greyscale artifacts

Artifacts:

Echoes that do not represent real interfaces such as

  • Missing echoes

  • Improperly located (misplaced) structures

  • Improper brightness, shape or size of structures

Basic assumptions of ultrasound:

  • Sound travels in straight lines

  • Echoes originate only from objects located on beam axis

  • Amplitude of returning echoes is related directly to reflecting properties of distant objects

  • Sound travels at 1540 m/s

Slice thickness

Dimension of US beam perpendicular to scan plane (3rd dimension)

  • Beam thickness includes cystic & solid structure, brightness is assigned as intermediate of the two creating debris in cystic structure, causes “fill-in”

  • Resolve with tissue harmonics

Acoustic speckle

Interference effects of scattered sound from distribution of all scatterers within tissue

  • From constructive & destructive wave interference produces pattern of bright & dark spots (granular appearance)

Reverberation

Multiple reflections occur between 2 strong reflectors or between transducer & strong reflector, may be interpreted as additional reflectors that do not represent real structures

  • Multiple reflectors displayed beneath real reflector seen at equal intervals with subsequent weaker reflections

Comet-tail

Trail of bright successive echoes caused by multiple echoes from small strongly reflective object

Ex. IUDs, surgical clips, calcifications…

Ring-down

Similar to comet-tail but caused by gas bubbles that oscillates at a resonance that creates echoes in image

Mirror Image

Occurs when specular reflectors & scatterers are close to highly reflective curved interface

  • Sound beam is redirected by specular reflector or secondary scaterer & creates improper location of echoes

    • Machine tries to straighten out the path 

Multipath artifact

Pulse that is redirected before returning to transducer causes false image to appear at deeper depth

Refraction

Bending of sound beam occur at interfaces between 2 media which have different propagation speed © 

  • Change in speed causes sound to bend & there is incorrect placement of echoes

  • Ex. edge shadowing

Double image artifact

Refraction of beam at fat-rectus abdominis muscle

  • Causes duplicated superior mesenteric artery or duplication of one border of foley’s catheter in the bladder

  • Can occur in colour or grey scale

grating/side lobe artifact

secondary , low-intensity beams outside main US beam

  • When outer beam encounter highly reflective structure echo returns to transducer & are incorrectly positioned

  • Structure appears as if it came from path of main beam

Grating lobe results form spacing & sequence of firing of array transducer

  • Tightly curved, convex array transducer are more susceptible as are higher frequency linear transducers

Propagation Speed artifact

US system is calibrated for propagation speed 1540m/s & assumes this speed through all tissues & assigns positions according

  • Longer it takes for echo to return, the deeper echo will be displayed

    • Incorrect axial location of echoes/structures of incorrect size & shape appears if propagation speed is not 1540m/s

    • If propagation speed > 1540m/s = closer than true position (smaller)

    • If propagation speed <1540m/s = farther than true position (larger)

  • Ex. appearance of break in diaphragm from liver lesion with <1540m/s

Range Ambiguity

Pulse may be emitted before all echoes from previous pulse have been received. Early echoes from pulse just emitted are simultaneously received with late echoes

  • US system incorrectly places echoes from distant reflectors too close to transducer

Shadowing

Reduction or elimination of echo amplitude from reflectors that lie behind strong reflector or strong absorber

  • Can be helpful in identifying gallstones

“Clean” shadowing: shadowing caused by strong absorbers

“Dirty” shadowing: shadowing from gas that reflects most of sound energy

Enhancement

Increase in echo brightness displayed distal to low attenuating structure

  • Helps distinguish solid from cystic lesions

Focal banding

Form of enhancement that occurs in region of tissue near focus on some US systems

  • Appears as overly bright area

Anisotrophy

Difference in echogenicity of structure caused by orientation to the US beam

  • Specular reflector perpendicular to US beam = inc. reflections

  • Specular reflector oblique to US beam = dec. reflections

Tendons…

Perpendicular to US beam = echogenic tendons

          Oblique to US beam = hypoechoic tendons (can mimic tendon tears)