Overview
This lecture explains acoustic artifacts in ultrasound, detailing their causes, types, visual appearances, and strategies for identification and management to ensure accurate imaging.
Introduction to Artifacts
- An artifact is any feature in an ultrasound image that does not accurately represent the true anatomy.
- Artifacts can create false anatomy, make real anatomy disappear, or distort the shape, size, location, or brightness of structures.
- Most artifacts result from incorrect assumptions made by the ultrasound machine or from mechanical/operator errors.
- Key machine assumptions leading to artifacts:
- Sound always travels at 1540 m/s (the average speed in soft tissue).
- Sound travels directly to a reflector and back.
- Reflections only come from structures along the main beam path.
- Sound does not change direction.
- The ultrasound beam is narrow in all dimensions.
- The machine cannot distinguish invalid echoes, so it displays all returning data, which can create artifacts.
- Artifacts can also result from mechanical issues (e.g., broken crystals causing dropout) or operator errors (e.g., excessive gain reduction), as well as external sources like other medical equipment.
Types of Artifacts
- Artifacts are categorized based on the invalid assumption and their effect on the image:
- Resolution artifacts: Affect image detail and clarity.
- Position artifacts: Cause anatomy to appear in the wrong location.
- Attenuation artifacts: Alter the brightness of structures.
- When studying artifacts:
- First, identify the artifact by its appearance.
- Next, understand which machine assumption is invalid and why the artifact occurs.
- Know how to correct or manage the artifact, or when to leave it if it aids diagnosis.
Resolution Artifacts
- Occur when the image detail does not accurately represent the true anatomy, often due to the beam not being narrow enough in all planes.
- Associated with poor axial, lateral, or elevational (slice thickness) resolution.
- Axial resolution artifact:
- Two objects parallel to the sound beam and closer than half the spatial pulse length appear as one.
- Common with low-frequency transducers.
- Minimized by using the highest frequency that allows adequate penetration.
- Lateral resolution artifact:
- Two objects perpendicular to the sound beam and closer than the beam width appear as one.
- Also called point spread artifact.
- Improved by placing the area of interest within the focal zone.
- Elevational (slice thickness) artifact:
- Occurs when the beam is too thick, causing echoes from above or below the imaging plane to appear within the image.
- Most noticeable in anechoic structures (e.g., gallbladder) where unexpected echoes appear.
- Also called partial volume or section thickness artifact.
- Reduced by using 1.5D array or annular transducers, or harmonics.
Position Artifacts
- These artifacts display anatomy in the wrong location due to changes in sound direction or propagation speed.
- Refraction artifact:
- Caused by bending of the sound beam at tissue interfaces with different propagation speeds.
- Produces a lateral, side-by-side replica of anatomy, degrading lateral resolution.
- Common near muscle layers or at the edge of structures.
- Mirror artifact:
- Occurs when sound reflects off a strong specular reflector (e.g., diaphragm) at an oblique angle.
- Creates a deep, exact replica of anatomy on the opposite side of the reflector, at equal distance.
- Often seen near the diaphragm, lungs, or vessel walls.
- Multi-path artifact:
- Sound bounces between multiple reflectors before returning to the transducer.
- Results in similar-appearing anatomy displayed deeper than the true structure.
- Common in areas with complex anatomy or near the bladder.
- Reverberation artifact:
- Sound repeatedly bounces between two strong reflectors, creating equally spaced, step-ladder-like echoes deeper in the image.
- Seen with needles, mechanical valves, or between the transducer and skin.
- Ring down (comet tail) artifact:
- Caused by sound bouncing between very small, closely spaced structures (e.g., air bubbles, crystals).
- Appears as a dense, bright line parallel to the sound beam.
- Useful for diagnosing air in the biliary tree or adenomyomatosis of the gallbladder.
- Lobe artifact:
- Occurs when sound energy escapes the main beam (side or grating lobes) and interacts with strong reflectors off-axis.
- Echoes from lateral anatomy are incorrectly displayed as if from the main beam, degrading lateral resolution.
- More apparent in anechoic areas like the gallbladder or heart chambers.
- Reduced by apodization (for array transducers) or sub-dicing.
- Speed error artifact:
- Occurs when sound travels through tissue at a speed different from 1540 m/s.
- Faster speeds cause structures to appear shallower; slower speeds make them appear deeper.
- Can make continuous structures look broken or displaced.
- Range ambiguity artifact:
- Happens when echoes from deep reflectors return after the next pulse is sent, causing them to be displayed at a shallower depth.
- More common at shallow imaging depths with high pulse repetition frequency (PRF).
- Corrected by increasing imaging depth.
Attenuation Artifacts
- Result from abnormal weakening or minimal weakening of sound as it passes through structures, affecting image brightness.
- Shadowing:
- Occurs behind highly attenuating structures (e.g., bone, stones, calcifications).
- Appears as anechoic (no echoes) or hypoechoic (darker) areas.
- Can obscure anatomy behind the shadow; sometimes necessary to adjust scanning angle or gain.
- Edge shadow:
- Caused by refraction and divergence of the sound beam at curved interfaces.
- Produces anechoic or hypoechoic lines behind the curve, parallel to the beam.
- Seen with rounded structures like the gallbladder or fetal head.
- Enhancement:
- Occurs behind low-attenuation structures (e.g., cysts, fluid-filled areas).
- Results in hyperechoic (brighter) areas due to increased sound energy behind the structure.
- Helps distinguish fluid-filled from solid pathology.
- Focal enhancement:
- A horizontal band of increased brightness at the focal point due to higher beam intensity.
- Easily corrected by adjusting time gain compensation (TGC) to match surrounding tissue.
Other Artifacts
- Speckle artifact:
- Grainy appearance caused by interference of returning echoes, especially in the near field.
- Represents tissue texture but can obscure detail.
- Reduced by using higher frequency transducers or machine settings like speckle reduction.
- Electronic and biological interference:
- External sources (e.g., other equipment, cautery tools) or patient motion (e.g., talking, stomach gurgling) can introduce noise or artifacts.
- Using different transducer frequencies or changing equipment setup can help reduce interference.
- Machine techniques to reduce artifacts:
- Spatial compounding: Combines images from different beam angles to reduce enhancement, shadow, and edge shadow.
- Frequency compounding: Merges images from different frequencies for improved image quality.
- Coded excitation: Uses complex pulses to reduce artifacts like range ambiguity.
Key Terms & Definitions
- Artifact: Any image feature not representing actual anatomy.
- Axial resolution: Ability to distinguish objects parallel to the sound beam.
- Lateral resolution: Ability to distinguish objects perpendicular to the sound beam.
- Elevational (slice thickness) resolution: Ability to distinguish objects at different heights relative to the beam.
- Refraction: Bending of sound at tissue interfaces with different speeds.
- Mirror artifact: Deep, exact replica of anatomy from oblique reflections off strong reflectors.
- Shadowing: Loss of echoes behind highly attenuating structures.
- Enhancement: Brighter echoes behind low-attenuation structures.
- Speckle: Grainy appearance from interference of echoes.
- Reverberation: Equally spaced echoes from repeated bouncing between strong reflectors.
- Comet tail (ring down): Dense, bright line from sound bouncing between small, closely spaced structures.
- Lobe artifact: Lateral echoes from side or grating lobes displayed as if from the main beam.
- Speed error: Displacement of structures due to sound traveling at speeds other than 1540 m/s.
- Range ambiguity: Deep echoes displayed at a shallow depth due to high PRF.
Action Items / Next Steps
- Practice identifying different artifacts using provided images and workbook activities.
- Complete the open-ended "nerd check" questions to reinforce understanding of the material.
- Review and apply techniques to correct or manage common artifacts during clinical scanning.
- Focus on recognizing artifacts by their appearance, understanding the underlying physics, and knowing when and how to adjust scanning technique or machine settings.