Cunitz BW et al, 2017: Characterizing the Acoustic Output of an Ultrasonic Propulsion Device for Urinary Stones.
Cunitz BW, Dunmire B, Bailey MR.
A noninvasive ultrasound (US) system to facilitate the passage of small kidney stones has been developed. The device incorporates a software-based US platform programmed with brightness mode and Doppler for visualizing stones, plus long duration focused pulses for repositioning stones using the same transducer. This paper characterizes the acoustic outputs of the ultrasonic propulsion device. Though the application and outputs are unique, measurements were performed based on the regulatory standards for both diagnostic US and extracorporeal lithotripters. The extended length of the pulse, time varying pressure output over the pulse, the use of focused targeting, and the need to regulate the output at shallow depths, however, required modifications to the traditional acoustic measurement methods. Output parameters included spatial-peak intensities, mechanical index (MI), thermal index, pulse energy, focal geometry, and target accuracy. The imaging and Doppler operating modes of the system meet the Food and Drug Administration acoustic power and intensity limits for diagnostic US device. Push mode operates at a maximum MI of 2.2, which is above the limit of 1.9 for diagnostic US, but well below any lithotripsy device and an ISPTA of 548 mW/cm2, which is below the 720-mW/cm2 limit for diagnostic US.
IEEE Trans Ultrason Ferroelectr Freq Control. 2017 Dec;64(12):1818-1827. doi: 10.1109/TUFFC.2017.2758647. Epub 2017 Oct 2. FREE ARTICLE
The major shortcoming of SWL, even when carried out with high quality lithotripters, is the presence of residual fragments. These are most commonly located in the lower calyces. This annoying outcome will in a substantial number of patients result in additional treatment with invasive techniques in order to clear the renal collecting system. The need of invasive follow-up accordingly has made SWL to a less attractive procedure and primary intervention with ureteroscopy or percutaneous surgery has increased while the use of SWL has decreased.
In order to maintain the non-invasive character of stone-removal several methods have been suggested to facilitate fragment elimination such as patient mobility/exercises, inversion, vibration and US-propulsion.
In this article the authors describe a new ultrasonic propulsive device with which identification of stones/stone fragments is combined with stone/fragment movement accomplished by focused ultrasound bursts of long duration and given intermittently with a 5 s pause. Compared with SWL the delivered energy is higher, but on the other hand fewer bursts are assumed to be necessary.
A detailed report of the underlying physics is given, which I made no attempt to scrutinize. We definitely need a reliable, effective and non-invasive method to improve fragment elimination and I am eagerly waiting for clinical results obtained with this device.