Attenuation

Keywords:
Amplitude
Intensity
Penetration
Incident Pulse
Refraction
Reflection
Scattering
Absorption
Inhomogenous/Heterogenous
Half Power Distance
Noise
Shadowing
Anechoic
Output Power
TGC

Attenuation is the reduction of amplitude and intensity as sound propagates or penetrates farther down into the medium. Amplitude is definded as the maximal cyclical change in a quantiy such as pressure (Mpa), whereas intensity is defined as the rate at which energy passess through a unit area (power over area). Both are indicators of strength of sound and are affected as penetration decrease.

Attenuation is caused by a number of factors: refraction, reflection, scattering, and absorption. Refraction is the change or shift in the direction of sound transmitted from the incident pulse after going through the tissue surface at an angle that may or may not be equal to the incident pulse angle (i.e. Oblique Incidence). Reflection is the reflected echo coming back to the sound source (i.e. transducer)from the tissue surface going in opposite direction to the incident pulse that may be perpendicular or oblique. Scattering is when sound hits an inhomogenous or heterogenous tissue surface and more reflected echoes are sent back to the sound source thus displaying a suboptimal image.

Sound scatter occurs with increasing frequences thus attenuation increases because penetration (depth of images) decreases. Frequency is inversely proportional to penetration, and as penetration decrease, amplitude and intensity become reduced. Therefore, lower frequencies are used for deeper penetration in which attenuation is decreased, whereas higher frequencies are used for structures located superficially requiring no deeper penetration. If, for example, a 5MHz frequency transducer is used on an overweight patient for an abdominal scan, sound scatter would increase thus the image displayed would be highly attenuated. The only way to fix this problem is to select a low frequency transducer, adjust output power, or TGC in order to decrease attenation and improve image resolution.

Absorption is the dominant factor contributing to attenuation of ultrasound in soft tissues, and is defined as the conversion of sound to heat. That is, absorption occurs when the tissue absorbs the sound and converts the echo to heat. As a result, the transferred energy (or heat) becomes lost and does not come back to the sound source thus a suboptimal image is displayed. The rest of the other factors contribute little to attenuation in most cases. The units for attenuation are in decibels (db), and is proportional to half of the frequency multiplied by the path length or wavelength. Therefore, as frequency and the path length or wavelength increases, attenuation increases.

Attenuation is higher in air (i.e. lung) and in bone than in other soft tissues (half power distance = 1-5cm) with a half power distance of 0.08 and 0.7cm, respectively. Attenuation occurs least in liquid or water due to a half power distance of 380cm. The half power distance refers to the distance that sound will travel to a particular tissue before its energy or amplitude is attenuated to half of its original value. Therefore, the presence of air and bone can interfere the ultrasound image by displaying increased noise or shadowing. In the presence of liquid or water, the image displayed in the monitor is anechoic.

In order to avoid attenuation, it is important for the tech to know the factors contributing to attenuation. The half power distances should also be noted and when scanning, the tech should avoid the presence of air or bone, which could interfere ultrasound image. Attenuation could also be avoided by adjusting the frequencies, output power, or TGC, which can be found on or near the machines to improve image resolution.