AUG 1, 2017Don’t Judge a Medical Shock Wave by its Cover
What makes the difference among different medical shock waves devices? The following are just a few characteristics.
The way by which shocks are generated (electrohydraulic, electromagnetic, piezoelectric, ballistic, etc.) The construction material of the reflector, which is the element that focuses the shock waves The geometry of the reflector (ellipsoidal, paraboloid, spherical, etc.) The treatment zone location relatively to the focal point, such as before, in or after the focal point, and also to the depth of penetration in skin or tissue, such as near the skin, inside the tissue, deep inside the tissue, etc. The type of treated tissue dictates the optimal location of the focal point in respect to the treatment zone Settings such as energy input value, frequency of shock waves, total number of pulses, etc., which is known as “dosage”
Based on the above parameters when using different shock wave devices, there are significant differences in the energy deposited in the tissue from the treatment zone, which differentiates them in performance. In other words, in order to obtain the maximum performance for the targeted application, the above parameters are to be tuned.
Shock waves travel unidirectional without loss via heat, which makes the medical focused shock wave technology a “cold” high energy therapy. Depending on energy setting, the focused shock waves are capable of producing tissue regeneration or tissue ablation in the “focal zone” or “targeted treatment zone” without producing any heat. Also, the focusing of shock waves without significant energy loss makes the focused shock waves efficient and capable to be concentrated superficially or deep inside the human or animal bodies. Any tissue depth penetration can be accomplished with focused shock waves based on reflector’s geometry, either shallower or deeper, and by varying the membrane’s height, as seen in the following figure.
Focused Shock Waves Different Penetration Based on Applicator’s Construction
In the focal zone or the targeted treatment zone, focused shock waves generate a pressure signal that has two major components – the “compressive phase”, where compressive pressures are generated inside the tissue, and the “tensile phase”, where negative pressures produce cavitation in any fluids present in the targeted treatment zone.
Typical Shock Wave Pressure Pulse
Focused shock waves are characterized by intensive compressive waves followed by significant cavitation generation and with very limited and localized and transient heat produced inside the tissue. Thus, one can talk about a “macro-level effect” generated by high compressive forces that can produce tissue micro-tears or tissue strains. During tensile phase, the collapse of large shock wave cavitation bubbles produces powerful high-speed jets with action within a few micrometers, which means at cellular level or “micro-level effect”.
Focused Shock Waves Effects
At macro and micro tissue levels, the synergetic action of the compressive pressures and cavitational high-speed jets seems to give faster and better results in healing wounds, chronic fractures, etc., for focused shock waves. In contrast, the “unfocused” shock waves produce much lower pressures inside the treated tissue and also significantly reduced cavitational action, since cavitation bubbles cannot grow to their full potential, thus producing lower therapeutic effects when compared to focused shock waves.
Unfocused Reflector and Its Targeted Treatment Zone
As seen from the above pictogram, for unfocused shock waves, the treatment zone should be completely before the “Focal Volume”. This is done in order to be able to treat an area larger than the cross section of the focal volume with one position of the applicator, and to avoid high energies found inside the focal volume, as it happens for focused shock waves. Due to lower energy delivered inside the unfocused treatment zone, the unfocused shock waves are also known as “soft shock waves”.
Practically, in the treatment with unfocused shock waves the focal volume should be completely out of the patient, which means that the focal point and its associated focal volume for such device should be far away from the unfocused treatment zone. To better visualize this, imagine a patient being treated in one room with unfocused shock waves with an applicator that has its focal volume positioned in the next room, for avoiding the focal volume to overlap with the patient in any treatment situation. For example, if the abdominal area is treated at the skin level from the left side, then the focal volume should be at all times outside the patient’s right side of the abdomen, which accounts for more than 20 inches or 500 mm for an average-size person. It means that the length of the unfocused treatment zone should be very large in order to accomplish such requirements.
For focused shock waves, the reflectors are approximately a half an ellipsoid, with a surface area depicted as A0 in the following picture. It is also known that the amount of “Energy” that is focused from the point of origin of the shock waves to the treatment zone is directly proportional with the reflective area of the reflector. Thus, the larger the area, the more energy is found in the focal volume.
Dependence of Unfocused Treatment Zone on Reflector’s Geometry
If the reflector area is reduced to 70% or 40% from the initial reflector’s surface area A0, consequently the “Energy” reflected towards focal volume becomes 70% or 40% of the energy for the reflector with A0 reflective surface. The reduced energy translates in reduced pressures and also focal volumes, as depicted in the above pictogram.
In case of unfocused shock waves the half-ellipsoidal reflectors cannot be used even for superficial treatments, due to the fact that the focused zone will be then placed inside the patient body, with unknown consequences. In this case, this is why it is most likely to utilize a reflector with reduced reflective area, where only a small fraction of the ellipsoidal area can be used. Therefore from the start, in the unfocused region, the unfocused applicators will produce significantly lower pressures gradients, when compared with those from half ellipsoid focal volume that represents the optimum for focused shock waves, assuming the same energy setting.
Pressures Comparison in Between Focused and Unfocused Zones
It means that the treatment can be gentler to the tissue when using unfocused shock waves. In the same time, treatments using unfocussed devices lose their therapeutic significance very fast, due to lower energy delivered inside the treatment zone, and thus requiring increased number of treatment sessions to deliver an equivalent amount of energy to the tissue and achieve results relatively close to those produced by fewer focused shock wave treatment sessions.
According to international standard on shock waves, for focused shock waves, the “Focal Volume” is defined as the -6dB Focal Volume that encompasses pressures from maximum pressure (Pmax) generated in geometrical focal point F2 of the treatment zone to half of that maximum pressures (Pmax/2). Another region of importance is the 5MPa Region that incorporates all pressures higher of 5MPa, which is defined as the minimum pressure generating some therapeutic effect inside living tissues.
To better understand the spatial relationship between -6dB Focal Volume and 5MPa Region when a low energy setting is used, the pressure distribution along the reflector’s longitudinal axis is presented in the following pictogram.
Pressure Distribution around Focal Point F2 for Low Energy Setting
In the “Focusing Region”, the shock waves focus towards the second focal point of the ellipsoid, F2, which transforms the sinusoidal waves generated in F1, the first focal point of the ellipsoid, into classic “saw-tooth” distorted pressures signals that are characteristic for shock waves. In this case, the Focusing Region runs from the reflector edge until a pressure of 13 MPa is reached, which represents the half-pressure of 26 MPa from the focal point F2. This is the region where the treatment zone should be place when unfocused shock waves are used.
The “Focused Region” is the region where the shock waves are focused and where the largest pressures and energies are created. The pressure signals from this region have high compressive pressures, very fast rise times, and significant tensile pressures that produce cavitation. The length of the Focused Region is equal with the length of the -6dB Focal Volume. Interesting to note is that for this example, the maximum pressure from F2 (26 MPa) is also the maximum pressure generated in the whole Focused Region.
The “Diffusing Region” is the region where the pressure shock waves are rapidly defocusing and transform themselves in distorted sinusoidal waves that have the positive and negative peak pressures relatively equal in size. The high pressures and energies decrease fast in this region, and generally are at values equivalent to those from the Focusing Region. This can be seen by analyzing the shape of the pressures signals, before and after the geometrical focal point F2.
As presented in the next pictogram, when the energy setting is increased, a higher pressure of 29 MPa is generated in the geometrical focal point F2, which stretches the length of the “Focused Region” or -6dB Focal Volume, when compared to the low energy setting presented before.
The length increase of the -6dB Focal Volume pushes the “Focusing Region” towards the reflector, and limits the available space for unfocused waves region. This shows that the energy setting can severely influence the available unfocused treatment zone length, which limits the treatment options for unfocused devices. This observation brings another important difference between focused and unfocused shock waves.
Pressure Distribution around the Focal Point F2 for High Energy Setting
For high energy setting, it is also interesting to note that the maximum pressure of 31MPa from the Focused Region is found at 6 mm away from F2, which indicates a shift in the focal point of the shock waves from the geometric focal point F2. This finding is common for focused shock wave devices, and it can be produced by the combination of reflector’s geometry and the energy setting for voltage discharge inside the reflector, at the point of origin for the focused shock waves.
For this case of high energy setting, the length of the 5MPa Region also increased significantly. Similarly, it shows that higher energy setting decreases the available length for the unfocused treatment zone, and most importantly it points out the possibility of a larger treatment zone with focused shock waves, which gives a considerable advantage for treatment options with focused shock waves.
Using unfocused shock waves means that living tissue may be treated with all that is not focused on the way to the focus. It looks such as “treatment energy arrows” are all over the place outside a target, due to a target aiming with an unfocused eye, as pictured below.
Focused and Unfocused Effects
The “blurry” aspect of the unfocused notion can easily be transferred to unfocused shock waves’ science and effects, which is in great contrast with the “clear” and “standardized” science behind the focused shock waves.
The “cover story” for unfocused devices benefits is the delivering of lower energies in a possible larger treatment zone, when compared to focused shock wave devices. This can be advantageous in treatment situations where the treatment zone is significantly large and actual low-pressure therapy reduces the patient’s possible pain sensation generated by shock waves. However, the increased number of treatment sessions, necessary to achieve the desired therapeutic results, may be an inconvenience to the patient from logistics, compliance, and financial point of view. In the same time, the construction of the unfocused applicator has its challenges that may reduce treatment possibilities, which can ultimately increase the total number of applicators needed to cover the range of treatments for a specific medical condition.
Therefore, each “cover story” involving shock waves needs to be seen from multiple angles, to understand all consequences, benefits, and disadvantages. Shock waves are never boring!!!
JUN 26, 2017The “Shock”-ing Truth about Pressure Waves
When extracorporeal shock wave technology was adopted in the medical field, the race for developing more performant and least costly devices started immediately. Due to this initiative, the shock wave devices were changed from expensive “water-bath” to “dry” devices. “Water bath” devices required the patient to be immersed in a bathtub filled with degassed water, while the newer “dry” devices were much more cost effective by allowing a therapy head filled with water to become the delivery system for shock waves while the patient was laying on a regular medical table. The change generated a significant price drop (80% to 90%) for the “dry” devices when compared to the “water-bath” systems.
The initial medical devices used a spark-gap discharge to produce pressure shock waves. These systems are known as “electrohydraulic” devices. The newer designs produced medical pressure waves by using one of three methods, “electromagnetic”, “piezoelectric”, or “concussive”. The electromagnetic devices use an electrical coil in close proximity to a metal plate. Piezo-crystal vibration was the next advancement in the field and led to the creation of piezoelectric devices. Concussive designs created an expanding wave front away from the concussion plate as a “radial wave”, sometimes also known as “soft”, ”unfocused/non-focused”, or “dispersive” waves. The new designs did not improved the performance and in some cases, it even dropped. However, their price was more attractive to physicians and patients, which constantly drove acceptance. At the forefront of the price drop are the radial devices, due to their relatively simple construction and durability, as they last large number of pulses without refurbishment.
Radial Devices’ principle of operation
As seen from above pictures, radial waves are created by accelerating a metal projectile inside a cylinder, known as an applicator body. The accelerated metal projectile then suddenly hits an impact body, concussion plate, or radial head that vibrates. These vibrations generate radial and unfocused pressure waves. Based on Physics principles, the energy decreases with the increase in radius from the source, which makes these radial waves have maximum pressure immediately at the exit from the applicator head at the impact body or point of origin. Therefore, upon entering the human body, they rapidly attenuate, thus the maximum effects of radial pressure waves are at skin surface. The radial waves can generate slight or strong side effects ranging from skin reddening to hematoma, depending on the energy/pressure generated by the radial shock waves.
Similar to ultrasound, the radial pressure waves can be described as ripples created when someone drops a rock in water. The intensity of the ripples, height of the ridges and their total number, depends on the size of the rock. The heavier the rock the higher the intensity.
Similarity of radial waves with water ripples
Just as the intensity of ripples-effect in water, for radial pressure waves the higher the pressure in their epicenter or origin, the larger their penetration and action in the tissue will be. The same principle of action applies for High Strikers, as another analogy to better understand the action and functioning of radial pressure waves.
Similarity of radial waves with High Striker
The punch’s intensity delivered by the hammer on High Striker moves the needle to various heights, which is similar to penetrations of radial pressure waves. Just to have a little fun with this analogy, strikingly enough, if the force or pressure is sufficient to get to the “1” mark for the High Striker, it can be equated to a penetration depth of 1 cm for radial waves. The “1” mark for the High Striker indicates a very weak guy or a “wimp”. By continuing the analogy with the High Striker, for a 2 cm penetration of radial waves, the Striker puts a person in the “daisy” category. A penetration of 3 cm gives a “big boy”, 4 cm translates in “big dog” and a 5 cm penetration makes you “he man”. Based on this analogy, the higher the initial pressure of radial waves, the more impressive the results should be, if not for the restrictive side effects, which might limit the radial waves to the “wimp” or “daisy” status of the High Striker.
Correspondingly, the radial waves in their epicenter can be compared to a boxing punch delivered during training to the punching bag.
Similarity of radial waves with the boxing punch
In order to avoid debilitating side effects, the intensity of the radial pressure waves should be careful tuned down to dodge a “knockout punch” that can end the good effects of the radial pressure waves, or continuing with analogies, ending a boxing match.
High intensity radial waves can produce a “Knockout”
Based on what we learned so far, the question that comes to mind is regarding the nature of radial pressure waves. Hence, are the radial waves truly shock waves or not?
To answer the question, manufacturers of such radial devices say “yes”, but the scientists say “no”. The debate reached the German Scientific Committee for Physics and Technology and the International Society for Extracorporeal Shock Wave Therapy. Based on these two scientific forums, a fundamental characteristic of medical shock wave devices is their ability to be focused, for achieving the maximum therapeutic effects at deep penetrations inside the human body and within their focus. Since the requirements for each treatment indication are different, it should be possible to adjust the depth of penetration for the focus to where the maximum shock wave energy should be found.
Differences in between focused shock waves and radial unfocused pressure waves
As seen from the above pictures, the radial pressure waves are not focused and have limited pressure or energy in order to not produce severe side effects such as a hematoma, which makes them NOT A SHOCK WAVE.
The German Scientific Committee for Physics and Technology and the International Society for Extracorporeal Shock Wave Therapy debated also about the specific parameters that characterize or ”makes” a shock wave and distinguish a shock wave from a pressure wave. Such parameters are the wave’s “rise time”, its “travel speed”, and the temporal duration of a pressure signal generated in the treatment zone, also known as “pulse duration”.
Comparison of key parameters in between focused shock waves and radial unfocused waves
The shock wave standards and theory states that pressure waves reach the shock wave status when the shock waves’ “rise time” gets very short (tens of nanoseconds). As seen from the above picture, the radial waves have a rise time of few microseconds (or few thousands of nanoseconds), which is enormous when compared to tens of nanoseconds for shock waves. This large “rise time” shows that radial waves are slow-developing events when compared to shock waves.
The “travel speed” for shock waves is constantly the speed of sound, which is 300 m/s (0.186 mile/s) in air, 1500 m/s (0.932 mile/s) in liquids and up to 9000 m/s (5.592 mile/s) in solids. In general, the amplitude of a pressure wave dictates its non-linear distortion during propagation, which can speed up the wave enough to reach the speed of sound. As seen in the above picture, the radial waves can produce pressure of few MPa to reduce side effects, which based on theory will require tens of meters for traveling distance in order to reach the speed of sound. Clearly that will not happen in reality and thus the radial pressure waves used in medical field do not reach speeds equal to the speed of the sound.
The third parameter, “pulse duration”, shows a fast or a slow event. Shock waves have a “pulse duration” less than 10 microseconds, whereas the “pulse duration” for radial waves is more than 1000 microseconds, which points out a very slow event for radial waves when compared to shock waves.
Based on these main parameters characteristics that distinguish a shock wave from a pressure wave, the radial pressure waves are NOT SHOCK WAVES.
Furthermore, the following figure illustrates the difference between an extracorporeal shock wave therapy (ESWT) device and a radial device in terms of energy distribution and depth of penetration.
Energy as a function of the depth of penetration for a radial pressure wave and a focused shock wave
Due to the radial propagation, there is no focus for radial pressure waves. Radial pressure waves are the strongest at the tissue entrance and dissipate very fast afterwards. Based on the above graph, the radial pressure waves are efficiently used only for less than 1 cm penetration. This result is also confirmed by the graphic display or mapping of the pressure distribution produced by a radial device with a flat impact body, or concussion plate, or radial head. The largest pressures of 1 MPa generated by the lowest energy setting for the radial device are found in a region that has a 10 mm radius (corresponding to the radial head radius) and below 1 cm (10 mm) in axial direction from applicator head (positioned at the left side of the map).
Pressure distribution produced by a radial device with flat concussion plate
(Adapted from Cleveland, Chitnis and McClure (2007))
In an effort to produce better penetrations, the radial devices’ manufacturers created a design for the impact body, or concussion plate, or radial head so that it has a concave surface to produce some focusing.
Pressure distribution produced by a radial device with concave concussion plate
(Adapted from Cleveland, Chitnis and McClure (2007))
In this case, the largest pressures of 7 MPa generated by the highest energy setting for the radial device are found in a region that has a radius of 5 mm (corresponding to the concave portion radius of the radial head) and below 2 cm (20 mm) in axial direction from the applicator head (positioned at the left side of the map). It means that the radial pressure waves’ penetration can be increased by using a higher energy setting and by designing the impact body, or concussion plate, or radial head with a concave surface to produce some focusing. However, the radial pressure waves are still have the highest impact at the entrance into the patient skin and decrease quickly after 1-2 cm of penetration.
The significant differences between acoustic pressure shock waves and radial pressure waves can also be seen from the output-effects produced by these technologies, as cavitation generated by the negative portion of their respective pressure profile in the treatment zone. As seen before in one of the previous pictures, the pressure signal created by a radial device has a much smaller region of negative pressures when compared to the large region produced by the pressure shock waves, which should translate in less cavitational activity for the radial devices.
Cavitation produced by shock waves (left) and by radial waves (right)
(Adapted from Cleveland, Chitnis and McClure (2007))
The above high-speed pictures of the cavitation bubbles confirm that for radial devices the cavitation phenomenon is greatly reduced and it is localized to the applicator head. In contrast for shock wave devices, the cavitation happens in a much larger volume known as focal region and at deeper penetration.
The German Scientific Committee for Physics and Technology and the International Society for Extracorporeal Shock Wave Therapy summarized in the following table their findings regarding the comparison of radial pressure waves (unfocused pressure wave therapy –UDWT) with the shock waves (extracorporeal shock wave therapy – ESWT).
Comparison of ESWT (Extracorporeal Shock Wave Therapy) with UDWT (Unfocused Pressure Wave Therapy or Radial Waves Therapy)
Extracorporeal shock wave therapy (ESWT) is designed as a therapeutic modality in which focused shock waves are used. The maximum energy is in the therapeutic zone and the depth of penetration can be adjusted. Radial pressure wave therapy does not meet these criteria and therefore it is not an ESWT. In conclusion, the “shock”-ing truth is that the radial devices cannot produce any proper shock waves and by designating the radial pressure waves as “shock wave therapy” is misleading.
JUN 7, 2017Shock Waves and Ultrasound – Apples and Oranges
Sound waves with frequencies above 18,000 Hz are called ultrasonic or ultrasound. Ironically, ultrasound waves although they are “sound” waves cannot be detected by the human ear that is capable of hearing only frequencies in between 20 Hz and 18,000 Hz (18 kHz).
However, in nature, ultrasound waves are generated in air and “heard back” by bats who are using the ultrasound waves for spatial orientation based on ultrasound reflection on possible obstacles or targeted “food”. This approach for orientation used by bats is called echolocation.
The first submarines were used during World War One, and submarine detection devices were developed to pinpoint their location using the same echolocation principle employed by bats, this time applied in water. As it happened with many other technologies from our lives, after its first military use the ultrasound was adapted for more peaceful purposes in medical and industrial fields, where the sound waves frequencies are in between 250 thousands and 15 million Hz (25 kHz to 15 MHz).
When ultrasound propagates, it has two perpendicular components – the transversal wave and the longitudinal wave, as seen from next figure.
The transversal wave moves particles perpendicular to direction of ultrasound propagation, in a sinusoidal pattern, while the longitudinal wave compresses the matter particles in the direction of propagation. The lateral sinusoidal move of matter particles, produced by ultrasound transversal component, creates friction in between different layers of particles, which generates heat and thus continuously reducing the ultrasound energy during its propagation. This is called absorption, which is energy lost by ultrasound as it overcomes the matter internal friction while traveling through it.
An increase in wave amplitude and frequency (frequency = 1/(wave period) will increase the amount of energy lost by ultrasound on its way to the target through heat absorption. Ultimately it translates in less penetration for the ultrasound. This is illustrated in next figure that shows the large period/lower frequency radial ultrasound waves travel further when compared to the small period/high frequency waves (see the size of the black arrows).
Lower Frequency Waves travel further and High Frequency Waves have less penetration
In medicine, to reduce the ultrasound heat loss/absorption rate, the non-continuous pulsed waves were developed besides the continuous ultrasound, as can be seen from the following figure.
The ultrasound used in medicine has a frequency range of 0.7 to 5.0 MHz. The low frequency ultrasound is used for diagnostic, the high frequency ultrasound is used for therapeutic and/or ablation of soft tissue. Diagnostic ultrasound is used in determining viability of pregnancy, diagnosis of gallbladder disease, detection of heart problems, and discovery of cysts and tumors. However, ultrasound is primarily associated with letting us know about pregnancy by visualizing the fetus starting from its early stages up to delivery, as seen from the following figure.
Therapeutic ultrasound or high frequency ultrasound is usually used for treating inflammation and soft tissue growth stimulation, whereas High Intensity Focused Ultrasound or HIFU is used when heat is extensively generated for ablation of unwanted tumors or cysts.
Similar to shock waves, a sound wave cannot travel by itself. It needs a medium for transmission (solid, liquid, gas). For medical applications ultrasound must enter from the air medium into the skin/fat, of a significantly higher density, and can produce a 100% reflection of the sound wave at the air-skin interface. If a coupling medium such as gel is used at skin interface (ultrasound gel has similar acoustic properties to skin or soft tissue), the reflection is reduced to 0.1%. This means that the sound energy will be transmitted through the skin barrier without any absorption, until it reaches tissues with high collagen content such as bone, periosteum, ligaments, capsules, fascia, tendons, and tissue interface (bursa). At the change from one medium to another, ultrasound energy is lost due to reflection or scattering of the sound beam on a reflecting surface, from different acoustic properties of the mediums.
Both ultrasound and shock wave devices are using ultrasound gel to couple their energy to human body. It is the main reason a lot of people consider that ultrasound and shock waves are the same type of technology. Differences between these technologies are many, from the functioning principle to the treatment targeted tissue, and their outcome efficiency.
From a higher perspective, a sound wave might look similar to a shock wave, yet the two are not the same. While a sound wave/ultrasound can be described as the ripples (sinusoidal waves) created when a small rock is dropped in water, a shock wave is faster and not as smooth. Due to their high intensity and faster nature, the shock waves look more similar to the V-shaped bow wave of a boat. The analogy of the V-shaped bow wave with shock waves is illustrated below by the shock waves produced with a bullet fired inside a water tank. Furthermore, the V-shaped bow wave is analogous to a shock wave formed by an airplane traveling faster than sound.
In contrast to ultrasound, shock waves travel nearly unchanged through fluids without any heat loss, and hence body’s soft tissues, exerting their effects only where there is a change in acoustic impedance along their path. As shock waves energy is not lost through heat on the path to their target, shock wave technology can be defined as “cold” technology, able to penetrate to any depth, a sharp contrast with the therapeutic ultrasound that produces heat and loses energy along the way to the target. Ultimately, because of unavoidable heat loss, ultrasound limits its depth penetration and thus treatment possibilities deep inside the human body. The following graph for High Intensity Focused Ultrasound (HIFU) shows that the maximum penetration may be 8 cm with 1 MHz ultrasound, and then drops exponentially to less than 2 cm for 3.5 MHz.
The increase in the ultrasound frequency produces a higher attenuation due to heat loss and ultimately reduced tissue penetration
Shock wave pressure signal (see below) lasts for 5 to 8 micro-seconds (5 to 8 x 10-6 seconds). For the sake of curiosity, if shock waves could be continuously generated one after another (as in continuous ultrasound), then that translates into a frequency of 125 to 200 kHz, which puts shock waves in the bracket of diagnostic ultrasound. In reality, shock waves are used only for therapeutic purposes and their max frequency is 10 Hz (10 shocks per second), another marked difference from ultrasound.
Shock wave characteristic pressure signal in the targeted treatment zone
Furthermore, ultrasound produces sinusoidal waves in the treatment area (alternating positive and negative pressures of equal values - up to 15 MPa/150 bars for high frequency ultrasound). This is different from focused shock waves that generate asymmetric distribution of pressure in the treatment zone, with high compressive pressures (up to 100 MPa/1000bars) for up to 3 micro-seconds, followed by negative pressures up to 15 MPa for the remaining 5 microseconds (tensile phase). Thus, all types of ultrasound produce much lower pressures inside the body and generate heat on the way to the treatment zone. This translates into small/limited penetrations.
Practically, shock waves are characterized by intensive compressive pressures and significant cavitation generation, with limited and localized heat produced inside the tissue by the collapse of the cavitational bubbles. There is a “macro effect” generated by high compressive forces producing tissue micro-tears and a “micro effect” given by the collapse of cavitation bubbles causing micro-jets in excess of 100 m/s. The synergetic effect of these two actions give faster and better therapeutic results for shock waves when compared to ultrasound.
Moreover, cavitation phenomenon generated by ultrasound is much lower in intensity or inexistent. This is given by the low ultrasound negative/tensile pressures, which makes the ultrasound cavitation bubbles smaller and thus generating less powerful micro-jets during their collapse, when compared to shock waves. Also, the ultrasound cavitation bubbles in many cases cannot grow to their full dimensions, since they are crushed by the immediate incoming positive cycle of the ultrasound, which reduces their therapeutic significance. This is valid for low intensity ultrasound (diagnostic ultrasound), high intensity ultrasound (therapeutic ultrasound), and High Intensity Focused Ultrasound – HIFU (ablation ultrasound).
Comparison of low intensity ultrasound with shock waves
Comparison of high intensity ultrasound with shock waves
Comparison of High Intensity Focused Ultrasound (HIFU) with shock waves
Finally, therapeutic ultrasound (frequencies in the range of 18 kHz to 1 MHz) cannot be focused, which is in contrast with shock waves that can be focused where the treatment is needed, regardless of depth inside the human body. High Intensity Focused Ultrasound (higher than 1 MHz) can be focused. However, HIFU generates significant amounts of heat and is applied only for tissue ablation. This is why cannot be used in the same applications as shock waves.
For medical applications, shock waves succeed in healing faster compared to ultrasound. This is done by supplying the treatment area with a high intensity energy in a short period of time and with synergistic effects at both macro and micro tissue levels. To obtain the same amount of energy from ultrasound (without the guarantee of healing success), it would be necessary to supply the relatively low power energy generated by ultrasound for much longer periods of time and in increased number of treatment sessions. The result of the longer treatment time would be storage of energy in tissue, with concomitant heating and tissue degradation, which practically eliminates this option. The alternative is to give small dosages of energy in increased number of treatments, which is harder to achieve due to poor patient compliance. Patients in general do not like to come by the doctor’s office for many treatments and the increased financial burden created by additional treatments represents another deterrent for majority of the patients, which results in poor compliance.
To summarize, below is presented a synopsis of major differences in between shock waves and different forms of ultrasound, which clearly demonstrates the title’s “apples and oranges” reference that is used “for two things that look the same but are fundamentally different”:
Unidirectional action generates no loss through heat – “cold” high energy therapy Long duration of Tensile Phase, when compared to ultrasound (7x to 10x longer), generates large cavitation bubbles of only one category: Gaseous (tensile phase expands gaseous mini-voids from body fluids) Collapse of larger shock wave cavitation bubbles generates powerful high speed jets with action within a few micrometers (cellular level) Any tissue depth penetration is based on reflector’s geometry Treat any type of tissue (hard, semi-soft or soft tissue) There are no limitations on treatment type (regeneration or ablation)
THERAPEUTIC /HIGH INTENSITY OR FOCUSED ULTRASOUND
Bi-directional action generates heat inside tissue reducing energy due to heat losses Cavitation by negative pressure generates small cavitation bubbles that are collapsed rapidly by next incoming ultrasound wave (they cannot reach their full potential): Gaseous (tensile phase expands gaseous mini-voids from body fluids) Vaporous (low negative pressures transforms fluid in vapor) Boiling (high temperatures generated during HIFU produce bubbles) Penetration depth is reduced by tissue’s ultrasound absorption, that generates heat Regenerative treatment or ablation treatment is effective only for soft tissue
MAR 28, 2017Shock Wave’s Modus Operandi
Kinematics, mechanics and the world of Physics…simple, yet so complex to observe phenomenon and express it in mathematical form. Arguably one of the best in the matter of mechanics, Sir Isaac Newton summarized as the third law of motion that for every action, there is an equal and opposite reaction. Forces always come in pairs - known as "action-reaction force pairs." Does this principle apply to shock waves? The answer is without exception, yes.
It is a fact that shock waves are capable to generate the “action” force via compressive pressure of its positive pressure signal, and high velocity jets (another “action” force) during the implosion of cavitational bubbles produced by tensile/negative pressure of the shock wave pressure signal (only when shock waves are traveling through liquids).
Typical Shockwave Pressure Pulse
However, it is even more complex when living organism are involved, and toned-down medical shock waves are acting at the cellular level or tissue level. The action-reaction principle is still applicable at the moment shock waves pass through tissue, which gives the instantaneous “action” of the shock waves and “reaction” of the tissue (macro level) and at cellular level (micro level). The less typical and interesting part is the “secondary reaction” or “delayed reaction”, with far more implications on the tissue and cells, as the medical professionals nicknamed “MOA” or mechanism of action.
When produced by explosion, the shock waves’ action-reaction effect is noticed immediately in close proximity to the point of origin, as seen in the one presented below, from archive photos of first nuclear explosion tests. The compressive force generated by acoustic pressure shock waves is the “action” that rolled/pushed the school bus for about 50 feet, and the “reaction” is the rolling motion of the bus that consumed completely the “action” force until the bus got to a complete rest. At the time when the bus stopped from its rolling motion, the shock wave front that started the “action” on the bus was further away, due to the fact that shock waves travel in air with 300 m/s (0.186 mile/s), thus shock wave “action” was then felt in other places.
What happens at large distances away from the explosion’s epicenter? It’s fast and furious.
When military tried to monitor one atomic bomb explosion, from the air and at a considered safe distance from explosion’s epicenter, the high energy-generated shock waves showed action farther than expected. To general surprise, the zeppelin used for observation turned into a “victim” of the shock wave action by crushing it and easily sending it down, as seen from picture below.
When they specifically travel through liquids and not atmosphere, the other possible “action” forces of shock waves are the cavitational jets produced by the collapse/implosion of the cavitation bubbles generated by the negative pressure of the shock waves in its tensile phase, as seen below.
Shock Wave Bubbles Implosion with Micro-jets
In many cases, cavitation action forces can generate undesirable consequences. In devices such as propellers and pumps, cavitation causes a great deal of noise, damage to components, vibrations, and a loss of efficiency. In domestic plumbing, when a pipe is suddenly closed at the outlet (downstream), the mass of water before the closure is still moving, thereby building up high pressure and a resulting shock wave that manifest as a loud banging resembling a hammering noise, known as “water hammer”, which can cause pipelines to break, if the pressure is high enough. The action forces produced by cavitation can produce “reaction forces” in materials surrounding the fluid, that can exceed the strength of the material, which can be devastating, as it shows in the below picture depicting the total destruction by cavitation of a headrace cement tunnel from a hydroelectric dam.
In medicine, the shock waves are used either to destroy kidney stones or to stimulate living tissue to repair and regenerate. The immediate “action” of the shock waves is practically to stretch the tissue and produce tissue strain, thus generating the immediate tissue “reaction” (macro level, immediate reaction). When cavitation bubbles produced in any of the body’s fluids (blood, interstitial fluid, urine, etc.) collapse, they produce micro-jets (the “action force”) that interact at micro-level with individual cells from the fabric of the tissue or the adjacent structures.
Regarding kidney stones destruction, the shock wave “action” forces exceed the kidney stone’s strength, thus producing stone fragmentation. For tissue regeneration medical application, the shock wave “action forces” are reduced in intensity in order to produce “reaction forces” at the macro and micro level and generate a cascade of “secondary body reactions”, as the reactive oxygen species (ROS) inside body fluids, expression of growth factors, angiogenic factors, inflammation modulation, improved microcirculation and oxygen supply that ultimately produce cell proliferation and differentiation.
The summary of all these reactions, demonstrated with numerous scientific publications results, are part of the shock waves mechanism of action or “MOA” inside the living tissue, as presented in the following movie.
In conclusion, shock waves definitely follow the nature principle of action and reaction, although shock waves have their own nuances when it comes to living tissue reaction: a “double reaction” (instantaneous and delayed) can be seen, can regenerate cells/tissues and constitute a non-invasive mean to add to our armamentarium of ways to keep one healthy, repair damaged tissue, which ultimately translates in a more productive life for both society and personal benefit.
FEB 23, 2017What Is A Shock Wave?
It is the theory that the Universe started with the “Big Bang”, which is the first cosmic scale shock wave that created the vast expanse of the stellar space. It was just the beginning, as planets, stars and galaxies still continue to form through collisions, explosions, implosions and other events which created other shock waves at cosmic scale.
When life appeared on Earth at cellular, multicellular level and complex organisms, there was a constant bombardment of meteors, intense volcanic activity, and sustained earthquakes, all sources of strong shock waves. As a reaction to the environmental generated shock waves, in time, living organisms adjusted to mechanical/pressure stimulus produced by shock waves similar to their reaction to other stimuli such as heat/cold, chemical, electrical, etc. This explains human body’s sustained reaction when subject to modulated shock waves, that translates in healing and regeneration due to cellular/tissue interaction.
Generally, an acoustic pressure shock waves is an audible and very strong pressure impulse in any elastic medium (air, water or solid), created by supersonic craft, lightning, explosions, earthquakes or other extreme phenomena that generate sudden and significant changes in pressure.
Explosions of any kind produce shock waves in air/water/solids, as can be seen from next videos:
Shock waves are very fast, invisible, powerful and propagate in any direction through all types of organic and inorganic matter. They travel with 300 m/s (0.186 mile/s) in air, 1500 m/s (0.932 mile/s) in liquids and up to 9000 m/s (5.592 mile/s) in solids, which makes them to reach almost instantaneous proximity targets and can travel large distances. Humans have used shock waves for their destructive power mainly in military purposes, but in the second half of the last century also for medical purposes (breaking of kidney stones and for tissue stimulation and regeneration).
The “beauty” of medical acoustic pressure shock waves is the harnessing and modulation of their power to be focused and pass through the human body without destroying soft tissue, when kidney stones are targeted, or for stimulating tissue regeneration (both hard tissues, as bone, and soft tissues, as skin and muscles). The focusing and propagation of shock waves is presented in the following picture captured using high speed photography.
The pressure profile of a shock wave is characterized by a sudden increase in compressive pressure (compressive phase) followed by an exponential decrease until the pressures get negative in the tensile phase of the shock waves. Medical shock waves are usually producing compressive pressures up to 100 MPa (1000 bar) that act on tissue macro level and negative/tensile pressures of up to -15 MPa (150 bar) that produce cavitation in fluids and act at cellular micro level.
In our divided world for any possible reasons, where the same event can be seen as “bad” or “good” depending on one’s opinion, it seems that acoustic pressure shock waves fit the same mold too. On one hand, shock waves can be used for their significant destructive power, whereas on the other hand they can heal the human body. The choice is always in front of us.