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One of the chief reasons for using ultrasound as a diagnostic modality relates to its superior safety profile, where the principle of ALARA (as low as reasonably achievable) yields great information at low energy. Yet, when ultrasound energy is employed at higher powers, it offers a number of therapeutic possibilities. Not needing the high frequencies required for imaging resolution also permits greater depth of penetration if desired.

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Therapeutic ultrasound interventions can be divided into two categories: low-power and high-power applications. The low-power group includes physiotherapy, sonophoresis and sonoporation (both methods of enhancing drug delivery), gene therapy, ultrasound-assisted thrombolysis, and bone healing. High-intensity focused ultrasound (HIFU), surgical energy devices, and lithotripsy are the more common applications in the high-power group.

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The physics of therapeutic ultrasound encompasses both thermal and nonthermal effects of acoustic energy. At high intensity or energy, heating of surrounding tissue from absorption dominates, while at lower intensities, the nonthermal or mechanical effects of cavitation, acoustic streaming, and microstreaming are observed. The latter effects are thought to result from scattering of the ultrasound energy, leading to mechanical changes in the medium (tissue).

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As described in detail in the physics chapter, as it travels through tissue the ultrasound wave is subject to absorption, scattering, reflection, rarefaction, etc. Absorption leads to the generation of heat within the tissue, the degree of which depends on the intensity of the incident wave. Intensity is simply defined as the power density within an area (spatial average), or at its peak (spatial peak). However, when considering pulsed ultrasound (which allows for delivery of greater power), one must take into account the duration of the pulse when describing intensity. One could, for example, describe the power delivered only over the duration of the pulse itself (pulse average), or averaged over a period to include the time between pulses (temporal average). From this discussion it can be seen then that intensity may be described in a number of ways, including ISATA (spatial average, temporal average), ISPTA (spatial peak, temporal average), or ISPPA (spatial peak, pulse average). Generally, in therapeutic ultrasound applications where tissue heating is the goal, the term ISATA is used as it gives the best sense of the net thermal effect of the ultrasound beam.

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A focused rise in temperature such as that afforded by ultrasound offers therapeutic benefits for perhaps a number of reasons. For example, hyperemia occurs as tissue is warmed, and the increased blood supply in a certain area may enhance the therapeutic effects of certain drugs or radiation therapy. Hyperthermia treatments in the temperature range of 43–50°C cause cessation of cell reproduction. Delivery of the heat must be sustained for up to an hour. HIFU results in a concentration of heat to temperatures of around 56°C, which causes instantaneous cell death (and thus need only be sustained for a few seconds).

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To summarize then the thermal effects of ultrasound ...

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