Pitfalls in Diagnostic Radiology || Ultrasound Imaging

3W.C.G. Peh (ed.), Pitfalls in Diagnostic Radiology,DOI 10.1007/978-3-662-44169-5_1, © Springer-Verlag Berlin Heidelberg 2015

Abbreviation

US Ultrasound

1.1 Introduction

An ultrasound image artifact is a structure seen within the ultrasound image which does not exist in the tissue being scanned. Ultrasound artifacts are either technology or technique related. The ultrasonologist or ultrasonographer should be aware of the existence and variety of artifacts to avoid misdiagnosis. Ultrasound technology works on the assumption that (1) the speed of sound propagation is the same for all tissues (1,540 m/s), (2) an ultrasound beam travels in a straight line, (3) the attenuation of sound is uni-form, (4) all echoes detected by the transducer have arisen from the transducer, and (5) the time taken after emission of a pulse for an echo to return to the transducer is directly related to the distance of the interface from the transducer (Feldman et al. 2009 ). These assumptions are not always correct and may lead to artifact formation. Artifact formation can take one of four forms as follows: (1) structures seen in the image that do not exist, (2) objects which should be seen in the image though are missing, (3) structures that are misplaced (misregistered) on the image, and (4) structures that are seen on the image but are of incorrect brightness, shape, and size (Feldman et al. 2009 ).

R. K. L. Lee , MBChB, FRCR • S. S. Y. Ho , PhD, RDMS • J. F. Griffi th , MD, MRCP, FRCR (*) Department of Imaging and Interventional Radiology , Prince of Wales Hospital, The Chinese University of Hong Kong , Hong Kong SAR , China e-mail: [email protected]; [email protected]; griffi [email protected]

1 Ultrasound Imaging

Ryan K. L. Lee , Stella S. Y. Ho , and James F. Griffi th

Contents

1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3

1.2 Artifacts Seen During Grayscale Ultrasound Imaging . . . . . . . . . . . . . . . . . . 4

1.2.1 Artifacts Due to Ultrasound Beam Characteristics . . . . . . . . . . . . . . . . . . 4

1.2.2 Artifacts Due to Multiple Echoes. . . . . . . . . 51.2.3 Artifacts Due to Velocity Errors. . . . . . . . . . 101.2.4 Artifacts Due to Attenuation Errors . . . . . . . 131.2.5 Artifacts Due to Improper

Machine Settings . . . . . . . . . . . . . . . . . . . . . 14

1.3 Artifacts Seen During Doppler Ultrasound Imaging . . . . . . . . . . . . . . . . . . 15

1.3.1 Artifacts Associated with Technical Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . 15

1.3.2 Artifacts Due to Patient Anatomy . . . . . . . . 171.3.3 Artifacts Due to Machine Factors . . . . . . . . 18

1.4 Artifacts Seen During Contrast Ultrasound Imaging . . . . . . . . . . . . . . . . . . 20

1.4.1 Color Blooming Artifact . . . . . . . . . . . . . . . 201.4.2 Increased Maximum Doppler

Shift/Systolic Peak Velocity. . . . . . . . . . . . . 20

Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

mailto: [email protected]




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1.2 Artifacts Seen During Grayscale Ultrasound Imaging

1.2.1 Artifacts Due to Ultrasound Beam Characteristics

1.2.1.1 Off-Axis Lobe Artifact Off-axis lobes include side lobe and grating lobe (Feldman et al. 2009 ). Side lobes are multiple beams of low amplitude that project radially from the main (primary) beam axis (Hedrick et al. 2005 ). Grating lobes are additional beams that are stronger than the side lobes of the individual ele-ments (Fig. 1.1 ). Both side and grating lobes are weaker than the primary lobes and, as such, do not normally give echoes of suffi cient strength to be displayed in the image. However, if a side lobe or a grating lobe encounter a highly refl ective inter-face (such as gas, bone, or a needle) outside the main beam, their echoes may be incorrectly dis-played on the image along the path of the main beam although they are usually weaker than the correct presentation of the structure (Fig. 1.2 ). Examples include bowel gas projected within the gallbladder or within the urinary bladder or fetal bone projected into adjacent amniotic fl uid. This artifact is also often seen during ultrasound-guided needle biopsy. An example of an off-axis lobe artifact results from bowel which is projected into an ovarian cyst (Fig. 1.3 ).

1.2.1.2 Beam Width Artifact This artifact occurs due to incorrect position of the focal zone (Feldman et al. 2009 ). A strong refl ector located within the widened beam beyond the margin of the transducer will gener-ate detectable echoes (Fig. 1.4 ). These echoes are assumed to have originated from the narrow imaging plane and are displayed as such. Beam width artifact is typically encountered with spu-rious echoes displayed in an anechoic area. For

example, refl ections from bowel generated by the edge of the beam are displayed inside the sagittal view of the urinary bladder where the beam is centered (Fig. 1.5 ).

Transducer

Grating lobe

Focus zone

Main beam

Side lobe

Fig. 1.1 Schematic diagram of an ultrasound beam. The primary (main) beam narrows as it approaches the focal zone and then diverges. Grating lobes and slide lobes are both off-axis lobes

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1.2.1.3 Slice Thickness Artifact This is similar to the beam width artifact but occurs due to the thickness of the beam which is 90° to the scan plane (Feldman et al. 2009 ). The slice of transducer will receive echoes from either side of the intended slice and will be included in

the displayed image. These artifacts are displaced in front of and behind the assumed plane of ori-gin. Increasing slice thickness will increase the number of artifactual echoes seen in the display. Slice thickness artifacts are typically seen in transverse views of the urinary bladder when adjacent structures are incorporated into the anechoic urinary bladder fl uid (Fig. 1.6 ).

1.2.2 Artifacts Due to Multiple Echoes

1.2.2.1 Reverberation (Comet Tail) Artifact

Multiple refl ections (reverberations) can occur between two strong parallel refl ectors or between the transducer and a strong parallel refl ector (Feldman et al. 2009 ). The echoes are refl ected back and forth repeatedly and may be strong enough to be detected by the instrument and dis-played (Fig. 1.7 ). The spuriously refl ected

Transducer

Fig. 1.2 Schematic diagram shows the side lobe artifact. The blue beams represent multiple side lobes of ultra-sound energy which encounter an object (a single black dot ). The display will assume that the echoes returning from this off-axis object have come from the main beam. As a result, the dot will be misplaced and multiplicated in the structure as shown

Fig. 1.3 US image shows the grating lobe artifact in which the bowel with reverberation artifacts ( white arrows ) is misregistered in an anechoic ovarian cyst ( black arrowheads ) which lies behind a gestational sac


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Transducer Transducer

Fig. 1.4 Schematic diagram shows the beam width artifact. The ultrasound image localization software assumes an imaging plane as shown by the dotted lines. Left diagram shows the effect of imaging an object of interest which is not positioned within the focal zone. Echoes from an object located in the peripheral fi eld ( black dot ) are displaced and seen to overlap the object of interest ( white circle ). Right diagram shows how this false image is eliminated by adjusting the focal zone so that it corresponds with the object of interest

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Fig. 1.5 Beam width artifact ( white arrow ) in the sagittal US image of the urinary bladder. The refl ections from bowel generated by the edge of the beam are displayed inside the sagittal view of the urinary bladder where the beam is centered

Fig. 1.6 Slice thickness artifact ( white arrows ) in the transverse US image of the urinary bladder. This artifact is typically seen in transverse views of the urinary bladder when adjacent structures are incorporated into the anechoic urinary bladder fl uid

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RealReverberationReverberationReverberationReverberation

Transducer

Fig. 1.7 Schematic diagram shows the reverberation artifact. Repeated refl ections between two interfaces cause reverberation of the displayed echoes


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echoes are displayed beneath the real refl ector at intervals equal to the distance between the trans-ducer and the real refl ector (Gibbs et al. 2009 ). Each subsequent refl ection is weaker than the previous one due to attenuation. Comet-tail arti-facts are usually seen in echo-free regions and are short, being less than 2 cm in length. Examples of reverberation artifacts include crystalline depos-its of cholesterol in the near wall of the gallblad-der, calcifi cation within tissues, or metallic structures. Reverberation artifact may also occur at the anterior peritoneal interface of the abdo-men (Fig. 1.8a, b ).

1.2.2.2 Ring-Down Artifact Ring-down (or resonance) artifact has a similar ultrasonographic appearance to comet-tail arti-fact but is generated by a different mechanism (Feldman et al. 2009 ). When the transmitted ultrasound beam passes through small gas or fl uid bubbles, the energy within the beam causes vibration of the gas or fl uid bubbles which, as a

result, emit and return sound waves back to the transducer (Fig. 1.9 ). Discrete echogenic lines or parallel bands of ring-down artifact extend from the point of origin. An example of ring-down artifact can be found with air or fl uid in the duo-denum (Fig. 1.10 ).

1.2.2.3 Mirror-Image Artifact This artifact occurs when an object is located directly in front of a highly refl ective smooth sur-face (Hedrick et al. 2005 ; Abu-Zidan et al. 2011 ). The ultrasound beam is totally refl ected by the strong refl ector towards the object. When the beam hits the object, part of the energy is refl ected back to the strong refl ector, which then redirects this echo towards the transducer. The true and false images are equidistant from but on opposite sides of the strong refl ector (Fig. 1.11 ). Mirror- image artifacts are most commonly seen where there is a large acoustic mismatch, such as an air- fl uid interface. The diaphragm, for example, is a strong refl ective interface, and the liver

a b

Fig. 1.8 ( a ) US image shows the comet-tail artifact ( white arrow ) from cholesterol crystals trapped in an Aschoff-Rokitansky sinus of the gallbladder wall. ( b ) US image

shows reverberation artifacts ( black arrowheads ) arising between the transducer-skin interface ( white arrow ) and strongly refl ective gas in a bowel loop ( black arrow )

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TransducerTransducer

Fig. 1.9 Schematic diagram shows the ring-down artifact which occurs when the main beam encounters air bubbles. The vibrations from the air bubbles cause a continuous source of sound energy that is transmitted back to the transducer for detection


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parenchyma is displaced above and below the diaphragm. Similarly, during scanning of the uri-nary bladder, the rectum can act as a specular refl ector leading to a mirror image of the urinary bladder being displayed posterior to the rectum (Fig. 1.12 ).

1.2.3 Artifacts Due to Velocity Errors

1.2.3.1 Velocity (Speed) Error Artifact The speed of sound varies within different tissues (Feldman et al. 2009 ). However, ultrasound tech-nology assumes that sound has a constant speed (1,540 m/s) within human tissue. If the average speed through the tissues is greater than 1,540 m/s, the returning echo will return faster to the transducer than calculated. The structure will thus be displayed more superfi cially on the image than expected. Conversely, if the average speed through the tissues is lower than 1,540 m/s, the structure will appear deeper than it is in reality

Fig. 1.10 Ring-down artifacts ( white arrow ) arising from air bubbles within the bowel seen on US

Transducer

Strong reflector

Falseimage

Object

Fig. 1.11 Schematic diagram shows the mirror-image artifact. A strong refl ector allows sampling of the object removed from the main beam as the sound beam does not follow a straight path

Fig. 1.12 US image shows the mirror-image artifact ( white arrow ) of a calcifi ed granuloma in the liver appear-ing above and below the diaphragm which acts as a strong refl ector

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(Fig. 1.13 ). Examples of velocity artifact include ultrasound imaging of a hyperechoic hepatic mass that leads to an ambiguous image of the dia-phragm or ultrasound imaging of a silicon breast implant that results in a distorted image of the fi brous capsule surrounding the implant (Fig. 1.14 ). The image posterior to the hyper-echoic lesion/silicone implant will appear deeper than it is in reality. As a result, the diaphragm or capsule appears to be distorted.

1.2.3.2 Refraction Artifact Refraction of the ultrasound beam at the bound-ary between two media with different acoustic

velocities (due to different density and elasticity) will result in misregistration and defocusing arti-fact (Feldman et al. 2009 ). In refraction artifact, a non-perpendicular incident ultrasound beam will refract and change direction at a boundary between two materials of different acoustic impedance. The ultrasound display process assumes that the beam travels in a straight line and thus misplaces the returning refracted echoes to the side of the true object location (Fig. 1.15 ).

Refraction artifact occurs when, for example, imaging the right kidney through the liver. The fat layer surrounding the liver bends the beam from a straight line. The kidney appears elon-gated since the refracted echoes of the kidney are misregistered and duplicated in the image (Fig. 1.16 ).

Double-image artifact (ghost image) is simi-lar in etiology to the refraction artifact (Hedrick et al. 2005 ). An example of double-image arti-fact is refraction of the ultrasound beam as its passes through the rectus abdominus muscles which act as lenses (Fig. 1.17 ). Multiple sam-pling of small objects by the redirected sound

Transducer

Reflector

3

1

2

Fig. 1.13 Schematic diagram shows the velocity error artifact. The refl ector is actually in position 1 . If the actual propagation speed is more than assumed, the refl ector will appear in position 3 . If the actual propagation speed is less than assumed, the refl ector will appear in position 2

Fig. 1.14 Velocity error artifact seen on US. The dia-phragm appears undulating ( black arrows ) due to a veloc-ity speed error caused by the heterogeneous hyperechoic liver mass ( white arrows )


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beam results in the duplication or triplication of these objects, leading to a ghost image (Fig. 1.18 ). In defocusing artifact (edge shadow-ing artifact), refraction produces shadowing at

the edges of a relatively large (compared to the ultrasound beam width) curved structure. The refraction occurs at the edge of the structure due

Transducer

False image

Object

Fig. 1.15 Schematic diagram shows the refraction arti-fact. Refraction of the ultrasound beam causes the object originally outside the beam path to be misplaced along the axis of the transducer

Fig. 1.16 Refraction artifact that leads to distortion of the kidney seen on US. The liver bends the sound beam and causes some distortion at the upper pole of the kidney ( white arrows )

Transducer

Object False imageFalse image

Fig. 1.17 Schematic diagram shows the ghost image artifact. Refraction at rectus muscles causes misplacement of a deep structure

Fig. 1.18 Ghost image artifact seen on US. Refraction at the rectus muscles causes double images of both the supe-rior mesenteric artery ( white arrows ) and the aorta ( black arrows )

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to the curved edge and difference acoustic impedance of the two tissue types involved. The ultrasound beam at the edge of the structure, such as a cyst or soft tissue mass, is deviated from its original path (Fig. 1.19 ).

1.2.4 Artifacts Due to Attenuation Errors

1.2.4.1 Acoustic Shadowing Artifact Acoustic shadowing artifact is caused by severe attenuation of the ultrasound beam at an inter-face, either by absorption or refl ection, resulting in reduced sound being transmitted beyond that interface (Feldman et al. 2009 ; Gibbs et al. 2009 ). This leads to an area of low-amplitude echoes behind an area of strongly attenuating tissue or a strongly refl ecting interface (Fig. 1.20 ). Acoustic shadowing artifact occurs at interfaces with large acoustic mismatch such as soft tissue and gas or soft tissue and bone (Fig. 1.21 ).

1.2.4.2 Acoustic Enhancement Artifact Acoustic enhancement artifact is caused by appli-cation of time-gain compensation to low- attenuating structures such as fl uid, resulting in unnecessary amplifi cation of echoes passing through this low-attenuating structure (Feldman

et al. 2009 ; Gibbs et al. 2009 ). This leads to an area of high-amplitude echoes behind an area of low-attenuating tissue (Fig. 1.22 ). Acoustic enhancement artifact occurs when scanning a

Fig. 1.19 Edge shadowing ( arrows ) is seen posterior to the fetal cranium on US

Transducer

Gallstone

Acousticshadowing

Fig. 1.20 Schematic diagram shows the acoustic shad-owing artifact. Acoustic shadowing occurs posterior to a strongly attenuating gallstone

Fig. 1.21 Acoustic shadowing ( white arrow ) is seen pos-terior to a gallbladder calculus due to a combination of refl ection and absorption on US


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low-attenuating structure such as a soft tissue mass or a cystic structure such as the gallbladder or urinary bladder (Fig. 1.23 ).

1.2.5 Artifacts Due to Improper Machine Settings

1.2.5.1 Range Ambiguity It is assumed that each pulse of echoes is received by the transmitter before the next pulse is emitted (Feldman et al. 2009 ). The instrument automatically reduces the pulse repetition fre-quency during imaging of deeper structures to avoid range ambiguity. If the returning fi rst pulse echoes from deep structures arrives after the second pulse has been transmitted, the returning fi rst pulse echoes are interpreted incorrectly as having originated from the most recently transmitted second pulse and will be incorrectly placed nearer to the transducer on the image. An example of range ambiguity is echoes within a cyst originating from a structure located deeper than the cyst (Fig. 1.24 ).

1.2.5.2 Equipment-Generated Artifact Incorrect use of the equipment control, such as gain or time-gain compensation, can result in recorded echoes being too bright or too dark (Feldman et al. 2009 ). This can affect image interpretation (Fig. 1.25 ).

Fig. 1.24 Range ambiguity in which the deeper liver parenchymal echotexture ( white arrow ) is displayed within the cyst on US

Fig. 1.23 Acoustic enhancement ( white arrow ) is seen posterior to a liver cyst on US

Transducer

Cyst

Acousticenhancement

Fig. 1.22 Schematic diagram shows the acoustic enhancement artifact. Acoustic enhancement occurs pos-terior to a low-attenuating cyst

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1.3 Artifacts Seen During Doppler Ultrasound Imaging

1.3.1 Artifacts Associated with Technical Limitations

1.3.1.1 Aliasing Artifact Aliasing artifact occurs when the velocity range exceeds the scale available to display and is only seen with color and spectral Doppler imaging (Rubens et al. 2006 ). Pulsed Doppler and color Doppler units have to recon-struct the complete Doppler shift signal from regularly timed samples of information. The sample rate is equal to pulse repetition fre-quency which limits the maximum velocity scale. The Nyquist limit is the upper limit of Doppler shift that can be detected accurately and is equal to one-half of the pulse repetition frequency. If the Doppler shift signal is higher than the Nyquist limit, aliasing artifact occurs with inaccurate display of color or spectral Doppler velocity. In color Doppler, the display “wraps around” the scale and overwrites the exiting data. In spectral Doppler, the velocity peak is cut off at the top end of the scale and is displayed at the lower end of the scale, i.e., below the baseline. Aliasing artifact is caused by high velocity fl ow leading to high-frequency

Doppler shift signals which exceeds the Nyquist limit. This can be overcome by increasing the velocity scale (pulse repetition frequency). Increasing the pulse repetition fre-quency is disadvantageous in that low veloci-ties cannot be accurately measured though is also advantageous in that the highest velocity area or area of abnormally high fl ow can be quickly localized (Fig. 1.26 ).

1.3.1.2 Range Ambiguity Artifact As for grayscale imaging, range ambiguity arti-fact will also occur in Doppler imaging with increasing pulse repetition frequency used to overcome an aliasing artifact (Rubens et al. 2006 ). This should be identifi ed in order to avoid the detection of frequency shifts deeper than their target object.

1.3.1.3 Blooming Artifact Blooming artifact is usually caused by abnor-mally high gain settings (Rubens et al. 2006 ). The color signal thus extends beyond the true grayscale vessel margin. This may mask luminal detail such as the presence of a thrombus. Color blooming artifact is also called “color bleed” (Fig. 1.27 ).

1.3.1.4 Directional Ambiguity/Indeterminate Flow Direction Artifact

This artifact in spectral Doppler tracing leads to the waveform being displayed with nearly equal amplitude above and below the spectral analysis baseline in a mirror-image pattern (Rubens et al. 2006 ). This is caused by the beam intercepting the vessel at a 90° angle, particularly for small vessels moving in and out of the imaging plane. True bidirectional fl ow is seen in pseudoaneurysms or when there is reversed diastolic fl ow in high resistance organs (such as in testicular torsion or deep venous thrombosis). True bidirectional is never simultaneously symmetric above and below the baseline and fl ow is seen to vary across the cardiac cycle (Fig. 1.28 ).

Fig. 1.25 Equipment-generated artifact due to incorrect time-gain control setting. The superfi cial part of the US image is too dark for accurate assessment


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Fig. 1.27 Blooming artifact seen on US. There is blooming artifact present in the left image . After adjustment of the color gain, the blooming artifact disappears, as shown in the right image

Fig. 1.26 Color Doppler aliasing. Longitudinal color Doppler US image of the internal carotid artery shows aliasing artifact ( white arrow ) at the arterial stenosis with spectra above and below the baseline. Aliasing artifact is caused by high velocity fl ow leading to high-frequency Doppler shift signals which exceeds the Nyquist limit

Fig. 1.28 Artifact from directional ambiguity caused by near 90° insonation angle in which the waveforms are dis-played with nearly equal amplitude above and below the baseline in a mirror-image pattern

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1.3.1.5 Artifact from Improper Doppler Angle with No Flow Artifact

The quality of Doppler signal degrades quite rapidly, and particularly so if the insonation angle is greater than 60° (Rubens et al. 2006 ). No Doppler signal is expected if the ultrasound beam is at 90° angle to the direction of fl owing blood. This Doppler angle effect artifact is seen in color, power, and spectral Doppler imaging (Fig. 1.29 ).

1.3.2 Artifacts Due to Patient Anatomy

1.3.2.1 Flash Artifact Movement of the patient, an organ, or the trans-ducer during Doppler imaging gives rise to a ran-dom color mosaic that obscures the grayscale image (Rubens et al. 2006 ). Power Doppler is most susceptible to this artifact. This artifact can be used to advantage to detect vessels with turbu-lent fl ow (such as in anastomotic sites, stenotic arteries, or arteriovenous fi stulae) which lead to the presence of this artifact in the adjacent soft tissues (Fig. 1.30a, b ).

1.3.2.2 Mirror-Image Artifact Mirror (or ghost) image artifact is caused by the same mechanism as that seen with grayscale imag-ing and is a feature of color, spectral, or power Doppler imaging (Rubens et al. 2006 ). For Doppler imaging, this artifact commonly occurs adjacent to the lung which is highly refl ective when, for exam-ple, examining the supraclavicular region. This leads to duplication of the subclavian artery or vein, and the false images are displayed deeper than the real images. Another example is duplica-tion of the common carotid artery which is some-times referred to a “carotid ghost” (Fig. 1.31 ).

1.3.2.3 Pseudofl ow Artifact The presence of fl uid fl ow other than that of blood can mimic real blow fl ow in color or power Doppler imaging (Rubens et al. 2006 ). However, no arterial or venous waveform will be seen if this apparent fl uid fl ow is subjected to spectral Doppler analysis. Examples of pseudofl ow artifact include fl uid fl ow in ascites, amniotic fl uid, or urine in ureteric jet emerging from the vesicoureteric junction. This ureteric jet artifact may be used to one’s advantage as the presence of ureteric jet helps exclude com-plete obstruction of the ureter (Fig. 1.32 ).

Fig. 1.29 Improper setting of high Doppler angle with artifactual absent fl ow in an artery ( white arrow ) ( left image ). Theoretically, no Doppler signal is detected if the

ultrasound beam is 90° to the direction of fl owing blood. After angle steering of the color box, color signals com-pletely fi ll the vessel lumen ( right image )


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1.3.3 Artifacts Due to Machine Factors

1.3.3.1 Twinkling Artifact Twinkling artifact can be seen behind any granu-lar (irregular or rough) refl ecting surface and is commonly caused by renal calculi, bladder calci-fi cation, and cholesterol crystals in the gallblad-der (Rubens et al. 2006 ). A mosaic of rapidly changing colors occurs which mimics tissue motion or fl uid fl ow just deep to a stationary strongly refl ecting interface in both color and power Doppler imaging (Fig. 1.33 ).

Fig. 1.31 Mirror-image artifact ( white arrow ) of the sub-clavian artery ( white arrowhead ) seen on US. It is formed by the air-fi lled lung apex posterior to the subclavian artery, acting as a highly refl ective acoustic interface

a

b

Fig. 1.32 ( a ) Artifact ( white arrow ) caused by turbulent uri-nary fl ow as a result of urinary jet at the vesicoureteric junction seen on US. ( b ) US image shows the pseudofl ow artifact ( white arrow ) within the gallbladder due to patient respiration

a b

Fig. 1.30 ( a ) Extensive fl ash artifact ( white arrow ) due to patient swallowing during US of the thyroid gland. ( b ) Flash artifact due to tissue vibration caused by the

high velocity fl ow from a pseudoaneurysm of the femoral artery seen on US

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Fig. 1.33 Twinkling artifact ( white arrow ) seen on color Doppler US is caused by renal calculus ( white arrowhead ) in the lower pole of the kidney


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1.3.3.2 Edge Artifact This artifact occurs at the margin of a strong, smooth refl ector (such as calcifi ed structures, e.g., gallstone or cortical bone), resulting in per-sistent color along the rim of the strong refl ector (Rubens et al. 2006 ). However, spectral tracing reveals no vascular fl ow pattern. Edge artifact is seen more commonly with power Doppler than color Doppler imaging (Fig. 1.34 ).

1.4 Artifacts Seen During Contrast Ultrasound Imaging

1.4.1 Color Blooming Artifact

Oversaturation of the color signal and color bloom-ing at the peak of contrast agent enhancement

can occur (Correas et al. 2001 ). It can be limited by reducing the color gain or by increasing the wall fi lter and the pulse repetition frequency (Fig. 1.35 ).

1.4.2 Increased Maximum Doppler Shift/Systolic Peak Velocity

Due to the effect of the contrast agent, the sys-tolic peak velocity can undergo an up to 50 % increase in peak velocity. This artifact is not, however, necessarily related to a change in blood fl ow. It has been suggested that the increase in velocity is an effect of the harmonic signal contribution from rupturing contrast agent microbubbles or a change in hemody-namics due to the contrast agent (Forsberg et al. 1994 ).

Conclusion

There are many causes of artifacts on both grayscale and Doppler ultrasound imaging. Radiologists should be aware of these arti-facts to avoid misinterpretation of pathology and, particularly with vascular imaging, to achieve accurate and reliable readings. It may not always be possible to precisely tell the source of the artifact since several arti-facts can produce the same aberrant effect on the image. Similarly, it may not always pos-sible to completely avoid a particular arti-fact. The important thing is to be aware of their presence and their wide-ranging appearances and avoid mistaking artifacts present on an ultrasound image as being a pathological lesion.

Fig. 1.34 Color Doppler US image shows the edge arti-fact ( white arrow ) of a gallbladder stone that results in a color rim along the edge of the gallstone (strong refl ector)

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References

Abu-Zidan FM, Hefny AF, Corr P (2011) Clinical ultrasound physics. J Emerg Trauma Shock 4:501–503

Correas JM, Bridal L, Lesavre A et al (2001) Ultrasound contrast agents: properties, principles of action, toler-ance, and artifacts. Eur Radiol 11:1316–1328

Feldman MK, Katyal S, Blackwood MS (2009) US arti-facts. RadioGraphics 29:1179–1189

Forsberg F, Liu JB, Burns PN et al (1994) Artifacts in ultrasonic contrast agent studies. J Ultrasound Med 13:357–365

Gibbs V, Cole D, Sassano C et al (2009) Ultrasound physics and technology: how, why and when. Churchill Livingstone/Elsevier, Edinburgh

Hedrick WR, Hykes DL, Starchman DE et al (2005) Ultrasound physics and instrumentation. Elsevier/Mosby, St Louis

Rubens DJ, Bhatt S, Nedelka S et al (2006) Doppler arti-facts and pitfalls. Radiol Clin North Am 44:805–835

Fig. 1.35 Color blooming artifact ( white arrow ) seen on the right US image. There is oversaturation of the color signal with color blooming at the peak of contrast agent

enhancement in the left portal vein. Left image is a color Doppler US image of the same portal vein before contrast administration


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Pitfalls in Diagnostic Radiology || Ultrasound Imaging