Artifacts (Artifact Recognition in Sonography)
📘 Table of Contents
Reverberation Artifacts
Reverberation Artifacts
Reverberation artifacts occur when an ultrasound pulse bounces back and forth between two strong reflectors (such as air, bone, or metallic objects) before returning to the transducer. This repeated reflection causes the system to register multiple echoes, displaying them as equally spaced lines.
Key Characteristics:
- Appearance: Multiple parallel, evenly spaced horizontal lines.
- Cause: Repeated reflection of sound between two strong surfaces, like air and probe face.
- Effect: Creates a "ladder-like" appearance, often seen in the bladder, lungs (comet tail), or when scanning over gas.
Clinical Example:
In lung ultrasound, reverberation artifacts produce characteristic “A-lines” — horizontal lines that help confirm normal aerated lung.
Diagram Suggestion:
Illustrate sound bouncing between the transducer and a reflector, generating multiple echo lines returning at different times, creating stacked horizontal artifacts.
Shadowing and Enhancement
🌗 Shadowing and Enhancement
Acoustic Shadowing
Occurs when a structure strongly absorbs or reflects ultrasound waves, preventing deeper structures from being visualized.
- Appearance: A dark (anechoic) band behind a dense object like bone, stone, or calcification.
- Cause: Total reflection or absorption of the beam (e.g., by gallstones, ribs, or gas).
- Clinical Use: Confirms presence of highly reflective or solid structures.
Posterior Acoustic Enhancement
Occurs when sound waves pass through a structure with very low attenuation (e.g., fluid), causing echoes behind it to appear brighter than surrounding tissue.
- Appearance: Brighter area just beyond a fluid-filled structure like a cyst or bladder.
- Cause: Sound travels easily through fluid, so more energy reaches deeper tissues.
- Clinical Use: Helps distinguish cystic from solid masses.
Diagram Ideas:
- Show a stone with a dark acoustic shadow behind it.
- Show a cyst with bright echoes behind it indicating enhancement.
Mirror Image Artifact
Mirror Image Artifact
The mirror image artifact occurs when ultrasound waves reflect off a strong reflector (like the diaphragm) and are redirected toward another structure, which then reflects back to the transducer. The machine assumes a straight path and misplaces the returning echoes, displaying a duplicate of the original structure deeper on the image.
Key Features
- Appearance: A duplicated image appears on the other side of a strong reflector (like the liver duplicated below the diaphragm).
- Common Sites: Liver, spleen, or fetal heart seen mirrored below the diaphragm or pleural line.
- Cause: Indirect reflection paths confused as straight-line echoes.
Example
In right upper quadrant scans, the liver may appear to have a duplicate structure “beneath” the diaphragm when a mirror image artifact is present.
Diagram Idea
- Show sound bouncing from the liver → diaphragm → back to liver → then to transducer.
- Illustrate a ghost liver image mirrored below the diaphragm line.
Refraction Artifact
Refraction Artifact
Refraction artifact occurs when ultrasound waves pass through tissues at an oblique angle and change direction due to differing propagation speeds. This bending of the wave results in misplaced or duplicated structures on the image, particularly lateral displacement of echoes.
Key Characteristics
- Cause: Refraction at tissue boundaries with different acoustic velocities (e.g., muscle–fat interface).
- Appearance: Structures may appear side-by-side or displaced laterally (shifted from their actual position).
- Common Sites: Abdominal wall, rectus muscle edge, or curved fluid–soft tissue boundaries.
Example
A cyst may appear duplicated or shifted to one side if the ultrasound beam passes at an angle through the rectus abdominis.
Diagram Idea
- Draw angled sound waves bending at a boundary between two different tissues.
- Show how a structure is mispositioned laterally due to the change in wave direction.
Side Lobe and Grating Lobe Artifacts
🔸 Side Lobe Artifacts
Side lobe artifacts are caused by smaller secondary beams emitted from the transducer elements at angles to the main beam. These artifacts occur when echoes from unwanted regions appear on the image.
1. ⚡ Causes
These side lobes reflect off tissue structures outside the primary imaging area, leading to inaccurate or false echoes.
2. Mitigation
Modern ultrasound systems use apodization and beamforming techniques to minimize the effects of side lobes, resulting in clearer and more accurate images.
Diagram Idea: Show main beam versus side lobes and how they can cause artifacts in the image.
🔴 Grating Lobe Artifacts
Grating lobe artifacts are similar to side lobes but occur due to the regular arrangement of transducer elements. They produce unwanted echoes from structures outside the primary field of view.
1. Causes
Grating lobes are generated when the element spacing in the transducer array leads to interference at certain angles. This causes unwanted echoes that distort the image.
2. Mitigation
To reduce grating lobes, advanced techniques such as electronic steering and optimized transducer array design are employed.
Diagram Idea: Show the main beam versus grating lobes with labeled angles of interference causing artifacts.
Speed Displacement Artifact
Speed Displacement Artifact
Speed displacement artifact occurs when the ultrasound system misinterprets the velocity of moving structures, leading to erroneous displacement of the object on the image.
1. Causes
This artifact is primarily caused by the Doppler effect when examining moving structures, such as blood flow or tissue displacement. If the velocity of the moving object is miscalculated or if the angle of insonation is too steep, it results in incorrect positioning of the object on the ultrasound image.
2. Impact on Imaging
Speed displacement artifacts can cause a structure to appear displaced or misaligned with its true location, leading to incorrect interpretations. This is especially significant in applications like vascular or cardiac imaging where precise positioning is crucial.
3. Mitigation
To minimize this artifact, the ultrasound operator must carefully choose the correct Doppler angle and avoid high-velocity motions in certain imaging planes. Additionally, proper system settings and calibration can reduce the risk of misinterpretation.
Diagram Idea: Show an image of moving blood flow or tissue with a highlighted area indicating the displacement due to speed miscalculation.
Range Ambiguity Artifact
Range Ambiguity Artifact
Range ambiguity artifact occurs when the ultrasound system misinterprets the distance of echoes from structures that are farther away due to insufficient sampling, leading to incorrect placement of objects within the image.
1. Causes
This artifact happens when the ultrasound pulse does not return to the transducer before the next pulse is emitted, causing echoes from deeper structures to be incorrectly assigned to closer locations. It is more likely to occur at high imaging depths or with high pulse repetition frequencies (PRF).
2. Impact on Imaging
The result of range ambiguity is that structures in the deeper areas of the body may appear falsely closer to the surface, distorting the image and potentially leading to inaccurate diagnoses, especially in applications like cardiac imaging or deep tissue imaging.
3. Mitigation
To reduce range ambiguity, ultrasound systems can adjust the pulse repetition frequency to allow for enough time for the echoes to return. Alternatively, imaging at shallower depths or using advanced techniques like coded excitation can help minimize the artifact.
Diagram Idea: Show an example where echoes from deeper structures are incorrectly assigned to shallower locations, illustrating how the artifact appears on the image.
Aliasing (Doppler Artifact)
Aliasing (Doppler Artifact)
Aliasing in Doppler ultrasound occurs when the velocity of the moving target (e.g., blood flow) exceeds the Nyquist limit, resulting in the misrepresentation of the velocity direction and producing a false or reversed signal on the Doppler waveform.
1. Causes
Aliasing happens when the Doppler frequency shift exceeds half the pulse repetition frequency (PRF). This occurs commonly in high-velocity flow, such as in arterial blood flow, where the Doppler shift exceeds the system's ability to accurately measure the velocity.
2. Impact on Imaging
The result of aliasing is a reversal of the Doppler waveform, where flow appears to be moving in the opposite direction, leading to inaccurate velocity measurements and potential diagnostic confusion. This can affect clinical decisions in vascular or cardiac assessments.
3. Mitigation
To minimize aliasing, operators can increase the Doppler angle to reduce the velocity component along the ultrasound beam, lower the imaging depth to increase the PRF, or use a higher-frequency transducer. Additionally, adjusting the scale on the Doppler machine can also help avoid aliasing.
Diagram Idea: Show a Doppler waveform with and without aliasing, highlighting the reversal of flow and the correct representation of flow velocity.
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