Artefacts Flashcards
What is an artifact by definition?
Any echo signal whose displaying position does not correspond to the actual position of a reflector in the body or whose displayed amplitude is not indicative of the reflecting or scattering properties of the region from which the echo originated (zagzebski,1996).
Causes of artifacts
1.Violation of assumptions e.g the rate of attenuation through soft tissue is constant. OR The transmitted wave returns along a straight path
2. Operator error
3. Equipment malfunction or error
What are the four main types of artifact?
1.Attenuation artifact
2.Beam path artifacts
3. Depth of origin artifacts
4. Beam dimension artifacts
For Attenuation artifact to occur there must be
The assumption that the attenuation is constant must be broken. The sound beam encounters structures with different attenuation
5 key assumptions ultrasound machines make that lead to artifacts
- There is a constant rate of attenuation 1dB/cm/MHz
- All echoes arise from a razor thin beam
3.The speed of sound in tissue is 1540m/s
4.The round trip time of a given echo is directly related to the depth of the reflector from the transducer. - The ultrasound beam travels in a straight line and reflects just once
What is acoustic shadowing? how do eliminate it?
Encounrerw tissues that attenuate more than surrounding structures. When a beam encounters a hyperechoic structure e g at a tissue to bone or tissue to prosthetic valve interface, the hypoechoic shadow can occur under the hyperechoic attenuating structure.
Can be reduced by adjusting the gain
What is acoustic enhancement?
When the transmitted beam travels through structures or tissues that attenuate less than surrounding structures.this creates hyperechoic areas distal to the weak reflecting structures
Advantages of acoustic enhancement?
Can help us differentiate fluid filled structures from solid masses
Ultrasound beam has how many planes? And at what point is it narrowest?
3 planes
Narrowest at the focal zone
What causes beam width artifact
Beam width artifacts occur when two structures are seen as one. If lateral resolution is poor. This can be improved by using a focal zone
What is slice thickness artifact
Occurs when the ultrasound beam samples tissues of different echgenicities within the thickness of the slice. This can lead to a blending of echoes from different structures.
Caused by
1.beam width: ultrasound beams are not infinitely thin.
2.Elevation plane: the third dimension of the U.S beam can include structures that are not in the direct path of the beam but can still contribute to the image
3. Scanning angle- structures at oblique angles to the beam can contribute echoes that are misrepresented in the final image
What is side lobe artifact?
False echoes that originated from secondary lobes of ultrasound energy, causing structures outside the main beam to appear in the image
Grating love artifact
Occurs due to periodic spacing of transducer elements causing spurious echoes from off axis structures to be misinterpreted in the ultrasound image
What’s the difference between slice thickness or beam width artifacts?
With slice thickness the structure causing the artifact is not seen in the display. The structure is located 90* perpendicular to the artifact.
Two types of depth of origin artifacts
1.Propagation speed artifacts: the machine accepts 1540m/s through tissues that are not and structures will appear in different places on the image
2.range ambiguity artefact: the second pulse is emitted before the first is received and the structure is displayed closer to the transducer
