Micro-ultrasound based characterization of cerebrovasculature following focal ischemic stroke and upon short-term rehabilitation

Notwithstanding recanalization treatments in the acute stage of stroke, many survivors suffer long-term impairments. Physical rehabilitation is the only widely available strategy for chronic-stage recovery, but its optimization is hindered by limited understanding of its effects on brain structure and function. Using micro-ultrasound, behavioral testing, and electrophysiology, we investigated the impact of skilled reaching rehabilitation on cerebral hemodynamics, motor function, and neuronal activity in a rat model of focal ischemic stroke. A 50 MHz micro-ultrasound transducer and intracortical electrophysiology were utilized to characterize neurovascular changes three weeks following focal ischemia elicited by endothelin-1 injection into the sensorimotor cortex. Sprague-Dawley rats were rehabilitated through tray reaching, and their fine skilled reaching was assessed via the Montoya staircase. Focal ischemia led to a sustained deficit in forelimb reaching; and increased tortuosity of the penetrating vessels in the perilesional cortex; with no lateralization of spontaneous neuronal activity. Rehabilitation improved skilled reaching; decreased cortical vascularity; was associated with elevated peri- vs. contralesional hypercapnia-induced flow homogenization and increased perilesional spontaneous cortical neuronal activity. Our study demonstrated neurovascular plasticity accompanying rehabilitation-elicited functional recovery in the subacute stage following stroke, and multiple micro-ultrasound-based markers of cerebrovascular structure and function modified in recovery from ischemia and upon rehabilitation.

Imaging Microbubbles With Contrast-Enhanced Endobronchial Ultrasound

OBJECTIVE

Endobronchial ultrasound (EBUS) is commonly used to guide transbronchial needle biopsies for the staging of lymph nodes in non-small cell lung cancer patients. Although contrast-enhanced ultrasound (CEUS) and microbubbles (MBs) can improve the diagnostic accuracy in tumors, the ability of contrast-enhanced EBUS (CE-EBUS) to image MBs has not yet been comprehensively evaluated. In this study, we assessed the ability of a CE-EBUS system (Olympus EU-ME2 PREMIER and BF-UC180F bronchoscope) to detect laboratory-synthesized MBs in comparison to clinical (Toshiba Aplio SSA-790A) and pre-clinical (VisualSonics Vevo 2100) CEUS systems in vitro and in vivo, respectively.

METHODS

Agar flow phantoms and reference tissue were used to assess CE-EBUS MB imaging in vitro, and A549 tumor-bearing athymic nude and AE17-OVA tumor-bearing C57BL/6 mice were used to assess MB detectability and perfusion in vivo, respectively.

RESULTS

Results revealed that despite the lower sensitivity of CE-EBUS to MB concentration in comparison to clinical CEUS, CE-EBUS yielded a similar contrast-to-tissue ratio (CTR) in vitro of 28.9 ± 4.5 dB for CE-EBUS, compared with 29.7 ± 2.6 dB for clinical CEUS (p < 0.05). In vivo, CE-EBUS generated a perfusion curve highly correlated with that obtained with the pre-clinical CEUS system (Pearson correlation coefficient = 0.927, p < 0.05). Moreover, CE-EBUS yielded a CTR 2.7 times higher than that obtained with the pre-clinical ultrasound system.

CONCLUSION

These findings together suggest that CE-EBUS can perform contrast imaging comparable to that produced by commercial pre-clinical and clinical ultrasound systems, with potential for clinical characterization of mediastinal lymph nodes in lung cancer patients.

The CathEye: A Forward-Looking Ultrasound Catheter for Image-Guided Cardiovascular Procedures

Catheter based procedures are typically guided by X-Ray, which suffers from low soft tissue contrast and only provides 2D projection images of a 3D volume. Intravascular ultrasound (IVUS) can serve as a complementary imaging technique. Forward viewing catheters are useful for visualizing obstructions along the path of the catheter. The CathEye system mechanically steers a single-element transducer to generate a forward-looking surface reconstruction from an irregularly spaced 2-D scan pattern. The steerable catheter leverages an expandable frame with cables to manipulate the distal end independently of vessel tortuosity. The tip position is estimated by measuring the cable displacements and used to create surface reconstructions of the imaging workspace with the single-element transducer. CathEye's imaging capabilities were tested with an agar phantom and an ex vivo chronic total occlusion (CTO) sample while the catheter was confined to various tortuous paths. The CathEye maintained similar scan patterns regardless of path tortuosity and was able to recreate major features of the imaging targets, such as holes and extrusions. The feasibility of forward-looking IVUS with the CathEye is demonstrated in this study. The CathEye mechanism can be applied to other imaging modalities with field-of-view (FOV) limitations and represents the basis for an interventional device fully integrated with image guidance.

Superharmonic and Microultrasound Imaging With Plane Wave Beamforming Techniques

Superharmonic contrast imaging (SpHI) suppresses tissue clutter and allows high-contrast visualization of the vasculature. An array-based dual-frequency (DF) probe has been developed for SpHI, integrating a 21-MHz, 256-element microultrasound imaging array with a 2-MHz, 32-element array to take advantage of the broadband nonlinear responses from microbubble (MB) contrast agents. In this work, ultrafast imaging with plane waves was implemented for SpHI to increase the acquisition frame rate. Ultrafast imaging was also implemented for microultrasound B-mode imaging (HFPW B-mode) to enable high-resolution visualization of the tissue structure. Coherent compounding was demonstrated in vitro and in vivo in both imaging modes. Acquisition frame rates of 4.5 kHz and 187 Hz in HFPW B-mode imaging were achieved for imaging up to 21 mm with one and 25 angles, respectively, and 3.5 kHz and 396 Hz in the SpHI mode with one and nine coherently compounded angles, respectively. SpHI images showed suppression of tissue clutter prior to and after the introduction of MBs in vitro and in vivo. The nine-angle coherently compounded 2-D SpHI images of contrast-filled flow channel showed a contrast-to-tissue ratio (CTR) of 26.0 dB, a 2.5-dB improvement relative to images reconstructed from 0° steering. Consistent with in vitro imaging, the nine-angle compounded 2-D SpHI of a Lewis lung cancer tumor showed a 2.6-dB improvement in contrast enhancement, relative to 0° steering, and additionally revealed a region of nonviable tissue. The 3-D display of the volumetric SpHI data acquired from a xenograft mouse tumor using both 0° steering and nine-angle compounding allowed the visualization of the tumor vasculature. A small vessel visible in the compounded SpHI image, measuring around [Formula: see text], is not visualized in the 0° steering SpHI image, demonstrating the superiority of the latter in detecting fine structures within the tumor.

Fine Pitch Flexible Printed Circuit Board Patterning for Miniaturized Endoscopic MicroUltrasound Arrays

Microultrasound (micro-US) has become an invaluable tool for preclinical research and in emerging applications in clinical diagnosis and treatment guidance. Several such applications can benefit from arrays with a small footprint and endoscopic form factor. However, critical challenges arise in making electrical connections to array elements in such spatial constraints. In this work, we describe a method to pattern a high-density flexible circuit cabling on a copper-on polyimide film, using laser ablation of a polymer resist and wet etching, and then demonstrate a connection to a micro-US array. We investigate laser ablation process parameters and evaluate the ability to consistently pattern continuous copper traces. A minimum 30- [Formula: see text] pitch was achieved with 5- [Formula: see text]-wide electrode lines, and continuity of a 5-m-long trace is demonstrated. A flexible circuit with 30-mm-long traces with 30- [Formula: see text] line and 30- [Formula: see text] space before fan-out was fabricated to connect in an interleaved manner to a 32-element array with 30- [Formula: see text] element pitch. Metal deposition and laser ablation were used to connect and pattern the element electrodes to the copper traces of the flexible circuit. Electrical and acoustic measurements show good yield and consistent impedance across channels. Element pulse-echo tests demonstrated device functionality; the two-way pulse had 43-MHz center frequency and 40% fractional bandwidth (-6 dB). The proposed manufacturing methods facilitate the prototyping and fabrication of flexible endoscopic or small-footprint micro-US devices.

Implementation of a Novel 288-Element Dual-Frequency Array for Acoustic Angiography: In Vitro and In Vivo Characterization

Acoustic angiography is a superharmonic contrast-enhanced ultrasound imaging method that produces high-resolution, 3-D maps of the microvasculature. Previous acoustic angiography studies have used twoelement, annular,mechanicallyactuated transducers(called "wobblers") to image microvasculature in preclinical tumor models with high contrast-to-tissue ratio and resolution, but these earlywobbler transducerscould not achieve the depth and sensitivity required for clinical acoustic angiography. In this work, we present a system for performing acoustic angiography with a novel dual-frequency(DF) transducer-a coaxially stacked DF array (DFA). We evaluate the DFA system bothin vitro andin vivo and demonstrate improvements in sensitivity and imaging depth up to 13.1 dB and 10 mm, respectively, compared with previous wobbler probes.

Characterization of an Array-Based Dual-Frequency Transducer for Superharmonic Contrast Imaging

Superharmonic imaging with dual-frequency imaging systems uses conventional low-frequency ultrasound transducers on transmit, and high-frequency transducers on receive to detect higher order harmonic signals from microbubble contrast agents, enabling high-contrast imaging while suppressing clutter from background tissues. Current dual-frequency imaging systems for superharmonic imaging have been used for visualizing tumor microvasculature, with single-element transducers for each of the low- and high-frequency components. However, the useful field of view is limited by the fixed focus of single-element transducers, while image frame rates are limited by the mechanical translation of the transducers. In this article, we introduce an array-based dual-frequency transducer, with low-frequency and high-frequency arrays integrated within the probe head, to overcome the limitations of single-channel dual-frequency probes. The purpose of this study is to evaluate the line-by-line high-frequency imaging and superharmonic imaging capabilities of the array-based dual-frequency probe for acoustic angiography applications in vitro and in vivo. We report center frequencies of 1.86 MHz and 20.3 MHz with -6 dB bandwidths of 1.2 MHz (1.2-2.4 MHz) and 14.5 MHz (13.3-27.8 MHz) for the low- and high-frequency arrays, respectively. With the proposed beamforming schemes, excitation pressure was found to range from 336 to 458 kPa at its azimuthal foci. This was sufficient to induce nonlinear scattering from microbubble contrast agents. Specifically, in vitro contrast channel phantom imaging and in vivo xenograft mouse tumor imaging by this probe with superharmonic imaging showed contrast-to-tissue ratio improvements of 17.7 and 16.2 dB, respectively, compared to line-by-line micro-ultrasound B-mode imaging.

High-Frequency Array-Based Nanobubble Nonlinear Imaging in a Phantom and In Vivo

There has been growing interest in nanobubbles (NBs) for vascular and extravascular ultrasound contrast imaging and therapeutic applications. Studies to date have generally utilized low frequencies (<12 MHz), high concentrations (>10⁹ mL^-1), and uncalibrated B-mode or contrast-mode on commercial systems without reporting investigations on NB signatures upon which the imaging protocols should be based. We recently demonstrated that low concentrations (10⁶ mL^-1) of porphyrin-lipid-encapsulated NBs scatter nonlinearly at low (2.5, 8 MHz) and high (12.5, 25, 30 MHz) frequencies in a pressure threshold-dependent manner that is advantageous for amplitude modulation (AM) imaging. Here, we implement pressure-calibrated AM at high frequency on a commercial preclinical array system to enhance sensitivity to nonlinear scattering of three phospholipid-based NB formulations. With this approach, improvements in contrast to tissue ratio relative to B-mode between 12.4 and 22.8 dB are demonstrated in a tissue-mimicking phantom, and between 6.7 and 14.8 dB in vivo.

Superharmonic Ultrasound for Motion-Independent Localization Microscopy: Applications to Microvascular Imaging From Low to High Flow Rates

Recent advances in high frame rate biomedical ultrasound have led to the development of ultrasound localization microscopy (ULM), a method of imaging microbubble (MB) contrast agents beyond the diffraction limit of conventional coherent imaging techniques. By localizing and tracking the positions of thousands of individual MBs, ultrahigh resolution vascular maps are generated which can be further analyzed to study disease. Isolating bubble echoes from tissue signal is a key requirement for super-resolution imaging which relies on the spatiotemporal separability and localization of the bubble signals. To date, this has been accomplished either during acquisition using contrast imaging sequences or post-beamforming by applying a spatiotemporal filter to the B-mode images. Superharmonic imaging (SHI) is another contrast imaging method that separates bubbles from tissue based on their strongly nonlinear acoustic properties. This approach is highly sensitive, and, unlike spatiotemporal filters, it does not require decorrelation of contrast agent signals. Since this superharmonic method does not rely on bubble velocity, it can detect completely stationary and moving bubbles alike. In this work, we apply SHI to ULM and demonstrate an average improvement in SNR of 10.3-dB in vitro when compared with the standard singular value decomposition filter approach and an increase in SNR at low flow ( [Formula: see text]/frame) from 5 to 16.5 dB. Additionally, we apply this method to imaging a rodent kidney in vivo and measure vessels as small as [Formula: see text] in diameter after motion correction.

In Vitro Superharmonic Contrast Imaging Using a Hybrid Dual-Frequency Probe

Superharmonic imaging is an ultrasound contrast imaging technique that differentiates microbubble echoes from tissue through detection of higher-order bubble harmonics in a broad frequency range well above the excitation frequency. Application of superharmonic imaging in three dimensions allows specific visualization of the tissue microvasculature with high resolution and contrast, a technique referred to as acoustic angiography. Because of the need to transmit and receive across a bandwidth that spans up to the fifth harmonic of the fundamental and higher, this imaging approach requires imaging probes comprising dedicated transducers for transmit and receive. In this work, we report on a new dual-frequency probe including two 1.7-MHz rectangular transducers positioned one on each side of a 20-MHz 256-element array. Finite element modeling-based design, fabrication processes and assembly of the transducer are described, as is integration with a high-frequency ultrasound imaging platform. Dual-frequency single-plane-wave imaging was performed with a microbubble contrast agent in flow phantoms and compared with conventional high-frequency B-mode imaging, and resolution and contrast-to-tissue ratio were quantified. This work represents an intermediate but informative step toward the development of dual-frequency imaging probes based on array technology, specifically designed for clinical applications of acoustic angiography.

Acoustic Behavior of Halobacterium salinarum Gas Vesicles in the High-Frequency Range: Experiments and Modeling

Gas vesicles (GVs) are a new and unique class of biologically derived ultrasound contrast agents with sub-micron size whose acoustic properties have not been fully elucidated. In this study, we investigated the acoustic collapse pressure and behavior of Halobacterium salinarum gas vesicles at transmit center frequencies ranging from 12.5 to 27.5 MHz. The acoustic collapse pressure was found to be above 550 kPa at all frequencies, nine-fold higher than the critical pressure observed under hydrostatic conditions. We illustrate that gas vesicles behave non-linearly when exposed to ultrasound at incident pressure ranging from 160 kPa to the collapse pressure and generate second harmonic amplitudes of -2 to -6 dB below the fundamental in media with viscosities ranging from 0.89 to 8 mPa·s. Simulations performed using a Rayleigh-Plesset-type model accounting for buckling and a dynamic finite-element analysis suggest that buckling is the mechanism behind the generation of harmonics. We found good agreement between the level of second harmonic relative to the fundamental measured at 20 MHz and the Rayleigh-Plesset model predictions. Finite-element simulations extended these findings to a non-spherical geometry, confirmed that the acoustic buckling pressure corresponds to the critical pressure under hydrostatic conditions and support the hypothesis of limited gas flow across the GV shell during the compression phase in the frequency range investigated. From simulations, estimates of GV bandwidth-limited scattering indicate that a single GV has a scattering cross section comparable to that of a red blood cell. These findings will inform the development of GV-based contrast agents and pulse sequences to optimize their detection with ultrasound.

Image-Guided Ultrasound Characterization of Volatile Sub-Micron Phase-Shift Droplets in the 20-40 MHz Frequency Range

Phase-shift perfluorocarbon droplets are designed to convert from the liquid to the gas state by the external application of acoustic or optical energy. Although droplet vaporization has been investigated extensively at ultrasonic frequencies between 1 and 10 MHz, few studies have characterized performance at the higher frequencies commonly used in small animal imaging. In this study, we use standard B-mode imaging sequences on a pre-clinical ultrasound platform to both image and activate sub-micron decafluorobutane droplet populations in vitro and in vivo at center frequencies in the range of 20-40 MHz. Results show that droplets remain stable against vaporization at low imaging pressures but are vaporized at peak negative pressures near 3.5 MPa at the three frequencies tested. This study also found that a small number of size outliers present in the distribution can greatly influence droplet performance. Removal of these outliers results in a more accurate assessment of the vaporization threshold and produces free-flowing microbubbles upon vaporization in the mouse kidney.

Quantification of Microvascular Tortuosity during Tumor Evolution Using Acoustic Angiography

The recent design of ultra-broadband, multifrequency ultrasound transducers has enabled high-sensitivity, high-resolution contrast imaging, with very efficient suppression of tissue background using a technique called acoustic angiography. Here we perform the first application of acoustic angiography to evolving tumors in mice predisposed to develop mammary carcinoma, with the intent of visualizing and quantifying angiogenesis progression associated with tumor growth. Metrics compared include vascular density and two measures of vessel tortuosity quantified from segmentations of vessels traversing and surrounding 24 tumors and abdominal vessels from control mice. Quantitative morphologic analysis of tumor vessels revealed significantly increased vascular tortuosity abnormalities associated with tumor growth, with the distance metric elevated approximately 14% and the sum of angles metric increased 60% in tumor vessels versus controls. Future applications of this imaging approach may provide clinicians with a new tool in tumor detection, differentiation or evaluation, though with limited depth of penetration using the current configuration.

Characterization of submicron phase-change perfluorocarbon droplets for extravascular ultrasound imaging of cancer

Because many tumors possess blood vessels permeable to particles with diameters of 200 nm, it is possible that submicron perfluorocarbon droplets could constitute a novel extravascular ultrasound contrast agent capable of selectively enhancing tumors. Under exposure to bursts of ultrasound of sufficient rarefactional pressure, droplets can undergo vaporization to form echogenic microbubbles. In this study, phase-change thresholds of 220-nm-diameter droplets composed of perfluoropentane were studied in polyacrylamide gel phantoms maintained at temperatures of 21-37°C, exposed to high-pressure bursts of ultrasound with frequencies ranging from 5-15 MHz and durations of 1 μs to 1 ms. The thresholds were found to depend inversely and significantly (p < 0.001) on ultrasound frequency, pulse duration, and droplet temperature, ranging from 9.4 ± 0.8 MPa at 29°C for a 1-μs burst at 5 MHz to 3.2 ± 0.5 MPa at 37°C for a 1-ms burst at 15 MHz. The diameters of microbubbles formed from droplets decreased significantly as phantom stiffness increased (p < 0.0001), and were independent of pulse duration, although substantially more droplets were converted to microbubbles for 1-ms pulse durations compared with briefer exposures. In vivo experiments in a mouse tumor model demonstrated that intravenously injected droplets can be converted into highly echogenic microbubbles 1 h after administration.

The effect of binding on the subharmonic emissions from individual lipid-encapsulated microbubbles at transmit frequencies of 11 and 25 MHz

Targeted microbubble imaging at ultrasound frequencies above 5 MHz has applications in both a preclinical context for a range of disease processes and clinically for the assessment of atherosclerosis and superficial tumors. Although the feasibility of ultrasound molecular imaging has been well demonstrated for a range of target molecules, little work has examined the effects of binding on microbubble oscillations, which is of potential relevance to improving the sensitivity, specificity, and quantification of bound-bubble detection. In this study we investigated the influence of binding on the subharmonic response of bubbles at transmit frequencies of 11 and 25 MHz. Individual bubbles were situated adjacent to a boundary in either a bound or an unbound state, optically sized and acoustically interrogated with pressures ranging from 0.02 to 1.2 MPa. At 11 MHz, unbound bubbles (n = 53) were found to have strong subharmonic activity for sizes between 2.4 and 2.6 μm, whereas bound bubbles (n = 50) were most active from 2.6 to 3.0 μm. Destruction thresholds were found to be lower for bound bubbles. At 25 MHz, bound-bubble (n = 57) activity was found to peak at 1.9 μm as compared to 2.1 μm in the unbound cases (n = 53), with a 20% increase in amplitude. Comparison with simulations indicates that both unbound and bound bubbles undergo compression-only behavior at 11 MHz, and expansion-dominated behavior at 25 MHz. Subharmonic emissions elicited from 0 radian transmit pulses were found to be π/2 radians out of phase with those elicited from a π radian transmit pulse, suggesting inefficient subharmonic preservation from pulse inversion schemes. With the appropriate postprocessed phase correction, an increase in the subharmonic amplitude of up to 60% was shown, depending on the bubble size and transmit frequency.

Investigating the subharmonic response of individual phospholipid encapsulated microbubbles at high frequencies: a comparative study of five agents

There are a range of contrast ultrasound applications above 10 MHz, a frequency regime in which nonlinear microbubble behavior is poorly understood. Lipid-encapsulated microbubbles have considerable potential for use at higher frequencies because they have been shown to exhibit pronounced nonlinear activity at frequencies up to 40 MHz. The objective of this work was to investigate the influence of agent formulation on the subharmonic response of lipid-encapsulated microbubbles at high frequencies with a view to providing information relevant to improving contrast agent design and imaging performance. An optical-acoustical setup was used to measure the subharmonic emissions from small (d < 3 μm) individual lipid-encapsulated microbubbles as a function of transmit pressure, size and composition. In this study, five agent formulations (Definity™, MicroMarker™ and three in-house agents manipulated to exhibit different levels of shell microstructure heterogeneity) were insonified at 25 MHz over a peak negative pressure (P(n)) range of 0.02-1.2 MPa. All agents exhibited distinctly different subharmonic behavior, both in terms of amplitude and active sizes. MicroMarker™ exhibited the strongest, broadest and most consistent subharmonic response, 22% greater in power than that of Definity™ and as much as 50% greater than the in-house formulations. No clear relation between in-house agents' shell microstructure and nonlinear response was found, other than the variability in the nonlinear response itself. An analysis of the response of MicroMarker™ bubbles suggests that these bubbles exhibit "expansion-dominated" oscillations, in contrast to "compression-only" oscillations observed for similar bubbles at lower frequencies (f < 11 MHz).

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.

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-microm pitch, 18-microm kerf width, and 100-microm thickness with isosceles triangular pillars in which the isosceles angle varied from 30 degrees to 60 degrees 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 degrees angle pillars has the lowest secondary pulse amplitude. The secondary pulse becomes larger when the pillar angle deviates from 45 degrees . To study whether the pillar height (which determines the resonance frequency) and aspect ratio would change the optimum angle, composites with 40-mum pitch, 15-microm kerf width, and 45-microm 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 degrees 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 degrees, 45 degrees, and 60 degrees 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 degrees design.

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-negativepressure) 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 highfrequency ultrasound to targeted contrast agents.

Acoustic and kinetic behaviour of definity in mice exposed to high frequency ultrasound

Microbubble contrast agents have shown clinical potential for characterising blood flow using 1 to 10 MHz ultrasound; however, scaling their use for similar applications in the mouse with high frequency ultrasound (20 to 60 MHz) has not been addressed. The goal was to determine the utility of microbubbles for mouse imaging with 30 MHz ultrasound by investigating their attenuation and backscatter characteristics as a function of concentration in vitro and dose response in vivo. The agent was exposed to a 30 MHz, 20% bandwidth pulse with a peak negative pressure of 244 kPa. In vitro results showed that the attenuation and backscatter increased linearly for concentrations between 2.8 x 10(6) and 28 x 10(6) bubbles per mL of deionized water. In vivo experiments where performed in the jugular vein of CD-1 mice and time intensity curves were acquired for doses between 10 and 100 microL kg(-1). These doses corresponded to the range of concentrations used in vitro. In vivo results showed that the peak enhancement of the agent increased linearly for doses between 10 and 60 microL kg(-1), the duration of enhancement varied between 200 to 300 s and the integrated enhancement (area under the curve) increased linearly up to 100 microL kg(-1). A maximum enhancement of 13 dB over the blood pool was observed for a dose of 100 microL kg(-1). The intra- and inter-mouse variabilities were 10% to 40% and indicate that further optimisations are required. These results suggest that quantitative contrast flow studies in the mouse using high frequency ultrasound are possible for doses between 10 and 60 microL kg(-1).

High-frequency subharmonic pulsed-wave Doppler and color flow imaging of microbubble contrast agents

A recent study has shown the feasibility of subharmonic (SH) flow imaging at a transmit frequency of 20 MHz. This paper builds on these results by examining the performance of SH flow imaging as a function of transmit pressure. Further, we also investigate the feasibility of SH pulsed-wave Doppler (PWD) imaging. In vitro flow experiments were performed with a 1-mm-diameter wall-less vessel cryogel phantom using the ultrasound contrast agent Definity and an imaging frequency of 20 MHz. The phantom results show that there is an identifiable pressure range where accurate flow velocity and power estimates can be made with SH imaging at 10 MHz (SH10), above which velocity estimates are biased by radiation force effects and unstable bubble behavior, and below which velocity and power estimates are degraded by poor SNR. In vivo validation of SH PWD was performed in an arteriole of a rabbit ear, and blood velocity estimates compared well with fundamental (F20) mode PWD. The ability to suppress tissue signals using SH signals may enable the use of higher frame rates and improve sensitivity to microvascular flow or slow velocities near large vessel walls by reducing or eliminating the need for clutter filters.

In vitro characterization of the subharmonic ultrasound signal from Definity microbubbles at high frequencies

Ultrasound microbubble contrast agents have been demonstrated to scatter subharmonic energy at one-half the driving frequency. At ultrasound frequencies in the 20-40 MHz range, the subharmonic offers the potential to differentiate the blood in the microcirculation from the surrounding tissue. It is unknown whether current contrast agents, manufactured to be resonant between 2 and 12 MHz, are ideal for subharmonic imaging at higher frequencies. We performed numerical simulations of the Keller-Miksis model for the behavior of a single bubble and experimental investigations of Definity microbubbles in water. The results supported the hypothesis that off-resonant bubbles, excited at their second harmonic, may be primarily responsible for the observed subharmonic energy. For frequencies between 20 and 32 MHz and 32 and 40 MHz, the optimal bubble diameters for the generation of subharmonics in vitro were determined experimentally to be 1.2-5 microm and less than 1.2 microm, respectively. Definity may be a suitable ultrasound contrast agent for subharmonic imaging at 20 MHz with peak-negative pressures between 380 and 590 kPa and pulses greater than or equal to four cycles in duration.

Noninvasive ultrasonic measurement of regional and local pulse-wave velocity in mice

Mouse models of human disease are increasingly used to study the nature of cardiovascular diseases such as atherosclerosis. The pulse wave velocity (PWV) provides an indirect measure of arterial stiffness and can be useful for characterizing disease progression. In this study, the PWV was measured noninvasively in the left common carotid artery of seven young mice using two image-guided approaches: a regional transit-time (TT) method and a local flow-area (QA) method. The QA approach measures the cross-sectional area and volume flow through the vessel using high frame-rate retrospective colour flow imaging. The QA method was found to correlate well with the TT method (r2=0.80, p<0.001). The mean difference between methods was 0.05+/-0.21 m/s. This study demonstrates the feasibility of measuring both regional and local PWV in mice using image-based high-frequency ultrasound methodologies.

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-microm 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 +/-30 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 microm. 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 microm 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 microm.

Model for the ultrasound reflection from micro-beads and cells distributed in layers on a uniform surface

A model predicting the reflection of ultrasound from multiple layers of small scattering spheres is developed. Predictions of the reflection coefficient, which takes into account the interferences between the different sphere layers, are compared to measurements performed in the 10-80 MHz and 15-35 MHz frequency range with layers of glass beads and spherical acute myeloid leukemia (AML) cells, respectively. For both types of scatterers, the reflection coefficient increases as a function of their density on the surface for less than three superimposed layers, at which point it saturates at 0.38 for glass beads and 0.02 for AML cells. Above three layers, oscillations of the reflection coefficient due to constructive or destructive interference between layers are observed experimentally and are accurately predicted by the model. The use of such a model could lead to a better understanding of the structures observed in layered tissue images.

Nonlinear ultrasound propagation through layered liquid and tissue-equivalent media: computational and experimental results at high frequency

Nonlinear propagation has been demonstrated to have a significant impact on ultrasound imaging. An efficient computational algorithm is presented to simulate nonlinear ultrasound propagation through layered liquid and tissue-equivalent media. Results are compared with hydrophone measurements. This study was undertaken to investigate the role of nonlinear propagation in high frequency ultrasound micro-imaging. The acoustic field of a focused transducer (20 MHz centre frequency, f-number 2.5) was simulated for layered media consisting of water and tissue-mimicking phantom, for several wide-bandwidth source pulses. The simulation model accounted for the effects of diffraction, attenuation and nonlinearity, with transmission and refraction at layer boundaries. The parameter of nonlinearity, B/A, of the water and tissue-mimicking phantom were assumed to be 5.2 and 7.4, respectively. The experimentally measured phantom B/A value found using a finite-amplitude insert-substitution method was shown to be 7.4 +/- 0.6. Relative amounts of measured second and third harmonic pressures as a function of the fundamental pressures at the focus were in good agreement with simulations. Agreement within 3% was found between measurements and simulations of the beam widths of the fundamental and second harmonic signals following propagation through the tissue phantom. The results demonstrate significant nonlinear propagation effects for high frequency imaging beams.

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-microm 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 ease 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% and 83%, respectively, and a -20 dB pulse width of -99 ns. The per-element peak acoustic power, for a +/- 30 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.

A model for reflectivity enhancement due to surface bound submicrometer particles

Submicrometer particles filled with liquid perfluorocarbon have been shown to increase the ultrasound reflectivity of surfaces onto which they bind and, consequently, are seen as potential targeted contrast agents. The objective of this study is to explain the reflectivity enhancement as a result of the presence of randomly distributed particles on a surface. A model is presented where the diffraction-weighted scattering of all particles is summed over the exposed surface. Experiments were performed at frequencies ranging from 15 MHz to 60 MHz, with glass microbeads and perfluorohexane particles deposited on the surface of agar and Aqualene, a rubber closely matched to water, to confirm the validity of the model. Results showed that the model predicts the surface density and the frequency dependence of the reflectivity enhancement up to a density corresponding to twice the maximum packing of spheres on a surface (200% confluence fraction) for glass beads and a fifth (20% confluence fraction) for perfluorohexane particles. This suggests the possibility of predicting signal enhancement due to a bound contrast agent in simple geometries.

Investigating perfluorohexane particles with high-frequency ultrasound

Submicron particles filled with liquid perfluorocarbon are currently being studied as a potential ultrasound-targeted contrast agent. The objective of this study was to evaluate the scattering properties of these particles. Sets of perfluorohexane-filled particles of different average sizes (300 nm to 1000 nm) were produced with a constant total volume fraction. The attenuation coefficient was measured in the 15- to 50-MHz frequency range and was found to increase smoothly with frequency and to be independent of the amplitude and bandwidth of the transmitted pulse. The values range from 0.31 to 0.64 dB/mm at 30 MHz for mean particle size ranging from 970 to 310 nm, respectively. The backscattering spectra of the particle solutions were measured and showed no sign of nonlinear scattering. The backscattering coefficient increased with the power 3.9 +/- 0.3 of the frequency. These results confirm that liquid perfluorocarbon droplets behave as linear Rayleigh scatterers.

High frequency nonlinear B-scan imaging of microbubble contrast agents

It previously was 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.

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.

Automatic three-dimensional reconstruction and characterization of articular cartilage from high-resolution ultrasound acquisitions

Three-dimensional (3D) high-resolution ultrasonography has proved to be useful for in vitro assessment of cartilage remodeling due to osteoarthritis. The diagnosis is performed by computation of the mean thickness of the cartilage, which reveals hypertrophy or thinning, and by 3D reconstruction of the data, which provides essential information about the size, extent, and localization of the lesion. In both cases, preliminary segmention of the cartilage is necessary. This article proposes an algorithm for automatic segmentation of the cartilage from 3D ultrasonic acquisitions of the rat patella, which includes the detection of the cartilage surface and the cartilage/bone interface. The method was designed on the assumption of regularity and smoothness of the interfaces. The use of a global threshold was sufficient to separate the patella area from the background. The cartilage/bone interface was detected by selection of regions of interest (ROIs) encompassing the interface, followed by the detection of the interface within these ROIs using the graph theory. The method was applied to 162 samples. The detection accuracy was judged to be very good or good in 99% of the cases for the cartilage surface and in 86% of the cases for the cartilage/bone interface. The mean cartilage thickness value in the central part of the patella obtained from the automatic detection method was compared to that obtained manually. The coefficient of correlation between the two measurements was 0.92. These results show that our method is reliable. Thus, fast processing of a large number of acquisitions and a more complete analysis of the cartilage surface become possible.

Comparison of psychosocial adaptation of advanced stage Hodgkin's disease and acute leukemia survivors. Cancer and Leukemia Group B

BACKGROUND

The purpose of this study was to compare the long-term psychosocial adaptation of Hodgkin's disease and adult acute leukemia survivors.

PATIENTS AND METHODS

Two hundred seventy-three Hodgkin's disease (HD) and 206 adult acute leukemia (AL) survivors were interviewed by telephone concerning their psychosocial adjustment and problems they attributed to having been treated for cancer, using identical research procedures and a common set of instruments. The following measures were used: Psychosocial Adjustment to Illness Scale (PAIS); Brief Symptom Inventory (BSI); current Conditioned Nausea and Vomiting triggered by treatment-related stimuli (CNVI); Indices of Employment, Insurance and Sexual Problems Attributed to Cancer; Negative Socioeconomic Impact of Cancer Index (NSI). All participants had been treated on one of nine Hodgkin's disease or 13 acute leukemia Cancer and Leukemia Group B (CALGB) clinical trials from 1966-1988, and had been off treatment for one year or more (mean years: HD = 5.9; AL = 5.6).

RESULTS

HD survivors' risk of having a high distress score on the BSI was almost twice that found for AL survivors (odds ratio = 1.90), with 21% of HD vs. 14% of AL survivors (P < 0.05) having scores that were 1.5 standard deviations above the norm, suggestive of a possible psychiatric disorder. HD survivors reported greater fatigue (POMS Fatigue, P = 0.01; Vigor Subscales, P = 0.001), greater conditioned nausea (CNVI, P < 0.05), greater impact of cancer on their family life (PAIS Domestic Environment, P = 0.004) and poorer sexual functioning (PAIS Sexual Relationships, P = 0.0001), than AL survivors.

CONCLUSIONS

Treatment-related issues may have placed HD survivors at a greater risk for problems in long-term adaptation than AL survivors.

Comparison of psychosocial adaptation and sexual function of survivors of advanced Hodgkin disease treated by MOPP, ABVD, or MOPP alternating with ABVD

BACKGROUND

Survivors of advanced Hodgkin disease, who were assigned randomly to treatment by mechlorethamine, vincristine, procarbazine, and prednisone (MOPP); doxorubicin, bleomycin, vinblastine, and dacarbazine (ABVD); or MOPP alternating with ABVD in a clinical trial of the Cancer and Leukemia Group B (protocol 8251), were compared in terms of their psychosocial adaptation and psychosexual function an average of 2.2 years after completion of treatment (range, 1-5 years). The study was undertaken to determine if there were differences among treatments in these functional areas as a consequence of differential long-term gonadal damage in the three regimens.

METHODS

Ninety-three disease-free survivors of advanced Hodgkin disease (56 men and 37 women) were studied (a minimum of 1 year after completion of treatment) by an interview conducted over the telephone. Standardized measures were used to assess their psychologic, sexual, family, and vocational functioning, including the following tests: the Psychosocial Adjustment to Illness Scale--Self Report, the Brief Symptom Inventory, the Profile of Mood States, and the Impact of Event Scale.

RESULTS

Contrary to expectation, no statistically significant differences in survivors' psychosocial adaptation or psychosexual function were found by treatment arm. Infertility (based on survivors' reports of medical test results and perceptions) and lower income 1 year before the diagnosis of cancer were significant predictors of poorer adjustment. Most survivors reported a range of problems that they attributed to having had cancer: 35%, proven or perceived infertility; 24%, sexual problems; 31%, health and life insurance problems; 26%, a negative socioeconomic effect; and 51%, conditioned nausea, associated with visual or olfactory reminders of chemotherapy.

CONCLUSIONS

No significant long-term advantage in psychosocial adaptation or psychosexual function was found for survivors of Hodgkin disease treated by the less gonadally toxic ABVD regimen 1 to 5 years after completion of treatment.

Hodgkin disease survivors at increased risk for problems in psychosocial adaptation. The Cancer and Leukemia Group B

BACKGROUND

The long-term psychosocial adaptations of 273 survivors of advanced Hodgkin disease were assessed to determine the nature and extent of problems experienced and to identify those at high risk for maladaptation.

METHODS

Hodgkin disease survivors were identified who initially had been treated in clinical trials within the Cancer and Leukemia Group B from 1966 to 1986, were currently disease free, and had completed treatment for a minimum of 1 year. All survivors had advanced Hodgkin disease (with disease diagnosed at a mean age of 28 years). Survivors were at a mean age of 37 years at the time of interview (6.3 years after treatment completion), and 60% were male. Survivors were interviewed over the telephone 7-10 days after questionnaires were mailed to them concerning their psychological, social, vocational, and sexual functioning.

RESULTS

Psychological distress was found to be elevated by one standard deviation (SD) above that of healthy subjects on the Brief Symptom Inventory, and 22% met the criterion suggested for a psychiatric diagnosis. In addition, the following problems were reported by survivors to be a consequence of having had Hodgkin disease: denial of life (31%) and health (22%) insurance, sexual problems (37%), conditioned nausea in response to reminders of chemotherapy (39%), and a negative socioeconomic effect (36%). Survivors found to be at high risk for maladaptation were: men earning less than $15,000 per year or who were currently unemployed; unmarried individuals; those with serious illnesses since treatment completion; and those who were less educated.

CONCLUSIONS

These findings suggest that including a routine assessment of these factors would help to target survivors in need of additional evaluation and treatment.

Quality of life assessment of Hodgkin's disease survivors: a model for cooperative clinical trials

With the growing number of patients surviving cancer, there has been an increasing concern with their long-term adaptation. Given the large number of Hodgkin's disease survivors, it was possible to conduct a study of their psychosocial adaptation. Two hundred seventy-three survivors of advanced Hodgkin's disease were interviewed over the telephone concerning the impact of cancer upon their lives, in terms of their psychological, social, and sexual functioning. The level of psychological distress was elevated by one standard deviation above normal on the Brief Symptom Inventory (BSI), for both men and women. Survivors in greater distress reported more problems in other areas of functioning, including sexual, social, vocational, and persistent conditioned nausea.