Ultrasound is sound with a frequency above 20 kHz, above the range of human hearing. Like all sound, it is a longitudinal wave: the particles of the medium vibrate back and forth along the direction the wave travels, making compressions and rarefactions.Medical ultrasound usually uses frequencies of about 1 to 15 MHz. It travels through soft tissue at about 1500 m s−1. When it reaches a boundary between two different tissues, part of it is reflected; the reflected pulses (echoes) are used to make images.
Ultrasound is non-ionising, so it is much safer than X-rays or gamma rays. It is used for scanning a fetus, the heart, the eye and many other organs, and it gives images in real time. It is not good at imaging bone or through air, such as in the lungs, because almost all of it is reflected there.The wave equation links frequency and wavelength:λ = c / f Worked example: find the wavelength of 3.0 MHz ultrasound in tissue where c = 1500 m s−1.λ = 1500 / 3.0 × 106 = 5.0 × 10−4 m = 0.50 mm Details smaller than about one wavelength cannot be seen, so a higher frequency gives a shorter wavelength and better resolution. But higher frequencies are attenuated more strongly by tissue, so they cannot reach deep organs. A compromise frequency is chosen for each scan.
Ultrasound is non-ionising, so it is much safer than X-rays or gamma rays. It is used for scanning a fetus, the heart, the eye and many other organs, and it gives images in real time. It is not good at imaging bone or through air, such as in the lungs, because almost all of it is reflected there.The wave equation links frequency and wavelength: