Many factors influence the quality of ultrasound images. However, before discussing the quality of ultrasound images, it is essential to know the ultrasound physics of how those images are produced.
According to ultrasonography, sound pulses penetrate through material tissues or media. Reflections from the objects in the media return to the transducer and appear as images on computer screens, recording devices, or printers. The source of sound waves is the ultrasound machine and a transducer.
Sound waves basic.
Parameters are characteristics of a sound wave.
There are seven parameters, such as period, frequency, amplitude, power, intensity, wavelength, and speed.
The period is the time for a wave to vibrate one cycle. The value is ranged from 0.1 to .05 microseconds or 0.0000001 to 0.0000005 seconds.
Frequency is the number of cycles of ultrasound waves during one second. It measures in Hz. In imaging ultrasound, frequency is in a range from 2MHz to 10MHz or 2-10 million per second. The name ultrasound is related to humans’ ability to hear sound waves. The audible frequency of the sound waves that people can hear lies between 20Hz and 20,000Hz. The frequency over 20,000Hz is called ultrasonic and gave a name to ultrasound waves.
Amplitude defines how big a sound wave is. Its value decreases as the wave goes through the medium.Power is another parameter that is the rate of the energy transferred. A technician can adjust both of these parameters.Intensity is the concentration of the energy in a sound beam. It determines how the power of a wave is distributed in space.
Amplitude, power, and intensity are directly related to each other and define the strength of the wave. When intensity increases, amplitude and power also increase. The wavelength, as the name suggests, defines the length of the sound wave. It measures from 0.15 to 0.8 mm.
Speed is the distance that sound wave travels via a medium in 1 sec. Speed measures in from 500m/s to 1,500m/s for soft tissue in a body for a typical clinical ultrasound. During an ultrasound session, settings, or parameters of the ultrasound system could be adjusted to achieve the best result when rendering images.
Why the medium is important for rendering clear images.
Sound cannot travel in a vacuum; it has to travel via a medium that could be of a different matter. For example, during an ultrasound session, the beam from a transducer has to penetrate tissues of the woman’s body. When a woman is obese, then the sound waves have to penetrate more body tissues. That would cause more interference, and the rendered images would have less quality under the same settings.
Knowing how parameters of sound waves correlate to each other is a key to the optimal settings during an ultrasound session. Period and frequency are reciprocal to each other. In other words, the smaller a period is, the higher a frequency and vice versa. Amplitude, power, and intensity are grouped together because they determine a magnitude of the sound wave. The technician could adjust those parameters.Speed, on the other hand, is determined by the medium through which a sound wave is passing through. A wavelength is to be determined by the source and the medium.
Medium is an essential factor that relates to the speed of the waves.
Its two characteristics are stiffness and density. Stiffness is defined by the object’s ability to resist the compression. An example could be a material that is resilient to squeezing. The stiffer the tissue, the more speed the sound waves have during the session. Density would have the opposite effect on speed. The higher density of the matter would cause a sound wave to travel at a slower speed.
That knowledge about the physics of sound waves tied with the practical experience would help a sonographer to obtain the best images of the baby during the session.