Ken-ichi Kawabata

Nanoparticles with Multiple Perfluorocarbons for Controllable Ultrasonically Induced Phase Shifting

In studying the feasibility of developing tissue-targeted contrast media that can be administered as liquids and vaporized by an external stimulus such as ultrasound, we have investigated the properties of emulsions of nanometer-size particles containing perfluoropentane and 2H,3H-perfluoropentane. We found that the ultrasound intensity required to induce echographically significant vaporization can be controlled by changing the ratio of 2H,3H-perfluoropentane to perfluoropentane and that the intensity threshold increases as this ratio increases. Significant azeotropic phenomena were not observed when the perfluorocarbon mixtures were heated, which indicates that mechanisms other than azotropy are involved in the threshold change. The vaporization of 2H,3H-perfluoropentane may have been due not only to ultrasound energy but also to the energy deposited by ultrasonically induced bubbles of perfluoropentane. Our results might lead to a phase-shift contrast agent with controllable ultrasound energy for phase shifting.

In vivo acceleration of ultrasonic tissue heating by microbubble agent

The ultrasonic power absorbed by a microbubble in its continuous wave response is estimated through numerically solving a version of the Rayleigh-Plesset equation. At an ultrasonic frequency of 3 MHz, a resonant microbubble, approximately 1.1 /spl mu/m in radius, showed an absorption cross section of about 0.005 mm/sup 2/ in its low power response. This estimation predicts that the tissue ultrasonic absorption will be doubled when such microbubbles are delivered to the tissue at a concentration of about eight bubbles/mm/sup 3/ in tissue. An exteriorized murine kidney was exposed to focused ultrasound at 3.2 MHz in degassed saline, and the tissue temperature change was measured. With an intravenous bolus administration of a microbubble agent, the ultrasonically induced temperature elevation was multiplied by up to five times. The enhancement in temperature elevation gradually decreased as the microbubble agent was eliminated from the body. The experimental results agreed with the prediction in the order of magnitude. This effect may have a potential use to enhance the throughput as well as the selectivity of focused ultrasound treatment.

In vitro and in vivo enhancement of sonodynamically active cavitation by second-harmonic superimposition

Acoustic cavitation, the primary mechanism of sonochemical effects, is known to be induced more easily by standing waves than by progressive waves. It has been found that acoustic cavitation can be an order of magnitude enhanced by superimposing the second harmonic on the fundamental. Significant synergistic effects between the fundamental and the second harmonic were observed in both in vitro and in vivo experiments employing a progressive wave field. Second-harmonic superimposition induces in vitro sonochemical reaction as well as fractional harmonic emission at a relatively low ultrasonic intensity even in a progressive wave field. The effect of second-harmonic superimposition was also investigated using exteriorized mouse livers suspended in degassed saline. The intensity threshold for the production of focal tissue damage, paired with fractional harmonic emission was significantly lowered by second-harmonic superimposition especially when a sonodynamically active agent had been administered to the mouse. Insonation with second-harmonic superimposition in combination with such administration may have potential use for selective tumor treatment.

Enhancement of sonodynamic tissue damage production by second-harmonic superimposition: theoretical analysis of its mechanism

Among the nonthermal effects of ultrasound, acoustic cavitation may have the highest potential for therapeutic applications if it can be somehow controlled. Recent in vitro and in vivo experiments have demonstrated that sonochemically active cavitation can be enhanced an order of magnitude by superimposing the second harmonic onto the fundamental in insonation. Moreover, they have shown that sonochemically active cavitation can be controlled with relative ease, thereby even in a progressive wave field. The effect of second-harmonic superimposition on the rectified diffusion through the gas-liquid interface of cavitated microbubbles is estimated theoretically. The theoretical rectified diffusion rate explained an asymmetric behavior of the threshold for producing sonodynamic tissue damage as a function of the fundamental and the second-harmonic amplitudes. The tissue damage was produced with a focused progressive wave in a liver lobe of a mouse administered with a sonodynamically active agent. The result suggests that the acceleration of the rectified diffusion is a primary mechanism of the enhancement of sonodynamically effective cavitation by second-harmonic superimposition.

Recent advances in sonodynamic approach to cancer therapy

Abstract Chemical agents such as porphyrins were found to be activated by ultrasound, producing significant antitumor effects. Hematoporphyrin (Hp) enhanced ultrasonically induced damage on sarcoma cells and shown a synergistic inhibitory effect on the tumor growth in combination with ultrasound at 2 MHz. Recently, other types of porphyrins such as protoporphyrin were also found to have such sonodynamic activities. Furthermore, it was found that sonochemical reactions can be greatly accelerated by superimposing the second harmonic onto the fundamental. The highest rate of iodine release from aqueous iodide was obtained at an acoustic intensity ratio between 1 MHz and 2 MHz of 1:1 while either one of the frequency components alone could not induce significant iodine release at the same total acoustic intensity. Second-harmonic superimposition in combination with sonodynamically active antitumor agents may have the potential for selective tumor treatment.

Studies on sonodynamic cancer therapy

Chemical agents such as hematoporphyrin have been found to have a sonochemical activity to induce anti-tumor effects. The same kind of sonochemical reactions producing active oxygen species has been demonstrated to be localized with a highly focused ultrasound in a specially developed tissue-mimicking phantom. These results suggest that the focused ultrasound irradiation to a tumor combined with administration of such a sonochemically active agent can be a potential low-invasive modality for cancer therapy.

Combination effect of photodynamic and sonodynamic therapy on experimental skin squamous cell carcinoma in C3H/HeN mice

We studied a combination of photodynamic therapy (PDT) and sonodynamic therapy (SDT) for improving tumoricidal effects in a transplantable mouse squamous cell carcinoma (SCC) model. Two sensitizers were utilized: the pheophorbide‐a derivative PH‐1126, which is a newly developed photosensitizer, and the gallium porphyrin analogue ATX‐70, a commonly used sonosensitizer. Mice were injected with either PH‐1126 or ATX‐70 i.p. at doses of 5 or 10 mg/kg.bw. At 24 (ATX‐70) or 36 hr (PH‐1126) (time of optimum drug concentration in the tumor) after injection, SCCs underwent laser light irradiation (88 J/cm2 of 575 nm for ATX‐70; 44 J/cm2 of 650 nm for PH‐1126) (PDT), ultrasound irradiation (0.51 W/cm2 at 1.0 MHz for 10 minutes) (SDT), or a combination of the two treatments. The combination of PDT and SDT using either PH‐1126 or ATX‐70 as a sensitizer resulted in significantly improved inhibition of tumor growth (92–98%) (additive effect) as compared to either single treatment (27–77%). The combination using PH‐1126 resulted in 25% of the treated mice being tumor free at 20 days after treatment. Moreover, the median survival period (from irradiation to death) of PDT + SDT‐treated mice (>120 days) was significantly greater than that in single treatment groups (77–95 days). Histological changes revealed that combination therapy could induce tumor necrosis 2–3 times as deep as in either of the single modalities. The combination of PDT and SDT could be very useful for treatment of non‐superficial or nodular tumors.

Sonodynamic effect of erythrosin B on sarcoma 180 cells in vitro.

The ultrasonically induced cytotoxic effect of erythrosin B (EB) on isolated sarcoma 180 cells was investigated. The tumor cells were suspended in an air-saturated phosphate buffered saline and exposed to ultrasound at 1.93 MHz in a standing-wave mode for up to 60 s in the presence and absence of EB. The rate of cell damage induction by ultrasound was enhanced by 4-5 times with 160-microM EB, while no cell damage was observed with EB alone. This enhancement was significantly inhibited by histidine. Sonochemical generation of active oxygen species in the presence of EB, measured by ESR spectroscopy, was also inhibited by histidine. These results indicate the involvement of a sonochemical mechanism.

Effect of second-harmonic superimposition on efficient induction of sonochemical effect

Abstract A new method of insonation for inducing sonochemical effects efficiently without using mechanical or acoustical agitation was investigated. When the second harmonic (1.5 MHz) was superimposed on the fundamental (750 kHz) at the optimum acoustical power ratio (1:1), the oxidation rate was one or more orders of magnitude higher than that obtained at the same total acoustic power when the fundamental or the second harmonic was used alone. Moreover, the second-harmonic superimposition reduced the threshold of acoustic power for the sonochemical reaction to half of the threshold when the fundamental alone was used.

Bubble Generation by Standing Wave in Water Surrounded by Cranium with Transcranial Ultrasonic Beam

Low-frequency ultrasound, typically less than 1 MHz, is suitable for enhancing thrombolysis because it penetrates the cranium effectively. However, intracerebral hemorrhages after transcranial insonation in clinical trials at 300 Hz have been reported. In this study, acoustic bubble formation in a standing wave with a 617 kHz ultrasonic beam in water surrounded by a contoured piece of a human cranium was detected by ultrasound B-mode imaging. This bubble formation was indirect evidence that standing-wave formation led to cavitational adverse effects in brain tissue at the place of reflection by transcranial insonation at a relatively low ultrasonic frequency. A way of suppressing cavitation after bubble formation was also investigated. The efficiency of nucleation of bubbles was highly dependent on pulse duration at a constant total acoustic power. The obtained result suggests that inertial cavitation can be suppressed while preserving the efficiency of thrombolysis by temporally changing the acoustic condition before resonant bubble formation.