Use of Intertransmission Coherence for Haze Artifact Suppression in Cardiovascular Synthetic Aperture Ultrasound Imaging.

Synthetic aperture (SA) beamforming is a principal technology of modern medical ultrasound imaging. In that the use of focused transmission provides superior signal-to-noise ratio (SNR) and is promising for cardiovascular diagnosis at the maximum imaging depth of about 160 mm. But there is a pitfall in increasing the frame rate to more than 80 frames per second (frames/s) without image degradation by the haze artifact produced when the transmit foci (SA virtual sources) placed within the imaging field. We hypothesize that the source of this artifact is a grating lobe caused by coarse (decimated) multiple transmission and manifesting in the low brightness region in the accelerated-frame-rate images. We propose an intertransmission coherence factor (ITCF) method suppressing haze artifacts caused by coarse-pitch multiple transmission. The method is expected to suppress the image blurring because the SA grating lobe signal is less coherent than the main lobe signals. We evaluated an ITCF algorithm for suppressing the grating artifact when the transmission pitch is up to four times larger than the normal pitch (equivalent to 160 frames/s). In in-vitro and in-vivo experiments, we demonstrated the strong relation of haze artifact with the grating lobe due to the coarse-pitch transmission. Then, we confirmed that the ITCF method suppresses the haze artifact of a human heart by 15 dB while preserving the spatial resolution. The ITCF method combined with focused transmission SA beamforming is a valid method for getting cardiovascular ultrasound B-mode images without making a compromise in the trade-off relationship between the frame rate and the SNR.

Focused Ultrasound and Lithotripsy.

Shock wave lithotripsy has generally been a first choice for kidney stone removal. The shock wave lithotripter uses an order of microsecond pulse durations and up to a 100 MPa pressure spike triggered at approximately 0.5-2 Hz to fragment kidney stones through mechanical mechanisms. One important mechanism is cavitation. We proposed an alternative type of lithotripsy method that maximizes cavitation activity to disintegrate kidney stones using high-intensity focused ultrasound (HIFU). Here we outline the method according to the previously published literature (Matsumoto et al., Dynamics of bubble cloud in focused ultrasound. Proceedings of the second international symposium on therapeutic ultrasound, pp 290-299, 2002; Ikeda et al., Ultrasound Med Biol 32:1383-1397, 2006; Yoshizawa et al., Med Biol Eng Comput 47:851-860, 2009; Koizumi et al., A control framework for the non-invasive ultrasound the ragnostic system. Proceedings of 2009 IEEE/RSJ International Conference on Intelligent Robotics and Systems (IROS), pp 4511-4516, 2009; Koizumi et al., IEEE Trans Robot 25:522-538, 2009). Cavitation activity is highly unpredictable; thus, a precise control system is needed. The proposed method comprises three steps of control in kidney stone treatment. The first step is control of localized high pressure fluctuation on the stone. The second step is monitoring of cavitation activity and giving feedback on the optimized ultrasound conditions. The third step is stone tracking and precise ultrasound focusing on the stone. For the high pressure control we designed a two-frequency wave (cavitation control (C-C) waveform); a high frequency ultrasound pulse (1-4 MHz) to create a cavitation cloud, and a low frequency trailing pulse (0.5 MHz) following the high frequency pulse to force the cloud into collapse. High speed photography showed cavitation collapse on a kidney stone and shock wave emission from the cloud. We also conducted in-vitro erosion tests of model and natural kidney stones. For the model stones, the erosion rate of the C-C waveform showed a distinct advantage with the combined high and low frequency waves over either wave alone. For optimization of the high frequency ultrasound intensity, we investigated the relationship between subharmonic emission from cavitation bubbles and stone erosion volume. For stone tracking we have also developed a non-invasive ultrasound theragnostic system (NIUTS) that compensates for kidney motion. Natural stones were eroded and most of the resulting fragments were less than 1 mm in diameter. The small fragments were small enough to pass through the urethra. The results demonstrate that, with the precise control of cavitation activity, focused ultrasound has the potential to be used to develop a less invasive and more controllable lithotripsy system.

High intensity focused ultrasound lithotripsy with cavitating microbubbles.

In the medical ultrasound field, microbubbles have recently been the subject of much interest. Controlling actively the effect of the microbubbles, a novel therapeutic method has been investigated. In this paper, our works on high intensity focused ultrasound (HIFU) lithotripsy with cavitating microbubbles are reviewed and the cavitation detection method to optimize the HIFU intensity is investigated. In the HIFU lithotripsy, collapse of the cloud cavitation is used to fragment kidney stones. Cloud cavitation is potentially the most destructive form of cavitation. When the cloud cavitation is acoustically forced into a collapse, it has the potential to concentrate a very high pressure. For the control of the cloud cavitation collapse, a novel two-frequency wave (cavitation control [C-C] waveform) is designed; a high-frequency ultrasound pulse (1-4 MHz) to create the cloud cavitation and a low-frequency trailing pulse (500 kHz) following the high-frequency pulse to force the cloud into collapse. High-speed photography showed the cavitation collapse on the stone and the shock-wave emission from the cloud. In vitro erosion tests of model and natural stones were also conducted. In the case of model stones, the erosion rate of the C-C waveform showed a distinct advantage with the combined high- and low-frequency waves over either wave alone. For the optimization of the high-frequency ultrasound intensity, the subharmonic acoustic pressure was examined. The results showed relationship between the subharmonic pressure from cavitating bubbles induced by the high-frequency ultrasound and eroded volume of the model stones. Natural stones were eroded and most of the resulting fragments were less than 1 mm in diameter. The method has the potential to provide a novel lithotripsy system with small fragments and localized cavitating bubbles on a stone.

Cloud cavitation control for lithotripsy using high intensity focused ultrasound.

Cloud cavitation is potentially the most destructive form of cavitation. When the cloud cavitation is acoustically forced into a collapse, it has the potential to concentrate a very high pressure, more than 100 times the acoustic pressure, at its center. We experimentally investigate a method to control the collapse of high intensity focused ultrasound (HIFU)-induced cloud cavitation to fragment kidney stones. Our study examines a novel two-frequency wave designed to control the cloud cavitation (cavitation control [C-C] waveform); a high-frequency ultrasound pulse (1 to 4 MHz) to create the cloud cavitation and a low-frequency trailing pulse (545 kHz) following the high-frequency pulse to force the cloud into collapse. High-speed photography has revealed that a localized distribution of the cloud cavitation can be produced within 1 mm on the solid surface by the high-frequency pulse. The low-frequency ultrasound was irradiated to the high-frequency-induced cloud cavitation. A subsequent shock wave emitted from the cloud cavitation was observed both in the shadowgraph photography and the remote hydrophone measurement. Furthermore, in vitro erosion tests of model and natural stones were conducted. In the case of model stones, the erosion rate of the C-C waveform showed a distinct advantage with the combined high- and low-frequency waves over either wave alone. Natural stones were eroded and most of the resulting fragments were less than 1 mm in diameter. The results show that the control of the cloud cavitation has untapped potential for the lithotripsy applications upon further optimization of the ultrasound parameters and complementary in vivo studies.