Molecular Acoustic Angiography: A New Technique for High-resolution Superharmonic Ultrasound Molecular Imaging

Abstract

Ultrasound molecular imaging utilizes targeted microbubbles to bind to vascular targets such as integrins, selectins, and other extracellular binding domains. After binding, these microbubbles are typically imaged using low pressures and multi-pulse imaging sequences. In this article, we present an alternative approach for molecular imaging using ultrasound which relies on superharmonic signals produced by microbubble contrast agents. Bound bubbles were insonified near resonance using a low frequency (4 MHz) and superharmonic echoes were received at high frequencies (25–30 MHz). While this approach was observed to produce declining image intensity during repeated imaging in both in vitro and in vivo experiments due to bubble destruction, the feasibility of superharmonic molecular imaging was demonstrated for transmit pressures which are sufficiently high to induce shell disruption in bound microbubbles. This approach was validated using microbubbles targeted to the αvβ3 integrin in a rat fibrosarcoma model (n=5), and combined with superharmonic images of free microbubbles to produce high contrast, high resolution 3D volumes of both microvascular anatomy and molecular targeting. Image intensity over repeated scans and the effect of microbubble diameter were also assessed in vivo, indicating that larger microbubbles yield increased persistence in image intensity. Using ultrasound-based acoustic angiography images rather than conventional B-mode ultrasound to provide the underlying anatomical information facilitates anatomical localization of molecular markers. Quantitative analysis of relationships between microvasculature and targeting information indicated that most targeting occurred within 50 µm of a resolvable vessel (>100 µm diameter). The combined information provided by these scans may present new opportunities for analyzing relationships between microvascular anatomy and vascular targets, subject only to limitations of the current mechanically-scanned system and microbubble persistence to repeated imaging at moderate mechanical indices.

Optimizing ultrasound molecular imaging of secreted frizzled related protein 2 expression in angiosarcoma

“…There were no perfusion differences in animals receiving control contrast. In our laboratory, Shelton et al demonstrated high-resolution super- harmonic ultrasound molecular imaging using avβ3 integrin in a rat fibrosarcoma model [20]. They demonstrated high-resolution 3D volumes of micro vascular anatomy, and when combined with ultrasound-based acoustic angiography, new opportunities for analyzing relationships between micro vascular anatomy and endothelial targets.…”

Assessment of Molecular Acoustic Angiography for Combined Microvascular and Molecular Imaging in Preclinical Tumor Models

“…Again, a B-mode (30 MHz) imaging volume was acquired, followed by a dual-frequency (transmit 4 MHz/receive 30 MHz, MI = 0.62) pre-scan, followed by injection of a bolus of VEGFR2-targeted microbubbles. After a 12 min wait, targeted images were acquired using a step size of 250 µm to ensure microbubbles were not destroyed when imaging the adjacent imaging plane [35]. Three more 3D scans were performed at an MI of 0.62 to ensure all bound microbubbles were destroyed before acquisition of a post-destruction volume.…”

“…However, it is possible to perform acoustic angiography with the same bubbles repeatedly through reduced acoustic pressures and minimized bubble acoustic exposure with low frame rates. By controlling the transmitted acoustic pressure and the step size between successive transmit pulses, we have previously demonstrated the ability to form high resolution, high contrast superharmonic molecular images which can be overlaid on microvascular images [35]. These imaging volumes might be useful for treatment planning and monitoring in highly vascularized tumors.…”

Dual-Frequency Piezoelectric Endoscopic Transducer for Imaging Vascular Invasion in Pancreatic Cancer

“…These “superharmonic” or “acoustic angiography” images of microvasculature have contrast-to-tissue ratios of ~25 dB [16, 19, 20] and can be used to quantify vascular characteristics in developing tumors [21–26]. We have also recently demonstrated the ability to use superharmonic signals to perform high resolution functional imaging, including molecular imaging [27, 28] and perfusion imaging [29]. …”