An ultrasound tomography system with polyvinyl alcohol (PVA) moldings for coupling: in vivo results for 3-D pulse-echo imaging of the female breast.

Full-angle spatial compounding (FASC) is a concept for pulse-echo imaging using an ultrasound tomography (UST) system. With FASC, resolution is increased and speckles are suppressed by averaging pulse-echo data from 360°. In vivo investigations have already shown a great potential for 2-D FASC in the female breast as well as for finger-joint imaging. However, providing a small number of images of parallel cross-sectional planes with enhanced image quality is not sufficient for diagnosis. Therefore, volume data (3-D) is needed. For this purpose, we further developed our UST add-on system to automatically rotate a motorized array (3-D probe) around the object of investigation. Full integration of external motor and ultrasound electronics control in a custom-made program allows acquisition of 3-D pulse-echo RF datasets within 10 min. In case of breast cancer imaging, this concept also enables imaging of near-thorax tissue regions which cannot be achieved by 2-D FASC. Furthermore, moldings made of polyvinyl alcohol hydrogel (PVA-H) have been developed as a new acoustic coupling concept. It has a great potential to replace the water bath technique in UST, which is a critical concept with respect to clinical investigations. In this contribution, we present in vivo results for 3-D FASC applied to imaging a female breast which has been placed in a PVA-H molding during data acquisition. An algorithm is described to compensate time-of-flight and consider refraction at the water-PVA-H molding and molding-tissue interfaces. Therefore, the mean speed of sound (SOS) for the breast tissue is estimated with an image-based method. Our results show that the PVA-H molding concept is applicable and feasible and delivers good results. 3-D FASC is superior to 2-D FASC and provides 3-D volume data at increased image quality.

Ultrasonic imaging of complex specimens by processing multiple incident angles in full-angle synthetic aperture focusing technique.

In the evaluation of large-scale metallic specimens, X-ray CT suffers from limited penetration, which results in artifacts in the reconstructed image. Data fusion of information obtained by different modalities allows correction of those artifacts. In this contribution, an approach is presented to provide complementary data of the inner pattern of the specimen by ultrasonic testing in immersion mode. To process an ultrasonic imaging full-angle synthetic aperture focusing technique, data are acquired along the a priori known contour of the specimen. Substantial discrepancies in speed of sound between the couplant and the material of the specimen lead to refraction effects which are corrected by a virtual source element method. Furthermore, several incident angles at each virtual source are utilized to achieve an enhanced detectability of inner structural edges. However, arising reverberations limit image quality and must be suppressed by predictive deconvolution. Additionally, a subspace analysis and projection method is utilized to remove echoes of the a priori known surface in the reconstructed image which potentially mask information of near-surface structures. In comparison with exclusively perpendicular insonification, resulting images show a significant enhanced possibility of detection for inner structural edges even in adverse orientations for ultrasonic imaging. Furthermore, surface echoes and reverberations are suppressed by the proposed filter methods in a reliable way.

Quantitative reconstruction of a disturbed ultrasound pressure field in a conventional hydrophone measurement.

Among various techniques enabling absolute measurements of ultrasound pressure, light refractive tomography (LRT) is so far the only one which is noninvasive, omnidirectional, and provides time-resolved results in pascals. By exploiting all these advantages, LRT shows suitability for investigations of ultrasound pressure fields, even in the case of adjacent medium boundaries which may cause considerable sound reflections. To demonstrate the potential research possibilities offered by this technique, we apply LRT to investigating the disturbance to a pressure field caused by a hydrophone. A commercial capsule hydrophone is placed in front of an ultrasound transducer excited by 1-MHz burst signals. We reconstruct the disturbed ultrasound pressure field between the hydrophone and the transducer in both spatial and temporal dimensions. Good agreement has been achieved between the reconstructed pressure field and the prediction made by a numerical simulation. Moreover, a comparison between the results provided by LRT and hydrophone shows that multiple reflections can jeopardize the reliability of hydrophone measurement when a hydrophone is placed very close to a medium boundary (e.g., <5 mm in our case). On the contrary, LRT achieves reasonable results at all distances. Finally, as a proof of the reliability of LRT, we compare the pressure amplitudes offered by LRT and hydrophone measurements at 27 mm away from the transducer in the absence of obstacles. The comparison shows a relative difference of 7.07%.

Magnetostrictive microelectromechanical loudspeaker.

A microelectromechnical-loudspeaker based on the magnetostrictive effect is presented. The membrane consists of a comb structure of monomorph bending cantilevers with an area of about 16 mm(2). Prototypes generate a sound pressure level (SPL) of up to 102 dB at 450 Hz with a total harmonic distortion of 2% inside a 2 cm(3) measurement volume. The fabrication process of the device as well as a coupled simulation model to calculate its sound pressure is introduced. The model reproduces the measurements and is employed to further optimize the loudspeaker membrane. As a result, a computed maximum SPL of 106 dB has been achieved with a -6 dB frequency range extending from 100 Hz to 2.6 kHz.

A reliability study of light refractive tomography utilized for noninvasive measurement of ultrasound pressure fields.

Light refractive tomography is an optical measurement technique that is able to provide absolute sound pressure values in specified volumes. Because of the simplicity of the measurement principle as well as the compactness of the measurement setup, light refractive tomography offers higher measurement performance and fewer error sources than light diffraction tomography. In this contribution, a numerically simulated ultrasound pressure field is exploited to determine the experimental parameters and to analyze the error sources as well as their influences on final results. After that, several ultrasound transducers excited with 1 MHz signals are investigated. The light refractive tomography results show good agreement with hydrophone measurements. Finally, we reconstruct 2000 transient states of the ultrasound pressure field within a volume of about 38 cm(3) after sending the burst signal. Without applying any smoothing to the resulting images, the reconstructed pressure field varies continuously in both spatial and temporal dimensions.

A method for characterizing stapes prostheses by their mechanical transfer function.

In this contribution, we present and evaluate a method for characterizing stapes prostheses by their mechanical transfer function. The measurements were carried out after a stapedotomy surgery was performed in three human temporal bones conserved in 4% formaldehyde. The inner ear was drained of fluid. Successively, one of three different stapes prostheses was inserted. After such preparation, the prosthesis piston movement compared to the incus movement is measured with laser vibrometry. The magnitude transfer function considered is defined as the amplitude of the prosthesis piston movement compared to the amplitude of the incus movement. Measurements were made in a frequency range from 500Hz to 4kHz. The measured amplitudes roughly ranged between 10nm and 100nm. A great advantage of the presented method is the fact that only a small portion of the ossicular chain influences the measurement, mainly the joint between the prosthesis and the incus. Furthermore, the usage of cadaver temporal bones allows for an automated measurement setup, long term experiments and the access of measurement positions inapproachable during in vivo measurements. With this method, the different kinds of prostheses could be evaluated on incuses of different diameters.

Optical 3-D metric measurements of local vocal fold deformation characteristics in an in vitro setup.

Understanding vocal fold dynamics presents an essential part in treating voice disorders as it is the prerequisite to appropriate medical therapy. Various physical and numerical models exist for simulation purposes, all relying on simplified material parameters. To improve current approaches, data of realistic tissue behavior, i.e., in natural surroundings, have to be considered in model development. An in vitro setup was proposed for tensile tests combined with an optical method for precise, local and metrical 3-D measurements of distinctive surface points. Compared to previous 3-D reconstruction methods, the accuracy was improved tenfold. Vertically applied forces versus resulting deformation were measured for ten porcine vocal folds. Deformation characteristics of mucosa and the two-layer structure of mucosa and muscle (MM) were investigated at three distinctive locations along the vocal fold edge. The spring rates were represented by an exponential function. For equal deflections, an increasing spring rate from posterior to anterior for MM was measured. For solely mucosa, the spring rate decreased from the posterior to the middle and subsequently increased again. The MM-layer presented a stiffer deformation behavior than mucosa. For deformations higher than 1.5 mm, the spring rates for MM were more than twice as high as for mucosa. The investigations display the importance of considering both multilayers and local differences for the improvement of vocal fold models.

Beam profile measurements using light refractive tomography.

A method for reconstructing the sound pressure profile from interferometer measurements using computed tomography is presented. In accordance with the term light diffraction tomography, in short we want to call the method light refractive tomography, because it is based on the sound pressure-induced refractive index change along the laser beam. We compare three tomographic reconstruction methods applicable to axisymmetric beam profiles, namely, the filtered back-projection, the Hankel Fourier method, and the onion peeling method, the latter based on Nestor and Olsens algorithm. Results are compared to hydrophone measurements. The performance of the tomographic algorithms are reflected by general considerations on the reconstruction of noise signals.

FEM-Based determination of real and complex elastic, dielectric, and piezoelectric moduli in piezoceramic materials.

We propose an enhanced iterative scheme for the precise reconstruction of piezoelectric material parameters from electric impedance and mechanical displacement measurements. It is based on finite-element simulations of the full three-dimensional piezoelectric equations, combined with an inexact Newton or nonlinear Landweber iterative inversion scheme. We apply our method to two piezoelectric materials and test its performance. For the first material, the manufacturer provides a full data set; for the second one, no material data set is available. For both cases, our inverse scheme, using electric impedance measurements as input data, performs well.

A coupled finite-element, boundary-integral method for simulating ultrasonic flowmeters.

Today's most popular technology of ultrasonic flow measurement is based on the transit-time principle. In this paper, a numerical simulation technique applicable to the analysis of transit-time flowmeters is presented. A flowmeter represents a large simulation problem that also requires computation of acoustic fields in moving media. For this purpose, a novel boundary integral method, the Helmholtz integral-ray tracing method (HIRM), is derived and validated. HIRM is applicable to acoustic radiation problems in arbitrary mean flows at low Mach numbers and significantly reduces the memory demands in comparison with the finite-element method (FEM). It relies on an approximate free-space Green's function which makes use of the ray tracing technique. For simulation of practical acoustic devices, a hybrid simulation scheme consisting of FEM and HIRM is proposed. The coupling of FEM and HIRM is facilitated by means of absorbing boundaries in combination with a new, reflection-free, acoustic-source formulation. Using the coupled FEM-HIRM scheme, a full three-dimensional (3-D) simulation of a complete transit-time flowmeter is performed for the first time. The obtained simulation results are in good agreement with measurements both at zero flow and under flow conditions.

Finite-element simulation of wave propagation in periodic piezoelectric SAW structures.

Many surface acoustic wave (SAW) devices consist of quasiperiodic structures that are designed by successive repetition of a base cell. The precise numerical simulation of such devices, including all physical effects, is currently beyond the capacity of high-end computation. Therefore, we have to restrict the numerical analysis to the periodic substructure. By using the finite-element method (FEM), this can be done by introducing periodic boundary conditions (PBCs) at special artificial boundaries. To be able to describe the complete dispersion behavior of waves, including damping effects, the PBC has to be able to model each mode that can be excited within the periodic structure. Therefore, the condition used for the PBCs must hold for each phase and amplitude difference existing at periodic boundaries. Based on the Floquet theorem, our two newly developed PBC algorithms allow the calculation of both, the phase and the amplitude coefficients of the wave. In the first part of this paper we describe the basic theory of the PBCs. Based on the FEM, we develop two different methods that deliver the same results but have totally different numerical properties and, therefore, allow the use of problem-adapted solvers. Further on, we show how to compute the charge distribution of periodic SAW structures with the aid of the new PBCs. In the second part, we compare the measured and simulated dispersion behavior of waves propagating on periodic SAW structures for two different piezoelectric substrates. Then we compare measured and simulated input admittances of structures similar to SAW resonators.