An In Vivo Validation of the Application of Acoustic Radiation Force to Enhance the Diagnostic Utility of Molecular Imaging Using 3-D Ultrasound

Abstract

For over a decade, the application of acoustic radiation force (ARF) has been proposed as a mechanism to increase ultrasonic molecular imaging (MI) sensitivity in vivo. Presented herein is the first noninvasive in vivo validation of ARF-enhanced MI with an unmodified clinical system. First, an in vitro optical-acoustical setup was used to optimize system parameters and ensure sufficient microbubble translation when exposed to ARF. 3D ARF-enhanced MI was then performed on 7 rat fibrosarcoma tumors using microbubbles targeted to αvβ3 and non-targeted microbubbles. Low-amplitude (< 25 kPa) 3D ARF pulse sequences were tested and compared to passive targeting studies in the same animal. Our results demonstrate that a 78% increase in image intensity from targeted microbubbles can be achieved when using ARF relative to the passive targeting studies. Furthermore, ARF did not significantly increase image contrast when applied to non-targeted agents, suggesting that ARF did not increase non-specific adhesion.

Targeted microbubble mediated sonoporation of endothelial cells in vivo

“…studies are ongoing to improve adherence of targeted microbubbles, including dual targeting [35], [36], varying ligand linker length [37], deflating microbubbles [38], or changing microbubble shell composition [39], whereas models reveal an optimal microbubble radius of 1 to 2 µm [36]. Furthermore, targeted microbubbles can be pushed toward the vessel wall to increase binding under flow conditions by acoustic radiation force [40], [41] or a magnetic field [42].…”

Quantitative Ultrasound Molecular Imaging in Large Blood Vessel Environments

“…Additionally, the image signal-to-noise ratio increased significantly due to the application of ARF [60]. In vivo validations have verified that the application of ARF enhanced the detection sensitivity and diagnostic utility of ultrasound molecular imaging [62], [63].…”

“…These ligands allow microbubbles to attach to specific regions of the vascular endothelium through the specific ligandreceptor bond, thereby enabling applications for both targeted molecular imaging [3], [7], [131], [132] and targeted gene/drug delivery [110], [133], [134]. In order to increase the binding efficacy of targeted microbubbles to potential binding sites, especially in large blood vessel environments, acoustic radiation force (ARF) is frequently applied [56], [59], [60], [62]. In addition, targeted microbubbles incorporated with multiple ligands can be used to further increase adhesion to the vessel wall [54].…”

“…In order to increase adhesion of targeted microbubbles to the vessel wall, multiple ligands can be incorporated on the microbubble shell [54]. Acoustic radiation force (ARF) is frequently applied to increase binding efficiency of microbubbles to potential sites, for example, in large vessel environments with high flow velocity and shear force [56], [59], [60], [62]. At present, ultrasound molecular imaging techniques are still mainly limited to pre-clinical applications.…”

“…First, increased binding efficiency can be achieved by incorporating multiple ligands into the targeted microbubble shell, for example, for detecting inflammation, microbubbles targeting both P-selectin and VCAM-1 [54]; and microbubbles incorporated with both P-and E-selectin antibodies have been used [94]. Secondly, the application of higher intensity acoustic radiation force (ARF) pulses within the microbubble imaging sequence has been studied as a means to force the microbubbles in closer proximity to the vessel wall and increase the attachment to potential binding sites [56], [58]- [60], [62]. The approach has been demonstrated ex vivo in large vessel environments where nonlinear detection and slow-time inter-frame filtering was used to detect adherent microbubbles [23], [50].…”