Applications of ultrasound in medicine for therapeutic purposes have been an accepted and beneficial use of ultrasonic biological effects for many years. Low power ultrasound of about 1 MHz frequency has been widely applied since the 1950s for physical therapy in conditions such as tendinitis or bursitis. In the 1980s, high pressure-amplitude shockwaves came into use for mechanically resolving kidney stones, and “lithotripsy” rapidly replaced surgery as the most frequent treatment choice. The use of ultrasonic energy for therapy continues to expand, and approved applications now include uterine fibroid ablation, cataract removal (phacoemulsification), surgical tissue cutting and hemostasis, transdermal drug delivery, and bone fracture healing, among others. Undesirable bioeffects can occur including burns for thermal-based therapies and significant hemorrhage for mechanical-based therapies (e. g. lithotripsy). In all these therapeutic applications for bioeffects of ultrasound, standardization, ultrasound dosimetry, benefits assurance and side-effects risk minimization must be carefully considered in order to insure an optimal benefit to risk ratio for the patient. Therapeutic ultrasound typically has well-defined benefits and risks, and therefore presents a tractable safety problem to the clinician. However, safety information can be scattered, confusing or subject to commercial conflict of interest. Of paramount importance for managing this problem is the communication of practical safety information by authoritative groups, such as the AIUM, to the medical ultrasound community. In this overview, the Bioeffects Committee outlines the wide range of therapeutic ultrasound methods, which are in clinical use or under study, and provides general guidance for assuring therapeutic ultrasound safety.
Ultrasonic energy can be a potent modality for generating biological effects. Given sufficient knowledge of the etiology and exposimetry, bioeffects can be planned for therapeutic purposes or avoided in diagnostic applications. For therapy, ultrasound can induce effects not only through heating, but also through nonthermal mechanisms including ultrasonic cavitation, gas body activation, mechanical stress or other undetermined nonthermal processes (Nyborg et al. 2002).
Starting from the diagnostic reference frame, ultrasound is usually produced from a piezoceramic crystal in very short, i.e., 1- to 5-cycle, pulses. Diagnostic ultrasound is often characterized by the center frequency of the pulses (typically in the 2–12 MHz range), which is usually a frequency inherent to the thickness of the ceramic crystal. As the pressure amplitude, the frequency, or the propagation length is increased, the ultrasound wave can distort, which could ultimately lead to a discontinuity or shock in the waveform. In regard to bioeffects, increasing frequency, nonlinear acoustic distortion, or pulse length can increase heating and enhance some nonthermal mechanisms, e.g., radiation force. Decreasing frequency increases the likelihood of cavitation and gas body activation. Increasing power or intensity tends to increase the likelihood and magnitude of all bioeffects mechanisms. Therapeutic ultrasound devices may use short bursts or continuous waves to deliver effective ultrasonic energy to tissues. Some devices operate at higher amplitude and therefore tend to produce shocked or distorted waves.
Ultrasound-induced heating is the result of the absorption of ultrasonic energy in biological tissue. For diagnostic ultrasound, temperature elevations and the potential for bioeffects are kept relatively low or negligible (Fowlkes et al. 2008) by carefully described indications for use, applying the ALARA (as low as reasonably achievable) principal, limited temporal average intensities, and generally short exposure durations. Therapeutic applications of ultrasonic heating therefore either utilize longer durations of heating with unfocused beams, or utilize higher intensity (than diagnostic) focused ultrasound. The use of unfocused heating, for example in physical therapy to treat highly absorbing tissues such as bone or tendon, can be moderated to produce enhanced healing without injury. Alternatively, the heat can be concentrated by focused beams until tissue is coagulated for the purpose of tissue ablation. Ultrasound heating which can lead to irreversible tissue changes follows an inverse time-temperature relationship. Depending on the temperature gradients, the effects from ultrasound exposure can include mild heating, coagulative necrosis, tissue vaporization, or all three.
Ultrasonic cavitation and gas body activation are closely related mechanisms which depend on the rarefactional pressure amplitude of ultrasound waves. Ultrasound transmitted into a tissue may have rarefactional pressure amplitudes of several megaPascals (MPa). This tensile stress is supported by the medium and, for example, a 2-MPa rarefactional pressure, which is common even for diagnostic ultrasound, represents a negative tension 20 times atmospheric pressure (i. e., 0.1 MPa). This high rarefactional pressure can act to initiate cavitation activity in tissue when suitable cavitation nuclei are present, or directly induce pulsation of pre-existing gas bodies, such as occur in lung, intestine, or with ultrasound contrast agents. Cavitation and gas body activation primarily cause local tissue injury in the immediate vicinity of the cavitational activity, including cell death and hemorrhage of blood vessels.
Other potential mechanisms for biological effects of ultrasound include the direct action of the compressional, tensile, and shear stresses. In addition, second-order phenomena, which depend on transmitted ultrasound energy, include radiation pressure, forces on particles and acoustic streaming. For high-power or high-amplitude ultrasound for therapy, several different mechanisms may be contributing concurrently to the total biological impact of the treatment. In addition to direct physical mechanisms for bioeffects, there are secondary physical, biological, and physiological mechanisms that cause further impact on the organism. Some examples are vasoconstriction, ischemia, extravasation, reperfusion injury, and immune responses (e.g., Alves et al. 2009, Hundt et al. 2007, Silberstein et al. 2008). Sometimes these secondary effects are greater than the direct insult from the ultrasound.
Intravascular catheters have been developed with MHz-frequency ultrasound transducers placed near the tip for enhancing dissolution of thrombi (Parikh et al. 2008). The catheter is placed into a deep vein thrombus and the ultrasound is directed radially into the thrombus. In addition, there are provisions for infusion of thrombolytic drugs, such as tissue plasminogen activator. The ultrasound accelerates the action of the thrombolytic drugs so that the total infusion dose of drugs and treatment times can be reduced significantly. The role of this method, and the full range of its clinical usefulness for thrombolysis is still being evaluated.
For transdermal drug delivery, the stratum corneum (≈ 10–30 μm) forms a barrier to passive drug diffusion for molecules which have a weight greater than 500 Da (Boucaud 2004). One effect of low-frequency ultrasound (<100 kHz) is its ability increase permeability of the stratum corneum, which is considered to be a primary barrier to protein diffusion (Pitt et al. 2004; Mitragotri and Kost 2004). The treatment can be monitored by measuring the electrical skin conductance (Farinha et al. 2006). Once a drug has traversed the stratum corneum, the next layer is easier to cross and subsequently the drug can reach the capillary vessels to be absorbed (Mitragotri et al. 1995). This skin permeabilization method may be useful for avoiding the multiple use of needles, for example, for delivery of heparin or insulin through the skin (Smith, 2008).
Low intensity pulsed ultrasound has therapeutic application to accelerate the healing of bone fractures including cases of nonunion (Gebauer et al. 2005). The characteristics of the pulsed ultrasound, for example, 1.5-MHz frequency with 30-mW/cm2 spatial average temporal average intensity, are in the range of diagnostic ultrasound. The biophysical mechanisms for the therapeutic action are uncertain for this application. Therapy involves multiple treatments of 20 min each day by applying the large flat transducer to the site of injury and continuing treatment for periods of months. Although the process appears to be safe and effective, the therapy is slow and its use is predominantly limited to management of non-healing fractures.
Therapeutic ultrasound methods provide a substantial armamentarium for medical practice. In addition, ultrasound brings fundamentally favorable safety characteristics to the clinic. For example, ionizing radiation with its dose accumulation and cancer risk is absent from ultrasound methods. Low energy exposures, below the threshold for a bioeffect, do not accumulate to produce the effect, even if repeated many times. The ultrasonic waves are dispersed and poorly transmitted in air: no lead gloves, aprons or other protective gear are needed for ultrasound diagnosis or therapy. However, this powerful modality does require attention to several safety factors in order to achieve the optimum benefit to risk ratio.
The operator of the equipment, for the most part, has little risk of harm from the machines, can remain in the treatment room and safely apply the ultrasound with hand held applicators for some applications. However, simple precautions should be followed for complete operator safety; for example, do not test therapeutic ultrasound equipment on oneself or others (as opposed to diagnostic ultrasound imaging which can be used on volunteer models for training purposes under medical supervision).
Ultrasound therapy machines are, of course, capable of causing substantial bioeffects; therefore, deliberate caution must be exercised to minimize injury for each patient. Patients should be fully informed of possible risks, as well as expected benefits.
Ultrasound therapy machines are typically complex and subject to deterioration or failure. Each machine should be monitored and tested on a regular basis for safe operation and verification of appropriate ultrasound fields to assure efficacious treatment.
Although no cumulative dose has been defined for any ultrasound therapy, unwanted bioeffects such as scarring from burns or vascular injury which occur during treatment can accumulate with repeated treatments, and this should be anticipated. For example, animal studies show permanent loss of renal functional mass with each lithotripsy and therefore recurrent treatments add injury to already compromised kidneys.
The benefits and potential risks associated with different therapeutic ultrasound methods vary widely and should be appreciated by the operator. For example, physical therapy ultrasound appears to have a low risk of harm in the hands of skilled physical therapists, but the expectation of therapeutic benefit is also low. Lithotripsy, in contrast, has the tremendous benefit of non-invasively treating a serious disease, which previously required major surgery, but it also has a risk of significant hemorrhage and longer-term kidney injury.
The search for new applications of this powerful tool should be pursued carefully, with thorough testing in appropriate animal models to identify possible human adverse events before clinical trials begin. Accurate and precise evaluation of acoustic fields in water and in situ should follow exposimetry and dosimetry procedures and numerical modeling previously recognized in the ultrasound literature. Means for monitoring heating or secondary mechanisms, such as acoustic cavitation, should be in place. Furthermore, in order to assure optimum patient benefit from therapeutic ultrasound, dedicated research should continually pursue better and safer methods to enhance present therapies and therapy monitoring.
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