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The stress patterns—two shears and one direct stress—that are maximally coupled to the electric field in piezoelectric ceramics are identified. This is achieved by a simplifying initial choice of coordinate axes in the ceramic's thermodynamic phase space and by a subsequent sequence of elementary coordinate changes—rotations and scale changes; simple physical arguments lead to the new coordinates. The utility of this perspective is pointed out in two important applications of piezoceramics in acoustic transducers: naval hydrophone sensors and pulse-echo ultrasonic transducers used in medical diagnostic imaging.

Combining a piezoelectric ceramic and a passive polymer to form a piezocomposite allows the transducer engineer to design new piezoelectrics that offer substantial advantages over the conventional piezoelectric ceramics and polymers. The rod composite geometry provides materials with enhanced electromechanical coupling and with acoustic impedance close to that of tissue; these factors yield transducers for medical ultrasonic imaging with high sensitivity and compact impulse response. The dice-and-fill technique produces piezocomposites that can be readily formed into complex shapes to facilitate focusing the ultrasonic beam. Proper design of the rod spacing yields materials that exhibit low crosstalk between array elements formed by patterning the electrode alone, without cutting between the elements. In this way, curved annular arrays have been made that provide high-quality clinical images of substantial diagnostic value to physicians. Included is an extensive bibliography of papers documenting the role of piezocomposites in ultrasonic imaging transducers. >

The attenuation slope with frequency from the frequency decrease along the ultrasonic echo signal is evaluated in tissue. When the local frequency from the reflected waveform is evaluated, many data segments yield unreliable values because of interference between overlapping echo pulses. The authors advance an analysis based on the singular-value decomposition (SVD) of the Wigner distribution of the signal, which provides a criterion to filter out data segments corrupted by interference. When applied to clinical data, this SVD-Wigner filtering process reduces the scatter in the local frequency estimate sufficiently to produce a reliable estimate of the frequency slope along the signal with about one third of the data needed by conventional short-time Fourier techniques. The improvement is not larger because these clinical data were taken on a scanner with a large sample volume, so that most data segments are perturbed by interference artifacts. Simulations show that in a more tightly focused system, with fewer scatterers contributing to the echoes, the reduction in required data can be appreciable. >

The authors report a transducer structure that combines two orthogonal phase arrays that enable the real-time scanning of two orthogonal sectors. This biplane phased array is formed by partially dicing the opposite faces of a composite piezoelectric plate in orthogonal directions. Individual elements of the orthogonal arrays show broad radiation patterns close to the theoretical expectations for isolated elements. Arrays formed by patterning the electrode alone, without cross dicing, show a narrower directivity pattern for a single element. This narrowing is a refractive effect due to the relatively high acoustic velocities in the composite material as compared with the sound velocity in the propagation medium. It is noted that the concept of the biplane phased array forms the basis for a new class of transducer systems that combine two transducer arrays in a single piezocomposite plate. >

The invention of methods to make new materials with an engineered microstructure that yields a negative Poisson ratio opens a new avenue to optimize the performance of piezoelectric-rod/polymer-matrix composites. Such negative Poisson's ratio materials can be used as the passive phase in the composite to redirect the external stress acting on the piezocomposite so that the resulting stress bearing on the piezoceramic rods produces a maximal piezoelectric response. To project the properties of such a piezocomposite, a simple physical model that assumes constant strain along the rods and constant stress in the perpendicular plane is used. Interesting opportunities to improve the performance of devices made from 1-3 piezocomposites are seen in results calculated for the thickness-mode electromechanical coupling constant relevant to pulse-echo ultrasonic applications and for the hydrostatic electromechanical coupling constant relevant to passive hydrophones. >

This overview describes the application of composite piezoelectric materials in acoustical imaging transducers. Attention is focused on one composite structure which has found particularly fruitful applications in acoustical imaging—the 1–3 piezocomposite structure consisting of long thin piezoceramic rods held parallel to each other by a polymer matrix. These piezocomposites may be viewed as ‘new’ materials with a set of ‘effective’ homogeneous materials properties whenever the spatial scale of the constituents in the composite structure is smaller than the wavelength of sound. From knowledge of the sub-wavelength ‘microstructure,’ we obtain an intuitive understanding of the origin of the piezocomposite’s properties, and see how the material properties can be tuned to specific transducer application needs. Commercial applications in medical ultrasonic diagnostics, non-destructive testing, and undersea imaging illustrate the practical exploitation of 1-3 piezocomposites.

Piezoelectric materials lie at the heart of ultrasonic transducers. These materials convert electrical energy into mechanical form when generating an interrogating acoustic pulse and convert mechanical energy into an electric signal when detecting its echoes. This paper first surveys the piezoelectric materials in current use: piezoceramics, such as barium titanate, lead zirconate titanate, and modified lead titanate; piezopolymers, such as polyvinylidene difluoride and its copolymer with trifluroethylene; and piezocomposites, consisting of piezoceramic rods in a passive polymer matrix. Each material system has properties which commend them for use in the present single element transducers, annular arrays, sequenced linear arrays, and steered phased arrays. Looking to the future, new transducer possibilities are opening up due to recent piezoelectric material developments, such as, for example, synthesis techniques for fine-grained high-density piezoceramics, electrostrictive relaxor ferroelectric ceramics, novel piezoceramic forming methods, piezoceramic fiber synthesis, piezoceramic/metal multilayer structures, composite acoustoelectric materials, ferroelectric thin film growth and processing, and new piezopolymers. These innovations lead to fabrication of conventional transducers at high frequencies, fine-scale piezocomposites, 11/2-D and 2-D arrays, small intravascular transducers, as well as provide opportunities for new ultrasonic imaging techniques, using pitch-catch and non-resonant traveling wave transducers.

1–3 piezoelectric-rod/passive-matrix composites offer advantages over the conventional piezoceramics and piezopolymers for the pulse-echo transducers used in medical ultrasonic imaging. Their benefits include high electromechanical coupling, acoustic impedance close to that of tissue, a wide range of dielectric constants, low dielectric and mechanical losses, an adjustable sound speed, low coupling to spurious oscillations, ease of subdividing into acoustically isolated array elements, and formability into complex curved shapes. Not all benefits are achieved simultaneously. In designing a material for a specific application, the material engineer can choose the piezoceramic, the passive matrix, their relative proportions and the spatial scale of the composite. We delineate the trade-offs in designing piezocomposites which enhance the performance of present ultrasonic transducers as well as make new transducer designs feasible.

We suppress the effects of noise and extract the intrinsic frequency variation from an ultrasonic echo using a singular value decomposition of the Wigner distribution of the signal. This method performs a non-linear filtering on the echo signal to reduce greatly the interference effects between overlapping echoes - correlated noise, as well as uncorrelated electronic noise. We can then evaluate the local frequency both accurately (i.e. without bias by noise) and efliciently (i.e. with a small data set). This analysis is carried out on the Wigner time-frequency function calculated from a segment of echo data. With the help of the singular value decomposition of thc Wigner distribution, we reject echo segments corrupted by interference and noise. In this way, correlated interference is suppressed much more efficiently than by averaging. We illustrate this approach on the traditional problem of estimating the attenuation slope from the downward shift in mean frequency with depth. Our analysis of simulated echoes shows that accurate estimates can be obtained from a single A-line. Moreover, we obtain the correct answer with two orders of magnitude less data than the conventional Fourier approach. The resulting savings in data acquisition time and computation time are substantial.

The performance of composite linear arrays with elements defined by electrode patterning alone is described. The prototype arrays (4.5 MHz, lmm pitch) show low cross coupling between neighboring elements, good sensitivity over a broad frequency range, and exceptionally short ringdown times. High quality images of a test object are produced by a composite array provided with electronic and mechanical lenses.

This paper surveys published research on the use of composite physicians. piezoelect.ric materials to make the acoustic transducers used in medical ultrasonic imaging. For medical imaging transducers, attention has focused on the 1-3, PZT-rod/polymer composite structure. The piezoelectric plates used in such ultrasonic transducers consist of a polymer matrix which holds together parallel thin rods of piezoelectric ceramic oriented perpendicular to the faces of the plate. By varying the properties, proportions and spatial scales of the polymer and piezoceramic constituents a rich variety of material properties are achieved. Of particular interest for medical imaging transducers is the ability to engineer materials whose electromechanical coupling is higher than that of conventional materials and whose acoustic impedance is close to that of tissue. Besides these basic advantages in material parameters. composites have properties that facilitate meeting other technological requirements. Flexible composite piezoelectrics can be formed into complex curved shapes for steering and focusing the acoustic waves. Transducer arrays can he made from composites by patterning the electrode alone -- cutting the piezoelectric is not required.

Single element transducers and linear arrays made from composite piezoelectric materials were characterized for application in medical ultrasonic imaging. The composite transducers showed a compact temporal response and high sensitivity over a broad frequency range. Prototype linear arrays with elements defined by an electrode pattern alone exhibited low acoustic cross talk between neighboring elements and overall performance equivalent to that of state of the art diced arrays.

The KLM transmission-line model is employed to analyze the performance of an ultrasonic transducer made from a composite piezoelectric material. The basic transducer parameters, including electrical and mechanical losses, are determined by analyzing the electrical impedance of the air-loaded composite disk in the vicinity of the thickness resonance. Using these parameters, the performance of a prototype 3 MHz composite transducer is calculated and found to be in good agreement with experimental data. On the basis of these results, the performance of the composite transducer with optimum acoustic and electrical matching is predicted. Our results reveal substantial promise for composite piezoelectric materials in making efficient broad-band ultrasonic transducers for medical imaging.

The spatial inhomogeneity of sound speed in tissue sets a fundamental limit on the ability to focus on ultrasonic beam in that tissue. The authors show how this speed inhomogeneity can be quantified from measurements of the differential angular scattering cross section. They give an expression for the statistical variations in acoustic path length as an integral of the correlation function-or, equivalently, the power spectrum-of density and compressibility fluctuations. Measurements of average differential scattering cross section yield values for the fluctuations in a model random medium and calf liver. Since the measured power spectra extend over only a finite wave-number window, they are fit to analytic functions to extend them over the whole region for which the integral must be evaluated. Results from the analysis of the model medium validate this approach, and those from calf liver provide insights into the limits of ultrasonic imaging resolution in living tissue. >