Finite Element Analysis of Temperature Rise from Catheter Ultrasound Ablation Transducers

The right image shows a schematic of a proposed catheter transducer for both real-time, 3-D (RT3D) echocardiography (112 element 2-D array shown in blue) and ultrasound ablation (86 element linear phased array in yellow). The 2-D array can image a pyramidal volume (as shown), while the linear phased array can produce an ablation beam (shown in red) that can be steered in azimuth.

The left image shows the results of finite element analysis of the temperature rise possible from the ablation array. The ablation array transmits into blood and is focused in the myocardium at a range of 20 mm, 10 mm off-axis. The contours are at 90%, 70%, 50%, 30%, and 10% of the focal temperature rise of 6.1°C.

Images courtesy of Kenneth L. Gentry, Mark L. Palmeri, Nasheer Sachedina, and Stephen W. Smith, Duke University, Biomedical Engineering, Durham, NC. See accompanying article, K. L. Gentry, M. L. Palmeri, N. Sachedina, and S. W. Smith "Finite-element analysis of temperature rise and lesion formation from catheter ultrasound ablation transducers," IEEE Trans. Ultrason., Ferroelect., Freq. Contr., vol. 52, no. 10, pp. 1713-1721, Oct. 2005.

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Computational Three-Dimensional Acoustic Tissue Models From Histologic Sections

Accurate three-dimensional representation of biological tissues may help in the identification of the tissue microstructure responsible for ultrasonic scattering. A novel approach was developed to reconstruct three-dimensional tissue models from adjacent photomicrographs of H&E-stained histologic sections.* The left panel displays a three-dimensional histologic volume (3DHV) reconstructed from a 43-section histologic dataset. The 43 3-µm thick sections were obtained from a mouse mammary tumor (EHS sarcoma). To obtain the 3DHV, the contrast of the sections was first equalized, and then sections were carefully aligned using an automatic non-rigid registration algorithm. Because seven (out of 43) sections were lost during sectioning they were interpolated using third-order Hermite interpolating polynomials. Each section in the reconstructed volume was then thresholded using a 7-level algorithm. The threshold algorithm was engineered under the supervision of a pathologist to guarantee that each level corresponded to a unique tissue microstructure. The right panel displays the resulting volume where seven arbitrary colors were used to represent the seven distinct microstructures. Acoustic impedance values were assigned to each recognized tissue microstructure. The resulting three-dimensional impedance map was then used as a computational phantom for ultrasonic tissue characterization. Both volumes are of size 218 × 156 × 129 µm.

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Tomographic Reconstruction using a Rotated Linear Array of Attenuation and Phase Shift through a Textile Composite

The front cover shows tomographic reconstructions of the attenuation and phase shift of a 2 MHz ultrasonic field that has propagated through regions of increasing porosity (left to right) in a textile composite. Images are constructed from measurements using a rotating linear array in which the phase-sensitive, tall, and narrow elements of the array act as parallel line integrators of the complex ultrasonic pressure field. The technique enables rapid visualization of phase-aberrated fields with good resolution in both the lateral and elevation directions.

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Ultrasonic Contrast Agent Shell Rupture Detected by Inertial Cavitation and Rebound Signals

Figures show echo waveforms and their time-frequency spectrogram for two single shelled microbubbles. On the left, after the end of the excitation, no acoustic emissions are detected. The microbubble is oscillating. On the right, at around 4.5 µs, a short-duration response is seen and a corresponding broadband signature is observed in the spectrogram. We conclude the shell has ruptured.

Images courtesy of Azzdine Y. Ammi, Laboratoire d’Imagerie Paramétrique, Centre National de la Recherche Scientifique, Paris, France. See accompanying article, A. Y. Ammi, R. O. Cleveland, J. Mamou, G. I. Wang, S. L. Bridal, and W. D. O’Brien, Jr., "Ultrasonic contrast agent shell rupture detected by inertial cavitation and rebound signals," IEEE Trans. Ultrason., Ferroelect., Freq. Contr., vol. 53, no. 1, pp. 126–136, Jan. 2006.

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Ferroelectric and Piezoelectric Properties for a Lead-free Transducer

The photograph (cover) illustrates the microstructure of potassium sodium bismuth titanate (K_xNa_1-x)_0.5Bi_0.5TiO₃, where x = 0.2, (0.2KNBT), as viewed in the scanning electron microscope (3500X). The ceramic is dense and of high quality. Ferroelectric (top right) and piezoelectric (bottom right) properties are reported at 25°C and 0.1 Hz. The effective piezoelectric strain coefficient was 325·10^-12 m/V for the 0.2KNBT ceramic, a lead-free transducer.

Images courtesy of James F. Carroll III, University of Illinois at Urbana-Champaign, Department of Materials Science and Engineering, Urbana, Illinois, USA. Portions of the research were carried out at The University of Tokyo, Research Center for Advanced Science and Technology, Meguro-ku, Tokyo, Japan. See accompanying paper, J. F. Carroll, III, D. A. Payne, Y. Noguchi, and M. Miyayama, "Field-induced strain behavior for potassium sodium bismuth titanate ceramics," IEEE Trans. Ultrason., Ferroelect., Freq. Contr., vol. 54, no. 12, pp. 2516-2522, Dec. 2007.

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Finite-difference Time Domain Simulation of Ultrasonic Wave Propagation in Bone

The front cover illustrates Finite-Difference Time-Domain (FDTD) simulations of wave propagation through bone specimens immersed in water. Such simulations are currently being performed on numerical models of bone by coupling a FDTD code with numerical three-dimensional (3-D) bone structures reconstructed from X-ray computed tomography data acquisitions.

Typical snapshots of wave propagation (bottom) are presented for three distinct applications (top): 3-D cancellous bone microstructure (left panel), 3-D human radius diaphysis (middle panel) and 2-D transverse cross-section of a human radius (right panel). Several logarithmic color scales were used to represent absolute values of the displacement velocity wave field, and to allow for a qualitative visualization of waves of different amplitudes combined with transparency effects to visualize bone. The possibility of applying the 3-D FDTD approach to actual bone structures provides a valuable tool to study transmission of ultrasound waves through cancellous and cortical bone and to elucidate the interaction mechanisms between ultrasound and bone structures. A 3-D snapshot of a 1-MHz quasi-plane wave propagating through a trabecular bone microstructure is shown on the left panel. A coherent ballistic plane wave is clearly seen, followed by a spatially incoherent scattered wave. Waves transmitted axially along the long axis of a human diaphysis are illustrated on the middle panel. Their nature are determined by the wavelength-to-cortical thickness ratio. Of particular interest are the leaky waves that can be detected with transducers placed on the skin using the so-called axial transmission technique. The right panel shows the transmission of a 1-MHz quasi-plane wave in a transverse cortical experimental configuration. Circumferential guided waves propagating in the cortex are shown together with a wave front directly transmitted through the medullary canal.

Images courtesy of Emmanuel Bossy and Pascal Laugier. Emmanuel Bossy is with the Laboratoire Photons et Matière, ESPCI/CNRS, Paris, France. Pascal Laugier is with Université Pierre et Marie Curie, Paris, France.

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Comparison of Microelastic Properties of Cortical Bone Assessed by Scanning Acoustic Microscopy with Degree of Mineralization Measured with Synchrotron Radiation Micro-Computed Tomography and with Nano-Indenter-Based Modulus.

Multimodal assessment of bone structural and material properties is essential for an accurate understanding of bone biomechanical competence. These images illustrate the feasibility of imaging tissue-level microelastic properties of cortical bone using scanning acoustic microscopy (SAM) at 200 MHz and 1 GHz, as well as comparing SAM-based measurements with site matched estimates of the elastic modulus and degree of mineralization of bone (DMB). The top panel shows an acoustic impedance (200-MHz SAM, 8-µm spatial resolution) fused with a DMB image (synchrotron radiation micro-computed tomography, SR-µCT, 10-µm spatial resolution) of a cortical bone specimen prepared from the diaphysis of a human femur. For a better illustration the acoustic impedance is represented in the dark foreground in a spatially limited region only. Following SAM acquisitions, nanoindentation was performed with a Berkovich tip and with an indentation depth of 2 µm within the subregion delimited by the yellow square. A magnified view of the subregion is obtained using a 1-GHz SAM (1.5-µm spatial resolution) (lower left). The optical microscope image of the same region (lower right) depicts the series of indents from which tissue elastic modulus values were derived for a face-to-face comparison with SAM measurements.

Images courtesy of Fabienne Rupin, Amena Saïed, Kay Raum and Pascal Laugier. F. Rupin, A. Saïed, and P. Laugier are currently with University Pierre et Marie Curie of Paris (France), Laboratoire d’Imagerie Paramétrique. K. Raum is with Martin Luther University of Halle-Wittenberg (Germany), Department of Orthopedics, Q-BAM Group.

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Visualization of Lesion Bonding in Axial-Shear Strain Elastograms of Benign (B) and Malignant (M) Breast Lesions

Axial-shear strain elastograms (color) superimposed on the corresponding sonograms (B&W) of benign (fig. a) and malignant (fig. b) breast lesions in vivo. Pixels in axial-shear strain elastograms with absolute magnitude greater than 50% of the applied axial strain (0.0025) and having corresponding correlation coefficient >0.75 were superimposed on the sonograms. Observe that the extent of the axial-shear strain region and its proximity to the sonographically visible lesion margin is different in the two lesions, probably due to the differences in the way they are bonded to their surrounding tissue; malignant lesions are generally tightly bonded and benign lesions are loosely bonded.

This work was supported in part by NIH program project grant P01-EB02105-12 awarded to the University of Texas Medical School at Houston. The author (AT) was also supported in part by the Electrical and Computer Engineering Department, University of Houston, through a teaching assistantship.

Image courtesy of Arun Thitaikumar and Jonathan Ophir. A. Thitaikumar and J. Ophir are with the University of Texas Medical School, Department of Diagnostic and Interventional Imaging, Ultrasonics Laboratory, Houston, TX. A. Thitaikumar is also with the University of Houston, Electrical and Computer Engineering Department, Houston, TX. The Authors thank Dr Brian Garra and his team (U. Vermont) for acquiring the sonographic data.

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Real-Time Ultrasound Guidance of Interventional Devices

A forward looking ring array of 2-D transducer elements was attached to the deployment catheter of a vena cava filter (left). The filter is pictured (top right) with the real-time image in a water tank (bottom right). The image clearly shows the main struts (MS) of the filter (F). The companion paper on page 2066 describes the design, fabrication, and testing of new transducers to explore the feasibility of using real-time 3-D ultrasound to guide the deployment of interventional devices.

Images courtesy of Edward D. Light, John F. Angle, and Stephen W. Smith. E. D. Light and S. W. Smith are with the Department of Biomedical Engineering, Duke University, Durham, NC. J. F. Angle is with the Department of Radiology, University of Virginia, Charlottesville, VA.

This work was supported by NIH Grant # HL 72840 and HL 64962. There was in kind support for the catheters from Cook Medical, Inc. (Bloomington, IN USA).

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