Week 9 Lecture 2: More B-Mode Imaging Artifacts

Depth Artifacts

• Occur due to the incorrect depth display of tissues.

• Types:

  • Propagation speed artifact

  • Reverberation artifact

    • Ring down artifact

    • Comet tail artifact

  • Range ambiguity artifact

• Violated assumptions:

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

  • The round trip time is 13 microseconds for every cm of depth.

Propagation Speed Artifact

• The ultrasound system calculates depth based on the propagation speed of tissue, assuming it's a constant (1540 m/s).

• If the actual propagation speed is slower, the returning echo takes longer to arrive, and the system displays the echoes deeper than they are.

• (u = 0.0022)

Range Ambiguity Artifact

• Occurs when all echoes are not received before the next pulse is sent.

• The system assumes all echoes are from the most recently transmitted pulse and places objects more superficially than they are.

• Often seen with superficial cystic structures.

• Can be mitigated by altering depth or PRF (Pulse Repetition Frequency) or using a lower frequency transducer.

• Superficial reflected sound energy arrives at the transducer before a second sound pulse is generated

• Deeper (delayed) arrive at the transducer after the transmit of the new pulse

• The system interprets the echoes returning from depth as being associated with the second pulse

• The system places these echoes in the image near field, assuming these are early returning pulses from the second pulse

• Alter depth or PRF/utilize lower frequency transducer

• 

Reverberation Artifacts

• Tissues are displayed multiple times at equally spaced depths due to repeated reflections.

• Only the first reflection is spatially correct.

• Characteristics:

  • Parallel to the sound beam’s main axis.

  • Decreasing in intensity.

  • Equidistant from each other.

• Echoes can appear between the transducer and a strong reflector or between two strong reflectors.

• Types:

  • Simple reverberation

  • Ring-down artifact

  • Comet-tail artifact

Simple Reverberation Artifact

• Sound bounces between the transducer and a strong specular reflector, or between two strong specular reflectors.

• Often called “reverb.”

• More common in superficial tissues where the sound beam has not been attenuated.

• Commonly noted in anechoic structures.

• 

Ringdown Artifact

• Reflectors are gas bubbles.

• Multiple echoes are seen due to sound reverberating back and forth between air/soft tissue interfaces and within gas bubbles.

• Strong, bright, repeating lines are observed deep to gas bubbles.

• 

Comet Tail Artifacts

• Multiple echoes from repeated reflections within a small but highly reflective object.

• The artifact decreases in amplitude and width (tapers and fades) as it gets deeper.

• Often seen with metal, crystalline materials.

• 

Beam Dimension Artifacts

• Types:

  • Beam width artifact

  • Slice thickness artifact

  • Side lobe/grating lobe artifact

• Violated assumptions:

  • All detected echoes originate from the axis of the main beam only, and beam dimensions are small in both section thickness (elevational) and lateral directions.

Beam Width Artifact

• A strong reflector located within the widened beam beyond the margin of the transducer will generate detectable echoes.

• These echoes are assumed to have originated from the narrow imaging plane and are displayed as such.

• Occurs because of beam width divergence beyond the focal zone.

• Typically seen in anechoic structures, e.g., gas shadowing seen “within” the bladder.

Slice Thickness Artifact

• Similar to the beam width artifact but occurs due to the thickness (elevational plane) of the beam.

• Often seen within anechoic structures at depth when adjacent bowel and bladder are contained within the same “slice thickness.”

• 

Side Lobe/Grating Lobe Artifact

• A strong reflector in a side lobe (single crystal transducer) or grating lobe (array transducer) can appear as an echo as if it arises from the central beam.

• Example: Bowel with reverberation artifacts is misregistered in an anechoic ovarian cyst.

• 

Beam Path Artifacts

• Types:

  • Refraction artifact

  • Mirror image artifact

• Violated assumptions:

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

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

Refraction Artifact

• The ultrasound system assumes that the beam has traveled in a straight line, but refraction has “bent” the beam due to Snell’s Law.

• Refraction artifacts cause lateral displacement of reflectors.

Mirror Image Artifact

• Duplication of a structure on either side of a strong specular reflector.

• The true reflector and the false object are equal distances from the “mirror,” with the false object placed deeper on the image.