Shengtao Lin

Quantifying Activation of Perfluorocarbon-Based Phase-Change Contrast Agents Using Simultaneous Acoustic and Optical Observation

Phase-change contrast agents in the form of nanoscale droplets can be activated into microbubbles by ultrasound, extending the contrast beyond the vasculature. This article describes simultaneous optical and acoustical measurements for quantifying the ultrasound activation of phase-change contrast agents over a range of concentrations. In experiments, decafluorobutane-based nanodroplets of different dilutions were sonicated with a high-pressure activation pulse and two low-pressure interrogation pulses immediately before and after the activation pulse. The differences between the pre- and post-interrogation signals were calculated to quantify the acoustic power scattered by the microbubbles activated over a range of droplet concentrations. Optical observation occurred simultaneously with the acoustic measurement, and the pre- and post-microscopy images were processed to generate an independent quantitative indicator of the activated microbubble concentration. Both optical and acoustic measurements revealed linear relationships to the droplet concentration at a low concentration range <10(8)/mL when measured at body temperature. Further increases in droplet concentration resulted in saturation of the acoustic interrogation signal. Compared with body temperature, room temperature was found to produce much fewer and larger bubbles after ultrasound droplet activation.

Optically and acoustically triggerable sub-micron phase-change contrast agents for enhanced photoacoustic and ultrasound imaging

To explore the extravascular space, sub-micron phase-change droplets show widespread interest in medical imaging and therapy with various modalities, such as ultrasound and photoacoustic. Existing studies (Wilson 2012, Wei 2014) on such dual-modality contrast agents have demonstrated the generation of both optical and ultrasound contrast after optical activation. However these studies did not explore the option of acoustic activation. Furthermore, high boiling point perfluorocarbons were used in these studies. A low boiling point may be preferred, to minimise un-wanted bioeffects, especially when activating in deeper tissues. In this study, we demonstrate a versatile phase-change sub-micron contrast agent that can provide three modes of contrast enhancement: 1) photoacoustic imaging contrast, 2) ultrasound contrast with optical activation, and 3) ultrasound contrast with acoustic activation. This would add versatility of vaporisation triggering, offering new possibilities in dual mode imaging, molecular imaging and drug delivery.

Effects of microchannel confinement on acoustic vaporisation of ultrasound phase change contrast agents

The sub-micron phase change contrast agent (PCCA) composed of a perfluorocarbon liquid core can be activated into gaseous state and form stable echogenic microbubbles for contrast-enhanced ultrasound imaging. It has shown great promise in imaging microvasculature, tumour microenvironment, and cancer cells. Although PCCAs have been extensively studied for different diagnostic and therapeutic applications, the effect of biologically geometrical confinement on the acoustic vaporisation of PCCAs is still not clear. We have investigated the difference in PCCA-produced ultrasound contrast enhancement after acoustic activation with and without a microvessel confinement on a microchannel phantom. The experimental results indicated more than one-order of magnitude less acoustic vaporisation in a microchannel than that in a free environment taking into account the attenuation effect of the vessel on the microbubble scattering. This may provide an improved understanding in the applications of PCCAs in vivo.

Cardiac imaging with high frame rate contrast enhanced ultrasound: In-vivo demonstration

This work presents the first in-vivo High-frame rate Contrast Enhanced Ultrasound (HFR CEUS) for cardiac application. The in-vivo acquisition has been made on a sheep. A coherent compounding of diverging waves combined with Pulse Inversion (PI) transmission allow a frame rate of 250 frame per seconds which is 8 times faster than standard CEUS acquisition in cardiac application. The proposed method improves the image contrast compared to the CEUS and allows a better tracking of fast movement of the heart.

Vaporising phase change ultrasound contrast agent in microvascular confinement

The sub-micron perfluorocarbon-based phase change contrast agents (PCCAs) have shown great potential for imaging both microvasculature and tumour microenvironment. However the effect of geometrical confinement on the acoustic vaporisation has not been explored. We report the investigation of the difference in PCCA-produced contrast increase after activation with and without a microvessel confinement using in vitro experiment phantom. The results demonstrated that the acoustic vaporisation of PCCAs in the 200-micron cellulose tube was significantly different from that in the open environment, with contrast enhancement suppressed by up to several fold.

Imaging of vaporised sub-micron phase change contrast agents with high frame rate ultrasound and optics

Phase-change ultrasound contrast agent (PCCA), or nanodroplet, shows promise as an alternative to the conventional microbubble agent over a wide range of diagnostic applications. Meanwhile, high-frame-rate (HFR) ultrasound imaging with microbubbles enables unprecedented temporal resolution compared to traditional contrast-enhanced ultrasound imaging. The combination of HFR ultrasound imaging and PCCAs can offer the opportunity to observe and better understand PCCA behaviour after vaporisation captures the fast phenomenon at a high temporal resolution. In this study, we utilised HFR ultrasound at frame rates in the kilohertz range (5-20 kHz) to image native and size-selected PCCA populations immediately after vaporisation in vitro within clinical acoustic parameters. The size-selected PCCAs through filtration are shown to preserve a sub-micron-sized (mean diameter  <  200 nm) population without micron-sized outliers (>1 µm) that originate from native PCCA emulsion. The results demonstrate imaging signals with different amplitudes and temporal features compared to that of microbubbles. Compared with the microbubbles, both the B-mode and pulse-inversion (PI) signals from the vaporised PCCA populations were reduced significantly in the first tens of milliseconds, while only the B-mode signals from the PCCAs were recovered during the next 400 ms, suggesting significant changes to the size distribution of the PCCAs after vaporisation. It is also shown that such recovery in signal over time is not evident when using size-selective PCCAs. Furthermore, it was found that signals from the vaporised PCCA populations are affected by the amplitude and frame rate of the HFR ultrasound imaging. Using high-speed optical camera observation (30 kHz), we observed a change in particle size in the vaporised PCCA populations exposed to the HFR ultrasound imaging pulses. These findings can further the understanding of PCCA behaviour under HFR ultrasound imaging.

Cardiac flow mapping using high frame rate diverging wave contrast enhanced ultrasound and image tracking

Contrast echocardiography (CE) ultrasound with microbubble contrast agents have significantly advanced our capability in assessing cardiac function. However in conventional CE techniques with line by line scanning, the frame rate is limited to tens of frames per second, making it difficult to track the fast flow within cardiac chamber. Recent research in high frame-rate (HFR) ultrasound have shown significant improvement of the frame rate in non-contrast cardiac imaging. In this work we show the feasibility of microbubbles flow tracking in HFR CE acquisition in vivo with a high temporal resolution and low MI as well as the detection of vortex near the valves during filling phases agreeing with previous study.

Acoustic wave sparsely-activated localization microscopy (AWSALM): super-resolution ultrasound imaging using acoustic activation and deactivation of nanodroplets

Photo-activated localization microscopy (PALM) has revolutionized the field of fluorescence microscopy by breaking the diffraction limit in spatial resolution. In this study, “acoustic wave sparsely activated localization microscopy (AWSALM),” an acoustic counterpart of PALM, is developed to super-resolve structures which cannot be resolved by conventional B-mode imaging. AWSALM utilizes acoustic waves to sparsely and stochastically activate decafluorobutane nanodroplets by acoustic vaporization and to simultaneously deactivate the existing vaporized nanodroplets via acoustic destruction. In this method, activation, imaging, and deactivation are all performed using acoustic waves. Experimental results show that sub-wavelength micro-structures not resolvable by standard B-mode ultrasound images can be separated by AWSALM. This technique is flow independent and does not require a low concentration of contrast agents, as is required by current ultrasound super resolution techniques. Acoustic activation and deactivation can be controlled by adjusting the acoustic pressure, which remains well within the FDA approved safety range. In conclusion, this study shows the promise of a flow and contrast agent concentration independent super-resolution ultrasound technique which has potential to be faster and go beyond vascular imaging.

High frame rate ultrasound imaging of vaporised phase change contrast agents

Phase-change ultrasound contrast agents (PCCAs), have shown promises for ultrasound-mediated diagnostics and therapeutics. High-frame-rate ultrasound imaging with microbubbles significantly improved temporal resolution and contrast compared to conventional contrast-enhanced ultrasound imaging, offering opportunities to observe and better understand PCCA behavior after vaporisation. In this study, high-frame-rate ultrasound was used to image the PCCAs immediately after vaporisation in vitro with clinical ultrasound parameters. Compared with microbubbles, the vaporised PCCAs have shown different temporal signal features with high-frame-rate ultrasound imaging: a significant decrease in the first tens of milliseconds after acoustic vaporisation while no evident change over time with microbubble signal. Simultaneous optical microscopy was operated to observe behaviors of the vaporised PCCAs. Highframe-rate ultrasound imaging enables improved understanding of the acoustic behavior of the vaporised PCCA populations at high temporary resolution.

Acoustic response of phase change contrast agents targeted with breast cancer cells immediately after ultrasonic activation using ultrafast imaging

Phase-change contrast agents (PCCAs) have advantageous properties in terms of smaller size, longer half-life and selective activation control compared to conventional microbubble contrast agents (MCAs), which make them ideal for ultrasound cancer imaging. [1] Acoustic signal from tumour site can be further enhanced by receptor targeted PCCAs. However there is still a lack of understanding of the behaviour of these targeted agents. In this work, we report the use of high frame rate (HFR) imaging to investigate the changes in acoustic signal of Folate Receptor (FR)-targeted versus control PCCAs with breast cancer cells immediately after acoustic activation.