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ultrasound physics

Periodic motion causes pressure waves in surrounding physical media. In the diagram, when the piston is shoved forward it compresses the medium. The compression travels through the medium. As the piston moves back and forth, it creates more compressions that travel through the medium like cars down a highway. The more quickly the piston moves back and forth, the closer one compression is to the next one.

Sound waves are made of high pressure and low pressure pulses traveling through a medium. The high pressure areas (compression) are where the particles have been squeezed together; the low pressure areas (rarefaction) are where the particles have been spread apart. The wavelength of sound is the distance between two successive high pressure pulses or two successive low pressure pulses. Wavelength of sound decreases as frequency increases. The sound we normally hear is from 20 to 20 000 cycles per second. Ultrasound means sound that has a higher frequency than our normal hearing. Ultrasound used for medical purposes is from one MHz (one million cycles per second) to 20 MHz. Ultrasound imaging does not usually use higher than 10 Mhz. Higher frequency ultrasound waves can form sharper images, but the images are fainter because tissues absorb higher frequency energy more readily. Just like any other type of sound, the higher the frequency of ultrasound, the shorter the wavelength. Ultrasound has a wavelength of about 1.5 mm.

The speed of ultrasound does not depend on its frequency. The speed of ultrasound depends on what material or tissue it is traveling in. The mass and spacing of the molecules of the material and the attracting force between the particles of the material all have an effect on the speed of the ultrasound as it passes through. Ultrasound travels faster in dense materials and slower in compressible materials. In soft tissue sound travels at 1500 m/s, in bone about 3400 m/s, and in air 330 m/s.

Over the past two decades ultrasound has undergone numerous advances in technology such as gray-scale imaging, real-time sonography, high resolution 7.5-10 MHz transducers, and color-flow Doppler and more.

Ultrasound is reflected at the boundaries between different materials. Ultrasound reflects very well wherever soft tissue meets air, or soft tissue meets bone, or where bone meets air. Frequency is unchanged as sound travels through various tissues. That means that in tissues where sound travels more slowly, wavelength decreases. Just as the spacing between cars on a highway narrows when they slow down for construction, the compression areas of a wave get jammed together when sound slows down.

Ultrasound waves are produced by a transducer. A transducer is a device that takes power from one source, converts the energy into another form, and delivers the power to another target. In this case the transducer acts like a loudspeaker and a microphone. The transducer converts electrical signals to ultrasound waves, and picks up the reflected waves converting them back into electrical signals. The electrical signals returned from the transducer are used to form pictures on a television screen.

Attenuation is loss of energy, expressed as change in intensity, as the energy travels through a medium. Ultrasound intensity is measured in watts per square centimeter. Decibels are used to express difference between ultrasound intensities. For example, when ultrasound becomes one hundred times less intense, the attenuation is -20 decibels (dB); when ultrasound is one thousand times less intense, the attenuation is -30 dB.

Rate of attenuation is called attenuation coefficient. For soft tissue, the attenuation coefficient is half the frequency per cm. For example, an 8 MHz signal will lose 4 dB per centimeter of travel.

Ultrasound has enjoyed considerable technical improvements that have resulted in the availability of high resolution real time gray scale imaging, tissue harmonic evaluation, and color and power and more.

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