| What is ultrasound? To begin, it's necessary to understand what sound is before one can understand ultrasound. Sound is our experience of the propagation of pressure waves through some physical elastic medium. Usually the medium is air, but liquids generally work even better. The pressure waves are generated from some type of mechanical disturbance. Sound is a transfer of power as well. Mechanical energy is being converted to a wave form that radiates energy away from the disturbance. Mechanical vibrations become vibrating pressure waves, transferring energy to the medium and to objects that the wave contacts. Human hearing is limited. If the vibrational frequency is too fast, too high a frequency, we can't hear it. This is ultrasound, vibrations too fast for us to hear. Vibrations Too Fast To Hear. Human hearing can't go beyond about 18,000 vibrations per second, or 18 kHz. There are mammals that can hear well above this. Bats and whales use echolocation that can reach frequencies above 100 kHz. Higher frequencies have a shorter wavelength. A smaller size allows them to reflect from objects more readily and to give greater information about those objects. Extremely high frequencies are difficult to generate and to measure. There is an upper boundary to usable ultrasound, around 10 MHz (10 million cycles per second) currently, although higher frequencies are possible. 18 kHz to 10 MHz Generally, the higher frequencies are used for medical imaging, such as investigating a fetus in the mother's womb. The lower frequencies, 1 MHz or less, having longer wavelengths and greater amplitude for a given input energy, produce greater disruption of the medium. Think of the sound system at a night club: the high frequencies "go right through your head," but the low frequencies actually cause your chest to resonate to the beat. Lower frequencies could affect the tissues being studied or introduce unwanted noise into the imaging process. So, what is medical ultrasound? Ultrasound diagnosis is a process which translates electric pulses to high frequency sound vibrations and then listens for the echoes. The devices -- or probes -- which both create the sound and listen for the echoes, use transducers made of piezoelectric materials. These materials have the wonderful property of vibrating when stimulated by electricity and of generating electricity when they are vibrated by sound. The electric pulses created in the transducers by the echoes are converted to digital representations which can then be converted into anything from a simple string of numbers to a picture on a video monitor. Modern medical ultrasound is simply a process where different types of sound waves are bounced right into your body. Once the reflections of the sound waves have been captured by a machine, the machine then transforms these reflections into an image which can be read. Ultrasound is an art which does not involve the use of radiation or any type of dyes that need to be injected or swallowed. It is safe, fast and very effective for certain types of medical imaging. 
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