Based on this assumption and simple application of the distance equation, the time for sound to travel 1 cm in the body is calculated to be approximately 6.5 μs. Ultrasound machines start with a simplified assumption that sound travels at 1,540 m/s (1.54 mm/μs). In essence, more dense tissues tend to be stiffer (higher bulk modulus) which results in higher propagation velocities. The propagation velocity is the speed at which a wave travels through a medium and is determined by the properties of the medium. Higher intensities increase sensitivity and penetration but also increase the risk of causing thermal bioeffect (an increase in temperature that can cause protein denaturing leading to cell death) and mechanical bioeffects referred to as cavitation (a rapid phase transition which creates cavities that can implode within the tissue).Īnother important wave parameter is the propagation velocity. The intensity is a measure of the distribution of the power per unit area, as indicated by Eq. The parameter that is measured to assess both the ability to improve signal strength and the risk of inducing a bioeffect is the intensity. Of course, because of the physical interaction with the medium, a higher acoustic pressure potentially increases the risk of causing tissue damage (referred to as a bioeffect). Higher transmit voltages produce higher acoustic pressure fields, increasing the strength of the reflected signal as well as increasing the maximum depth of penetration. In order to produce the sound waves, a piezoelectric transducer is excited by an electrical signal, referred to as the transmit voltage. The power of the wave relates to the pressure developed within the tissue.
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