By National Council on Radiation Protection and Measurements

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4. Adding another 3 dB to make 6 dB represents a power ratio of 2' or 4 and an amplitude ratio of 2. In fact, any addition of decibels represents a multiplication of the corresponding amplitude or power ratios. Note also that a change of 10 dB changes the power by a factor of 10; a change of 20 dB changes the amplitude by a factor of 10 and the power by a factor of 100. Each addition of 20 dB changes the amplitude by another factor of 10 and the power by another factor of 100. 5 ACOUSTIC PROPERTIES OF TISSUES / 27 specified equipment a t a particular location in the body.

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.

Each bright echo-spot then traces a line, and rhythmic displacements of the line indicate, for example, heart-valve function. In a B-scan, the one-dimensional A-scan image is also formed by brightness modulation, but the transducer is moved or scanned over the tissue, and the oscilloscope trace is positioned along the y-axis according to the line of sight of the transducer. This procedure produces a pictorial two-dimensional representation of a plane section through the body, and many variations of this technique have been developed.

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