Real-Time Technique for Improving Molecular Imaging and Guiding Drug Delivery in Large Blood Vessels: In Vitro and Ex Vivo Results

Abstract

Ultrasound-based molecular imaging employs targeted microbubbles to image vascular pathology. This approach also has the potential to monitor molecularly targeted microbubble-based drug delivery. We present an image-guided drug delivery technique that uses multiple pulses to translate, image, and cavitate microbubbles in real time. This technique can be applied to both imaging of pathology in large arteries (sizes and flow comparable to those in humans) and guiding localized drug delivery in blood vessels. The microbubble translation (or pushing) efficacy of this technique was compared in a variety of flow media: saline, viscous saline (4 cp), and bovine blood. It was observed that the performance of this approach was marginally better (by 6, 4, and 2 dB) in viscous saline than in bovine blood with varying levels of hematocrit (40%, 30%, and 10%). The drug delivery efficacy of this technique was evaluated by in vitro and ex vivo experiments. High-intensity pulses mediated fluorophore (DiI) deposition on endothelial cells (in vitro) without causing cell destruction. Ex vivo fluorophore delivery experiments conducted on swine carotids of 2 and 5 mm cross-section diameter demonstrated a high degree of correspondence in spatial localization of the fluorophore delivery between the ultrasound and composite fluorescence microscopy images of the arterial cross sections.

Quantitative Ultrasound Molecular Imaging in Large Blood Vessel Environments

“…Due to the high flow rate and shear forces, ARF is essential in large vessels to push microbubbles towards the distal vessel wall. The combination of ARF and a slow-time filtering technique resulted in real-time ultrasound molecular imaging in a swine carotid artery, ex vivo [50]. The combination of ARF and singular value filtering resulted in enhanced real-time ultrasound molecular imaging in large vessels [23].…”

“…tumor) has been demonstrated, studies were performed in large blood vessel environments with application of ARF in vitro and ex vivo [23], [45], [49], [50], [64]. Due to the high flow rate and shear forces, ARF is essential in large vessels to push microbubbles towards the distal vessel wall.…”

“…Low-pass filtering in slow-time dimension is designed to suppress highfrequency circulating microbubble signals while retaining low-frequency adherent microbubble signals [45]- [48]. Similarly, slow-time averaging is designed to cancel out more random signals derived from circulating microbubbles while retaining stationary signals from adherent microbubbles [49], [50]. "Dwell time" is another threshold-determined parameter used to quantify the inter-frame signature of microbubbles.…”

“…Current methods for separating signals arising from molecularly bound adherent microbubbles, versus tissue and freely circulating microbubbles, are primarily designed for small vessels and generally rely on frequency domain filtering schemes [46], [47], [49], [50], [114]. For example, Gessner et al achieved high contrast-to-tissue ratio imaging using a dual-frequency transducer [114].…”

“…consisted of harmonic imaging and slow-time filtering with a wideband customized transducer to enhance sensitivity [47]. In addition, Patil et al demonstrated molecular imaging in large vessels in vitro and ex vivo using the combination of pulse inversion and a slow-time averaging strategy [49], [50]. In each of these methods, the elimination of tissue signal is attempted by first using non-linear extraction methods (e.g.…”