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 Figure 6. Top: Normalized transient stress in model stones di- rectly after impingement of ultrasound bursts of 170, 285, and 800 kHz. Middle: Corresponding fractures formed in artifi- cial cylindrical stones in a water tank after a short exposure to burst wave lithotripsy (BWL) at each frequency. Bottom: Cor- responding fragments produced at each frequency. Reprinted with permission from Maxwell et al. (2015) ©The Journal of Urology.
developed using acoustic radiation force to reposition kidney stones in the urinary tract (May et al., 2016b). In ultrasonic pro- pulsion, a transducer is placed against the skin and ultrasound imaging is performed simultaneously with the same transducer or with a transducer that includes separate imaging and ther- apy components. The stone is then seen to move in real time.
There are conceivably many clinical uses for repositioning stones, including moving a painful obstructing stone, relo- cating a stone before breaking it, and expelling stone frag- ments during SWL or BWL to assess when the stone is fully comminuted. The acoustic radiation force (F) applied to the stone by ultrasonic propulsion can be estimated as the pow- er (W) divided by the sound speed (c) of the fluid. The force stems from the acoustic energy being absorbed or reflected by the stone. If all the energy is reflected 180° directly back to the source, the force doubles because the energy was not only temporarily absorbed but reflected in the opposite di- rection. Properties of the stone, such as density and sound speed, affect the degree of reflection, and the stone mate- rial’s attenuation coefficient describes the degree of absorp- tion. However, stone material properties are less variable and therefore have less impact on the net force than do stone shape and, particularly, mass (Chuong et al., 1993; Cleve- land and McAteer, 2007). Assuming an absorbed or reflected power of 1 W is reasonable and is about 1/200 of the power for a maximum diagnostic pulse average intensity of 190 W/ cm2 encountering a large-stone cross section of ~1 cm2. The estimated displacement for a pulse duration of 0.1 s is 0.7
cm, which is similar to the hop that is observed in experi- ments (Janssen et al., 2017).
Thus it is reasonable that average intensities below the maxi- mum limit for diagnostic ultrasound could be used to move kidney stones, although a more accurate calculation of the radiation force becomes more complicated. Sapozhnikov and Bailey (2013) describe a model that simulates an arbitrary beam (ultrasound is usually focused) on an elastic spheri- cal stone. Interestingly, this model shows that an ultrasound beam 20% broader than the stone imparts the most force on the stone, although one might expect any energy not directly impacting the stone to be wasted. The wave passing across or over the stone appears to couple energy into the stones pri- marily as shear waves in a process similar to that described for breaking stones (Sapozhnikov et al., 2007).
An ultrasonic propulsion system has been built and tested in humans, and stones were observed to reposition within the kidney. Stones were successfully repositioned in 14 of 15 sub- jects. The main clinically relevant findings were that 4 of 6 pa- tients with postlithotripsy fragments passed collectively over 30 fragments; in 4 subjects, what appeared on clinical imag- ing to be one large stone requiring surgery was in fact a pile of passable small fragments; and 1 subject with a 1-cm ob- structing stone felt relief from the stone movement procedure (Harper et al., 2016). Based on the observation of debris col- lections, the probe was redesigned to generate a longer pulse and a broader beam to sweep the stones like a leaf blower (Janssen et al., 2017). That system is now very similar in de- sign to the BWL probes and has been shown to have synergy with BWL. Especially in the confined space of the kidney or ureter, BWL may crack the stone, but ultrasound propulsion separates the broken fragments from the main stone.
Significant advancements have been made in kidney stone detection and management with ultrasound. Changes in image-processing techniques have improved the appearance and sizing of stones with grayscale ultrasound, and the ad- dition of twinkling has the potential to make stones easier to detect. Revisions to current technologies such as stone tracking and acoustic bubble coalescence as well as new technologies including ultrasonic propulsion and BWL have the potential to change the current clinical practice for man- aging kidney stones. The end result is a suite of technologies that may diagnose and manage kidney stones on Earth or in space with fewer complications and faster relief for those who experience kidney stones.
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