Michael S. Hughes

Detection of individual microbubbles of ultrasound contrast agents: imaging of free-floating and targeted bubbles.

Rationale and Objectives:During echo examinations with microbubble contrast, individual “dots” of ultrasound reflection can be visualized. To address the question whether these signals represent individual microbubbles, very dilute suspensions of ultrasound contrast agents or individual microbubbles attached to Petri dishes were prepared and studied by ultrasound imaging. Methods:Microbubble suspensions were diluted in saline and evaluated by a clinical ultrasound imaging system. Microbubble concentration was verified by Coulter counter. Single microbubble preparation on a Petri dish was established by streptavidin–biotin interaction under microscopy control and subjected to ultrasound imaging. Results:Ultrasound of dilute microbubble dispersions demonstrated distinct white foci; concentration of these sites was consistent with signals from individual microbubbles as determined by Coulter. Individual microbubbles immobilized on polystyrene were also visualized by ultrasound. Conclusion:Ultrasound medical systems can resolve backscatter signals from individual microbubbles of ultrasound contrast, both in solution and in the targeted immobilized state, implying picogram sensitivity.

On the applicability of Kramers-Kronig relations for ultrasonic attenuation obeying a frequency power law

In the recent literature concern has been raised regarding the validity of Kramers-Kronig relations for media with ultrasonic attenuation obeying a frequency power law. It is demonstrated, however, that the Kramers-Kronig dispersion relations for application to these types of media are available. The developed dispersion relations are compared with measurements on several liquids, and agreement is found to better than 1 m/s over the experimentally available bandwidth. A discussion regarding the validity of these dispersion relations, in particular how the dispersion relations relate to the so-called Paley-Wiener conditions, forms the conclusion.

Detection of individual microbubbles of an ultrasound contrast agent: fundamental and pulse inversion imaging.

The use of ultrasound contrast materials in diagnostic imaging has been steadily increasing, with several agents recently approved for clinical application (1). When contrast echo imaging is performed, individual “speckles” of contrast can be often observed in the interrogated tissues. These white foci may represent the images of individual micron-sized bubbles. This implies exceptional detection sensitivity of ultrasound imaging with contrast agents. The capability of echo imaging to detect individual microbubbles is important for the quantification of the amount of bubbles in the tissues, determination of microvascular volume and targeted microbubble imaging. In order to test the ability of ultrasound imaging to detect individual microbubbles, dilute dispersions of microbubbles were prepared and evaluated by ultrasound imaging in vitro.

Targeting and ultrasound imaging of microbubble-based contrast agents.

Preparation and characterization of targeted microbubbles (ultrasound contrast agents) is described. Specific ligands were attached to the microbubble shell, and ligand-coated microbubbles were selectively attached to various targets, using either an avidin-biotin model system or an antigen-antibody system for targeting to live activated endothelial cells. Firm attachment of microbubbles to the target was achieved. Forces necessary to detach microbubbles from the target were estimated to exceed dozens of pN. Microbubbles were bound to the target even in the rapidly moving stream of the aqueous medium. Down to 20 ng of the ultrasound contrast material on the target surface could be detected by the ultrasound imaging with a commercial medical imaging system. At high bubble density on the target surface, strong ultrasound image attenuation was observed.

Broadband through-transmission signal loss measurements of Albunex® suspensions at concentrations approaching in vivo doses

Broadband normalized signal loss of commercially available Albunex®, an ultrasonic contrast agent, was measured in vitro at concentrations approaching those which may be found in vivo for clinical doses. The measurements were made using a novel specimen chamber, careful material handling procedures, and a broadband square wave pulser system. Results were obtained over the full bandwidth of the experimental system (1 to 20 MHz) at concentrations up to 1.9×106 microspheres/mL. Further results were obtained over a partial bandwidth of the experimental system at concentrations up to 1.5×107 microspheres/mL. The frequency-dependent signal loss exhibited a peak for all concentrations investigated. In the meaningful bandwidth of the system, the signal loss (expressed in dB) was directly proportional to microsphere concentration. The experimental results for normalized signal loss were compared with predictions from a linear single-scattering model for encapsulated bubbles. The experimental data was used to estim...

Nanomedicine opportunities for cardiovascular disease with perfluorocarbon nanoparticles

Nanomedicine promises to enhance the ability of clinicians to address some of the serious challenges responsible for cardiovascular mortality, morbidity and numerous societal consequences. Targeted imaging and therapy applications with perfluorocarbon nanoparticles are relevant to a broad spectrum of cardiovascular diseases, ranging from asymptomatic atherosclerotic disease to acute myocardial infarction or stroke. As illustrated in this article, perfluorocarbon nanoparticles offer new tools to recognize and characterize pathology, to identify and segment high-risk patients and to treat chronic and acute disease.

Differential forms of the Kramers-Kronig dispersion relations

Differential forms of the Kramers-Kronig dispersion relations provide an alternative to the integral Kramers-Kronig dispersion relations for comparison with finite-bandwidth experimental data. The differential forms of the Kramers-Kronig relations are developed in the context of tempered distributions. Results are illustrated for media with attenuation obeying an arbitrary frequency power law (/spl alpha/(/spl omega/) = /spl alpha//sub 0/ + /spl alpha//sub 1/ |/spl omega/|/sup y/). Dispersion predictions using the differential dispersion relations are compared to the measured dispersion for a series of specimens (two polymers, an egg yolk, and two liquids) exhibiting attenuation obeying a frequency power law (1.00 /spl les/ y /spl les/ 1.99), with very good agreement found. For this form of ultrasonic attenuation, the differential Kramers-Kronig dispersion prediction is found to be identical to the (integral) Kramers-Kronig dispersion prediction.

Frequency and concentration dependence of the backscatter coefficient of the ultrasound contrast agent Albunex

A broadband ultrasonic measurement system has been utilized to characterize the concentration and frequency dependence of in vitro suspensions of Albunex® microspheres at concentrations ranging from 1.7×105 to 2.1×107 microspheres/mL and over a bandwidth of 1–16 MHz. The apparent backscattered power (not compensated for effects due to attenuation) was shown to increase with dose for lower concentrations of microspheres, but then to decrease rapidly with increasing concentration where attenuation effects become significant. Measurements of signal loss demonstrated that the attenuation grew exponentially with increasing concentration, so that a doubling of the number of microspheres led to a doubling of the value of the attenuation coefficient measured in dB/cm. This relationship was demonstrated over the entire system bandwidth. Compensation of the apparent backscattered power for the attenuation yielded the backscatter transfer function. This quantity was shown to be linearly proportional to concentration...

Characterization of digital waveforms using thermodynamic analogs: detection of contrast-targeted tissue in vivo

We describe characterization of backscatter from tumor tissue targeted with a nanoparticle-based ultrasound contrast agent in vivo using analogs of thermodynamic quantities. We apply these waveform characteristics to detection of tumor neovasculature in tumors implanted in athymic nude mice, which were imaged using a research ultrasound scanner over a 2-hour period after injection of alphanubeta3-targeted perfluorocarbon nanoparticles. Images were constructed from backscattered ultrasound using two different approaches: fundamental B-mode imaging and a signal receiver based on a thermodynamic analog (Hc). The study shows that the thermodynamic analog is capable of detecting differences in backscattered signals that are not apparent with the B-mode approach

On a time-domain representation of the Kramers–Krönig dispersion relations

The development of Kramers-Kronig dispersion relations is typically carried out in the frequency domain. An alternative approach known as the time-causal theory develops dispersion relations for media with attenuation obeying a frequency power law through analysis in the time domain [T. L. Szabo, J. Acoust. Soc. Am. 96, 491-500 (1994)]. Although both approaches predict identical dispersion relations, it is perceived that these two approaches are distinct from each other. It is shown, however, that the time-causal theory is in essence a time-domain formulation of the Kramers-Kronig dispersion relations for the special case of media with attenuation obeying a frequency power law. Additionally, it is shown that time-domain representations of the Kramers-Kronig dispersion relations are available for a broader class of media than simply those with power law attenuation. The time-causal theory and the Kramers-Kronig dispersion relations can be viewed as two complementary, yet equivalent, approaches to the study of dispersion.