Two-Dimensional Imaging Artifacts

Imaging Artifacts: Basic Principles

Learning Objectives
Definition of Imaging Artifacts

  • False or misleading information introduced by the imaging system.

  • An echo signal whose displayed position does not correspond to the reflector's actual position.

  • Any unintended information on an image that doesn't represent the object.

  • An incorrect representation of anatomy or motion.

  • A feature of the displayed image that doesn't correspond to the nature of the reflecting tissues.

In simpler terms, an artifact is:

  • Something that shouldn't be there.

  • Something missing.

  • Something in the wrong place.

  • Something of the wrong brightness.

Categories of Imaging Artifacts

  1. Instrument-Related:

    • Miscalibration of the ultrasound machine.

    • Broken crystals in the transducer.

    • Faulty monitors.

  2. Technique-Related:

    • Improper operation of the ultrasound equipment.

    • Poorly trained operators.

    • Inexperienced operators unaware of artifacts or how to avoid them.

  3. Beam-Tissue Interactions:

    • Occur when basic assumptions about ultrasound and its interaction with tissue are violated.

Assumptions Made by the Ultrasound Machine

  1. Constant Rate of Attenuation:

    • Assumes a constant attenuation rate through soft tissue of 1 \,\text{dB/cm/MHz}.

    • Reality: Attenuation rates vary with different tissue types.

  2. Echo Origin:

    • Assumes all echoes arise from the centre of a razor-thin ultrasound beam.

    • Reality: Echoes arise throughout the full width and thickness of the beam and from secondary beams.

  3. Propagation Speed:

    • Assumes a constant propagation speed of sound through soft tissue of 1540 \,\text{m/s}.

    • Reality: Propagation speed varies depending on the structure encountered (up to 10% difference).

  4. Round Trip Time and Depth:

    • Assumes the round trip time of an echo pulse is directly related to the depth of the reflector.

    • Reality: Round trip time can be affected by factors like bouncing of the ultrasound beam between interfaces.

  5. Beam Path:

    • Assumes the ultrasound beam travels in a straight line and reflects only once.

    • Reality: Bouncing and multiple reflections can occur.

Types of Beam-Tissue Interaction Artifacts

  1. Attenuation Artifacts

  2. Beam Dimension Artifacts

  3. Depth of Origin Artifacts

  4. Beam Path Artifacts

Attenuation Artifacts

  • As the ultrasound beam passes through the field of view, its intensity weakens (attenuation).

  • The machine assumes a constant attenuation rate throughout soft tissue (1 \,\text{dB/cm/MHz}).

  • Artifacts occur when the beam encounters structures with different attenuation rates.

Acoustic Enhancement

  • Occurs when the beam passes through a structure that is less attenuating than surrounding tissue.

  • Distal structures appear brighter (increased echogenicity).

  • Commonly seen distal to fluid-filled structures.

  • Example: Pericardial effusion causing enhancement of structures behind it.

Shadowing

  • Occurs when the beam passes through a structure that is more attenuating than surrounding tissue.

  • Distal structures are either missing or appear with decreased echogenicity.

  • Commonly seen distal to prosthetic valves, calcium, and bone.

  • Example: Shadows distal to calcification of the mitral and aortic annuli.

Clinical Usefulness of Attenuation Artifacts

  • Acoustic enhancement helps differentiate cystic from solid lesions.

  • Acoustic shadowing helps identify calcification.

Disadvantage of Attenuation Artifacts

  • May obscure real structures from the image.

Violated Assumption

  • Constant rate of attenuation through soft tissue of 1 \,\text{dB/cm/MHz}.

  • Structures distal to less attenuating structures appear brighter.

  • Structures distal to more attenuating structures appear weaker or absent.

Beam Dimension Artifacts

  • The ultrasound machine assumes echoes originate from the central axis of a razor-thin beam.

  • In reality, echoes come from the full width/thickness of the 3D beam and secondary beams.

  • Echoes are displayed as though they originated from the centre of the main beam.

Common Types

  • Beam Width Artifacts

  • Slice Thickness Artifacts

  • Side Lobe/Grating Lobe Artifacts

Beam Width Artifacts

  • Echoes generated from reflectors within the beam's width.

  • Echoes continue as long as the reflector is within the beam.

Types of Beam Width Artifacts

1.  Single reflector appears wider than it is.
2.  Two reflectors appear as one.
3.  Strong reflectors at the edge of the beam appear within an echo-free space.

  • Two Separate Reflectors: If reflectors are closer than the beam width, they appear as one.

  • Single Dot Reflector: Appears wider (width equals beam width at that depth).

  • Strong Reflector at Beam Edge: Displayed as if from the central axis, appearing within echo-free spaces; often seen with gas or bowel.

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Clinical Example

  • Aortic leaflets appear longer than they should due to beam width.

  • Tissue mimicking phantom shows filaments smeared in the far field.

  • Beam width is narrowest just beyond the focal point.

Slice Thickness Artifacts

  • The ultrasound beam has a thickness (elevation plane), but machines perceive it as very thin.

  • Structures within the full thickness are detected and compressed onto a 2D image.

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Clinical Examples

  • Rib seen transecting the left ventricular cavity because it's being detected in front of the primary imaging plane.

  • Structure in the left atrium cavity turns out to be an amplatzer device over the atrial septum when viewed from another plane.

  • Apparent mass on the noncoronary cusp of the aortic valve is actually a slice thickness artifact.

Differentiating Real from Artifact

  • Real structures are seen in two planes. Artifacts are typically not.

Side Lobes/Grating Lobes

  • Secondary beams of ultrasound energy exist outside the main beam.

  • Echoes from secondary beams are assumed to have originated from the centre of the main beam.

  • These echoes are lower in amplitude.

  • Grading lobe artifact arises from the posterior wall of the inferior vena cava and appears as a membrane across the right atrium.

Differentiating Beam Dimension Artifacts

  • Slice thickness: Offending structure not seen in the imaging plane, but seen at 90 degrees to it.

  • Beamwidth/sidelobe: Offending structure is seen.

  • Beamwidth Artifacts: Can only be as wide as 1/2 of the beamwidth so aren't very wide.

  • Sidelobe Artifacts: Wider than beamwidth artifacts, decreasing amplitudes across the width.

Violated Assumption

  • All echoes arise from the centre of a razor-thin beam.

  • Smearing of structures(Beam width artifact).

  • Structures in front of or behind the imaging plane being detected(Slice thickness artifact).

  • Echoes arising from secondary ultrasound beams being detected(Side lobe or grading lobe artifacts).

Depth of Origin Artifacts

  • Structures appear at the wrong depth.

  • Determined by the time taken for the signal to return to the transducer.

  • d = (c \cdot t) / 2
    where:-

    • d = depth

    • c = propagation speed (assumed to be 1540 \,\text{m/s})

    • t = time

  • If the propagation through a medium faster than 1540 \,\text{m/s}, the structure will appear closer to the transducer than it really is. Conversely, if the propagation is slower, it will appear further away.

  • If the propagation speed through a structure is greater than 1540 \,\text{m/s}, then the structure will appear shorter than it really is. Conversely, if the propagation is slower, the structure will appear longer.

Clinical Example

  • Starr Edwards prosthetic valve (ball-cage valve) in the aortic position. The silastic ball appears elongated which suggests the propagation speed through it is slower than 1540 \,\text{m/s}.

Violated Assumptions

  • Propagation speed in soft tissue is 1540 \,\text{m/s}.

  • Round trip time is directly related to the depth of the reflector.

    • When violated, structure appears too close/short (faster propagation) or too far/long (slower propagation).

Beam Path Artifacts

  • Echoes displayed in the wrong place.

  • Duplication of a structure.

Types of Beam Path Artifacts

  • Reverberation Artifacts

  • Refraction Artifacts

  • Mirror Artifacts

Reverberation Artifacts

  • Large acoustic mismatch between two closely spaced interfaces oriented perpendicular to the ultrasound beam.

  • Bouncing of the beam between these interfaces.

    • Occurs between transducer/skin, anatomic surfaces, or artificial surfaces (mechanical heart valves).

  • The signal takes longer to return to the transducer.

  • Weaker intensity as bouncing occurs between interfaces.

  • Appears somewhat like the rungs of a ladder.

Clinical Usefulness

  • Assessment of mechanical prosthetic valves.

  • Example: Dual artifacts associated with each disc in a normally functioning bileaflet tilting disc valve.

  • Reverberation artifact of the lateral disc remains fixed over the entire cardiac cycle in abnormal prosthetic valve.

Refraction Artifacts

  • Bending of the ultrasound beam.

Conditions for Refraction Artifacts

1.  The ultrasound beam must strike an interface between two media at an angle other than 90 degrees.
2.  The propagation speeds of the two media on either side of this interface must be different.

  • The image display is based on the assumption that the ultrasound beam travels in a straight line.

  • Misplacement of echoes.

  • Edge shadows(narrow shadow directly beneath the margins of the rounded structure).

  • Duplication of structures (lens artifact).

  • Duplication of structures are most commonly seen in the subcostal views.

Clinical example

  • There appears to be two left ventricles.

Avoiding Lens Artifact

  • Move away from the offending structure so that the ultrasound beam does not strike the offending structure at an oblique angle.

Mirror Artifacts

  • Duplication of a structure because there is a large acoustic impedance mismatch between an interface and a specular reflector.

  • Structures located in front of the mirror are interrogated and displayed twice.

  • Example: Duplication of heart structures, in particular mitral valve, posterior to the pericardium.

Differentiating Refraction Duplication from Mirror Artifact

  • If the duplication is side by side, this is likely to be a refraction artifact.

  • If the duplication is behind the real structure, then this is likely to be a mirror artifact.

  • The mirror will also be seen between the two duplicated structures.

Violated Assumptions for Beam Path Artifacts

  • Round trip time is directly related to the depth of the reflector from the transducer.

  • The ultrasound beam travels in a straight line and reflects just once.

  • Reverberation Artifact: Bouncing of the beam between two strong reflectors.

  • Refraction Artifact: Bending of the beam away from its original path.

  • Mirror Artifact: Reflection of the beam when it strikes a large specular reflector.

Determining if a Structure is Real or an Artifact

  • As a general rule of thumb, an artifact is not usually seen in two imaging planes, whereas real structures are seen in at least two imaging planes.

Ultrasound Machine Assumptions

  1. The rate of attenuation is 1\,\text{dB/cm/MHz}.

  2. Echoes arise from the centre of a razor-thin beam.

  3. The propagation speed is 1540 \,\text{m/s}.

  4. The round trip time equals the depth of the reflector.

  5. The beam travels in a straight line and reflects only once.

Artifact Summary

  • Attenuation rate is not 1\,\text{dB/cm/MHz}: Refraction artifacts, acoustic enhancement/shadowing, improper brightness.

  • Echoes do not arise from the centre of a razor-thin beam: Beam dimension artifacts, beam width, slice thickness, sidelobe/grating lobe artifacts, added structures, most obvious in echo-free spaces.

  • Propagation speed is not 1540\,\text{m/s} and round trip time does not equal the distance of a reflector: Propagation speed artifacts, structures in the wrong place or of the wrong size.

  • Round trip time does not equal distance to reflector and the beam doesn't travel in a straight line/reflect only once: Reverberation, refraction, or mirror artifacts, duplication of structures or structures in the wrong place.