Artifacts in Ultrasound.pptx( by Jalil Hanafi)

Artifacts details

1. ULTRASOUND ARTIFACTS

2. MUHAMMAD YOUSAF MS MIT* (UOL) ARDMS(USA) B.Sc( Hons ) MIT(UHS)

3. Ultrasound imaging artifacts are echoes on the display that do not properly represent the structures being imaged. Ultrasound images are filled with acoustic artifacts. Some, like acoustic enhancement , are helpful. Others, such as reverberation , are a hindrance. The design of an ultrasound scanner makes certain assumptions about the way sound interacts with tissues. However, these assumptions are often contradicted by the way in which the sound beam actually behaves in tissue. This results in errors in the presentation of the echo information.

4. These assumptions include: 1. sound travels only in straight lines (NOT TRUE - sound may be refracted or reflected before returning to the transducer). 2. echoes originate only from objects located along the central beam axis (NOT TRUE - echoes may originate from anywhere in the beam cross-section including the edges of the beam. Echoes may also originate from side lobes). 3. the amplitude of returning echoes is directly related to the reflection amplitude of reflectors (NOT TRUE - the amplitude of echoes at the transducer is affected by several factors, including angle, attenuation and wave interference.) 4. reflector distance is proportional to the go-return time of 13 us/cm (NOT TRUE – the go-return time may be longer or shorter than normal depending on the actual speed of sound in the medium)

5. There are several different ways of categorizing acoustic artifacts. One common way is to group them according to their effect on the display . In this system four categories are typically used: 1. ECHOES ADDED TO THE DISPLAY. These are echoes in the display that do not represent actual interfaces in the scanned structure. They are not real. 2. ECHOES MISSING FROM THE DISPLAY. These are echoes that should be displayed, but are not. 3. ECHOES THAT ARE IMPROPERLY LOCATED. These are real echoes that are seen in the display, but appear in the wrong location. 4. ECHOES OF IMPROPER BRIGHTNESS, SHAPE, OR SIZE. Finally, some artifacts can be caused by improper control settings, or by equipment malfunction.

6. BEAM WIDTH ARTIFACT One of the most common artifacts seen in clinical practice is the “fill-in” artifact caused by poor beam width. In this artifact, additional low-level echoes are displayed inside cystic structures. If the beam width is wide when imaging a cystic structure, the edge of the beam may lie outside the cystic structure and within the surrounding soft tissue. This results in echoes from the edge of the beam returning to the transducer. However, since the system always assumes that all echoes are returning from along the central beam axis, it writes these echoes along the beam axis where they are displayed within the cyst. The result is low-level echoes being seen within the cystic structure.

7. Typically, these low-level echoes are seen at the edges of cystic structures. However, small cysts may even be completely filled in with beam width artifact. Pseudo sludge in the normal gallbladder is a common example of “fill-in” echoes seen in a structure which should have an anechoic lumen. Thankfully, it is usually easy to distinguish these artifacts from real echoes in real-time. 1) Repositioning the transducer and focusing will often help remove this artifact.

8. SLICE THICKNESS The beam width perpendicular to the direction of the scan plane is known as the "slice thickness". Slice thickness produces low-level fill-in artifact that is identical to beam width. The only difference is that the slice thickness echoes that are filling in the cystic structures come from tissue lying perpendicular to the plane of section, while the echoes from beam width artifact arise from tissue lying within the plane of section. For the sonographer , both look identical This artifact is a result of inherent characteristics of the transducer, and apart from trying a different transducer, cannot be eliminated. Slice thickness artifact can also be called section thickness and volume averaging artifact.

9. 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.

10. REVERBERATION Reverberation (reverb artifact) is a very common artifact caused by echoes “bouncing back and forth” between two strong interfaces. Reverberative artifact is seen as multiple, equally-spaced linear echoes with a stepladder like pattern.

11. When echoes return to the transducer, the instrument assumes that all of the echo energy is absorbed by the transducer and that a single echo is produced for each interface in the beam path. In reality, a significant portion of the echo energy may be reflected from the transducer back into the patient to produce a second or third echo from the same interface. The machine displays the first echo (the real echo) at its correct location in the image. In the meantime, the echo reflects from the interface a second time and a second echo is returned to the transducer. This reverberation may occur several more times and continue until the energy in the ultrasound pulse is too weak to be detected.

12. The first echo displayed is real, but any subsequent echoes received from the same reflector result in additional echoes being displayed which are not real. Since each successive reverberative echo takes longer to be detected by the transducer, it is displayed deeper in the image. Because the distance between the transducer and interface remains constant, the axial separation between successive reverberations in the image is always the same. Reverb artifact is commonly seen in the near field since the transducer face may act as one of the strong reflectors. It is also often seen at the edge of images in the near field due to poor contact between the transducer and the skin.

13. Gas-containing structures can also produce reverberations because they are strong reflectors. In real-time reverb artifacts from bowel gas can be quite dynamic, since bowel is frequently peristalsing . Important note: reverb artifact is a different artifact than ring-down. Both can be seen behind gas. However, they have different underlying causes and different appearances.

14. Rectification: Increase the amount of gel used . Harmonic imaging Use a stand-off pad. Reduce the gain. Move the position of the transducer Increased transducer pressure

15. COMET TAIL Comet tail artifact is a form of short range reverberation originating from strong reflectors that lie close together. Common causes of comet tail are metal (surgical staples, surgical clips, and IUCDs) as well as cholesterol crystals and thin layers of calcium. The mechanism is the same as reverberative artifact, with the only difference being the close proximity of the two reflectors. In reverb, the two reflectors are usually a centimeter or so apart. In comet tail, the reflectors are only millimeters apart. Unlike reverb, comet-tail can be a helpful artifact that enables us to identify metal, cholesterol crystals, or thin layers of calcium.

16. RING-DOWN Ring-down is an extremely common artifact. Ring-down is an artifact produced by gas, and only by gas. Ring-down is known as a “resonance” artifact because it is generated when a small column of fluid between the gas bubbles resonates when struck with a pulse of ultrasound. When this happens, the sliver of fluid between the gas bubbles vibrates and sends a stream of echoes back to the transducer. When received by the transducer, the stream of echoes is displayed as a bright streak of echoes originating at the gas and extending to the back of the display.

17. Because both artifacts produce a streak of bright echoes, many early authors mistook comet tail for ring-down and vice versa. The two are produced by different phenomena and have different appearances. Ring-down is only seen behind gas, and usually extends much deeper on the screen. Comet tail can also be seen behind gas, but is also seen behind metal, cholesterol crystals, and thin layers of calcium. Comet tail is also usually much shorter. Please try to keep ring-down and comet-tail distinct and use the terms appropriately!

18. REFRACTION Refraction is “bending” of the beam. It occurs at oblique incidence when there is a change in propagation speed across the interface. Refraction of the sound beam can cause several artifacts including duplication artifacts as well as edge shadows . Refractive duplication most commonly occurs in the midline of the abdomen or pelvis due to refraction by both left and right rectus abdominus muscles. The beam is bent by refraction at the lens shaped rectus muscle causing structures deep to the muscle to be improperly located lateral to their true position. The most common structures to be artifactually duplicated are the SMA and aorta in the abdomen and gestational sacs and IUCDs in the uterus.

19. Like most other artifacts, refractive duplication can usually be eliminated by repositioning the transducer.

20. MULTIPATH Multipath describes a situation in which the path of the pulse and the path of the returning echo are different (see diagram). Multipath results in echoes being displayed deeper on the screen than they should be since the go-return time is longer than it should be. Mirror image is the most common example of a multipath artifact.

21. MIRROR IMAGE Mirror image is a very common artifact. Mirror image is the duplication of echoes on the other side of a very strong reflector. In abdominal scanning the most common location for this artifact is adjacent to the diaphragm. When imaging the right upper abdomen we might expect to see nothing superior to the diaphragm since normal air-filled lungs are perfect reflectors of sound and should not allow any sound to cross that interface. However, this is not the case. We typically see many mid-level echoes similar to liver echoes superior to the diaphragm.

22. These echoes are an artifact - a mirror-image artifact of liver echoes that are reflected from the diaphragm back into the liver and then back to the transducer. Since the system always assumes the echoes originate from along a straight path, it is unaware of the multipath process, and thus writes the echoes deeper on the screen, which in this case places them superior to the diaphragm. We have even had cases of mirror image duplication of a first trimester pregnancy in a thin patient due to the pelvic bones acting as a mirror!

23. SECONDARY LOBES All transducers emit a main beam as well as several secondary beams which are directed on all sides of the main beam. There are two types of secondary beams, namely side lobes and grating lobes. Side lobes are secondary beams that travel in directions different from the main beam. They are produced by all transducers Grating lobes are a second type of secondary beam emitted only by arrays. Arrays produce grating lobes due to the geometric arrangement of the small crystal elements in these transducers.

24. Side lobes and grating lobes are typically 30 dB less intense (1000 times weaker) than the main beam, but if the side lobe happens to interact with a strong reflector it can generate weak artifact echoes which may be written into the ultrasound image. These echoes can sometimes be seen as a linear streak adjacent to a strong reflector and extending into an anechoic region such as the bladder. They can also have a similar in appearance to beam width artifact “filling-in” normally anechoic structures such as the bladder or a cyst with weak echoes.

25. Arrays can reduce side lobe formation by a process called apodization . Array apodization varies the voltage amplitudes to the individual elements in the array, giving slightly lower voltages to the outside elements. Modern arrays can also incorporate a design feature called sub-dicing to suppress grating lobes. Sub-dicing involves cutting the crystals into smaller sections. This changes the geometric arrangement of the crystals in the array and reduces grating lobe formation.

26. ACOUSTIC SHADOWING An acoustic shadow is the dark vertical band seen deep to a strong attenuating structure. The strong attenuator weakens the pulse resulting in no echoes (or much weaker echoes) being returned to the transducer from interfaces that lie deep to the strong attenuator. A gallstone is a common example of a strong attenuator that casts an acoustic shadow. Shadowing can be both a helpful or a hindering artifact.

27. As an disadvantage , shadowing may make it difficult to see structures that lie deep to the attenuator. Common examples of this are shadowing from the bones in a 2nd or 3rd trimester fetus obscuring the underlying anatomy, or shadowing from calcific plaque obscuring the lumen of an artery. Bowel gas also commonly causes shadowing that can obscure abdominal anatomy. As an advantage , shadowing can be a very useful artifact. For example, the shadowing associated with gallstones, renal stones, and calcification is most helpful in confirming the nature of these strong attenuators.

28. Shadowing can be made more prominent by focusing and using a higher frequency . If you are struggling to get a shadow from a small gallstone, focus the beam at the tone and try a higher frequency.

29. STONE SHADOW VS. GAS SHADOW Stones and gas are both associated with acoustic shadowing. However, the sonographic appearance of the shadows seen deep to these two media usually differ significantly, allowing shadowing associated with gas to be distinguished from shadowing associated with stones. Air is an almost perfect reflector of ultrasound, reflecting in excess of 99% of the incident sound energy. Consequently, you expect a very strong shadow to be seen deep to gas. However, gas also produces other artifacts such as reverberations and ringdown . These artifacts are superimposed on the shadow, resulting in what is called a “dirty shadow”.

30. In addition, since much of the gas within the body is within peristalsing bowel, realtime observation of the changing nature of bowel gas can help distinguish it from non-changing shadowing seen behind stones Stones produce shadowing by both absorption and reflection, with about 80% of the incident energy being absorbed. This results in a relatively “clean” shadow since there are few, if any, other superimposed artifacts. A “clean” shadow is blacker and has sharper margins than a “dirty” shadow.

31. EDGE SHADOWING Edge shadowing is the formation of shadows at the edges of structures. Edge shadowing is very common. Edge shadowing is due to reflection and refraction of the beam. At the edge of a structure the beam is at extremely oblique incidence. This permits both reflection and refraction to occur, causing the beam to be bent away from the central beam axis. Since sound energy is lost along the central beam axis a shadow is produced along the central beam axis at the edge of the structure.

32. ACOUSTIC ENHANCEMENT Acoustic enhancement is another very common acoustic artifact. It can also be called “through transmission”. Enhancement is the opposite of shadowing. It is the display of brighter echoes deep to a structure with low attenuation. Enhancement deep to a mass indicates the mass has lower attenuation than the surrounding tissue. Enhancement is most commonly seen behind fluid, because most body fluids are much less attenuating than soft tissue. Enhancement is one of the two most important criteria used to diagnose a cystic mass.

33. Why is enhancement displayed? Enhancement is displayed if the attenuation in the structure is lower than the surrounding tissue. When this happens, the beam energy passing through the low attenuator is not weakened to the same degree as the energy passing through the adjacent soft tissue. Thus, the echoes deep to the low attenuator will be stronger because the incident energy deep to the low attenuator remains stronger than the energy in the surrounding soft tissue. As a result, these stronger echoes are displayed more brightly on the screen.

34. The degree of enhancement is dependent on several factors. For example, the greater the attenuation difference between the structure and the surrounding tissue, the greater the degree of enhancement. Thin watery fluids such as bile or urine are very low attenuators, and display strong enhancement. Blood, while still a fluid, is not as low an attenuator as thin fluid. Thus, while we can usually see enhancement behind large hematomas, enhancement is not commonly seen behind blood vessels.

35. Large low attenuating masses will also have more obvious enhancement than smaller ones since the beam is travelling through the larger structure for longer. The longer the path length through a low attenuator, the greater the enhancement. It is much easier to see enhancement behind large cysts than behind small cysts. Remember that the sonographer can accentuate the difference in attenuation by using a higher frequency. In situations where it is important to display the enhancement, the sonographer should use as high a frequency as possible and focus the beam.

36. One note of caution. If a cystic mass is located anterior to a highly reflective or highly absorbing structure, the effects of enhancement in the cyst may not be evident in the image. The diagram shows a liver cyst against the diaphragm. There is no liver tissue between the cyst and the diaphragm. As the sound beam leaves the cyst, it is totally reflected by the diaphragm/lung interface, preventing enhancement to be seen. Remember that them echoes seen superior to the normal air/diaphragm interface represent mirror image artifact, not real tissue.

37. Speckle is the artifactual dot pattern displayed from tissue. As the beam propagates in soft tissue it is being scattered by thousands of tiny scattering interfaces. As a result, the echoes from these scatterers interfere with each other as they return to the transducer. Constructive and destructive wave interference occurs with the result that the echoes that arrive at the transducer no longer represent individual scatterers , but rather represent an “interference pattern” from the soft tissue. (This is one reason that the echoes from the parenchyma of liver and spleen look so similar, or that the echoes from thyroid and testis look so similar: they represent an “echo pattern”, not the actual echoes from individual interfaces )

38. Speckle is an acoustic artifact. It is acoustic noise. Any technique that can help reduce speckle, such as coded excitation, spatial compounding, frequency compounding, and post-processing will improve the signal-to-noise ratio, improve contrast resolution, and improve image quality.

39. Speed of Sound Error An ultrasound system calculates the distance to the interface using the Range Equation: The calibration speed used in the equation is 1.54 mm/us, the average speed of sound in ‘soft tissue’. The Range Equation works well and accurately positions most echoes in the display.

40. However, a speed of sound error can occur if the propagation speed in the medium does not equal the calibration velocity used in the range equation. For example, if the propagation speed in the tissue is slower than soft tissue, then all the go-return times will be longer than they should be, with the result that the echoes will be displayed farther from the transducer than they should be. Similarly, if the propagation speed in the medium is faster than 1.54 mm/us, all the go-return times will be shorter than they should be, with the result that the echoes will be displayed closer to the transducer than they should be.

41. The most common cause for speed of sound error is a large fatty mass. The speed of sound in fat is 1.44 mm/us, approximately 6% slower than soft tissue. Sound travelling through a large fatty mass will take 6% longer to return to the transducer than normal. Thus echoes returning from interfaces deep to the fatty mass will be mispositioned 6% farther from the transducer than they should be.

42. Remember, the effect of a change in propagation speed is on the goreturn time. Slower propagation speeds result in longer go-return times which mis -position echoes farther from the transducer than they should be. Faster propagation speeds result in shorter go-return times which misposition echoes closer to the transducer than they should be. Slower - farther. Faster - closer!

43. In the vast majority of situations the ultrasound system will accurately position the echo data. However, a sonographer must always be aware that positioning errors can and do occur. The slower speed of sound in fatty tumors can mis -position interfaces farther from the transducer. The higher speed of sound in metal can misposition echoes closer to the transducer.

44. RANGE AMBIGUITY An ultrasound system calculates the distance to a reflector based on the echo go-return time. The go-return time “clock” is reset each time a pulse is transmitted and all echoes are assumed to be generated by the last transmitted pulse. If a second pulse is transmitted before all the echoes return, then the clock will be reset causing the later returning echoes to have erroneously short go-return times since their go return time will be measured from the transmission of the second pulse. As a consequence, the range equation will mis -locate all these echoes closer to the transducer than they should be. This is termed range ambiguity.

45. Range ambiguity should not occur In gray scale imaging because the system is programmed to always wait at least as long as the maximum go-return time set by the depth of field. For example, if the depth of field was set at 2 cm, then the system would wait at least 26 us between pulses (2 cm x 13 us/cm = 26 us). This would ensure that all echoes from the maximum imaging depth have returned before any subsequent pulses are transmitted.

46. Similarly, if the depth of field was 20 cm, then the system would wait (20 cm x 13 us/cm = 260 us) between pulses to ensure that all echoes were received before sending the next pulse. The depth of field limits the minimum PRP (the time between the start of one pulse and the start of the next pulse), and thus sets the maximum PRF (pulse repetition frequency). In this way, range ambiguity is avoided.

47. MINIMIZING CONFUSING ARTIFACTS Most acoustic artifacts are caused by the interaction of the beam and tissue. They occur when that interaction breaks one of the design principles of the ultrasound system. Therefore, the sonographer’s best method to both identify and minimize acoustic artifacts is to scan the region of interest from multiple windows. Moving the transducer to a different window will often remove or, at least, minimize the artifact. Real interfaces should be seen consistently from multiple windows.

48. Artifacts are often only seen from a specific window where the conditions for the production of the artifact are just right. So, scan the region of interest from multiple points of view. In so doing, you will be less likely to be confused by acoustic artifacts.