Jay A. Williams

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.

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.

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 times 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 45deg 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 -63deg, 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.

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.

A kerfless 30 MHz linear ultrasonic array

In this study a 30 MHz kerfless linear array design was developed and was compared to a 30 MHz piezo-composite array of identical dimensions. A 2-D finite element model (PZFlex) was used to optimize and compare the performance of both arrays. The modeled results suggested that the kerfless array with two matching layers would have a two-way single element sensitivity 7 dB greater than that of the composite array with one acoustic matching layer. However analysis of one-way directivity patterns for single array elements showed a significant reduction in acceptance angle for the kerfless design due to the expected increase in electromechanical crosstalk. The modeled -6 dB acceptance angles were 28o and 50o for the kerfless array element and composite array element, respectively. Based upon these encouraging results kerfless and composite 30 MHz arrays were fabricated and tested. On average the sensitivity of the kerfless array echo response was only 0.8 dB higher than that of the composite array. The average echo center frequency and bandwidth was slightly higher than that of the composite array, whereas the maximum measured crosstalk for the kerfless array was 14 dB larger than that of the composite array. As expected, this crosstalk increase resulted in a reduction of element acceptance angle. The measured -6dB acceptance angle was 22o for the kerfless array and 35o for the composite array. Based upon these results it is expected that kerfless array technology will provide a viable alternate for high frequency linear array designs but should be considered only when an adequate composite is not available.

Dual-element needle transducer for intravascular ultrasound imaging

Abstract. A dual-element needle transducer for intravascular ultrasound imaging has been developed. A low-frequency element and a high-frequency element were integrated into one device to obtain images which conveyed both low- and high-frequency information from a single scan. The low-frequency element with a center frequency of 48 MHz was fabricated from the single crystal form of lead magnesium niobate-lead titanate solid solution with two matching layers (MLs) and the high frequency element with a center frequency of 152 MHz was fabricated from lithium niobate with one ML. The measured axial and lateral resolutions were 27 and 122  μm, respectively, for the low-frequency element, and 14 and 40  μm, respectively, for the high-frequency element. The performance of the dual-element needle transducer was validated by imaging a tissue-mimicking phantom with lesion-mimicking area, and ex vivo rabbit aortas in water and rabbit whole blood. The results suggest that a low-frequency element effectively provides depth resolved images of the whole vessel and its adjacent tissue, and a high-frequency element visualizes detailed structure near the surface of the lumen wall in the presence of blood within the lumen. The advantages of a dual-element approach for intravascular imaging are also discussed.

Fabrication of 20 MHz convex array transducers for high frequency ophthalmic imaging

20 MHz 192-element convex array transducers have been designed, fabricated and tested. Major design parameters are 111 µm pitch, 24 mm radius of curvature, 52° maximum view angle, 7 mm elevation width, and 30 mm geometric focus. A 1–3 composite was fabricated with a ceramic post width of 28 µm and a polymer kerf width of 9 µm. The ceramic volume fraction was approximately 57%. Three ceramic posts formed one element in the azimuth direction. A custom designed flexible circuit was aligned and attached to a scratch-diced composite and 75 Ω micro-coaxial cables. A single epoxy matching layer and a backing block were used for acoustic matching. The pulse-echo response for a typical element acquired at the axial peak, 28.0 mm, matched with a designed geometrical focusing at 30 mm, had a center frequency of 24.4 MHz and 64% of −6 dB fractional bandwidth. The estimated insertion loss at 24 MHz after compensating for attenuation and reflection was 35 dB and the maximum crosstalk was −17 dB and −22 dB for the nearest and for the next-nearest element respectively.

Development of high frequency linear arrays using interdigital bonded composites

This paper describes the development of an interdigital bonded (IB) 1-3 composite for use in a 30MHz 256-element geometrically focused ultrasonic array. The composite was manufactured using two IB operations in the elevation direction and one IB operation as well as one additional dicing step in the azimuth direction. The resultant composite elements had 14mum x 19mum wide posts separated by 8mum wide kerfs. The diced inter-element kerfs were 14mum. The finished array had a 50mum (1lambda ) azimuth pitch and 2mm elevation aperture focused to 8mm. The measured pulse-echo center frequency was 30MHz and -6dB bandwidth was 55%. The maximum combined electrical and mechanical crosstalk was less than -27 dB over the range of 10-50MHz. The measured transmit -3dB directivity was 36deg. The measured insertion loss was 29dB after compensating for attenuation and diffraction.

Crosstalk reduction for high-frequency linear-array ultrasound transducers using 1-3 piezocomposites with pseudo-random pillars

The goal of this research was to develop a novel diced 1-3 piezocomposite geometry to reduce pulse-echo ring down and acoustic crosstalk between high-frequency ultrasonic array elements. Two PZT-5H-based 1-3 composites (10 and 15 MHz) of different pillar geometries [square (SQ), 45° triangle (TR), and pseudo-random (PR)] were fabricated and then made into single-element ultrasound transducers. The measured pulse-echo waveforms and their envelopes indicate that the PR composites had the shortest -20-dB pulse length and highest sensitivity among the composites evaluated. Using these composites, 15-MHz array subapertures with a 0.95λ pitch were fabricated to assess the acoustic crosstalk between array elements. The combined electrical and acoustical crosstalk between the nearest array elements of the PR array subapertures (-31.8 dB at 15 MHz) was 6.5 and 2.2 dB lower than those of the SQ and the TR array subapertures, respectively. These results demonstrate that the 1-3 piezocomposite with the pseudo-random pillars may be a better choice for fabricating enhanced high-frequency linear-array ultrasound transducers; especially when mechanical dicing is used.

A study of 1–3 pseudo-random pillar piezocomposites for ultrasound transducers

The goal of our research is to develop a new diced 1-3 piezocomposite geometry to reduce pulse-echo ring down and acoustic cross-talk between high frequency ultrasonic array elements. Two PZT-5H based 1-3 composites (10 and 15 MHz) with different pillar geometries (square (SQ), 45° triangle (TR), and pseudo-random (PR)) were fabricated and then made into single element ultrasound transducers. The measured pulse-echo waveforms and their envelopes indicate that the PR composites had the shortest -20 dB pulse length and strongest sensitivity among all the composites. Using these composites, 15 MHz array sub-apertures with a 0.95 λ pitch were fabricated to measure the acoustic cross-talk between array elements. The combined electrical and acoustical crosstalk between the nearest array elements of the PR array sub-apertures (-32 dB @ 15 MHz) was 6 dB and 2 dB lower than that of the SQ and the TR array sub-apertures, respectively. These results demonstrate that the 1-3 piezocomposite with the pseudo-random pillars may be a good choice for fabricating enhanced high frequency linear array ultrasound transducers; especially when mechanical dicing is used.