Clinical applications of ultrasound have increased as ultrasonography has progressed from its early images to sophisticated diagnostic options. A brief overall view of current general medical applications of ultrasound imaging is presented, and three general areas are discussed in greater detail.

Advances in ultrasound transducer technology can lead to improvements in the assessment of skin disease. In clinical research, noninvasive methods of assessing skin structure in both health and disease are clearly needed. Ultrasound imaging of the skin before and after treatment can add on important objective assessment in the clinical evaluation of new therapeutic agents that are designed to reverse disease- induced changes in the skin. The goal of this paper is to review the current applications and drawbacks of ultrasound technology in the evaluation of skin disease.

Echography has become a valuable diagnostic tool in ophthalmology. Ultrasonic biomicroscopy (UBM) in particular may be applied to the evaluation of small lesions of the anterior segment of the eye. Disease processes such as conjunctival and iris melanoma, other forms of neoplasia, intraocular cysts, narrow angle glaucoma, and intraocular foreign bodies can be diagnostically evaluated and followed longitudinally by UBM. Combining UBM with spectroscopy may become useful in determining cell type origins of a variety of tumors. Eye banking also has an increased need for UBM in corneal tissue banking. The recent development of the Laser In Situ Keratomileusis procedure has allowed corneal surgeries to create a partial thickness flap of tissue in the cornea, remove tissue from the base of the cornea with excimer laser ablation, and replace the hinged flap. This causes a substantial change in refractive error while thinning the cornea and leaving an interface within the corneal stroma. The ability to detect this type of surgery is essential in eye banking. Ultrasonic pachymetry to determine central thickness and biomicroscopy to detect the presence of an interface are essential in avoiding the use of these corneas for transplantation purposes. Determining the topography of the preserved corneas is another potential application for ultrasonography. Using this information to reduce optical aberration after transplant is crucial in improving visual performance post transplantation. A review of the anatomy of the eye, pathology of ocular diseases relevant to UBM, and principles of eye banking will be presented.

PZN-PT single crystals exhibit an ultra-large longitudinal bar mode coupling coefficient (k₃₃) of approximately 0.94. The thickness mode coupling coefficient (k_t), however, is just comparable to soft PZT ceramics. In order to utilize the advantages of single crystals, it is desirable to incorporate them into a 1 - 3 composite structure. The single crystals possess a very large effective radial mode-coupling coefficient (k_p) and are elastically soft compared with PZT ceramics. As a result, 1 - 3 single crystal polymer composites exhibit a very large k_p of approximately 0.7 and only moderately enhanced k_t (approximately 0.8) when typical epoxies are used as the matrix material.

The ferroelectric copolymer polyvinylidene fluoride- trifluoroethylene, P(VDF-TrFE), has been shown to possess electrical and acoustic properties of interest for high frequency ultrasonic transducer applications. It has been used successfully in high frequency (10 - 200 MHz) medical imaging and nondestructive testing applications where its inherent properties of low acoustic impedance, high compliance, and broadband acoustic properties outweigh its poorer dielectric and electro-mechanical coupling properties which are lower than electroceramics. This paper briefly reviews successful high frequency P(VDF-TrFE) ultrasonic transducer applications and focuses on the design and testing of ultrasound transducer devices based on new spin- cast P(VDF-TrFE) copolymer films. Particular attention is given to (PVDF-TrFe) materials characterization and selection, ultrasonic transducer design and device packaging for high frequency applications. Ultrasonic testing of such high frequency transducers is also briefly discussed.

In this paper, the elastic properties of passive materials (matching, backing and lens materials) for ultrasound transducers are explored at room temperature in the frequency range of 25 - 65 MHz using the ultrasonic spectroscopy method. Alumina/EPO-TEK 301 and tungsten/EPO- TEK 301 composites were fabricated and measured. Experimental results display a monotonic rise in acoustic impedance of the composites with the addition of the particle filler. However, there was an attenuation peak occurring at about 8% volume fraction of particle filler. The acoustic impedance of the compositions was modeled. And additional passive materials were fabricated and measured. The measured results showed that materials having high attenuation also had large velocity dispersion, and low attenuation materials displayed low velocity dispersion.

Utilizing transducers with center frequencies and bandwidths both up to 100 MHz high frequency ultrasound allows high resolution imaging in fluids and tissue with a resolution down to approximately 10 micrometers . In addition to the increased resolution for medical imaging the backscatter properties of biological tissue are of considerable diagnostic advantage which change more rapidly with increasing frequency than the spatial resolution does. Because of the increasing attenuation of ultrasound in tissue at higher frequencies only small near surface areas can be imaged. For applications in dermatology recent research activities and instrument developments have included B-Scan systems (skin: thickness of layers, tumors, inflammatory diseases), flow visualization concepts (diagnosis of the cutaneous microcirculation), and tissue characterization (tumorous skin areas). As transducer and array technology has limitations high frequency imaging systems mainly utilize mechanically scanned single element transducers. They require special scanning procedures as well as signal processing techniques in order to optimize resolution, range, and signal-to-noise ratio. The paper will give an overview of these techniques and will also present some examples of applications in dermatology.

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.

Methods for fabricating and modeling high frequency 2-2 composites and arrays are presented. The composites are suitable for arrays and small aperture single element devices operating above 20 MHz. Coupling coefficients above 0.65 and lateral mode frequencies near 60 MHz were achieved with this composite. Backing and matching materials were prepared to provide up to 70% bandwidth and coaxial cable was used to impedance match the elements to a 50 ohm source. A TPX lens was fabricated and bonded to the face to provide focusing in the elevation direction. Three prototype 4 element 30 MHz linear arrays were designed and built. The designs were analyzed in a time domain finite element analysis program and excellent agreement between theory and experiment was achieved.

We have previously described 2D arrays of several thousand elements operating up to 5.0 MHz for transthoracic cardiac imaging. Lately, there has been interest in developing catheter based intracardiac imaging systems to aid in the precise tracking of anatomical features for improved diagnoses and therapies. We have constructed several arrays for real time intracardiac volumetric imaging based upon two different designs; a 10 X 10 equals 100 element 5.0 MHz forward looking 2D array, and a 13 X 11 equals 143 element 5.0 MHz 2D array for side scanning applications.

Trans-Esophageal Echocardiographic probes were introduced around 1982. The development ranges over the years from single plane, phased array transducers at low frequency to today's multiplane probes with a large number of elements. Further miniaturization and higher frequencies allow paediatric applications. Recent development includes a probe with 48 elements at 7 MHz with a shaft diameter of only 5 mm.

The influence of the mechanical coupling between the elements (cross-coupling) on the final performances of a piezoelectric array transducer is a well-known matter. Different solutions have been proposed to reduce, as much as possible, its effects. The approaches studied are not very generalist, because normally they are related to a specific transducer design. The model we proposed is based on a 2D model of the array, which is able to describe the vibration of each element (piezoceramic and polymer) both in the radiation (thickness) and the lateral (width) direction and, therefore, to take the coupling between the elements into account.

A new apodization technique to enhance the contrast resolution of medical ultrasonic imaging systems is presented. It can easily be implemented on any commercial transducer array. The array aperture windowing in the transmitting mode is performed without modifying the driver voltage over the whole array, but only varying the driving pulse length from an element to another. A design curve has been determined which transforms any conventional amplitude apodization into a `time apodization'. Experimental and numerical results are given for a 3.5 MHz convex array and for a 2.5 MHz phased array. They demonstrate the effectiveness of this method in reducing the off-axis energy in the radiated pattern. Moreover, they point out that `time apodization' improves also the range resolution, since the overall length of the ultrasonic pulse transmitted by the probe is reduced.

Our objective in apodizing piezoelectric ceramic discs was to produce discs that vibrate more intensively in the central region than in the region near the edge in order to generate acoustic fields with minimum diffraction effects. A spherical poling electrode was used to format the electrical field across the ceramic disc in order to achieve a polarization stronger in the central region and weaker in the edges. The electrode radius was previously determined by simulation with finite element method. The frequency spectrum of the apodized ceramic discs showed that the resonance and the anti-resonance frequencies shifted to larger values. Ultrasound transducers were constructed with the apodized ceramics and with normal commercial ceramics in order to compare their acoustic fields. The apodized transducers showed an average value of the electromechanical coefficient of 0.576 while for the non-apodized transducers this value was 0.597. Their outputs were measured in a water tank, with a point hydrophone, and showed that the time duration of the pulses generated by the apodized transducers were shorter than the ones generated by the conventional transducers, even though the acoustic pressure output intensities were similar. We have also mapped their apodized and non-apodized transducers. The far field of the apodized transducers showed a smoother decay and extended to larger distances from their face, compared to that of the non- apodized transducers.

Acoustic bullet waves are a class of solutions of the scalar wave equation which have the property of maintaining their shape upon propagation, without spreading in space and in time. This is a very interesting issue that suggests the use of acoustic bullets in many fields, among which there is acoustical imaging. In this work, a necessary and sufficient condition for the generation of acoustic bullets is presented and an impulse response for the case of axial symmetry is obtained. These aspects shed light onto the peculiarities of acoustic bullets and give also hints for the practical realization of this kind of fields. Numerical simulations are also presented in order to analyze the departures from the ideal case when using a finite- dimensional aperture. In particular, the performances of 2D transducer arrays, sparse and not, are considered in the simulations.

A 256 element ultrasonic phased array was constructed from thermal treatment of deep seated tissue. The 1.1 MHz array had a 10 cm radius of curvature and a 12 cm diameter. The elements formed a planar projection grid of 0.65 X 0.65 cm² such that the focal range of the array was approximately +/- 1 cm from the natural focus of the array both in the focal plane and +/- 2 cm along the array axis. The array was driven with phased continuous wave signals to both shift individual foci and to create multiple focus patterns. The goals of this study were to demonstrate that an array with many elements has the ability to coagulate large volumes of deep seated tissue in a single sonication and to experimentally compare the in vivo thermal measurements of large focal volume sonications to those predicted in a simple simulation model. It was found that the array could coagulate thigh muscle volumes of 3 - 5 cm³ in a twenty second sonication.

A spatially phased `doily' transducer (patent pending) forms a steered beam along one axis of the transducer without the need for electronically applying phase shifts to an array of elements. In its simplest form, the transducer consists of two channels from spatially shaped cosine and sine apertures, where the sine channel is shifted 90 degree(s) and then combined with the cosine channel. The transducer produces a beam steered to a desired angle at a specific design frequency in either receive or transmit operation. Several `doily' transducers have been fabricated and tested using low-lateral mode transducer materials such as Polyvinylidene Fluoride and 1 - 3 composite, with electroplated electrodes to form the cosine and sine apertures. The apertures were also geometrically shaped to suppress sidelobes along the steering axis. These `doily' transducers formed beams that were steered as far as 54 degree(s) with typical sidelobe levels of -20 dB.

A 96-channel phased array probe using a single-plate 0.91Pb(Zn_1/3Nb_2/3)O₃-0.09PbTiO₃ (PZNT 91/9) single-crystal transducer with dimensions of 14 X 20 mm has been fabricated to realize greater sensitivity and broader bandwidth properties. The center frequency of the probe, 3.5 MHz, was selected to include the bandwidths of two conventional lead zirconate titanate (PZT) ceramic probes with center frequencies of 2.5 and 3.75 MHz. A solder paste was used to connect a flexible printed circuit to the PZNT 91/9 transducer. The echo amplitude of the PZNT 91/9 probe revealed good linearity in a practical voltage range. The echo amplitude of the PZNT 91/9 probe is about 6 dB higher than those of the two PZT probes, and the fractional bandwidth is 30 and 25 percentage points wider, respectively. Ultrasonic image obtained by the PZNT 91/9 probe realized both the penetration of the 2.5-MHz PZT probe and the resolution of the 3.75-MHz PZT probe.

Diffracting grating transducers are interdigitated grating- like structures of thin elements spaced at a few wavelengths apart. By changing the phase or the frequency of the driving signal, two beams whose angle differs by a known quantity are produced. Doppler measurements in the blood stream at the two beam angles enable the estimation of angle- independent blood velocity. The performance of diffracting grating transducer in term of efficiency and bandwidth has been optimized by finite element analysis. Two different arrays operating at 5 MHz and 10 MHz were fabricated and modeled. The simulated and measured insertion losses for each transducer were compared. To broaden the bandwidth, matching layers were included in the design. The insertion losses for different driving modes were also computed. The results may shed light as to how the performance of the diffracting grating transducer may be improved to make better angle independent Doppler velocity measurements.

The tonpilz configuration is applied to a transducer operating in the megahertz frequency range. The KLM model is used to design the transducer using readily available components. The construction techniques used are the same as those applied to standard high frequency transducers. Modeled and measured pulse-echo results display a high level of agreement, but impedance and sensitivity comparisons are less promising.

It is widely accepted that a 2D array would be advantageous in medical ultrasound imaging. Such an array would be steerable in both the azimuth and elevation directions. One of the limitations on the practicality of 2D arrays is the electronic channel count. Simple brute force extension of conventional systems to such large systems is not practical. Increasing the number of connections to the transducer elements through the coaxial cable to the probe becomes prohibitive. Increasing the electronics of conventional beamforming systems by a factor of four or eight would be expensive and excessively power consuming. By duplexing, it is possible to double the number of effective channels, however, there is a need for further reduction in the number of channels needed to achieve a practical 2D array.

A flexible test system is described that has the capability to characterize very high frequency ultrasound transducer arrays up to 128 elements. The system has the capability of testing single element transducers as well as groups of array elements. The RF front end electronics consists of pulser circuit, preamp and time gain compensation circuit. The pulser circuit is a high voltage, high speed, switching circuit designed for -60 V pulse amplitude and pulse width as low as 10 ns. The preamp is a low noise, wide bandwidth amplifier and the time gain compensation circuit has a low noise figure and a bandwidth of 75 MHz. Custom, miniaturized PCB's have been fabricated and tested for the RF electronics. The data acquisition system has the capability to synchronously sample 8 channels at 250 MHz, or 16 channels at 125 MHz with 8 bit resolution. Multiplexing and demultiplexing units have been designed and tested for all the 128 channels. The demultiplexers are suitable for frequencies up to more than 100 MHz, and the multiplexers have a bandwidth of 700 MHz with good off isolation and cross talk. The multiplex/demultiplex architecture gives the test system the capability to perform synthetic aperture processing as well as dynamic apodization. An adapter board interfaces the external components to the PC (digital I/O card). A software control structure for control and synchronization of the system components for 128 elements has been designed and developed. Results are shown for the characterization of individual 50 MHz transducers as well as element responses for a 30 MHz array.

A handheld ultrasound imaging device, one that weighs less than five pounds, has been developed for diagnosing trauma in the combat battlefield as well as a variety of commercial mobile diagnostic applications. This handheld device consists of four component ASICs, each is designed using the state of the art microelectronics technologies. These ASICs are integrated with a convex array transducer to allow high quality imaging of soft tissues and blood flow in real time. The device is designed to be battery driven or ac powered with built-in image storage and cineloop playback capability. Design methodologies of a handheld device are fundamentally different to those of a cart-based system. As system architecture, signal and image processing algorithm as well as image control circuit and software in this device is deigned suitably for large-scale integration, the image performance of this device is designed to be adequate to the intent applications. To elongate the battery life, low power design rules and power management circuits are incorporated in the design of each component ASIC. The performance of the prototype device is currently being evaluated for various applications such as a primary image screening tool, fetal imaging in Obstetrics, foreign object detection and wound assessment for emergency care, etc.

A simple yet novel method has been developed which is useful for visualizing pressure distributions in acoustic wave fields. The method utilizes the power of contemporary computers to store entire waveforms from the numerous points interrogated in a planar scan. The most useful type of planar scan for the concept is made in the xz plane. The z- axis typically corresponds to the beam axis of a transducer, and the x-axis is an arbitrary axis normal to it. The technique to be discussed will yield moving images of instantaneous acoustic pressure in the xz plane. The concept of looking down at the surface of a body of water, with ripples moving away from a wave generator, is a good analogy.

Ensuring that an ultrasound imager complies with all aspects of the FDA 510(k) regulations is a complex task, because there are hundreds of thousands of discrete operating conditions available to the sonographer. Accurate measurements require `peaking' of the hydrophone in azimuth and elevation, and acquiring data as a function of range. Thus it is necessary to characterize the acoustic field in 3 dimensions. It is simply impossible to measure the imager's output under each condition, so algorithmic means are needed to reduce the dimensionality of the problem. Even when simple linear dependencies (such as pulse repetition frequency) are taken into account, the time to obtain Thermal and Mechanical Indices for a new probe is formidable. We must also repeat the experiment each time changes are made to the transmitter hardware, or its waveforms. In this paper, we explore how to speed the acquisition of data used for estimation of the output labeling parameters by guiding the water-tank measurements with a beam simulator.

Creare is developing microfabrication techniques to manufacture low-cost, multi-dimensional ultrasonic transducer arrays with single- and multi-layer piezoelectric elements for low impedance and high sensitivity. The manufacturing approach is scaleable for fabrication of transducer arrays in the frequency range of 10 - 50 MHz in dense or sparse array configurations. Our approach employs the following processes: (1) Physical Vapor Deposition (PVD or sputtering) of high-quality, piezoelectric films using reactive sputtering of metallic targets and (2) Novel use of state-of-the-art photolithography and masking to provide the interlayer electrodes, element interconnections, and array element fabrication. To date, Creare has successfully demonstrated that piezoelectrically active thick films of PZT material can be deposited by using a reactive sputtering approach. In addition, these thick, multi-layer PZT films have been formed into high aspect ratio elements using dicing to fabricate a 12 MHz transducer. Array designs based on these films show that expected performance should meet the requirements for high resolution biomedical imaging.

Broadband signals are commonly used in ultrasonic spectroscopy to measure the frequency dependent attenuation characteristics of lossy solid media. Compared to narrowband signals, broadband signals are preferred since they do not require tedious frequency scanning and extensive data reduction efforts. Typically these broadband signals take the form of a pulse. Although the spectral range of a pulse is wide, the spectral resolution is limited by the duration of the signal. By employing signals with large time- bandwidth-products, the overall accuracy and resolution of ultrasonic spectroscopy can be improved. Expressions for the interaction of longitudinal waves, with large time- bandwidth-product, and isotropic materials are developed. The approach is effective for evaluating material with signals optimized for a frequency resolution and range of interest, but can also be used when thin materials (<EQ (lambda) ) are characterized by pulse signals. Using these expressions, the acoustical properties of wave-speed and attenuation can be determined when density and thickness are measured. Explicit account is made for diffraction corrections, multiple echo contributions, and interface scattering losses. The formalism is compared with the traditional analysis approach to illustrate the improved accuracy of the new technique, detailing where diffraction correction and multiple echo effects can become significant. Measured attenuation spectra are presented for common plastic materials as well as for a castable polyurethane commonly used in ultrasonic transducer fabrication.

We report a set of measured material properties of Pb(Zn_1/3Nb_2/3)O₃-PbTiO₃(PZN-PT) multi-domain single crystals. While deriving these properties, it was assumed that the rhombohedral PZN-PT crystals have 4 mm symmetry when it is poled along the [001] of the original cubic direction. Samples with three pair of faces oriented in [001], [010] and [100] were made for the resonance measurements and additional samples with the orientation of [001], [110] and [110] were made for the ultrasonic measurements. Complete sets of the material constants were obtained for PZT-4.5%PT and PZN- 8%PT. The coupling coefficient k₃₃ is 94% for the PZN- 8%PT and 90% for the PZN-4.5%PT. A piezoelectric constant d₃₃ of 2300pC/N was obtained for the PZN-8%PT sample. These values are very encouraging for designing broadband ultrasonic transducers. Based these measurement results, the directional dependence of phase velocities and electromechanical coupling coefficients were analyzed. The calculations indicate that the coupling coefficient k₃₁ and k₃₃ has a maximum in the [110] and [001], respectively, a result consistent with the direct measurements. Errors in these measurements were also analyzed.