Ultrasound therapy is one of the most common physiatrist techniques, which uses the biological effects produced by ultrasounds for therapeutic and aesthetic purposes. An ultrasound is defined as an acoustic vibration with frequencies above the audible limit, namely higher than 20,000 Hz. Ultrasounds are transmitted in the form of compression/decompression waves and are produced artificially through a piezoelectric effect, exploiting either a quartz or a ceramic disc. By applying electrical charges to the faces of a quartz lamina, the crystal is compressed; by inverting this, the direction expansion is obtained. When the quartz is subjected to an alternating electrical field, it is possible to obtain an alternation of compression and expansion of the crystal, producing a series of vibrations that can be used for therapy. When ultrasonic waves travel through tissues, they lose a certain amount of their energy, and this process is known as attenuation. For most tissues, attenuation increases as frequency increases, so a 1.0 MHz signal penetrates more deeply than a 3.0 MHz signal due to lower attenuation in the tissue.
Attenuation in tissues is caused by several mechanisms, which can include absorption, ray divergence and deflection.
Absorption is the main cause of ultrasound attenuation. Ultrasound energy is absorbed by tissues and is then converted into heat. Divergence of the ray is the rate at which the ray is dispersed by the transducer. Divergence of the ray decreases as the frequency increases (a higher frequency signal, therefore, has a more focused ray). Deflection includes the processes of reflection, refraction and dispersion.
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