Molecular imaging with ultrasound relies on microbubble contrast agents (MCAs) selectively adhering to a ligand-specific target. Prior studies have shown that only small quantities of microbubbles are retained at their target sites, therefore, enhancing contrast sensitivity to low concentrations of microbubbles is essential to improve molecular imaging techniques. In order to assess the effect of MCA diameter on imaging sensitivity, perfusion and molecular imaging studies were performed with microbubbles of varying size distributions. To assess signal improvement and MCA circulation time as a function of size and concentration, blood perfusion was imaged in rat kidneys using nontargeted size-sorted MCAs with a Siemens Sequoia ultrasound system (Siemans, Mountain View, CA) in cadence pulse sequencing (CPS) mode. Molecular imaging sensitivity improvements were studied with size-sorted alphavbeta3-targeted bubbles in both fibrosarcoma and R3230 rat tumor models. In perfusion imaging studies, video intensity and contrast persistence was approximately 8 times and approximately 3 times greater respectively, for "sorted 3-micron" MCAs (diameter, 3.3 +/- 1.95 microm) when compared to "unsorted" MCAs (diameter, 0.9 +/- 0.45 microm) at low concentrations. In targeted experiments, application of sorted 3-micron MCAs resulted in a approximately 20 times video intensity increase over unsorted populations. Tailoring size-distributions results in substantial imaging sensitivity improvement over unsorted populations, which is essential in maximizing sensitivity to small numbers of MCAs for molecular imaging.

The purpose of this paper is to provide the biomedical imaging community with details of a new high resolution contrast imaging approach referred to as “acoustic angiography.” Through the use of dual-frequency ultrasound transducer technology, images acquired with this approach possess both high resolution and a high contrast-to-tissue ratio, which enables the visualization of microvascular architecture without significant contribution from background tissues. Additionally, volumetric vessel-tissue integration can be visualized by using b-mode overlays acquired with the same probe. We present a brief technical overview of how the images are acquired, followed by several examples of images of both healthy and diseased tissue volumes. 3D images from alternate modalities often used in preclinical imaging, contrast-enhanced micro-CT and photoacoustics, are also included to provide a perspective on how acoustic angiography has qualitatively similar capabilities to these other techniques. These preliminary images provide visually compelling evidence to suggest that acoustic angiography may serve as a powerful new tool in preclinical and future clinical imaging.

Purpose Microbubble contrast agents are currently implemented in a variety of both clinical and preclinical ultrasound imaging studies. The therapeutic and diagnostic capabilities of these contrast agents are limited by their short in-vivo lifetimes, and research to lengthen their circulation times is ongoing. In this manuscript, observations are presented from a controlled experiment performed to evaluate differences in circulation times for lipid shelled perfluorocarbon-filled contrast agents circulating within rodents as a function of inhaled anesthesia carrier gas. Methods The effects of two common anesthesia carrier gas selections - pure oxygen and medical air – were observed within five rats. Contrast agent persistence within the kidney was measured and compared for oxygen and air anesthesia carrier gas for six bolus contrast injections in each animal. Simulations were performed to examine microbubble behavior with changes in external environment gases. Results A statistically significant extension of contrast circulation time was observed for animals breathing medical air compared to breathing pure oxygen. Simulations support experimental observations and indicate that enhanced contrast persistence may be explained by reduced ventilation/perfusion mismatch and classical diffusion, in which nitrogen plays a key role by contributing to the volume and diluting other gas species in the microbubble gas core. Conclusion: Using medical air in place of oxygen as the carrier gas for isoflurane anesthesia can increase the circulation lifetime of ultrasound microbubble contrast agents.

Purpose:To determine if the morphologies of microvessels could be extracted from contrast material-enhanced acoustic angiographic ultrasonographic (US) images and used as a quantitative basis for distinguishing healthy from diseased tissue. Materials and Methods:All studies were institutional animal care and use committee approved. Three-dimensional contrast-enhanced acoustic angiographic images were acquired in both healthy (n = 7) and tumor-bearing (n = 10) rats. High-spatial-resolution and high signal-to-noise acquisition was enabled by using a prototype dual-frequency US transducer (transmit at 4 MHz, receive at 30 MHz). A segmentation algorithm was utilized to extract microvessel structure from image data, and the distance metric (DM) and the sum of angles metric (SOAM), designed to distinguish different types of tortuosity, were applied to image data. The vessel populations extracted from tumor-bearing tissue volumes were compared against vessels extracted from tissue volumes in the same anatomic location within healthy control animals by using the two-sided Student t test. Results:Metrics of microvascular tortuosity were significantly higher in the tumor population. The average DM of the tumor population (1.34 6 0.40 [standard deviation]) was 23.76% higher than that of the control population (1.08 6 0.08) (P , .0001), while the average SOAM (22.53 6 7.82) was 50.73% higher than that of the control population (14.95 6 4.83) (P , .0001). The DM and SOAM metrics for the control and tumor populations were significantly different when all vessels were pooled between the two animal populations. In addition, each animal in the tumor population had significantly different DM and SOAM metrics relative to the control population (P , .05 for all; P value ranges for DM, 3.89 3 10 27 to 5.63 3 10 23 ; and those for SOAM, 2.42 3 10 212 to 1.57 3 10 23). Conclusion:Vascular network quantification by using high-spatialresolution acoustic angiographic images is feasible. Data suggest that the angiogenic processes associated with tumor development in the models studied result in higher instances of vessel tortuosity near the tumor site. Kruse and Ferrara (16), and the design of the prototype imaging transducer has been previously presented by Gessner et al (17). The purpose of this analysis was to test our hypothesis that the morphologies of microvessels could be extracted from US images and used as a quantitative basis for distinguishing healthy from diseased tissue volumes in a rat model. Materials and Methods Animal and Contrast Agent PreparationAll animal protocols used in this prospective study were approved by our institutional animal care and use committee. Animals used in this prospective study were female Fischer 344 rats (approximately 250 g, Charles River Laboratories, Durham, NC). A fibrosarcoma tumor model was established through the implantation of a 1-mm 3 sample of tissue in the right flank region of the animals as previously described (18). Animals were imaged after the tumor had grown larger than 0.5 cm ...