Ryan Gessner

Improving sensitivity in ultrasound molecular imaging by tailoring contrast agent size distribution: In vivo studies

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 αvß3-targeted bubbles in both fibrosarcoma and R3230 rat tumor models. In perfusion imaging studies, video intensity and contrast persistence was ≈8 times and ≈3 times greater respectively, for “sorted 3-micron” MCAs (diameter, 3.3 ± 1.95 μm) when compared to “unsorted” MCAs (diameter, 0.9 ± 0.45 μm) at low concentrations. In targeted experiments, application of sorted 3-micron MCAs resulted in a ≈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.

Advances in Molecular Imaging with Ultrasound

Ultrasound imaging has long demonstrated utility in the study and measurement of anatomic features and noninvasive observation of blood flow. Within the last decade, advances in molecular biology and contrast agents have allowed researchers to use ultrasound to detect changes in the expression of molecular markers on the vascular endothelium and other intravascular targets. This new technology, referred to as ultrasonic molecular imaging, is still in its infancy. However, in preclinical studies, ultrasonic molecular imaging has shown promise in assessing angiogenesis, inflammation, and thrombus. In this review, we discuss recent advances in microbubble-type contrast agent development, ultrasound technology, and signal processing strategies that have the potential to substantially improve the capabilities and utility of ultrasonic molecular imaging.

Acoustic angiography: a new imaging modality for assessing microvasculature architecture

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.

Effect of anesthesia carrier gas on in vivo circulation times of ultrasound microbubble contrast agents in rats

PURPOSEnMicrobubble 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 on going. 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.nnnMETHODSnThe 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.nnnRESULTSnA 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.nnnCONCLUSIONnUsing medical air in place of oxygen as the carrier gas for isoflurane anesthesia can increase the circulation lifetime of ultrasound microbubble contrast agents.

An in vivo validation of the application of acoustic radiation force to enhance the diagnostic utility of molecular imaging using 3-d ultrasound.

For more than 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. 3-D ARF-enhanced MI was then performed on 7 rat fibrosarcoma tumors using microbubbles targeted to α(v)β₃ and nontargeted microbubbles. Low-amplitude (<25 kPa) 3-D ARF pulse sequences were tested and compared with 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 nontargeted agents, suggesting that ARF did not increase nonspecific adhesion.

High-resolution, high-contrast ultrasound imaging using a prototype dual-frequency transducer in-vitro and in-vivo studies

Recently, there has been a great interest in the capabilities of high-resolution ultrasound imaging. One of the applications is imaging blood vessels to assess tumor growth and response to therapy. Due to their nonlinear response, microbubble contrast agents scatter ultrasound energy at frequencies higher and lower than the imaging frequency. To maximize ultrasound scatter, the imaging frequency should be near the microbubble resonant frequency. Previously, this was not possible with high-frequency imaging systems because most contrast agents resonate at 1–10 MHz and the systems pulse at higher frequencies. We have developed a unique dual-frequency confocal transducer which excites microbubbles at low frequencies, near their resonance, and detects their emitted high-frequency energy at greater than 15 MHz. With this imaging approach, we have attained an average improvement in contrast-to-tissue ratio of 12.3 dB over standard b-mode imaging for MI between 0.5 and 0.65 with spatial resolution near that of the high-frequency element (30 MHz). This method is less susceptible to tissue motion corruption than power-Doppler because it does not rely on signal-decorrelation. Because of this, dual-frequency imaging can be used for flow imaging without respiratory gating.

Hybrid dual frequency transducer and Scanhead for micro-ultrasound imaging

We report on the design, assembly and evaluation of a dual frequency mechanically scanned transducer for ultrasound bubble manipulation and real time high frequency imaging applications. A low frequency 2 MHz annulus was designed to fit on the outside of a commercially available 30 MHz, 100% bandwidth single element fixed focus transducer such that the axial axes of each transducer would be substantially aligned. The acoustic pressure and beam characteristics of the low frequency transducer were characterized as a function of transmit center frequency and drive voltage. The focal depths of the low and high frequencies were measured to be 11.8–12.1mm and 12.7mm for transmit pulses of 2–3MHz and 30 MHz respectively. The probe was integrated into a commercially available Visualsonics RMV Scanhead and could achieve a frame rate of 20 fps. The dual frequency probe provided an average of ≫ 12dB enhancement of contrast to tissue ratios in rat kidney and rat xenograft Fibrosarcoma models.

A Pilot Study to Assess Markers of Renal Damage in the Rodent Kidney After Exposure to 7 MHz Ultrasound Pulse Sequences Designed to Cause Microbubble Translation and Disruption

Acoustic radiation force has been proposed as a mechanism to enhance microbubble concentration for therapeutic and molecular imaging applications. It is hypothesized that once microbubbles are localized, bursting them with acoustic pressure could result in local drug delivery. It is known that low-frequency, high-amplitude acoustic energy combined with cavitation nuclei can result in bioeffects. However, little is known about the bioeffects potential of acoustic parameters involved in radiation force and microbubble destruction pulse sequences applied at higher frequencies. In this pilot study, rat kidneys are exposed to high-duty cycle, low-amplitude pulse sequences known to cause substantial bubble translation due to radiation force, as well as high-amplitude short pulse sequences known to cause microbubble destruction. Both studies are performed at 7 MHz on a clinical ultrasound system, and implemented in three-dimensions (3-D) for entire kidney exposure. Analysis of biomarkers of renal injury and renal histopathology indicate that there was no significant renal damage due to these ultrasound parameters in conjunction with microbubbles within the study group.

An in-vivo evaluation of the effects of anesthesia carrier gases on ultrasound contrast agent circulation

Both clinical and preclinical ultrasound imaging studies use microbubble contrast agents. Short in-vivo lifetimes limit the diagnostic and therapeutic capabilities of these contrast agents, and currently there is research interest in how to lengthen their circulation times. Differences have been observed in contrast agent circulation times during preclinical studies depending on the type of inhaled anesthesia carrier gas. This paper presents the observations from a controlled experiment that evaluated differences in circulation times for lipid shelled perfluorocarbon-filled contrast agents circulating in rodents as a function of inhaled anesthesia carrier gas. Both pure oxygen and medical air are considered and we discuss the persistence of the contrast agents as determined by ultrasound video intensity. The results show a significantly longer in-vivo contrast agent circulation for animals breathing medical air as compared to breathing pure oxygen. Simulations show enhanced contrast persistence may be explained by classical diffusion and that nitrogen plays a critical role by contributing to the volume and diluting the other gas species in the microbubble gas core.

Effects of contrast agents and high intensity focused ultrasound on tumor vascular perfusion

High intensity focused ultrasound (HIFU) is a treatment modality which causes localized tissue ablation and is further enhanced when microbubble or nanodroplet contrast agents are present. While it is known that HIFU ablation results in diminished blood perfusion, these effects have never been mapped with high-resolution 3-D imaging. We aimed to longitudinally and volumetrically evaluate the effects of microbubbles and nanodroplets on tumor vessel perfusion following HIFU treatment. Rats bearing flank fibrosarcoma tumors were injected with 1.5 × 109 microbubbles (nu2009=u20093), nanodroplets (nu2009=u20093), or control/no-injection (nu2009=u20093) during targeted HIFU (125W/cm2, 1 MHz, 2 MPa, 10% duty cycle, 1 Hz PRF, 3 min). This HIFU power is ~99% less than clinical norms. Tumor perfusion was assessed by contrast enhanced 3-D acoustic angiography before, immediately following, and 72 h after HIFU application. No significant changes in tumor perfusion were observed when HIFU was applied with nanodroplet or control injections. H...