Ryosuke Iwasaki

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.

Feasibility of real-time treatment feedback using novel filter for eliminating therapeutic ultrasound noise with high-speed ultrasonic imaging in ultrasound-guided high-intensity focused ultrasound treatment

In the conventional ultrasonic monitoring of high-intensity focused ultrasound (HIFU) treatment, a significant interval between HIFU shots is required when monitoring target tissue to avoid interference between HIFU noise and RF echo signals. In our previous study, a new filtering method to eliminate only HIFU noise while maintaining tissue signals intact was proposed, and it was shown that the thermal coagulation could be detected during simultaneous HIFU irradiation through off-line processing. In this study, the filtering method and a real-time coagulation detection algorithm were implemented in an ultrasound imaging system, whose use for sequential exposure with multiple foci was demonstrated similarly to a commercial HIFU ablation system. The coagulation was automatically detected by the proposed method during real-time simultaneous HIFU irradiation, and the HIFU exposure time was controlled according to the changes in the tissue. The results imply that ultrasonic monitoring with the filtering and detection methods is useful for true real-time detection of changes in the tissue due to thermal coagulation during HIFU exposure.

Elimination of therapeutic ultrasound noise from pre-beamformed RF data in ultrasound imaging for ultrasound-guided high-intensity focused ultrasound treatment

In conventional ultrasonic monitoring of high-intensity focused ultrasound (HIFU) treatment, a significant interval between consecutive HIFU shots is set for monitoring target tissue to avoid the interference of HIFU noise with RF echo signals. Thus, it is difficult to detect changes in tissue on the order of milliseconds, which are required to dynamically control the HIFU exposure. In this study, a new filtering method to eliminate the HIFU noise in the RF signals before beamforming is proposed. The CW response was estimated from RF signals with no pulse response to the imaging exposure, and the estimated CW response was subtracted from the entire RF signal to selectively eliminate the HIFU noise for each channel of the array probe before dynamic focusing was applied. The HIFU noise was selectively eliminated by this method when it existed. The results imply that the proposed filtering method is useful for true real-time detection of changes in tissue due to thermal coagulation during HIFU exposure.

Singular value decomposition of received ultrasound signal to separate tissue, blood flow, and cavitation signals

High-intensity focused ultrasound is a noninvasive treatment applied by externally irradiating ultrasound to the body to coagulate the target tissue thermally. Recently, it has been proposed as a noninvasive treatment for vascular occlusion to replace conventional invasive treatments. Cavitation bubbles generated by the focused ultrasound can accelerate the effect of thermal coagulation. However, the tissues surrounding the target may be damaged by cavitation bubbles generated outside the treatment area. Conventional methods based on Doppler analysis only in the time domain are not suitable for monitoring blood flow in the presence of cavitation. In this study, we proposed a novel filtering method based on the differences in spatiotemporal characteristics, to separate tissue, blood flow, and cavitation by employing singular value decomposition. Signals from cavitation and blood flow were extracted automatically using spatial and temporal covariance matrices.

Selective detection of cavitation bubbles by triplet pulse sequence in high-intensity focused ultrasound treatment

Acoustic cavitation bubbles are known to enhance the heating effect in high-intensity focused ultrasound (HIFU) treatment. The detection of cavitation bubbles with high sensitivity and selectivity is required to predict the therapeutic and side effects of cavitation, and ensure the efficacy and safety of the treatment. A pulse inversion (PI) technique has been widely used for imaging microbubbles through enhancing the second-harmonic component of echo signals. However, it has difficulty in separating the nonlinear response of microbubbles from that due to nonlinear propagation. In this study, a triplet pulse (3P) method was investigated to specifically image cavitation bubbles by extracting the 1.5th fractional harmonic component. The proposed 3P method depicted cavitation bubbles with a contrast ratio significantly higher than those in conventional imaging methods with and without PI. The results suggest that the 3P method is effective for specifically detecting microbubbles in cavitation-enhanced HIFU treatment.

Prediction of thermal coagulation by short-pulse pre-exposure for cavitation-enhanced ultrasonic heating

Targeting the ultrasound beam and predicting the thermal coagulation in advance are important for high-intensity focused ultrasound (HIFU) treatment. Cavitation bubbles are known to enhance ultrasonic heating, however, temporal and spatial control of their generation is not simple. In our previous study, a method utilizing acoustic radiation force to predict thermal coagulation was suggested. In this study, it was investigated whether the proposed method works effectively even for the cavitation-enhanced ultrasonic heating and can be used to ensure both safety and efficiency of the treatment.

Study on ultrasound sequence with scanning the focus to generate reactive oxygen species efficiently for sonodynamic therapy

In the process of sonodynamic treatment, reactive oxygen species (ROS) are generated to damage cancer cells, and therefore the reproducible efficiency of ROS generation is significant for the treatment. Our previous study demonstrated that trigger HIFU (High-intensity focused ultrasound) exposure sequence, consisting of a high-intensity short trigger pulse and a medium-intensity long sustaining burst, was effective for enhancing the efficiency of ROS generation. To generate ROS more efficiently, trigger HIFU sequences with focus scanning were tested in this study. Considering the diffusion of the precursors of ROS, the optimum time of scanning the focus was studied. A KI method and high-speed camera were used to measure the amount of ROS and confirm the distribution of cavitation bubbles in the aqueous solution. The high-speed camera observation confirmed that cavitation bubbles were generated around each focal point similarly in all sequences. The scanning sequence with less intermission time between the...

Basic study on ultrasonic monitoring using 1.5-dimensional ultrasound phased array for ultrasound-guided high-intensity focused ultrasound treatment

In conventional ultrasonic monitoring of high intensity focused ultrasound (HIFU) treatment, it has been difficult to track the target region when the tissue to be treated deviates from the imaging plane along the elevation axis of the 1-D probe. A 2-D phased array probe providing 3-D imaging capability requires a large number of elements and it is very expensive to build a system to drive all channels simultaneously. A 1.5-D phased array probe can provide a good compromise between conventional 1-D and 2-D phased array probes and has significant cost and implementation advantages. In this study, a new 1.5-D phased array probe consisting of 64×4 elements was designed and developed to track tissue motion within mm order along the elevation direction and the elevational displacement range where the tissue tracking is effective was investigated.

Simultaneous observation of cavitation bubbles generated in biological tissue by high-speed optical and acoustic imaging methods

Acoustic cavitation bubbles are useful for enhancing the heating effect in high-intensity focused ultrasound (HIFU) treatment. Many studies were conducted to investigate the behavior of such bubbles in tissue-mimicking materials, such as a transparent gel phantom; however, the detailed behavior in tissue was still unclear owing to the difficulty in optical observation. In this study, a new biological phantom was developed to observe cavitation bubbles generated in an optically shallow area of tissue. Two imaging methods, high-speed photography using light scattering and high-speed ultrasonic imaging, were used for detecting the behavior of the bubbles simultaneously. The results agreed well with each other for the area of bubble formation and the temporal change in the region of bubbles, suggesting that both methods are useful for visualizing the bubbles.

Elimination of therapeutic ultrasound noise in phase modulated high intensity focused ultrasound treatment

Ultrasound image-guided High Intensity Focused Ultrasound (HIFU) has been become one of the potential surgical techniques for the treatment of the malignant tumor. The ultrasound image during the treatment is severely corrupted by the reflected HIFU wave because HIFU and imaging transducers are simultaneously activated. In our previous study, a new method to selectively eliminate only HIFU noise during the treatment was suggested, but it cannot sufficiently reduce the HIFU noise when the HIFU exposure is phase-modulated with a period less than a few milliseconds. In this study, a new method to selectively eliminate the HIFU noise even when the HIFU exposure is modulated with a certain modulation period. In this study, “multiple HIFU exposure” with a certain modulation period was employed. The focus of HIFU was moved every 25μs sequentially for six positions with spacing proper for thermal conduction. the RF signals of the same modulation period were received with (RF2) and without (RF1) transmitting an im...