1. Field of the Invention
The invention relates to devices and methods for performing Extracorporeal Shock Wave Lithotripsy (ESWL). More specifically, the invention relates to methods and devices for performing ESWL in which a spherical-sector shock wave (non-focused and diverging) is applied to non-invasively disintegrate kidney stones, gallstones or other calculi within living bodies.
2. Relates Art
There are various known methods for destroying stone-like concretions (or "calculi") within living bodies using shock waves generated outside the body.
a. Ellipsoidal Shock Wave Focus Method
The first of these methods is the ellipsoidal shock focus method, which is illustrated in FIG. 1. An ellipsoid is formed by the inner surface 102 of ellipsoidal shock focus element 112 as well as by an imaginary continuation of the ellipsoidal curve along a dotted line indicated as 104, separated by circular rim 106.
Ellipsoid 102/104 has two foci. At the first, "source focus" 108 of the ellipsoid 102/104 is located a shock source such as high explosive or electrodes in a fluid medium 116. At the second, "target focus" 110 of the ellipsoid 102/104, is located the target calculus 128. In an early embodiment, the entire portion of a body 130 containing the target calculus 128 was immersed in a bath 118 having sides 124. In later embodiments, the large bath with sides 124 was replaced by a shock transfer pad or body surface shock mitigator belt 122.
In operation, the shock source located at source focus 108 is activated. This activation is accomplished by exploding a small charge of high explosive using means commonly known in the art, or by applying a high electrical potential 136 across electrode 132 and 134 so as to cause an arc and a gas generation reaction which creates a shock wave that emanates from source focus 108.
Whichever particular means of shock source is employed, a shock wave front emanates from source focus 108 in all directions. That portion of the spherical shock front which intersects that inner face 102 of ellipsoidal shock focus element 112 reflects off the inner surface 102. Exemplary paths of individual portions of this shock wave are indicated in FIG. 1. For example, acoustic energy traveling along paths 138 and 140 reflects off ellipsoidal surface 102 so as to travel paths 142 and 144, respectively. As is known in the are,t the use of an ellipsoidal inner surface allows energy emanating from one ellipsoidal focus to reflect off the ellipsoidal surface itself and be substantially refocused at the second, target focus 110 of the ellipsoid as can be seen by the convergence of pathways 142 and 144.
In the first embodiment of this ellipsoidal shock focus method (employing a bath 118 with sides 124), the shock waves travel directly from source focus 108 through a fluid medium 116 to penetrate the body-fluid interface 114 before converging in the neighborhood of target focus 110. In the second embodiment, the shock waves travel from a fluid medium 116 through a fluid 120 in the interior of shock transfer pad or body surface coupling belt 122 before penetrating the body-fluid interface 114.
i. Unpredictability and Uncontrollability of Peak Localization, Intensity, and Duration
The ellipsoidal shock focus method has received approval from the Food and Drug Administration (FDA) in the United States. However, the ellipsoidal focus method has certain drawbacks and disadvantages. These drawbacks and disadvantages stem from the fact that ellipsoidal shock focus devices require the convergence of shock wave energy from a relatively large surface (truncated ellipsoid 102) to a small volume (near target focus 110).
For a variety of reasons, to be described immediately below, the focusing of shock energy into a small volume of body tissue is an uncertain and potentially dangerous science. First, the use of high explosives or electric arcs creates a supersonic shock wave whose characteristics are much less predictable and much less controllable than, for example, an analogously generated acoustic wave. Second, the velocity of a shock wave need not be constant, but may vary with the passage of time after the creation of the initial shock, and may vary with spatial direction based on asymmetries in the means of creating the shock source.
Furthermore, the incident angle of shock waves is not necessarily equal to the reflection angle of the shock waves, so that the mathematical model of reflection pathways off the ellipsoid itself may not be followed in practice.
The attempt of the ellipsoidal focus method to focus energy at a mathematical singularity point is further frustrated by the possible existence of thermal gradients along the shock wave pathway, nonuniform distribution of gas and mineral solutes in the liquid, and particulate suspensions.
The mathematical model of the ellipsoidal focus method involves the generation of a shock wave from a first mathematical singularity point and focusing the shock wave at a second mathematical singularity point. The practical considerations described above result in a focus of shock waves near the target focus 110 having properties tangibly at variance with the ideal mathematical model. The practical inability to achieve pinpoint localization results in variability and unpredictability in three important parameters: (1) localization of the focused shock waves; (2) peak intensity at the shock wave focus; and (3) duration of the focusing of shock waves.
Variability and unpredictability of the localization of the focused shock wave has apparently caused physical damage to the body tissue in the area of the target calculus. For example, treatment of kidney stones using this ellipsoidal focus method has commonly resulted in internal bleeding in the kidney, indicating that some of the shock wave pressure peaks were focused outside the target calculus, despite the use of expensive aiming equipment. The demonstrated presence of secondary injury apparently accompanies a failure to completely disintegrate target calculi in a significant number of patients, despite the application of 500-2000 "shots" of focused shock waves. Aiming accuracy is thus crucial to the success of the ellipsoidal focus disintegration effort.
The necessity of repeating shock wave treatments 500-2000 times indicates that the properties of shock wave focus provide only statistical, and not determinative, operation. The variability and unpredictability of shock wave focus localization, peak magnitude, and shock duration are apparently compensated for only by subjecting the patient to such a large number of "shots".
In addition to the bleeding resulting from secondary injury adjacent the target calculus, physical damage near the body-fluid interface 114 (FIGS. 1), as well as a second body-fluid interface 148 at the far side of the body, are issues of great concern, especially when dealing with shock wave repetitions of such quantity. In addition to the potential density discontinuities present at the body-fluid interfaces, the ESWL practitioner must be wary of gaseous bubbles 146 within the body 130 being treated. Such gaseous bubbles 146 may comprise the air within the alveoli of the lungs, as well as gases within the alimentary canal of the subject.
For reasons not yet completely understood, the use of the ellipsoidal shock focus method has resulted in permanent high blood pressure in certain patients.
Finally, patients undergoing treatments involving these 500-2000 shots must be placed under heavy anesthesia. Placement under heavy anesthesia is, in and of itself, an added risk which many patients cannot tolerate.
ii. Limitations on Quantitative Description
The variability and unpredictability of the localization, peak magnitude, and duration of the shock wave focus results in the lack of accurate quantitative description of the ESWL process. Quantitative descriptions of the process are highly desirable, since the effect of a given experiment or treatment could yield valuable information as to the quantity and magnitude of shock pulses necessary to safely disintegrate any given particular type of calculus. The lack of accurate quantitative description engenders a continued necessity to experiment with each individual patient, and each individual calculus type, sometimes resulting in potentially dangerous "overkill" treatments.
b. Focused Piezoelectric Array Method
Another method of extracorporeal shock wave lithotripsy is commonly used in France and Germany. This second method involves the generation of shock waves from a concave spherical surface whose inner face is covered with piezoelectric elements which are fired so as to produce converging shock waves which are focused in the target calculus. See, for example, U.S. Pat. No. 4,617,931 to Dory. This method, commonly referred to as the focused array method, does not produce as sharp a focus as the ellipsoidal shock wave focus method, described above. The focused array system reportedly has not caused the internal bleeding which is characteristic of the ellipsoidal focused shock wave method, and less anesthesia has been found necessary.
c. Electromagnetically Driven Piston Method
Another method of extracorporeal shock wave lithotripsy employs an electromagnetically driven water piston that generates a plane wave in a tube. An acoustic lens focuses the plane wave's acoustic energy on a target. See "Extracorporeal Shock Wave Lithotripsy: An Update" by Fuchs and Chaussy, Endourology, Vol. 2, No. 2, pp. 1-8 (1987).
d. Conclusion
It is therefore desirable to employ an ESWL device and method which overcome the disadvantages of focused shock waves which are used in known ESWL systems and methods. Elimination of the attendant dangers and uncertainties of individual treatment, as well as the lack of quantitative descriptions for future predictability of shock wave localization, peak magnitude, and duration, are also desirable goals for progress of extracorporeal shock wave lithotripsy.