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Welcome to the Duke University
Ultrasound Transducer Group

Professor Stephen Smith

Alumni

Richard Goldberg

Multi-layer piezoelectric ceramics for medical ultrasound 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 signal-to-noise ratio (SNR) because their impedance is much higher than the electrical impedance of the transmit and receive circuitry. It is hypothesized that the SNR can be improved by using an N layer structure of piezoelectric ceramic with the layers connected acoustically in series and electrically in parallel. Further, it is hypothesized that such multi-layer ceramics (MLC's) can be accurately modeled by a simplified circuit model, the KLM transmission line model, and finite element analysis. For the MLC, the clamped capacitance is multiplied by a factor of N$/sp2$ and the electrical impedance by 1/N$/sp2$ compared to a single layer element of the same dimensions. The hypothesis was tested by analysis, development and performance measurement of MLC array transducers. Prototype 2-D array elements were fabricated from 19 layer PZT-5A, operating at 1 MHz. The electrical impedance was 50 $/Omega,$ compared to 14 k$/Omega$ for a single layer control element. As a result, the pulse-echo SNR increased by 26.3 dB for the MLC compared to the control when driving a coaxial cable load on receive. Also, a 2.25 MHz, 3 x 43 phased-array transducer was fabricated using 3 layer PZT-5H material. The MLC was manufactured using thick film technology with plated-through vias to electrically interconnect the electrode layers. The MLC elements had an impedance of 115 $/Omega,$ compared to 830 $/Omega$ for single layer control elements. For the low impedance MLC elements, the pulse-echo SNR increased by 6.8 dB when driving a coaxial cable load. 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 multi-layer PZT can be used to form images at a higher frequency and a greater penetration depth than single layer arrays. These results were consistent with simulations from the KLM model and finite element analysis. For future applications of MLC's, further improvement in SNR can be obtained using a hybrid design for a sparsely sampled array. Several hybrid array designs were analyzed.

Richard Davidsen

Design and fabrication of multiplexed two-dimensional transducer arrays using electrostrictive ceramic materials

Two-dimensional array transducers are essential for real time volumetric imaging. The implementation of receive mode parallel processing requires the transducer be designed with a wide transmit beam to allow multiple simultaneous receive beams. Unfortunately, this requirement has an associated increase in pulse-echo beamwidth that reduces contrast of subtle lesions. When the volumetric system is used for B-mode scanning, the wide transmit beam degrades image quality unnecessarily. It is hypothesized that a multiplexed two-dimensional array transducer can be developed to improve image quality of a volumetric scanner when it is used for B-mode scanning, while maintaining the capability of real time volumetric imaging. It is further hypothesized that 2-D array multiplexing can be achieved using the bias controlled sensitivity of electrostrictive relaxor ferroelectric materials. The hypothesis was tested by analysis, development and evaluation of sparse random arrays and relaxor ferroelectric materials. Random array patterns with Gaussian and uniform element distributions were analyzed by computer simulation. A random array pattern was designed for each mode of the multiplexed transducer. Beamwidth was controlled by distribution of the 192 transmitter and 64 receivers, whereas the average sidelobe amplitude was determined by the number of elements. Bar mode piezoelectric properties of two prototype relaxor ferroelectric materials were measured and used in KLM transducer modeling. The materials exhibited high dielectric permittivity, but low electromechanical coupling and high dielectric loss. Bias circuits were designed to allow operation of the relaxor transducer with the existing phased array system. The 3.5 MHz, 82 x 82 multiplexed transducer was fabricated with 428 active elements. Element and array performance were in good agreement with predictions. Multiplexing was successfully accomplished with performance comparable to commercial multiplexors. The multiplexing system was integrated with the Duke phased array scanner and allowed rapid multiplexing between element patterns. Images of test phantoms indicated the element pattern for B-mode imaging had improved contrast when compared to the element pattern for volumetric imaging.

Loriann Davidsen

Deformable array transducers for phase aberration correction in medical ultrasonic imaging

Phase aberrations due to inhomogeneities in the acoustic velocity of human tissues degrade medical ultrasound images by disrupting the ultrasound beam focus. Near field phase correction algorithms compensate for aberrating tissue located close to the transducer by adjusting the electronic phase delays used to steer and focus the ultrasound beam. In order to correct the two-dimensional phase aberrations in tissue using this technique, a two-dimensional array is necessary. However, two-dimensional arrays are a complex option for phase correction due to their large number of elements and poor sensitivity. Instead of using a full two-dimensional array, a new technique is proposed which uses a deformable transducer of significantly fewer channels for two-dimensional phase correction. Phase correction in azimuth is achieved by altering the electronic phase delay of the elements. However, phase correction in elevation is achieved by tilting the elements in elevation with a piezoelectric actuator. Simulations of phase aberration correction using a deformable array transducer were compared to electronic correction with a 2-D array. The results have shown that a deformable 1 x N or 2 x N array transducer can approach the image quality of a 4 x N two-dimensional array. A prototype 1 x 32 deformable array was developed using a RAINBOW actuator for both the element deflection and the generation of ultrasound. The prototype array was characterized with measurements of vector impedance, pulse-echo sensitivity, and bandwidth. Phase correction in elevation was simulated by tilting the elements on-line to alter the B-scan image. Measured cyst contrast of a tissue mimicking phantom increased from 0.76 with half the elements tilted to 0.86 for the corrected case with all the elements aligned. To improve the future performance of the deformable array while minimizing the fabrication effort, 2-D and 3-D finite element analysis (FEA) was developed to predict the characteristics of the deformable array. Since the deformable array combines a mechanical actuator with a medical ultrasound transducer, both the low frequency acutator and ultrasonic characteristics of the array were modeled and were well matched to experimental results.

Charles Emery

Signal-to-noise ratio of transducer arrays for medical ultrasound

Linear array transducers for medical ultrasound would be valuable for field use and emergency room treatment. Such a portable ultrasouund scanner requires reducing the size and weight of the system and minimizing the amount of power necessary for scanning. For example, integrating the transmit and receive circuitry by decreasing the conventional transmit voltage from 100 V to 15 V reduces the size of the scanner. Unfortunately, the transmit sensitivity is reduced because of the reduction in transmit voltage. In receive mode, the linear array element combined with the loading effects of the coaxial cable limit the received signal-to-noise ratio (SNR). Therefore, the pulse-echo SNR of a linear array has to be optimized to make imaging feasible. It was hypothesized the pulse-echo SNR could be improved using a hybrid array. In transmit, the poor sensitivity in a portable ultrasound scanner and a 2-D array was improved by using multilayer lead zirconate titanate (PZT) ceramic elements which reduced the element input impedance and increased the power coupled into the body. In addition to decreasing the element impedance, a reduction in the transmitter source impedance from the typical value of 50 $/Omega$ to 7 $/Omega$ further increased transmit sensitivity. The received SNR was improved by using a single layer PZT element combined with a high impedance preamplifier adjacent to the element. The low impedance transmitters were also located in the handle with the receive preamplifiers. The hypothesis was tested by fabricating and testing a 5 MHz hybrid linear phased array. The 5 MHz hybrid array which consisted of 64 elements had alternating multilayer transmit and single layer receive elements. A low voltage transmitter with an output impedance of 7 $/Omega$ and a high impedance preamplifier were placed adjacent to the transmit and receive elements. B-scan images of tissue phantoms as well as abdominal images using the Siemens SI-1200 scanner further confirmed the hypothesis. To further improve the ultrasound scanner maneuverability, an optoelectronic transmitter was investigated to replace the conventional electronic transmitter. Flexible light weight fiber optics replaced the coaxial cable. A photosensor in the handle switched wide bandwidth pulses across the transducer element upon detection of an optical signal. Feasibility studies were performed using a 500 mW AlGaAs laser diode and a silicon photoconductive switch (PCSS). Finally, a 48 channel ultrasound system with 16 optoelectronic transmitters and 32 conventional electronic receivers was designed to interface with a Siemens SI-1200 scanner and a 2.25 MHz linear array. Transmit signal measurements and B-scan images of cysts and tumors in tissue mimicking phantoms were performed to compare the optoelectronic transmitter to the conventional electronic transmitter. (Abstract shortened by UMI.)

Warren Lee

Real-time three-dimensional intracardiac ultrasound imaging using two-dimensional catheter arrays

Intracardiac echocardiography (ICE) is a minimally invasive imaging technique in which a miniaturized ultrasound transducer is mounted in the tip of a catheter, enabling image acquisition from within the heart. To date, many applications of ICE involve the guidance of cardiac interventional procedures, such as ablation treatment of atrial fibrillation, guidance of atrial septal puncture. Current commercially available ICE systems offer monoplanar imaging, i.e. imaging in a 2-D plane, acquired using a mechanically rotating single transducer element, or with a linear phased array of elements. With these 2-D imaging configurations, it is often difficult to orient the imaging catheter in such a way that both the cardiac anatomy and interventional device are aligned in the same imaging plane. The development of a real-time three-dimensional (RT3-D) ICE system addresses these shortfalls. This work describes the design, simulation, fabrication and testing of miniaturized, 2-D phased array transducers mounted in the tips of catheters to enable RT3-D ICE. The transducers are constructed on multi-layer polyimide interconnect circuits and incorporate high density cabling interconnections within the catheter lumen. Advances in interconnection and cabling technology are described which have enabled up to a 5X increase in channel density over previous designs. These advances have facilitated both the miniaturization of the devices and image quality improvements necessary for the devices to become clinically useful. Several designs are discussed, including a 9 Fr (3.0 mm O.D.), 70 element 7 MHz side-viewing 2-D catheter array on a silicon substrate, and a 7 Fr (2.3 mm O.D.), 112 element 5 MHz side-viewing 2-D catheter array. RT3-D images of cardiac anatomy are presented, obtained during in vitro and in vivo studies with a sheep model. The design, fabrication and testing of forward-viewing RT3-D ICE probes is also described. The forward-viewing probes contain an additional working lumen through which interventional devices are delivered. The forward viewing probes contain 112 elements operating at 5 MHz. Combining the imaging catheter with a working lumen in a single device may simplify cardiac interventional procedures by allowing clinicians to easily visualize cardiac structures and simultaneously direct interventional tools in a RT3-D image.

David Mills

Multi-layer composite transducer arrays for improved signal-to-noise ratio and bandwidth in medical ultrasound

Increasing transducer bandwidth and signal-to-noise ratio (SNR) is fundamental to improving the quality of medical ultrasound images. In this dissertation, I describe a range of array transducers using new materials to improve both parameters. These new materials are stacked multi-layer composites of piezoelectric ceramics and polymer epoxies. The first transducer consists of 2 layers of posts (piezoelectric ceramic, PZT-5H) surrounded by soft epoxy. Experimentally, this 2 layer 1-3 composite transducer, yielded increased pulse-echo SNR by 5.2 dB and increased -6 dB bandwidth by a factor of 1.3, compared to the PZT-5H control. However, this structure required precision alignment of the posts greater than 90% of the post pitch (0.125mm) and a thin film bond line between the layers. Thus, I developed a new multi-layer structure that will not require post alignment and would ideally be fabricated using thick film technology capable of volume production. Starting from a PZT-5H multi-layer transducer, cuts were made through the top layer and back-filled with epoxy, forming a PZT/epoxy composite layer on top of PZT layers, referred to as a multi-layer composite hybrid transducer. Finite element simulations (FEM) showed that for a 2 MHz phased array element with a single acoustic matching layer, the 3 layer hybrid structure increases the pulse-echo SNR by 11 dB compared to a single layer PZT-5H control element and increases -6 dB pulse-echo fractional bandwidth from 46% to 65%, a factor of 1.4, for the hybrid element. I fabricated a hybrid transducer array and obtained improvement in SNR by 11 dB over a PZT-5H control and increased -6 dB bandwidth from 54% to 59%, a factor of 1.1. However, the material properties of currently available thick film multi-layer transducers limit the performance of these hybrid arrays and need further refinement before simulated results can be matched experimentally. Additional FEM simulations were performed to further improve the transducer array designs. These simulations showed that for a 2 MHz phased array element with a single matching layer, the improved 3 layer hybrid structure increased the pulse-echo SNR by 16 dB and -6 dB pulse-echo fractional bandwidth from 58% to 75%, a factor of 1.3, for the hybrid element versus the PZT-5H control. Analogous FEM simulations of single crystal material (PZN-8%PT), showed increased pulse-echo SNR by only 3.1 dB versus the PZT-5H control and a -6 dB bandwidth of 108%.

Edward Light (website)

Two-Dimensional Arrays for Real Time Volumetric Imaging

The design, fabrication, and evaluation of two dimensional array transducers for real time volumetric imaging are described. The transducers we have previously described operated at frequencies below 3 MHz and were unwieldy to the operator because of the interconnect schemes used in connecting to the transducer handle. Several new transducers have been developed using new connection technology. A 40 x 40 = 1600 element 3.5 MHz array with –6 dB fractional bandwidth of 63%, 50 Ohm insertion loss of –63 dB, and a –6 dB pulse-echo angular response of 35° was fabricated with 256 transmit and 256 receive elements. A 60 x 60 = 3600 element 5.0 MHz array with –6 dB fractional bandwidth of 50%, a 50 Ohm insertion loss of -69 dB, and a –6 dB pulse-echo angular response of 18° was constructed with 248 transmit and 256 receive elements. An 80 x 80 = 6400 element 2.5 MHz array with a –6 dB fractional bandwidth of 54%, a 50 Ohm insertion loss of -64 dB, and a –6 dB pulse-echo angular response of 14° was fabricated with 256 transmit and 208 receive elements. An 11 x 13 = 143 element 5.0 MHz array with a –6 dB fractional bandwidth of 50%, a 50 Ohm insertion loss of -64 dB , and a –6 dB pulse-echo angular response of 36° for use in an intracardiac catheter was constructed with 51 transmit and 30 receive elements. All the transducers were used to generate real time volumetric images in phantoms and in vivo using the Duke University real time volumetric imaging system which is capable of generating multiple planes at any desired angle and depth within the pyramidal volume.

Jesse Yen

Real-time rectilinear 3-D ultrasound imaging

Current real-time volumetric scanners use a 2-D array to scan a pyramidal volume consisting of many sector scans stacked in the elevation direction. This scan format is primarily useful for cardiac imaging to avoid interference from the ribs. However, a real-time rectilinear volumetric scanner with a wider field of view close to the transducer could prove more useful for abdominal, breast, or vascular imaging. This work describes the design and development of the first real-time rectilinear 3-D ultrasound scanner. The system featured three sparse 2-D array designs producing increasing image quality. The first array was a 5 MHz Mills cross array. It consisted of a 2 x 94 transmit arm and a perpendicular 94 x 2 receive arm. The Duke prototype 3-D ultrasound scanner, T4, was modified for real-time rectilinear volumetric imaging by changing the beamformer and display software. The field of view was 30 x 8 x 60 mm. The second transducer was a 5 MHz 2-D periodic array having 169 transmitters and 256 receivers. The receivers measured 0.3 x 0.3 mm and were spaced every 2.4 mm in azimuth and elevation. The transmitters had dimensions of 2.4 x 2.4 mm and a pitch of 2.4 mm. The Model 1 3-D scanner built by Volumetrics Medical Imaging (Durham, NC) was modified for real-time rectilinear volumetric scanning by changing the beamformer and display software. This array had an increased field of view of 30 x 30 x 60 mm. To improve the imaging quality of the periodic array, a new 5 MHz array was built having 1024 receivers and used 4:1 receive mode multiplexers. The receivers were 0.6 x 0.6 mm and had a staggered distribution to suppress grating lobes. Simulations indicated a 13 dB increase in pulse-echo sensitivity and another 13 dB decrease in grating lobe levels compared to the periodic array. Images of tissue phantoms and in vivo showed significant improvement in penetration and contrast compared to the periodic array. Images from a 1.5 cm diameter cyst in a phantom showed a 12 dB improvement in sensitivity and a 6 dB improvement in contrast.

Jason Zara

Ultrasound and optical scanners using micromachine technology

Numerous applications benefit from both high frequency ultrasound imaging and optical imaging techniques. High frequency ultrasound imaging is used in intracardiac and intravascular imaging as well as imaging the skin, eye, and small animals for genetic studies. Potential uses of optical scanners range from bar code scanners and laser printers in industry to corneal resurfacing and optical coherence tomography in medicine. A major issue in developing these systems is steering the acoustic and optical beams. This work describes the design, fabrication and testing of new types of ultrasound and optical beam scanning devices. These devices are fabricated from polyimide films using photolithography and use a linear polyimide MEMS actuator to mechanically scan the beams. This actuator, the integrated force array (IFA), is a network of hundreds of thousands of micron scale deformable capacitors that electrostatically contract with an applied voltage. Forward viewing tables pivoting on cantilever hinges and side scanning structures tilting on torsion hinges were fabricated on polyimide substrates with tables 1.125 mm and 2.25 mm wide. These structures were modeled using one dimensional beam theory and ANSYS finite element analysis prior to fabrication. For the ultrasound probes, PZT transducers were fabricated on these tables that operate at 20 MHz and 30 MHz and yielded insertion losses of 20-26 dB with fractional bandwidths of 34-49%. The transducer assemblies driven by MEMS actuators produced sector scans of 20-45° in air at resonant frequencies of 32 Hz to 90 Hz and sector scans in fluid of 6-8°. Both forward viewing and side scanning devices were then used in conjunction with a single channel high frequency ultrasound system to make real time images of wire phantoms. The optical scanning devices had gold-coated silicon mirrors mounted on the table in place of the PZT transducers. Only side scanning devices were developed for optical applications. For environmental protection, the devices were conformally coated with 500 angstroms of parylene. These devices demonstrated optical scan angles up to 146° for applied voltages up to ±50 volts. These devices were also used to steer a laser beam in a prototype bar code scanner to demonstrate functionality.

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