A. General Principles
ESWL is a non-invasive technique for destroying biliary and renal concretions, i.e., stones, using acoustic shock waves. The shock waves are generated externally to the patient and focused on the site of the stone. During ESWL treatment the stones are fragmented into pieces small enough to either pass out of the body through normal excretory channels (ureter or bile ducts) or respond favorably to chemical dissolution treatment.
The use of sound waves for ultrasound and electrohydraulic lithotripsy by direct contact with the target was well established prior to ESWL. However, the use of shock waves in ESWL is different. Ultrasound consists of sinusoidal waves of defined wave length, with alternating positive and negative deflections. Shock waves consist of a single positive pressure front of multiple frequencies with a steep onset and a gradual decline.
Shock waves undergo less attenuation than ultrasound waves when propagated through water or body tissue. As a result, shock waves can be transmitted through water and into the body with little loss of energy or damage to tissue.
The use of shock waves in the medical field for the destruction of urinary stones is based on the following properties:
1. Shock waves give rise to mechanical stress in brittle materials, such as human kidney stones. PA1 2. Shock waves lead to disintegration of such brittle material. PA1 3. Shock waves generated by the underwater discharge of a capacitor can be reliably reproduced. PA1 4. Shock wave energy can be propagated through water bath and body tissue to the stone with minimal energy loss or damage to tissue. PA1 5. Shock waves can be precisely focused by integrating the energy source with a suitable reflecting system. PA1 1. Each signal is windowed, i.e., chopped, around the arrival time of any reflected pressure waves. The duration of this window is such that the windowed signal comprises the prominent features of the reflected shock wave. PA1 2. The average amplitude of the data points comprising each signal is subtracted from each data point in that signal. In so doing, each signal is shifted such that the average value of the data points in that signal is zero. This operation is equivalent to removing any DC-shift in the signal. PA1 3. The amplitudes of the data points comprising each signal are divided by the signal's maximum positive amplitude within the chosen window. (The maximum positive value of the output from all of the sensors will be identical after this operation.) This normalization scheme facilitates the subsequent analyses. PA1 4. The windowed signal from each sensor is time-shifted with respect to the signals from each of the other sensors, in turn, in such a way that the correlation between the signals is maximized for each combination of sensors. The time shift for maximum correlation is equal to the difference in the time of arrival of the reflected pressure waves at the two sensors. These differences in arrival times (delays in arrival at one sensor with respect to the other) are then stored in the computer's memory for each sensor pairing. Thus, for a system of four pressure sensors, six delay times are calculated. PA1 5. The delay times for each sensor pairing are used to determine the coordinate components of a vector from the geometric focus of the lithotripter to the stone location.
Based on these principles, repeated shock wave stress will eventually exceed the comprehensive strength of the stone and lead to its disintegration.
Once the focused shock wave reaches the stone, the pressure front is partially reflected at the front surface of the stone, thus producing compressive and tensile components, which leads to buildup of a high-pressure gradient and causes disintegration of the stone's front surface. A portion of the wave continues through the stone and is reflected at the rear surface, where the same effect takes place. The disintegration of the outer layers exposes new surfaces that in turn are broken into fragments. This process eventually results in the complete disintegration of the entire stone.
In addition to the compressive forces and the negative tensile forces, cavitation microjets contribute to calculus fragmentation. Acoustic cavitation occurs when the tensile forces exceed ambient pressure, pulling apart the liquid and creating a bubble that collapses with the return of positive pressure.
B. Lithotripter Designs
Shock waves are created by converting energy into an acoustic form. The currently available extracorporeal shock wave lithotripters use energy sources that are electrohydraulic, piezoelectric, electromagnetic, or explosive in nature. Shock waves are focused on the stone by ellipsoidal reflectors, shaped array, or lens. Localization of the stone is by x-ray and/or ultrasound studies.
With the electrohydraulic method, electricity is discharged into water across a gap between two electrodes. The temperature of the water rises rapidly to form steam and then a plasma. A compressive pressure pulse results from expansion of the heated gases, followed by a negative pressure pulse as the gas bubble collapses. In the piezoelectric machines, an electrical field is applied across a piezoelectric crystal, changing the external dimensions of the crystal. Pressure waves are produced by the movement of the crystal. Multiple crystals are used in the machines for reliability and ease of construction. In the electromagnetic generators, a magnetic field is generated by current flow through a wire or coil. Magnetic materials are attracted or repelled by this field, turning electrical energy into mechanical and acoustic energy. The pressure wave is created by movement of a flexible membrane from passage of current through a fixed coil.
Focusing is accomplished geometrically by an ellipsoidal reflector in many machines. The shock waves created at the first focus of the ellipsoid by the generation system are reflected by the ellipsoidal reflector to arrive at the second focus simultaneously creating a shock wave at the final focal point. In the Siemens electromagnetic lithotripter, the sound waves are focused by biconcave acoustic lens. In the piezoelectric lithotripters, the crystals are shaped as part of a sphere to focus the energy.
In order for the shock wave to effectively destroy the stone, the site of the stone must be coincident or nearly coincident with the final focal point of the generated shock wave. This is accomplished by first, localizing the site of the stone in relation to the geometric focal point of the shock wave and, then, targeting (or adjusting) the location of the stone so as to be coincident with the focal point of the shock wave.
The targeting of the stone in relation to the focal point of the shock wave is conventionally accomplished by physically adjusting the position of either the patient or the shock wave generating system, until the location of the stone is determined to be coincident with the geometric focal point of the shock wave generating system. Although somewhat cumbersome, this method of adjustment has proven useful in conventional lithotripsy.
Difficulty has existed in locating the stone and accurately identify that location in relation to both the geometric focal point and, more importantly, the acoustic focal point of the machine. Stones are typically located using ultrasonography or x-ray flourography, with the image of the stone being displayed on a videoscope or x-ray image. A computer generated image, denoting the geometric focal point of the shock wave, may be superimposed on the ultrasound video display or x-ray image as a targeting aid. However, this technique of localizing and targeting has proven inadequate in several respects.
Ultrasound has the advantage of being able to be used continuously, but the disadvantage that stones cannot be visualized in certain locations. Moreover, ultrasound requires a considerable learning curve and sometimes it is difficult to determine the degree or fragmentation, in part due to the formation of an echogenic fragment line. Stone localization by ultrasound may be difficult in the kidney in the presence of a percutaneous nephrostomy tube, multiple calculi in the renal pelvis, or partial staghorn calculi. X-ray fluoroscopy is less commonly used and present added health concerns in that some people want strongly to avoid any exposure to x-rays. In addition, fluoroscopy is unable to visualize radiolucent stones such as those common in the gallbladder.
More importantly, the conventional techniques for localizing the stone frequently do not allow effective targeting. In order for the lithotripsy to effectively destroy the stone, the target center or the stone should be positioned as closely as possible to the focal point of the lithotripter. To the end, it is considered advantageous to be able to keep the stone in a region where the peak pressure at the stone site during lithotripsy is at least 50% of the pressure at the focus. For currently available lithotripters, the area in the focal plane described by this 50% isobar can be as small as 3 mm.sup.2, as reported by Coleman and Saunders (Ultrasound in Medicine and Biology, 15(3):213-227. (1989)). Conventional techniques of ultrasonography and flourography are problematic when targeting with such a small focal area for a variety of reasons, e.g., imprecision in manually adjusting the physical position of the patient and/or the shock wave focal point, displacement of the acoustic focal point of the generated shock waves from the geometric location due to diffraction effects, and stone visualization errors introduced due to diffraction effects on ultrasound imaging systems. As a result, extracorporeal lithotripsy procedures currently involve the application of between 1500 and 4000 shock waves to the stone site in order to reduce stone fragments to a size that can safely pass through the patient's system, or to be treated successfully using dissolution agents. Such a large number of shock waves prolong the treatment, may require multiple treatments, increase treatment cost, can produce tissue damage, and may require the administration of anesthesia to the patient.
In seeking to reduce the number of shocks applied, conventional targeting techniques have evolved along two diverging paths. One philosophy of improving lithotripters has been to apply more powerful shook waves and increase the volume of the acoustic focal point of the shock waves. This approach has the disadvantage of applying more shock wave energy to the tissue near the stone, with the attendant risk of tissue damage. The other philosophy of lithotripter development is to decrease the shock wave amplitude and decrease the volume of the focal point, the argument being that this method reduces the risk of tissue damage while maintaining the ability to destroy stones. A further advantage of this philosophy is that it makes possible anesthesia-free lithotripsy. However, a reduced focal point volume exacerbates existing targeting problems.
Other improvements in localizing and targeting stones have been recently reported. See Kuwahara et al, "Initial Experience Using a New Type Extracorporeal Lithotripter With An Anti-Misshot Control Device," J. of Lithotripsy and Stone Disease, Vol. 3, No. 2, pp. 141-146 (1991). This device is based noon the use of piezoelectric transducers to generate the shock wave, but the piezoelectric transducers are also used in a rudimentary way to assist in localizing the stone. Initial localizing of the stone is performed using an ultrasound sonoprobe installed in the shock wave generator. Ultrasound pulses are directed from the shock wave generator toward the geometric focal point of the shock wave. The sound levels, if any, reflected from the focal point region are detected by the piezoelectric transducers (now acting as a microphone). The signal is judged as a hit or miss based upon the amplitude of the reflected wave, i.e., a high level indicates a hit and a low level indicates a miss. A predetermined threshold is established in a comparator to judge whether the signal is a hit or a miss.
Simply stated, the Kuwahara et al. technique merely provides "yes/no", trial-and-error, information on whether the stone has been accurately sited and targeted. Kuwahara et al. failed to recognize or disclose any guidance adjustment mechanism for determining the direction and amount of movement which is needed to accurately locate and target the stone. Moreover, the technique of Kuwahara et al. is limited to lithotripters which are based upon piezoelectric shock wave generators. Kuwahara et al. does not disclose a guidance adjustment mechanism which is useful in other types of lithotripters.