Introduction to the Physics and T echnology of Extracorporeal Shock Wave Therapy (ESWTCSWT) STORZ MEDICAL AG, Kreuzlingen, Switzerland Kreuzlingen, October 2003 Physics and Technology of ESWTCSWT t Table of contents Definition of Shock Waves .........................................................................................................................3 Generation of Shock Waves .......................................................................................................................3 Requirements for Shock Waves for Medical Applications ...........................................................................5 Power...................................................................................................................................................................6 The properties of shock wave energy ...............................................................................................................7 Cavitation........................................................................................................................................................9 Dynamic Range, Reproducibility and Dosing Capability ........................................................................................10 Localization ........................................................................................................................................................10 Design Concept of MODULITH ® SLX .......................................................................................................12 XRay Localization...............................................................................................................................................12 Ultrasound Localization.......................................................................................................................................13 Versatility............................................................................................................................................................13 Design Concept of MODULITH ® SLK .......................................................................................................13 Therapy Source ...................................................................................................................................................14 Ultrasound Localization.......................................................................................................................................14 XRay Localization...............................................................................................................................................14 Multifunctional Workstation ...............................................................................................................................15 Design Concept of MINILITH ® SL1 ...........................................................................................................15 Design Concept of MODULITH ® SLC (Cardio)..........................................................................................16 Therapy Source ...................................................................................................................................................17 Ultrasound Localization.......................................................................................................................................17 Energy Release...................................................................................................................................................17 ECG Synchronization ..........................................................................................................................................17 Physics and Technology of ESWTCSWT 3 Definition of Shock Waves In physical terms, shock waves are highenergy waves with a high amplitude, characterized by extremely short buildup times. Acoustic shock waves used for medical applications are generated by processes that are similar to explosions, displacing the mass surrounding them, such as detonations of explosives. In nature, such processes can be observed in lightning, for instance, which is characterized by the instantaneous heating of a spark channel, thus displacing the surrounding air similar to an explosion. The disturbance of the uniform ambient pressure is radiated in the form of blast waves or shock waves (thunder). Shock waves are characterized by a surgetype pressure distribution. Similar surges in the distribution of pressure can also be obtained by generating steepflanked highamplitude sound waves within a medium. This process can be compared, even if only to a limited extent, to a sine sea wave, which, initially mildly swinging, grows increasingly steeper on a flat, sandy beach until it eventually breaks. However, this is where the comparison ends. Admittedly, the shock waves currently used for medical applications are generated and propagated in water; contrary to transverse waves occurring on the surface of the sea, however, shock waves are longitudinal waves and cannot break. Whether the shock waves used for medical applications are shock waves in the strict physical sense of the word, still remains unanswered. Fig. 1: Sine wave and steepflanked shock wave Be that as it may, medical shock waves are pulseshaped acoustic waves with a high amplitude and thus correspond to the established definition of the term shock wave. Interestingly, the first shock waves used for medical purposes were generated by a spark discharge applying the aforementioned principle of the lightning stroke. In view of the fact that shock waves, contrary to natural lightning strokes, are intended to develop their effects in the human body, they are normally generated in water, as the acoustic properties of water are similar to those found in living tissue. The highenergy waves can thus be introduced into the body without any significant reflection losses. Generation of Shock Waves A patent application for the first shock wave generator to be used for the treatment of brain tumours was filed in the United States byF. Rieber as long ago as 1947. Fig. 2: Patent application filed by F. Rieber In the early 1960s, research into shock waves gained fresh impetus from material analysis. Industry and the medical profession started collaborating. In February 1980, the first kidney stone was successfully disintegrated at the Großhadern Hospital in Munich 1 . Owing to the astonishing success of this method of treatment, extracorporeal shock wave therapy soon became the number one choice in the treatment of 1 Chaussy, C., Schmiedt, E., Jocham, D., Brendl, W., Forssmann, B., Walther, V. (1982): First clinicalexperience with extracorporeally induced destruction of kidney stones by shock waves; Journal of Urology, 1982127, pp. 417420 Physics and Technology of ESWTCSWT 4 kidney and ureter stones and was also used to disintegrate gallstones as well as pancreatic stones and stones in the salivary glands. Early lithotripters required the patient to be immersed in a misshapen bath tub surrounded by a mass of technical apparatus. Today, the patient is positioned on a comfortable patient table and not in direct contact with the water. In fact, the shock waves are generated in water inside the therapy head which is attached to the patients body by means of a closed water cushion, i.e. also dry, simply applying some contact gel or a thin film of water. Shock waves can be generated in different ways; however, they all have one thing in common and that is an electric storage capacitor with variable high voltage which is charged and subsequently rapidly discharged by means of various electroacoustic transducers. The four methods employed in the generation of shock waves for medical applications are the electrohydraulic, the piezoelectric and the electromagnetic mechanism, with the latter being subdivided into systems employing flat coils and acoustic lenses and systems using cylindrical coils and paraboloidaltype reflectors. Fig. 3: Generation principles of shock waves The electrohydraulic systemshown on the left in Fig. 3 employs a spark gap. High voltage is applied to two opposing electrodes positioned inside the water bath about one millimetre apart. The spark arcing across causes the surrounding water to evaporate. The pressure wave induced by the steam bubble is reflected by an ellipsoidal acoustic mirror. Even today, this system is still employed with considerable success. However, the disadvantage of this method is that substantial pressure fluctuations (approx. 50%) may occur between individual shock waves. In addition to this drawback, the spark discharge becomes increasingly uncontrollable due to the consumption of the electrodes. This leads to substantial fluctuations in the average value of the generated pressure, thus limiting the service life of this system to a few thousand shock waves. Piezoelectric systemsmake use of the fact that polycrystalline piezoelectric ceramic elements expand or, depending on the highvoltage polarization, contract when subjected to a highvoltage pulse. Due to the spherical arrangement of a great number of piezoelectric crystals, the waves thus generated are focused on the centre, i.e. the focus, of the spherical arrangement. The advantages offered by this system of shock wave generation are its focusing accuracy, its long service life and the fact that due to the relatively low acoustic power treatment can generally be performed without anaesthetic. Its disadvantage lies in the insufficient power it often produces despite the mass of sophisticated technical equipment involved. The patient thus has to be subjected to repeated treatments to obtain the desired effect. In addition to this, Xray localization systems are rather difficult to integrate into piezoelectric systems. The first of the two electromagnetic systemsshown in Fig. 3 is characterized by a strong pulsed current flowing through a flat coil, thus generating a rapidly changing magnetic field. An opposing magnetic field is induced in the metal membrane above the coil, thus causing the membrane to be pushed away from the coil. The initially flat waves are focused by means of a lens that is arranged above the coil. Physics and Technology of ESWTCSWT 5 The cylindrical source 2 patented by STORZ MEDICAL and shown on the right in Fig. 3 also employs the electromagnetic principle of shock wave generation. The heart of this system is a cylindrical coil. The cylindrical membrane is pushed away from the coil by the induction of a magnetic field and accelerated radially outwards by a pulsed current, thus initially generating a cylindrical wave perpendicular to the cylinder axis. The cylindrical wave is reflected by the paraboloidaltype reflector and transformed into a spherical wave that is focused concentrically onto the focal point. Fig. 4: Patented STORZ MEDICAL cylindrical source The use of the cylindrical source described above has brought about significant benefits in clinical practice. Firstly, the cylindrical designoffers sufficient space for the integration of an inline localization unit. Secondly, the required energy is introduced into the patients body over a large skin area, thus reducing pain to a minimum. The particular geometry of this system allows a precisely defined focal point with high energy densities to be obtained. The cylindrical source can be built in such a way that the focal point is located well clear of the therapy head. This allows the shock waves to penetrate deep into the tissue, and thus the treatment of obese patients as well – particularly important in urology. 2 Wess, O., Marlinghaus, E.H., Katona, J. (1990): A new Design of an Optimal Acoustic Source for Extracorporeal Lithotripsy; in: Burhenne, Joachim (ed.): Biliary Lithotripsy II;Year Book Medical Publishers, Inc.; Chicago, pp. 211214; ISBN 0815113757 Fig. 5: Treatment of an obese patient The cylindrical source is easily adapted also for cardiac indications. Fig. 5a: Cardiac treatment Todays lithotripters are increasingly equipped with electromagnetic sources. Requirements for Shock Waves for Medical Applications As with any drug or medication employed in the treatment of diseases, it is the dosage that determines the success of the therapy. On the one hand, the effects produced have to be vigorous enough to obtain the desired result. On the other hand, however, possible side effects are to be minimized or, where possible, excluded. On the basis of these conditions, the technical requirements to be met by shock wave systems will be examined in detail. • The powerof the source has to be sufficient. • Adequate measures have to be taken to minimize side effects. • It must be possible for the dynamic range, the reproducibilityand the dosing capabilityto be tailored to the clinical conditions. • A localization methodis to be used to allow shock waves to be applied to precisely defined regions of the patients body. Physics and Technology of ESWTCSWT 6 Power Discussions dealing with the efficiency of different shock wave apparatus generally concentrate on the energy and energy flux density as these two parameters are considered to determine the power of a shock wave source. However, no universally valid definition of the underlying criteria has been published so far. This issue thus needs to be dealt with in greater detail, and investigations need to be carried out into the characteristic physical parameters of the shock wave field. The temporal and spatial pressure distribution is the most important of all criteria, as all other quantities can be derived from this parameter. Therefore, this will be dealt with first. Fig. 6: Temporal pressure distribution The above graph shows a typical temporal pressure distribution. Such a curve is obtained by introducing a suitable pressure probe, a socalled hydrophone, into the shock wave field and triggering a single pressure pulse. Optical glassfibre hydrophones are nowadays stateoftheart measurement probes for shock waves. Only with this complex and expensive equipment a sufficiently accurate recording of the extremely high pressures, the short rise and fall times, and also the negative tensile parts can be achieved. Sometimes values obtained with the older needle of membrane hydrophones made of PVDF can still be found. The pressure increases from the ambient pressure up to several hundred or thousand bar and subsequently decreases within one microsecond. This is followed by postpulse oscillations, i.e. tensile components, which need to be minimized as they are considered to be one of the major causes of pain and possible tissue damage. The amplitude of the shock wave can be changed by varying the amount of electric energy supplied. However, the general form of the temporal pressure distribution remains virtually unaffected by these changes. Fig. 7: Spatial pressure distribution The spatial pressure distribution varies according to the geometry of the shock wave source. As can be seen in Fig. 7, the pressure field can be illustrated in the form of a threedimensional area. The peak of this 3D area corresponds to the focal point. Moving away from the focus, the pressuredecreases more or less steeply, which shows that a considerable amount of pressure can still be measured outside the actual focus. Owing to the acoustic energy contained in these areas, these pressures cannot be ignored. In physical terms, the focus corresponds to the peak pressure. The pressure values that are higher than half the peak pressure constitute the focal zone, which means that only the top of the threedimensional area cut off at half the peak pressure is taken into consideration. In case of the MODULITH ® SLX, the focus size as defined above is 6 mm lateral and 28 mm in axial direction. With the MODULITH ® SLK dimensions of 4 mm (lateral) and 50 mm (axial) can be found. Eventually, the MINILITH SL1 features a diameter of 2.4 mm and an axial extension of 25 mm. Cardiac application (MODULITH ® SLC) utilizes a specialically matched focal zone. However, the definition of a focal zone alone does not furnish a satisfactory description of the shock wave field as the measuring procedure simply uses the pressure values down to half the peak pressure, regardless of the absolute peak pressure. Physics and Technology of ESWTCSWT 7 The other important parameters of shock wave fields used for medical applicationsare the acoustic energy contained in tissue and the tissuespecific energy flux density or the shock wave intensity in the focal zone 3 . The properties of shock wave energy In view of the fact that shock waves can be generated by using different methods (Fig. 3), parameters such as kilovolt (kV) figures or specific settings on the apparatus employed are not suitable for defining the shock wave energy. Regrettably, kV values are still widely encountered in this context due to the lack of knowledge of meaningful parameters. dt (t) p c A E ⋅ ⋅ = ∫ 2 ρ Unit of measurement: Millijoule A = wave surface p = pressure ρ= density of the propagation medium c = propagation speed in said medium t = time The above formula defines the total energy contained in the shock wave, without explaining, however, whether this energy is concentrated on a small area (focus) or spread over a large surface. 3 Wess, O., Ueberle, F., Dührssen, R.N., Hilcken, D., Krauss, W., Reuner, Th., Schultheiss, R., Staudenraus, I., Rattner, M., Haaks, W., Granz, B.: Working Group Technical Developments – Consensus Report in High Energy Shock Waves in Medicine, Thiem Verlag, Stuttgart, Germany, 1997 Fig. 8: Focusing of a shock wave As shown in Fig. 8, the shock wave spreads out as it is radiated from the therapy head and is subsequently concentrated onto the focus.The distribution of the shock wave energy over a large entry area allows unnecessary pain and side effects to be reduced and the effects to be confined to the focal zone. Damage to skin and tissue layers situated in front of the actual therapy zone can thus be avoided, without reducing the efficacy of the shock wave energy in the area to be treated. The following rule applies to the focus: the better the energy can be concentrated over a small target area, the greater will be the effect. Consequently, the efficacy of the shock wave at the focus is dependent on the intensity of the energy flux density at this point. The energy flux density, also referred to as energy density, is the energy per unit area and is defined by dividing the energy by the area: dt (t) p c A E ⋅ ⋅ = ∫ 2 1 ρ Unit of measurement: Millijoule per square millimetre (mJmm 2 ) As shown in Fig. 8, the energy flux density can be increased dramatically by focusing the shock wave, provided that the shock wave energy, which initially spreads over a large surface, is concentrated on a very small area. The energy flux density or intensity can be drawn upon as one of the parameters – though not the only one – that determine the efficacy of shock waves used for medical applications. Physics and Technology of ESWTCSWT 8 The qualities of shock waves produced by various lithotripters used in urology were compared in a research study 4 commissioned by the Italian Department of Health. Among other things, to describe the shock wave quality this study drew upon the peak pressure and the maximum intensity (i.e. energy flux density) obtained at the focus with the maximum energy level. Fig. 9: Focus pressure Fig. 10: Shock wave intensity at the focus The chart above clearly illustrates that the cylindrical source developed by STORZ MEDICAL stands out from all other tested sources in that it has led to the best results with regard to both the aforementioned parameters. 4 Buizza, A., Dell’Aquila, T., Giribona, P., Spagno, C. (1995): The performance of different pressure pulse generators for extracorporeal lithotripsy: a comparison based on commercial lithotripters for kidney stones; Ultrasound in Medicine Biology, 21:2, pp. 259272 Side effects As mentioned above, a successful treatment can only be performed if the desired efficacy of the shock waves is not accompanied by intolerable side effects. In the field of urology, for example, side effects such as different types of haematomata or, occasionally, interactions with the cardiac rhythm (extrasystoles) have occurred. Tests carried out on animals have revealed that the application of shock waves may lead to serious damage to the lungs. This is due to the fact that shock waves are almost entirely reflected and hence develop destructive forces when they reach boundary layers (tissueair), at which the acoustic properties change drastically. However, when shock waves reach the boundary layer of a kidney stone, it is precisely these forces which cause the stone to be disintegrated without damaging the surrounding tissue unacceptably. Side effects can be kept within acceptable limits if users make sure that the high intensity zones of the shock wave field are not targeted on vulnerable tissue (e.g. the lungs), and that the energy flux density and the total energy applied reach therapeutic values only in the zones to be treated. In all other zones, these values have to be kept below the permissible values to avoid damage. The generally accepted maximum value is at present 0.03 mJmm². This value has been measured on bloodfilled umbilical cords. 5 From a technical point of view, a favourable distribution of the shock wave field is obtained by producing sources with large apertures that distribute the energy entering the body over a large skin surface. Another positive feature of these largeaperture sources is that they are characterized by an astonishing focusing accuracy and thus ensure the best possible energy flux densities or intensities. Itshould be mentioned, however, that the energy density and intensity are limited by technical and, to a certain extent, anatomical factors. Shock wave sources should be used that feature a maximum aperture and a maximum aperture angle at which the shock waves converge towards the focal point in order to optimize the efficacy of the shock waves while at the same time minimizing undesired side effects. Again, the cylindrical source developed by STORZ MEDICAL stands out from other sources in 5 Steinbach, P., Hofstaedter, F., Nicolai, H., Roessler, W., Wieland, W. (1993): Determination of the energydependent extent of vascular damage caused by highenergy shock waves in an umbilical cord model; Urological Research, 21, pp. 279282 Physics and Technology of ESWTCSWT 9 that it meets these requirements better than any other shock wave generator. This is clearly shown in Fig. 11, which is taken from the aforementioned study. The diagram below confirms that the STORZ MEDICAL source features the highest ratio between the focus pressure and the pressure at the point of entry into the body (in this case 50 mm in front of the focus). Fig. 11: Focus pressure and pressure at the skin surface Being rapid pressure fluctuations occurring in water, shock waves are not visible. However, they can be made visible by using suitable Schlieren photography apparatus. The images in Fig. 13 show the shock waves on their way to the focal point at various times after leaving the source. These images show a twodimensional sectional view of the threedimensional, spherical segmentshaped wave surface illustrated in schematic form in Fig. 12. Fig. 12: Propagation of waves The wavefronts have a thickness of about 1 mm and propagate in the direction of the focal point at a velocity of approx. 1500 ms. After having passed through the focal point, the wavefronts diverge and dissipate, their amplitude gradually weakening. Fig. 13: Propagation of waves – schlieren photographs Cavitation On closer inspection of the last few images in Fig. 13, cavitation bubbles can be seen on the centre line of the images. Cavitation occurs when shock waves are followed by tensile waves, which blow open the fluid. Bubbles are formed around microscopically small cavitation centres and expand within a few microseconds to different maximum diameters (about 1 mm). Having reached their maximum size, they generally collapse within 100 microseconds, merging with the original cavitation bubble and sending out a secondary spherical shock wave. Cavitation bubbles contribute to the disintegration of kidney stones. However, theyare also considered to be responsible for tissue damage. Bubbles located near acoustic boundary layers (stone surface, vascular walls, pulmonary alveoli, etc.) cannot collapse pointsymmetrically. The initially spherical bubble is dented and forms an intensive fluid jet with a diameter of a few tenths of a millimetre which impacts on the boundary surface at a velocity of several hundred metres per second. This jet can erode stone surfaces and puncture the walls of smaller vessels (formation of microhaemorrhages). This again calls for the use of shock wave sources with large apertures. Using such sources, the cavitation threshold values are only exceeded near the focus zone, whereas the skin surface is exposed to cavitation only to a limited degree or not at all. Physics and Technology of ESWTCSWT 10 Dynamic Range, Reproducibility and Dosing Capability A generous dynamic range is essential in order to be able to handle different indications and to take into account patients differing pain thresholds. It must be possible for the energy levels and energy flux densities to be adjusted to specific conditions within generous limits. Experts often refer to highenergy and lowenergy shock waves. However, so far no precise definition has been established stating which energy flux densities are covered by these two terms. Various study groups have suggested the following classification for orthopaedic applications 6 : Mainz7 : low energy: 0.08 mJmm 2 medium energy: 0.28 mJmm 2 high energy: 0.6 mJmm 2 Kassel 8 : low energy: < 0.12 mJmm 2 high energy: > 0.12 (bis 0,38) mJmm 2 For different fields of shock wave applications (cardiology, urology) specific energy settings may be required. In order to tailor the treatment to be performed to the target zone and the patients tolerance to pain, the doctor needs to know how much energy is applied throughout the entireduration of the treatment. This means that it must bepossible for every emission of shock waves to be precisely dosed and reproduced. The description of the individual methods employed in the generation of shock waves has shown that not all systems are equally effective in this respect. In fact, the study commissioned by the Italian Department of Health has confirmed that it is the electromagnetic shock wave generators that produce the most stable values, even in the higher pressure range. 6 The energy levels that can be set when using the MINILITH cover both the highenergy and the lowenergy values listed in the above classification. In the lower energy range, which generally does not require any anaesthetic, the energy levels can be precisely adjusted. When using higher energy levels, the energy supplied by the MINILITH is vigorous enough to allow even pseudarthrosis to be treated successfully. 7 Rompe, J. D., Universität Mainz, auf dem Süddeutschen Orthopädenkongress, 25. 28. April 1996 in BadenBaden 8 Siebert, W., Orthopädische Klinik Kassel, auf dem Süddeutschen Orthopädenkongress, 25. 28. April 1996 in BadenBaden Fig. 14: Stability at focus Localization The use of a reliable localization system is absolutely essential when performing shock wave therapy, all the more so as serious side effects produced by shock waves cannot always be excluded. Today, there are really only two systems suitable for this purpose: ultrasound or Xray localization systems. It is up to the doctor to decide which system to use. However, this decision should always be dependent on the specific indication to be treated. Various localization methods, which are independent of the type of system used, are offered by the individual manufacturers. A distinction is usually made between offline and inline localization, the latter being characterized by the fact that localization is performed through the therapy head. Fig. 15: Offlineinline localization When using offline ultrasound localization systems, the targeted zone may not correspond to the zone actually subjected to shock waves 9 . Just like optical 9 Wess, O., Stojan, L., Rachel, U.K. (1995): Untersuchungen zur Präzision der Ultraschallortung in vivo am Beispiel der extrakorporal induzierten Lithotripsie Investigations into the accuracy of in vivo ultrasound localization on the basis of extracorporeally induced lithotripsy in: Chaussy, Christian (ed.): Die Stoßwelle, Forschung und Klinik Shock waves, research and clinical applications; Attempto Verlag, pp. 3744; ISBN 389308228X Physics and Technology of ESWTCSWT 11 boundary surfaces, the boundary layers between different types of tissue may refract the ultrasonic waves. The deviations from the linear direction of propagation of the ultrasonic waves, which is an indispensable requirement to ensure precise localization, are not so small that they can be neglected. In the case of penetration depths of about 100 mm, which are quite common in the treatment of kidney stones, deviations of 10 mm and more may occur. These difficulties can be almost entirely excluded when using inline localization systems. It goes without saying that none of the systems available today can guarantee a 100% localization accuracy. However, inline systems allow deviations to be reduced to a minimum, as the shock waves propagate through the same tissue areas as the inline ultrasonic waves and thus undergo similar refraction. This is of utmost importance for cardiac applications. When shock waves are applied in orthopaedics to treat disorders concerning human postural or locomotor systems, it will hardly ever be possible to detect and localize structures sited in the centre of the shock wave field if the ultrasound transducer is located outside the shock wave axis (offline arrangement). Unfavourable anatomical conditions and bone structures that interfere with the target area and cannot be penetrated by ultrasonic waves render the localization of the target area difficult or even impossible if a lateral transducer is used. Consequently, none of the shock wave apparatus currently available for orthopaedic applications are equipped with offline localization units. Despite the fact that after having performed an anatomically oriented localization of the treatment zone, minor corrections are frequently carried out after consultation with the patient, a precise localization is still indispensable. Furthermore, for various indications, such as calcareous tendinopathy or pseudarthrosis, it may be advisable to use an Xray localization system instead of or in addition to ultrasound localization. Again, inline fluoroscopic Xray localization offers considerable advantages compared with an offline arrangement. This system ensures an axial projection of the target area sited in a central position on the shock wave axis and allows deviations from the cross hairs to be detected and corrected with maximum accuracy. Again, the central opening of the electromagnetic cylindrical source, which can be attributed to the design, offers ideal conditions for an inline Xray localization, which would be difficult or impossible to perform with other types of systems. Fig. 16: Inline Ultrasound and Xray localization The following chapter takes a closer look to the different concepts of shock waves devices of STORZ MEDICAL. • MODULITH® SLX ................. Page 12 • MODULITH® SLK ................. Page 13 • MINILITH® SL1 ..................... Page 15 • MODULITH® SLC .................. Page 16 Physics and Technology of ESWTCSWT 12 Design Concept of MODULITH® SLX The MODULITH ® SLX is a modular system that is comprised of a therapy unit with cylindrical source, a patient table and an Xray andor ultrasound localization unit. The MODULITH ® SLX is available in three different versions: • MODULITH ® SLXMX featuring a highly versatile, highquality Xray system • MODULITH ® SLXMXF featuring an astonishingly compact Xray system • transportable MODULITH ® SLX featuring a mobile Xray Carm Therapy Unit The patented STORZ MEDICAL cylindrical source stands out from other systems in that it uses the best technology for the generation of shock waves available today. Energy flux densities of between 0.2 mJmm² and 2.0 mJmm² make the MODULITH ® SLX ideally suited both for the gentle treatment of children and for the vigorous disintegration of ureter stones, which are generally difficult to crush, or extremely hard kidney stones and gallstones. In addition to this, the MODULITH ® SLX is equipped with a unique patient table specially developed to meet the highest demands in terms of ease of operation and comfortable patient positioning. An extremely resistant supporting sheet that is transparent to Xrays, ultrasound and shock waves ensures that not only normal weight patients but also obese patients and small children are held in a comfortable and safe position. The patient table has a generous range of movement (both motordriven and manual) in three directions, which allows treatment to be performed without having to move the patient. However, the use of this versatile patient table is not limited to the field of lithotripsy. In fact, it is equally suited for a variety of diagnostic examinations and therapeutic applications. Fig. 17: MODULITH ® SLX energy flux densities energy levels XRay Localization All apparatus are characterized by the fact that Xray localization is carried out inline directly through the source. This is done by using the proven TTS technique (Through The Source) which allows the target area to be localized much more quickly and reliably. Any focusing inaccuracies are detected immediately throughout the treatment and can be corrected without having to interrupt the therapy. Two airbags are used to force the water out of the Xray path during localization, thus ensuring an optimum image quality. Fig. 18: STORZ MEDICAL MODULITH ® SLXMXS Apart from inline (p.a.) fluoroscopy, the compact Xray system of the MODULITH SLXAX is equally suited for 30° projections. In addition to this, the highly flexible Xray system of the MODULITH SLXUX allows +45° projections in lateral direction and 0° to 45° projections in craniocaudal direction to be performed. Physics and Technology of ESWTCSWT 13 Fig. 19: STORZ MEDICAL MODULITH ® SLX transportable Ultrasound Localization All apparatus can be equippedwith an optional inline ultrasound localization unit which can be easily inserted into the central cylindrical opening of the therapy source. Ultrasound and Xray localization can be performed simultaneously. Furthermore, it is possible for the 3.5 MHz ultrasound transducer to be removed from the MODULITH ® SLX and used as an independent diagnostic unit. Fig. 20: Ultrasound localization Versatility The apparatus that are included in the MODULITH ® SLX series are more than just optimum lithotripters. They can equally be used as versatile urological workstations. For this purpose, the apparatus can be combined with various accessories, among which the Trendelenburg cushion, leg supports, urosink or infusion rod. Fig. 21: STORZ MEDICAL MODULITH ® SLXMX Design Concept of MODULITH® SLK The MODULITH ® SLK follows a completely new and revolutionary design: A medium sized shock wave source offering a wide energy range for all indications of shock waves is mounted on a flexible, easytomove, articulated arm. Fig. 22: MODULITH ® SLK workstation Different components of the treatment setup like uro table, Xray Carc, ultrasound device etc. can be chosen from a variety of brands and models according to the specific needs and requirements of the hospital. These components can not only be used for ESWL Diagnostics and urological procedures can also be performed utilising parts of the ESWL setup. An endoscopic tower completes the urolgical workstation. And a detached use of Xray Carc or ultrasound device for example is also easily possible. For stone treatment the different components are simply regrouped — no mechanical connections are to be made. Physics and Technology of ESWTCSWT 14 Due to its flexibility the MODULITH ® SLK allows also interdisciplinary utilisation. Besides urology, shock wave therapy in RheumatologyOrthopaedics is just as well possible as stone disintegration in Gastroenterology and ENT. Therapy Source MODULITH ® SLK is equipped with the STORZ MEDICAL proprietary shock wave source with cylindrical coil and parabolic reflector. This allows complete control over the shock wave application. The desired energy is accurately generated and precisely reproduced from shock to shock. With a penetration depth of between 0 and 150 mm the MODULITH ® SLK is prepared to tackle any challenge the patient’s pathology and anatomy may confront it with. With this source it realises an energy range unequalled in the world of shock wave technology. From pain treatment in rheumatology to treatment of ureteric stones in urology or pseudarthrosis in orthopaedics — the MODULITH ® SLK offers the proper energy settings for every shock wave therapy. The uncompromising avoidance of disposable materials, the absence of an energy absorbing lens and continuous water conditioning evades time consuming service breaks between treatments, results in low running costs and guarantees the MODULITH ® SLK is always ready for action. Ultrasound Localization Ultrasound localization is the fastest way to localize a target and to ensure permanently that the right target is hit — without any exposure to radiation. The STORZ MEDICAL cylinder source allows an ultrasound transducer to be integrated within the shock wave source: localization and therapy follow the same path. This is what we call inline. Osseous structures or tissues with gas inclusions are easily detected. When moving the therapy head the ultrasound transducer moves as well. So, the perfect path towards the target is easily found — both for diagnostic ultrasonic waves and for shock waves. Examining the anatomy of a human body with ultrasound not only means choosing the proper orientation of the transducer but also to perform a searching motion on the skin to find the best place to scan through. Only by finding this ‘keyhole’ can a good ultrasound image be obtained. The therapy head of the MODULITH ® SLK can be adjusted to this keyhole almost like a handheld transducer XRay Localization When utilising shock waves in medical applications, the path of the pressure pulse towards its target is always of great importance. Obstacles such as bones or gas filled intestines willdiminish the energy transferred to the target and thus affect the treatment success. For most targets there is a path which is free of obstacles and it can be used as long as the shock wave source can be placed wherever required by the patient’s anatomy. For this reason, STORZ MEDICAL has invented the Lithotrack ® positioning system — the key technology to computeraided shock wave therapy (CAST). Fig. 23: Working principle of Lithotrack ® positioning system An optical link between the Carm and therapy head is established. The camera ‘looks’ at the shock wave source and in doing so can determine the position and orientation of the shock wave head. On the basis of this data the processing and display module generates a virtual reality scenery. Physics and Technology of ESWTCSWT 15 Fig. 24: Lithotrack ® display By moving the therapy head and watching the corresponding movements of the focus on the Lithotrack ® display, the physician can adjust the focus of the shock wave to the centre of the Xray with a sure and quick hand. Multifunctional Workstation A full featured uro table and a fluoroscopic Xray Carc form the nucleus of a multipurpose urological workstation. An ultrasound device for diagnostic purposes and therapy (punctures) further extends the possibilities for the attending physician. The addition of the shock wave device MODULITH ® SLK with its Lithotrack ® system and its support for inline ultrasound creates a fully featured treatment unit for extracorporeal shock wave therapy. Combining endoscopic devices by Karl Storz GmbH, Tuttlingen, with this treatment unit ends up in a stateoftheart urological workstation meeting all professional and economic challenges of today’s health care. Design Concept of MINILITH® SL1 The MINILITH ® developed by STORZ MEDICAL is the result of many years of extensive experience in the field of kidney stone lithotripsy and the successful implementation of the specific requirements to be met by orthopaedic apparatus for extracorporeal shock wave therapy, as has been outlined above. The MINILITH ® is characterized by its compact size and astonishing mobility, which allow the system to be transferred to different locations. The apparatus simply has to be plugged in and does not require any further installation procedures. Fig. 25: STORZ MEDICAL MINILITH ® SL1 The unrestricted movement of the therapy head is due to it being suspended on gimbals on a lockable, articulated arm. It comprises a 7.5 MHz inline integral ultrasound transducer with an infinitely adjustable scanning plane that can be rotated through 360°. When performing general ultrasound examinations, the transducer can be easily removed from the central therapy head opening and used independently of the therapy unit itself. In addition to this, the therapy head has a precise adjustment feature which allows the focal point to be precisely moved in two directions without having to disconnect the therapy head. Physics and Technology of ESWTCSWT 16 Fig. 26: Therapy head Owing to its astonishing mobility and manoeuvrability, the therapy head can bepositioned for use on the most diverse regions of the patients body and is thus suitable for an almost unlimited variety of indications. One application is shown in the following figure. Fig. 27: MINILITH ® SL1 Finally, the MINILITH ® SL1 provides well dosaged uniform energy pulses with a wide dynamic range of 0.005 to 0.50 mJmm² in 20 steps. Very gentle treatments as well as consequent powerful applications in case of pseudarthrosis are matching all different medical needs. Fig. 28 illustrates the available energy range of the MINILITH ® . Fig. 28: MINILITH ® SL1 energy flux densities energy levels Furthermore, the MINILITH ® stands out from other apparatus in the same category because its inline ultrasound localization system can be complemented by a standard mobile Carm to perform inline Xray localization in the treatmentof pseudarthrosis, for example. Design Concept of MODULITH® SLC (Cardio) Fig. 28: MODULITH ® SLC For shock wave therapy in cardiology a stringent control of the shock wave field parameters is mandatory. The delicate cardiac system and vital functions of the heart require total control of all shock wave parameters to provide medical efficiency without causing serious side effects. Physics and Technology of ESWTCSWT 17 The concept of the MODULITH ® SLC is technically matched to the following medical requirements: • Precise control of spatial shock wave field dimensions • Precise targeting and simultaneous position control by ultrasound • Precise control and reproducibility of energy release • Precise synchronisation with cardiac cycle Therapy Source As all shock wave devices of STORZ MEDICAL the MODULITH ® SLC is equipped with its proprietary shock wave source with cylindrical coil and parabolic reflector. The focal distance and aperture configuration is matched to cardiac anatomy. The shock wave focus is precisely defined to provide shock wave agitation on only predetermined areas of the heart muscle. Targeting is controlled by manual fine adjustments within millimetre precision. Fig. 29: Fine adjustment Ultrasound Localization Inline ultrasound coaxially integrated in the centre of the shock wave source provides continuous position control during shock wave treatment. Fig. 30: Inline ultrasound Energy Release Special care is taken for incremental adjustment of shock wave energy in order to guaranty exactly the amount of shock wave exposure required. Fig. 31: Dynamic range ECG Synchronization As known from kidney stone treatment shock waves may interfere with cardiac cycle and cause arrhythmia and premature heart beats. Exact synchronisation of shock wave release and refractory phase of the cardiac cycle is guaranteed by aid of ECG triggering Fig. 32: ECG control STORZ MEDICAL AG ∙UNTERSEESTR. 47 ∙CH8280 KREUZLINGEN ∙TELEFON: +4171677 45 45 ∙TELEFAX: +4171677 45 05 EMAIL: INFOSTORZMEDICAL.COM ∙INTERNET: WWW.STORZMEDICAL.COM
Trang 1Extracorporeal Shock Wave Therapy (ESWT/CSWT)
STORZ MEDICAL AG, Kreuzlingen, Switzerland
Kreuzlingen, October 2003
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Table of contents
Definition of Shock Waves 3
Generation of Shock Waves 3
Requirements for Shock Waves for Medical Applications 5
Power 6
The properties of shock wave energy 7
Cavitation 9
Dynamic Range, Reproducibility and Dosing Capability 10
Localization 10
Design Concept of MODULITH® SLX 12
X-Ray Localization 12
Ultrasound Localization 13
Versatility 13
Design Concept of MODULITH® SLK 13
Therapy Source 14
Ultrasound Localization 14
X-Ray Localization 14
Multifunctional Workstation 15
Design Concept of MINILITH® SL1 15
Design Concept of MODULITH® SLC (Cardio) 16
Therapy Source 17
Ultrasound Localization 17
Energy Release 17
ECG Synchronization 17
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Definition of Shock Waves
In physical terms, shock waves are high-energy waves
with a high amplitude, characterized by extremely
short build-up times Acoustic shock waves used for
medical applications are generated by processes that
are similar to explosions, displacing the mass
sur-rounding them, such as detonations of explosives In
nature, such processes can be observed in lightning,
for instance, which is characterized by the
instantane-ous heating of a spark channel, thus displacing the
surrounding air similar to an explosion The
distur-bance of the uniform ambient pressure is radiated in
the form of blast waves or shock waves (thunder)
Shock waves are characterized by a surge-type
pres-sure distribution Similar surges in the distribution of
pressure can also be obtained by generating
steep-flanked high-amplitude sound waves within a
me-dium This process can be compared, even if only to a
limited extent, to a sine sea wave, which, initially
mildly swinging, grows increasingly steeper on a flat,
sandy beach until it eventually breaks However, this is
where the comparison ends Admittedly, the shock
waves currently used for medical applications are
generated and propagated in water; contrary to
transverse waves occurring on the surface of the sea,
however, shock waves are longitudinal waves and
cannot break Whether the shock waves used for
medical applications are shock waves in the strict
physical sense of the word, still remains unanswered
Fig 1: Sine wave and steep-flanked shock wave
Be that as it may, medical shock waves are pulse-shaped acoustic waves with a high amplitude and thus correspond to the established definition of the term "shock wave"
Interestingly, the first shock waves used for medical purposes were generated by a spark discharge apply-ing the aforementioned principle of the lightnapply-ing stroke In view of the fact that shock waves, contrary
to natural lightning strokes, are intended to develop their effects in the human body, they are normally generated in water, as the acoustic properties of water are similar to those found in living tissue The high-energy waves can thus be introduced into the body without any significant reflection losses
Generation of Shock Waves
A patent application for the first shock wave genera-tor to be used for the treatment of brain tumours was filed in the United States by F Rieber as long ago as
1947
Fig 2: Patent application filed by F Rieber
In the early 1960s, research into shock waves gained fresh impetus from material analysis Industry and the medical profession started collaborating In February
1980, the first kidney stone was successfully disinte-grated at the Großhadern Hospital in Munich1 Owing
to the astonishing success of this method of treat-ment, extracorporeal shock wave therapy soon be-came the number one choice in the treatment of
1
Chaussy, C., Schmiedt, E., Jocham, D., Brendl, W., Forssmann, B., Walther, V (1982): "First clinical experience with extracorporeally induced destruction of kidney stones by shock waves"; Journal of Urology, 1982/127, pp 417-420
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kidney and ureter stones and was also used to
disin-tegrate gallstones as well as pancreatic stones and
stones in the salivary glands
Early lithotripters required the patient to be immersed
in a misshapen bath tub surrounded by a mass of
technical apparatus Today, the patient is positioned
on a comfortable patient table and not in direct
con-tact with the water In fact, the shock waves are
generated in water inside the therapy head which is
attached to the patient's body by means of a closed
water cushion, i.e also "dry", simply applying some
contact gel or a thin film of water
Shock waves can be generated in different ways;
however, they all have one thing in common and that
is an electric storage capacitor with variable high
voltage which is charged and subsequently rapidly
discharged by means of various electroacoustic
trans-ducers The four methods employed in the generation
of shock waves for medical applications are the
elec-trohydraulic, the piezoelectric and the
electromag-netic mechanism, with the latter being subdivided
into systems employing flat coils and acoustic lenses
and systems using cylindrical coils and
paraboloidal-type reflectors
Fig 3: Generation principles of shock waves
The electrohydraulic system shown on the left in
Fig 3 employs a spark gap High voltage is applied to
two opposing electrodes positioned inside the water
bath about one millimetre apart The spark arcing
across causes the surrounding water to evaporate
The pressure wave induced by the steam bubble is
reflected by an ellipsoidal acoustic mirror Even today,
this system is still employed with considerable success
However, the disadvantage of this method is that
substantial pressure fluctuations (approx 50%) may
occur between individual shock waves In addition to
this drawback, the spark discharge becomes increas-ingly uncontrollable due to the consumption of the electrodes This leads to substantial fluctuations in the average value of the generated pressure, thus limiting the service life of this system to a few thousand shock waves
Piezoelectric systems make use of the fact that
polycrystalline piezoelectric ceramic elements expand
or, depending on the high-voltage polarization, con-tract when subjected to a high-voltage pulse Due to the spherical arrangement of a great number of pie-zoelectric crystals, the waves thus generated are fo-cused on the centre, i.e the focus, of the spherical arrangement The advantages offered by this system
of shock wave generation are its focusing accuracy, its long service life and the fact that due to the relatively low acoustic power treatment can generally be per-formed without anaesthetic Its disadvantage lies in the insufficient power it often produces despite the mass of sophisticated technical equipment involved
The patient thus has to be subjected to repeated treatments to obtain the desired effect In addition to this, X-ray localization systems are rather difficult to integrate into piezoelectric systems
The first of the two electromagnetic systems shown
in Fig 3 is characterized by a strong pulsed current flowing through a flat coil, thus generating a rapidly changing magnetic field An opposing magnetic field
is induced in the metal membrane above the coil, thus causing the membrane to be pushed away from the coil The initially flat waves are focused by means of a lens that is arranged above the coil
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The cylindrical source2 patented by STORZ MEDICAL
and shown on the right in Fig 3 also employs the
electromagnetic principle of shock wave generation
The heart of this system is a cylindrical coil The
cylin-drical membrane is pushed away from the coil by the
induction of a magnetic field and accelerated radially
outwards by a pulsed current, thus initially generating
a cylindrical wave perpendicular to the cylinder axis
The cylindrical wave is reflected by the
paraboloidal-type reflector and transformed into a spherical wave
that is focused concentrically onto the focal point
Fig 4: Patented STORZ MEDICAL cylindrical source
The use of the cylindrical source described above has
brought about significant benefits in clinical practice
Firstly, the cylindrical design offers sufficient space for
the integration of an in-line localization unit
Sec-ondly, the required energy is introduced into the
patient's body over a large skin area, thus reducing
pain to a minimum The particular geometry of this
system allows a precisely defined focal point with high
energy densities to be obtained The cylindrical source
can be built in such a way that the focal point is
lo-cated well clear of the therapy head This allows the
shock waves to penetrate deep into the tissue, and
thus the treatment of obese patients as well –
particu-larly important in urology
2
Wess, O., Marlinghaus, E.H., Katona, J (1990): "A new Design of an
Optimal Acoustic Source for Extracorporeal Lithotripsy"; in: Burhenne,
Joachim (ed.): Biliary Lithotripsy II; Year Book Medical Publishers, Inc.;
Chicago, pp 211-214; ISBN 0-8151-1375-7
Fig 5: Treatment of an obese patient
The cylindrical source is easily adapted also for cardiac indications
Fig 5a: Cardiac treatment
Today's lithotripters are increasingly equipped with electromagnetic sources
Requirements for Shock Waves for Medical Applications
As with any drug or medication employed in the treatment of diseases, it is the dosage that determines the success of the therapy On the one hand, the effects produced have to be vigorous enough to obtain the desired result On the other hand, how-ever, possible side effects are to be minimized or, where possible, excluded On the basis of these condi-tions, the technical requirements to be met by shock wave systems will be examined in detail
• The power of the source has to be
suffi-cient
• Adequate measures have to be taken to
minimize side effects
• It must be possible for the dynamic range, the reproducibility and the dosing
capa-bility to be tailored to the clinical
condi-tions
• A localization method is to be used to
al-low shock waves to be applied to precisely defined regions of the patient's body
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Power
Discussions dealing with the efficiency of different
shock wave apparatus generally concentrate on the
energy and energy flux density as these two
parame-ters are considered to determine the power of a shock
wave source However, no universally valid definition
of the underlying criteria has been published so far
This issue thus needs to be dealt with in greater
de-tail, and investigations need to be carried out into the
characteristic physical parameters of the shock wave
field The temporal and spatial pressure distribution is
the most important of all criteria, as all other
quanti-ties can be derived from this parameter Therefore,
this will be dealt with first
Fig 6: Temporal pressure distribution
The above graph shows a typical temporal pressure
distribution Such a curve is obtained by introducing a
suitable pressure probe, a so-called hydrophone, into
the shock wave field and triggering a single pressure
pulse Optical glass-fibre hydrophones are nowadays
state-of-the-art measurement probes for shock waves
Only with this complex and expensive equipment a
sufficiently accurate recording of the extremely high
pressures, the short rise and fall times, and also the
negative tensile parts can be achieved Sometimes
values obtained with the older needle of membrane
hydrophones made of PVDF can still be found
The pressure increases from the ambient pressure up
to several hundred or thousand bar and subsequently
decreases within one microsecond This is followed by
post-pulse oscillations, i.e tensile components, which
need to be minimized as they are considered to be
one of the major causes of pain and possible tissue
damage The amplitude of the shock wave can be
changed by varying the amount of electric energy
supplied However, the general form of the temporal pressure distribution remains virtually unaffected by these changes
Fig 7: Spatial pressure distribution
The spatial pressure distribution varies according to the geometry of the shock wave source As can be seen in Fig 7, the pressure field can be illustrated in the form of a three-dimensional area The peak of this 3-D area corresponds to the focal point Moving away from the focus, the pressure decreases more or less steeply, which shows that a considerable amount of pressure can still be measured outside the actual focus Owing to the acoustic energy contained in these areas, these pressures cannot be ignored
In physical terms, the focus corresponds to the peak pressure The pressure values that are higher than half the peak pressure constitute the focal zone, which means that only the top of the three-dimensional area cut off at half the peak pressure is taken into consid-eration In case of the MODULITH® SLX, the focus size
as defined above is 6 mm lateral and 28 mm in axial direction With the MODULITH® SLK dimensions of
4 mm (lateral) and 50 mm (axial) can be found Even-tually, the MINILITH SL1 features a diameter of 2.4 mm and an axial extension of 25 mm Cardiac application (MODULITH® SLC) utilizes a specialically matched focal zone
However, the definition of a focal zone alone does not furnish a satisfactory description of the shock wave field as the measuring procedure simply uses the pressure values down to half the peak pressure, regardless of the absolute peak pressure
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The other important parameters of shock wave fields
used for medical applications are the acoustic energy
contained in tissue and the tissue-specific energy flux
density or the shock wave intensity in the focal zone3
The properties of shock wave energy
In view of the fact that shock waves can be generated
by using different methods (Fig 3), parameters such
as kilovolt (kV) figures or specific settings on the
ap-paratus employed are not suitable for defining the
shock wave energy Regrettably, kV values are still
widely encountered in this context due to the lack of
knowledge of meaningful parameters
dt (t) p c
A
⋅
ρ Unit of measurement: Millijoule
A = wave surface
p = pressure
ρ = density of the propagation medium
c = propagation speed in said medium
t = time
The above formula defines the total energy contained
in the shock wave, without explaining, however,
whether this energy is concentrated on a small area
(focus) or spread over a large surface
3
Wess, O., Ueberle, F., Dührssen, R.-N., Hilcken, D., Krauss, W.,
Reuner, Th., Schultheiss, R., Staudenraus, I., Rattner, M., Haaks, W.,
Granz, B.: "Working Group Technical Developments – Consensus
Report in High Energy Shock Waves in Medicine", Thiem Verlag,
Stuttgart, Germany, 1997
Fig 8: Focusing of a shock wave
As shown in Fig 8, the shock wave spreads out as it is radiated from the therapy head and is subsequently concentrated onto the focus The distribution of the shock wave energy over a large entry area allows unnecessary pain and side effects to be reduced and the effects to be confined to the focal zone Damage
to skin and tissue layers situated in front of the actual therapy zone can thus be avoided, without reducing the efficacy of the shock wave energy in the area to
be treated
The following rule applies to the focus: the better the energy can be concentrated over a small target area, the greater will be the effect Consequently, the effi-cacy of the shock wave at the focus is dependent on the intensity of the energy flux density at this point
The energy flux density, also referred to as energy density, is the energy per unit area and is defined by dividing the energy by the area:
dt (t) p c A
⋅
= 1 ∫ 2
ρ
Unit of measurement: Millijoule per square millimetre (mJ/mm2)
As shown in Fig 8, the energy flux density can be increased dramatically by focusing the shock wave, provided that the shock wave energy, which initially spreads over a large surface, is concentrated on a very small area The energy flux density or intensity can be drawn upon as one of the parameters – though not the only one – that determine the efficacy of shock waves used for medical applications
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The qualities of shock waves produced by various
lithotripters used in urology were compared in a
re-search study4 commissioned by the Italian Department
of Health Among other things, to describe the shock
wave quality this study drew upon the peak pressure
and the maximum intensity (i.e energy flux density)
obtained at the focus with the maximum energy level
Fig 9: Focus pressure
Fig 10: Shock wave intensity at the focus
The chart above clearly illustrates that the cylindrical
source developed by STORZ MEDICAL stands out from
all other tested sources in that it has led to the best
results with regard to both the aforementioned
pa-rameters
4
Buizza, A., Dell’Aquila, T., Giribona, P., Spagno, C (1995): "The
performance of different pressure pulse generators for extracorporeal
lithotripsy: a comparison based on commercial lithotripters for kidney
stones"; Ultrasound in Medicine & Biology, 21:2, pp 259-272
Side effects
As mentioned above, a successful treatment can only
be performed if the desired efficacy of the shock waves is not accompanied by intolerable side effects
In the field of urology, for example, side effects such
as different types of haematomata or, occasionally, interactions with the cardiac rhythm (extrasystoles) have occurred Tests carried out on animals have revealed that the application of shock waves may lead
to serious damage to the lungs This is due to the fact that shock waves are almost entirely reflected and hence develop destructive forces when they reach boundary layers (tissue/air), at which the acoustic properties change drastically However, when shock waves reach the boundary layer of a kidney stone, it is precisely these forces which cause the stone to be disintegrated without damaging the surrounding tissue unacceptably
Side effects can be kept within acceptable limits if users make sure that the high intensity zones of the shock wave field are not targeted on vulnerable tissue (e.g the lungs), and that the energy flux density and the total energy applied reach therapeutic values only
in the zones to be treated In all other zones, these values have to be kept below the permissible values to avoid damage The generally accepted maximum value is at present 0.03 mJ/mm² This value has been measured on blood-filled umbilical cords.5
From a technical point of view, a favourable distribu-tion of the shock wave field is obtained by producing sources with large apertures that distribute the energy entering the body over a large skin surface Another positive feature of these large-aperture sources is that they are characterized by an astonishing focusing accuracy and thus ensure the best possible energy flux densities or intensities It should be mentioned, how-ever, that the energy density and intensity are limited
by technical and, to a certain extent, anatomical fac-tors Shock wave sources should be used that feature
a maximum aperture and a maximum aperture angle
at which the shock waves converge towards the focal point in order to optimize the efficacy of the shock waves while at the same time minimizing undesired side effects Again, the cylindrical source developed
by STORZ MEDICAL stands out from other sources in
5
Steinbach, P., Hofstaedter, F., Nicolai, H., Roessler, W., Wieland, W
(1993): "Determination of the energy-dependent extent of vascular damage caused by high-energy shock waves in an umbilical cord model"; Urological Research, 21, pp 279-282
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that it meets these requirements better than any
other shock wave generator
This is clearly shown in Fig 11, which is taken from
the aforementioned study The diagram below
con-firms that the STORZ MEDICAL source features the
highest ratio between the focus pressure and the
pressure at the point of entry into the body (in this
case 50 mm in front of the focus)
Fig 11: Focus pressure and pressure at the skin surface
Being rapid pressure fluctuations occurring in water,
shock waves are not visible However, they can be
made visible by using suitable Schlieren photography
apparatus The images in Fig 13 show the shock
waves on their way to the focal point at various times
after leaving the source These images show a
two-dimensional sectional view of the three-two-dimensional,
spherical segment-shaped wave surface illustrated in
schematic form in Fig 12
Fig 12: Propagation of waves
The wavefronts have a thickness of about 1 mm and propagate in the direction of the focal point at a velocity of approx 1500 m/s After having passed through the focal point, the wavefronts diverge and dissipate, their amplitude gradually weakening
Fig 13: Propagation of waves – schlieren photographs
Cavitation
On closer inspection of the last few images in Fig 13, cavitation bubbles can be seen on the centre line of the images Cavitation occurs when shock waves are followed by tensile waves, which blow open the fluid
Bubbles are formed around microscopically small cavitation centres and expand within a few microsec-onds to different maximum diameters (about 1 mm)
Having reached their maximum size, they generally collapse within 100 microseconds, merging with the original cavitation bubble and sending out a secon-dary spherical shock wave
Cavitation bubbles contribute to the disintegration of kidney stones However, they are also considered to
be responsible for tissue damage Bubbles located near acoustic boundary layers (stone surface, vascular walls, pulmonary alveoli, etc.) cannot collapse point-symmetrically The initially spherical bubble is dented and forms an intensive fluid jet with a diameter of a few tenths of a millimetre which impacts on the boundary surface at a velocity of several hundred metres per second This jet can erode stone surfaces and puncture the walls of smaller vessels (formation
of microhaemorrhages) This again calls for the use of shock wave sources with large apertures Using such sources, the cavitation threshold values are only ex-ceeded near the focus zone, whereas the skin surface
is exposed to cavitation only to a limited degree or not at all
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Dynamic Range, Reproducibility and
Dosing Capability
A generous dynamic range is essential in order to be
able to handle different indications and to take into
account patients' differing pain thresholds It must be
possible for the energy levels and energy flux densities
to be adjusted to specific conditions within generous
limits Experts often refer to 'high-energy' and
'low-energy' shock waves However, so far no precise
definition has been established stating which energy
flux densities are covered by these two terms Various
study groups have suggested the following
classifica-tion for orthopaedic applicaclassifica-tions6:
Mainz7:
low energy: 0.08 mJ/mm2
medium energy: 0.28 mJ/mm2
high energy: 0.6 mJ/mm2
Kassel8:
low energy: < 0.12 mJ/mm2
high energy: > 0.12 (bis 0,38) mJ/mm2
For different fields of shock wave applications
(cardi-ology, urology) specific energy settings may be
re-quired
In order to tailor the treatment to be performed to
the target zone and the patient's tolerance to pain,
the doctor needs to know how much energy is
ap-plied throughout the entire duration of the treatment
This means that it must be possible for every emission
of shock waves to be precisely dosed and reproduced
The description of the individual methods employed in
the generation of shock waves has shown that not all
systems are equally effective in this respect In fact,
the study commissioned by the Italian Department of
Health has confirmed that it is the electromagnetic
shock wave generators that produce the most stable
values, even in the higher pressure range
6
The energy levels that can be set when using the MINILITH cover both
the high-energy and the low-energy values listed in the above
classifi-cation In the lower energy range, which generally does not require any
anaesthetic, the energy levels can be precisely adjusted When using
higher energy levels, the energy supplied by the MINILITH is vigorous
enough to allow even pseudarthrosis to be treated successfully
7
Rompe, J D., Universität Mainz, auf dem Süddeutschen
Orthopäden-kongress, 25 - 28 April 1996 in Baden-Baden
8
Siebert, W., Orthopädische Klinik Kassel, auf dem Süddeutschen
Orthopädenkongress, 25 - 28 April 1996 in Baden-Baden
Fig 14: Stability at focus
Localization
The use of a reliable localization system is absolutely essential when performing shock wave therapy, all the more so as serious side effects produced by shock waves cannot always be excluded
Today, there are really only two systems suitable for this purpose: ultrasound or X-ray localization systems
It is up to the doctor to decide which system to use
However, this decision should always be dependent
on the specific indication to be treated Various local-ization methods, which are independent of the type
of system used, are offered by the individual manu-facturers A distinction is usually made between off-line and in-off-line localization, the latter being character-ized by the fact that localization is performed through the therapy head
Fig 15: Off-line/in-line localization
When using off-line ultrasound localization systems, the targeted zone may not correspond to the zone actually subjected to shock waves9 Just like optical
9
Wess, O., Stojan, L., Rachel, U.K (1995): "Untersuchungen zur Präzision der Ultraschallortung in vivo am Beispiel der extrakorporal induzierten Lithotripsie" [Investigations into the accuracy of in vivo ultrasound localization on the basis of extracorporeally induced li-thotripsy] in: Chaussy, Christian (ed.): Die Stoßwelle, Forschung und Klinik [Shock waves, research and clinical applications]; Attempto Verlag, pp 37-44; ISBN 3-89308-228-X