Ultrasound Physics: Artifacts Vocabulary

Vocabulary terms and definitions related to imaging artifacts in ultrasound physics, including descriptions of artifacts, imaging assumptions, and noise.

Artifact

An error in imaging, resulting in reflections that are not real, not seen, or have incorrect shape, size, position, or brightness.

Hyperechoic

Portions of an image that are brighter than surrounding tissues, or tissues that appear brighter than normal.

Hypoechoic

Portions of an image that are not as bright as surrounding tissues, or tissues that appear less bright than normal.

Anechoic

An extreme form of hypoechoic, meaning entirely without echoes (echo-free).

Isoechoic

Describes structures with equal echo brightness.

Homogeneous

A portion of tissue or an image that has similar echo characteristics throughout.

Heterogeneous

A portion of tissue or an image that has differing echo characteristics throughout.

Assumption 1 of Imaging Systems

Sound travels in a straight line.

Assumption 2 of Imaging Systems

Sound travels directly to a reflector and back.

Assumption 3 of Imaging Systems

Sound travels in soft tissue at exactly $1,540\,m/s$.

Assumption 4 of Imaging Systems

Reflections arise only from structures positioned in the beam's main axis.

Assumption 5 of Imaging Systems

The imaging plane is very thin.

Assumption 6 of Imaging Systems

The strength of a reflection is related to the characteristics of the tissue creating the reflection.

Reverberation

Multiple, equally spaced echoes caused by the bouncing of the sound wave between two strong reflectors positioned parallel to the ultrasound beam.

Comet Tail (Ring Down)

A solid hyperechoic line directed downward created when closely spaced reverberations merge; it is also called ring down artifact.

Shadowing

A hypoechoic or anechoic region extending downward from a highly attenuating structure, preventing the display of true anatomic structures.

Edge Shadow (Shadowing by Refraction)

A hypoechoic region extending down from the edge of a curved reflector where the sound beam refracts and diverges, resulting in a drop in intensity.

Enhancement

A hyperechoic region beneath tissues with abnormally low attenuation; it is entirely unrelated to the speed of sound.

Focal Enhancement (Focal Banding)

A hyperechoic side-to-side region of an image caused by increased intensity in the focal zone of a strongly focused beam.

Mirror Image Artifact

Created when sound reflects off a strong reflector (mirror) and is redirected toward a second structure, placing a second copy of the structure deeper than the real one.

Crosstalk

A mirror image artifact that specifically appears on a spectral Doppler display.

Speed Error (Range Error Artifact)

Created when a sound wave propagates at a speed other than $1.54\,km/s$; it results in the correct number of reflectors placed at incorrect depths.

Lobe Artifact

Artifact appearing when sound energy is transmitted in a direction other than along the beam's main axis; it degrades lateral resolution.

Side Lobes

Lobe artifacts created by a single crystal transducer, such as a mechanical probe.

Grating Lobes

Lobe artifacts created by array transducers.

Subdicing

The process of dividing each PZT element into small pieces to reduce grating lobe artifact.

Apodization

The process of differential excitation (applying higher voltages to center elements and lower voltages to outer elements) to reduce grating lobes.

Refraction Artifact

A second copy of a reflector appearing side by side or at the same depth as the true reflector, created when a pulse changes direction during transmission.

Slice Thickness (Section Thickness)

Artifact where reflections from structures above or below the assumed imaging plane appear in the image; also called partial volume artifact.

Lateral Resolution Artifact (Point Spread)

Occurs when a pair of side-by-side reflectors are closer than the beam width, causing two objects to appear as one or as a wide line.

Axial Resolution Artifact

Created when a long pulse strikes two structures parallel to the beam that are closer than one-half the spatial pulse length.

Multipath

Artifact created when sound pulses glance off a second structure on the way to or from the primary reflector, making the transmit path length different from the return path length.

Temporal Resolution

The ability to precisely locate a moving structure at all times; it is best with high frame rates.

Spatial Resolution

Related to the overall detail in an image; determined by factors including line density, axial resolution, and lateral resolution.

Pixel Density

The factor that determines spatial resolution in a digital display; higher density results in better resolution.

Range Ambiguity Artifact

Occurs when a reflecting structure deeper than the imaging depth is interpreted as being created by the second pulse and placed at a shallow location.

Acoustic Speckle

Noise resulting from the constructive and destructive interference of small sound wavelets, creating a grainy tissue-like texture.

Clutter

A form of noise consisting of false echo signals arising from locations outside of the main sound beam, such as side lobes or grating lobes.

Harmonic Imaging

Imaging technique that reduces noise content and increases the signal-to-noise ratio by selectively distinguishing meaningful reflections from noise.