Patrick David Lopath

Hifu applications with feedback control

High intensity focused ultrasound heating is provided by applying therapeutic ultrasonic waves using an array of transducers. Cavitation is detected by using some or all of the transducers in the array to detect ultrasonic waves emanating from or reflected from the patients body or the interface. A feedback signal is generated based on such detection. If the feedback signal indicates the presence of cavitation, the therapeutic ultrasonic waves are terminated or altered. Obstacles to ultrasound transmission, such as bony structures or bubbles at the interface between the transducer array and the patient can be detected using the same transducers.

Relaxor-based single-crystal materials for ultrasonic transducer applications

Relaxor ferroelectric single crystals of Pb(Zn1/3Nb2/3)O3 (PZN), Pb(Mg1/3Nb2/3)O3 (PMN) and their solid solutions with normal ferroelectric PbTiO3 (PT) were investigated for ultrasonic transducer applications. Crystals offer adjustable properties not only by compositional tailoring but also by domain state engineering associated with different crystallographic orientation, which is not achievable in polycrystalline materials. Longitudinal coupling coefficients (k33) as high as 94% and dielectric constants (K3T) in the range of 3500 - 6000 were achieved with low dielectric loss (less than 1%) using <001> oriented rhombohedral crystals of (1-x)PZN-xPT and (1-y)PMN-yPT, where x less than 0.09 and y less than 0.35. Dicing direction as well as poling direction were critical for high coupling under laterally clamped condition. Dicing parallel to the (001) yields 90% of laterally clamped coupling (kbar) out of 94% longitudinal coupling (k33) for PZN-8%PT. On the other hand, samples diced parallel to (110) exhibited no dominant mode present. Thickness coupling (kT) as high as 64% and low dielectric constant (K3T) less than 600 with low loss (less than 1%) could be achieved using tetragonal crystals of (1-x)PZN-xPT and (1-y)PMN-yPT, where x greater than 0.1 and y greater than 0.4. The performance gains associated with these ultra-high coupling coefficients and range of dielectric constants are evident in relation to broader bandwidth and electrical impedance matching. Specifically, rhombohedral crystals offer the possibility of extremely broad bandwidth devices for transducer arrays and tetragonal crystals for single element transducers. Transducer simulation was performed using the KLM model. The pulse/echo response simulated a 124% bandwidth subdiced array element with a center frequency of 10 MHz. An optimized array design of the same geometry constructed of PZT 5H displays an 87% bandwidth.

Innovations in piezoelectric materials for ultrasound transducers

Piezoelectric materials lie at the heart of ultrasonic transducers. For transducers used in medical imaging (3¿7 MHz), PZT-5H ceramics offer high electromechanical coupling (k33 ¿ 75%), resulting in good bandwidth and sensitivity. As transducer arrays become smaller with increasing frequency, the development of high permittivity ( ¿RT > 7,000 vs. 3,400 for PZT-5H), piezoelectrics based on polycrystalline PMN-PT, provide improved electrical impedance matching. Advanced medical diagnostic techniques, including contrast and harmonic imaging, have taken advantage of the recent development in single crystal Relaxor-PTs that offer coupling k33¿s > 90% and subsequently, significant increases in bandwidth. For small animal, ophthalmology and cellular imaging, higher resolution is demanded, thus requiring transducers operational in the range of 20¿100 MHz. Advancements in ceramic processing include pore-free and fine-grain (¿1 micron) piezoelectric ceramics of PT and PZT, being an ¿enabling¿ technology, allowing the fabrication of high frequency single element and annular arrays. Innovations in the fabrication of high frequency arrays (¿ 30 MHz) include tape casting and sol-gel molding techniques. Of particular significance, DRIE (deep reaction ion etching), has demonstrated the ability to mill out ultrafine features, allowing 1¿3 crystal-polymer composites operational at frequencies ¿ 60 MHz, far beyond that achieved by current state-of-the-art dicing.

Ultrasonic transducers using piezoelectric single crystal perovskites

Solid solutions of the relaxor-based Pb(Zn/sub 1/3/Nb/sub 2/3/)O/sub 3/ (PZN) and Pb(Mg/sub 1/3/Nb/sub 2/3/)O/sub 3/ (PMN) systems with PbTiO/sub 3/ (PT) have been grown in single crystal form. The piezoelectric and dielectric properties of several compositions are reported along various crystallographic directions. The piezoelectric transducer model developed by Kimholtz, Leedom and Matthaei (KLM) was employed to study the behavior of these materials as ultrasonic resonators. Extremely high piezoelectric coupling coefficients (k/sub 33/>94%) and a range of dielectric constants (3000-5000) have been observed in these systems on the rhombohedral side of the morphotropic phase boundary (MPB). Relatively low dielectric constants (/spl sim/1000) and high thickness mode coupling (kt>63%) were observed as typical of tetragonal formulations. The ability to tailor the dielectric and piezoelectric constants with composition and crystal orientation allows the design of very large bandwidth ultrasonic transducers for applications ranging from medical diagnostic imaging to high frequency single element ultrasound backscatter microscopy.

High frequency 1-3 composite transducers fabricated from alkoxide-derived PZT fibers

Research in obtaining higher frequency 1-3 composite transducers for ultrasonic imaging applications has prompted the need for finer scale composite structures. Fine scale fibers of lanthanum doped lead zirconate titanate (PLZT) were derived from a continuous alkoxide based sol-gel spinning process for incorporation into 1-3 type composites. Single fiber filaments, ranging from 10 to 60 /spl mu/m in diameter, were fabricated. Microstructural and electrical characteristics of these fibers were determined under different pyrolysis and sintering conditions to obtain high quality dense fibers with dielectric constants up to 1100. Fibers were then incorporated into 1-3 composites using an epoxy matrix with volume fractions ranging from 10 to 40%. Resonance frequencies between 15 and 40 MHz could be obtained with thickness coupling coefficients up to 70%. Transducers were fabricated using these composites and evaluated for their pulse/echo characteristics. These transducers, without optimization, had center frequencies of 40 MHz with a bandwidth of 54% and a round-trip insertion loss of 47 dB.

Single crystal Pb(Zn/sub 1/3/Nb/sub 2/3/)O/sub 3//PbTiO/sub 3/ (PZN/PT) in medical ultrasonic transducers

Solid solutions of Pb(Zn/sub 1/3/Nb/sub 2/3/)O/sub 3/ (PZN) and PbTiO/sub 3/ (PT) have been grown in single crystal form to sizes large enough to permit the fabrication of a 5 MHz phased array. Transducer construction techniques employed differ from standard ceramic arrays. Crystallographic alignment is found to be a significant contributor to piezoelectric performance. The crystal structure and phase diagram play a major role in processing steps requiring heating such as poling and electroding. A complex composite structure is utilized to allow for efficient operation while maintaining structural integrity. The pulse-echo performance and insertion loss of this array are found to be superior to a similarly constructed PZT 5H array.

Clinical blood flow measurements using diffraction-grating transducers

Changing the phase or frequency of the signal driving a diffraction-grating transducer produces beams at known, multiple angles; these multiple angle beams can be used to make angle-independent Doppler measurements. This paper addresses three questions in developing a diffraction-grating instrument for measuring blood flow: how efficient and broadband can these transducers be, how close can such transducers be placed to blood vessels, and can the Doppler spectrum shape be used to determine the velocity profile.

Relaxor‐based single‐crystal materials for ultrasonic transducer applications

Relaxor ferroelectric single crystals of Pb(Zn1/3Nb2/3)O3 (PZN), Pb(Mg1/3Nb2/3)O3 (PMN) and their solid solutions with normal ferroelectric PbTiO3 (PT) were investigated for ultrasonic transducer applications. Longitudinal coupling coefficients (k33) as high as 94% and dielectric constants (K3T) in the range of 3500–6000 were achieved with low dielectric loss ( 0.1 and y>0.4. The performance gains associated with these ultra‐high coupling coefficients and range of dielectric constants...

High frequency transducers for ultrasonic backscatter microscopy

Very high frequency (VHF) ultrasound (> 20 MHz) has recently gained much attention as an effective non-invasive means to diagnose ocular and dermatological lesions. These ultrasonic backscatter microscopy systems have advanced rapidly with the electronics industry; however the VHF transducers are often the limiting factor in the overall image quality. This overview examines a number of the issues facing the high frequency transducer designer, including active material selection, passive components, acoustic and electrical matching and accurate characterization. Modified lead titanate was used as an active material to examine the use of a transmission line transformer to improve the electrical match between 25 and 40 MHz transducers and 50 (Omega) electronics. At 40 MHz, the transformer was shown to improve response only modestly, increasing bandwidth a few percent and insertion loss a few dB. At 25 MHz, -6 dB bandwidth improved almost 60%; however, peak sensitivity increased only 1 dB. PZT fiber composites were investigated to determine the effect of various volume fractions and passive backings on high frequency response. Parylene C was shown to be an effective matching layer for these composites, improving bandwidth of the 30% volume fraction transducers by 20% and sensitivity by 5 dB. Finally, a test setup, taking advantage of state of the art in linear positioners, is presented, addressing the issues pertinent to high frequency transducer characterization.

Single-crystal PZN/PT transducers

Single crystal Pb(Zn113Nb2i3)03 (PZN) I PbTiO3 (PT) solid solutions have been investigated as the active materials in medical diagnostic transducers. The compositionally controlled properties of this crystal system allow the application of this material to transducers which span the frequency range of modern diagnostic transducers. Phased array elements operating at 5MHz were constructed from rhombohedral PZN I 4.5% PT single crystals and compared to PZT 5-H elements. The single crystal element displayed similar sensitivity over a broader bandwidth response. Transducer construction techniques employed differ from standard ceramic arrays. Crystallographic alignment was found to be a significant contributor to piezoelectric performance. The crystal structure and phase diagram play a major role in processing steps requiring heating such as poling and electroding. Emphasizing the versatility of the PZN/PT single crystals, a 20 MHz single element was also constructed, using tetragonal PZN /12% PT. Its response was compared to theoretical pulseechoes generated via the KLM model.