Elastic Sonic Boom in Soft Materials

The cover image shows experimental evidence of an elastic sonic boom in soft materials. The ultrasonic remote generation of a supersonic moving source generates an intense shear wave propagating in a Mach cone. Such Mach waves are sensitive to medium elasticity inhomogeneities and have potential medical applications.

Image courtesy of Jeremy Bercoff, Mickael Tanter, and Mathias Fink. The cover contributors are with Laboratoire Ondes et Acoustique, Paris, France. See article J. Bercoff, M. Tanter, and M. A. Fink,“Supersonic shear imaging: A new technique for soft tissue elasticity mapping,” IEEE Trans. Ultrason., Ferroelect., Freq. Contr., vol. 51, no. 4, pp. 396–409, Apr. 2004.

Since publication of this paper, the company Supersonic Imagine was created to apply this technology to breast cancer diagnosis.

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Finite Element Modeling and Experimental Characterization of Crosstalk in 1-D CMUT Arrays

The front cover shows the commercial package LS-DYNA simulation result of crosstalk displacements in the 1-D CMUT array immersed in soybean oil. All array elements are biased in the conventional mode, and the center element at the origin is excited with a pulse. In the time-spatial representation, three components of crosstalk propagating with different phase velocities and signal strengths are observed. The fastest crosstalk component is the weakest, with -65 dB displacement amplitude relative to the transmitter, and is identified as Lamb wave mode S0. A slightly slower component, observed at -40 dB level, is the Lamb wave mode A0. The main crosstalk mechanism is also the slowest component: dispersive guided mode propagating in the fluid-solid interface. Although the transmitter element has a center frequency of 5.8 MHz with 130% fractional bandwidth, the dispersive guided mode is observed with the maximum amplitude at a frequency of 2.1 MHz, and has a cut-off frequency of 4 MHz. The average crosstalk between array elements is -24.4 dB.

Image courtesy of Baris Bayram. He was with Stanford University, Edward L. Ginzton Laboratory, Stanford, CA, when the research was performed. See article on page 418.

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A Miniaturized Catheter 2-D Array for Real-Time 3-D Intracardiac Echocardiography

Top left and right: A 112 channel, miniature 2-D array constructed on a high-density, multi-layer flexible interconnect. Bottom left: Transducer attached to ribbon cables and final packaging into a 2.3 mm (7 Fr) catheter. Bottom right: Real-time 3-D in vivo image of an electrophysiological catheter guided into the coronary sinus.

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Rayleigh Waves Escaping from a SAW Resonator

Laser-interferometric scan of a longitudinal leaky SAW (LLSAW) resonator on YZ-LiNbO₃: (a) reflectivity of the sample surface, (b) relative amplitude of the vertical surface-vibration component. The measurement reveals first and second order Rayleigh-wave beams escaping from the resonator. These beams are generated by a periodic system of sources, i.e., by the oscillating charges accumulated at the tips of the finger electrodes.

Image courtesy of Olli Holmgren, Helsinki University of Technology, Espoo, Finland. See accompanying paper on page 861.

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Demonstration of Ultrasound-Assisted Dye Delivery to a Cell Monolayer

Transillumination (left) and epifluorescence (right) images of fluorescent fragments adherent to melanoma cells cultured on a cover slip. These images were acquired after ultrasound exposure of the cells to a pulse sequence designed for drug delivery which uses radiation force and gas-filled drug delivery vehicles similar to ultrasound contrast agents.

Images courtesy of Michaelann J. Shortencarier, Paul A. Dayton, Susannah H. Bloch, Patricia A. Schumann, Terry O. Matsunaga, and Katherine W. Ferrara, University of California, Davis, Department of Biomedical Engineering, Davis, CA. See article M. J. Shortencarier, P. A. Dayton, S. H. Bloch, P. A. Schumann, T. O. Matsunaga, and K. W. Ferrara, "A method for ultrasound-localized drug delivery using gas-filled lipospheres," IEEE Trans. Ultrason., Ferroelect., Freq. Contr., vol. 51, no. 7, pp. 822-831, 2004.

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Evolution of Phase Aberration Correction using Ultrasound Radiation Force and Vibrometry Optimization

Images of the velocity measured by a laser vibrometer of a spherical target as the phase of each element of a ten element annular array is adjusted to maximize the radiation force induced vibration. The sphere had a diameter of 1.59 mm and was embedded in a gelatin phantom placed in a water tank. The phantom was placed perpendicular to the ultrasound propagation direction. The ultrasound beams were manually defocused by adjustments of the focusing phase values simulating a random phase aberrator. The top left image was acquired when the transducer was in a defocused state. The magnitude and distribution of the vibration velocity evolves to a better focus as each element of the array is optimized by changing the phase of the AM signal applied to each element and using the sphere's velocity for feedback. The image in the bottom right was acquired when the transducer was originally focused using a needle hydrophone and compares very well with the result obtained from the progressive optimization of all ten elements using the ultrasound radiation force shown immediately to the left.

Images courtesy of Matthew W. Urban, Miguel Bernal, and James F. Greenleaf, Mayo Clinic College of Medicine, Department of Physiology and Biomedical Engineering, Rochester, MN. See accompanying article on page 1142.

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Novel Display Modes for Microbubble Characterization

The pictures illustrate the movement of microbubbles in a highly diluted ultrasound contrast agent. Imaging ultrasound pulses (4-cycle @ 4 MHz, PRF=1 kHz, 1.6 MPa peak-negative pressure), produced by a single element transducer with 80 mm focal distance (f-number of 3), apply radiation force on the bubbles. The M-Mode display on the top shows grey-scale-coded echo amplitudes as a function of time and distance from the transducer. Each trace corresponds to the echoes of a single bubble, its slope depending on the acceleration impressed by the radiation force. The bottom pictures show the Doppler velocities calculated from the echoes detected at 128 distinct depths (Multigate Doppler Mode). In the picture on the left, each spotlight corresponds to a different bubble. One bubble has high instantaneous velocity (21 mm/s), and various bubbles have lower velocities. The picture on the right shows the history of the same bubble velocities over a total interval of 2 seconds.

Images courtesy of Francesco Guidi, Hendrik J. Vos, and Piero Tortoli, University of Florence, Electronics and Telecommunications Dept., Firenze, Italy. See accompanying paper on page 1333.

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Ultrasonic Tissue Characterization of Atherosclerosis by a Speed-of-Sound Microscanning System

The amplitude (upper left), speed of sound (upper right), attenuation (lower left) and thickness (lower right) of the coronary arteries were measured with a new concept of speed-of-sound microscanning system using fast Fourier transform of a single pulsed wave instead of burst waves used in conventional SAM systems.

Images courtesy of Yoshifumi Saijo, Naohiro Hozumi, Kazuto Kobayashi, Nagaya Okada, Esmeraldo D. Santos Filho, Hidehiko Sasaki, Tomoyuki Yambe and Motonao Tanaka. Y. Saijo, E.D Santos Filho, H. Sasaki, T. Yambe and M. Tanaka are with Department of Medical Engineering and Cardiology, Institute of Development, Aging and Cancer, Tohoku University, Sendai, Japan. N. Hozumi is with Aichi Institute of Technology. K. Kobayashi and N. Okada are with Honda Electronics Co. Ltd. See accompanying paper on page 1571.

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Photograph Through a Microscope of a 40 MHz Linear Array

Linear arrays are difficult to fabricate for operating frequencies above 20 MHz since the dimensions of the array scale with the ultrasound wavelength. We have fabricated a 40 MHz array with 40 μm element-to-element pitch. Photolithography was used to define the electrodes and wire-bonding was used to make the electrical contacts. The array electrodes were defined on the surface of a 1-3 PZT5H-composite material.

Image courtesy of Jeremy A Brown, F. Stuart Foster, Andrew Needles, Emmanuel Cherin, and Geoffrey R Lockwood. J. A. Brown, F. S. Foster, A. Needles, and E. Cherin are with Sunnybrook Health Sciences Centre, Imaging Research, Toronto, Ontario, Canada. G. R. Lockwood is with Queen’s University, Physics, Kingston, Ontario, Canada. See companion article on page 1888 of this issue.

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1.156-GHz Self-Aligned Vibrating Micromechanical Disk Resonator

Scanning electron micrograph of a 20 micron-diameter polysilicon micromechanical disk resonator fabricated on a 10 Ω-cm silicon substrate via a process that combines surface-micromachining with a sidewall sacrificial spacer step capable of achieving sub-100 mm lateral gaps between the disk and its polysilicon electrodes. This device has been demon-strated with on-chip Q’s of 7,890 and 5,160 in vacuum and air, respectively, at its 734.6 MHz second mode frequency; and Q’s > 2,650 at its third mode 1.156 GHz frequency.

Laser-induced Ultrasound Image of Human Forearm

A human forearm was illuminated with short laser pulses to generate stress transients in absorbing structures, such as blood vessels. The transients were recorded with an ultrasound device (left) to reconstruct a cross sectional laser-induced ultrasound image (25x20 mm, right) showing a vessel at 7 mm depth. The ultrasound image in the left frame shows the color-flow image of the blood vessel identified as such at 7 mm in the right image.

Image courtesy of Joël J. Niederhauser, Inst. of Appl. Phys., Univ. of Bern, Switzerland, (e-mail: j.n@switzerland.org).

See article J. J. Niederhauser, M. Jaeger, R. Lemor, P. Weber, M. Frenz, "Combined ultrasound and optoacoustic system for real-time high-contrast vascular imaging in vivo," IEEE Trans Med. Imag., vol. 24, no. 4, pp. 436-440, 2005.

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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 on page 2516.

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