Nature and Properties of Ultrasound Waves
Humans can hear sound waves with frequencies between 20 Hz and 20 KHz. Frequencies higher than this range are termed as ultrasound. A sound wave can be described as a mechanical, longitudinal wave comprised 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. The amplitude of these cyclic changes can be measured in any of three acoustic variables.1-3
- Pressure: Routinely measured in pascals
- Density: Units of mass per unit volume (eg, kg/cm3)
- Distance: Units of length (eg, 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 above acoustic variables. Amplitude is equal to the difference between average and the maximum (or minimum) 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 ultrasound (US).
The operator can modify all of the above parameters. Note that this 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/cm2) is assigned the value of 0 dB. An increase of 3 dB represents a two-fold increase in intensity while an increase of 10 dB represents a ten-fold increase in intensity. This means that a sound with an intensity of 120 dB is one trillion 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 ultrasound system, these parameters cannot be manipulated by the operator.
- Period: The duration of a single cycle. Typical values for clinical ultrasound are 0.1 to 0.5 microseconds (μs).
- Frequency (f): The number of cycles per unit time. One cycle per second is 1 hertz (Hz). Ultrasound (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 (λ): The distance traveled by sound in 1 cycle (0.1 to 0.8 mm)