Plane-Wave Contrast Imaging: A Radiation Force Point of View.

Radiation force is known to produce microbubble axial displacements by an amount that depends on the transmit burst frequency, amplitude, and length, as well as the pulse repetition frequency (PRF). In the standard focused-imaging mode, the actual PRF experienced by each microbubble is low, because it is of the order of the frame rate (i.e., usually tens of Hertz). In the plane-wave imaging mode, however, the actual PRF is considerably higher, as it is equivalent to the transmit PRF (kiloHertz range). Furthermore, the radiation pressure is expected to be almost uniform over the field of view, and typically lower than the peak pressure experienced in the focused transmit (TX) mode. We have experimentally investigated the possible effects of radiation force in the plane-wave mode. Here, we report on preliminary findings that show that the acoustic radiation force is negligible only at lower TX levels. At higher TX amplitudes, the bubble displacements due to radiation force are comparable to those obtained for focused waves at the same PRF. In addition, the radiation force is nearly uniform over the field of view and increases as the TX burst central frequency approaches the resonance frequency of size-isolated microbubbles.

Click Conjugation of Cloaked Peptide Ligands to Microbubbles.

Interest in the use of targeted microbubbles for ultrasound molecular imaging (USMI) has been growing in recent years as a safe and efficacious means of diagnosing tumor angiogenesis and assessing response to therapy. Of particular interest are cloaked microbubbles, which improve specificity by concealing the ligand from blood components until they reach the target vasculature, where the ligand can be transiently revealed for firm receptor-binding by ultrasound acoustic radiation force pulses. Herein, a bio-orthogonal "click" conjugation chemistry is introduced to decorate the surface of cloaked 4-5-µm-diameter microbubbles as part of a sterile and reproducible production process. Azido-functionalized antagonists for the angiogenic biomarkers αVß3 integrin (cRGD) and VEGFR2 (A7R) proteins were conjugated to bimodal-brush microbubbles via strain-promoted [3 + 2] azide-alkyne cycloaddition (SPAAC) click chemistry. Ligand conjugation was validated by epifluorescent microscopy, flow cytometry, and Fourier-transform infrared spectroscopy. Sterility was validated by bacterial culture and endotoxin analysis. Additionally, clinically normal dogs receiving escalating microbubble doses were shown to experience no pathologic changes in physical examination, complete blood count, serum biochemistry profile, or coagulation panel. This bio-orthogonal microbubble conjugation process for cloaked peptide ligands may be leveraged for future USMI studies of tumor angiogenesis for translation to preclinical and clinical applications.