E. Cherin

A NEW ULTRASOUND INSTRUMENT FOR IN VIVO MICROIMAGING OF MICE

We report here on the design and evaluation of the first high-frequency ultrasound (US) imaging system specifically designed for microimaging of the mouse. High-frequency US or US biomicroscopy (UBM) has the advantage of low cost, rapid imaging speed, portability and high resolution. In combination with the ability to provide functional information on blood flow, UBM provides a powerful method for the investigation of development and disease models. The new UBM imaging system is demonstrated for mouse development from day 5.5 of embryogenesis through to the adult mouse. At a frequency of 40 MHz, the resolution voxel of the new mouse scanner measures 57 microm x 57 microm x 40 microm. Duplex Doppler provides blood velocity sensitivity to the mm per s range, consistent with flow in the microcirculation, and can readily detect blood flow in the embryonic mouse heart, aorta, liver and placenta. Noninvasive UBM assessment of development shows striking similarity to invasive atlases of mouse anatomy. The most detailed noninvasive in vivo images of mouse embryonic development achieved using any imaging method are presented.

High frequency nonlinear B-scan imaging of microbubble contrast agents

It was previously shown that it is possible to produce nonlinear scattering from microbubble contrast agents using transmit frequencies in the 14-32 MHz range, suggesting the possibility of performing high-frequency, nonlinear microbubble imaging. In this study, we describe the development of nonlinear microbubble B-scan imaging instrumentation capable of operating at transmit center frequencies between 10 and 50 MHz. The system underwent validation experiments using transmit frequencies of 20 and 30 MHz. Agent characterization experiments demonstrate the presence of nonlinear scattering for the conditions used in this study. Using wall-less vessel phantoms, nonlinear B-scan imaging is performed using energy in one of the subharmonic, ultraharmonic, and second harmonic frequency regions for transmit frequencies of 20 and 30 MHz. Both subharmonic and ultraharmonic imaging modes achieved suppression of tissue signals to below the noise floor while achieving contrast to noise ratios of up to 26 and 17 dB, respectively. The performance of second harmonic imaging was compromised by nonlinear propagation and offered no significant contrast improvement over fundamental mode imaging. In vivo experiments using the subharmonic of a 20 MHz transmit pulse show the successful detection of microvessels in the rabbit ear and in the mouse heart. The results of this study demonstrate the feasibility of nonlinear microbubble imaging at high frequencies

High-resolution, high-contrast ultrasound imaging using a prototype dual-frequency transducer: In vitro and in vivo studies

With recent advances in animal models of disease, there has been great interest in capabilities for highresolution contrast-enhanced ultrasound imaging. Microbubble contrast agents are unique in that they scatter broadband ultrasound energy because of their nonlinear behavior. For optimal response, it is desirable to excite the microbubbles near their resonant frequency. To date, this has been challenging with high-frequency imaging systems because most contrast agents are resonant at frequencies in the order of several megahertz. Our team has developed a unique dual-frequency confocal transducer which enables low-frequency excitation of bubbles near their resonance with one element, and detection of their emitted high-frequency content with the second element. Using this imaging approach, we have attained an average 12.3 dB improvement in contrast-to-tissue ratios over fundamental mode imaging, with spatial resolution near that of the high-frequency element. Because this detection method does not rely on signal decorrelation, it is not susceptible to corruption by tissue motion. This probe demonstrates contrast imaging capability with significant tissue suppression, enabling high-resolution contrast-enhanced images of microvascular blood flow. Additionally, this probe can readily produce radiation force on flowing contrast agents, which may be beneficial for targeted imaging or therapy.

Performance and Characterization of New Micromachined High-Frequency Linear Arrays

A new approach for fabricating high frequency (>20 MHz) linear array transducers, based on laser micromachining, has been developed. A 30 MHz, 64-element, 74-mum pitch, linear array design is presented. The performance of the device is demonstrated by comparing electrical and acoustic measurements with analytical, equivalent circuit, and finite-element analysis (FEA) simulations. All FEA results for array performance have been generated using one global set of material parameters. Each fabricated array has been integrated onto a flex circuit for case of handling, and the flex has been integrated onto a custom printed circuit board test card for ease of testing. For a fully assembled array, with an acoustic lens, the center frequency was 28.7 MHz with a one-way -3 dB and -6 dB bandwidth of 59% arid 83%, respectively, arid a -20 dB pulse width of -99 ns. The per-element peak acoustic power, for a plusmn30 V single cycle pulse, measured at the 10 mm focal length of the lens was 590 kPa with a -6 dB directivity span of about 30 degrees. The worst-case total cross talk of the combined array and flex assembly is for nearest neighboring elements and was measured to have an average level -40 dB across the -6 dB bandwidth of the device. Any significant deviation from simulation can be explained through limitations in apparatus calibration and in device packaging

Experimental characterization of fundamental and second harmonic beams for a high-frequency ultrasound transducer

In the diagnostic frequency range, nonlinear imaging has been shown to improve image contrast and decrease artefacts. The extension of these techniques to high-frequency imaging (>15 MHz) was investigated. The second harmonic beam at 40 MHz of a high-frequency focused transducer (aperture 6 mm, focal distance 10 mm, f-number 1.67) was measured experimentally in water, in transmission and pulse-echo, and compared with the fundamental beams at 20 MHz and 40 MHz. Measurements were performed at peak negative pressures of 0.8 to 4.7 MPa. Transmission measurements were performed with a custom hydrophone with a 25microm spot size to limit beam averaging. Over the range of peak negative pressures, the transmitted harmonic (40 MHz) beam had an average lateral beam width (-3 dB) of 77 microm and an average depth-of-field of 0.93 mm, whereas the fundamental beam had a corresponding beam width of 137 microm and a depth-of-field of 1.59 mm. The harmonic beam showed a 3-dB decrease in side lobe levels. Preliminary second harmonic images of mouse tissue in vitro are presented and compared to fundamental imaging at 20 and 40 MHz.

Harmonic imaging at high frequencies for IVUS

The authors investigated the properties of harmonic fields at high frequencies. A 40 MHz focused broadband transducer was excited at 20 and 40 MHz. The response from 20 MHz was measured at 20 and 40 MHz. The response from 40 MHz was determined at 40 MHz. Hydrophone measurements revealed that the harmonic field was proportional to the square of the fundamental field. The harmonic field increased 10 dB over the focal zone with respect to the fundamental field at 40 MHz. Harmonic images of an excised human femoral artery gave significantly better resolution and more detail than the fundamental image. The fundamental at the double frequency gave similar resolution, but higher attenuation than the harmonic. Additional advantages of harmonic imaging are discussed.

2F-1 Fabrication and Performance of a High-Frequency Geometrically Focussed Composite Transducer with Triangular Pillar Geometry

A single-element, 40 MHz, 3 mm diameter transducer was fabricated with a geometric focus at 9 mm. The transducer was based on a piezo-composite substrate with triangular-shaped composite pillars. The transducer produced pulses with a two-way bandwidth of 55 %. The bandwidth and impedance magnitude were in agreement with that predicted using finite element modeling. A one-way radiation pattern was collected using a needle hydrophone. The oneway -3 dB beamwidth at the geometric focus was measured to be 120 mum and the -3 dB depth-of-field was 2.5 mm This is in good agreement to the theoretical predictions of 112.5 mum and 2.4 mm The triangular-pillar composite transducer was then compared to a transducer with square composite pillars. A 12 dB reduction in the amplitude of the secondary resonance was found for the triangular- pillar composite.

Reflection from bound microbubbles at high ultrasound frequencies

Targeted contrast agents and ultrasound imaging are now used in combination for the assessment and tracking of biomarkers in animal models in vivo. These applications have triggered interest in the understanding and prediction of the ultrasound echoes from contrast agents attached to cells. This study describes the reflection enhancement due to microbubbles bound on a gelatin surface. The reflection enhancement was measured using ultrasound pulses at high frequency (40 MHz) and low pressure (38 kPa peak-negative pressure) allowing a linear approximation to be applied. The observed reflection coefficient increased with the number of microbubbles, until reaching saturation at 0.9 when the surface coverage fraction was 35%. A multiple scattering model assuming that the targeted microbubbles are confined within an infinitesimally thin layer appeared suitable in predicting the reflection coefficient even at very high surface densities. These results could permit the optimization of the sensitivity of high frequency ultrasound to targeted contrast agents.

Performance and characterization of high frequency linear arrays

A new approach for fabricating high frequency (> 20 MHz) linear array transducers, based on laser micromachining, has been developed. The capabilities of this approach will be presented using a 30 MHz 64-element, 74-micron pitch and 8- micron kerf design. Each fabricated array has been integrated onto a flex circuit for ease of handling and the flex has been integrated onto a custom circuit board for ease of testing. Examples of measured characteristics of arbitrary array elements are as follows: Electrical impedance, measured in air, of about 120 Ohms with -20 degrees of phase. All transducer elements were acoustically tested using a +/- 30V single cycle drive pulse and a 40µm needle hydrophone. The average center frequency was found to be 28.1 +/- 0.7 MHz with a 1-way bandwidth of 83 +/- 1.2 %. The average peak-to-peak pressure measured at an axial distance of 10 mm from a transducer element was 590 +/- 24 kPa. The combined acoustic and electrical cross talk for nearest neighbours, averaged across the bandwidth of the device was determined to be -40 dB. Simulations of the array characteristics based on finite element analysis performed with PZFlex show good agreement with experimental results. Synthesized images based on the measured performance of the array elements for a given fabricated transducer will also be presented.

Geometry effect on piezo-composite transducers with triangular pillars

High frequency transducers/arrays made of piezo- composite materials have the advantages of lower acoustic impedances, which better match tissue and a flexibility that allows focusing without the use of an acoustic lens. However, developing a high-frequency piezo-composite material for such arrays is still a challenge due to the extremely small pillar dimensions required to avoid the interference from the lateral resonances. Recently, success in developing high-frequency transducers made of piezo-composite materials with triangular pillars has been reported. The use of triangular pillar piezo-composite material was shown to suppress lateral resonances that appear in square pillar composites. To further understand how the geometry of the pillars affects the lateral resonance, piezo- composite materials with different triangular pillar angles are investigated in this work. The performance of composite transducers with triangular pillar angles of 30deg, 40deg, 45deg, 50deg and 60deg were simulated using PZFlex. The electrical impedances of these different transducers show large differences in lateral resonances. The lateral resonances cause a secondary pulse to appear after the main pulse in the time response and ripples in the pass-band of the frequency response. This secondary pulse will produce a ghost in imaging and need to be suppressed. The simulation results show that the composite with a 45deg pillar angle has the lowest secondary pulse amplitude (-22 dB below the main pulse). The secondary pulse becomes larger when the angle deviates from 45deg. Composites with 30deg and 60deg angles have secondary pulse amplitude -15 dB and -10 dB below the main pulse amplitude, respectively. Experimental composite samples have also been fabricated and acoustical and impedance measurements compared with simulation predictions. Three composite transducers with pillar angles of 30deg, 45deg and 60deg were made from PZT5H and Epotek 301 epoxy. The electrical impedances and the pulse echoes were measured to compare with theoretical predictions.