Showing papers in "IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control in 1996"

A surface micromachined electrostatic ultrasonic air transducer

Abstract: Airborne ultrasound has many applications such as, ranging, nondestructive evaluation, gas flow measurement, and acoustic microscopy. This paper investigates the generation and detection of ultrasound in air at a few MHz. Conventional plane piston lead zirconium titanate (PZT) based transducers perform poorly for this application due to the lack of proper matching layer materials. Electrostatic, or capacitive, transducers promise higher efficiency and broader bandwidth performance. The device structure in this work consists of a capacitor where one plate is a circular silicon nitride membrane coated with gold and the other is a rigid silicon substrate. By applying a voltage between the membrane and the silicon substrate, an electrostatic force is exerted on the membrane which sets it in motion, thus generating a sound wave in air. Presented here is an electrical equivalent circuit model for electrostatic transducers which is based on the early work of Mason (1942). The electrostatic transducers were designed and constructed for operation at 1.8 and 4.6 MHz. The transducers were fabricated using standard micromachining techniques. An optical interferometer was used to measure the peak displacement of the 1.8 MHz electrostatic transducer at 230 /spl Aring//V. A transmit-receive system was built using two electrostatic transducers. The system had a signal to noise ratio of 34 dB at a transducer separation of 1 cm. Each transducer had a 3-dB bandwidth of 20%, and a one-way insertion loss of 26 dB. There is excellent agreement between the measured device performance and theoretical predictions.

Young's modulus measurements of soft tissues with application to elasticity imaging

Abstract: Ultrasound elasticity imaging is a promising method that may eventually allow early detection of many tissue pathologies. However, before elasticity imaging can be applied to its numerous potential clinical applications, the quantitative accuracy of tissue elasticity measurements must be established. Simple 1-D ultrasound elasticity measurements were performed on muscle and liver and compared with independent and established mechanical measurements to investigate both the accuracy and consistency of ultrasound elasticity measurements. In addition, some interesting properties of soft tissue and aspects of the measurement process which should be considered in elasticity measurements are discussed.

Ultrasonic beam focusing through tissue inhomogeneities with a time reversal mirror: application to transskull therapy

Abstract: Time reversal of ultrasonic fields allows a very efficient approach to focus pulsed ultrasonic waves through lossless inhomogeneous media. Time reversal mirrors (TRMs) are made of large transducer arrays, allowing the incident field to be sampled, time reversed, and reemitted. Time reversal method corrects for phase, amplitude, and even shape aberration and thus, is more efficient than time shift compensation techniques. However, this technique needs the knowledge of the Green's function of a dominant scatterer available in the medium. Aberration correction for ultrasonic hyperthermia could be achieved by combining TRM with an artificial acoustic source or sensor implanted inside the treatment volume. In this paper, time reversal method has been experimentally applied to the focusing through the skull bone. It is shown that the skull induces severe attenuation of ultrasound and reduces the efficiency of the time reversal approach. Then, an amplitude correction method is proposed to focus through an attenuating layer located close to the array. This method consists in inversing the amplitude modulation and then time reversing these signals. Finally, this method is combined with numerical backpropagation to compensate for an attenuating layer located some distance away from the transducer array.

A 100-MHz ultrasound imaging system for dermatologic and ophthalmologic diagnostics

Abstract: A major design problem concerning high-frequency broad-band ultrasound imaging systems is caused by the strong dispersive attenuation of the tissue, which gives rise to images with inhomogeneous resolution and poor signal to noise ratio (SNR). To address the noise problem, strongly focused transducers with high energy density in a narrow focal region are utilized, which also provide more isotropic images due to improved lateral resolution. To account for the short depth of the focal area two suitable imaging conceptions are used: 1) synthetic aperture concept and 2) B/D-scan concept. To avoid the inhomogeneity of the images, different transmitter signals for each depth are applied, which are pseudoinversely prefiltered according to the transfer function of the tissue. To gain signal energy required for inverse filtering, a pulse compression technique with nonlinearly frequency modulated chirp signals is utilized. These procedures have been implemented in an ultrasound imaging system, which has been developed in the authors' laboratory for eye and skin examinations, It can be used with transducers in a frequency range from 20 to 250 MHz.

Transcutaneous measurement and spectrum analysis of heart wall vibrations

Abstract: For the noninvasive diagnosis of heart disease based on the acoustic and elastic characteristics of the heart muscle, it is necessary to transcutaneously measure small vibration signals, including components with an amplitude of less than 100 /spl mu/m, from various parts of the heart wall continuously for periods of more than several heartbeats in a wide frequency range up to 1 kHz. Such measurement, however, has not been realized by any ultrasonic diagnostic methods or systems to date. By introducing the constraint least-square approach, this paper proposes a new method for accurately tracking the movement of the heart wall based on both the phase and magnitude of the demodulated signal to determine the instantaneous position of the object so that the vibration velocity of the moving object can be accurately estimated. By this method, small vibrations of the heart wall with small amplitudes less than 100 /spl mu/m on the motion resulting from a heartbeat with large amplitude of 10 mm can be successfully detected with sufficient reproducibility in the frequency range up to several hundred Hertz continuously for periods of about 10 heartbeats. The resultant small vibration is analyzed not only in the time domain, but also in the frequency domain. As confirmed by the preliminary experiments herein reported, the new method offers potential for research in acoustical diagnosis of heart disease.

Optimizing the radiation pattern of sparse periodic linear arrays

Abstract: We have developed a method for designing sparse periodic arrays. Grating lobes in the two-way radiation pattern are avoided by using different element spacings on transmission and reception. The transmit and receive aperture functions are selected such that the convolution of the aperture functions produces a desired effective aperture. A desired effective aperture is simply an aperture with an appropriate width, element spacing, and shape such that the Fourier transform of this function gives the desired two-way radiation pattern. If a synthetic aperture approach is used, an exact solution to the problem is possible. However, for conventional imaging, often only an approximation of the desired effective aperture can be found. Different strategies for obtaining these approximate solutions are described. The radiation pattern of a sparse array designed using the effective aperture concept is compared experimentally with the radiation patterns of a dense array, and sparse arrays with periodic and random element spacing. We show that the number of elements in a 128-element linear array can be reduced by at least four times with little degradation of the beam forming properties of the array.

Typing of prostate tissue by ultrasonic spectrum analysis

Abstract: Prostate cancer is the highest-incidence cancer and second-leading cancer killer of men in the U.S. Diagnosis now relies virtually exclusively on core-needle biopsy, guided by transrectal ultrasound (TRUS). Because of the limitations of TRUS in detecting suspicious regions, biopsy often fails to sample cancer that is present or to determine that extracapsular cancer exists, which results in false-negative biopsies or inappropriate prostatectomies. Therefore, the authors conducted this study to investigate the use of spectrum analysis of radio frequency (RF) echo signals as a possible means of reducing the number of false-negative biopsies and inappropriate prostatectomies. This method utilizes databases of parameters derived from normalized power spectra of RF echo signals and histologically proven tissue types to determine ranges of parameter values associated with tissue types of interest. Typing an unknown tissue is performed by comparing the parameter values of the unknown to the value ranges of specific tissue types in the database. The authors' results provide encouraging preliminary discriminant-function distributions that suggest an excellent potential for differentiating cancerous from noncancerous prostate tissue far superior in terms of sensitivity and specificity than means now used to determine whether biopsy is required. In addition, the authors developed images using color to indicate the most likely tissue type throughout the tissue cross section as determined by comparisons with database parameter values. These images showed excellent correlation with histology.

Noninvasive real-time multipoint temperature control for ultrasound phased array treatments

Abstract: A method for noninvasively estimating spatiotemporal temperature changes in samples using diagnostic ultrasound, and using these as inputs to a multipoint ultrasound phased array temperature controller, is presented in this paper. This method is based on a linear relationship between the apparent tissue echo pattern displacements and temperature, as seen along A-lines acquired with diagnostic ultrasound when the sample is heated by external heating fields. The proportionality constant between echo displacement and temperature is determined by the local change in speed of sound due to temperature and the linear coefficient of thermal expansion of the material. Accurate estimation of the displacements and proportionality constant yields accurate calibrated high-resolution (1 mm spatial, sub-/spl deg/C) noninvasive sample temperature estimates. These are used as inputs to a multipoint temperature controller, capable of controlling ultrasound phased array treatments in real-time. Phantom and in vitro results show that the noninvasively estimated temperature values can effectively be used to control ultrasound hyperthermia treatments, almost replacing invasive thermocouple measurements. The mathematical background and assumptions of the noninvasive temperature estimator and the controller are presented in this paper, together with experimental results showing estimator and controller performance and limitations. To the best of our knowledge, this paper presents the first demonstration of real-time treatment control based entirely on noninvasive temperature estimates.

Optimizing the radiation pattern of sparse periodic two-dimensional arrays

Abstract: A method for reducing the number of elements in a 2-D array while minimizing degradation of the beam forming properties is described. The method relies on selecting a different arrangement of elements when the array is transmitting energy and when the array is receiving energy. The transmit and receive aperture functions are chosen to minimize the difference between the effective aperture of the sparse array and the effective aperture of a desired dense array. In a companion paper [see ibid vol. 43, pp. 7-14, 1996], the design of sparse linear arrays using the effective aperture method was described. Here, we extend this method to the design of 2-D arrays. Comparisons of the radiation patterns of a dense 2-D array and sparse 2-D arrays with random and periodic element spacing are given. Using the effective aperture method, we show that the number of elements in a 64/spl times/64 2-D array can be reduced by more than six times, and the elements in a 128/spl times/128 array can be reduced by more than 12 times, with little effect on the beam forming properties of the arrays.

Lateral displacement estimation using tissue incompressibility

Abstract: Using the incompressibility property of soft tissue, lateral displacements can be reconstructed from axial strain measurements. Results of simulations and experiments on gelatin-based tissue equivalent phantoms are compared with theoretical displacements, as well as estimates derived from traditional speckle tracking. Incompressibility processing greatly improves the accuracy and signal-to-noise ratio (SNR) of lateral displacement measurements compared with more traditional speckle tracking.

Noninvasive surgery of prostate tissue by high-intensity focused ultrasound

Abstract: Modern ultrasound transducer material and matching layer technology has permitted us to combine the ultrasound visualization capability with production of high-intensity focused ultrasound (HIFU) on the same ceramic crystal. This development has lead to the design of a transrectal probe for noninvasive surgery of prostate tissue by HIFU. The combined capability using the same ceramic crystal simplifies treatment planning, targeting, and monitoring of tissue before and during the HIFU treatment. This mechanically scanning transrectal probe was introduced for clinical use in 1992 for noninvasive surgery of the prostate to treat benign prostatic hyperplasia (BPH) condition. This paper reviews major steps progressing from conception to the present clinical trial status of the HIFU device. During these clinical studies generation of microbubbles and cavitation were observed. Data from microbubble generation, temperature monitoring in tissue, and autopsy of HIFU-treated animal prostates are presented. Results of human clinical studies are briefly summarized to indicate performance of the device.

Ultrasonic linear motor using surface acoustic waves

Abstract: The first success in the operation of an ultrasonic linear motor at HF band driving frequency using the Rayleigh wave is described. The substrate material is a 127.8/spl deg/ Y-cut LiNbO/sub 3/ wafer whose diameter is three inches. Four interdigital transducers (IDT's) are arranged to excite x- and y-propagation waves in both directions. The dimensions of the IDT are 25 mm aperture size, 400 /spl mu/m pitch, 100 /spl mu/m strip width, and 10 pairs. The operation area is about 25 mm square, The driving frequency is about 9.6 MHz in the x direction and about 9.1 MHz in the y direction. The most important point of the success is the shape of the contact surface and slider materials. For the contact materials, small balls about 1 mm in diameter are introduced to obtain sufficient contact pressure around 100 MPa. The use of ruby balls, steel balls, and tungsten carbide balls is investigated. Each slider has three balls to enable stable contact at three points. The maximum transfer speed is about 20 cm/s. The transfer speed is controllable by changing the driving voltage.

Sparse random ultrasound phased array for focal surgery

Abstract: Ultrasound phased arrays offer several advantages over single focused transducer technology, enabling electronically programmable synthesis of focal size and shape, as well as position. While phased arrays have been employed for medical diagnostic and therapeutic (hyperthermia) applications, there remain fundamental problems associated with their use for surgery. These problems stem largely from the small size of each array element dictated by the wavelength employed at surgical application frequencies (2-4 MHz), the array aperture size required for the desired focal characteristics, and the number of array elements and electronic drive channels required to provide RF energy to the entire array. The present work involves the theoretical and experimental examination of novel ultrasound phased arrays consisting of array elements larger than one wavelength, minimizing the number of elements in an aperture through a combination of geometric focusing, directive beams, and sparse random placement of array elements, for tissue ablation applications. A hexagonally packed array consisting of 108 8-mm-diameter circular elements mounted on a spherical shell was modeled theoretically and a prototype array was constructed to examine the feasibility of sparse random array configurations for focal surgery. A randomly selected subset of elements of the prototype test array (64 of 108 available channels) was driven at 2.1 MHz with a 64-channel digitally controlled RF drive system. The performance of the prototype array was evaluated by comparing field data obtained from theoretical modeling to that obtained experimentally via hydrophone scanning. The results of that comparison, along with total acoustic power measurements, suggest that the use of sparse random phased arrays for focal surgery is feasible, and that the nature of array packing is an important determinant to observed performance.

Ultrasound surgery: comparison of strategies using phased array systems

Abstract: In this paper the possibility of using phased array generated multiple-focus patterns to reduce the overall treatment time in ultrasound surgery while restraining prefocal heating was investigated. This was done by comparing through computer simulation the performance of different possible schemes, i.e., single-focus scans, multiple-focus scans, and simultaneous multiple focusing without scanning, when used to "ablate" a 10/spl times/10/spl times/10 mm/sup 3/ tissue volume 100 mm deep. In all cases, 41 foci were used to cover the treatment volume. Multiple-focus scans were arranged into nine groups which were scanned in a raster fashion, as with single-focus scans. Keeping the treatment time constant, the maximum intensities, maximum thermal doses, dose distributions, and prefocal necrosis zones for the different schemes were compared. It was found that the nonscanned simultaneous multiple-focus case required the smallest maximum intensity and dose, and resulted in the most even dose distribution. Single-focus raster scanning of individual lesions, as currently used with fixed-focus transducers, gave the worst results. These results show that multiple-focus patterns help considerably in reducing the maximum intensity and dose, and in generating a more even dose distribution assuming the same treatment time and prefocal heating. Alternatively, multiple-focus patterns can be used to significantly reduce treatment time while keeping the maximum intensity and prefocal heating below predetermined limits.

A new coded-excitation ultrasound imaging system. I. Basic principles

Abstract: A new approach to ultrasound imaging with coded-excitation is presented. The imaging is performed by reconstruction of the scatterer strength on an assumed grid covering the region of interest (ROI). Our formulation is based on an assumed discretized signal model which represents the received sampled data vector as a superposition of impulse responses of all scatterers in the ROI. The reconstruction operator is derived from the pseudo-inverse of the linear operator (system matrix) that produces the received data vector. The singular value decomposition (SVD) method with appropriate regularization techniques is used for obtaining a robust realization of the pseudo-inverse. Under simplifying (but realistic) assumptions, the pseudo-inverse operator (PIO) can be implemented using a bank of transversal filters with each filter designed to extract echoes from a specified image line. This approach allows for the simultaneous acquisition of a large number of image lines. This could be useful in increasing frame rates for two-dimensional imaging systems or allowing for real-time implementation of three-dimensional imaging systems. When compared to the matched filtering approach to similar coded-excitation systems, our approach eliminates correlation artifacts that are known to plague such systems. Furthermore, the lateral resolution of the new system can exceed the diffraction limit imposed on conventional imaging systems utilizing delay-and-sum beamformers. The range resolution is compared to that of conventional pulse-echo systems with resolution enhancement (our PIO behaves as a pseudo-inverse Wiener filter in the range direction). Both simulation and experimental verification of these statements are given.

Displacement and strain imaging of coronary arteries with intraluminal ultrasound

Abstract: Tissue elasticity can be estimated from displacement and strain images acquired under controlled deformation. We extend this approach for coronary arteries, deformed and imaged by an integrated angioplasty balloon and ultrasonic imaging probe. Because the lumen cross section of a severely occluded artery is not circular, we have also developed a technique to perform all motion computations in the reference frame of the lumen's geometric center. This coordinate system is independent of the imaging catheter and consequently referencing to this frame removes artifacts associated with probe motion within the balloon during deformation. Displacements and strains estimated by phase-sensitive correlation-based speckle tracking were used to distinguish arterial plaques in simulated coronary arteries of differing elastic moduli: hard, soft, and homogenous. We have also applied these methods to images of a homogeneous gelatin phantom collected with the integrated probe. The maximum phantom displacement was about 40 pm, and the maximum radial normal strain was about 4% (absolute value). The spatial dependence of these quantities shows good agreement with theoretically predicted values.

Transurethral ultrasound array for prostate thermal therapy: initial studies

Abstract: This study presents the initial evaluation of an applicator designed for transurethral ultrasound thermotherapy (TUST) of prostate tissue in the treatment of benign prostatic hyperplasia (BPH) and cancer. A tubular multitransducer applicator, consisting of four piezoceramic tubes (2.5 mm diameter, 6 mm long, 6.8 MHz) under separate power control, was designed to fit within a semiflexible water-cooled temperature-regulated delivery catheter to be placed within the prostatic urethra during therapy. Sonication patterns were tailored to produce power depositions which avoid nontargeted tissues, such as the rectum. Computer simulations have demonstrated that 1.4-2.0 cm radial therapeutic zones (temperatures /spl ges/50-55/spl deg/C, thermal doses >300 EM/sub 43/) with concurrent sparing of the urethral mucosa can be produced within prostate tissue having blood perfusion as high as 10 kg m/sup -3/ s/sup -1/ within 15-30 min. Acoustic distributions and power output measurements of a prototype applicator have demonstrated acoustic power levels approaching 10 W per each sectored transducer segment are attainable, with beam profiles collimated within the transducer length and with desired circumferential distributions. In vivo thermal dosimetry characterizations of these transurethral applicators have indicated that therapeutic temperatures between 50 and 90/spl deg/C are attainable, controllable in the longitudinal and circumferential directions, and have effective radial heating. These results clearly indicate that transurethral ultrasound applicators have potential to provide improved spatial localization and control of the heating distribution over existing transurethral thermal therapy techniques for both hyperthermia and thermal coagulative therapy of the prostate.

Feasibility of using ultrasound phased arrays for MRI monitored noninvasive surgery

Abstract: The purpose of this paper was to evaluate the in vivo feasibility of using phased arrays for MRI guided ultrasound surgery. Two different array concepts were investigated: a spherically curved concentric ring array to move the focus along the central axis and a spherically curved 16 square element array to make the focus larger. Rabbit thigh muscles were exposed in vivo in a 1.5 T MRI scanner to evaluate the array performance. The results showed that both of the arrays performed as expected, and the focus could be moved and enlarged. In addition, adequate power could be delivered from the arrays to necrose in vivo muscle tissue in 10 s. This study was the first implementation of phased arrays for MRI guided ultrasound surgery. The results demonstrate that phased arrays have significant potential for noninvasive tissue coagulation.

Micromachined piezoelectric force sensors based on PZT thin films

Abstract: A micromachined lead zirconate titanate (PZT) force sensor for scanning force microscope (SFM) is conceptualized by its piezoelectricity. The fabrication procedure is interpreted, and mechanical characteristics of the micromachined PZT force sensors with various lengths are studied in this paper. A compact SFM is constructed by using the piezoelectric PZT sensor. A very clear image is taken by this SFM. The current study of the micromachined PZT force sensor can be considered as a breakthrough of design of SFM as well as a good example of integrated piezoelectric microdevices.

Applications of through-air ultrasound for rapid NDE scanning in the aerospace industry

Abstract: This work describes two separate applications of air-coupled ultrasonic nondestructive evaluation (NDE) for rapid inspection of aerospace components. First, air-coupled transducer arrays are used for through transmission scanning of relatively large preproduction test samples. In this case a focused array system was designed to facilitate faster inspection rates and at the same time match the resolution and performance of available water jet apparatus. The second approach is geared toward in situ inspection and involves single-sided scanning using a dual transducer configuration for generation of surface, shear, and Lamb waves within the test specimen. In both cases, the difficulties of practical air-coupled inspection are discussed and the design solutions presented for each application. Piezocomposite transducer technology, in conjunction with narrow-band low-noise electronics, constitutes the basis for achieving the required signal to noise ratio (SNR) to ensure robust operation within the industrial environment. A number of scan images, performed on realistic samples, are shown and the results compared with those obtained using alternative methods. Excellent image quality is demonstrated from real time scanning and without recourse to any off-line data processing.

Enhancement of sonodynamic tissue damage production by second-harmonic superimposition: theoretical analysis of its mechanism

Abstract: Among the nonthermal effects of ultrasound, acoustic cavitation may have the highest potential for therapeutic applications if it can be somehow controlled. Recent in vitro and in vivo experiments have demonstrated that sonochemically active cavitation can be enhanced an order of magnitude by superimposing the second harmonic onto the fundamental in insonation. Moreover, they have shown that sonochemically active cavitation can be controlled with relative ease, thereby even in a progressive wave field. The effect of second-harmonic superimposition on the rectified diffusion through the gas-liquid interface of cavitated microbubbles is estimated theoretically. The theoretical rectified diffusion rate explained an asymmetric behavior of the threshold for producing sonodynamic tissue damage as a function of the fundamental and the second-harmonic amplitudes. The tissue damage was produced with a focused progressive wave in a liver lobe of a mouse administered with a sonodynamically active agent. The result suggests that the acceleration of the rectified diffusion is a primary mechanism of the enhancement of sonodynamically effective cavitation by second-harmonic superimposition.

Characterization of air-coupled transducers

Abstract: This paper describes a theoretical and experimental study for determination of the through-air system impulse response and insertion loss with different air-coupled ultrasonic transducers. Wide-band piezopolymer transducers (PVDF) are employed in both transmission and reception modes and their behavior assessed by means of mathematical modeling and experiment. Specifically, a linear systems approach, modified to include the influence of attenuation in the propagation medium, was used to design suitable PVDF transducers for wide-band operation in air. Suitable devices were then manufactured for determination of the transmission and reception response characteristics of piezocomposite and electrostatic transducers when operating in the air environment. A range of transducers was evaluated, including 1-3 connectivity composites of different ceramic volume fraction and mechanical matching conditions, in addition to electrostatic devices of varying design. To complement the investigation, relative performances for narrow-band operation are also presented under transmission and transmit-receive conditions. Despite the obvious measurement difficulties, good agreement between theory and experiment was observed and the methodology is shown to provide a convenient and robust procedure for comparison of through-air transducers operating in the frequency range 50 KHz to 2 MHz. Although highly resonant, the most effective composite transducers under consideration demonstrate an improvement in two-way insertion loss of 22.4 dB and 11.5 dB over a corresponding electrostatic pair, under narrow-band and wide-band operation, respectively.

Absolute backscatter coefficient over a wide range of frequencies in a tissue-mimicking phantom containing two populations of scatterers

Abstract: The acquisition and interpretation of in vivo ultrasonic measurements in tissue encounter problems associated with limited access to the region of interest, intermixed scattering structures with different characteristic dimensions, and system-dependent effects. This work addresses these problems by adapting and testing a technique for measuring the absolute attenuation and the absolute backscatter coefficient (effective backscatter cross section per unit volume of material), as a function of frequency, in a single-transducer backscatter configuration. The frequency-dependent attenuation and backscatter coefficients of a tissue-mimicking gelatin phantom containing a random distribution of two populations of scatterers were measured, Three transducers with different center frequencies and focusing characteristics were used in order to verify that system-dependent effects were removed by the technique and to investigate the change in the measured parameters across a broad range of frequencies (2 to 60 MHz). A spherical autocorrelation model was applied to measurements of the backscatter coefficient in order to estimate the size of scatterers. Measurements demonstrate that the backscatter and attenuation properties of a mixture of two distinct intermixed scatterer-size populations change as a function of the frequency range across which the model is applied. Comparison of both the magnitude and the frequency dependence of the experimental results with the theoretical prediction of the backscatter coefficient showed good agreement.

Treatment planning for hyperthermia with ultrasound phased arrays

Abstract: Treatment planning for ultrasound phased arrays suggests a strategy for hyperthermia therapy which satisfies therapeutic conditions at the target and spares other sensitive anatomical structures. To predict both desirable and harmful interactions between ultrasound and important structures such as the tumor, bones, and air pockets, a hyperthermia treatment planning system has been developed for ultrasound phased arrays. This collection of treatment planning routines consists of geometric and thermal optimization procedures specific to ultrasound phased arrays, where geometric treatment planning, combined with thermal treatment planning and three-dimensional visualization, provides essential information for the optimization of individual patient treatments. A patient image data set for cancer of the prostate, a difficult target situated in the midst of multiple pelvic bone obstructions, illustrates the geometric treatment planning algorithm and other tools for treatment analysis. The results indicate that the analysis of complex three-dimensional relationships between the applicator, anatomical structures, and incident fields provides an important means of predicting treatment limiting conditions, thereby allowing the hyperthermia applicator to electronically adapt to individual patients and specific sites.

Assessing the influence of pillar aspect ratio on the behavior of 1-3 connectivity composite transducers

Abstract: This paper describes a theoretical study, using finite element analysis, into the influence of the ceramic pillar aspect ratio on the behavior of 1-3 connectivity composite transducers. The main objective is to provide working design guidelines for the transducer engineer, with a view toward the cost-effective manufacture of thickness drive and hydrostatic devices. Modal and harmonic analyses are performed to ascertain the conditions under which the composite behaves as a homogenous material, under different values of volume fractions, passive filler material, pillar shape, and distribution. Consequently, a set of criteria is generated to determine analytically the range of aspect ratios for which the composite material behaves homogeneously in the thickness dimension. The influence of polymer loss on these criteria is discussed, along with effects of practical encapsulation and protective layers. Where possible, real data are provided to supplement theoretical predictions, with reasonable correlation between theory and experiment.

Intracavitary ultrasound phased arrays for noninvasive prostate surgery

Abstract: The feasibility of using intracavitary ultrasound phased arrays for thermal surgery of the prostate was investigated. A simulation study was performed which demonstrated the ability of phased arrays to generate necrosed tissue volumes over anatomically appropriate ranges (2-6 cm deep and >6 cm axially) and investigated the effects of varying frequency, sonication time, maximum temperature, and blood perfusion on the necrosed tissue volume. An advantage that phased arrays have over geometrically focused transducers is that they are able to electronically scan a single focus over a specified range very quickly. This study demonstrated that the necrosed tissue volume may be increased by more than a factor of 100 by using electronic scanning. Scan parameters that were investigated included foci spacing, scan width, perfusion, maximum temperature, and unequal weighting of the foci. An optimization was performed to select the foci weighting parameters such that a uniform thermal dose was achieved at the focal depth, providing a more uniformly heated target volume. Finally, the ability of linear ultrasound phased arrays to create necrosed tissue lesions was demonstrated experimentally in fresh beef liver using a single stationary focus and single focus scans generated by an aperiodic 0.83-MHz 57-element linear ultrasound phased array.

Spectral velocity profiles for detailed ultrasound flow analysis

Abstract: The operation of a novel ultrasound multigate instrument capable of computing in real-time the fast Fourier transform (FFT) of Doppler signals detected from 64 equally spaced range cells is presented. The new system provides up to 50 velocity profiles per second, which are displayed in such a manner that information about the full spectral content of Doppler signals at all the investigated depths is continuously monitored over a PRF-wide frequency range which can be set arbitrarily between -PRF and +PRF. Experimental results are presented, which demonstrate that the true velocity profile can be accurately detected through the computation of "local" maximum velocities obtained by properly correcting the maximum frequency of each spectrum. There is also a discussion on how the results of multigate analysis are influenced by the sample volume length, a parameter which can be usually set by modifying the duration of the transmitted burst. In particular, it is shown that, in regions close to the vessel walls, the shear rate can be measured with a spatial resolution related to the spacing between subsequent range cells and not to the sample volume length.

Estimation of blood velocity with high frequency ultrasound

Abstract: In order to map blood velocity in small regions near the transducer, we evaluate the performance of the wideband maximum likelihood (WMLE) strategy and infinite impulse response (IIR) filters for blood velocity estimation with a transducer center frequency of 38 MHz. Using a short transmitted pulse and the narrow lateral beam width obtained using this frequency, we show that velocities smaller than 1 mm/s can be estimated reliably. In addition, using both changes in the location and magnitude of the peak of the RF correlation, vessels as small as 40 /spl mu/m can be visualized in the RF signal and distinguished from stationary tissue. The experimental system also provides the opportunity to examine changes in flow and in the vessel wall over a cardiac cycle.

Reduction of the grating lobes of annular arrays used in focused ultrasound surgery

Abstract: An annular array with variable focus, used in focused ultrasound surgery, generates grating lobes responsible for undesirable lesions. It is known that the amplitude and the location of the lobes depend on both the geometry and the frequency of the transducer. A new procedure based on the use of a wide-band CW signal, i.e., a signal phase modulated by a pseudorandom code, is proposed to reduce the amplitude of these lobes. The theoretical study enables us to determine the location and the amplitude of these lobes and to simulate the effect of the transmitted signal bandwidth. In particular, a simple analytical relation gives the intensity ratio between the grating lobes and the main lobe. This equation shows that this ratio is inversely proportional to the number of rings and to the bandwidth of the transmitted signal. A system was developed and tested with two transducer arrays of 35- and 150-mm diameter, respectively. The simulations and experiments demonstrate the validity of the theoretical study and the efficacy of the proposed procedure. In conclusion, it is possible to reduce the grating lobes without geometric modification of the array by increasing the bandwidth of the transmitted signal.

Ultrasonic beam steering through inhomogeneous layers with a time reversal mirror

Abstract: Adaptive time delay focusing techniques allow an efficient correction of the effects due to an inhomogeneous layer close to the transducer array. If the layer is far from the array, these techniques are no longer appropriate to correct the diffraction effects between the layer and the transducer array. This problem was overcome by the use of acoustic time reversal mirrors. In this technique, the Green's function of a dominant scatterer available in the medium is recorded in digital memories and used to focus on the scatterer in both transmit and receive modes. We present in this paper an extension of this technique to focus, in the presence of an aberrating layer, not only on the dominant scatterer, but also around it in order to image the surrounding zone. From the knowledge of the Green's function needed to focus on the initial scatterer, we calculate the new Green's function matched to the new point of interest. The algorithm uses the concept of time reversal propagation, and we shall present here theoretical and experimental results obtained with this technique. Finally, the knowledge of each Green's function matched to each new desired focal point allows the realization of a B-scan image of the zone surrounding the reflector.