Characterizing the Acoustic Output of an Ultrasonic Propulsion Device for Urinary Stones

Abstract

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.

Fig. 1

Ultrasonic propulsion system hardware. The ultrasound system (left) consists of the Verasonics data acquisition system, computer, keyboard, touchscreen monitor, and a C5-2 curved linear array (right).

Fig. 2

Schematic of the Push burst. A 50 ms burst consists of 81 pulses. Each pulse is 4500 μs long followed by 165 μs of off time; a duty cycle of 73%.

Fig. 3

Lateral dithering of the Push burst. A dithering cycle consists of 17 pulses. The initial pulse is centered at the user selected focal position. Each pair of pulses after is delivered at alternating increasing lateral distance from the axis in ±0.25 mm steps. After the 17^th pulse, at - 2.0 mm, the dithering cycle repeats starting on axis. The dithering sequence is applied over the 81 pulses of a single 50 ms Push burst.

Fig. 4

Block diagram of the custom assembled scanning tank and system for the AP&I measurements.

Fig. 5

Illustration of the modified acquisition of a 50 ms burst. (a) The decrease in amplitude associated with the capacitor drain across the five dithering cycles. Within each dithering sequence there is an additional decrease as the treatment focus moves away from the hydrophone with dithering. (b) Acquisition across one dithering cycle (17/81 pulses); the entire 50 ms pulse is not shown. The red boxes illustrate the portion of the waveform that is acquired by the oscilloscope. The average cycle amplitude measured within the acquisition window is applied across the 450 us pulse.

Fig. 6

Replica of the figure used to illustrate I_SPBA in Herman and Harris [14].

Fig. 7

The peak positive pressure map showing the relative pressure and target error over the acoustic field. The length of the arrow represents the error in distance between the true and measured peak location. The direction of the arrow represents the location of the expected (true) target location relative to the measured target location. The local pressure maps shown to the right provide an amplified view of the target accuracy. At 7 cm on axis there is no error. At 10 cm depth, 7 cm off axis the peak positive pressure is at (−1.8, −4) relative to the expected location (0, 0).

Fig. 8

(top) True pressure waveform without (blue) and with (green) the FOPH noise reduction. (bottom) View of several cycles in a low signal section. The nonlinearity of the waveform are discussed in detail in Ref. .

Fig. 9

Peak positive and negative pressure at 7 cm. (a) Low power - XZ plane. (b) Low power - YZ plane. (c) Maximum output - XY plane. The color scale is in units of MPa. The black line is the −6 dB contour.

Fig. 10

Peak positive acoustic pressure. The waveform was obtained by averaging the 115 waveforms captured in a windowed 50 μs acquisition. The green line represents the rise time and the red line represents the compressional pulse duration

Fig. 11

Pulse intensity integral in the XY plane at a depth of 7 cm. The results are after correction and summation of the 81 pulses. The units for the color scale are J/cm².

Fig. 12

Illustration of the burst average calculation for the ultrasonic propulsion device. Herman and Harris [14] assume each pulse is the same amplitude, which is not the case for the investigational device.