# Physics: Attenuation

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The loss of ultrasound energy as it travels through a medium (such as tissue) is called attenuation. The loss of ultrasound energy is expressed as change in ultrasound intensity.

The units of ultrasound intensity are watts per centimeter squared.

Decibels are the units for describing the difference between ultrasound intensities.

Decibels are used because they are small numbers (called logarithms) that can describe large changes in intensity.

For example, when the intensity of sound becomes one thousand times softer, the attenuation is minus 30 decibels.

When the intensity of sound decreases to one half of the original value, the attenuation is minus 3 decibels.

The distance that ultrasound travels in order for the intensity to decrease to half the original value is called: half intensity depth. In decibels, half intensity depth is the distance ultrasound travels to achieve a three decibel loss.

The rate of attenuation of ultrasound in soft tissue is one half the frequency per centimeter. The rate of attenuation is called the attenuation coefficient. For example, the attenuation coefficient of a 12 MHz transducer in soft tissue is 6 dB per centimeter.

The half intensity depth using this particular 12 MHz transducer is the distance the ultrasound travels to achieve a three decibel loss. Since we know that there is a 6 decibel loss after the ultrasound travels a centimeter. The half intensity depth is 0.5 centimeter. The contents and links on this page were last verified on October 31, 2006.