Chapter 12 Image Artifacts

1. Chapter 12Image Artifacts

2. Assumptions for two-dimensional echo mapping Violations of these assumptions produce artifacts in the image: • The transmitted wave travels along a straight-line path from the transducer to the object and back to the transducer. • Attenuation of sound in tissue is uniform along the path. • Beam dimensions are small in both section thickness and lateral directions. • All detected echoes originate from the axial of the main beam only.

3. Continue • All received echoes are derived from the most recently transmitted pulse. • The ultrasound wave travels at the rate of 1540 m/s in tissue; thus the distance to the interface is determined from the time of flight. • Each reflector contributes a single echo when interrogated along a single scan line. • The amplitude of the echo is derived from the object scanned alone and is directly related to the reflective properties of the object.

4. Types of image artifacts • Partial volume • Attenuation • Banding • Reverberation • Comet tail • Resonance • Multipath reflections • Mirror image • Refraction • Ghost image • Side lobes • Range ambiguity • Velocity error

5. Partial volume • Finite beam width creates a partial volume artifact related to the simultaneous sampling of tissues with different acoustic properties.

6. Partial volume Slice thickness artifact illustrated as debris at the base of the bladder.

7. Attenuation • The rate of intensity loss may vary greatly among different types of tissue. • Enhancement and shadowing affect the brightness of the displayed echoes obtained along weakly or strongly attenuated beam paths.

8. Attenuation Enhancement distal to fluid surrounding a testicle.

9. Banding • Focusing characteristics of the transducer may create a banding artifact, which is a region of increased brightness caused by greater intensity in the focal zone.

10. Banding Focal zone banding artifact (bright area on the image).

11. Reverberation • When reverberations or multiple reflections from an interface are present, additional echoes from this interface are recorded in the image. • The amount of sound energy reflected from an interface depends on the acoustic impedances of the two media.

12. Reverberation A, for a single transmitted pulse, B, the echoes along one line of sight are separated equally in time, C, two-dimensional mapping of a reverberation artifact.

13. Reverberation Reverberation artifact arrow in the bladder.

14. Comet tail • Multiple internal reflections within a small but highly reflective object create a series of echoes. • The series of echoes is expressed as multiple small bands, called comet tails, in the image.

15. Comet tail Internal reflections give rise to multiple echoes from an object.

16. Comet tail Comet tail artifact arrow distal to a strong reflector.

17. Resonance • A closely related phenomenon, called a ring-down artifact, occurs when a small gas bubble resonates, resulting in a continuous emission of ultrasound.

18. Mirror image • Mirror image artifacts are produced when an object is located directly in front of a highly reflective surface at which near-total reflection occurs. • This artifact can also occur if the object is offset from a curved strong reflector, or the object and strong reflector are oriented at an angle.

19. Mirror image A strong reflector allows sampling of the object along lines of sight in which the sound beam does not follow a straight path.

20. Mirror image Mirror image artifact of a Foley catheter placed in the bladder.

21. Sampling of the tissue-fluid interface A, proper placement of the tissue-fluid interface, labeled1 in D. B, proper placement of the strong reflector, labeled 2 in D. C, improper placement of tissue/fluid interface by multiple path reflections, labeled 3 in D.

22. Refraction • Refraction of the ultrasound beam at a boundary between two media with different velocities causes two types of artifacts, misregistration and defocusing. • Misregistration may distort size and shape of the object. • Defocusing in this connotation describes a loss of multiple beam coherence.

23. Refraction Refraction along the beam path cause the object to be mispositioned in the image.

24. Refraction Bending of the sound beam by the liver causes the spatial assignment for the pole of the kidney to be in error.

25. Defocusing Loss of beam coherence. A, velocity of curved structure is less than surrounding soft tissue. B, velocity of curved structure is greater than surrounding soft tissue.

26. Ghost image • A special case of refraction artifact, called the ghost image, is caused by altered paths of the sound beam as it passes through the overlying rectus muscles. • The rectus muscles act as a lenses, causing refraction of the ultrasound beam that leads to duplication of a small object distal to the muscles.

27. Ghost image Refraction at the edge of a strong reflector causes misplacement of the gestational sac (sampling positions 1 and 2). At positions 3 and 4 the gestational sac is correctly registered in the image.

28. Side lobes • Side lobes, which are present with all transducers, result from interference phenomena attributed to nonthickness mode vibrations and multiple source beam formation.

29. Side lobes A highly reflective object along the path of a secondary lobe produces an echo that is incorrectly assigned a location in the direct of the main beam.

30. Range ambiguity • When the area of interest is limited in depth by high PRFs (pulse repetition frequency), structures beyond the indicated range may be depicted in the image.

31. Range ambiguity A, at low pulse repetition frequencies the measured time for the echo is proper. B, at high PRFs the time between the most recently transmitted pulse and the echo does not correspond to the actual depth.

32. Range ambiguity When the range of scanning is limited by a high pulse repetition frequency, deep lying structures are incorrectly placed near the transducer in the image.

33. Velocity error • Errors in calibration of the velocity or scanning through tissues (bone, lens of the eye, cartilage, fluid, silicone and fat) that have different velocities of sound can cause an artifact called a velocity error, or propagation error.

34. Velocity error A, uniform velocity along the beam path. B, low velocity structure along the beam path causes the interface to be displaced distal to the true location. C, high velocity structure along the beam path causes the interface to be displaced proximal to the true.

35. Distance measurement • The measurement of distance is usually more accurate along the direction of propagation than along the direction perpendicular to the beam path.

36. Temporal resolution • Frame rate determines the temporal resolution. • Fast-moving structures require high frame rates (50 frame per second). • Abdominal imaging may be satisfactory at a frame rate as low as 4 per second.

37. Environmental interference • Instrument noise caused by environmental electrical or radiofrequency interference from other electronic devices in the vicinity also can create artifacts in the image. • Manufacturers have improved the shielding design to reduce the effects of environmental interference.