The acoustic window or field is the area defined by the pathway of the ultrasound beam between the transducer and the acoustic reflector. The sound reflection to skin boundary should be minimized with an ultrasound gel where this gel acts as an acoustic window through which the image is seen.
Acoustic window refers also to the optimal placing of the transducers so that the areas of interest are clearly imaged.

See also Transforaminal Window, Transcranial Window, Transorbital Window and Transtemporal Window.

An image artifact is any image attribute, which is not present in the original imaged object. An image artifact is sometime the result of an improper operation of the imager, and in other times a consequence of natural processes or properties of the human body.
Artifacts in diagnostic ultrasound are a reflection or an echo, which appears on the display and represents the real anatomical structure not correctly. An artifact can be a false, multiple or misleading information introduced by the imaging system or by interaction of ultrasound with the adjacent tissue.

Artifacts in ultrasound can be classified as to their source like e.g.:
Image artifacts can occur in each medical ultrasound. Then an interpretation of the image is complicated and can eliminate the structural information of objects looking for.

See also Ultrasound Imaging Procedures.

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.

Ultrasound waves are reflected when there is a change in acoustic impedance. The larger the change, the more ultrasound is reflected. Microbubbles have an enormous difference in acoustic impedance as compared to surrounding fluid due to the large differences in density, elasticity and compressibility.
At low acoustic power (mechanical index less than 0.1), the mechanism of ultrasound reflection is that of Rayleigh scattering and the microbubbles may be regarded as point scatterers. The scattering strength of a point scatterer is proportional to the sixth power of the particle radius and to the fourth power of the ultrasound frequency;; the echogenicity of such contrast agent is therefore highly dependent upon particle size and transmit frequency. The backscattered intensity of a group of point scatterers is furthermore directly proportional to the total number of scatterers in the insonified volume. The concentration of the contrast medium is of importance.

See also Backscatter Energy, Cross-section Scattering.

(CEUS) Contrast agents increase the reflection of ultrasonic energy, improve the signal to noise ratio and caused by that the detection of abnormal microvascular and macrovascular disorders. Contrast enhanced ultrasound is used in abdominal ultrasound (liver sonography) as well as in cerebrovascular examinations e.g., for an accurate grading of carotid stenosis. The used contrast agents are safe and well tolerated.

The quality of the enhancement depends on:
The additional use of ultrasound contrast agents (USCAs) may overcome typical limitations like poor contrast of B-mode imaging or limited sensitivity of Doppler techniques. The development of new ultrasound applications (e.g., blood flow imaging, perfusion quantification) depends also from the development of pulse sequences for bubble specific imaging. In addition, contrast enhanced ultrasound improves the monitoring of ultrasound guided interventions like RF thermal ablation.

See also Contrast Enhanced Doppler Imaging, Contrast Harmonic Imaging, Contrast Imaging Techniques and Contrast Pulse Sequencing.