Adaptive windowing in contrast-enhanced intravascular ultrasound imaging

Intravascular ultrasound (IVUS) is one of the most commonly-used interventional imaging techniques and has seen recent innovations which attempt to characterize the risk posed by atherosclerotic plaques. One such development is the use of microbubble contrast agents to image vasa vasorum, fine vessels which supply oxygen and nutrients to the walls of coronary arteries and typically have diameters less than 200 µm. The degree of vasa vasorum neovascularization within plaques is positively correlated with plaque vulnerability. Having recently presented a prototype dual-frequency transducer for contrast agent-specific intravascular imaging, here we describe signal processing approaches based on minimum variance (MV) beamforming and the phase coherence factor (PCF) for improving the spatial resolution and contrast-to-tissue ratio (CTR) in IVUS imaging. These approaches are examined through simulations, phantom studies, ex vivo studies in porcine arteries, and in vivo studies in chicken embryos. In phantom studies, PCF processing improved CTR by a mean of 4.2 dB, while combined MV and PCF processing improved spatial resolution by 41.7%. Improvements of 2.2 dB in CTR and 37.2% in resolution were observed in vivo. Applying these processing strategies can enhance image quality in conventional B-mode IVUS or in contrast-enhanced IVUS, where signal-to-noise ratio is relatively low and resolution is at a premium

Refraction Correction in 3D Transcranial Ultrasound Imaging

We present the first correction of refraction in three-dimensional (3D) ultrasound imaging using an iterative approach that traces propagation paths through a two-layer planar tissue model, applying Snell’s law in 3D. This approach is applied to real-time 3D transcranial ultrasound imaging by precomputing delays offline for several skull thicknesses, allowing the user to switch between three sets of delays for phased array imaging at the push of a button. Simulations indicate that refraction correction may be expected to increase sensitivity, reduce beam steering errors, and partially restore lost spatial resolution, with the greatest improvements occurring at the largest steering angles. Distorted images of cylindrical lesions were created by imaging through an acrylic plate in a tissue-mimicking phantom. As a result of correcting for refraction, lesions were restored to 93.6% of their original diameter in the lateral direction and 98.1% of their original shape along the long axis of the cylinders. In imaging two healthy volunteers, the mean brightness increased by 8.3% and showed no spatial dependency

On the Relationship Between Microbubble Fragmentation, Deflation and Broadband Superharmonic Signal Production

Acoustic angiography imaging of microbubble contrast agents utilizes the superharmonic energy produced from excited microbubbles, and enables high-contrast, high-resolution imaging. However, the exact mechanism by which broadband harmonic energy is produced is not fully understood. In order to elucidate the role of microbubble shell fragmentation in superharmonic signal production, simultaneous optical and acoustic measurements were performed on individual microbubbles at transmit frequencies from 1.75 to 3.75 MHz and pressures near the shell fragmentation threshold for microbubbles of varying diameter. High-amplitude, broadband superharmonic signals were produced with shell fragmentation, while weaker signals (approximately 25% of peak amplitude) were observed in the presence of shrinking bubbles. Furthermore, when imaging populations of stationary microbubbles with a dual-frequency ultrasound imaging system, a sharper decline in image intensity with respect to frame number was observed for 1 μm bubbles than for 4 μm bubbles. Finally, in a study of two rodents, increasing frame rate from 4 to 7 Hz resulted in a decrease in mean steady-state image intensity of 27% at 1000 kPa and 29% at 1300 kPa. While the existence of superharmonic signals when bubbles shrink has the potential to prolong the imaging efficacy of microbubbles, parameters such as frame rate and peak pressure must be balanced with expected re-perfusion rate in order to maintain adequate contrast during in vivo imaging

Molecular Acoustic Angiography: A New Technique for High-resolution Superharmonic Ultrasound Molecular Imaging

Ultrasound molecular imaging utilizes targeted microbubbles to bind to vascular targets such as integrins, selectins, and other extracellular binding domains. After binding, these microbubbles are typically imaged using low pressures and multi-pulse imaging sequences. In this article, we present an alternative approach for molecular imaging using ultrasound which relies on superharmonic signals produced by microbubble contrast agents. Bound bubbles were insonified near resonance using a low frequency (4 MHz) and superharmonic echoes were received at high frequencies (25–30 MHz). While this approach was observed to produce declining image intensity during repeated imaging in both in vitro and in vivo experiments due to bubble destruction, the feasibility of superharmonic molecular imaging was demonstrated for transmit pressures which are sufficiently high to induce shell disruption in bound microbubbles. This approach was validated using microbubbles targeted to the αvβ3 integrin in a rat fibrosarcoma model (n=5), and combined with superharmonic images of free microbubbles to produce high contrast, high resolution 3D volumes of both microvascular anatomy and molecular targeting. Image intensity over repeated scans and the effect of microbubble diameter were also assessed in vivo, indicating that larger microbubbles yield increased persistence in image intensity. Using ultrasound-based acoustic angiography images rather than conventional B-mode ultrasound to provide the underlying anatomical information facilitates anatomical localization of molecular markers. Quantitative analysis of relationships between microvasculature and targeting information indicated that most targeting occurred within 50 µm of a resolvable vessel (>100 µm diameter). The combined information provided by these scans may present new opportunities for analyzing relationships between microvascular anatomy and vascular targets, subject only to limitations of the current mechanically-scanned system and microbubble persistence to repeated imaging at moderate mechanical indices

Ex Vivo Porcine Arterial and Chorioallantoic Membrane Acoustic Angiography Using Dual-Frequency Intravascular Ultrasound Probes

The presence of blood vessels within a developing atherosclerotic plaque has been shown to be correlated to increased plaque vulnerability and ensuing cardiac events, however, detecting coronary intraplaque neovascularizations poses a significant challenge in the clinic. In this paper, we demonstrate in vivo a new intravascular ultrasound imaging method using a dual-frequency transducer to visualize contrast flow in microvessels with high specificity. This method uses a specialized transducer capable of exciting contrast agents at a low frequency (5.5 MHz) while detecting their nonlinear superhamonics at a much higher frequency (37 MHz). In vitro evaluation of the approach was performed in a microvascular phantom to produce 3D renderings of simulated vessel patterns and to determine image quality metrics as a function of depth. Furthermore, the ability of the system to detect microvessels is demonstrated both ex vivo using porcine arteries and in vivo using the chorioallantoic membrane of a developing chicken embryo with optical confirmation. Dual-frequency contrast specific imaging was able to resolve vessels of a similar size to those found in vulnerable atherosclerotic plaques at clinically relevant depths. The results of this study adds growing support for further evaluation and translation of contrast specific imaging in intravascular ultrasound for the detection of vulnerable plaques in atherosclerosis

Intravascular forward-looking ultrasound transducers for microbubble-mediated sonothrombolysis

Effective removal or dissolution of large blood clots remains a challenge in clinical treatment of acute thrombo-occlusive diseases. Here we report the development of an intravascular microbubble-mediated sonothrombolysis device for improving thrombolytic rate and thus minimizing the required dose of thrombolytic drugs. We hypothesize that a sub-megahertz, forward-looking ultrasound transducer with an integrated microbubble injection tube is more advantageous for efficient thrombolysis by enhancing cavitation-induced microstreaming than the conventional high-frequency, side-looking, catheter-mounted transducers. We developed custom miniaturized transducers and demonstrated that these transducers are able to generate sufficient pressure to induce cavitation of lipid-shelled microbubble contrast agents. Our technology demonstrates a thrombolysis rate of 0.7 ± 0.15 percent mass loss/min in vitro without any use of thrombolytic drugs

The Grizzly, February 5, 2004

Take Notice of New Member Education Standards • AES Team with Upromise to Help Ease the Repayment of Student Loans • Unpredictable Democratic Primaries • Myrin Library Update: Media Services Re-opens • Opinions: New Member Education: Yes or No?; Is Blackboard Worth the Fuss?; Max and Erma\u27s New Hot Spot in Town • Men\u27s Lacrosse: 2004 Season Outlookhttps://digitalcommons.ursinus.edu/grizzlynews/1552/thumbnail.jp

The Grizzly, January 27, 2005

Ashley Lynn McCaleb: Scholar, Daughter, Friend • I Want to be Made Into an RA • Students Take Action at the Presidential Inauguration • New Member Education Returns • Fatal Blow • Myrin Undergoes Major Changes • Tsunami Relief Efforts Taking Shape at Ursinus • Ursinus Students Make Requests for The Facebook • Opinions: Is Mother Nature Trying to Tell us Something?; Bush\u27s New Cabinet Appointments may not be the Wisest • Bears Battling Without Stanton • Dedication and Attitude Forms a True Competitorhttps://digitalcommons.ursinus.edu/grizzlynews/1575/thumbnail.jp

High-Redshift Starbursting Dwarf Galaxies Revealed by GRB Afterglows

We present a study of 15 long-duration gamma-ray burst (GRB) host galaxies at z>2. The GRBs are selected with available early-time afterglow spectra in order to compare interstellar medium (ISM) absorption-line properties with stellar properties of the host galaxies. In addition to five previously studied hosts, we consider new detections for the host galaxies of GRB050820 and GRB060206 and place 2-sigma upper limits to the luminosities of the remaining unidentified hosts. We examine the nature of the host galaxy population and find that (1) the UV luminosity distribution of GRB host galaxies is consistent with expectations from a UV luminosity weighted random galaxy population with a median luminosity of =0.1 L*; (2) there exists a moderate correlation between UV luminosity and SiII 1526 absorption width, which together with the observed large line widths of W(1526)>1.5 Ang for a large fraction of the objects suggests a galactic outflow driven velocity field in the host galaxies; (3) there is tentative evidence for a trend of declining ISM metallicity with decreasing galaxy luminosity in the star-forming galaxy population at z=2-4; (4) the interstellar UV radiation field is found ~ 35-350 times higher in GRB hosts than the Galactic mean value; and (5) additional galaxies are found at < 2" from the GRB host in all fields with known presence of strong MgII absorbers, but no additional faint galaxies are found at < 2" in fields without strong MgII absorbers. Our study confirms that the GRB host galaxies (with known optical afterglows) are representative of unobscured star-forming galaxies at z>2, and demonstrates that high spatial resolution images are necessary for an accurate identification of GRB host galaxies in the presence of strong intervening absorbers.Comment: 24 emulateapj pages, 24 figures, ApJ in press; full-resolution version available at http://lambda.uchicago.edu/public/tmp/ghost.pd