Trans-Stent B-Mode Ultrasound and Passive Cavitation Imaging

Abstract

Angioplasty and stenting of a stenosed artery enable acute restoration of blood flow. However, restenosis or a lack of re-endothelization can subsequently occur depending on the stent type. Cavitation-mediated drug delivery is a potential therapy for these conditions, but requires that particular types of cavitation be induced by ultrasound insonation. Because of the heterogeneity of tissue and stochastic nature of cavitation, feedback mechanisms are needed to determine whether the sustained bubble activity is induced. The objective of this study was to determine the feasibility of passive cavitation imaging through a metal stent in a flow phantom and an animal model. In this study, an endovascular stent was deployed in a flow phantom and in porcine femoral arteries. Fluorophore-labeled echogenic liposomes, a theragnostic ultrasound contrast agent, were injected proximal to the stent. Cavitation images were obtained by passively recording and beamforming the acoustic emissions from echogenic liposomes insonified with a low-frequency (500 kHz) transducer. In vitro experiments revealed that the signal-to-noise ratio for detecting stable cavitation activity through the stent was greater than 8 dB. The stent did not significantly reduce the signal-to-noise ratio. Trans-stent cavitation activity was also detected in vivo via passive cavitation imaging when echogenic liposomes were insonified by the 500-kHz transducer. When stable cavitation was detected, delivery of the fluorophore into the arterial wall was observed. Increased echogenicity within the stent was also observed when echogenic liposomes were administered. Thus, both B-mode ultrasound imaging and cavitation imaging are feasible in the presence of an endovascular stent in vivo. Demonstration of this capability supports future studies to monitor restenosis with contrast-enhanced ultrasound and pursue image-guided ultrasound-mediated drug delivery to inhibit restenosis.

Introduction

Drug-eluting and bare-metal stents are available for atheroma intervention after balloon angioplasty, with stent choice being dictated by patient comorbidity factors. Neo-intimal hyperplasia and vascular smooth muscle cell proliferation are important mechanisms for arterial stenosis in endogenous atheroma and restenosis within the stent post-implantation. Drug-eluting stents aim to reduce restenosis, but typically delay re-endothelization, thus necessitating long-term anti-platelet therapy (Cutlip et al., 2001, Lüscher et al., 2007). Bare metal stents permit re-endothelization and reduce the risk of thrombosis, but are subject to restenosis.

Approaches to deliver therapeutics to the vascular wall may allow re-endothelization after stent deployment while inhibiting restenosis. For example, PPARγ (peroxisome proliferator-activated receptor γ) agonists, such as rosiglitazone, have local anti-inflammatory effects in the arterial wall (Beckman et al. 2003). Like many potential therapeutics, secondary effects of systemic PPARγ agonists have limited their clinical utility (Nissen and Wolski, 2007, Zinn et al., 2008). Thus, a spatially and temporally targeted therapeutic delivery approach is attractive. An intrinsically echogenic liposome formulation has been developed that can be loaded with therapeutics (Britton et al., 2010, Buchanan et al., 2010, Huang et al., 2009, Shaw et al., 2009, Tiukinhoy et al., 2004) and serve as an ultrasound contrast agent. Echogenic liposomes (ELIP) have been loaded with both rosiglitazone and the endothelial nitric oxide synthase gene and have been reported to have an effect in stabilizing atheroma after balloon angioplasty (Huang et al., 2007, Huang et al., 2009). When exposed to ultrasound, the microbubbles within ELIP oscillate volumetrically. The oscillation (also referred to as cavitation [Leighton 1997]) can induce beneficial bio-effects. The ability to deliver therapeutics to the vascular wall can be enhanced when the ELIP oscillate in a particular low-amplitude mode known as stable cavitation (Hitchcock et al. 2010).

For therapeutic delivery at the site of stent placement, a number of conditions are required. To achieve stable cavitation, particular in situ ultrasound pressures are required (Bader and Holland, 2013, Radhakrishnan et al., 2013). Heterogeneity of the overlying tissue makes it difficult to ensure a priori that the proper ultrasound pressure amplitude is achieved at the desired location. Techniques that rely on the passive detection of cavitation emissions from the microbubbles can be used as a feedback mechanism. Passive cavitation detection techniques implement a receive-only transducer to measure the cavitation emissions (Atchley et al. 1988). The cavitation activity may be induced by a separate therapy transducer. Passive cavitation detection techniques have been used to inform the therapeutic ultrasound insonation scheme for ultrasound-enhanced thrombolysis (Hitchcock et al. 2011). To optimize the cavitation activity throughout a targeted vessel, the cavitation activity needs to be mapped throughout the vessel. Passive cavitation imaging uses an array-based ultrasound system in receive-only mode to measure and beamform the cavitation emissions. The resulting image is a map of the cavitation activity (Gyöngy and Coussios, 2010, Haworth et al., 2012, Salgaonkar et al., 2009), which can be overlaid on a standard B-mode image to create duplex passive cavitation images. The B-mode image provides anatomic information, and the overlaid cavitation activity can provide therapeutic guidance. However, previous studies have not determined if this technique is robust enough to provide feedback through an endovascular stent.

This study had three aims. The first aim was to determine whether ultrasound can penetrate through a stent to induce stable cavitation emissions from ELIP that perfuse the lumen of the stent. The second aim was to determine whether stable cavitation emissions can be detected through a stent in a porcine model. The third aim was to determine the feasibility of beamforming the cavitation emissions to form a B-mode and cavitation duplex image. If all aims are achieved, then image-guided therapeutic delivery within the stent lumen is feasible. This technique would help guide the treatment of flow-limiting atheroma.