Rei Asami

Acoustic Response of Microbubbles Derived from Phase-Change Nanodroplet

An in vitro feasibility test for a novel ultrasound therapy using a type of superheated perfluorocarbon droplet, phase-change nanodroplet (PCND), was performed in gel phantoms with the goal of high selectivity and low invasiveness. Measurements of broadband signal emission revealed that a triggering ultrasound pulse (peak negative pressure of 2.4 MPa) reduces the pressure threshold for cavitation induced by a subsequent ultrasound exposure at an order of magnitude from 2.4 to 0.2 MPa. The maximum allowed interval between the two ultrasound exposures for inducing cavitation with 100- and 1,000-cycle triggering ultrasound was about 100 and 500 ms, respectively. The echo signal increases induced by the triggering ultrasound with 100- and 1000-cycles were enhanced and suppressed by the subsequent ultrasound exposure, respectively. This different behavior seemed to be due to the presence of enlarged free bubbles, which should be avoided for the localization of therapeutic effects.

Acoustic Signal Characterization of Phase Change Nanodroplets in Tissue-Mimicking Phantom Gels

To apply superheated perfluorocarbon nanodroplets to tumor diagnosis and treatment, acoustic signals formed upon the vaporization of droplets in tissue-mimicking phantoms were measured with 3 MHz ultrasound. A characteristic impulse wave was associated with phase change induction with 100 cycles of ultrasound at a 9 MPa peak negative pressure, observed as an intensity change in B-mode imaging. In addition, the subsequent impulse waves were observed only from the phase change induction in gels. Therefore, these subsequent impulse waves were suggested to be characteristic features of the vaporization of superheated nanodroplets at rigid boundaries. Furthermore, such impulse waves reflect the properties of surrounding environments such as elasticity and viscosity in the gel. The findings of this study would lead to a novel microscale tissue characterization method applicable to local sites.

Sustaining Microbubbles Derived from Phase Change Nanodroplet by Low-Amplitude Ultrasound Exposure

To improve the short lifetime of microbubbles induced upon application of triggering ultrasound pulse to a phase change nanodroplet (PCND), the effect of low-pressure continuous ultrasound for sustaining microbubbles was studied in a gel phantom. A pulse with 100 cycles of 1.1 MHz ultrasound with a peak negative pressure of 2.4 MPa was used for the generation of microbubbles while superimposing a bubble-sustaining ultrasound at a frequency of 1.1 MHz with a relatively low-pressure amplitude. It was found that a peak negative pressure in the range from 0.01 to 0.1 MPa was suitable for sustaining the microbubbles without inducing cavitation. The presence of sustained bubbles could be echographycally observed as a beam-shaped brightness change. Moreover, the sustained microbubbles induced cavitation upon additional application of ultrasound pulse at a peak negative pressure of 0.2 MPa. The results obtained suggested that not only the lifetime but also the activity of the microbubbles can be controlled.

Cavitation assisted HIFU with phase-change nano droplet

A preliminary study for therapeutic application of phase change nano droplet (PCND) in combination with low frequency ultrasound was performed. With gel phantom, it was found that the presence of PCND enhanced temperature elevation induced by 1.1-MHz ultrasound using albumin as the indicator of protein coagulation. Not like 2.2-MHz ultrasound, coagulation volume increased drastically as exposure time prolonged, suggesting the increase of numbers of bubbles which are responsible for the temperature elevation. Acoustic signal analysis showed that very violent cavitation is induced when PCND and 1.1-MHz ultrasound used. Animal experiments with mice tumor tissues were also performed and it was found that tissue erosion and necrosis were simultaneously generated. Results obtained showed the potential usefulness of PCND for mixed mechanism (HIFU & cavitation) therapy.

Phase Change Nanodroplets and Microbubbles Generated from Them as Sources of Chemically Active Cavitation

Aiming at a multidisciplinary tumor treatment with thermal and chemical mechanisms, the effect of phase change nanodroplets (PCNDs) on inducing reactive chemical species through cavitation was evaluated in vitro. By using the reaction yield of the oxidation of iodide ions to tri-iodide ions as a measure, the effect of PCNDs and PCND-derived microbubbles were investigated. The presence of PCNDs reduced the intensity threshold for chemical reaction by at least 40%. Furthermore, the threshold with PCND-derived microbubbles was 5 times lower than that with PCNDs alone. The interval of exposing the phase change trigger, which is used to generate microbubbles from PCNDs, was found to be shorter than 0.01 s for efficient induction of a chemical reaction. The reaction yields were independent of PCND concentration, thus PCND-derived microbubbles are not considered to work directly in the generation of chemical species undergoing the reaction.

2A-3 Enhanced and Site-Specific HIFU Treatment with Phase-Change Nano Droplet

A preliminary study for advanced HIFU therapy system was performed which enabled echographic marking and enhancing of the ultrasonic energy absorption efficiency at site of interest. As a main component of this system, we are developing phase change nano-droplet, a liquid precursor of a microbubble that changes into a bubble upon applying ultrasonic pulses. In this study, we investigated the effect of nano-droplet in temperature rise induced by HIFU exposure using gel phantom. It was found that HIFU exposure to nano-droplet suspended in polyacrylamide gel resulted in approximately 2.5 times greater temperature elevation when nano-droplet concentration was more than 10 muM. The concentration was in the same order as concentration required for the generation of echographycally significant microbubbles. Also, it was suggested that not the droplet itself but the induced microbubble by the phase change pulse is responsible for the enhancement of the temperature elevation. Such results were promising for the potential effectiveness of our approach for the site-specific HIFU.

Acousto-chemical manipulation of drug distribution: In vitro study of new drug delivery system

In vitro experiments were conducted for developing a novel drug delivery system (DDS). In the previous study, we achieved destruction of tissue structures by the combined use of pulsed ultrasound and liquid microbubble precursors, i.e., phase change nanodroplets (PCNDs). Here, fundamental studies were carried out to investigate the possibility of a DDS utilizing the phenomena. Gels with aqueous regions filled with PCNDs and sham drugs (black ink particles) were used as tissue mimicking samples with locally injected PCNDs and drugs. The exposure of samples to focused ultrasound pulses at 1.1 MHz with an acoustic intensity of 3.2 kW/cm2 was found to cause the transport of the sham drug toward the transducer. The transport speeds were about 0.34 and 0.1 mm/s, in and out of the focal zone of the transducer, respectively. Focal zone shifting during the exposure with an appropriate timing resulted in an enhanced drug transport while maintaining the initial speed of 0.34 mm/s. Our results suggested that the spatial distribution of locally injected drugs can be manipulated using ultrasound pulses when drugs are injected with PCNDs. As a control experiment, the effect of Sonazoid®, a commercially available microbubble contrast agent, was investigated. No drug transport was observed using Sonazoid® at a bubble number density one order of magnitude higher than that used in the PCND experiments. The pressure resistance properties of PCND and Sonazoid® might be related to the absence of the drug transport phenomena in case of Sonazoid®.

1F-3 Site-Specific Contrast Imaging with Locally Induced Microbubbles from Liquid Precursors

In studying a site-targeted contrast agent that can be administered as nano-particle (liquid droplet) and change its phase from liquid to gas by an external stimulus such as ultrasound to yield microbubbles, we have investigated if the phase shift can be achieved by using short pulses produced by a medical ultrasound scanner and probe. As expected in a preliminary study, we could induce phase shift by using pulsed ultrasound with several MPa negative peak pressure in in vitro experiments by using a linear probe with the frequency range of 3-8 MHz. It was found that the higher the ultrasound frequency, the lower the threshold, which indicated a thermal mechanism in phase shift. However, exposed total energy did not seem to be important to induce the phase shift because increasing wave cycle numbers from 2 to 8 did not lower the threshold significantly. Also it was suggested that negative peak pressure amplitude seemed to play a major role and positive peak did not. In vivo experiments with mice tumor exhibited that microbubble can also be generated in living bodies with ultrasound pulses generated by the ultrasound scanner at the same negative peak pressure amplitude as in in vitro experiments at the frequency of 7.8 MHz

Repeatable vaporization of optically vaporizable perfluorocarbon droplets for photoacoustic contrast enhanced imaging

Perflurorocarbon droplets that can be vaporized optically over multiple laser pulse applications are formulated. Photoacoustic contrast agents (PCAs) are important in visualizing molecular targets, deeply seated lesions, and vasculature. A novel PCA consisting of optically vaporizable perfluorocarbon droplets (OPDs) was investigated in this study. It has been reported that OPDs vaporize and emit a unique acoustic signal stronger than that from a light absorber alone, but that it only happened once at the time of vaporization. Our goal is to obtain repeatable vaporization of OPDs in order to sustain a high photoacoustic signal caused by vaporization over multiple laser pulse applications. By changing the contents of droplets to a compound with a higher boiling point, repeatable vaporization was successfully accomplished. Three times higher photoacoustic signals were produced repeatedly by droplets that exhibit transient vaporization behavior, namely PFH droplets within the size range of 0.2-1 μm.

Controlled induction of mechanical bioeffects with pulsed ultrasound and chemical agents

We aim to develop a novel low-invasive and effective treatment method of pancreatic tumor with synergistic effects of ultrasound and locally injected chemical agents such as anti-tumor drugs. In the treatment, controlling the spatial and temporal distribution of the agents by mechanical effects of ultrasound (cavitation) is a key process. To apply the mechanical effects to pancreases, a deep-seated organ, sensitizers for reducing the required acoustic intensity are essential. As preliminary experiments, we evaluated a formulation of superheated perfluorocarbon droplet, phase change nano droplet (PCND) as such a sensitizer. Effects on destruction of tissue structures were investigated ex vivo. It was found that ultrasound pulses with intensity higher than 2.2 kW/cm2 was enough to induce mechanical effects of ultrasound in chicken breast tissues in the presence of PCND while no damages were observed in the absence of PCND. Increasing pulse intensity or PCND concentration increased destructed volume. It was found that pulse duration should be longer than 100 cycles and increasing duration more than 300 cycles does not increase the destructed volume significantly. Results obtained suggested that PCND works as a sensitizer for inducing mechanical effects in tissues.