Susanne M. Stieger

Direct observations of ultrasound microbubble contrast agent interaction with the microvessel wall

Many thousands of contrast ultrasound studies have been conducted in clinics around the world. In addition, the microbubbles employed in these examinations are being widely investigated to deliver drugs and genes. Here, for the first time, the oscillation of these microbubbles in small vessels is directly observed and shown to be substantially different than that predicted by previous models and imaged within large fluid volumes. Using pulsed ultrasound with a center frequency of 1 MHz and peak rarefactional pressure of 0.8 or 2.0 MPa, microbubble expansion was significantly reduced when microbubbles were constrained within small vessels in the rat cecum (p<0.05). A model for microbubble oscillation within compliant vessels is presented that accurately predicts oscillation and vessel displacement within small vessels. As a result of the decreased oscillation in small vessels, a large resting microbubble diameter resulting from agent fusion or a high mechanical index was required to bring the agent shell into contact with the endothelium. Also, contact with the endothelium was observed during asymmetrical collapse, not during expansion. These results will be used to improve the design of drug delivery techniques using microbubbles.

Angiogenic Response to Bioactive Glass Promotes Bone Healing in an Irradiated Calvarial Defect

Localized radiation is an effective treatment modality for carcinomas, yet the associated reduction of the host vasculature significantly inhibits the tissues regenerative capacity. Low concentrations of bioactive glass (BG) possess angiogenic potential, and we hypothesized that localized BG presentation would increase neovascularization and promote healing in an irradiated bone defect. An isolated calvarial region of Sprague-Dawley rats was irradiated 2 weeks before surgery. Bilateral critical-sized defects were created and immediately filled with a BG-loaded collagen sponge or an empty sponge as an internal control. Histological analysis of calvaria collected after 2 weeks demonstrated greater neovascularization within the defect in the presence of BG than with collagen alone. Noninvasive ultrasound imaging at 4 weeks detected less contrast agent in the brain below BG-treated defects than in the nearby untreated defects and images of treated defects acquired at 2 weeks. The reduced ability to detect contrast agent in BG-treated defects suggested greater attenuation of ultrasound signal due to early bone formation. Micro-computed tomography imaging at 12 weeks demonstrated significantly greater bone volume fraction within BG-treated defects than in controls. These results suggest that neovascularization induced by localized BG delivery promotes bone regeneration in this highly compromised model of bone healing and may offer an alternative approach to costly growth factors and their potential side-effects.

Ultrasound radiation force modulates ligand availability on targeted contrast agents.

Radiation force produced by low-amplitude ultrasound at clinically relevant frequencies remotely translates freely flowing microbubble ultrasound contrast agents over distances up to centimeters from the luminal space to the vessel wall in order to enhance ligand-receptor contact in targeting applications. The question arises as to how the microbubble shell might be designed at the molecular level to fully take advantage of such physical forces in targeted adhesion for molecular imaging and controlled therapeutic release. Herein, we report on a novel surface architecture in which the tethered ligand is buried in a polymeric overbrush. Our results, with biotin-avidin as the model ligand-receptor pair, show that the overbrush conceals the ligand, thereby reducing immune cell binding and increasing circulation persistence. Targeted adhesion is achieved through application of ultrasound radiation force to instantly reveal the ligand within a well-defined focal zone and simultaneously bind the ligand and receptor. Our data illustrate how the adhesive properties of the contrast agent surface can be reversibly changed, from stealth to sticky, through the physical effects of ultrasound. This technique can be combined with any ligand-receptor pair to optimize targeted adhesion for ultrasonic molecular imaging.

Imaging of angiogenesis using Cadence contrast pulse sequencing and targeted contrast agents.

OBJECTIVES Low-power multipulse contrast ultrasound imaging provides a promising tool to quantify angiogenesis noninvasively when used with contrast agents targeted to vascular markers expressed by the angiogenic endothelium. Targeted ultrasound contrast agents, with a diameter on the order of micrometers, cannot extravasate and therefore are targeted solely to receptors expressed by the vascular endothelium. The aim of this study was to evaluate the potential of a low-power multipulse imaging sequence, Cadence(TM) contrast pulse sequencing (CPS), combined with targeted contrast agents to quantify angiogenesis. MATERIAL AND METHODS Targeted microbubbles were prepared by conjugating echistatin via biotin-avidin linkage to the surface of a phospholipid microbubble shell. The density of echistatin present on the shell was confirmed with flow-cytometry and quantified by total fluorescence. The binding of targeted microbubbles was evaluated in vitro by quantifying the adherence of targeted microbubbles to rat aortic endothelial cells, compared with control (nontargeted) microbubbles. The circulation time and adherence of targeted microbubbles was evaluated in vivo in a Matrigel model in rats and compared with control microbubbles using CPS in addition to a destructive ultrasound pulse. RESULTS Using only the low-power CPS pulse, the echo intensity produced in the neovasculature of the Matrigel pellet was significantly greater with targeted microbubbles than with the control contrast agent (p < 0.001). Combining CPS with the destructive pulse, the processed image was significantly different in intensity (p < 0.001) and spatial extent between targeted and control agents (p < 0.001). When the morphology of the histological sample and ultrasound image correlated, the microvessel density count and the percentage of the circular area enhanced by ultrasound were correlated (p < 0.05). CONCLUSION Low-power multipulse imaging in combination with targeted echistatin-bearing microbubbles facilitated a noninvasive, quantitative evaluation of early angiogenesis during real-time imaging. The addition of high-intensity destructive pulses facilitated estimation of the spatial extent of angiogenesis.

Quantitative evaluation of perfusion and permeability of peripheral tumors using contrast-enhanced computed tomography.

Rationale and Objectives:Our purpose was to validate contrast-enhanced computed tomography (CECT)-derived quantitative measures of perfusion and permeability against gold standard techniques of fluorescent microspheres and Evans Blue dye, respectively. Materials and Methods:Normal and tumor-bearing (R3230AC) Fischer 344 rats were used. CECT perfusion measurements of normal and tumor tissue were compared with quantitative fluorescent microsphere perfusion measures. CECT permeability measurements from tumors were compared with semiquantitative Evans Blue Dye permeability estimates. CT images were obtained precontrast and an imaging plane was selected. Serial, stationary images were obtained every 2 seconds for 2 minutes after intravenous bolus of iodinated contrast. Permeability and perfusion were measured by applying Patlak analysis to time-density data from normal tissue or tumor and femoral artery. Results:There was good correlation between fluorescent microsphere and CECT measurements of perfusion (r2 = 0.681, P ≪ 0.001) and between Evans Blue Dye and CECT measurements of permeability (r2 = 0.873, P = 0.0007). Conclusions:CECT provides useful, quantifiable measures of perfusion and permeability in peripheral tumors.

9B-4 Microbubble Oscillations in Gel Phantom and Ex Vivo Preparation Validate Proposed Mechanisms for Contrast-Based Drug Delivery

The use of ultrasound contrast agents for gene and drug delivery has shown much promise in many recent studies, while simultaneously raising questions regarding the safety of methods used. There is currently no consensus on the optimal safe operating regime, since the mechanisms for contrast- enhanced drug delivery are not well-understood. Here, the mechanisms for increased permeability are investigated by using high-speed microscopy to directly observe microbubbles in a vessel phantom and ex vivo preparation. The vessel phantom was a small block (30 mm times 20 mm times 3 mm) of 0.75% agarose gel with a 200-mum diameter cylindrical channel along the length of the block. For ex vivo work, the preparation used was an excised rat cecum with a cannulated ileocolic vein to allow for microbubble perfusion. Images of microbubble oscillation were acquired using a strobe imaging system with a 30 nsec flash provided by a copper vapor laser. Images of vessel wall disruption and tunnels formed in the wall were acquired with 1, 2.25, and 5 MHz center frequencies using microbubble concentration, duty cycle, and rarefactional pressures that were similar to those used for in vivo drug delivery experiments. Observations from in vitro and ex vivo studies indicate that expansion of microbubbles within small vessels is smaller than in an infinite fluid and that bubbles persist over many transmitted cycles and pulses. In vitro studies over a long pulse train indicate that microbubbles create pores within the tissue phantom and travel long distances along the beam axis. The width and depth of tunnels created by microbubbles at varied frequencies indicate a stronger dependence on insonation frequency than suggested by mechanical index.