Ultrasound Physics: Artifacts

Comprehensive vocabulary flashcards covering the types, causes, and characteristics of artifacts in ultrasound physics as described in the lecture notes.

Artifact

Essentially an error in imaging, resulting in reflections that are not real, not seen on the image, have incorrect shape or size, incorrect position, or incorrect 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.

Soft Tissue Propagation Speed Assumption

The assumption that sound travels in soft tissue at exactly $1,540\,m/s$.

Reverberation

Multiple, equally spaced echoes caused by the bouncing of the sound wave between two strong reflectors positioned parallel to the ultrasound beam, resembling a ladder or Venetian blind.

Comet Tail

Also known as ring down artifact; it appears as a solid hyperechoic line directed downward, created when closely spaced reverberations merge.

Shadow

A hypoechoic or anechoic region extending downward from a highly attenuating structure, appearing the same color as the image background.

Edge Shadow

Also called shadowing by refraction; a hypoechoic region extending down from the edge of a curved reflector caused by the beam refracting and diverging, resulting in a drop in intensity.

Enhancement

A hyperechoic region beneath tissues with abnormally low attenuation, appearing brighter and the same color as the foreground.

Focal Enhancement

Also called focal banding; a side-to-side hyperechoic region of an image caused by increased intensity at the focus.

Mirror Image Artifact

A replica or second copy of a structure that incorrectly appears on the image deeper than the real structure, created when sound reflects off a strong reflector.

Crosstalk

A mirror image artifact specifically appearing on a spectral Doppler display.

Speed Error Artifact

Also called range error artifact; created when a sound wave propagates through a medium at a speed other than $1,540\,m/s$, appearing as a step-off where reflectors are at incorrect depths.

Speed Error (Slower Medium)

When the medium's speed is slower than soft tissue, go-return time is too long, the system assumes reflectors are far, and they are placed too deep on the image.

Speed Error (Faster Medium)

When the medium's speed is faster than soft tissue, go-return time is too short, the system assumes reflectors are close, and they are located too shallow on the image.

Lobes

Artifacts appearing when sound energy is transmitted in a direction other than along the beam's main axis, creating a second copy of the true reflector side by side at the same depth.

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 where elements closer to the beam center are excited with higher voltages than outermost elements to reduce grating lobes.

Refraction Artifact

Created when a sound pulse changes direction during transmission (transmission with a bend), resulting in a second copy of the reflector at the same depth as the true reflector.

Slice Thickness Artifact

Also called section thickness or partial volume artifact; it occurs because the imaging plane has a three-dimensional thickness, filling in hollow structures like cysts.

Lateral Resolution Artifact

Also called point spread artifact; occurs when side-by-side reflectors are closer than the beam width, appearing as one reflection or a wide line.

Axial Resolution Artifact

Created when a long pulse strikes structures parallel to the beam that are closer than $\frac{1}{2}$ the spatial pulse length, resulting in only one reflection.

Multipath

Created when sound pulses glance off a second structure on the way to or from the primary reflector, resulting in different transmit and return path lengths.

Temporal Resolution Artifact

The inability to accurately position moving reflectors due to low frame rates.

Spatial Resolution

The overall detail in an image, determined by line density, axial resolution, lateral resolution, and pixel density.

Range Ambiguity Artifact

Occurs when a reflecting structure deeper than the imaging depth is placed at a shallow location; it is eliminated by increasing the pulse repetition period (PRP).

Acoustic Speckle

Noise resulting from the constructive and destructive interference of small sound wavelets, giving the image a grainy appearance.

Clutter

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

Harmonic Imaging

A method used to reduce noise and increase the signal-to-noise ratio by selectively distinguishing meaningful reflections.