Attenuation is the reduction of power, for example due to the passage through a medium or electrical component. In ultrasound imaging, attenuation means the decrease in amplitude and intensity as a sound wave travels through a medium. In ultrasound attenuation is often characterized as the half-value layer, or the half-power distance. These terms refer to the distance that ultrasound will travel in a particular tissue before its energy is attenuated to half its original value.

Attenuation originates through:
A thick muscled chest wall will offer a significant obstacle to the transmission of ultrasound. Non-muscle tissue such as fat does not attenuate acoustic energy as much. The half-value layer for bone is still less than muscle, that's why bone is such a barrier to ultrasound.

See also Attenuation Coefficient, and Derated Quantity.

Axial resolution is the minimum separation between two interfaces located in a direction parallel to the beam (objects above and below each other) so that they can be imaged as two different interfaces. The axial space resolution directly relates with the wave frequency, but higher frequencies have lower penetration into tissues.
The axial resolution is inversely proportional to the frequency of the transducer depending on the size of the patient. The higher the frequency the lower the axial resolution in large patients. This state results from the rapid absorption of the ultrasound energy with lower penetration. Lower frequencies are utilized to increase depth of penetration.

See also Damping.

To calculate the echo position, a constant sound speed of 1538.5 m/sec is assumed. Tissue penetration is frequency depended, if the frequency increases, the imaging depth decreases. The range resolution defines the depth. Ultrasound propagating in tissue is attenuated due to scattering and absorption. The attenuation is proportional to depth and frequency and is typically in the range from 0.5 to 1 dB/(MHz cm).

See also Attenuation Coefficient, Proximity Sensor, and Echo Ranging.

Diagnostic ultrasound imaging has no known risks or long-term side effects. Discomfort to the patient is very rare if the sonogram is accurately performed by using appropriate frequencies and intensity ranges. However, the application of the ALARA principle is always recommended.
There are reports of low birth weight of babies after applying more than the recommended ultrasound examinations during pregnancy. Women who think they might be pregnant should raise this issue with the doctor before undergoing an abdominal ultrasound, to avoid any harm to the fetus in the early stages of development.
Since ultrasound is energy, sensitive tissues like the reproductive organs could possibly sustain damage if vibrated to a high degree by too intense ultrasound waves. In diagnostic ultrasonic procedures, such damage would only result from improper use of the equipment.

Possible ultrasound bioeffects:
The thermal effect is controlled by the displayed thermal index and the mechanical index indicates the risk of cavitation.
An ultrasound gel is applied to obtain better contact between the transducer and the skin. This has the consistency of thick mineral oil and is not associated with skin irritation or allergy.
Specific conditions for which ultrasound may be selected as a treatment may be attached with higher risks.

See also Ultrasound Imaging Procedures, Fetal Ultrasound and Obstetric and Gynecologic Ultrasound.

Due to the absorption of ultrasound, heating of tissue (including bone) can occur. For this reason, the sonographer should follow the ALARA principle to minimize the potential for ultrasonic heating of tissue during for example M-mode ultrasound. The thermal effect of Doppler ultrasound flow examinations is significantly greater.

See also Thermal Index and Ultrasonic Power.