Week 9: lecture 1

Definition of Artifact

• Any visible structure on an image that does not accurately demonstrate the true presence, location, or reflectivity of that structure.

Categories of Artifacts

• Image detail resolution artifacts

• Locational artifacts

• Attenuation artifacts

• Doppler artifacts

Causes of Artifacts

• Ultrasound system assumptions

• Equipment failure/faults

• Inappropriate equipment settings

• Electrical/magnetic interference

Assumptions Leading to Artifacts

• Assumption 1: The transmitted wave travels along a straight-line path from the transducer to the object and back.

• Assumption 2: All detected echoes originate from the axis of the main beam only, and the depth of objects is proportional to the round-trip time.

• Assumption 3: The ultrasound wave travels in all soft tissues at a constant rate of 1540 m/s.

• Assumption 4: The attenuation of sound in tissue does not vary.

• Assumption 5: All received echoes are derived from the most recently transmitted pulse.

• Assumption 6: The amplitude of the echo is related to the reflective characteristics of each object.

Importance of Assumptions

• If any of the assumptions are violated, an ultrasound artifact will occur.

• Artifacts can be seen in B-mode and Doppler imaging.

Pros & Cons of Artifacts

• Pro: Some artifacts are desirable and can indicate tissue characteristics, aiding in diagnosis.

  • Example: Shadowing may suggest calcification, and enhancement may indicate cystic change.

• Con: Some artifacts are undesirable and may obscure or distort anatomy.

  • They must be recognized to prevent misinterpretation of the image and potential misdiagnosis.

B-mode Artifacts

Types of B-mode Artifacts

• Shadowing

• Enhancement

• Edge shadowing

• Propagation speed artifact

• Reverberation artifact

• Slice thickness artifact

• Beam width artifact

• Side lobe/grating lobe artifact

• Refraction artifact

• Mirror image artifact

Categories of B-mode Artifacts

• Attenuation artifacts

• Depth artifacts

• Beam dimension artifacts

• Beam path artifacts

• Equipment and electrical artifacts

ATTENUATION ARTIFACTS

Types of Attenuation Artifacts

• Posterior acoustic shadowing

• Posterior acoustic enhancement

• Edge refraction shadowing

Violated Assumptions

• Assumption 4: The attenuation of sound in tissue does not vary.

• Assumption 6: The amplitude of the echo is related to the reflective characteristics of each object.

Attenuation Coefficient

• Attenuation (a) = α x f (MHz) x L (cm)

• Table of Medium-Acoustic Values:

  • Various materials, densities, velocities, impedance, and attenuation coefficients are provided in the table.

Posterior Acoustic Shadowing

• Definition: Increased attenuation causes transmitted intensity deep to a region to be reduced compared to the intensity of tissues around it at the same depth.

• Classifications:

  • Clean Shadowing: Posterior to calcification or bone.

    • Due to high percentage of absorption & reflection with no transmission.

  • Dirty Shadowing: Posterior to air-filled structures.

    • Due to a high percentage of reflection and a small percentage of transmission with secondary reflections.

• Reduction: Can be reduced with compound imaging.

Edge Refraction Shadowing

• Occurs at the edges of rounded structures.

• The sound beam is reflected or refracted, and the sound beam is deflected or broadened.

• Can be reduced with compound imaging.

Posterior Acoustic Enhancement

• Definition: Decreased attenuation causes transmitted intensity deep to a region to be strengthened compared to the intensity of tissues around it at the same depth.

• Minimized by compound imaging.

• Often a beneficial artifact for tissue characterization.