Emmanuel Cherin

Fabrication and Performance of a 40-MHz Linear Array Based on a 1-3 Composite with Geometric Elevation Focusing

The fabrication and performance of a 256-element high-frequency (40-MHz) linear array is described. The array was fabricated using a high-frequency 1-3 PZT-polymer composite material developed in our laboratory. The spacing of the pillars in the composite was chosen to match the 40-mum center-to-center element spacing of the array electrodes. The element electrodes were created using photolithography, and connections to the electrodes were made using ultrasonic wire bonding. The array was focused in the elevation direction by geometrically shaping the composite material using a cylindrical die with a 6-mm radius of curvature. The resulting transducer produced pulses with a -6 dB two-way bandwidth of 50% and a peak-to-peak pressure of 503 kPa when excited with a plusmn30 V monocycle pulse. The measured one-way ( -6 dB) directivity for a single array element was 24 degrees and the -3 dB one-way elevation beamwidth was measured to be 130 mum. The radiation pattern for a focused 64-element subaperture was measured by mechanically translating the aperture above a needle hydrophone. A -3 dB one-way beamwidth of 97 mum was found at a depth of 6 mm. The one-way radiation pattern decreased smoothly to less than -30 dB at a lateral distance of 640 mum.

Effect of triangular pillar geometry on high- frequency piezocomposite transducers

Piezocomposite materials are used extensively in biomedical transducer array fabrication. However, developing high-frequency piezocomposite materials for imaging systems is still a challenge due to the extremely small pillar dimensions required to avoid the interference from lateral resonances. The use of triangular pillar piezocomposite material has been shown to suppress lateral resonances that appear in square pillar composite designs. To further understand how the geometry of the pillars affects the lateral resonances, piezocomposite materials with triangular pillars of different angles have been simulated and fabricated. Simulations were performed on composite transducers of 70-?m pitch, 18-?m kerf width, and 100-?m thickness with isosceles triangular pillars in which the isosceles angle varied from 30? to 60? using a finite-element analysis. By varying the pillar geometry, the composite transducers show large differences in lateral resonances. The simulation results demonstrate that the composite with 45? angle pillars has the lowest secondary pulse amplitude. The secondary pulse becomes larger when the pillar angle deviates from 45?. To study whether the pillar height (which determines the resonance frequency) and aspect ratio would change the optimum angle, composites with 40-?m pitch, 15-?m kerf width, and 45-?m thickness were also simulated. Finally, the composite with triangle pillars was compared with composites with square and round pillars. The simulation results show that the 45? triangular pillar geometry is, for high-frequency applications, the best configuration among all investigated in this work. Composite samples have also been fabricated to confirm results from finite-element modeling. Acoustical and electrical measurements were carried out to compare with theoretical predictions. Three composite transducers with pillar angles of 30?, 45?, and 60? were fabricated using a dice-and-fill technique. The measured electrical impedances and one-way pulse responses agreed well with the theoretical predictions and confirm the optimal nature of the 45? design.

Comparison of nonlinear and linear imaging techniques at high frequency

High frequency second harmonic imaging was compared with fundamental imaging in terms of lateral resolution, depth of field and image quality. In order to assess the fundamental beam at 20 MHz, the harmonic beam at 40 MHz, and the fundamental beam at 40 MHz of a 40 MHz broadband transducer, transmission measurements were performed using a hydrophone. The beam profile obtained using each of these techniques was estimated in a tissue-mimicking phantom. Image quality was evaluated in phantom and mouse tissue in vitro.

Acoustical characterization of submicron particles of perfluorocarbon in solution [ultrasound targeted contrast agent]

Determining the acoustical properties of submicron particles of perfluorocarbon in solution is important to explain their response to ultrasound pulses when bound to a target. In this study, we present measurements of the frequency-dependent attenuation at different particle sizes, pressures and bandwidths. Backscattering spectra of the particles probed with narrowband pulses at 33 MHz are also shown. This study demonstrates the linearity of submicron particles made of liquid perfluorocarbon at high frequency.

Ultrasonic measurement of backscatter from embryonic mouse red blood cells in vivo

The mouse is a tremendously valuable animal model system in which to explore both normal development and human disease models. Because humans share approximately 95% of their genes with the mouse a large number of diseases such as congenital heart disease, cancer and atherosclerosis are now being studied. The present study focuses on the formation of the embryonic mouse heart and in particular on changes in backscatter intensities from the developing blood system. significant changes in brightness of the ultrasound images of the embryonic mouse heart at different ages of gestation have been observed. We hypothesize that these changes are due to the early development of Red Blood Cells (RBCs). In order to quantify these changes, measurements of the frequency dependence of the backscatter coefficient from blood within heart chambers were performed in utero and in vivo. For this calculation 100 radio-frequency lines were collected from the chambers of the embryonic mouse heart positioned at the focal zone of the probe, using a 40 MHz ultrasound biomicroscope (VisualSonics VS40). These signals were corrected for diffraction effects and the system transfer function in order to estimate the backscatter coefficient.

Theoretical and experimental high-frequency nonlinear ultrasound propagation through multilayered media

We present a new model of nonlinear ultrasound propagation through multilayered liquid/tissue media. This model includes the effects of diffraction, attenuation, and nonlinearity, with refraction and energy conservation at layer boundaries. It is capable of simulating pulsed and continuous wave propagation from sources of arbitrary geometry and excitation. Using this model, the acoustic field of a high-frequency circular focused transducer (1.5 mm aperture radius, f# 2.5) was simulated for two configurations, water only, and water-tissue phantom-water, and compared to the field measured with a high-frequency needle hydrophone, for 10 different source amplitudes of a transmitted Gaussian pulse (f/sub 0/=20 MHz, fractional bandwidth=35%), ranging from 0.05 to 1 MPa. Very good agreement was found between simulated and measured fields in terms of relative amplitudes of the fundamental, second and third harmonics, beam widths and depth of fields, given the uncertainty in the experimental transmitted pulse amplitudes and hydrophone sensitivity above 40 MHz. Our model is capable of simulating realistic finite-amplitude propagation from high-frequency transducers through multilayered biological media. Its remarkable accuracy and efficiency makes it a very useful tool for the study of nonlinear ultrasonic fields, its well as for transducer design optimization.

ECG-triggered retrospective colour flow imaging

Colour flow imaging systems estimate flow velocities by measuring the phase/time shift between backscattered signals. Using a high-frequency single-element transducer, colour flow imaging of blood velocities has been accomplished in mice by sweeping the transducer over a region of interest. This technique, however, has limitations: tissue clutter artifacts are induced by the sweep-velocity; spatio-temporal artifacts occur when visualizing pulsatile flow; the accuracy of flow velocity estimation is limited. To overcome these limitations, a retrospective colour flow imaging technique (RCFI) has been developed, using the ECG to trigger the acquisition of M-mode data at successive lateral positions of the transducer, followed by retrospective reconstruction of colour flow images. This technique was validated using a phantom with a sinusoidally varying velocity profile, and compared to the swept-scan technique in mouse carotid arteries.

Dual frequency imaging of microbubbles using 1.7-MHz transmit stacks parallel to a 21-MHz receive array

The concept of contrast imaging using microbubble superharmonic signals was introduced in 2002[1]. A number of implementations with detection above 10 MHz have been reported, in particular by our group using a 25–30 MHz single element receive transducer concentric with a low 2–4 MHz transmit ring[2]. In the present work, the implementation of dual frequency (DF) imaging on a Vevo 2100 (VisualSonics, Toronto) was investigated (Fig.1-A).

Subharmonic behavior of targeted and untargeted lipid encapsulated microbubbles at high ultrasound frequencies.

Molecular imaging with ultrasound contrast agents (microbubbles) has recently gained interest as a feasible technique for disease‐specific imaging, with applications ranging from intravascular ultrasound to small animal imaging. The attachment of targeting ligands to the microbubble shell enables a selective accumulation of bound microbubbles around a target site. The ability, however, to differentiate between the nonlinear signal from bound microbubbles and from unbound, circulating agent still remains a challenge. This study conducts a size‐per‐size comparison of the acoustic nonlinear response of individual streptavidin‐coated MicroMarker microbubbles either bound (BMM) or adjacent (UBMM) to a compliant agarose/biotin gel surface. Bubbles were optically sized and insonified at 25 MHz over a range of pressures and pulse bandwidths. The subharmonic (nonlinear) response between unbound (n = 24) and bound (n = 29) bubbles was found to differ significantly, with UBMM bubbles having a higher propensity to in...

A model for the reflectivity enhancement due to particles of perfluorocarbon on a surface

The objective of this study is to explain the reflectivity enhancement due to the presence of randomly distributed particles on a surface. A model is presented where the scattering of all particles is weighted by the diffraction pattern of the transducer and summed over the whole surface. Experimental confirmation of the model was performed with glass microbeads beads and perfluorohexane particles on surfaces of agar and Aqualene at frequencies ranging from 15 MHz to 60 MHz. The model predicted correctly the surface density and the frequency dependence of the reflectivity enhancement. It could be applied to calculate the enhancement from targeted ultrasound contrast agents bound to a surface target.