PHYSICS 2 ARTIFACTS ✅

Artifact

An echo that does NOT represent real anatomy; can appear as structures that aren't real, missing anatomy, wrong location/depth, or wrong brightness/shape; key clue = disappears when you change angle or imaging plane.

Machine Assumptions

Sound travels in a straight line; speed = 1540 m/s (1.54 mm/µs);

echoes come from the beam axis only;

reflection strength equals tissue characteristics;

imaging plane is infinitely thin.

Dual Nature of Artifacts

Artifacts can mislead diagnosis (bad) OR help confirm diagnoses (good);

shadowing helps detect stones;

enhancement confirms fluid-filled structures.

Artifact Classification

Four types:

Propagation artifacts,

attenuation artifacts,

resolution artifacts,

Doppler artifacts.

Axial Resolution Artifact

Occurs when two reflectors are closer together than SPL/2;

they appear as ONE structure instead of two;

reflectors must be parallel to the beam axis;

fix = use higher frequency transducer.

SPL (Spatial Pulse Length)

Number of cycles × wavelength; determines axial resolution;

shorter SPL = better axial resolution = fewer axial artifacts;

improved by using higher frequency.

Lateral Resolution Artifact

Occurs when two reflectors are closer together than the beam width; they appear merged into one structure;

reflectors must be perpendicular to the beam axis;

fix = narrow beam, proper focal zone, higher frequency.

Elevational Resolution Artifact

Caused by beam thickness in the elevation plane;

adds false echoes inside structures;

example: cyst appears to contain debris when it doesn't;

fix = harmonic imaging.

Contrast Resolution

Ability to distinguish different shades of gray;

improved by increasing bit depth/gray scale or using B-scale;

artifacts occur when gray shade differences are too subtle to display.

Spatial Resolution (Overall)

Combined image detail from pixel density, line density, and display monitor lines;

improved by write magnification;

pixel density itself cannot be changed by the operator.

Speckle

Grainy 'salt and pepper' noise throughout the image;

caused by small amplitude sound waves interfering with each other;

fix = harmonic imaging.

Refraction

Beam bends at a tissue interface, producing a duplicate side-by-side image of a structure;

fix = change transducer angle.

Mirror Image Artifact

A duplicate of a real structure appears deeper than the actual structure;

caused by a strong reflector (most commonly the diaphragm);

fix = change angle or adjust gain.

Multipath Artifact

Sound beam takes multiple different paths back to the transducer;

causes echoes to appear at the wrong depth;

machine assumes single straight-line path.

Reverberation Artifact

Sound bounces repeatedly between two strong reflectors;

produces evenly spaced lines;

only the first echoes are real;

fix = use alternative acoustic window, decrease near-field TGC.

Comet Tail Artifact

A type of reverberation;

short, tapering posterior echoes;

caused by metal or cholesterol crystals;

distinguishing feature = tapers and shortens with depth.

Ring Down Artifact

A type of reverberation; long echoes that expand posteriorly;

caused by gas bubbles;

distinguishing feature = continues and expands rather than tapering.

Propagation Speed Error

Occurs when tissue speed differs from the assumed 1540 m/s;

faster medium → structure appears too shallow;

slower medium → structure appears too deep.

Focal Banding

Increased image intensity caused by multiple focal zones creating overlapping areas of brightness;

fix = decrease number of foci or change foci location.

Side Lobes

False echoes outside the main beam produced by a single-element transducer;

creates artifactual echoes inside fluid-filled structures.

Grating Lobes

False echoes outside the main beam produced by array transducers;

similar effect to side lobes but specific to arrays;

fix = apodization or subdicing.

Apodization

Technique to reduce side lobes and grating lobes;

excites center elements with higher voltages and outer elements with weaker voltages.

Subdicing

Process of cutting a single transducer element into many smaller sub-elements;

helps reduce grating lobes.

Shadowing

Dark area posterior to a strongly attenuating or reflecting structure;

two types = clean and dirty; very important clinically.

Clean Shadowing

Dark anechoic shadow posterior to a highly reflective smooth interface;

produced by stones; shadow has sharp, clear edges.

Dirty Shadowing

Cloudy or echogenic shadow posterior to a structure;

produced by gas;

less well-defined than clean shadowing.

Enhancement (Posterior Acoustic Enhancement)

Bright area posterior to a weakly attenuating structure;

confirms fluid-filled structures.

Aliasing

Most common Doppler artifact; occurs when blood flow velocity exceeds the Nyquist limit;

waveform appears reversed or wrapped around baseline.

Nyquist Limit

Maximum measurable Doppler shift before aliasing occurs; = PRF ÷ 2.

HPRF (High Pulse Repetition Frequency) Doppler

Special mode of PW Doppler using multiple sample volumes along the same scan line;

main goal = reduce aliasing.

Crosstalk

Mirror image of the spectral Doppler waveform appearing on the opposite side of the baseline;

caused by Doppler gain set too high.

Range Ambiguity

PRF set too high → next pulse sent before previous echoes return → echoes appear to come from wrong depth.

Ghosting (Clutter)

Low-frequency Doppler shifts caused by slow-moving tissue and vessel walls;

fix = wall filter.

Twinkle Artifact

Rapidly changing color signal appearing posterior to a strong scatterer; helps detect stones.

Flash Artifact

Sudden burst of false color throughout the image caused by probe or patient motion; not related to actual blood flow.

Wall Filter

Removes low-frequency Doppler signals from slow-moving tissue and vessel walls;

if set too high, eliminates real slow-flow signals.

What is an artifact and what is the key clinical clue that something is an artifact?

An echo that does not represent real anatomy;

key clue = it disappears or changes when you change the transducer angle or imaging plane.

What are the 4 main machine assumptions that, when violated, produce artifacts?

Sound travels in a straight line;

speed = 1540 m/s;

echoes come only from the beam axis;

reflection strength = tissue characteristics; imaging plane is infinitely thin.

Can artifacts ever be useful clinically?

Yes — shadowing detects gallstones/kidney stones; enhancement confirms fluid-filled structures (cysts, bladder); twinkle artifact helps detect stones.

What are the 4 categories of ultrasound artifacts?

Propagation artifacts, attenuation artifacts, resolution artifacts, Doppler artifacts.

When does an axial resolution artifact occur and how do you fix it?

When two reflectors are closer together than SPL/2 and parallel to the beam; they merge into one; fix = use higher frequency transducer to shorten SPL.

When does a lateral resolution artifact occur and how do you fix it?

When two reflectors are closer together than the beam width and perpendicular to the beam; they merge; fix = narrow beam, proper focal zone, higher frequency.

What causes the slice thickness artifact and what is the classic example?

Beam thickness in the elevation plane adds false echoes from outside the imaging plane into the image; classic example = a simple cyst appears to contain internal debris.

How do you fix the slice thickness artifact?

Use harmonic imaging (produces narrower beam); use 1.5D arrays or disc-shaped elements that produce thinner slices in the elevation plane.

What is the difference between axial and lateral resolution artifacts in terms of reflector orientation?

Axial resolution artifact = reflectors parallel to the beam; lateral resolution artifact = reflectors perpendicular to the beam.

What causes speckle and how do you reduce it?

Small amplitude sound waves interfere with each other producing grainy noise; fix = harmonic imaging.

What is the mirror image artifact and what commonly causes it?

A duplicate of a real structure appears at a greater depth than the actual structure; most commonly caused by the diaphragm acting as a strong reflector.

What is the difference between comet tail and ring down artifacts?

Both are reverberation types; comet tail = short, tapering echoes caused by metal or cholesterol; ring down = long, expanding echoes caused by gas bubbles.

What does a reverberation artifact look like and what causes it?

Evenly spaced parallel lines deeper in the image; caused by sound bouncing repeatedly between two strong reflectors; only the first echoes are real.

How do you fix a reverberation artifact?

Use an alternative acoustic window; decrease near-field TGC; change transducer angle.

What happens with propagation speed error and in which direction does the error go?

If tissue speed is faster than 1540 m/s → structure appears too shallow; if tissue speed is slower → structure appears too deep.

What causes focal banding and how do you fix it?

Multiple focal zones create overlapping areas of intensity; fix = decrease the number of foci or change foci locations.

What is the difference between side lobes and grating lobes?

Side lobes = produced by single-element transducers; grating lobes = produced by array transducers (multiple elements); both create false echoes outside the main beam.

What is apodization and what does it fix?

Excites center elements with higher voltages and outer elements with weaker voltages; reduces side lobes and grating lobes.

What is subdicing?

Cutting a single transducer element into many smaller sub-elements; helps reduce grating lobes.

What is refraction and what does it produce on the image?

Beam bends at a curved tissue interface; produces a side-by-side duplicate of a structure.

What is the difference between clean and dirty shadowing?

Clean shadowing = sharp, anechoic shadow produced by stones; dirty shadowing = cloudy, echogenic shadow produced by gas.

What machine assumption does posterior acoustic enhancement violate?

The assumption that reflection strength is related only to tissue characteristics;

What structures produce posterior acoustic enhancement?

Fluid-filled structures — cysts, bladder, gallbladder (when no stones), any anechoic structure.

Why is shadowing clinically important?

It helps detect highly reflective or attenuating structures — gallstones, kidney stones, calcified arteries.

What is the most common Doppler artifact and what causes it?

Aliasing; occurs when blood flow velocity exceeds the Nyquist limit; waveform appears wrapped around or reversed.

What is the first step to fix aliasing in spectral Doppler?

Adjust/shift the baseline; then increase PRF; can also lower transducer frequency or use CW Doppler.

What is HPRF Doppler and why is it used?

High Pulse Repetition Frequency Doppler — a special PW Doppler mode; allows higher PRF to raise the Nyquist limit.

What causes crosstalk and what does it look like?

Doppler gain set too high or beam angle close to 90°; produces a mirror image of the spectral waveform.

What is range ambiguity and when does it occur?

PRF set too high → next pulse transmitted before echoes return → echoes appear at the wrong depth.

What is ghosting (clutter) and how do you fix it?

Low-frequency Doppler signals from slow-moving tissue; fix = increase wall filter setting.

What is the twinkle artifact and why is it useful?

Rapid flickering color signal posterior to a strong scatterer like a kidney stone; helps detect stones.

What causes flash artifact?

Sudden probe or patient motion during color Doppler scanning; produces a burst of false color across the image.

What happens if the wall filter is set too high?

Real slow-flow blood signals near the baseline are eliminated, missing clinically important low-velocity flow.

What is the key difference between aliasing and range ambiguity as Doppler artifacts?

Aliasing = PRF too LOW → Nyquist exceeded; range ambiguity = PRF too HIGH → echoes appear wrong depth.