Ultrasound Artifacts and Imaging Characteristics

A complete set of vocabulary terms covering ultrasound imaging characteristics, assumptions, and various types of artifacts.

Anechoic

Without echoes.

Hyperechoic

Brighter than surrounding tissue.

Hypoechoic

Not as bright as surrounding tissue.

Isoechoic

Equal echo brightness.

Homogeneous

Similar echoes throughout.

Heterogeneous

Different echo brightness throughout.

Artifact

Error in imaging that appears in some views and disappears in others, whereas actual anatomy appears in both views.

Assumption #1

Sound travels in a straight line.

Assumption #2

Sound travels directly to a reflector and back.

Assumption #3

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

Assumption #4

Reflections arise only from the beam's main axis.

Assumption #5

Thin imaging plane.

Assumption #6

Strength of a reflection is related to characteristics of the tissue that creates the reflection.

Reverberation

Equally spaced echoes in multiples parallel to the beam's main axis caused by the bouncing of the sound wave between two strong reflectors; the first 2 reflections are real.

Shadow

Hypoechoic/anechoic region behind a structure with high attenuation; breaks assumption #6 and is unrelated to the speed of sound.

Comet Tail

Also known as Ring Down Artifact; reverberation with the "spaces squeezed out" appearing as a solid hyperechoic line caused by closely spaced reverberations or resonance.

Edge Shadow

Also called shadowing by refraction; shadowing on the edge of a structure caused by sound hitting a curved reflector resulting in beam divergence.

Enhancement

Hyperechoic region beneath a structure with low attenuation; breaks assumption #6.

Focal Enhancement

Also called focal banding; a form of enhancement displaying a side-to-side hyperechoic band at the focus where sound is most intense.

Mirror Image

A replication of anatomy located deeper than the real structure, created when a beam hit a strong reflector; breaks assumptions #1 and #2.

Crosstalk

A mirror artifact that specifically appears on spectral Doppler.

Speed Error

Also called range error artifact; occurs when sound travels at a speed other than $1,540\,m/s$, resulting in reflectors displayed at incorrect depths.

Faster than soft tissue error

The system assumes the reflector is shallower and underestimates the distance because the go-return time is too short.

Slower than soft tissue error

The system assumes the reflector is deeper and overestimates the distance because the go-return time is too long.

Side Lobes

Lobe artifacts created by a single crystal transducer (mechanical probes).

Grating Lobes

Lobe artifacts seen in phased array probes with multiple elements; reduced by apodization and subdicing.

Subdicing

Dividing each PZT element into small pieces to reduce grating lobes.

Refraction

A side-by-side copy of anatomy at the same depth as the true reflector, caused by a pulse changing direction during transmission via oblique incidence and different propagation speeds.

Lateral Resolution Artifact

Also called point spread artifact; occurs when two reflectors are closer than the width of the sound beam and appear as one.

Slice Thickness Artifact

Also known as section thickness or partial volume artifact; occurs when the beam dimension is larger than the reflector size, filling in anechoic structures like cysts.

Axial Resolution Artifact

Created when two long pulses strike objects placed one in front of the other; only one reflector appears if objects are closer than $\frac{1}{2}$ the spatial pulse length.

Multipath

An artifact that occurs when a pulse hits a second reflector on the way to or from a reflector; it cannot be specifically identified on the image.

Range Ambiguity Artifact

Occurs when deep reflectors are displayed in a shallow location because returned echoes are displayed later than expected; eliminated by increasing the Pulse Repetition Period.

Noise

Small amplitude echoes resulting from electrical interference, signal processing, and reflections that affect low-level hypoechoic regions.

Temporal Resolution Artifact

Relates to the capability to precisely position move structures; determined by frame rate.

Speckle

Also called acoustic speckle; a form of noise caused by small amplitude sound waves inhibiting one another; reduced by spatial compounding.

Spatial Resolution Artifact

Relates to the detail of an image; determined by line density, axial and lateral resolution, and pixel density in digital displays.

Harmonic Imaging

A technique that reduces noise and increases the signal-to-noise ratio.