Updated: Feb 27, 2020
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).
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.
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).
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
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