Shin Yoshizawa

Ultrasonic Coagulation of Large Tissue Region by Generating Multiple Cavitation Clouds in Direction Perpendicular to Ultrasound Propagation

High-intensity focused ultrasound (HIFU) therapy is attracting attention as a minimally invasive therapeutic modality. However, it has a problem of a long treatment time. To improve the efficiency of the treatment, we developed a method of coagulating a large region at one time utilizing multiple clouds of cavitation. It is known that acoustic cavitation generated in the focal region of HIFU enhances tissue heating. In this study, cavitation clouds were generated at three positions in the direction perpendicular to ultrasound propagation using high-intensity ultrasound pulses. The tissue in the vicinity of the cavitation clouds was coagulated simultaneously with nonspherically focused ultrasound waves at a relatively low intensity. A high-speed camera was used to observe such behavior of cavitation clouds in a tissue-mimicking gel and to optimize the sequence, and the coagulation performance of the sequence was confirmed with an experiment using excised tissue. The result suggests that the HIFU treatment time is significantly shortened by employing the proposed method.

Monitoring of Lesion Induced by High-Intensity Focused Ultrasound Using Correlation Method Based on Block Matching

High-intensity focused ultrasound (HIFU) is a noninvasive therapeutic application that focuses ultrasound to the target tissue, such as a malignant tumor, and thermally coagulates it. Monitoring methods for evaluating the formation of thermal lesions induced by HIFU are required to perform safe and accurate HIFU treatment. However, the conventional ultrasonic B-mode image incurs difficulties in assessing the formation of HIFU-induced lesions. In this study, ultrasound RF signals were acquired during HIFU exposure. A correlation coefficient was used to evaluate the changes occurring in the RF signals backscattered from the thermal lesion. Also, a block matching algorithm has been implemented to compensate the tissue motion during HIFU exposure. The experimental results show that correlation coefficients in the focal spot decreased significantly with HIFU exposure, which indicates that the backscattered RF signals changed owing to tissue coagulation.

Thermal Simulation of Cavitation-Enhanced Ultrasonic Heating Verified with Tissue-Mimicking Gel

High-intensity focused ultrasound (HIFU) causes selective tissue necrosis through heating and is used as a noninvasive treatment in cancer therapy. However, there is a problem that it takes several hours to treat a large tumor. To shorten the treatment time, there is need for the development of a highly efficient method. It is known that cavitation bubbles generated by HIFU enhance the heating effect of ultrasound. In this study, the enhancement of the heating effect due to cavitation was considered in the bio-heat transfer equation (BHTE) by increasing the absorption coefficient in the region of generated cavitation. The absorption coefficient was calculated by curve fitting between the temperature rise at the focal point in the experiment and that in the simulation. The results show that the increased absorption can simulate the enhancement of the temperature rise by cavitation bubbles.

Coagulation of Large Regions by Creating Multiple Cavitation Clouds for High Intensity Focused Ultrasound Treatment

High-Intensity focused ultrasound (HIFU) is attracting attention as a minimally invasive therapeutic modality. However, it has a problem of a long treatment time. To improve its treatment throughput, we developed a method of coagulating a large region at one time utilizing multiple clouds of cavitation. It is known that acoustic cavitation generated in the focal region of HIFU enhances tissue heating. In the experiment in this study, the focal distance was changed using an annular array transducer. Cavitation clouds were generated at three positions within a fraction of a second by high-intensity ultrasound pulses. The tissues in the vicinity of the cavitation clouds were coagulated simultaneously with non-spherically focused ultrasound waves at a relatively low intensity. The result suggests that the HIFU treatment time is significantly shortened by employing the proposed method.

Optical Phase Contrast Mapping of Highly Focused Ultrasonic Fields

The most common method of measuring an ultrasonic pressure field is a hydrophone scan. However, this method has a long scanning time and disturbs the acoustic field. In this study, we used an optical phase contrast method for the measurement. Because this method uses light, fast and noninvasive measurement can be performed. The projections of an ultrasonic pressure field were obtained with a charge-coupled device (CCD) camera, and the three-dimensional (3D) acoustic pressure field was reconstructed using a computed tomography (CT) algorithm from these projections. The result was compared with that of hydrophone measurement and demonstrated the successful reconstruction of a focal ultrasonic pressure field.

Analysis of Temperature Rise Induced by High-Intensity Focused Ultrasound in Tissue-Mimicking Gel Considering Cavitation Bubbles

High-intensity focused ultrasound (HIFU) causes a selective temperature rise in tissue and is used as a noninvasive method for tumor treatment. However, there is a problem in that it typically takes several hours to treat a large tumor. The development of a highly efficient method is required to shorten the treatment time. It is known that cavitation bubbles generated by HIFU enhance HIFU heating. In this study, the enhancement of the heating effect by cavitation was estimated in a numerical simulation solving a bio-heat transfer equation (BHTE) by increasing the absorption coefficients in and out of the volume of cavitation bubbles. The absorption coefficients were obtained by a curve fitting the temperature rise near the focal point between experiment and simulation. The results show that cavitation bubbles caused the increase in ultrasonic absorption not only in but also near the volume of cavitation bubbles.

Enhancement of Focused Ultrasound Treatment by Acoustically Generated Microbubbles

Microbubbles, whether introduced from outside the body or ultrasonically generated in situ, are known to significantly enhance the biological effects of ultrasound, including the mechanical, thermal, and sonochemical effects. Phase-change nanodroplets, which selectively accumulate in tumor tissue and whose phase changes to microbubbles can be induced by ultrasonic stimulation, have been proposed for high-intensity focused ultrasound (HIFU) tumor treatment with enhanced selectivity and efficiency. In this paper, a purely acoustic approach to generate microbubble clouds in the tissue to be treated is proposed. Short pulses of focused ultrasound with extremely high intensity, named trigger pulses, are used for exposure. They are immediately followed by focused ultrasound for heating with an intensity similar to or less than that of normal HIFU treatment. The localized generation of microbubble clouds by the trigger pulses is observed in a polyarylamide gel by a high-speed camera, and the effectiveness of the generated clouds in accelerating ultrasonically induced thermal coagulation is confirmed in excised chicken breast tissue. The use of second-harmonic superimposed waves as the trigger pulses is also proposed. The highly reproducible initiation of cavitation by waves with the negative peak pressure emphasized and the efficient expansion of the generated microbubble clouds by waves with the positive peak pressure emphasized are also observed by a high-speed camera in partially degassed water.

Staircase-Voltage Metal–Oxide–Semiconductor Field-Effect Transistor Driver Circuit for Therapeutic Ultrasound

An important component of all ultrasound systems is the high-voltage pulse generator used to excite the transducer. We created a new type of metal–oxide–semiconductor field-effect transistor (MOSFET) switching circuit for therapeutic ultrasound applications, on the basis of the staircase-voltage drive concept. The staircase drive can significantly decrease the harmonics in comparison with the conventional square-wave drive. Since it does not require a resonant circuit to improve the efficiency, the drive frequency can be freely changed and the total size of the drive circuit can be made smaller. Using a diode in series to the drain of each MOSFET, a staircase-voltage driver was created with a relatively simple configuration. The experimental results of operating a prototype driver, as well as the results from circuit simulation, are discussed.

High-speed observation of bubble cloud generation near a rigid wall by second-harmonic superimposed ultrasound

Cavitation bubbles are known to accelerate therapeutic effects of ultrasound. Although negative acoustic pressure is the principle factor of cavitation, positive acoustic pressure has a role for bubble cloud formation at a high intensity of focused ultrasound when cavitation bubbles provide pressure release surfaces converting the pressure from highly positive to negative. In this study, the second-harmonic was superimposed onto the fundamental acoustic pressure to emphasize either peak positive or negative pressure. The peak negative and positive pressure emphasized waves were focused on a surface of an aluminum block. Cavitation bubbles induced near the block were observed with a high-speed camera by backlight and the size of the cavitation generation region was measured from the high-speed images. The negative pressure emphasized waves showed an advantage in cavitation inception over the positive pressure emphasized waves. In the sequence of the negative pressure emphasized waves immediately followed by the positive pressure emphasized waves, cavitation bubbles were generated on the block by the former waves and the cavitation region were expanded toward the transducer in the latter waves with high reproducibility. The sequence demonstrated its potential usefulness in enhancing the effects of therapeutic ultrasound at a high acoustic intensity.

Basic study of intrinsic elastography: Relationship between tissue stiffness and propagation velocity of deformation induced by pulsatile flow

We proposed an estimation method for a tissue stiffness from deformations induced by arterial pulsation, and named this proposed method intrinsic elastography (IE). In IE, assuming that the velocity of the deformation propagation in tissues is closely related to the stiffness, the propagation velocity (PV) was estimated by spatial compound ultrasound imaging with a high temporal resolution of 1 ms. However, the relationship between tissue stiffness and PV has not been revealed yet. In this study, the PV of the deformation induced by the pulsatile pump was measured by IE in three different poly(vinyl alcohol) (PVA) phantoms of different stiffnesses. The measured PV was compared with the shear wave velocity (SWV) measured by shear wave imaging (SWI). The measured PV has trends similar to the measured SWV. These results obtained by IE in a healthy male show the possibility that the mechanical properties of living tissues could be evaluated by IE.