Characterizing Probe Tip Movement in the Swiss Lithoclast

Read following paper to get more imformation.

Songlin Zhu, Juhn Kourambas, Ravi Munver, Pei Zhong, and Glenn M. Preminger, “Characterization of Tip Movement of the Lithoclast Flexible Probe”, Journal of Endourology, Vol. 13, Sept. 1, 1999

Henry; Nicole; Hassan; Roberto: “Handpiece for use in lithotripsy”, U.S. Patent: 5,868,756, 1999

Favre; Robert: “Device for acting by ultrasonic vibrations on an object ”, U.S. Patent: 5,160,336, 1990

Hirt; Joachim; Merkle; Wolfgang: “Device for removal of calculi”,  U.S. Patent: 6,511,485, 2003
 

    The Swiss Lithoclast, developed by EMS SA (a Switzerland company), is a pneumatically driven lithotrisy device for the fragmentation of ureteral and renal calculi. Compressed air is used to driven a projectile which in turn impinges on a Lithoclast probe, typically passed though a cystoscope or ureteroscope's working channel. The impact energy from the vibrating projectile is transmitted through the probe and delivered to the target calculus, which is placed in contact with the distal end of the probe. The probe come in a rigid and flexible version and in varied diameters to work in conjunction with endscope instruments. The particular interest is the flexible version of the probe typically used for enhanced treatment of upper and middle pole renal calculi. The current flexible probe indicated that it can manage the upper pole renal stone. For extended application to mid and lower pole stone, the ureteroscope and the coaxial probe need to deflect to access the mid and lower pole. In an attempt to better understand the mechanics of actual stone fragmentation and quantifiable parameters critical to fragmentation, a laser-optical technique was developed to quantify the probe tip movement and a high-speed photographic system was set up to view the tip-stone interaction.

    To trace the probe tip movement in real time, a laser-optical system was introduced and set up. In this system, a He-Ne laser was used as a light source. Because the original laser beam is about 1mm and less then required beam, a beam expander was used to form a large collimated uniform beam (see Fig. 1 and Fig. 2). As the probe tip travels through the test area, a portion of the laser beam will be blocked. With a telescope in combination with the a objective lens, the image of the test area and the probe tip was projected onto an opaque screen. A slit was cut in the screen such that the width of the probe tip was less then the width of the probe tip image. Hence, as the probe tip advances, the intensity of the laser light transmitted through the slit will be proportionally reduced. The variation in the transmitted laser light intensity was detected by a photo-detector and recorded on an oscilloscope before being transferred to a computer with a GPIB interface card.

Figure 1 Schematic diagram of optical system for measuring the probe tip displacement

Figure 2 Photographs of optical system for measuring the probe tip displacement and imaging tip-stone interaction

    The measurement system was calibration by moving a rigid Lithoclast probe affixed to a translation stage in discrete stages through the test area and recording the corresponding transmitted laser light intensity. A linear correlation between the tip displacement and the transmitted laser light intensity was obtained (see Fig 3). The accuracy of the measurement system was further confirmed by comparing directly with the tip displacement determined by high-speed photography.

Figure 3 Calibration apparatus and calibration curve

    The Swiss Lithoclast flexible probe was evluated. The The flexible probe was employed through an actively deflectable ureteroscope. Tip displacement was measured at five different angles of deflection (0, 12, 24, 33, 48 degree). Figure 4 shows the tip displacement curves at 2.0 bar of driven pressure for various deflection angles. In general, a similar profile of the tip displacement curves was obtained, and the maximum tip displacement was found to decrease with deflection angle. Table 1 summarizes the maximum tip displacement and tip velocity at different air pressures and deflection angles. The results clearly demonstrate a pressure- and deflection-dependent response of the tip movement.

Figure 4 The probe tip displacement at different deflection angle. The probe is the 0.89mm flexible probe.

Table 1 Maximum tip displacement and tip velocity of the Lithoclast flexible probe
 

Pressure	Maximum	Tip	Displacem	ent	(mm)	Maximum	Tip	Velocity	(m/s)
(bar)	0 degree	12 degree	24 degree	33 degree	48 degree	0 degree	12 degree	24 degree	33 degree	48 degree
2.0	0.608	0.587	0.541	0.505	0.439	20.268	16.308	18.069	18.260	14.557
2.5	0.682	0.649	0.570	0.552	0.487	20.271	19.663	19.913	19.428	15.427

    A high-speed imaging system was used to capture a series of picture of interaction between the Lithoclast probe tip and an artificial stone. The stone was made of plaster of paris. To acquire a trigger signal, I set up an optical system, which is similar to that used to measure probe tip movement, at the proximal end of the probe near the hand piece. The stone place in a cubic chamber with one side open and the probe tip placed in contact with the stone. A U-shape of soft tubing, used as a elastic buffer, was put between the stone and the chamber wall to prevent excessive bouncing of the stone.
    Figure 5 shows the selected high speed images taken at different instances after the trigger signal at 0 degree of deflection angle. The number above each picture is the delay between trigger and imaging time. Initially, the probe was seen to move forward and plugged into the stone because the compressing pressure generated by the collision. The initial stone movement was too small to be noticed presumably because of its inertia. In this period, the impact momentum was transferred into stone. Subsequently, the advancement of the probe tip slowed down and the stone started to move gradually. When the stone could not be compressed further, both the stone and the probe tip were relative motionless and the probe tip plugged into the stone with a maximum distance of 0.337mm. After this critical point, the probe tip and stone separated with each other, but the probe still kept moving forward. When the probe tip pulled out from the stone, some plaster powders removed from the stone could be noticed. Eventually, when the kinetic energy of the probe was exhausted, the probe stopped moving. The maximum tip movement was found to be about 0.471mm, which is much less then that without interaction of the stone (about 0.886mm). Afterward, the probe bounded back because the O-ring released the energy store when it was compassed. The total time interval of the interaction between the probe tip and the stone is about 100 microsecond.

Figure 5 High-speed photographs of tip-stone interaction at 0 deflection angle. Driven pressure was 2.5 bar

    Figure 6 shows the tip displacements with and without tip-stone interaction. Tip displacement with interaction was determined from high-speed imaged shown in Figure 5. Due to the interaction with the target stone, duration of tip excursion is shortened (from 145.6 microsecond to 133.1 microsecond) and the displacement curve around maximum tip movement is flatted. This result indicates a substantial loss of the original tip kinetic energy, presumably transferred into the energy that cause deformation of stone material, kinetic energy of the stone, friction loss, O-ring compression etc.

Figure 6 Comparison of the probe tip displacement with and without tip-stone interaction

    The process of the probe tip movement at 33 degree of deflection angle is similar to that at 0 degree, except in the later period of the interaction (Fig. 7). In this later period, as the probe tip pulled out from the stone alone it axis, it also tilted  up anti-clockwise to band probe further. Because of this tilting movement of the probe tip, most plaster powders were seen to be pushed up from the stone. The maximum tip displacement and maximum distance that tip plugged into the stone is 0.392mm and 0.327mm.

Figure 7 High-speed photographs of tip-stone interaction at 33 deflection angle. Driven pressure was 2.5 bar

    A software used to processing the tip movement data was developed by using Matlab. For easy of use, a Graphical User Interface (GUI) is built in software (see Fig. 8).

Figure 8  A program to streamline the whole data processing procedure