Nature and Properties of Ultrasound Waves
Humans can hear sound waves with frequencies between 20 and 20,000 hertz. Frequencies higher than this range are called ultrasound (US). A sound wave can be described as a mechanical, longitudinal wave composed of cyclic compressions and rarefactions of molecules in a medium. This is in contrast to electromagnetic waves, which do not require a medium for propagation. Three acoustic variables identify sound waves:
Pressure: force within an area (measured in pascals)
Density: mass within a volume (measured in kg/cm3)
Distance: motion measured in length (e.g., millimeters, centimeters)
Three parameters can be used to describe the absolute and relative strength (“loudness”) of a sound wave:
Amplitude: The amount of change in one of the previously mentioned acoustic variables. Amplitude is equal to the difference between the average and the maximum values of an acoustic variable (or half the “peak-to-peak” amplitude).
Power: The rate of energy transfer, expressed in watts (joules/second). Power is proportional to the square of the amplitude.
Intensity: The energy per unit cross-sectional area in a sound beam, expressed in watts per square centimeter (W/cm2). This is the parameter used most frequently when describing the biological safety of US.
The operator can modify the three parameters described, but it should be noted that modifying these parameters is not the same as adjusting receiver gain, which is a postprocessing function.
Changes (usually in intensity) can also be expressed in a relative, logarithmic scale known as decibels (dB). In common practice, the lowest-intensity audible sound (10-12 W/M2) is assigned the value of 0 dB. An increase of 3 dB represents a two-fold increase in intensity, and an increase of 10 dB represents a ten-fold increase in intensity. This means that a sound with an intensity of 120 dB is 1 trillion (1012) times as intense as a sound of 0 dB.
Four additional parameters that are inherent to the sound generator (transducer) and/or the medium through which the sound propagates are also used. When referring to a single transducer (piezoelectric) element in a pulsed US system, these cannot be manipulated by the operator:
Period: The duration of a single cycle. Typical values for clinical US range from 0.1 to 0.5 microseconds (μs).
Frequency (f): The number of cycles per unit time. One cycle per second is one hertz (Hz). US is defined as a sound wave with a frequency greater than 20,000 Hz. Values that are relevant in clinical imaging modalities such as echocardiography and vascular ultrasound range from 2 to 15 megahertz (MHz).
Period and frequency are reciprocals. Period = 1/f.
Wavelength and frequency are inversely proportional and are related ...