Introduction

In most imaging the word ªartefactº is taken to meancomponents of the image that are generated in the im-aging process but not properly indicative of the struc-tures studied. They can be: not real; missing; wronglyplaced; or changed in size, shape or brightness. In ultra-sound artefacts can be roughly divided into those ap-pearing mainly because the actual sound beam differsfrom the ideal beam assumed by the machine and arte-facts caused mainly by the signal processing. Whateverthe cause of the artefacts it is important for everyonedealing with ultrasound to recognise the pitfalls createdso as to avoid misdiagnosis [1, 2]. The other side of thecoin, equally important, is that a good knowledge of theartefacts and the mechanisms behind them will preventactual pathology from being discarded as artefactual, asituation which is potentially even more dangerous.

Grey-scale sonography

There are four incorrect assumptions made by the ma-chine about the properties of the sound beam:

1. The beam is narrow and uniform in width in both theimage plane and the orthogonal plane.

2. Attenuation of the sound in the tissue is uniform.3. The speed of sound is the same in all types of tissue in

the image.4. The beam travels directly and in a straight line to and

from its target.

Beam width and slice width artefacts

The scanner will assume razor-thin signals in both thelateral and the orthogonal plane. The true beam, how-ever, will be complex near the transducer, narrowed byfocusing and then fanning out deep to the focal zone.

Eur. Radiol. (2001) 11: 1308±1315DOI 10.1007/s003300100914 ULTRASOUND*

Anders Nilsson Artefacts in sonography and Doppler

Published online: 19 May 2001� Springer-Verlag 2001

*Categorical Course ECR 2002

A. Nilsson ())Department of Radiology,Academic Hospital, Uppsala,Sweden and Department of Radiology,Faculty of Medicine and Health Sciences,United Arab Emirates University, Al Ain,UAEE-mail: anders.nilsson@uaeu.ac.aePhone: +9 71-370395 41Fax: +9 71-3767 2067

Present address: A. Nilsson, Department ofRadiology, Faculty of Medicine and HealthSciences, United Arab Emirates University,P.O. Box 17666, Al Ain, UAE

Abstract A working knowledge ofthe most common artefacts insonography is essential in order toavoid not only misdiagnosis of arte-facts as pathology but also failure todetect pathology ªin disguiseº. Thebasic artefacts affecting all sonogra-phy and some of the ones particularto Doppler and contrast ultrasoundare discussed.

Keywords Artefacts ´Sonography ´ Doppler

The intensity of the sound will be highest in the centreof the beam and decrease progressively with a Gaussiandistribution in both planes:

1. The lateral thickness of the beam will cause a smallpinpoint reflector to be smeared laterally and thusimaged as a short line. The extent of this artefact de-pends on the slice thickness (focusing) and also onthe echogenicity of the reflector as a strong reflectorwill produce echoes above the machine's thresholdfurther away from the centre of the beam. This willalso limit the lateral spatial resolution as two adja-cent reflectors may merge due to the smearing.

2. Echoes beside the centre of the beam may be de-picted. In the lateral plane this will cause echoes tobe wrongly placed in the image, e.g. when gas in ad-jacent bowel produces echoes depicted within thegallbladder (Fig. 1). This is also a kind of smearingand is fairly easy to detect as the artefact-producing,usually strong, reflector is within the image plane. Inthe orthogonal plane, however, the reflector is out-side the image plane and therefore not seen in itselfmaking the artefact more difficult to detect [3, 4]. Anexample is a strong echo from gas lateral to the liverdepicted within the liver parenchyma on a longitudi-nal scan producing the appearance of a liver abscess.

3. A further problem is caused by side lobes and, forphased-array transducers, grating lobes. These aresound lobes sent out at an angle to the main beam.They are of low intensity, up to 10% of the ultra-

sound energy, but if a strong reflector lies in the di-rection of a side lobe it will be depicted as lying in theimage plane [5].

Both, items 2 and 3 above, also decrease image contrastby superimposing low-level echoes like noise on theimaged structures.

Attenuation

In soft tissue the major part of the attenuation is causedby absorption, i.e. the coherent vibration of tissue par-ticles that constitutes sound will become random andthus turn into heat. The attenuation is proportional tothe frequency and is approximately 1 dB/cm tissue foreach megahertz. As the sound is absorbed and to a less-er degree reflected when it travels through the tissue,the image should be darker further away from thetransducer as less sound has reached these regions. Thisis corrected by amplification of the weaker echoes fromthe far field and is controlled with the time-gain com-pensation (TGC). However, the setting of the TGCcurve operates across the entire image at a certain depthand artefacts will result where there are attenuationdifferences in the more superficial tissues.

Acoustic enhancement is actually not really en-hancement but lack of attenuation. It will appear as abright band behind an area with increased throughtransmission of sound, e.g. behind a cyst (Fig. 2) [6]. The

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Fig.1 Due to beam width, the strong echoes from the bowel arecausing artefacts within the gallbladder. This compromises imagequality and can sometimes be interpreted as gallbladder pathology

Fig.2 Acoustic enhancement seen as a bright band behind thegallbladder. Also note the narrow shadows (arrows) on both sidesof the enhancement. These are caused by shadowing behind thecurved edges of the gallbladder

widely used name for this phenomenon should not beconfused with the enhancement in vessels and paren-chyma caused by the injection of ultrasound contrastagents. A common misconception is that an enhance-ment only occurs behind an unechoic structure. How-ever, as reflection is only responsible for a small part ofthe attenuation, it can also appear behind hypoechoicsolid structures, e.g. lymphoma in the liver and indeedeven hyperechoic structures (i.e. a lot of reflection butlittle absorption) like fluids containing cholesterol orother crystals.

Acoustic shadowing, conversely, appears as a darkband behind a highly attenuating area. This can becaused by:

1. Increased absorption as behind areas with fibrosis orfat, e.g. the ligamentum teres.

2. Increased reflection as at a tissue±gas interfacewhere approximately 90 % of the sound is reflected.When this much sound is reflected there is often re-verberation artefacts (see below) causing a grey ªfill-inº of the shadow behind the reflector. This is knownas ªdirty shadowingº and is common behind bowelgas (Fig. 3).

3. A combination of these two mechanisms as seen be-hind gall- and kidney stones. In these instances thereis little or no reverberation artefact and the shadow isuniformly dark [7].

4. A forth kind of shadowing appears as a narrow darkband behind the edge of a rounded surface such as a

cyst wall or the gallbladder (Fig. 2). There are twoexplanations for the artefact; refraction of the soundbeam as it hits the rounded surface dispersing thesound or simply increased attenuation throughslightly thicker tissue. Whatever its cause, it is im-portant to recognise because, for example, shadowsfrom the curved surfaces of the gallbladder neck andcystic duct may imitate an impacted gallstone(Fig. 4).

Speed of sound calibration

The scanner is calibrated for a sound transmission of1540 m/s. This holds more or less true for most tissuesbut not for fat (1450 m/s) and not for materials such assilicone (600 m/s). When the sound transmission isslower than the calibrated speed, the echo will takelonger to return to the transducer and the reflector willthus be depicted deeper in the image than its true posi-tion [8]. This is usually not a problem in clinical scanningbut may cause some distortion in the image, e.g. pro-ducing a notch in the contour of the left kidney as theupper pole is imaged through the spleen and the lowerpole through layers of fat.

Another problem arising from differences in thespeed of sound propagation is that the beam will be re-fracted, i.e. its direction changed, when passing fromone type of tissue to another (Fig. 5).

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Fig.3 ªDirty shadowingº behind a segment of the transverse co-lon (arrows). Even though there is no image information behindthe strongly reflective surface between tissue and gas reverbera-tion, artefacts prevent the shadow from being uniformly dark

Fig.4 A narrow shadow (arrows) is caused by the curved surfacesat the neck of the gallbladder. The differential diagnosis betweenthis artefact and a small impacted gallstone may be difficult

Direction of the sound beam in tissue

The depth at which a reflector is plotted in the image isdependent on the time delay between the emission ofthe sound pulse and its return to the transducer. Arte-facts occur not only when the speed of sound signifi-cantly differs from the calibrated 1540 m/s but alsowhen the received signal has bounced (i.e. been reflect-ed) or has deviated (i.e. been refracted) within the tis-sue.

Reflection from surfaces parallel to the transducersurface

An echo from a strong reflector may, upon its return tothe transducer, bounce back from the surface, hit thereflector a second time and then return again to thetransducer. This produces a ghost image at double thedepth of the actual reflector, e.g. in pelvic scans thebladder may be duplicated in the far field creating theillusion of a hypoechoic mass.

If the reflector is strong, the sound may be reflectedback and forth many times creating multiple echoes thatproduce parallel stripes, called reverberations, in theimage (Fig. 6). If the reflectors are close, these stripescan merge causing a noise-like veil over the near field(Fig. 7). This reduces the image contrast.

Reverberations also appear behind any two parallelsurfaces. Each time the beam is reflected back and forthbetween the surfaces, a little sound returns to the trans-ducer producing multiple ghost echoes. As the sound isabsorbed and returned to the transducer the ghost ech-oes become progressively weaker. Since a weak echoappears narrower, a ªring downº or comet tail-likeartefact is created behind metal or plastic structures,small gas bubbles (as gas in the intrahepatic bile ducts;Fig. 8) or small cystic spaces such as the Aschoff-Roki-tansky sinuses in adenomyomatosis of the gallbladder.In foamy gas like in the duodenal bulb reverberationartefacts can be stronger creating a bright band extend-ing deep into the far field [9].

Reflections from a surface not parallel to the transducersurface

A mirror image may be created if the sound is reflectedfrom a flat, strong reflector to a target and back to thetransducer, again via the flat reflector (Fig. 9). Thiscommonly happens over the diaphragm where soundreflected from the tissue±air surface of the pleura canreach a target within the liver and, upon return to thetransducer, produce a ghost image of the liver structuresplaced within the thorax (Fig. 10) [10].

Sound that is reflected from a flat reflector angledaway from the transducer but not reaching a definedechogenic target will of course not produce a ghost im-

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a b

Fig.5a, b The propagation of the sound beam over a surface be-tween fat (c) and other soft tissues (b). The beam emitted from thetransducer (a) is refracted when it crosses the surface (d) and thusdeviates from the course assumed by the scanner. It is reflectedfrom the target (e) and returned to the transducer. The scannerassumes sound propagation along a straight line and thus place thetarget at an erroneous position (f). Apart from being displacedlaterally there is also an error in the depth of the resulting image aspart of the sound path has been through fat where the speed ofsound is less than 1540 m/s. The direction in which the beam is re-fracted will be different when sound passes from other tissue tofat (a), or vice versa (b)

Fig.6 Sound emitted from the transducer (a) is returned from thereflector (b). Upon return to the transducer surface some of thesound is reflected back into the tissue. It again reaches the reflectorand return to the transducer. This second pulse travels double thedistance and thus takes twice as long as the original pulse to reachthe transducer. The resulting artefactual echo (c) is thus placed atdouble the depth. This process can be repeated, especially whenthe reflector is strong, creating multiple artefacts, each one weakerthan the previous one (d)

age, but neither will the reflected sound be returned tothe transducer. This will decrease the echogenicity ofthe tissue and can be seen when imaging muscles andtendons with the fibres at an angle to the beam (Fig. 11).

Refraction

A sound beam passing between two tissues with a dif-ferent speed of sound is deviated from its true direction

(unless the angle of incidence is 90 �). In the body thishappens every time the sound passes a surface betweenfat and other soft tissues (Fig. 5). It decreases the spatialresolution and is particularly important to recognisewhen imaging deep in the lower part of the abdomenwhere the triangular fat pad between the rectus musclesmay act as a prism stretching the lateral measurement ofstructures deep in the pelvis. The artefact is less obviousin the upper part of the abdomen, but duplication of themesenteric artery can sometimes be seen. Moving thetransducer removes the prism effect in cases of doubt[11].

Signal processing in the scanner

All imaging systems are affected by noise. Structurednoise may appear in the image as patterns of stripes or

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Fig.7 Multiple echoes within the urinary bladder caused by re-verberation artefacts near the surface (short arrows) and noise dueto a faulty time-gain compensation setting (long arrows)

Fig.8 Artefactual echoes behind small gas bubbles in the intrahe-patic bile ducts

Fig.9 Drawing of the mechanism behind the mirror image arte-fact. Sound emitted from the transducer (a) reaches the target (c)via a flat, strong reflector (b) and returns along the same path tothe transducer. The scanner, assuming propagation along a straightline, places the echo in the wrong position (d)

Fig.10 Mirror image artefact placing intrahepatic structuresabove the diaphragm

flashes. They are usually caused by electrical interfer-ence, e.g. from adjacent machines, and are easily recog-nised as artefactual. Random noise is produced in allelectrical circuits. In ultrasound scanners it is usually ata very low level and thus only seen at high levels of am-plification, i.e. in the far field or with a too high TGCsetting (Fig. 7). The noise is easily seen in anechoic ar-eas but otherwise just causes a decrease in image con-trast. A widely used method to improve the signal-to-noise ratio is by frame averaging where the scanner willsuperimpose a number of images. This improves thedelineation of the real image components, whereas thenoise is averaged out. If too many frames are averagedand/or the transducer is moved too fast, images that arenot identical are added compromising image quality.

When the scanner is set at a too high pulse repetitionfrequency (PRF) range ambiguity is encountered [12].This occurs when one pulse is emitted before all theechoes from the previous pulse have returned. Thescanner cannot determine if a signal is an early echofrom one pulse or a late echo from the previous onemaking it impossible to determine the depth of the re-flector. Images from reflectors deep in the image maythus be positioned superficially. These images can, es-pecially when superimposed on an anechoic area such asthe urinary bladder, cause serious confusion. They dis-appear with a lower PRF.

When new image-producing software is introduced,there may also appear artefacts not previously noted.These artefacts are caused by the image processing,

rather than the properties of sound propagation. Theycan cause serious misdiagnoses and one should alwaysbe aware of the possibility of such artefacts appearingwhen using newly implemented software.

Doppler

Since the properties of the sound beam are the same forDoppler as for basic ultrasound, the same artefacts re-lated to beam shape and sound propagation apply. Themost important ones are:

1. Attenuation. The ability of the scanner to pick uplow flow, i.e. receive a signal above the sensitivitythreshold, is decreased with depth. A vessel deep inthe abdomen may therefore appear without a de-tectable flow even though flow is depicted in moresuperficial vessels. A lower frequency will penetratebetter or the signal may be improved by the injectionof an ultrasound contrast agent.

2. Beam width. The Doppler sampling volumes, e.g. thegate in spectral Doppler, is not a paper-thin squarebut more in the shape of a pear or a teardrop. Thus,flow in a vessel adjacent to the imaged structure canbe detected by the scanner and will, of course, bedepicted in the image plane creating the illusion offlow where there is none. This is usually not a prob-lem in clinical practise but is important to bear inmind when scanning regions with numerous vessels.

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a b

Fig.11a, b Difference in re-sulting echogenicity when flatreflectors, such as muscle fi-bres, are imaged a parallel tothe surface or b at an angle. Inthe latter case the sound is re-flected away from the trans-ducer causing a hypoechoic ap-pearance

A spectral Doppler trace is often helpful in deter-mining if the flow profile is consistent with the vesselinterrogated.

3. Reflection. Same as with B-mode ultrasound thesoundbeam may be reflected once or more causing re-verberations and ghost images of vessels. An equiva-lent to theabovedescribedmirror imageartefactat thediaphragm is when flow in the intrahepatic vessels maybe depicted as colour flow above the diaphragm.

4. Refraction. This causes the same kind of misplace-ment of structures (here flow signals) as in grey-scaleimaging.

There are also artefacts that are unique to Doppler, themost common being aliasing [13]. This occurs in bothspectral and colour Doppler when the PRF is too low tosample the Doppler shift adequately. (The Doppler fre-quency shift may not exceed one-half of the PRF.) Flowthat is too fast, i.e. that gives a too high Doppler shift, isdisplayed as fast but reversed flow as if the spectraltracing had gone off the scale and reappeared on theother side of the baseline. In colour Doppler this ap-pears as a mosaic of colours. It can be dealt with by in-creasing the pulse repetition frequency, increasing theDoppler angle (which will decrease the Doppler fre-quency shift) or by shifting the baseline. When dealingwith aliasing by increasing the PRF a problem withrange ambiguity [12, 13] (see above) may be encoun-tered. Many scanners deal with this problem by showingmultiple gates in the spectral Doppler image when thePRF is too high but in colour Doppler flow may be du-plicated in incorrect positions.

A situation similar to aliasing in spectral Doppler, i.e.with unidirectional flow producing a trace on both sidesof the baseline, may occur when the gain is too high(overload of the system), the Doppler angle is too highor the gain so low that the system cannot detect the signof the Doppler shift.

As the beam to flow angle increases, the frequencyshift is reduced giving a poorer signal. The accuracy ofthe angle measurement decreases and the angle correc-tion factor increases, the latter logarithmically; thus,Doppler angles above 60� should be avoided [14]. Thedeterioration of the signal strength affects all kinds ofDoppler, including power Doppler, albeit the deterio-ration in this case probably has more to do with the im-plementation of filters in the scanner [15]. Another im-portant aspect of beam to flow-angle problems is thatthe angle-in-case is the angle between the flow (vessel)and the propagation of the sound beam, not the surfaceof the image (in most instances the skin). When using asector scanner to image a vessel parallel to the skin, flowcan easily be towards the transducer at one end of theimage and away from it at the other.

In both colour and power Doppler ªbloomingº arte-facts may appear as colour outside, mainly deep to, the

vessels. This is most likely caused by reverberation and/or a too high gain setting. Tissue motion artefacts ap-pear when the scanner mistakes moving tissue for flow.In spectral Doppler a low-frequency (but high-ampli-tude) shift may come from pulsating vessel walls andsurrounding tissue. In colour Doppler they are oftencaused by respiratory movement in the upper abdomen;it may be annoying but is seldom a diagnostic problem.In power Doppler, however, they appear as flashes ofcolour across the image especially, of course, when thescanner is set at a high sensitivity and with a high gainsetting. In addition, they are more likely to appear inhyperechoic regions of the image [16]. This may causeproblems when the artefact is discreet. Artefactual col-

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Fig.12 Flow is detected in a branch of the portal vein, but arte-factual colour is also seen in the hyperechoic tissue adjacent to theliver. This could be mistaken for a tortuous continuation of the in-trahepatic vessels, i.e. a shunt, but a spectral Doppler trace willyield no flow information

Fig.13 Tissue vibration causing colour to be displayed in theperivascular tissue around a stenosis in an arteriovenous fistula fordialysis

our limited to a narrow hyperechoic band, e.g. the liga-mentum teres, is easily mistaken for flow (Fig. 12). Anattempt to get a spectral Doppler trace from the regionwill, however, fail and is often a good way of confirmingthe authenticity of a low flow identified with powerDoppler. A special kind of tissue motion artefact is theso-called perivascular artefact. This appears as colour (amosaic in colour Doppler or a single hue in power

Doppler) in perivascular soft tissues around vessels withturbulent flow (Fig. 13). It is thought to be caused bytissue vibration and is, although artefactual in its nature,an important sign of, for example, stenotic vessel seg-ments [17].

Colour may also appear outside vessels even withoutapparent tissue motion. This artefact can be noted be-hind small, focal, hyperechoic areas made up of indi-vidual reflectors, e.g. parenchymal calcifications orcatheters in fluid-filled spaces. It has been named theªtwinkling signº (Fig. 14) [18].

In colour and power Doppler the picture elementsare fewer but larger than with grey-scale imaging. TheDoppler pulses are also often made longer to improvethe accuracy of the measurements leading to a de-creased resolution. Colour images should therefore notbe used for measurement, e.g. of vessel diameter.

Finally, special artefacts may occur when ultrasoundcontrast agents are used. Because flow creating Dopplersignals too weak to be detected without contrast arenow seen, there will be an apparent spectral broadeningwhich will affect absolute values but, of course, not in-dices and ratios. Bubbles collapsing will give rise toshort peaks in a spectral trace and signals emitted by thisbubble destruction will, possibly together with rever-beration between bubbles, cause a more pronouncedblooming artefact in colour and power Doppler.

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Fig.14 An experimental set-up showing the twinkling artefactbehind a hyperechoic structure (in this case a forceps) in a water-bath

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