By National Council on Radiation Protection and Measurements
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Additional resources for Biological effects of ultrasound : mechanisms and clinical implications
3. Medical Ultrasound Fields In this section (Section 3) information is given on the characteristics of ultrasound fields generated by equipment used in medicine. 1. 2 distribution patterns in space are described for continuous fields generated by circular transducers, both unfocused and focused; also discussed are space-time distributions for single transducers and linear arrays during pulse-mode and scanning operations. , the bio-effect potential inherent in the fields when applied to human patients.
As beam proceeds from source to receiver, different parts of the beam travel at different speeds, cl, c2, etc. 3. Attenuation coefficients obtained by phase-independent methods are the preferred quantities to use in making predictions of intensity reduction for ultrasound propagating in mammalian tissues. However, most of the attenuation data available comes from measurements with phase-dependent receivers, and virtually all diagnostic instruments are of the phase-dependent type. Therefore, until phase-independent receivers become more readily available, phase-dependent measurements will remain the only attenuation data available for many tissues.
2. Under conditions common in medical ultrasound, the field has characteristics that cannot be explained by linear theory alone. Even if the ultrasound is generated by a transducer which vibrates sinusoidally, any or all of the following may occur: a) The mean pressure at a point is raised or lowered; b) Objects in the field are acted on by steady forces; C) Particles suspended in a fluid are set into steady translatory or rotary motion; 20 / 2. BASIC PHYSICS OF ULTRASOUND d) Steady circulatory flow is set up i n fluid media; e) Harmonics are generated, so that the waveform becomes distorted and the attenuation becomes intensity-dependent; and f ) Cavitation phenomena appear.