Andre Bezanson

Fabrication of a miniaturized 64-element high-frequency phased array

We have developed a 40 MHz, 64-element phased array transducer packaged into a 2.5mm by 3.1mm endoscopic form factor. The array is a forward looking kerfless design based on PMN-32%PT with an element-to-element pitch of 38 microns. In order to achieve such a miniaturized form factor, a novel technique of wire bonding the array elements to a miniature flex circuit oriented in line with the forward looking ultrasound beam was developed. A technique of scratch dicing the back of the array was also implemented in order to improve directivity of the array elements The array was fabricated with a single P(DVF-TrFE) matching layer and a TPX lens for passive elevation focussing to a depth of 7mm. The two-way -6dB pulse bandwidth was measured to be 55% and the average electromechanical coupling (kT) for the individual elements was measured to be 0.62. The one-way directivity pattern from single elements was measured to be +/25 degrees, which was shown to be a large improvement compared to an identical kerfless array. The -3dB elevation focus resulting from a TPX lens was measured to be 152 microns at the focal depth (~7mm).

A low cost open source high frame-rate high-frequency imaging system

In this paper a low cost open source approach to high-frequency ultrasound imaging is described. This complete imaging system is based around four core components: A single-element geometrically focused imaging transducer, a low cost high frame-rate mechanical scanner, a field programmable gate array (FPGA) controlled pulser-receiver unit, and a data acquisition system running open source interface software. The single-element imaging transducer is spherically curved composite based on Lithium Niobate that has a centre frequency of 45 MHz, a bandwidth of 65%, and an insertion loss of -19dB. The mechanical scanning mechanism is based on a 45 mm long PZT bimorph attached to an extension arm. The mechanism can scan up to a 10 mm displacements at 100 Hz and is driven with a low cost Arduino microcontroller. The mechanism is mounted in an enclosed probe holder filled with deionized water. The FPGA accurately controlling the variable timing of the pulser-receiver unit is a Xilinx Virtex V and the data acquisition hardware consists of an off the shelf AlazarTech PCIe digitizing card and a PC. The hardware communication, GUI/plotting libraries, and data collection is all controlled with an open source Python application we have named OpenHiFUS.

Experimental and finite element model directivity comparison between PZT and PMN-PT kerfless arrays

This paper describes a preliminary study in comparing the directivity patterns of kerfless linear arrays when the piezoelectric substrate is changed from conventional PZT-5H to the next generation PMN-PT32% single crystal piezoelectric. The directivity patterns were compared both through finite element modeling and experiment. It was also found that a significantly better directivity pattern for PMN-PT32% could be measured when the unelectroded substrate between the elements was locked into what we presume is a different morphotropic phase than the active elements. The -6 dB one-way directivity was measured to be ± 18° degrees for the PZT kerfless array, ± 12° degrees for the PMN-PT array with a single uniform phase throughout the crystal, and ± 52° degrees for the PMN-PT crystal with what we assume are different mechanical properties in the substrate between the elements.

A low-cost high frame-rate piezoelectric scanning mechanism for high-frequency ultrasound systems

This paper characterizes a low cost high-frequency two dimensional imaging system based on a single-element focused imaging transducer and a novel mechanical scanning mechanism. As previous designs have utilized costly electromagnetic motors, for the mechanical scanning translation, this mechanical scanning mechanism provides a highly simplified alternative. The mechanical scanning mechanism utilizes a single piezoelectric bimorph as the actuating element. Laser Doppler analysis of the transducer motion has shown that transducer deflections greater than 10 mm can be obtained for a large range of frequencies, from 40 to 120 Hz. This, in turn, results in a maximum theoretical frame rate of 240 fps. Peak deflections greater than 20 mm can be obtained at the scanning mechanism resonance (110 Hz). Additionally, an analysis of the transducer phase relative to the input voltage has shown that high phase stability (+/- 1.4 degrees) potentially eliminates the need for position encoders, further reducing the device complexity and cost.

A Sub-Nyquist, Variable Sampling, High-Frequency Phased Array Beamformer

A digital receive beamformer implementing a “one sample per pixel” variable sampling technique is described. The sampling method reduces the required sampling rates by a factor of 3, and reduces the data capture rate by a factor of 2, in comparison with the previous systems based on variable sampling. The sampling method is capable of estimating broadband pulse envelopes accurate for bandwidths up to 83.0%. This beamforming method has been implemented on a field-programmable gate array with maximum transmit and receive delay errors measured to be less than ±1.0 ns. The beamformer was tested and verified on a previously described 45-MHz 64-element phased array. The system generates images with 128 lines, 512 pixels per RF line, and 2 transmit focal zones. The system generates images with approximately 55 dB of dynamic range and was tested by imaging wire targets submersed in a water bath, wire targets embedded in a tissue phantom, and real-time in vivo imaging of a human wrist.

Design and preliminary experimental results for a high-frequency crossed electrode phased array, based on a reconfigurable Fresnel lens

A reconfigurable Fresnel lens approach for 3D imaging on crossed electrode array is described. A Fresnel lens can provide a focus comparable to traditional beamforming if the Fresnel focus is used in one direction, combined with conventional beamforming in the other direction. For example, in azimuth the Fresnel pattern is applied on transmit and dynamic beamforming is completed on receive. A second feature to our crossed electrode design arises from the fact that although a Fresnel lens can create a tight lateral focus, inherently long axial pulses result in poor axial resolution. This is because the phase delays are quantized to 0 or 180 degrees, and at wide steering angles the element path distance spans multiple wavelengths. We have developed a split and delay subaperture technique that maintains axial resolution throughout the image. At 0, 15 and 25 degrees the axial resolution using a single aperture was 64, 180 and 321 um. After splitting the aperture into 8 sub-apertures the resolution is 53, 64 and 77 um. The beamwidths using this technique are comparable to a traditional array and agree well with simulation suggesting that this Fresnel approach shows promise for 3D imaging.

Fabrication of a high-frequency phased array with sparse Vernier array element spacing for grating lobe suppression

This work presents the design, fabrication and characterization of a 50 MHz Vernier array. The Vernier array was based on a 128 element transducer with half-wavelength pitch, where every third element was used for transmit, every forth element was used for receive and unused elements were left inactive. The array was a forward looking kerfless design based on a PZT-5H substrate with an element-to-element pitch of 19 microns, and the probe was packaged in a 2.5 mm by 3.1 mm endoscopic form factor. The array was fabricated with a single P(VDF-TrFE)-copolymer matching layer and a polymethylpentene (TPX) lens for passive elevation focusing to a depth of 6 mm. To generate beam profiles, images and videos, the transducer was connected to an in-house developed 64-channel, high-frequency phased array beamformer. Near real-time radiation patterns and images were collected at a frame rate of 10 Hz. The performance of the Vernier array was directly compared to that of a previously developed phased array transducer with approximately one-wavelength pitch. Both transducers possessed similar two-way beamformed pulse bandwidths of 60%. At large steering angles the Vernier array suppressed the grating lobe levels 15 dB over the previously developed phased array, however, as a result of the sparseness of the Vernier array, the measured two-way sensitivity was 18.2 dB lower than the phased array with the fully active aperture. Experimental measurements were in good agreement with the theoretical predictions of 20dB grating lobe suppression and 22dB lower sensitivity than the phased array. Comparison images were generated of wire phantoms in a water bath as well as wire phantoms situated in a tissue phantom in order to assess the tradeoff between lower grating lobe levels at the expense of lower sensitivity.

Fabrication and performance of a miniaturized 64-element high-frequency endoscopic phased array

We have developed a 42 MHz, 64-element phased array transducer packaged in a 2.5 mm by 3.1 mm endoscopic form factor. The array is a forward looking semi-kerfed design based on a 0.68Pb(Mg1/3Nb2/3)O3-0.32PbTiO3 (PMN-32%PT) single-crystal wafer with an element-to-element pitch of 38 microns. In order to achieve a miniaturized form factor, a novel technique of wire bonding the array elements to a polyimide flexible circuit board oriented parallel to the forward looking ultrasound beam and perpendicular to the array was developed. A technique of dicing partially into the back of the array was also implemented in order to improve the directivity of the array elements. The array was fabricated with a single layer P(VDF-TrFE)-copolymer matching layer and a polymethylpentene (TPX) lens for passive elevation focusing to a depth of 7 mm. The two-way -6 dB pulse bandwidth was measured to be 55% and the average electromechanical coupling (kT) for the individual elements was measured to be 0.62. The one-way -3 dB directivity from several array elements was measured and found to be ± 20 degrees, which was shown to be an improvement over an identical kerfless array. The -3 dB one-way elevation focus resulting from the TPX lens was measured to be 152 microns at the focal depth, and the focused lateral resolution was measured to be 80 microns at a steering angle of 0°. To generate beam profiles and images the probe was connected to a Visualsonics Vevo 2100 imaging platform which was reprogrammed to allow for phased array transmit beamforming and receive data collection. The collected RF data was then processed offline using a Matlab script to generate sector images. Focused one-way transmit radiation patterns were collected with a needle hydrophone. Two-way images were generated with a dynamic range of 60 dB for wire phantoms in water and a tissue-equivalent medium. Finally, ex-vivo tissue images were generated of porcine brain tissue.