Integrated processing of contrast pulse sequencing ultrasound imaging for enhanced active contrast of hollow gas filled silica nanoshells and microshells

Abstract

In recent years, there have been increasing developments in the field of contrast-enhanced ultrasound both in the creation of new contrast agents and in imaging modalities. These contrast agents have been employed to study tumor vasculature in order to improve cancer detection and diagnosis. An in vivo study is presented of ultrasound imaging of gas filled hollow silica microshells and nanoshells which have been delivered intraperitoneally to an IGROV-1 tumor bearing mouse. In contrast to microbubbles, this formulation of microshells provided strong ultrasound imaging signals by shell disruption and release of gas. Imaging of the microshells in an animal model was facilitated by novel image processing. Although the particle signal could be identified by eye under live imaging, high background obfuscated the particle signal in still images and near the borders of the tumor with live images. Image processing techniques were developed that employed the transient nature of the particle signal to selectively filter out the background signal. By applying image registration, high-pass, median, threshold, and motion filtering, a short video clip of the particle signal was compressed into a single image, thereby resolving the silica shells within the tumor. © 2012 American Vacuum Society.

Figure 1

(Color online) CPS Imaging of Gas Filled Silica Microshells. 100 μg/ml of gas filled 2 μm silica microshells were imaged in a thin-walled acoustically transparent chamber using CPS mode. (a) CPS image of microparticles at 0.39 MI. Each bright spot was a single imaging event; the white arrow points to a single imaging event. At low MI, the received signal intensity from each imaging event was low, resulting in dim images where the particle signal may be difficult to observe. (b) CPS image of microparticles at 1.9 MI. The large density of bright spots corresponded to a large number of particles being imaged. The received signal intensity was much stronger than at low MI, resulting in much brighter images. (c) CPS image of traditional microbubbles at a concentration of 10⁸ microbubbles/ml which closely correlated in particle count to 100 μg/ml of the silica microshells.

Figure 2

SEM images of particles preand post-HIFU. Scanning electron microscopy (SEM) images of (a) 500 nm and (b) 2 μm particles after synthesis but before receiving any treatment or experimentation. SEM images of (c) 500 nm and (d) 2 μm particles after undergoing 2 min of high intensity focused ultrasound (HIFU). The appearance of semispherical half-shells (white arrows) only after HIFU was consistent with the assumption that the particles were fractured during high mechanical index imaging.

Figure 3

(Color online) In vivo Silica Particle CPS Imaging. CPS imaging was performed 1 h after IP injection of 2 μm gas filled silica particles in a mouse tumor model. A sample of frames were chosen, each frame sequentially four frames apart, to show that (1) the particle signal appeared as transient bright spots (P: green arrows) and (2) under high MI CPS imaging, there was significant nonlinear tissue response that persisted over time (T: blue arrows).

Figure 4

(Color online) Microparticle The images shown above illustrate the change of the integrated intensity of the CPS image signal after image processing steps were incrementally added: (a) MC, (b) MC + HP filter, (c) MC + HP + M&T filters, and (d) MC + HP + M&T + GM filters. In each image, the particle signal was rendered as a heat map overlaid on a grayscale B-mode background. Each heat map was normalized such that the brightest signal within each image appears white. The green arrow points to the tumor. As each image processing step was added from (a)–(d), the tumor signal became increasingly dominant relative to background tissue signal. MC: motion correction, HP: high-pass filter, M&T: median and threshold filter; GM: global motion filter.