artifacts

ARTIFACT IN ULTRASOUND IMAGING

DEFINITION

  • Artifact: A part of an image seen on display that does not accurately represent anatomy.

IMAGE DISCREPANCIES

  • Common Issues in Artifacts:

    • Not real

    • Improper brightness

    • Improper shape

    • Improper size

    • Improper position

CAUSES OF ARTIFACTS

  • Violation of Assumptions

  • Equipment Malfunction or Design Flaws

  • Physics of Ultrasound: Not using the correct adjustments.

  • Interpreter Error: Refers to anatomic pitfalls.

  • Operator Error: Includes poor angles of insonation or wrong probe placement.

ASSUMPTIONS IN ULTRASOUND

  1. Sound travels in a straight line.

  2. Sound travels directly to the reflector and back.

  3. Sound travels at exactly 1,540 m/s.

  4. Reflections arise from structures positioned along the beam’s main axis.

  5. The imaging plane is extremely thin.

  6. The strength of the reflection is related to the characteristics of the tissues creating the reflections.

DYNAMICS OF ARTIFACTS

  • Artifacts can appear and disappear depending on the view.

  • Artifacts vanish when corrective measures are taken. Actual anatomy remains unchanged.

  • If artifacts fail to change, the function of the imaging instrument may be in question. Consulting a service engineer could be necessary.

CHARACTERISTIC TERMS IN ULTRASONOGRAPHY

  • Echogenicity: The extent to which a structure gives rise to reflections of ultrasonic waves.

  • Hyperechoic: Portions of an image that are brighter than surrounding tissues; also termed echodense or echogenic.

  • Hypoechoic: Portions of an image that are less bright than surrounding tissues or tissues that appear less bright than normal.

  • Anechoic: Areas which do not display sound, appearing as black regions in an image.

  • Isoechoic: Having the same echogenicity as surrounding tissue; also called sonoluscent.

TYPES OF IMAGE ECHOES

  • Homogeneous: Similar echotexture in a structure or within tissues.

  • Heterogeneous: Different echotexture in a structure or within tissues, mixing various echogenicities.

ARTIFACT EXAMPLES IN IMAGING

  • Hypoechoic Mass: Illustrated as seen in the testes; homogeneous appearance.

  • Heterogeneous Appearance: Seen in the kidney, where hyperechoic medulla indicates a distinction from surrounding tissues.

REVERBERATION ARTIFACT

  • Definition: Result of sound bouncing between two strong reflectors that are positioned parallel to the ultrasound beam or between the transducer and a strong reflector.

    • Visual representation resembles a "ping-ponging" effect, appearing like rungs of a ladder or a Venetian blind.

    • The number of bounces corresponds to the number of echoes (like ladder steps).

    • Assumption of direct sound travel to a reflector and back is invalid.

CHARACTERISTICS OF REVERBERATION

  • Multiple images equally spaced and parallel to the sound beam’s main axis.

  • Located at successively greater depths.

  • The first and second reflectors closest to the transducer are real; all others are artifacts.

REVERBERATION ARTIFACT VISUALS

  • Illustration depicting how reverberations create multiple representations of a rib, as well as echoes spaced along the depth.

COMET TAIL OR RING DOWN ARTIFACT

  • Definition: A variation of reverberation artifact with space compressed, appearing as a solid hyperechoic line directed downward; also referred to as flashlight artifact.

  • Creation: Happens due to closely spaced reverberations merging, typically arising when reflecting surfaces have very high propagation speeds (e.g., metal objects like mechanical heart valves).

  • Characteristics:

    • Solid hyperechoic band extending downward.

    • Located parallel to the main axis of the beam.

    • The assumption of sound traveling directly to the reflector and back is violated.

COMET TAIL AND RING DOWN EXAMPLES

  • Comet tail artifact from gas-filled bowel.

  • Ring down observed from bubbles in an intrauterine saline injection.

SHADOW ARTIFACT

  • Definition: Appears as a hypoechoic or anechoic region extending downward from a strong attenuating medium.

  • Occurs when attenuation is greater in the tissues above the shadow than in surrounding environments. This may be caused by interfaces like bone and soft tissues or gas.

CHARACTERISTICS OF SHADOW ARTIFACT

  • Appears hypo or anechoic (background color).

  • Develops due to excessive attenuation.

  • Leads to the absence of true anatomical representation on the scan.

  • Shadows are parallel to the beam’s main axis and helpful in diagnosing certain pathologies like stones.

  • Independent of sound propagation speed.

CLEAN AND DIRTY SHADOWING

  • Clean Shadowing: Thought to be produced by sound-absorbing materials like stones.

  • Dirty Shadowing: Caused by sound-reflecting materials (e.g., abdominal gas) or infections from bacteria generating gas.

REFRACTION ARTIFACT

  • Definition: Occurs when the sound beam changes direction during transmission.

  • Caused by:

    • Oblique angles of incidence.

    • Different propagation speeds in two mediums.

  • Degrades lateral resolution.

  • A second copy of the true reflector appears side-by-side with the true reflector at the same depth.

  • Often seen in scans through the rectus abdominis muscle, where extra beams are sent off-axis, and the system errors by assuming the beam was straight.

REFRACTION ARTIFACT EXAMPLES

  • Images demonstrating how refraction artifact may cause duplicated aortic images side by side at the same depth.

EDGE SHADOW ARTIFACT

  • Definition: A type of shadowing caused by refraction at the edge of a curved reflector.

  • Appears as a hypoechoic region extending from the edge of a curved structure.

  • Created when the beam diverges, reducing intensity.

CHARACTERISTICS OF EDGE SHADOWING

  • Generally hypo or anechoic due to diverging beams.

  • Extends downwards from the curved reflectors' edges.

  • Prevents visualization of true anatomy.

ENHANCEMENT ARTIFACT

  • Definition: The opposite of shadowing; occurs behind tissues with low attenuation, appearing as hyperechoic echoes from deeper structures.

  • Helpful in diagnosing cystic structures and unrelated to speed of sound.

EXAMPLES OF ENHANCEMENT

  • Illustrated enhancement artifacts noted in gallbladder and ovarian cysts.

FOCAL ENHANCEMENT ARTIFACT

  • Definition: A special form of enhancement appearing as hyperechoic regions from side to side. It resembles incorrect time gain compensation (TGC) settings.

  • Most prominent at the focus due to high intensity from beam focusing.

MIRROR IMAGE ARTIFACT

  • Definition: When a structure appears in two places on an ultrasound image due to sound bouncing off a strong reflector (mirror).

  • Assumption of direct sound beam travel is violated here.

CHARACTERISTICS OF MIRROR IMAGE

  • Presence of a second copy of the true reflector.

  • The second copy appears deeper than the actual anatomy.

  • The mirror lies directly between the artifact and true anatomy, displaying reflected objects equally spaced from the mirror plane.

SPEED OR RANGE ERROR ARTIFACT

  • Definition: Occurs when sound travels through a medium at speeds other than soft tissue speed (1,540 m/s).

    • Results in split or step-off appearances of structures at incorrect depths:

    • Fast Medium: Structures appear too shallow.

    • Slow Medium: Structures appear too deep.

RANGE AMBIGUITY ARTIFACT

  • Definition: Arises when reflections from deep structures arrive after the next pulse is transmitted, causing reflections to appear too shallow.

  • Can be resolved by increasing pulse repetition period (PRP) or lowering pulse repetition frequency (PRF).

LOBE ARTIFACTS

  • Appear due to transmitted acoustic energy outside the beam's main axis, creating unnecessary reflections at the same depth of true anatomy.

  • Two types: Side Lobes (from single crystal/ mechanical transducers) and Grating Lobes (from array transducers).

CHARACTERISTICS OF LOBE ARTIFACTS

  • Degrade lateral resolution and exhibit stronger amplitude than may be expected.

CORRECTING LOBE ARTIFACTS

  • Changing the angle of insonation can help discern true anatomy from lobe artifacts since real structures will always be visible regardless of angle.

SLICE THICKNESS ARTIFACT

  • Definition: Also known as partial volume artifact, occurs due to measurable thickness of the ultrasound beam.

  • Affected by slice thickness resolution – the thicker the slice, the greater the likelihood of displaying reflectors outside the actual imaging plane.

LATERAL AND AXIAL RESOLUTION ARTIFACTS

  • Lateral Resolution Artifact: Occurs when beam width is wider than the distance between two reflectors, causing them to appear as one.

  • Axial Resolution Artifact: Happens when long pulses strike closely spaced structures causing one reflection to appear for multiple structures. Improvement can be achieved using high-frequency transducers.

TEMPORAL RESOLUTION ARTIFACT

  • Definition: Relates to the ability to determine the position of reflectors relative to time, directly influenced by frame rate. Poor resolutions lead to decreased positioning accuracy.