Overview
This lecture covers acoustic artifacts in ultrasound imaging, explaining their causes, classifications, and visual features, with emphasis on recognizing, understanding, and managing these artifacts in clinical practice.
Introduction to Acoustic Artifacts
- Artifacts are image features that do not represent true anatomy.
- Artifacts can create fake anatomy, hide real anatomy, or distort size, shape, location, or brightness.
- Most artifacts result from incorrect machine assumptions or operator/mechanical errors.
- Ultrasound machines commonly assume sound travels at 1540 m/s in tissue, which is not always true.
Classification of Artifacts
- Artifacts are grouped as resolution, position, and attenuation artifacts.
- Recognize artifacts by appearance, understand the invalid assumption, and know correction techniques.
Resolution Artifacts
- Resolution artifacts occur when image detail fails to match true anatomy.
- Poor axial resolution merges closely spaced reflectors parallel to the beam; use higher frequency to minimize this.
- Poor lateral resolution merges objects perpendicular to the beam that are closer than the beam width; place the focus at the area of interest.
- Poor elevational (slice thickness) resolution displays echoes from outside the imaging plane; use 1.5D arrays, annular arrays, or harmonics.
Position Artifacts
- Position artifacts display anatomy in incorrect locations due to sound direction changes or beam assumptions.
- Refraction artifact creates a lateral, side-by-side duplicate of real anatomy.
- Mirror artifact creates a deep, equally distant replica of anatomy opposite a strong reflector (e.g., diaphragm).
- Multi-path artifact creates similar-appearing, deeper structures due to sound bouncing between multiple reflectors.
- Reverberation artifact displays equally spaced, parallel lines from sound bouncing between two strong reflectors.
- Ring down (comet tail) artifact causes dense, bright lines from sound trapped by closely spaced structures (e.g., air bubbles).
- Lobe artifact displays echoes from anatomy lateral to the main beam path.
- Speed error artifact results in misplaced anatomy due to tissue speeds differing from 1540 m/s.
- Range ambiguity artifact occurs when echoes from deep structures appear in shallow images due to high pulse rates.
Attenuation Artifacts
- Shadowing appears as anechoic or hypoechoic regions behind highly attenuating structures (e.g., stones, bones).
- Edge shadowing results from refraction and divergence at curved surfaces, producing bands behind them.
- Enhancement appears as hyperechoic (bright) regions behind low attenuating structures (e.g., cysts, fluid).
- Focal enhancement produces a horizontal band of increased brightness at the beam focal point.
Other Artifacts and Noise
- Speckle adds a grainy appearance due to wave interference; higher frequency transducers and speckle reduction techniques minimize it.
- Electronic, biological, or environmental interference can introduce noise into images.
- Modern techniques like spatial compounding, frequency compounding, and coded excitation help reduce artifacts.
Key Terms & Definitions
- Artifact — Any image feature not representing true anatomy.
- Axial Resolution — Ability to distinguish close reflectors along the beam path.
- Lateral Resolution — Ability to distinguish objects perpendicular to the beam path.
- Elevational Resolution (Slice Thickness) — Ability to distinguish objects above/below the imaging plane.
- Anechoic — Lacking echoes; appears black on an image.
- Hypoechoic — Having fewer echoes; appears darker.
- Hyperechoic — Having more echoes; appears brighter.
- Specular Reflector — A large, smooth structure that reflects sound well (e.g., diaphragm).
- Refraction — Bending of the sound beam as it passes through media of different speeds.
- Shadowing — Dark region behind a highly attenuating structure.
- Enhancement — Bright region behind a weakly attenuating structure.
- Speckle — Grainy texture caused by interference.
Action Items / Next Steps
- Practice identifying artifact types using provided workbook images.
- Answer open-ended review questions in the workbook (nerd check questions).
- Review unit 15a for artifact reduction techniques.
- Adjust transducer angle, gain, depth, and focus as needed when artifacts are detected.