ArtIfacts in usg In general the aim of any imaging modality is…. - PowerPoint Presentation

1. ArtIfacts in usg

2. In general the aim of any imaging modality is…. Improve SNR. Avoid saturation of the signal. Avoid loss of useful information. Reduce artifacts. Optimize resolution. Limit examination time.

3. ARTIFACTS Defined as a feature appearing in an ultrasound image that does not correspond to or represent an actual anatomical or pathological structure.

4. Classified into three types :

5. The ultrasound machine makes various assumptions in generating an image. These include:The ultrasound beam only travels in a straight line with a constant rate of attenuation. with constant speed speed (1540m/s.) as infinitely thin beam with all echoes originating from its central axis. in all body tissuesThe depth of a reflector is accurately determined by the time taken for sound to travel from the transducer to the reflector and return.

6. ARTIFACTS RELATED TO EQUIPMENT

7. ARTIFACTS RELATED TO EQUIPMENT ARTIFACTUAL NOISEIt is caused by electrical interference from nearby equipment.

8. MAIN BANG ARTIFACT Many echoes from the skin –transducer interface in the immediate subcutaneous tissue. More common with the old transducers. The matching layers in the new transducers prevent this. Too much Near Gain also causes the artifact. Disadvantages : The superficial lesions will be hidden.

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10. Correction technique: Decrease the near gain. Use a higher frequency transducer, Use stand-off pads to evaluate superficial structures.

11. SIDE LOBES Produce confusing echoes that arise from sound beams that lie outside the main ultrasound beam. Create impression of structures or debris in fluid filled structures. Eliminated by repositioning the transducer and proper focusing.The ultrasound beam exits the transducer as a complex three-dimensional bowtie shape with additional off-axis low-energy beams.These off axis beams also reflect corresponding to imaging object and reach probe and so cause artefact.

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13. The ultrasound beam exits the transducer as a complex three-dimensional bowtie shape with additional off-axis low-energy beams, which are referred to as side lobes and grating lobes (3,4) . Normally echoes produced from these two beams by the objects that are in their tract are not imaged as they are not sufficient in magnitude. A strong reflector located outside of the main ultrasound beam may generate echoes that are detectable by the transducer. These echoes will be falsely displayed as having originated from within the main beam. So the reflector should be strong and is within the tract of side lobes.

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15. GRATING LOBES Periodic spacing of elements by phased array, more commonly linear array. Grating lobe travel at an angle to the main beam .Depending on whether it hits after or before the main beam a curvilinear echo seen . Correction is by using different transducer or acoustic window

16. ARTIFACTS RELATED TO TECHNIQUE

17. ARTIFACTS RELATED TO TECHNIQUE NOISE Noise is created by excessive Gain. Disadvantage: excessive gain causes artifactual echoes in the fluid filled structures. Correction is by the reduction of the gain.

18. TGC PROBLEMS Artifacts can occur due to poor TGC adjustments. It can lead to excessive or few echoes from a depth. A type of poor TGC technique is BANDING.

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20. Correct adjustment

21. VEILING Bands of increased echogenicity seen at different depths. Occurs if multiple focal zones are utilized simultaneously.Disadvantage : Mass can be created or overlooked at the interfaces.Correction: Use only one focal zone. Adjust the TGC.

22. F1F2Apparent mass

23. ARTIFACTS CAUSED BY MOVEMENT Breathing. Operator scanning speed. Operator pressure.

24. ARTIFACTS CAUSED BY SOUND-TISSUE INTERACTION ARTIFACTS FROM STRONGLY REFLECTIVE STRUCTURESGas and Bone do not conduct the sound well.When the sound strikes these structures two effects can occur: 1.SHADOWING.2.REVERBERATION. The disadvantages are that the structures in the distal field are masked. It is however helpful in identifying structures like the calculi.

25. Reverberation Artifact.Reverberation artifact produces a series of parallel false images of flat interfaces that happen to lie parallel to the skin. They are the result of repeated reflections of the incident ultrasound pulse between the transducer and the flat surface or between two such surfaces within the body.

26. Reverberation Artifact.

27. The difficulty in measuring the thickness of the superficial intima–media layer of the carotid artery results from this artefact.

28. Shadowing Artifact

29. Shadowing occurs when little or no ultrasound can penetrate an interface and results in a dark band over the deeper tissues.Causes of shadowing 1.Absorption: calculi eg. Shadowing by fibrous tissue like scars, scirrhous breast carcinoma, Fibroid in uterus. 2. Reflection: e.g. tissue-gas interfaceShadowing Artifact

30. PROPOGATION VELOCITY ARTIFACTWhen US beam passing through a fat containing lesion, delayed echo return.Delay in echo return interpreted as deeper target.The image shows a mis-registration artifact.Ex., Broken diaphragm sign of hepatic lipoma,Shadowing at areas of multiple interfaces ex renal medulla,

31. PROPOGATION VELOCITY ARTIFACT

32. More generally, the different velocities at multiple small tissue–fat interfaces produce minor deviations of the ultrasound beam that defocus and disperse it. The clinical effect of this is loss of contrast, the image appearing noisy and blurred.Propagation velocity artefact 2

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35. Mirror ArtefactDue to reflection by curved specular reflector surface.It is fallacy of usg machine misinterpreting that - all echoes returning to transducer arise from structure along the axis. -an echo returns to the transducer after a single reflection.

36. Mirror Artefact

37. Enhancement Effect In area of low attenuation, stronger echoes arise from deeper tissue because they receive stronger pulses as compared to surrounding structures behind normal soft tissue that causes over amplification and bright echoes.

38. Cyst attenuates less US wavesSoft tissue attenuates more than cyst and so less US waves available to image deep tissueAs you go deep, tissue will be darker.Tissue behind cyst receives and is imaged by more number of US waves

39. Comet Tail ArtifactOccurs due to a very strong acoustic interface as gas bubble or metallic structure.It appears as a dense echogenic line extending through the image.It is diagnostically important in identifying surgical clips and air pockets

40. Comet Tail Artifact

41. SPLIT IMAGE/REFRACTION ARTIFACT It is also known as the Refraction artifact. Bending of the sound beam occurs so that target not along the axis of transducer are insonated. There reflections are then detected and displayed in the image. Structures that lie outside the volume appear in the image. Example- vanishing twin phenomenon Correction is by Scanning from site other than midline

42. (a) shows the refraction of incident US beam as it travels between two adjacent tissues with different sound propagation velocities (C1 and C2). (b) The object in the path of the refracted portion of the beam is misplaced because the processor assumes a straight path of the beam.

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44. Conclusion..Artifacts are common in US. Artifacts are unavoidable and occur secondary to intrinsic physical properties of the ultrasound beam. Recognition of these unavoidable artifacts is important because they may be clues to tissue composition and aid in diagnosis.The ability to recognize and remedy potentially correctable artifacts is important for image quality improvement.

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