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 Table of Contents  
REVIEW ARTICLE
Year : 2018  |  Volume : 13  |  Issue : 5  |  Page : 9-16

Artefacts in musculoskeletal ultrasound


Department of Radiology, Cancer Institute (WIA), Chennai, Tamil Nadu, India

Date of Web Publication1-Aug-2018

Correspondence Address:
Dr. Susila Krishnan
Department of Radiology, Cancer Institute (WIA), Adyar, Chennai, Tamil Nadu
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0973-3698.238195

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  Abstract 


Artefacts in musculoskeletal (MSK) ultrasound (US) comprise artefacts that mimic an abnormality while scanning normal structures and artefacts that occur together with abnormal conditions at grey-scale and Doppler imaging. Some of these can be avoided by correct scanning technique while other artefacts are the result of inherent characteristics of different tissues which may help in making a correct diagnosis. The origin and examples of common grey-scale and power/colour Doppler artefacts in ultrasound are explained with particular reference to MSK ultrasound. The commonly encountered Doppler artefacts such as the flash/twinkle artefacts and pseudo flow artefacts and their importance for image interpretation are discussed. Anisotropy is the sonographic artefact that is direction dependent and can be mistaken for a partial tear while scanning a tendon or nerve. Aliasing and random noise are well known artefacts, which depend upon grey-scale and overall gain. Mirror image artefacts refer to any highly reflecting smooth surface which may act as an acoustic mirror. In rheumatology, the mirrors will nearly always be bone surfaces. Blooming artefact displays colour outside of a vessel and makes vessels appear larger. Artefacts can also be due to ultrasound beam characteristics (side-lobe artefact), multiple echoes (reverberation, comet-tail, and mirror artefacts), and attenuation errors and enhancement, among others. Not all artefacts are confusing or unwanted. Certain artefacts are beneficial in assisting with the diagnosis. A better understanding of proper imaging techniques will allow correction or minimization of many of these artefacts.

Keywords: Artefact, musculoskeletal, ultrasonography, ultrasound


How to cite this article:
Krishnan S. Artefacts in musculoskeletal ultrasound. Indian J Rheumatol 2018;13, Suppl S1:9-16

How to cite this URL:
Krishnan S. Artefacts in musculoskeletal ultrasound. Indian J Rheumatol [serial online] 2018 [cited 2018 Dec 15];13, Suppl S1:9-16. Available from: http://www.indianjrheumatol.com/text.asp?2018/13/5/9/238195




  Introduction Top


Musculoskeletal (MSK) ultrasound (US) uses high-resolution linear transducers and images are produced assuming that the returning echoes faithfully represent the tissue characteristics of the structures below. When significant differences in them occur, artefacts are produced. The term artefact is used to describe any unwanted image information generated in the process of image formation.[1]

Some artefacts arise due to improper scanning technique and are therefore avoidable. Others are generated by the physical limitations of the modality and are related to the physical properties of the US beam and the propagation of sound in the matter, such as the US beam characteristics, the presence of multiple echo paths, velocity errors, and attenuation errors.

In ultrasound, artefacts may cause visualisation of a nonexisting structure on the displayed image or nonvisualisation of an existing structure. US artefacts may also display the existing structures in an incorrect location, size, or brightness.[2] MSK ultrasound artefacts are made up of those artefacts that mimic an abnormality while scanning normal structures and those that occur together with abnormal conditions at grey-scale and Doppler imaging.

Understanding artefacts are critical to avoid errors in interpretation because some of these can be overcome by correct scanning technique while other artefacts are the result of inherent characteristics of different tissues and often provide important diagnostic information.

Incorrect US beam assumptions that lead to artefacts:[3]

  • US beam travels in a straight line
  • All echoes detected are from the main US beam
  • Attenuation of sound in tissue is uniform
  • The speed of sound is same in all types of tissue at 1540 m/s
  • Each reflector in the body produces only one echo
  • The depth of an object imaged is directly related to the time it takes for the US echo to return to the transducer.


Understanding the underlying physics of US and the assumptions used for US image formation is important for understanding US artefacts.[4]


  Artefacts Associated With Beam Characteristics Top


The US beam consists of main US beam that narrows as it nears the focal zone and diverges distal to it. Accompanying them are additional undesirable low energy off-axis beams known as the side lobes and grating lobes which also propagate sound, reflect, and return echoes to the transducer similar to the main beam and cause artefacts [Figure 1]. US image processing assumes that the echoes detected originated from within the main US beam. A structure that is strongly reflective and located outside of the main US beam may generate echoes that are detected by the transducer and may be interpreted as being originated from within the main beam,[5],[6] giving rise to artefacts.
Figure 1: Diagrammatic representation of the ultrasound beam showing the main beam, side lobes and grating lobes. The main beam narrows as it approaches the focal zone and then diverges. (Reproduced with permission from: Feldman MK, Katyal S, Blackwood MS. US artifacts. Radiographics. 2009; 29(4): 1179-1189. RSNA 2009)

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Beam width artefact

Beam-width artefact occurs when the US beam is too wide with respect to the imaged structure. They are caused due to the widening of the main beam beyond the focal spot. Echoes are generated by a highly reflective object situated outside of the transducer margin but inside of the widened divergent distal beam. In addition, if the beam is too wide, one portion of the beam may be interacting with a fluid-filled structure while another portion of the beam interacts with adjacent soft tissues that may create spurious echoes within the cystic structures [Figure 2] and reduced contrast at the lesion borders.[2] This artefact can be reduced by adjusting the focal zone at the level of the region of interest or by placing the transducer at the center of the examined structure.[5],[7]
Figure 2: Beam-width artefact: Long-axis Grey-scale ultrasound image shows spurious echoes within the distended bursa

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Side lobe artefact

Side lobes are multiple beams of low-amplitude US energy that project radially from the main beam axis, which is seen mainly with linear transducers.[2] Strong reflectors present in the path of these low-energy, off-axis beams may create echoes detectable by the transducer.[5] These echoes will be displayed as having originated from within the main beam and appear as faint specular or diffuse artefactual echoes within the beam. As with beam-width artefact, this phenomenon is most likely to be recognized as extraneous echoes present within an expected anechoic structure such as the bladder,[5] simple cyst, or fluid-filled bursa [Figure 3]. Specular artefacts occur adjacent to curved, highly reflective surfaces, such as the diaphragm, bladder, and gallbladder (GB) while diffuse echoes tend to occur adjacent to bowel gas.[7]
Figure 3: Side-lobe artefact: (a) short-axis and (b) long-axis Grey-scale image shows faint extraneous echoes within the anechoic urinary bladder (arrow)

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Anisotropy

Anisotropy is a US imaging artefact that appears as decreased echogenicity of the examined soft tissue when the US beam is not directed perpendicular to the examined anatomical structure.[2] In other words, this is the effect that makes a tendon appear bright when it runs at 90° to the US beam but dark when the angle is changed [Figure 4]. It occurs notably when imaging muscles and tendons and may lead to erroneous interpretation of a tendon tear or tendinopathy when there is none.
Figure 4: Anisotropy: (a) short-axis Grey-scale image of the distal supraspinatus tendon showing the normal echogenic appearance when the probe is perpendicular to the tendon, (b) abnormal hypoechoic appearance of the tendon at the same anatomical location as in (a) when the probe is tilted and not held perpendicular to the tendon due to anisotropy and (c) long-axis image of the same shows abnormal hypoechoic appearance of the distal supraspinatus tendon due to anisotropy which can mimic tendinopathy and/or tear

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A common cause of false-positive diagnoses of rotator cuff tears is anisotropy or angle-dependent appearance of tissue structures. The rotator cuff appears echogenic when the US beam insonates at 90° to the long axis of the tendon fibers because the beam is then reflected maximally. The more the angle deviates from this angle, the fewer reflected sound waves will be detected by the transducer. Tendon insertions, where most rotator cuff tears occur, are most vulnerable to the anisotropic artefact due to their curved course.[8] To overcome anisotropy, simple heel-toe maneuvering with manual tilting or angulation of the transducer may be used or the beam steering feature that tilts and angulates the US beam to steer the entire B-mode image may be used.[9],[10]

Speckle

Speckle refers to the inherent granular appearance of tissues and occurs due to the constructive and destructive interference of US waves interacting with the microstructures in tissue.[11] It appears as noise within tissues and reduces image contrast and lesion detectability. Speckle can be reduced with the postprocessing algorithm of speckle reduction imaging in most US machines of today.


  Artefacts Due to Velocity Errors Top


US image processing assumes that sound travels at a constant speed of 1540 m/s in human tissue. This does not hold good in clinical sonography, as the US beam may encounter a variety of materials such as air, fluid, fat, soft tissue, and bone.

Speed displacement artefact

If an echo travels through the examined structure with slower velocity than through the soft tissues, a longer time will be needed for that echo to return to the transducer. The delayed return of the echo will manifest as the imaged structure being displayed deeper than its true anatomical depth.[2],[5] The US system interprets the delay as the object being deeper in the scan, leading to the apparent disruption of the outline of the structure imaged [Figure 5]. This is termed a speed displacement artefact and mostly encountered when the incident US beam encounters a focal area of fat.
Figure 5: Speed displacement artefact: Transverse Grey-scale image of the liver displays diffuse heterogeneity of the liver parenchyma with an area of fatty infiltration in the right lobe (arrows) just superficial to discontinuity at the liver's diaphragmatic interface (arrowhead), secondary to speed displacement artefact

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Refraction

Sound is refracted the same way as light when it passes from one medium to another; it changes direction. This occurs when the US beam encounters an interface obliquely. Refraction alters the direction of the US beam and causes objects that are not in the assumed path of the beam to be displayed as if they are. When an incident US beam encounters the interface between two different tissues at <90°, it changes direction.[12] This causes incorrect positioning of objects on the resulting image or even their duplication (ghosting) when the angle of incidence is acute [Figure 6]. This can be minimized by placing the transducer probe perpendicular to the object being imaged. This commonly occurs at the interface between the rectus abdominis muscles and abdominal wall adipose tissue; at the interface between liver or spleen and adjacent adipose tissues.[12]
Figure 6: Refraction: (a) Grey-scale image and (b) diagrammatic representation showing misregistration due to refraction artefact. Change in direction of the US beam at the interface with bladder as opposed to the beam passing through the fetal head (arrows in a) results in misregistration and ghosting of the fetal head (arrowheads in a)

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Refractile shadowing, edge shadowing, or critical angle shadowing

Edge shadowing is seen at curved surfaces of tendons, diaphysis of bones, and large vessels along their lateral margins [Figure 7], and also at the retracted torn ends of tendons and muscle. This occurs from a combination of refraction and reflection. Clinically, this is helpful when imaging torn tendons as this is indicative of a full thickness tear [Figure 8]. The distance between the acoustic shadowing deep to the rounded edges of the proximal and distal stumps of the tendon will accurately reflect the degree of tendon retraction.[13] Edge shadowing can be decreased or eliminated by changing the angle of insonation.[11]
Figure 7: Critical angle shadowing: long axis ultrasound image of epidermoid cyst showing lack of echoes deep to the lateral margins of the lesion (*) appearing as linear dark bands

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Figure 8: Refractile/Critical angle shadowing: (a) Lateral radiograph showing calcification at the site of TendoAchilles tear, (b) long-axis Grey-scale image showing discontinuity of the tendon fibers with refractile shadowing seen deep to a full thickness tear of the TendoAchilles (TA) and (c) calcification (*) with posterior shadowing at the torn retracted end of the tendon with echogenic gap between the torn edge and calcaneum (calc)

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  Artefacts Due to Attenuation Errors Top


Increased through transmission

As US beam travels through the body, its energy becomes attenuated secondary to absorption and scatter.[14] Fluid-containing structures attenuate the sound much less than solid structures, so the strength of the sound pulse is greater after passing through fluid than through an equivalent amount of solid tissue.[12] Also referred to as posterior enhancement or acoustic enhancement, this artefact is seen when the sound wave passes through a fluid-filled structure like a cyst or bursa resulting in increased acoustic enhancement of the structure deeper to it and is seen as increased echogenicity (whiteness) posterior to the cystic area [Figure 9].
Figure 9: Posterior acoustic enhancement: increased through transmission (posterior enhancement) seen as increased echogenicity/whiteness (*) deep to the complex cystic lesion

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Posterior acoustic shadowing

Acoustic shadowing refers to the reduction in echo strength distal to a highly attenuating or reflective object.[11] Hence, the echoes returning from structures beyond such a structure will also be diminished. During imaging, shadowing appears as a hypoechoic or anechoic band occurring deep to a highly attenuating structure. This artefact occurs at interfaces with bone, calcifications, foreign bodies, and gas.

Clean shadowing occurs with bone, stones, and calcifications because most of the sound energy is absorbed by these structures and very little passes beyond and the resulting posterior shadowing is anechoic or “clean” [Figure 10]. On the other hand, dirty shadowing occurs behind gas and is due to the high degree of reflection at gas/tissue interfaces. Dirty shadowing is commonly seen distal to a highly reflecting surface such as gas [Figure 11], whereby multiple secondary reflections produce low-level echoes that appear within the shadow, similar to the ring-down artefact.[15]
Figure 10: Posterior acoustic shadowing: “Clean” shadowing in the long-axis Grey-scale image of calcification in gout (*)

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Figure 11: Posterior acoustic shadowing: secondary reflections at gas/tissue interface within this dilated bowel loop producing low-level echoes within the shadow deep to the gas, accounting for the “dirty” appearance (*)

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  Artefacts Associated With Multiple Echoes Top


Reverberation

Posterior reverberation artefact results in multiple echoes beneath the reflecting surface at regularly spaced intervals. This happens due to the presence of two highly reflective parallel surfaces, and the primary US beam is reflected back and forth repeatedly before returning to the transducer for detection.[5],[14] This artefact is made up of a first reflection which is the only spatially correct one, followed by a series of bright bands parallel to the main beam, decreasing in brightness and equidistant from one another and are commonly seen posterior to the biopsy needle during MSK interventions [Figure 12]. Reverberation artefact may be minimized by selecting a different imaging plane to avoid the reflective surfaces or by changing the angle of insonation.[11]
Figure 12: Reverberation: long-axis Grey-scale image shows reverberation artefact-multiple reflections causing multiple linear bright echoes posterior to the biopsy needle equidistant from each other and decreasing in brightness

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Comet-tail artefact

A special subtype of reverberation artefact caused by highly reflective interfaces that are so closely spaced to each other that their individual echoes are not discernible. In addition, attenuation of more delayed echoes results in a progressively decreased amplitude and width with increasing depths.[11] The result is an artefact caused by the principle of reverberation but with a triangular shape tapering distally resembling the tail of a comet. This commonly occurs due to cholesterol deposits along gallbladder wall and inspissated colloid in a thyroid nodule, which are diagnostic of adenomyomatosis GB and colloid nodule, respectively. These also occur in the presence of metal such as surgical staples, needles, clips, and sutures [Figure 13].
Figure 13: Comet-tail artefact: triangular echogenic shadow (arrow) which trails off distally deep to the prostatic biopsy needle; imaged when the angle of the incident beam is almost perpendicular to the needle

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Ring-down artefact

Although similar in appearance to comet-tail artefact, ring-down artefacts typically occur due to the resonance (vibration) of fluid trapped among gas bubbles after being bombarded with US. These resonant vibrations produce a continuous, although decaying, sound wave transmitted back to the receiver, appearing as a streak or series of parallel bands deep to a focus of gas [11] [Figure 14]. These artefacts are useful in identifying abnormal foci of air as in biliary air, portal venous gas or infections, and abscesses by gas-forming organisms.
Figure 14: Ring-down artefact: Grey-scale short-axis image showing a series of parallel bands in the interface of the liver and lung (arrows)

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Mirror image artefact

Imaged structures immediately adjacent to highly reflective acoustic interfaces may appear duplicated. This is secondary to reverberation at their interface as a result of scattering of the US waves which are redirected toward the structure imaged.[2] On the US images, this artefact results in a second copy of the image as an inverted, duplicated, distorted, and incompletely portrayed mirror image equidistant from but deep to the strongly reflective interface [Figure 15]. The typical example is the mirror image of liver seen deep to the highly reflective interface of the diaphragm. This artefact may be reduced by decreasing the gain or changing the angle of insonation.[11]
Figure 15: Mirror image artefact: short (a) and long-axis (b) Grey-scale images showing mirror image of the NG tube (arrows) traversing a segment of circumferential wall thickening of the cervical esophagus. Posterior Reverberation artefact is seen deep to the tube in the short-axis image (arrowhead)

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  Doppler Artefacts Top


Many Doppler artefacts are secondary to limitations in the system and can be overcome with correct usage and adjustment of the system controls. Artefacts that appear in the B-mode image such as reflection effects, mirroring, and shadowing also occur in the Doppler signal.

Twinkle artefact is a discrete focus of alternating colors with or without an associated-color comet-tail artefact. It is dependent on US machine settings (color-write priority, Grey-scale gain, and pulse repetition frequency [PRF]), motion of the object scanned with respect to the transducer, and equipment used.[12] Commonly seen in urinary stones, twinkle artefact is due to noise generated in calcification [Figure 16].
Figure 16: Twinkle artefact: short axis Grey-scale and Doppler image of pediatric neuroblastoma showing multiple calcific foci with comet-tail artefact (a) with rapidly fluctuating mix of Doppler signals representing twinkling artefact (b)

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Background noise is caused by excessive Doppler gain. There is a random distribution of color within the color box because noise has a random frequency shift. To achieve optimum image quality, the gain settings should be increased to display noise pixels and then scaled down till all the noise disappears [Figure 17].
Figure 17: Noise: increase in color gain in this Doppler long-axis image resulted in increased noise and degradation of image

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Blooming artefact occurs when there is color bleed outside of the vessel margins [Figure 18]. This can also be eliminated by adjusting the gain setting. Decreasing the Doppler gain minimizes the blooming artefact, but the weakest signal from the smallest vessels may go undetected.[2]
Figure 18: Color bleed outside of the vessel wall gives rise to blooming artefact which can be eliminated by adjusting the gain setting

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Aliasing

Aliasing occurs when the frequency exceeds the Nyquist limit, which is two times the PRF. In other words, aliasing occurs when the PRF is less than twice the Doppler frequency shift. Increasing the PRF or decreasing baseline will diminish aliasing.

Reducing the depth of the sample volume (gate) will allow an increase in the PRF. Reducing the transducer frequency, increasing the PRF and reducing the Doppler angle all help in coping with aliasing.

Spectral aliasing produces a wrap-around effect on the Doppler display with the peaks of the waveforms “cut off” and displayed on the opposite side of the baseline [Figure 19]. Color Doppler aliasing projects the color of reversed flow within central areas of highest velocity.[15] In this way, aliasing may be a useful indicator of high-velocity flow thereby aiding in stenosis detection.
Figure 19: Aliasing: Aliasing displayed on a spectral Doppler waveform. When the Nyquist limit is exceeded, the waveform “wraps around,” (displayed below the baseline) resulting in an inability to measure velocity accurately

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Flash artefact

Flash is the sudden burst of color that appears within the frame. It is caused by transducer motion or patient breathing or motion of an anatomic structure secondary to an external force (such as the pulsation of an adjacent artery or the heart), which causes motion of the reflectors within a structure of interest and results in color flash in the absence of actual blood flow. This spurious appearance of blood flow results in flash artefact [Figure 20]. Hypoechoic and anechoic structures are more susceptible to flash artefact.
Figure 20: Flash artefact: Color Doppler US image shows flash of color within the ascitic fluid due to patient breathing

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Pseudoflow artefact

Pseudoflow is also related to motion, but to that of fluid rather than of blood within a vessel. It is similar to flash artefact and appears such as real blood flow at color or power Doppler US, but in the absence of a vascular structure [Figure 21]. The color or power Doppler signal will appear as long as the fluid motion continues. Spectral analysis reveals flow that is atypical for a normal vessel.[11]
Figure 21: Pseudoflow artefact: (a) similar to flash artefact and related to motion, but of fluid rather than that of blood within a vessel as in this dermoid cyst (b) spectral analysis reveals a pattern inconsistent with that of flow within a vessel

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Artefacts related to aliasing, motion, and echogenic interfaces are common while using Doppler in MSK US as Doppler and imaging “compete” when used together. Certain general principles that may be followed to avoid and minimize these artefacts are.[15]

  • Smaller Doppler angle representing the actual flow vector preferred
  • Sample volume (color box) used for interrogation to be kept as small as possible
  • Spectral and color scales to be optimized to avoid errors
  • Doppler gain to be adjusted to avoid excessive signal.



  Conclusion Top


Artefacts are an integral part of imaging because of the nature of US interaction with tissues and cannot be completely avoided. Furthermore, artefacts do not appear singularly; one image may contain many types of artefacts and in various combinations. Artefacts must be recognized as part of the image and not mistaken for “real” echoes. Conversely, it is important not to mistake a real finding for an artefact. Not all artefacts are confusing or unwanted. Certain artefacts are beneficial in assisting with the diagnosis. A better understanding of proper imaging techniques will allow correction or minimization of many of these artefacts.

Acknowledgment

The author would like to thank Dr. A Gafoor and Mr. P Cantin for their guidance and support.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

1.
Bolvig L, Fredberg U, Rasmussen OS. Textbook of Musculoskeletal Ultrasound – For Beginners and Trained. E Book Edition; Munksgaard Danmark, Copenhagen. 2013. p. 10-14.  Back to cited text no. 1
    
2.
Taljanovic MS, Melville DM, Scalcione LR, Gimber LH, Lorenz EJ, Witte RS. Artefacts in musculoskeletal ultrasonography. Semin Musculoskelet Radiol 2014;18:3-11.  Back to cited text no. 2
    
3.
Gimber DH, Melville DM, Klauser AS, Witte RS, Arif-Tiwari H, Taljanovi MS. Artefacts at musculoskeletal US: Resident and Fellow Education Feature. Radiographics. 2016;36:479-80.  Back to cited text no. 3
    
4.
Baad M, Feng Lu, Raiser I, Paushter D. Clinical Significance of US artefacts. RadioGraphics 2017;37:1408-23.  Back to cited text no. 4
    
5.
Feldman MK, Katyal S, Blackwood MS. US artefacts. Radiographics. 2009;29:1179-89.  Back to cited text no. 5
    
6.
Abreu I, Roriz D, Barros M, Moreira A, Alves FC. B-mode Ultrasound Artefacts. Educational Exhibit. C-2135, ECR2015. [doi: 10.1594/ecr2015/C-2135].   Back to cited text no. 6
    
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Scanlan KA. Sonographic artefacts and their origins. AJR Am J Roentgenol 1991;156:1267-72.  Back to cited text no. 7
    
8.
Rutten MJCM, Jager GJ, Blickman JG. US of the Rotator Cuff: Pitfalls, Limitations and Artefacts. RadioGraphics 2006;26:589-604.  Back to cited text no. 8
    
9.
Jacobson JA. Fundamentals of Musculoskeletal Ultrasound. 2nd ed. Philadelphia, PA: Elsevier Saunders; 2012.  Back to cited text no. 9
    
10.
Klauser AS, Peetrons P. Developments in Musculoskeletal Ultrasound and Clinical Applications. Skeletal Radiol. 2010 Nov; 39:1061-71.  Back to cited text no. 10
    
11.
Campbell SC, Cullinan JA, Reubens DJ. Slow Flow or No Flow? Color and Power Doppler US Pitfalls in the Abdomen and Pelvis. RadioGraphics 2004;24:497-506.  Back to cited text no. 11
    
12.
Hindi A, Peterson C, Barr RG. Artefacts in Diagnostic Ultrasound. Reports in Medical Imaging 2013;6:29-48.   Back to cited text no. 12
    
13.
van Holsbeeck MT, Introcaso JM. Musculoskeletal Ultrasound, 3rd Edition: Jaypee Brothers Medical Publishers (P) Ltd, India; 2016. p. 12-25.  Back to cited text no. 13
    
14.
Bushberg JT, Seibert JA, Leidholdt EM, Boone JM. The essential physics of medical imaging. 2nd ed. Philadelphia, Pa: Lippincott Williams & Wilkins, 2002;469-553.  Back to cited text no. 14
    
15.
Paushter DM, Primer and Clinical Significance of Artefacts in Ultrasound. UCCME Radiology Review, RSNA/AAPM 2015.  Back to cited text no. 15
    


    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11], [Figure 12], [Figure 13], [Figure 14], [Figure 15], [Figure 16], [Figure 17], [Figure 18], [Figure 19], [Figure 20], [Figure 21]



 

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