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rtifacts and Pitfall Errors Associated Withltrasound-Guided Regional Anesthesia. Part II:Pictorial Approach to Understanding

nd Avoidance

rian D. Sites, M.D., Richard Brull, M.D., F.R.C.P.C., Vincent W. S. Chan, M.D.,.R.C.P.C., Brian C. Spence, M.D., John Gallagher, M.D.,ichael L. Beach, M.D., Ph.D., Vincent R. Sites, M.D., Sherif Abbas, M.D.,

nd Gregg S. Hartman, M.D.

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he use of real-time ultrasound guidance in re-gional anesthesia is growing in popularity. Par-

mount to the successful and safe use of ultrasounds the appreciation and accurate interpretation ofommon ultrasound-generated artifacts. An artifacts any perceived distortion, error, or addition causedy the instrument of observation (signal proces-or).1 Imaging artifacts can be considered displayhenomena, and, therefore, can potentially compli-ate the planned procedure. There are 4 genericategories of imaging artifacts:2 (1) Acoustic: errorn presentation of ultrasound information; (2) An-tomic: error in interpretation (often called “pitfall”rror); (3) Optical illusion: error in perception; and4) Other: electrical noise.

This article builds on the fundamental principlesf ultrasound physics that are discussed in Part I ofhis article.3 The objective of this article is to de-cribe and illustrate many of the acoustic and ana-omic artifacts commonly encountered by the re-ional anesthesiologist. In the process, we will offernderlying physical explanations and describe prac-ical tips on how to negotiate these often misleadinghenomena.

From the Departments of Anesthesiology(B.D.S., B.C.S., J.G.,.L.B., G.S.H.) and Orthopedic Surgery (B.D.S.), Dartmouthedical School, Dartmouth-Hitchcock Medical Center, Leba-

on, NH; Department of Anesthesia and Pain Medicine (R.B.,.W.S.C., S.A.), Toronto Western Hospital, University of To-onto, Toronto, Ontario, Canada; and Department of RadiologyV.R.S.), Tufts University School of Medicine, Lahey Clinic, Bur-ington, MA.

Accepted for publication May 18, 2007.Reprint requests: Brian D. Sites, M.D., Regional Anesthesia,artmouth-Hitchcock Medical Center, One Medical Centerrive, Lebanon, NH 03756. E-mail: [email protected]© 2007 by the American Society of Regional Anesthesia and

ain Medicine.

p1098-7339/07/3205-0011$32.00/0doi:10.1016/j.rapm.2007.08.001

Regional Anesthesia and Pain Medicine, Vol 32, No

coustic Artifacts

Acoustic artifacts associated with performing re-ional anesthesia can be further subdivided into 2ajor categories: (1) Missing or falsely perceived

tructures; and (2) degraded images. The readerhould note that different imaging artifacts mayave the same underlying physical mechanism.

issing Structures or Falsely Perceived Objects

Overgain and undergain artifacts. Inappro-riately low gain settings may result in the apparentbsence of an existing structure (i.e., “missing struc-ure” artifact), whereas inappropriate high gain set-ings can easily obscure existing structures.

Example 1: Figure 1 represents two identical im-ges of the interscalene brachial plexus with the over-ll gain set too high (Fig 1A) and too low (Fig 1B).Example 2: The incorrect use of the time gain

ompensation (TGC) dials can create an imageherein existing structures appear absent. In Figure, the fourth TGC button was turned down too low,ffectively abolishing the image of the nerve roots.hen the TGC was set correctly (Fig 3), the C5

hrough C7 nerve roots can easily be seen.Clinical pearls: Gain adjustments are fraught with

otential artifact generation. Take advantage of the abil-ty to control overall gain levels and TGC. Adjust the gainettings to optimally define the structure of interest. Thesual pattern of the TGC dials is depicted in Figure 3B,ith a gradual increase in signal intensity; the near field

ain is turned down and the far field gain is increased inprogressive fashion.Lateral resolution artifacts. Lateral resolu-

ion refers to the system’s ability to distinguish 2bjects from one another when they exist in aateral to medial relationship.

Example 1: Figure 4B demonstrates the common
eroneal and tibial nerves as they appear in short
5 (September–October), 2007: pp 419–433 419

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420 Regional Anesthesia and Pain Medicine Vol. 32 No. 5 September–October 2007

xis in the popliteal fossa. Given that these struc-ures are superficial, the system is set to a highrequency (12 MHz). In addition, the ultrasoundeam is electronically focused (creating the narrow-st beam as possible) at the exact depth of theerves. The two nerves clearly appear as separateyperechoic structures. Figure 4A demonstrateshat happens when the frequency is reduced (8Hz) and the beam is incorrectly focused proximal

o the neural structures. Now the neural imagesppear less like 2 distinct structures due to theegradation in lateral (and axial) resolution. Figure 5emonstrates the degradation in the resolution ofhe popliteal sciatic nerve with incorrect focusettings.

ig 1. Incorrect overall gain settings. This is a short axisiew of the brachial plexus. (A) Overall gain set too high.B) Overall gain set too low. AS, anterior scalene muscle;5, C6, C7, the fifth through seventh cervical nerve roots;S, middle scalene muscle.

ig 2. Incorrect use of the time gain compensation dials.A) The ultrasound image is the short axis view of thenterscalene brachial plexus. The nerve roots are nowpparently missing. The arrow points to the band ofypoechoic tissue created by the incorrect use of the

ourth time compensation button. (B) Note that theourth time gain compensation dial is turned down, cre-ting the band of undergain that eliminates the C5, C6,nd C7 nerve roots. AS, anterior scalene muscle; MS,

fiiddle scalene muscle.

Clinical pearls: Electronically focus the ultrasoundeam on the structure of interest. Select the highest fre-uency probe that still allows adequate penetration to theepth of the structure of interest. Many ultrasound ma-hines have the ability to generate multiple focal zones forhe simultaneous imaging of several structures at differentepths.Acoustic shadowing. Acoustic shadowing oc-

urs when a structure has a larger attenuation co-fficient than the tissue that lies deep to it, causinghe deeper tissue to appear far less echogenic thanormal. Acoustic shadowing occurs most notablyhen seeking a target that lies deep to bone. The

ppreciation of acoustic shadowing is critical to theerformance of safe and effective ultrasound-guidedegional anesthesia. We will therefore present 5 ex-mples spanning the spectrum of regional anesthesia.Example 1: This is a case of acoustic shadowing

aused by bone during spinal imaging in prepara-ion for a neuraxial blockade. The acoustic shadowenerated by the adult spinous process impedes theenetration of the ultrasound waves, resulting inhe inability to reliably visualize the ligamentumavum, epidural space, and dura (Fig 6). The larger

ntervertebral spaces combined with less denselyssified childhood bone cause less acoustic shadow-ng and enable visualization of these importanttructures.4

Clinical pearls: The acoustic shadows produced by thepinous processes and lamina can help the operator iden-ify the midline when performing neuraxial blockade,specially in obese patients. The operator may opt foreedle insertion either above or below the level of anypinous process. Traditional endpoints for confirmation oforrect needle location into the epidural or intrathecalpace should always be employed.

Example 2: Acoustic shadowing created by the

ig 3. The correct time gain compensation (TGC) set-ings. (A) The C5 through C7 nerve roots are now easilyeen with the TGC settings. (B) The diagonal arrow indi-ates the typical pattern for the TGC dials, with a progres-ive increase (right shift) for the deeper structures. AS,nterior scalene muscle; MS, middle scalene muscle.

rst rib in the supraclavicular region. Acoustic shadow-

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Ultrasound-Generated Artifacts • Sites et al. 421

ng can lead to the erroneous assumption that therexist no structures of interest in an anechoic (black)egion. In the performance of a supraclavicular nervelock, the goal of the ultrasound exam is to visualizehe subclavian artery, the first rib, and the trunks orivisions of the brachial plexus. The important struc-ure to avoid contact with is the pleura which shouldxist immediately below the first rib. However, be-ause the rib shields the pleura from the ultrasoundeam, no anatomical information is provided (Fig 7).Clinical pearls: Never insert the needle into the ane-

hoic region deep to the first rib because the location of theleura cannot be identified.Example 3: Acoustic shadowing created by air

uring local anesthetic injection. In this dramaticase of acoustic shadowing, a popliteal sciatic nervelock was aborted secondary to the presumed dam-

ig 4. This image demonstrates the impact of lateral resolnd tibial nerve (TN) in short axis in the popliteal fossa.2 MHz because the structures are superficial. The beamote that you can clearly delineate 2 separate nerves. (A)

een in (B). In this image, the frequency was reduced torrowheads indicate the two nerves. The arrows indicate

ig 5. This is the short axis image of the sciatic nerve inhe popliteal fossa showing the impact of the focus onmage quality of a single structure. In (A), with the focuset correctly, the sciatic nerve is well defined. In (B), theocus is set incorrectly, thereby greatly degrading themage. The focus location is indicated by the {. With thisarticular system, there are multiple focal zones, explain-
ng the three icons on the right side of the screen. t
ge to the sciatic nerve. Fig 8 shows the sciaticerve before the injection. Fig 9 shows the nervefter 10 mL of local anesthetic was injected. Theciatic nerve appears to have been physically dis-ected in half. The most likely explanation for thiscoustic shadow is an air bubble located at the tipf the needle. In the video loop of this block,hen the needle is removed from the patient, theerve appears to suddenly reform (see Appendix,ideo 1). A small amount of air serves as theerfect medium to generate a dropout shadow, asir does not conduct ultrasound.

on the ability to image the common peroneal nerve (CP)short axis image of these nerves. The frequency is set attronically focused at the depth of the neural structures.urate imaging of the same structures, which were easily

z and the focus was placed superficial to the nerves. Thecal zone (narrowest point) of the ultrasound beam.

ig 6. Ultrasound image of the structures seen in theumbar region during the performance of an ultrasound-uided epidural placement. The bony structures, namelyhe spinous process (SP) and the lamina (L) act astrong specular reflectors. Ultrasound is unable to passhrough the bone and a hypoechoic acoustic shadowdrop out) is thus generated deep to the bone. Thisrecludes the ability to easily image the spinal cord. TP,

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Clinical pearls: When you suspect that the anatomy isrroneous secondary to needle or air-induced acoustichadowing, simply move the needle or reposition therobe slightly to allow the ultrasound waves to bypass thebstruction from air or metal. Further, the operatorhould pay careful attention to remove all air from thenjection syringes.

Example 4: This is an example of acoustic shad-wing created by a 19-gauge needle during an out-f-plane approach to place a continuous femoralerve catheter. In this example, the needle crossedhe ultrasound beam in a perpendicular mannerreating a linear acoustic shadow transecting the

ig 7. Ultrasound image of the structures seen during theerformance of a supraclavicular nerve block. Note thatlthough it is very easy to visualize the subclavian arterySA), the nerves, and the first rib, one cannot image theleura or lung. This is because the first rib acts like apecular reflector and a complete acoustic shadow (droput) occurs deep to the rib. The arrows indicate theivisions of the brachial plexus.

ig 8. Ultrasound image of the sciatic nerve as seen inhort axis in the popliteal fossa. The nerve appears as aarge and round hyperechoic structure. The triangles in-
icate the needle as it is approaching the nerve. f
erve (Fig 10). This acoustic shadow is helpful be-ause it tells the operator that the needle is correctlyligned in the lateral-medial plane. However, be-ause the exact location of the needle tip is un-nown (in the anterior-posterior plane), an alter-ative endpoint for injection or catheter threading

s necessary (such as nerve stimulation).Clinical pearls: Search for an acoustic shadow

transecting the target nerve) when inserting a needlesing the out-of-plane technique. This will help to con-rm the correct lateral-medial location of the needle rel-tive to the target nerve. The operator should appreciatehe limitations of the out-of-plane technique, primarily,

ig 9. After 10 ml of local anesthetic injection, the nerveppears to split in half. An acoustic shadow was mostikely generated by an air bubble at the needle tip. Thiscoustic shadow is easy to see passing right through theerve and into the anterior tissues.

ig 10. Ultrasound image of the infrainguinal structuress seen in short axis. There is an acoustic shadow (arrow)ssociated with the needle being advanced using the out-f-plane technique. The nerve appears falsely to haveeen separated into 2 structures. The dropout shadow isn indication that the needle tip or shaft has crossed theltrasound beam. FA, femoral artery; FV; femoral vein; N,

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he inability to confirm the real-time exact location of theeedle tip. See Figure 6 of Part I3 for the comparison of

n-plane versus out-of-plane techniques.Example 5: Acoustic shadowing may not only

ide important structures or distort normal anat-my (as in examples 1-4), it may also be a sign oferious patient pathology. For example, calcifiedrterial plaques act as strong specular reflectors thatenerate distinct acoustic shadows deep to theffected artery. Because most ultrasound-guidederve blocks make use of the intimate association oferves and blood vessels as an important referenceoint, the regional anesthesiologist will inevitablye confronted with the primary discovery of vascu-ar pathology. In addition, if a transarterial tech-ique is contemplated, one may wish to choosenother approach if acoustic shadowing is identifiedelated to a blood vessel of interest. Figure 11 showsn acoustic shadow associated with an atheroscle-otic plaque of the right femoral artery discovereduring the placement of a single injection femoralerve block. Figure 12 shows another acoustichadow associated with an atherosclerotic lesion ofhe right carotid artery discovered during the place-ent of an interscalene catheter. The hyperechoic

laque is also well visualized.Clinical pearls: If an acoustic shadow is identified re-

ated to a blood vessel, further clinical evaluation may beeeded to assess for significant vascular disease. This maye most important for patients in whom the regionalnesthesiologist may traumatize the blood vessel duringhe performance of the nerve block.

Acoustic enhancement. Acoustic enhance-ent occurs when a region behind a weakly atten-ating structure produces stronger echoes than

ig 11. Ultrasound image of the femoral artery in shortxis showing an atherosclerotic lesion. This pathologyas identified during the performance of a femoral nervelock. Note the hyperechoic plaque and the acoustichadow. Calcium deposits in the plaque act as strong

specular reflectors. FA, femoral artery.

hose observed from adjacent tissues. Enhancementrtifacts commonly occur when ultrasound wavesass relatively unattenuated through blood vesselsweak attenuators), resulting in false enhancementf the adjacent deeper tissue. Because many periph-ral nerves are associated with large blood vessels,coustic enhancement is a common finding.Examples 1 and 2: Enhancement artifact in the

egion of the infraclavicular and axillary brachiallexus can be most misleading as the tissue immedi-tely behind the posterior wall of the axillary arteryften appears very hyperechoic and is easily mistakenor either the radial nerve (axillary block; Fig 13) orhe posterior cord (infraclavicular block; Fig 14).

Clinical pearls: Because acoustic enhancement canake it difficult to definitively visualize either the radialerve or the posterior cord of the brachial plexus, it maye important to use an additional confirmation techniqueuch as nerve stimulation.

Absent blood flow when blood flow actuallyxists. Doppler is an important technology increening for vascularity of a region. However, as isrue for 2-dimensional imaging, the assessment oflood flow has the potential for artifact generation.he major concern for the regional anesthesiologists to falsely conclude that a structure is not a bloodessel when no flow is seen. As explained in Part I,3

oppler technology allows for the assessment ofoth velocity and directionality of blood flow. Theomplicating factor is that for accurate analysis,lood flow should be parallel to the ultrasoundeam.5 This is based on the Doppler equationPart I, Fig 63). In most cases, the regional anesthe-

ig 12. Ultrasound image of the carotid artery in longxis showing an atherosclerotic lesion. This pathologyas identified during the performance of an interscalene

atheter. Note the hyperechoic plaque and the character-stic acoustic shadow.

iologist is imaging blood vessels on short axis, and,

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herefore, the blood flow is completely perpendic-lar to the ultrasound beam. As the angle of inci-ence of the beam and the blood flow approaches0 degrees, the cosine of this angle approaches zerosee Doppler equation3), thereby creating an arti-act of no flow.

Clinical pearls and example: Figure 15 shows the leftarotid artery and internal jugular vein (IJ) as imaged onhort axis in the mid neck. Figure 15A reveals no bloodow in the IJ, whereas Figure 15B clearly suggests bloodow. The only difference between Figures 15A and B ishat the probe handle was tilted slightly cephalad, thushanging the angle of incidence (between the ultrasoundeam and the blood flow) from near 90 degrees to lesshan 90 degrees. This change in angulation allows foraximal return (to the probe) of the ultrasound waves

hat have shifted frequency (i.e., Doppler shift). Thisoppler shift is represented as blood flow on the screenisplay. The operator should note that the appearance oflood flow will also be dependent on the scale set tovaluate velocities and the color gain settings. In thisxample, the scale was set at 10 cm/s which is a relativelyow setting for a large artery. Low scales and higher colorain will tend to increase the sensitivity to detect flow.

egraded Images

Image degradation may be the result of user in-erface issues, but can often be the result of a phe-omenon known as reverberation. Reverberationsccur as the result of ultrasound waves bouncingack and forth between two strong specular reflec-ors. The result is usually multiple linear and hy-erechoic areas emanating distal to the reflectingtructures. When multiple reverberation artifactsre merged, this has been referred to as the comet

ig 13. Ultrasound image of the structures in short axiss seen in the axillary fossa. Acoustic enhancement of theissue immediately posterior to the blood vessel is indi-ated by the arrow. This hyperechoic area can easily beistaken for the radial nerve (R) which is actually lying

djacent to the artery. A, axillary artery; M, medianerve; MCN, musculocutaneous nerve; U, ulnar artery.

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Needle reverberation artifact. Example 1:lthough there are many reverberation artifactsescribed in the cardiac literature7 the most rele-ant reverberation artifact for the regional anesthe-iologist involves 2 specular reflectors: the probetself and the block needle. A depiction of an 18-auge needle inserted in-plane with the ultrasoundeam is elsewhere in this issue (Part I, Fig 5A3).nder the needle are seen multiple and equally

paced linear densities that represent ultrasoundaves bouncing back and forth within the lumen of

he needle. When the ultrasound energy finallyeturns to the probe to be processed, a duplicatemage of the needle will be displayed on the screen.his duplicate image will appear deeper than the pri-ary (actual) needle because more time has elapsed

or the ultrasound energy to return to the probe. Theechanism of the reverberation artifact is depicted in

igure 16.Example 2: Figure 17 illustrates the in-plane ap-

roach for a musculocutaneous nerve block. Theeedle is inserted through the biceps muscle. Due toreverberation artifact involving the needle, the

2-gauge b-bevel needle appears larger than ithould. In addition, the exact tip location is ob-cured secondary to this size discrepancy.

Clinical pearls: Reverberation artifacts occur to a largeregree when the needle is completely perpendicular to theltrasound beam. Therefore, if the angle of incidence isdjusted to less than 90 degrees, there will be less artifactillustrated in Fig 5B of Part I3). The operator shouldeep in mind that as the needle becomes less perpendic-

ig 14. Ultrasound image of the structures in short axiss seen in the infraclavicular fossa. Acoustic enhance-ent of the tissue posterior to the blood vessel can be

een. This generates confusion as to the exact location ofhe posterior cord (P) of the brachial plexus. A, axillaryrtery; L, lateral cord; M, medial cord; PMa, pectoralisajor muscle; PMi, pectoralis minor muscle; V, axillary

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lar to the ultrasound beam, the needle shaft will alsoecome more difficult to visualize. The other helpful tactics to reduce the far gain in an effort to darken theuplicate images of the needle.

ig 15. Ultrasound image of the short axis view of the internappears to be no blood flow in the IJ. This artifact was generatehe blood flow approached 90 degrees. Blood flow, however, cahis will create an angle of incidence greater than 90 degrees,

ig 16. Reverberation artifact, detailed in a stepwise man-er. Each number above the needle (top) has a correspond-

ng number on the ultrasound screen (bottom) to graphi-ally represent the result of different reverberation events.he original ultrasound beam contacts the needle and iseflected back to the probe correctly (1). In addition, part ofhe ultrasound beam penetrates the hollow needle and iseflected back to the probe from the distal wall of the needle2). However, a component of the ultrasound beam be-omes “stuck” within the needle lumen because the needlealls are highly reflective barriers. This signal component is

eflected between the needle walls several times beforeescaping” back to the probe (3), (4). Thus, the probe inter-rets these later occurring signals as objects distal to the needle
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Tissue reverberation artifact. Example 1:ultiple reverberations generated by the pleura in

he infraclavicular region can produce a comet tailign (Fig 18A). Hyperechoic streaks (“comets”) em-nating from a strong specular reflector such as theleura should alert the provider that the plannedoint of entry is too medial or that the plane ofmaging is too oblique, thereby prompting the re-ositioning of the probe prior to needle insertion.igure 18B is an example of the comet tail signssociated with the subclavian artery identified dur-ng a supraclavicular block.

Example 2: A linear artifact is generated when ainear tissue plane participates in a reverberationrocess which generates a reproduction of the

ar vein (IJ) being analyzed with color flow Doppler. (A) Thereuse the angle of incidence between the ultrasound beam andisualized by tilting the probe handle toward the patient’s head.me blood flow will be revealed (B). CA, carotid artery.

ig 17. Ultrasound image of the musculocutaneouserve (N) imaged in short axis in axillary fossa. Theeedle (only a 22-gauge b-bevel) can be seen in long axis

ust anterior to the nerve. The reverberation artifactakes the needle appear larger than it is and obscures the

xact tip location. Local (L) anesthesia can be seen sur-ounding the nerve. The triangles indicate the needle

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mage deep to its actual location. Figure 19 rep-esents the performance of an interscalene nervelock. The C5 nerve root can be easily visualizedrior to injection (Fig 19A). Postinjection (needleot present), there appears to be a physical struc-ure now present within the center of the C5 nerveoot (Fig 19B). This artifact most likely resultedrom a reverberation process associated with theelative echogenicity of a fascial layer above therachial plexus. Following the injection of local an-sthesia, the ultrasound waves are now reflectedack and forth between this fascial layer and an-ther specular reflector (most likely the probe it-elf). The end result is the depiction of the fascialayer at an erroneous position through the C5 nerveoot.

Clinical pearls: The comet tail sign (example 1) can beelpful as an alert to the provider that a strong speculareflector (e.g., bone or pleura) exists in the region of theroposed needle trajectory. To help confirm that a suspi-ious structure is a reverberation artifact, increase theressure applied to the probe against the skin. This pres-ure application should move (or eliminate) a linearrtifact to a more superficial location (because the distanceo the primary specular reflector has been decreased).ollowing probe pressure in example 2, the artifact dis-ppeared. If there truly were a lesion within the C5 nerveoot, then it would still appear at the original locationithin the C5 nerve root.

ig 18. (A) The comet sign visualized during preparationor an infraclavicular blockade. The streaks (“comets”)ndicated by the arrows represent the convergence of

ultiple reverberations generated by the first rib imagedbliquely in the infraclavicular region. The unlabeledrrow indicates the comet tail. Slight repositioning of therobe to the sagittal plane causes these artifacts to disap-ear. (B) Classic comet tail sign involving structures com-only seen during a supraclavicular block. AA, axillary

rtery; AV, axillary vein; PL, pleura; PMJ, pectoralis ma-
or; SA, subclavian artery. t
Example 3: “Double-barreled subclavian artery.”similar artifact has been reported in the cardiac

ltrasound literature for the aorta. Ultrasound mayounce back and forth within the lumen of anrtery, generating a duplication of the 2-dimen-ional image of the subclavian artery with the arti-act existing deep to the actual structure. This du-lication will also occur with Doppler flow.igure 20 shows a mirror image artifact involvinghe subclavian artery seen during the performancef a supraclavicular block.Clinical pearls: When performing a supraclavicular

lock, neural structures should be found adjacent to theore superficial (nonartifact) artery. If a needle is in-

ig 19. (A) Ultrasound image of the interscalene brachiallexus imaged in short axis during the performance of annterscalene nerve block. Only the fifth cervical nerveoot (C5) is visualized. Prior to the injection, the C5 nerveoot was visualized and appeared normal. (B) Followingnjection, there appeared to be a new structure within theenter of the nerve root. This represents a linear artifactenerated by a reverberation event involving a moreyperechoic structure anterior to the nerve root. Withhe application of probe pressure, the artifact disap-eared. MS, middle scalene; SCM, sternocleidomastoiduscle.

ig 20. A reverberation artifact showing a duplication ofhe subclavian artery (SA) found during a supraclavicularlock. The artifact exists for both the 2-dimensional im-ge and the Doppler analysis. The artifact is the structure
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erted towards the deeper (artifact), a pneumothorax willikely ensue.

Bayonet artifact. The term Bayonet artifactas coined by Gray et al.8 and first reported with

espect to a transarterial axillary plexus block. An ex-mple of a Bayonet artifact is illustrated in Figure 12aptured during a popliteal sciatic block performedith the patient in the supine position. The needle

s inserted in-plane and almost completely perpen-icular to the ultrasound beam. The ultrasoundmage suggests that the needle itself appears brokenr bent. This degraded image artifact is generatedue to the subtle differences in the speed of ultra-ound in various biological tissues. Although weenerally assume that ultrasound travels at 1,540eters per second inside the human body, the re-

lity is that ultrasound wave velocity varies slightlyith different tissues.9 In Figure 21, the needle

ravels through muscle and into the adipose tissueurrounding the sciatic nerve. The apparent bend inhe needle occurs because the speed of ultrasoundn adipose tissue is slower than that in the sur-ounding muscle. Therefore, the needle appears toend away (anterior) from the probe. When ultra-ound velocity is reduced, it takes longer for theaves to return to the probe. Based on the basic

ormula of distance equals velocity multiplied byime, the machine will register the part of the nee-le in the adipose tissue as deeper than the part ofhe needle in the muscle. This creates the bending

ig 21. Ultrasound image of the sciatic nerve (SN) in theopliteal fossa imaged in short axis. The hyperechoicound sciatic nerve can easily be seen. The needle isnserted from the lateral aspect of the patient. As theeedle enters the perineural adipose tissue, it appears toend in an anterior fashion. This artifact is generated byhe subtle differences in the velocity of ultrasound indipose and muscle tissue. The arrowheads indicate theeedle. The arrow indicates the bending point on theeedle.

ppearance. The direction of the bend in the needle i

s opposite to that presented by Gray8 in which theeedle bends toward the probe, because the speedf ultrasound in blood is faster than in the sur-ounding nerve sheath.

Probe-skin artifact. Because air does not con-uct ultrasound, the probe faceplate must fully con-act the skin without any interfacing air. This is theeason that a conductive gel is often placed betweenhe probe faceplate and the skin. Two situations canrise that generate significant dropout. The firstase can occur when only a portion of the probeootprint is able to make contact with the skin dueo size mismatch between the breadth of the foot-rint and the anatomical region to be imaged, asommonly occurs during tibial nerve block at thenkle or ulnar nerve block at the wrist. The latter isllustrated in Figure 22. This may preclude the abil-ty to image the progress of the needle if it is in-erted from the side of the dropout shadow. Whenomplete contact is made between the entire probeootprint and the skin (Fig 23), the dropout shadows eliminated. The second case of dropout can occurhen air pockets are contained between the plasticrobe cover and the probe footprint.Clinical pearls: Careful probe positioning, appropriate

robe selection, and placement of generous amounts ofltrasound transmission gel should eliminate most of thekin-to-probe dropout artifacts.

natomic Artifacts

Anatomic artifacts are tissue structures–eitherormal or aberrant–that may resemble the targeterve and thus mislead the operator into pursuinghe wrong target. These errors in interpretation areften referred to as “pitfall errors.” The commonolutions to all pitfall errors are to: (1) trace thearget nerve along its expected anatomic course;nd (2) use a peripheral nerve stimulator as andjunct to confirm the target’s identity.

ig 22. The incorrect position of the probe on the skinA) can generate a large dropout shadow (B) which couldomplicate the performance of an ulnar nerve (UN) block
n the mid forearm. UA, ulnar artery.

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endons and Muscles

Chief among the pitfall errors is mistaking a ten-on for the target nerve. The ultrasound appearancef nerves and tendons can appear indistinguishablyimilar. With experience, the ultrasonographer willppreciate the subtle hyperechoic continuous fibrilshat comprise a tendon compared with the hypo-choic fascicles inside a nerve.10 Flexor tendons can beasily mistaken for the median nerve when perform-ng a distal forearm median nerve block as depicted inigure 24. In the forearm, the tendons appear morerregular and less oval in comparison with the medianerve.Muscles can confuse the performance of an ul-

rasound-guided nerve block as well. This com-only occurs in the popliteal fossa. The common

eroneal, tibial, and sciatic nerves have very similarltrasound appearances when compared with theurrounding biceps femoris, semitendinosus, andemimembranosus muscles. One can appreciate thisimilarity in Figure 25, depicting an image of theopliteal fossa after the division of the sciatic nerve.he operator can easily distinguish the nerve fromhe surrounding muscle by appreciating the inter-ositioned adipose tissue (indicated in Fig 25). Ad-

pose tissue is more hypoechoic than either theerve or the muscle, and, therefore, creates a dis-ernable interface. In our experience, the mosthallenging popliteal blocks to perform are in ath-etes with well developed muscles and little adiposeissue. The sciatic nerve of an athlete as imagedroximally in the popliteal fossa is shown elsewheren Figure 3 of Part I.3 The difficulty in distinguishinghe sciatic nerve from the surrounding muscle isue to the paucity of surrounding adipose tissue.

ig 23. The correct way to place the probe on the skinA) to generate a complete image without dropout (B).he is important because the needle is commonly in-erted in-plane from the ulnar (i.e., medial) side of therobe. In situations similar to Figure 23, the approachingeedle may not be visualized. UA, ulnar artery; UN, ulnar
erve. s
Muscles can actually be confused for major bloodessels. Figure 26 shows the anatomy commonlyncountered during a supraclavicular approach tohe brachial plexus. In this patient, a structure ap-ears in long axis situated anterior to the brachiallexus. Due to the tubular and anechoic appear-nce, this structure appears virtually identical to aarge caliber blood vessel. This structure is the infe-ior belly of the omohyoid muscle and Dopplernterrogation will reveal no blood flow. The inferiorelly of the omohyoid muscle is a helpful landmarkn localizing the nerves which usually lie just pos-erior to the muscle.

ig 24. Ultrasound image of the median nerve (N) im-ged in short axis in the distal forearm. Note the similarityn echogenicity of the tendons and nerve. The trianglesndicate the tendons. The nerve becomes rounder andeparates from the tendons as the probe is moved moreroximally in the arm.

ig 25. Short axis ultrasound images of the structuresound in the popliteal fossa including the common pero-eal (CPN) and the tibial (TN) nerves. Note the similarityf the nerves to the surrounding muscle. The only aspectf this image that supports the ability to identify theerves is the small layer of adipose tissue that separatesuscles from nerves. BFM, biceps femoris muscle; STM,

emitendinosus muscle.

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lood Vessels

Blood vessels are not usually mistaken for nervesecause there are several key identifying featureshat differentiate one from the other. In general,erves are echogenic, whether hypoechoic or hy-erechoic relative to the surrounding tissue, whilelood vessels are uniformly anechoic structures.econd, arteries can be seen to pulsate and resistompression. Conversely, veins do not pulsate andre easily compressible. Importantly, nerves appearonpulsatile and are not compressible. Finally,olor Doppler can detect blood flow within an im-ged structure, thereby positively identifying it as alood vessel. Color Doppler may be especiallymportant to help differentiate artery from nervehen performing an ultrasound-guided inter-

calene brachial plexus blockade. Unlike mostther nerves in the body, the roots of the brachiallexus can appear perfectly round and anechoic.

small caliber noncompressible artery in theeck may be dangerously mistaken for a nerveoot, as evidenced in Figure 27. In this case, coloroppler promptly identified the hazardous vascu-

ar structure. In addition, the supraclavicularlock in Figure 28 and Video 2 (see Appendix)as aborted in favor of an infraclavicular block

econdary to anomalous hypervascularity surround-ng the brachial plexus.

ig 26. Short axis view of the brachial plexus and sub-lavian artery in the supraclavicular fossa. The trianglesutline an anechoic structure resembling a large bloodessel imaged in long axis. This structure is actually thenferior belly of the omohyoid muscle. The divisions ofhe brachial plexus are sandwiched between the omohy-id muscle anteriorly and the first rib posteriorly, as
ndicated in the cartoon. A, subclavian artery. a
Sometimes, however, even color Doppler can failo identify a blood vessel, as was the case in Figure9, which depicts a thrombosed internal jugularein. A thrombosed vein is noncompressible withnternal hypoechoic shadows, not dissimilar fromhe appearance of a nerve. Whenever basic ma-euvers (compression), defining features (internalchogenicity), and color Doppler fail to identify atructure of interest, the operator must remembero trace the structure proximally and distally as wells use a nerve stimulator for definitive confirma-ion. In addition, it is useful to scan the anticipatedath of the needle with color Doppler prior to nee-le insertion. This will help (as in Figure 28) to

ig 27. (A) The interscalene brachial plexus imaged inhort axis. Color Doppler interrogation (B) revealed thathat we thought was the C5 nerve root was actually a

mall artery (noncompressible with light probe pressure).

ig 28. Short axis view of the supraclavicular fossa.creening with color flow Doppler is an important safetyddition to any nerve block. Here we discovered unex-ected and anomalous arterial vascularity. This vascular-ty was not appreciated until we scanned the area witholor Doppler. The procedure was aborted and changed to
n infraclavicular block. A, subclavian artery.

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dentify any unnamed and unsuspected vasculartructures.

ymph Nodes

For the regional anesthesiologist, inflamed lymphodes may complicate ultrasound-guided periph-ral nerve blockade in the cervical, axillary, andnguinal regions. There are several sonographic fea-ures that can help to differentiate lymph nodesrom nerves. Inflamed lymph nodes are generallyarge noncompressible, well circumscribed, roundnechoic structures with small internal hypoechoic

ig 29. An occluded internal jugular vein found duringhe performance of an interscalene nerve block. Coloroppler flow demonstrated no flow and the vessel didot compress with light probe pressure. Note the hypere-hoic thrombus found in the normally anechoic lumen.A, carotid artery.

val-shaped opacities, which are representative ofntranodal necrosis.11 Figure 30 depicts inguinalymphadenopathy incidentally encountered uponerforming a femoral nerve block in a patient pre-enting for a total knee arthroplasty. Upon re-ognition of the lymphadenopathy, the operatorsecided to perform a single injection procedure

nstead of the originally planned continuous femo-al nerve catheter. Figure 31 shows a lymph nodehat was initially mistaken for a venous thrombosisnvolving the saphenous vein in the infrainguinalegion. Finally, lymph nodes have a very similarppearance to nerves in the axilla and as such canonfound the performance of an ultrasound-guidedxillary block. In these situations a nerve stimulatoray be indispensable to physiologically distinguishlymph node from nerve.

erves

How can we be fooled by the target nerve itself?ommonly, the operator can easily identify the tar-et nerve in a particular location, but while manip-lating the probe to optimize the image quality, theerve suddenly disappears. There are 2 explana-ions for this apparent vanishing act.

First, peripheral nerves are highly mobile struc-ures that can change position with even smallmounts of pressure routinely applied to administerperipheral nerve block. This phenomenon was

ptly described by Retzl and colleagues who dem-nstrated the varying location of the median nerveelative to the axillary artery with gentle pressurepplied by the ultrasound probe.12 Further, text-

Fig 30. Ultrasound appear-ance of lymphadenopathyfound in the inguinal regionduring the performance of afemoral nerve block. Lymphnodes (LN) often have aheterogeneous ultrasoundappearance. They may beround, oval, and havesepti (as in this case).They can easily be con-fused for neural tissue oroccluded veins (Fig 31).

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ooks traditionally designate the area posterior tohe axillary artery as the location of the radialerve, but ongoing experience with ultrasound iseaching us that the location of the radial nerve isighly variable. Further, as Figure 32 shows, thelnar nerve is often located at a significant distance

rom the axillary artery. In this patient, the ulnarerve was 3.5 cm away from the axillary arteryuring the performance of an axillary block. Nervetimulation was used to confirm that this was trulyhe ulnar nerve.

ig 31. Ultrasound appearance of an inguinal lymphode discovered during the performance of a continuous

emoral nerve catheter. We originally believed this struc-ure to be an occluded saphenous vein. However, patencyf the saphenous vein was later confirmed. The anechoicuter ring of the lymph node (indicated by the arrows)epresents perinodal edema, whereas the more echogenicnterior represents the soft tissue aspects of the node. FA,emoral artery.

ig 32. Ultrasound ap-earance of the axillaryrachial plexus as imagedn short axis. The operatordentified the ulnar nerve.5 cm away from the ax-llary artery (a). Due to thisistance from the artery,erve stimulation wassed to confirm that thistructure was truly the ul-ar nerve (U). M, medianerve.

Secondly, there is the issue of reflection and re-raction of ultrasound energy. The only way anmage can be displayed on the ultrasound screen isf ultrasound reflects off of a nerve and successfullyeturns to the probe. If a significant portion of theltrasound energy bounces off of the nerve and isransmitted away from the probe, then the image ofhe nerve will either be degraded or completelyissing. Figure 33 shows how subtle angulations of

he probe can either degrade or optimize the neuralmage. The amount of ultrasound reflected or re-racted depends on the angle at which the ultra-ound beam hits the interface between differentbjects (media). As the angle of incidence nears0 degrees (perpendicular to the structure of in-erest), a higher percentage of reflected ultra-ound successfully returns to the probe. There-ore, the operator should sweep the beamhrough an arc to try to generate a situationhich optimizes the image.

dema

The regional anesthesiologist performing extremitylocks will inevitably encounter patients with signifi-ant peripheral edema. This edema may physicallyistort the neural anatomy (depth change), mask hy-oechoic structures, dilute the local anesthetic injec-ion, and possibly alter nerve stimulation thresholds.igure 34 shows mid calf edema in a patient having aid calf saphenous nerve block. In this case the op-

rators were unable to find the saphenous nerve oraphenous vein due to the severity of the edema.

Alternatively, edema may sometimes facilitatehe image localization of a nerve. That is, a nervehat appears primarily hyperechoic (e.g., sciatic),ay become prominently outlined by the hypo-

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choic edema fluid. This is analogous to the injec-ion of ultrasound contrast media to facilitate nerveocalization or cardiac imaging.13

onclusion

Knowledge of ultrasound related artifacts is anxciting educational opportunity for the regionalnesthesiologist. At the very least, these artifacts areuisances and provoke intellectual curiosity. At theery worst, a misinterpretation of an ultrasound-enerated artifact may result in a negative patientutcome. It should be emphasized that as our com-unity’s experience grows with these relativelyew techniques, there will undoubtedly be addi-ions to this preliminary database of artifacts.

Our anecdotal and collective experiences are thathe appreciation of the existence, the understand-ng of the mechanisms, and the recognition of the

ig 33. Short axis view of the ulnar nerve in the midorearm. The angle of incidence between the ultrasoundeam and the nerve is important for optimal imaging.ubtle angulations can result in dramatic variations inmage quality. The right hand side of the Figure depicts aituation where a subtle tilting motion of the probe re-ults in degradation of the image, presumably from aeduction in effective reflection and an increase in refrac-ion. The left hand side of the figure represents the idealituation with optimal imaging. The white arrow indi-
ates the ulnar nerve.
ppearances of the aforementioned artifacts sup-orts the efficient and safe conduct of ultrasound-uided regional anesthesia.

ppendix

upplementary material (video files)

Supplementary videos can be found in the onlineersion of this article at www.rapm.org (doi:0.1016/j.rapm.2007.08.001).

References

1. Webster’s Online Dictionary. Available at http://www.websters-online-dictionary.org/definition/artifact. Ac-cessed May 1, 2007.

2. Kremkau F, Taylor K. Artifacts in ultrasound imag-ing. J Ultrasound Med 1986;5:227-237.

3. Sites BD, Brull R, Chan VWS, Spence BC, GallagherJ, Beach ML, Sites VR, Hartman GS. Artifacts andpitfall errors associated with ultrasound-guided re-gional anesthesia. Part I: Understanding the basicprinciples of ultrasound physics and machine opera-tions. Reg Anesth Pain Med 2007;32:412-418.

4. Rapp HJ, Folger A, Grau T. Ultrasound-guided epi-dural catheter insertion in children. Anesth Analg2005;101:333-339.

5. Perinno A, Reeves S, eds. A Practical Approach toTransesophageal Echocardiography. Philadelphia: Lip-pincott Williams & Wilkins; 2003:77-92.

6. Thickman DI, Ziskin MC, Goldenberg NJ, Linder BE.Clinical manifestations of the comet tail artifact.J Ultrasound Med 1983;2:225-230.

7. Blanchard DG, Dittrich HC, Mitchell M, McCann HA.Diagnostic pitfalls in transesophageal echocardiogra-

ig 34. Ultrasound appearance of the soft tissues of theid calf during the performance of a saphenous nerve

lock. Because the ultrasound approach to the saphenouserve block relies in the visualization of the greater sa-henous vein, edema (indicated by arrows) can com-ound this block by obscuring the landmark vein. Edemaas the same anechoic appearance as any vein. This pro-edure was aborted.

phy. J Am Soc Echocardiogr 1992;5:525-540.

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http://10.1016/j.rapm.2007.08.001

http://www.websters-online-dictionary.org/definition/artifact

http://www.websters-online-dictionary.org/definition/artifact

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8. Gray AT. Bayonet artifact during ultrasound-guidedtransarterial axillary block. Anesthesiology 2005;102:1291-1292.

9. Anderson ME, McKeag MS, Trahey GE. The impactof sound speed errors on medical ultrasound imag-ing. J Acoust Soc Am 2000;107:3540-3548.

0. Silvestri E, Martinoli C, Derchi LE, Bertolotto M,Chiaramondia M, Rosenberg I. Echotexture of pe-ripheral nerves: correlation between US and histo-logical findings and criteria to differentiate tendons.
Radiology 1995;197:291-296.
1. Ahuja A, Ying M. Grey-scale sonography in assess-ment of cervical lymphadenopathy: review ofsonographic appearances and features that mayhelp a beginner. Br J Oral Maxillofac Surg 2000;38:451-459.

2. Retzl G, Kapral S, Greher M, Mauritz W. Ultrasono-graphic findings of the axillary part of the brachialplexus. Anesth Analg 2001;92:1271-1275.

3. Goldberg BB, Liu JB, Forsberg F. Ultrasound contrastagents: a review. Ultrasound Med Biol 1994;20:319-
333.

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