ULTRASOUND ARTEFACTS

INTRODUCTION

*An ultrasound artifact is a structure in an image which  does not directly similiar with actual tissue being  scanned.

*Artifact assumes different forms including :

•Structures in the image that are  not actually present

•Objects that should be represented but are missing from the image.

•Structures which are misregistered on the image.

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TYPES OF ARTEFACTS

  1. REVERBERATION
  2. ACOUSTIC SHADOWING
  3. ACOUSTIC ENHANCEMENT
  4. EDGE SHADOWING
  5. BEAM WIDTH ARTIFACT
  6. SLICE THICKNESS ARTIFACT
  7. SIDE LOBE ARTIFACT
  8. MIROR IMAGE
  9. DOUBLE IMAGE
  10. EQUIPMENT-GENERATED ARTIFACT
  11. REFRACTION ARTIFACT

REVERBRATION

•  This is the production of false echoes due to repeated  reflections between two interfaces with a high acoustic  impedance mismatch.

•The echo from the interface is received by the transducer  and displayed on the image.

•Some of the energy in the returned echo is reflected at the  transducer face, and return to the reflecting interface as if  it was a weak transmitted pulse, returning as a second  echo.

•As the time taken for the second echo to arrive is twice  that taken by the depth.

•This sequence of reflection and transmission can occur many times, with the third echo taking three times as long  to return to the transducer and being displayed at three  times the depth, and so on.

•The reverberation echoes will be equally spaced because  the time for each additional echo is multiple of the time of  return of the first echo.

• These reverberation echoes will be strong because of the  high acoustic mismatch.

This artifact will be seen at the skin-transducer interface  and behind bowel gas.

Rectification:

•Increase the amount of gel used.

•Use a stand-off pad.

•Reduce the gain.

•Move the position of the transducer.

ACOUSTIC SHADOWING

•This appears as an area of low amplitude echoes behind an  area of strongly attenuating tissue.

•It is caused by severe attenuation of the beam at an  interface, resulting in very little sound being transmitted  beyond.

•The attenuation can be due to either absorption or  reflection of the sound waves, or a combination of the  two.

•Acoustic shadowing will occur at interfaces with large  acoustic mismatch such as:

1.Soft tissue and gas

2.Soft tissue and bone or calculus

ACOUSTIC ENHANCEMENT

•This artifact appears as a localized area of increased echo  amplitude behind an area of low attenuation. •On a scan it will appears as an area of increased brightness,  and can commonly be seen distal to fluid-filled structures  such as the urinary bladder, GB or a cyst.

•The artifact arises due to the application of the time-gain  compensation(TGC) to areas of low attenuating structures  such as fluid.

•It is caused by the low level of attenuation of the beam as it  passes through fluid relative to the greater attenuation of  the beam in the adjacent more solid tissue.

•This artifact can often be an useful diagnostic aid,  particularly when scanning a soft-tissue  mass or cyst  containing low level echoes.

•These echoes may often cause the structure to disappear in the image as it blend into the surrounding echo pattern.

EDGE SHADOWING

•A combination of refraction and reflection occurring at the  edges of rounded structures will result in edge shadowing  artifact.

•It arises due to refraction of the beam caused by both the  curvature of the rounded edges and difference in speed of  two materials.

•When the ultrasound beam reaches the rounded edge of a  structure, reflection will occur, with an angle of incidence  equal to the angle of reflection.

•The outer part of the beam will be totally reflected, but  the reminder of the beam passes through the rounded  structure and is refracted.

•This combination of reflection and refraction of the  beam at the edges of a rounded structure results in a  thin strip of tissue behind the edge not being insonated  and causes a shadow.

BEAMWIDTH ARTIFACTS

•This artifact can be demonstrated by scanning a point  reflector in a phantom, where the display will clearly  portray this as a line.

•During routine scanning, the artifact can be seen when  spurious echoes  are displayed in an echo-free area.

•Correct positioning of the focal zone will help to reduce  this artifact.

•The focal zone is controlled by electronically narrowing the beam.

SLICE THICKNESS ARTIFACTS

•These occurs due to the thickness of the beam, and are  similar to beam width artifacts.

•These artifacts will typically be seen in transverse views of  the urinary bladder when structures adjacent to the slice  through the bladder being scanned will be incorporated  into the image.

•These echoes are then displayed as if they were arising  from within the bladder.

•Although the appearance of this artifact is similar to the  beam width artifact, the differentiating factors is that the  reflector causing the slice thickness artifact will not be  seen on the display.

SIDE LOBE ARTIFACTS

•The energy within the ultrasound beam exists as several  side lobes radiating at a number of angles from a central  lobe.

•Echoes are generated by these side lobes in addition to  the main lobe, but all the returning echoes are assumed  by the transducer to have arisen from the central axis of  the main lobe.

•Side lobe echoes will therefore be misregistered in the  display.

•This artifact can often be seen in area such as the urinary  bladder and may also arise within a cyst.

MIRROR IMAGE ARTIFACTS

•These artifacts results in a mirror image of a structure  occurring in an ultrasound display.

•They arise due to specular reflection of the beam at a  large smooth interface.

An area close to a specular reflector will be imaged twice,  once by the original ultrasound beam and once by the  beam after it has reflected off the specular reflector.

•Mirror image artifacts are most commonly seen where  there is a large acoustic mismatch, such as a fluid-air  interface.

•Typically this artifact can occur during the scanning of a  full bladder, when air in the rectum behind the bladder  act as specular reflector and mirror image of the  bladder is displayed posteriorly.

•It will then have the appearance of a large cyst behind  the bladder.

•It can also be seen when scanning the liver, and the  diaphragm act as a specular reflector.

DOUBLE IMAGE ARTIFACTS

•This image is caused by refraction of the beam and may  occur in areas such as the rectus abdominis muscle on the  anterior abdominal wall. •

In the transverse plane the edges of the muscle act as a  lens and the ultrasound beam to be refracted and this  causes the single structure to be interrogated by two  separate refracted beams.

•Two sets of echoes will therefore be returned and these  will cause display of two structures in the image.

EQUIPMENT GENERATED ARTIFACT

•Incorrect use of the equipment controls can lead to artifact  appearing.

•Misuse of controls such as the gain or TGC can result in  echoes being recorded as too bright or too dark. •Care must be taken when setting these controls, to ensure  an even brightness throughout the image.

•If the dynamic range control is incorrectly set, this can lead  to an image which has too much contrast, and result in the  loss of subtle echo information.

•Gain must be in medium level.

•Blurring of a moving image can occur if the frame rate is  too low or if the persistence is too high.

•It is important to ensure that the frame rate is capable of  recording a moving structure at the speed.

•Use of multiple focal zones can give rise to a prominent  banding effect within the image.

REFRACTION ARTIFACT

•The refraction is the change of the sound direction on  passing from one medium to another.

•In ultrasound, refraction is due to sound velocity  mismatches combines with oblique angles of incidence,  most commonly with convex scanheads.

•When the ultrasound wave crosses at an oblique angle the  interface of two materials, through which the waves  propagate at different velocities, refraction occurs, caused  by bending of the wave beam.

•Refraction artifact cause spatial distortion and loss of  resolution in the image.

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