Artifacts

Attenuation-

the weakening of sound while it propagates 

Shadowing/ enhancement are attenuation artifacts

What affects shadow-

reduction in echo amplitude from reflectors that lie behind a strongly reflecting or attenuating structures; can also occur behind the edges of objects that are not strong attenuators- may be the defocusing action of a refracting curved surface

What increases Shadowing?

highly attenuating structures (bone, gas, calcified stones, high impedance)

What decreases shadowing?

adjust TGC, angle, standoff pad, adjust the frequency); amplitude is too low. Compound Imaging can eliminate shadowing. 

Lobes

Weaker beams that come of the beam’s main axis

Lobes USF

 degrade lateral resolution. A copy of the true reflector. Artifact and the true reflector will be side by side at the same depth. Grating lobes duplicate structures laterally to the true ones. GL artifacts are normally weaker than the correct presentation of the structure. 

What can fix reverb?

harmonics

Reverb defined:

Multiple reflections between a structure and the transducer, between structures, or within a structure. Echoes bouncing back and forth. Equally spaced and positioned parallel to the beam

Three types of reverb

Mirror, ringdown, comet tail

Reverb USF

resembles a venetian blind or a ladder; appear in multiples, equally spaced, parallel to beam, increasing depth, becoming weaker as it travels. Displayed beneath the real reflector. 

Lateral and axial artifacts

Lat(point spread artifact): Refraction, Lobes,

Ax: Prop speed (Speed Error), enhancement, shadowing, mirror, ringdown, comet tail

Aliasing-

 most common artifact with doppler, caused by undersampling (intermittent, insufficient) of the doppler shifts. 

Has pulses

NL= PRF/2; eliminate by Using CWD, adjust the scale (PRF), change angle, lower frequency, shift baseline; 

Enhancement

 strengthening of echoes from reflectors that lie behind a weakly attenuating structure

Enhancement USF-

 hyperechoic, result of too little attenuation, beneath a structure with abnormally low attenuation. Results from amplitudes that are too high

Refraction

 bending of the transmitted beam. Degrades Lateral resolution. To cause refraction there must be oblique angles, and different prop speeds.

Why does refraction occur?

when a sound wave strikes a boundary obliquely and the media on either side of the boundary have different prop speeds. 

Refraction USF

Duplicate the structure, change the size, shape and location; copy is side by side or at the same depth as the true reflector,

Range ambiguity

PRF is too high, machine does not know where the sample is coming from. It adds an extra sample gate.  occurs when a pulse is emitted before all the echoes from the previous pulse have been received. Early echoes from the last pulse are received simultaneously with late echoes from the previous pulse. 

What causes range ambiguity

High PRF, and multiple samples appear as a result.

- Eliminated by increasing PRP. Longer PRP means deeper imaging and decreased PRF

Speckle-

 granular appearance of images that is caused by the interference of echoes from the distribution of scatterers in tissue. 

Speckle characteristics:

Can make images look grainy

Caused by interferences of scatter

Interference is constructive and destructive. 

Decreases contrast resolution.

Prop speed is also called

Speed error artifact

 Faster propagation speed =

 shorter depth (reflector placed too close together)

Slower propagation speed=

 longer depth ( reflector is placed too far away)

Prop speed- Speed Error Artifact

Occurs when the assumed value for speed of sound in soft tissues is incorrect. 

6 assumptions:

1. Sound travels in a straight line

2. Sound travels directly to a reflector and back

3. Sound travels in soft tissue at exactly 1,540 m/s (1.54 mm/ms)

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

5. Imaging plane is very thin

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

Shadow by refraction-

Edge Shadow;  it is an attenuation artifact. Occurs behind the edges of objects that are not strong attenuating. Defocusing action of a refracting curved surface

Why does refraction occur?

Sound beam refracts (transmits with a bend) at the edge of a curved reflector. 

Partial volume thickness-

Slice Thickness

 the beam width perpendicular to the scan plane (3rd dimension). The fill-in of small cystic spaces.

USF of partial volume thickness

fill in of small cystic spaces, Fixed focus and tissue harmonics along with a 1 ½ dimensional array can be used to fix this. 

What malfunctions can cause artifacts?

Machine and electrical

Artifacts 

 inncorrect representations of anatomy or function. Error in imaging. 

advantage and disadvantage of CWD

advantage: does not alias; disadvantage: range ambiguity ( poor depth resolution)

advantage and disadvantage of PWD

advantage: good depth resolution; disadvantage: subject to aliasing.

Clutter

 artifact that results from tissue, heart wall or valve, or vessel wall motion. A form of Noise artifact.

How to fix clutter

 increase wall filter to eliminate ghosting/bleeding/ clutter

Shadowing and enhancement are attenuating artifacts

Mirrored image

sound reflects off a strong reflector and is redirected toward a second structure. 

USF of mirrored image

 Appears as a copy of a true reflector, artifact appears deeper than the true reflector, a bright reflector, the mirror, lies on a straight line between the artifact and the transducer, true reflector and artifact are equal distances from the mirror.  Image is flipped posteriorly to the true image

What causes a mirrored image?

 sound beams reflect off a strong reflector

Cross talk-

 electronic duplication of the spectral information

Form of mirrored image

How to fix cross talk

 turn down gain, adjust angle. 

where does cross talk appear?

mirror artifact that occurs on spectral doppler display. 

Lateral resolution-

 refers to beam diameter and occurs when a pair of side-by-side reflectors are closer than the width of the sound beam. Two objects appear as one. 

Focal banding-

 a form of enhancement; - Brightening of echoes caused by the intensity in the focal region of a beam because the beam is narrow there. 

Prominent hyperechoic horizontal line across the screen at the depth of the focus

What causes ringdown and what causes comet tail-

comet tail : metal

   Ringdown- gas/air

Reverberation results from

 reflecting off surfaces that are located in a medium w/ a very high prop speed.

AVG SPEED OF SOFT TISSUE

 1.54 mm/ms

1,540m/s

WHAT DOES SLICE THICKNESS ARTIFACT DO TO IMAGES-

fills in small cystic spaces.

what does scatter redirect?

sound

Lateral resolution is also known as:

point spread artifact- Move an object, duplicate an object - spreads the small reflector to display as a wide line rather than a narrow dot.