Tissue-ultrasound interactions: Absorption to imaging artifacts

Abstract

Attenuation determines fundamental limitations in sonography. It directly limits the imaging depth (penetration) and indirectly limits the detail, contrast, and temporal resolutions by limiting the frequency. These resolutions improve with increasing frequency but penetration requirements determine the upper limit on frequency in each imaging situation because attenuation (and therefore penetration) is frequency dependent. Attenuation in soft tissue is dominated by absorption (typically about 90%). Absorption occurs primarily at the molecular level. Sonographic artifacts occur when one of the pulse-echo imaging assumptions is violated. These assumptions are that sound travels in straight lines, that echoes originate only from objects located on the beam axis, that the amplitude of returning echoes is related directly to the reflecting or scattering properties of distant objects, and that the distance to these objects is proportional to the round-trip travel time (13 μs/cm of depth). If scattering strength of an object is high but its absorption is low, a hyperechoic presentation will occur with distal enhancement. A more common occurrence is hyperechoic with distal shadow indicating high echogenicity and attenuation, therefore high absorption. Examples of several imaging artifacts will be shown with discussion of the propagation assumptions violated.