pt 2 Notes on Anatomic Imaging Artifacts

Anatomic Imaging Assumptions

• Brightness (gray level) of displayed echoes should proportionately represent the echogenicity of the object.
• Violation of this assumption can lead to improper display of echo strengths, resulting in artifacts.

Types of Artifacts

• The two primary forms of artifacts are:
• Shadowing
• Enhancement

Shadowing

• Defined as the reduction in echo amplitude from reflectors located behind a strongly reflecting or attenuating structure.
• Strongly attenuating/reflecting structures weaken the sound that reaches distal areas, causing echoes from this region to appear darker (like a shadow).
• Examples of shadowing structures include:
• Calcified plaque
• Bone
• Stone
• Shadowing can also occur beyond the edges of non-strongly attenuating objects due to:
• Defocusing caused by refracting curved surfaces.
• Destructive Interference due to portions of ultrasound pulses passing through tissues with different propagation speeds, resulting in phase mismatches.
• The beam's intensity decreases past the edges of the structure, weakening echoes.

Enhancement

• Defined as the strengthening of echoes from reflectors behind a weakly attenuating structure.
• Shadowing and enhancement lead to reflectors being inaccurately represented on images with amplitudes that are too low (shadowing) and too high (enhancement).
• Brightening of echoes can also be linked to increased intensity in the focal region of the beam due to its narrow width, termed focal enhancement or focal banding.
• Banding may also result from incorrect settings for gain and time gain compensation.
• Both shadowing and enhancement can provide useful information regarding the characteristics of masses and structures.
• Shadowing can be reduced through spatial compounding, which uses multiple approaches to the same anatomical site, allowing for more accurate imaging by enabling the beam to circumvent the attenuating structure.

Noise

• Various sources, both internal and external, can contribute to noise, which may also produce artifacts.