Difficulty in obtaining well focused efficient ultrasound transducers has limited the development of new high frequency applications of B-mode imaging. This paper describes a method for fabricating high frequency (53 MHz) spherically focused lead zirconate titanate (PZT) transducers. A transducer is fabricated by bonding a malleable backing layer onto a thin plate of PZT and then pressing the plate into a spherically shaped well. The backing layer evenly distributes stresses across the material when it is pressed into the well. Local concentrations of stress which lead to fracture are avoided and the material can be deformed without macroscopic cracking. The characteristics of a 2 mm diameter 53 MHz PZT transducer with a 4 mm focal length are described. A lateral beam width of 68 μm and a 12 dB depth of field of 1.5 mm were obtained. The minimum two-way insertion loss of the system was -25 dB and the 6 dB bandwidth of the pulse echo response was 30%. An image of a resolution phantom and an in vivo skin image illustrate the excellent imaging characteristics of the transducer     

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 -38 degrees, 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.     

A 100-200 MHz ultrasound biomicroscope

The development of higher frequency ultrasound imaging systems affords a unique opportunity to visualize living tissue at the microscopic level. This work was undertaken to assess the potential of ultrasound imaging in vivo using the 100-200 MHz range. Spherically focused lithium niobate transducers were fabricated. The properties of a 200 MHz center frequency device are described in detail. This transducer showed good sensitivity with an insertion loss of 18 dB at 200 MHz. Resolution of 14 /spl mu/m in the lateral direction and 12 /spl mu/m in the axial direction was achieved with f/1.14 focusing. A linear mechanical scan system and a scan converter were used to generate B-scan images at a frame rate up to 12 frames per second. System performance in B-mode imaging is limited by frequency dependent attenuation in tissues. An alternative technique, zone-focus image collection, was investigated to extend depth of field. Images of coronary arteries, the eye, and skin are presented along with some preliminary correlations with histology. These results demonstrate the feasibility of ultrasound biomicroscopy In the 100-200 MHz range. Further development of ultrasound backscatter imaging at frequencies up to and above 200 MHz will contribute valuable information about tissue microstructure.     

Fabrication and evaluation of a single-element Bi(0.5)Na(0.5)TiO(3)-based ultrasonic transducer

This paper discusses the fabrication and characterization of a single-element ultrasonic transducer with a lead-free piezoelectric active element. A piezoelectric ceramic with composition of 0.88Bi(0.5)Na(0.5)TiO(3)-0.08Bi(0.5)K(0.5)TiO(3)- 0.04Bi(0.5)Li(0.5)TiO(3) was chosen as the active element of the transducer. This composition exhibited a thickness coupling coefficient (kt) of 0.45, a dielectric constant of 440 (at 1 kHz), and a longitudinal piezoelectric coefficient (d(33)) of 84 pC?N(-1). To make the transducer, the ceramic was sandwiched between an epoxy-tungsten backing layer and a silver epoxy matching layer. An epoxy lens was also incorporated into the transducer?s design to focus the ultrasound beam. The focused transducer with a center frequency of about 23 MHz demonstrated a -6-dB bandwidth of 55% and an insertion loss of -32 dB; the -20-dB pulsed length was measured to be 150 ns. A phantom made of copper wires (30 μm in diameter) was utilized to investigate the imaging capability of the transducer. The results indicated that the fabricated transducer, with a lateral resolution of 260 μm and a relatively high depolarization temperature, could be considered as a candidate for replacement of lead-based ultrasonic transducers.     

Low-loss, multimode 5-IDT SAW filter

Longitudinally coupled resonator filters provide unbalanced-balanced operation with wide bandwidth, low loss, and high suppression levels. However, reducing the insertion loss in the 1.8-2.2 GHz range remains a challenging problem because at high frequencies the resistive losses arising from the relatively wide aperture of the filter may degrade the performance. A 5-interdigital transducer (IDT) filter has six gaps at which the periodicity of the grating is broken, resulting in additional loss due to scattering into the bulk. In this paper, we show that replacing the gaps between the transducers with short transducer sections having their pitch different from that of the main transducers reduces the insertion loss of the device. We present devices with balun operation at 1842 MHz with wide bandwidth of 4.5% and -40 dB suppression, with a minimum insertion loss less than 1 dB in the best devices, and a maximum insertion loss of -1.2 dB in the passband. The passband is quite flat, with <1 dB ripple. We also discuss the layout of the contact pads and the connections, and its effect on the device performance and balance characteristics.     

Characterization of high-frequency, single-element focused transducers with wire target and hydrophone

In this paper, a wire-target technique was used for lateral beam profile measurements for a single-element, focused transducers in the very high-frequency range (35-60 MHz). Two wire targets made from 9-cm long tungsten wires with diameters of 8 microm and 20 microm were used as the pulse-echo targets to measure the lateral beam profiles at the focal plane of two single-element, focused transducers, a spherically focused 40 MHz transducer and a lens-focused in-house lithium niobate (LiNbO3) 60 MHz transducer. For comparison, measurements on the same transducers were performed by three small-aperture hydrophones with geometrical diameters varying from 37 microm to 150 microm. Tomographic reconstruction of the acoustic field from the spherically focused transducer also was conducted. Results obtained with the wire-target technique are comparable to those obtained with small-aperture hydrophones in characterizing lateral radiation patterns of a single-element, focused transducer in the high-frequency range (35-60 MHz). However, the wire-target method may overestimate pulse length because of the additional attenuation caused by the return path. Compared to small-aperture hydrophones, the wire-target technique is simpler and more cost effective. Its major advantage, however, is in the frequency range above 100 MHz in which commercial hydrophones are not yet available.     

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.     

A miniaturized catheter 2-D array for real-time, 3-D intracardiac echocardiography

The design, fabrication, and characterization of a 112 channel, 5 MHz, two-dimensional (2-D) array transducer constructed on a six layer flexible polyimide interconnect circuit is described. The transducer was mounted in a 7 Fr (2.33 mm outside diameter) catheter for use in real-time intracardiac volumetric imaging. Two transducers were constructed: one with a single silver epoxy matching layer and the other without a matching layer. The center frequency and -6 dB fractional bandwidth of the transducer with a matching layer were 4.9 MHz and 31%, respectively. The 50 omega pitch-catch insertion loss was 80 dB, and the typical interelement crosstalk was -30 dB. The final element yield was greater than 97% for both transducers. The transducers were used to acquire real-time, 3-D images in an in vivo sheep model. We present in vivo images of cardiac anatomy obtained from within the coronary sinus, including the left and right atria, aorta, coronary arteries, and pulmonary veins. We also present images showing the manipulation of a separate electrophysiological catheter into the coronary sinus.     

High-frequency transducers based on integrated piezoelectric thick films for medical imaging

A screen-printed PZT thick film with a final thickness of about 40 microm was deposited on a porous PZT substrate to obtain an integrated structure for ultrasonic transducer applications. This process makes it possible to decrease the number of steps in the fabrication of high-frequency, single-element transducers. The porous PZT substrates allow high acoustic impedance and attenuation to be obtained, satisfying transducer backing requirements for medical imaging. The piezoelectric thick films deliver high electromechanical performance, comparable to that of standard bulk ceramics (thickness coupling factor over 45%). Based on these structures, high-frequency transducers with a center frequency of about 25 MHz were produced and characterized. As a result, good sensitivity and axial resolution were obtained in comparison with similar transducers integrating a lead titanate (PT) disk as active material. The two transducers were integrated into a high-frequency imaging system, and comparative skin images are shown.     

Characterization of lead zirconate titanate ceramics for use in miniature high-frequency (20-80 MHz) transducers

The material properties of lead zirconate titanate (PZT) ceramics for operation in the thickness mode at frequencies as high as 80 MHz are reported. Each of the ceramics tested showed a reduction in k (t) with increasing frequency. In a fine-grained PZT, values of k(t) as high as 0.44 were measured at 80 MHz. The effects of grain size were also evident in the measurement of frequency dependent mechanical losses. Experimental and theoretical analysis of a 1 mmx1 mm, 45 MHz PZT transducer verified the validity of the measurements of the properties and demonstrated excellent insertion loss and bandwidth characteristics. The minimum insertion loss of -17.5 dB is in good agreement with theory and is a marked improvement over the performance of polymer devices. Details on the fabrication and testing of high frequency ceramic transducers are described.     

Bandwidth and resolution enhancement through pulse compression

A novel pulse compression technique is developed that improves the axial resolution of an ultrasonic imaging system and provides a boost in the echo signal-to-noise ratio (eSNR). The new technique, called the resolution enhancement compression (REC) technique, was validated with simulations and experimental measurements. Image quality was examined in terms of three metrics: the eSNR, the bandwidth, and the axial resolution through the modulation transfer function (MTF). Simulations were conducted with a weakly-focused, single-element ultrasound source with a center frequency of 2.25 MHz. Experimental measurements were carried out with a single-element transducer (f/3) with a center frequency of 2.25 MHz from a planar reflector and wire targets. In simulations, axial resolution of the ultrasonic imaging system was almost doubled using the REC technique (0.29 mm) versus conventional pulsing techniques (0.60 mm). The -3 dB pulse/echo bandwidth was more than doubled from 48% to 97%, and maximum range sidelobes were -40 dB. Experimental measurements revealed an improvement in axial resolution using the REC technique (0.31 mm) versus conventional pulsing (0.44 mm). The -3 dB pulse/echo bandwidth was doubled from 56% to 113%, and maximum range sidelobes were observed at -45 dB. In addition, a significant gain in eSNR (9 to 16.2 dB) was achieved.     

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 (PbTiO₃) ceramic, poly(vinylidene fluoride) (PVDF) film, and lithium niobate (LiNbO₃) 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 Ω for the PZT fiber composite and PbTiO₃ 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 ₃ and PbTiO₃ transducers provided better image contrast than the other materials     

80-MHz intravascular ultrasound transducer using PMN-PT free-standing film

[Pb(Mg(1/3)Nb(2/3))O(3)](0.63)[PbTiO(3)](0.37) (PMN-PT) free-standing film of comparable piezoelectric properties to bulk material with thickness of 30 μm has been fabricated using a modified precursor coating approach. At 1 kHz, the dielectric permittivity and loss were 4364 and 0.033, respectively. The remnant polarization and coercive field were 28 μC/cm(2) and 18.43 kV/cm. The electromechanical coupling coefficient k(t) was measured to be 0.55, which was close to that of bulk PMN-PT single-crystal material. Based on this film, high-frequency (82 MHz) miniature ultrasonic transducers were fabricated with 65% bandwidth and 23 dB insertion loss. Axial and lateral resolutions were determined to be as high as 35 and 176 μm. In vitro intravascular imaging on healthy rabbit aorta was performed using the thin film transducers. In comparison with a 35-MHz IVUS transducer, the 80-MHz transducer showed superior resolution and contrast with satisfactory penetration depth. The imaging results suggest that PMN-PT free-standing thin film technology is a feasible and efficient way to fabricate very-high-frequency ultrasonic transducers.     

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.     

Pre-focused lead titanate > 25 MHz single-element transducers from hollow spheres

Miniature lead titanate (PT) hollow spheres with diameters in the 1 to 10 mm range and wall thicknesses of 20 to 120 μm have been fabricated. Shell sections were used as components of pre-focused transducers. Spheres are produced using a new sacrificial core technique that produces hundreds of spheres with a more uniform wall thickness than those produced by earlier methods. Shells produced from these spheres were found to have a wall thickness variation of about 10%. Despite this variation, bulk properties were estimated from capacitance and impedance data. Shells tested in this work had dielectric constants (1 kHz) near 280 with loss factor of <2% and d₃₃ values of 68 pC/N. Thickness coupling coefficients averaged 0.51 with mechanical quality factors of <15. A transducer fabricated from these sections of spheres had a round-trip insertion loss of -20.1 dB at the center frequency of 39.8 MHz and a 6 dB bandwidth of 33%     

Design and fabrication of PZN-7%PT single crystal high frequency angled needle ultrasound transducers

A high-frequency angled needle ultrasound transducer with an aperture size of 0.4 x 0.56 mm2 was fabricated using a lead zinc niobate-lead titanate (PZN- 7%PT) single crystal as the active piezoelectric material. The single crystal was bonded to a conductive silver particle matching layer and a conductive epoxy backing material through direct contact curing. A parylene outer matching layer was formed by vapor deposition. Angled needle probe configuration was achieved by dicing at 45 degrees to the single crystal poling direction to satisfy a clinical request for blood flow measurement in the posterior portion of the eye. The electrical impedance magnitude and phase of the transducer were 42 Omega and -63 degrees , respectively. The measured center frequency and the fractional bandwidth at -6 dB were 43 MHz and 45%, respectively. The two-way insertion loss was approximately 17 dB. Wire phantom imaging using fabricated PZN-7%PT single crystal transducers was obtained and spatial resolutions were assessed.