Carotid Artery Scanning:The Basics, the Pitfalls andCorrect Doppler Techniques
By: Angela Mills, RVS, BA• Angela Mills joined CardioServ in April 2011; accepting the role as Clinical Director.• Her responsibilities include clinical development, clinical education and training and advising on Ultrasound and Vascular Accreditation.• Angela is a Registered Vascular Specialist and holds a Bachelor of Business Administration Degree.• Angela has held a position at Palms West Hospital, Florida since 2001 where she continues to use her sonography skills and patient care.• Currently Angela is a member of SDMS, ACR, South Florida Echo Society and CCI.
Course ObjectivesUpon completion of this education activity, the participantswill be able to : • Identify the Carotid Artery and Vertebral Artery • Identify Proximal, Mid, and Distal in an Artery • List the recommended Carotid Artery Protocol • Identify and Describe each Artery’s Waveform • Identify and Describe Abnormal Waveform • Demonstrate Proper Sample Volume Placement • Apply Proper PW Doppler Measurements • Demonstrate Proper Angle Techniques • Apply Proper Angle Corrections • Identify and Describe Color Flow Patterns • Apply Corrections for Optimization of Color Flow Images • Evaluate the Carotid Artery for Disease
Agenda• Vessel Orientation and Identification• Carotid Artery Protocol• Waveforms• Sample Volume Placement• Doppler Measurements• Angle correction• Color flow
A comprehensive understanding of the anatomy ofthe circulatory system is essential for qualitydiagnostic vascular ultrasound including thecorrect identification of vessel(s) and anunderstanding of the vessel location beinginterrogated. Many vessels can be identified bythe waveform characteristics or landmarks withinthe body.Proximal in an Artery is closest to the heart sincethe blood is traveling away from the heart.Proximal in a Vein is the opposite since itterminates in the heart. Many Sonographersmislabel not only the name of the vessel but alsothe location within the vessels.
Proximal: Position nearest the center of the body,situated nearest to the point of origin or at thebeginning of a structure.Arteries originate at the heart - blood flow within anartery is traveling away from the heart so the proximallocation in an artery refers to the closest point of theartery to the heart.Proximal in a Vein is the opposite since it terminates inthe heart- blood flow within a vein is traveling towardsthe heart.Correct labeling of both the name of the vessel and thelocation of the vessel is needed.
Understanding the true location of the Proximal, Mid, and Distalsections of each vessel is central to a correct and systematic scanof the carotid arteries.CCAProximal:Proximal CCA will be closest to the clavicle bone. The CCAoriginates on the Left at the Aortic Arch and on the Right from theInnominate artery.Distal:The Distal CCA is located just before the vessel bifurcates into theInternal Carotid and External Carotid Arteries. The CCA DistalDoppler should be taken 2cm from the bulb.Mid:The Mid CCA will be directly between these two points. Locationwill vary depending on the patient’s neck and vessel length.
ICA:The ICA needs to be evaluated Proximal, Mid, and Distal. Muchlike the CCA the length of the artery that is visible will vary perperson.Proximal:Proximal will be just after the bifurcation.Distal:Distal will usually be as far as you can scan superiorly.Mid:Mid will depend on the patient and fall between the Proximal andDistal. The ICA terminates at the Circle of Willis.
ECA:• The ECA originates at the Bifurcation and terminates superiorly as it supplies blood to the face and tongue and external parts of the head.• The ECA usually runs medial and the ICA usually runs lateral as you travel up the neck.• Only one Doppler is necessary for this artery.
Vertebral artery:• The Vertebral artery runs posteriorly and is seen by sweeping lateral to the CCA or ICA.• The shadowing seen from the vertebrae will help to identify that you are in the correct place.• Only one PW doppler is needed to verify the direction of flow.
ECA D ECA M ECA P CCA D CCA M CCA P ICA D ICA M ICA P BULB
Standard ProtocolThe suggested protocol may be modified by each institution withrespect to their clinical settings and requirementsPatient PositionThe patient may lie down in the supine or semi-supine positionwith the head slightly hyperextended and rotated 45 away fromthe side being examined.TransducerHigher-frequency linear transducers (>7 MHz) are ideal forassessment of the intima-media thickness and plaquemorphology, while lower-frequency linear transducers (<7 MHz)are preferred for Doppler examination. In a short muscular neck, ifimaging with a linear transducer is impossible, a curved-arraytransducer (<7 MHz) may be helpful to document the anatomy ofthe carotid bifurcation with color Doppler US.
Carotid Artery ProtocolThe same protocol will be followed on both the left and the rightsides of the Carotids:TRANSVERSEStart with transverse in order to survey the entire Carotid Arterythis will give the sonographer and the radiologist an idea of whatlies ahead. (Plaque, Tortuosity, Occlusion) Start at the claviclebone and scan superiorly. The patients head will be tilted slightlyaway from the probe. Documentation of disease is required whenpresent.TRANSVERSE• CCA 2D• Bulb 2D• Bulb 2D/ with Color• ICA/ECA Bifurcation 2D• ICA/ECA Bifurcation 2D with Color
Carotid Artery ProtocolSAGITAL CCA:Move onto Sagittal views of the CCA. Make sure to evaluate theProximal, Mid, and Distal. Start at the most inferior aspect of theCCA and move the probe superiorly until the bifurcation is seen.If plaque is seen additional images are required.SAGITAL CCA:• Prox 2D• Prox Color• Prox Doppler (PSV and EDV measurement)• Mid 2D• Mid Color• Mid Doppler (PSV and EDV measurement)• Distal 2D• Distal Color• Distal Doppler (PSV and EDV measurement)
Carotid Artery ProtocolBULB:When scanning the Bulb make sure to carefully evaluate forplaque. Due to the flow separation or irregular flow patternsin the Bulb it is susceptible to plaque build up. It is importantto show the CCA open leading to the Bulb and then the Bulbopen leading to the Bifurcation.BULB:• Bulb 2D• Bulb Color
Carotid Artery ProtocolSAGITAL ICA:When scanning the ICA it is important to understand that this isthe main artery feeding the brain. The ICA runs slightly lateral tothe ECA and does not have branches. The waveform is lowresistance. Strict evaluation of this artery must be followed dueto its importance in stenosis evaluation.SAGITAL ICA:• Prox 2D• Prox Color• Prox Doppler (PSV and EDV measurement)• Mid 2D• Mid Color• Mid Doppler (PSV and EDV measurement)• Distal 2D• Distal Color• Distal Doppler (PSV and EDV measurement)
Carotid Artery ProtocolSAGITAL ECA:When evaluating the ECA only one measurement is necessaryfor waveform evaluation. Most do not grade the amount ofstenosis seen in this branch of the Carotid. If the ECA is above125 cm/s the Radiologist will note the increased flow. Thewaveform will change when occlusion is present.SAGITAL ECA:• 2D• Color• Doppler (PSV and EDV measurement)
Carotid Artery ProtocolVERTEBRALS:The final artery to evaluate is the Vertebral artery. The vertebralartery originates from the Subclavian on the left and theBrachiocephalic on the right. This artery runs in the vertebra andterminates at the Basilar artery. Probe placement to visualizethis artery can vary. The patient can look forward chin up; theprobe placed lateral to the trachea; and then angle slightlylateral. Another option is from the Bulb begin to sweep lateral.The Vertebral artery will be marked with shadowing from thevertebrae. Only one Doppler to evaluate flow direction.VERTEBRALS:• 2D• Color
ADDITIONAL IMAGES REQUIRED WHERE DISEASE IS PRESENTSTENOSIS:When stenosis is present documentation of the severity of stenosis shouldbe made. Additional PW Doppler should be taken in order to calculatethe area of highest stenosis. First document: • 2D Image Proximal to disease • Color Image Proximal to disease • PW Doppler with Color images proximal to diseaseNext walk your sample volume through the disease stopping at the areawith the highest velocity and document: • 2D Image in area of disease • Color Image in area of disease • PW Doppler with Color images in area of diseaseFinally documentation distal to the stenosis should be made. Be awarethat the post-stenotic turbulence will cause spectral broadening. This willhelp to diagnose the degree of narrowing. • 2D Image distal to disease • Color Image distal to disease • PW Doppler with Color images distal to disease
PLAQUE:When plaque is present the degree of narrowing may be requested. Thiscan be documented by area % stenosis or diameter % stenosis. TheDiameter is most used and found to be most accurate in a study byNASCET North American Symptomatic Carotid Endarterectomy Trial. Theformula used is (1-N/D)100=%N is at the narrowest segmentD is at the true diameter away from the Bulb.OCCLUDED ARTERIES:When declaring that an artery is occluded proper documentation is required.Make sure that the display of color and Doppler flow is not being blocked byartifact or poor probe placement. Once the determination has been made ofproper location and optimization the follow images must be documented: • 2D Image of disease • Color Doppler of disease • Power Color Doppler of disease • PW Doppler with Color of disease (gate should be widened to help detect flow)
WAVEFORMS:CCA:• The CCA waveform has characteristics of both low and high resistant waveforms but mainly low like that of the ICA.• If a distal vessel is occluded the waveform of the CCA will change. • If the ICA is occluded the CCA waveform will display high-pulsatility features. • If the ECA is occluded the CCA waveform will display a low resistant wave form, minimal change or no change at all.• If the obstruction is proximal to the CCA a low-amplitude, dampened waveforms will be seen.
ICA:• The ICA has a low resistant waveform• The ICA requires much stricter evaluation of the entire length of the artery.• When evaluating the artery be sure to scan as distal as possible without jeopardizing the quality of the image.• The ICA feeds the brain and can cause serious harm if plaque breaks free.• Determining the location of stenosis and severity is extremely important.• When occlusion or severe stenosis is present in the ICA the ICA and ECA waveforms can appear very similar.
ECA:• The ECA has a high resistant waveform• The ECA is not evaluated in as much detail as the ICA and CCA.• Only one PW Doppler is required for the ECA protocol which can be sampled at any area within the vessel.• The ECA has many branches where the ICA has none.• The ECA can take on the appearance of the ICA when occlusion occurs. When this occurs a temporal tap may be used to differentiate the two. • To perform a Temporal Tap during the PW Doppler of the ECA the Tech will tap just anterior to the ear causing a ripple in the waveform. (slide will be shown)VERT:• Low resistant waveform
Example of a CCA Distal Waveform Both ICA and ECA characteristics seen
Example of a ECA Proximal Waveform High resistance, pulsatile, low flow diastole
Example of a ICA Proximal WaveformMore high pitched and continuous than the ECA, Low resistance, high diastolic flow
Sample Volume SizeThe size and position of the sample volume, which can be controlled by theoperator, will also affect the proportion of the vessel exposed to theultrasound waves.• A small sample volume placed in the center of a large vessel may not detect any of the flow near the vessel wall at all• A larger sample volume, which could cover the whole depth of the vessel would detect the flow near the anterior and posterior walls but not the lateral walls.• The size sample volume will therefore affect the range of Doppler frequencies detected and should be taken into account when interpreting the degree of spectral broadening.• A narrow beam with a small sample volume placed in the center of the vessel may detect only the fast-moving blood and therefore, in normal circumstances, would not demonstrate much spectral broadening.• However, in the presence of disease, increased spectral broadening may be seen due to the presence of turbulent flow.
Examples of Sample Volume Size affecting the range of DopplerFrequencies A larger sample volume, which could cover the whole depth of the vessel, would detect the flow near the anterior and posterior walls but not the lateral walls. A small sample volume placed in the center of a large vessel may not detect any of the flow near the vessel wall at all.
The sample volume should be less than a third of the size of thevessel. Try to use 1-3mm, smaller preferred. The sample needs toalways be in the center of flow. The Angle should be parallel to thevessel wall or flow stream when stenosis or disturbance is present.
Example of the Sample Volume and Angle placement in area of stenosis.Normally the angle will be parallel to the vessel walls but when plaque ispresent the direction of blood flow will now govern the angle. The anglewill need to be parallel to the blood flow versus the vessel wall.
Do you know where to place your sample volume on a tortuous vessel?
Two examples of where to placeyour sample volume and anglewhen dealing with tortuousvessels.To the right shows Prox and belowshows Prox and Distal.
This one is tricky! An accurate Proximal measurement is not obtainable becausethe vessel is immediately tortuous off the bulb.Possible Techniques:• Move more distal into the artery• Angle the color box• Use the heel/toe movement to change the angle of the artery• The yellow and red marker below represents a good location for the mid vessel measurement.
Peak Systolic Velocity and End Diastolic Velocity Measurements • PSV and EDV should be carefully measured in each of the three locations for the CCA and the ICA (Proximal, Mid, Distal) • Once all three measurements have been made the location with the highest velocity for the CCA and the ICA will be used to calculate the ICA/CCA ratio. • The ICA/CCA Ratio is used in determining the severity of stenosis. • The PSV measurement is obtained at the peak of the waveform. • In the presence of irregular heart beats avoid early and late beats. Where possible, the velocity should be measured on the second or subsequent cardiac cycle of a string of consecutive regular cycles. • Measure the velocity that is the average representation of the patient’s waveforms. (Example to follow)
PSV and EDV Measurements continued…• The EDV measurement is obtained at the end for the waveform. In the presence of Bi-Phasic flow do not measure below the baseline.• Do Not measure the dip if it appears after the EDV• Do Not measure spikes in the waveform. Make sure you are measuring the peaks.• The PSV measurement is obtained at the peak of the waveform.
PSV and EDV Measurements continued…• Certain machines include an auto measure feature: The auto measure does not always measure correctly.• Correcting auto measurements: When making corrections after the cursor has been auto placed - pay close attention to the labeled cursor. Some equipment will prompt for the EDV measurement first. Accidentally switching your measurements (measuring the PSV as the EDV, and vice versa) will calculate incorrect ratios.• The auto measure feature can be removed to allow for manual placement of the cursors.
When a patient has an irregular heart rate what is the correct technique to measure the PSV? It is important to measure the average velocity. Due to the delayed filling process and then rapid contraction of the heart the initial waveform will appear much higher than the waveforms following that initial contraction.Never measure thewaveformimmediatelyfollowing a delay, asseen here.The first PSVand EDV is the onlymeasureablewaveforms.
Is this measurement correct? NOAlways use the average PSV and never after the delaywhen the velocity is at its highest.
Doppler AngleThe Doppler Angle is the angle of the Doppler beam with respect to thedirection of blood flow; also called the angle of incidence, or angle theta(θ) from the Doppler equation. The optimal angle for vascular duplexscanning is usually considered to be 45 to 60 ; angles greater than 60lead to significant errors in velocity measurement. Ninety degrees isthe worst Doppler angle, since it gives little or no frequency shift. AsSonographers we are usually not inside the vessel but on the surface ofthe skin. Therefore, Zero degrees, although it gives you the maximumpossible frequency shift, is not always obtainable.• Most places choose to stay as close to 60 as possible for standardization.• All diagnostic criteria for stenosis currently being used in the field is based on doppler samples taken at 60 .• Most newer machines allow angle correction and steering of the sample gate for maximum doppler range.
Correct: Angle correction and Sample placementand doppler measurements.
The angle in this image is at 31 . If the color box wascentered the angle would correct closer to 60
With the color box steered to the center as seen here in redthe angle can now be adjusted to the optimal degree forultrasound PW Doppler between 45 and 60 .
Effect of Doppler Examination Angle.All 8 images were acquired from the same location inthe same artery within the same examination periodusing the Doppler equation to correct for thegeometric angle between the ultrasound beam andthe artery axis. All duplex Doppler ultrasoundinstruments use this method to adjust the velocitymeasurement value reported.
Blood flow: TOWARDS the probeAngle is Lower than Angle is higher than45º and can be Normal Doppler Angles 60º and can becorrected with between 45º and 60º. corrected withheel/toe movement. 9 cm/s difference between the heel/toe movement.23cm/s less than the two. 28 cm/s more than the50 measurement 60 measurement
Blood flow: AWAY FROM the probeAngle is Lower than Angle is higher45º and can be Normal Doppler Angles between than 60º and cancorrected with 45º and 60º. 20 cm/s difference be corrected withheel/toe movement. between the two. heel/toe movement.17 cm/s less than the 17 cm/s more than50 angle. the 60 angle.
Heel/Toe Technique:This is accomplished by a combination of smalladjustments in the transducer. Placing pressure to thefront or back of the probe. This technique is used tocorrect the angle to the vessel flow. Sometimes a slightheel/toe movement of the transducer can correct thevessel angle enough to obtain a 60 angle and remainparallel with the walls.
Joint Recommendations for Reporting Carotid Ultrasound Investigations in the United KingdomIt was recognized that the ideal situation would be to measure allvelocities at a fixed angle, but that it was not always possible toachieve such a specific alignment either by steering the Dopplerbeam or by ‘‘heel and toeing’’ the probe. Using an angle of 45º -60º will minimize these effects and should ensure any error in thevelocity measurements due to Doppler angle alignment is lessthan 10%.The Working Group recommend that the Doppler angle should bein the range of 45º-60º with proper correction / calibration appliedusing the angle correction cursor. In the case of a tortuous vesselthe cursor should be aligned to the tangent of curvature at thepoint of measurement. In the case of an eccentric jet within astenosis the angle cursor should be aligned to the jet. Taken from and article posted by Elsevier
Why do we use color flow doppler?Four basic uses for color flow doppler in the Carotid: 1. To visualize flow patterns quickly. 2. Identifying flow in a vessel that appears occluded. 3. Visualize stenotic jets for angle correction. 4. To recognize anatomy that is difficult to identify.The need for correct PW Doppler angles is discussed a lot withinthe context of vascular ultrasound but it is just as important toadjust the color box.
Adjusting the Color BoxThe color box can be adjusted using the same controls and featuresused to enhance PW Doppler.Such as: • Color gain • Packet Size • PRF or Scale or Flow • Line density Rate • Transmit Power • Wall Filters • Maps • Baseline • Smoothing • Size • Persistence • Shape • Spatial Filter • Direction • Gray-scale Vs. Color • Invert write priority.
Example of Color Box Corrections: • When a vessel is identified the color box must be adjusted to the correct size, position, and angle. • If the vessel dips the color box will need to be centered. • If the vessel is straight or at a slight angle the color box will be angled in the same direction as the vessel angle. Whether it be left of right and with new machines the range is everything in between. Older machines just have left, right, and center. • The color represents the direction of flow. • The directional color map is controlled by the sonographer. If the invert button is selected the blue and red will reverse.
In order to enhance color filling the color scale(PRF) or colorgain may need to be corrected accordingly.• High velocity of flow: Increase the scale (PRF), decrease the color gains.• Low velocity of flow: Decrease the scale (PRF), Increase the color gains.Using both the scale and the gain can overcompensate in somecases. Only adjust one or the other may be all that is needed.The wall filter can be adjusted if needed due to wall motion onthe vessel.***Note: When a vessel is stenotic aliasing may occur andcorrecting the image may not be possible in severe cases.
What corrections could be used to enhance color flow in this image?Where would be the best sample volume placement?
Some vessels can be extremely deep and challenging to correctly quantify.In this image the color box (in yellow) could be centered and the bestpossible sample volume placement is demonstrated with the yellow dashes.
Another example of a tortuous vessel: Where would the bestsample volumes be placed?
An attempt at the best placement of angles within the vessel(A) approximately 60 and (B) and (C) will be close to 45 . A B C
This image demonstrates proper placement for the Proximal ICA.How would Doppler of the Mid ICA be performed on the sameimage?
1. Center the color box over the next section of vessel 2. Place the cursor and angle parallel with the vessel wall (45 ) 3. Center your sample volume (All changes seen in red)45
Good representation of color fill and proper color boxsteer; since the vessel dips the box is nearly centered.
Note that this color box is steered slightly left due to thevessel is already at a slight angle. The blood traveling towardthe probe is Blue and away from the probe is Red. This isidentified by the color map to the right.
Correcting Color aliasing: This is the same vessel one demonstratingaliasing, one not. What technique was used?Adjusting the angle of the color box eliminates the aliasing.
E–F, Increasing the color PRF from 14.4 cm/s to 43.3cm/s resolves the aliasing – number below color mapping.
Adjustment of the Color gain.Color Doppler image obtained When the color gain is lowered to 66%, thewith the color gain set at anatomy of the bifurcation is demonstrated80%shows marked turbulence in more accurately. The improvedboth the ICA and the ECA. But demonstration of the anatomy aids accurateno luminal narrowing is evident. placement of the sample volume box on the narrowest segment, with subsequent alignment of the Doppler angle parallel to the flow vectors.
Power DopplerPower Doppler increases diagnostic confidence when verifying orexcluding occlusion of vessels.Power Doppler imaging has recently gained attention as an additionalcolor flow imaging technique that overcomes some of the color Dopplerlimitations discussed. Limitations of conventional color Doppler include: Angle dependence Aliasing Difficulty in separating background noise from true flow in slow-flow states.Power Doppler’s increased sensitivity to flow is valuable in low-flowstates and when optimal Doppler angles cannot be obtained. Longersegments of vessels and more individual vessels can be visualized withpower Doppler than with conventional color Doppler.RadioGraphics – The Journal of Continuing Medical Education in Radiology Volume 17, pages 499-513
Power Doppler allows detection of trace flow that was not seen withcolor flow Doppler. String Flow in the ICA – Near Occlusion
THE ENDThank you forreviewing thispresentation.
References:http://www.cardiovascularultrasound.com/search/results?terms=carotid%20scanninghttp://search.medscape.com/news-search?newSearchHeader=1&queryText=carotid+ultrasoundhttp://radiographics.rsna.org/content/25/6/1561.figures-onlyhttp://www.ajronline.org/content/177/1/53.figures-onlyhttp://www.vascularweb.org/searchcenter/pages/vwebresults.aspx?k=carotid%20imageshttp://radiographics.rsna.org/content/25/6/1561/F31.expansion.htmlhttp://www.sdms.org/http://radiographics.rsna.org/content/25/6/1561.fullhttp://www.bmus.org/policies-guides/CarotidRecommendationsPublishedPaperCO.pdfIntroduction to Vascular Ultrasound; Zwiebel; W.B. Saunders CompanyIntroduction to Vascular Scanning, Aguide for the complete beginner; Donald P. Ridgway, RVT;Davies publishingGoogle Images