2173850-arterial-doppler

8/7/2019 2173850-Arterial-Doppler

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ARTERIAL DOPPLER

Peripheral arterial disease is a common and serious disorder, with a prevalence ofapproximately 4.5% to 9%. Compromise of arterial flow due to stenoses and occlusionscan result in limb ischemia, which may manifest as claudication, rest pain, local tissue

loss (ulceration), and, potentially, amputation. Treatment options include medicaltherapy, bypass surgery, and various percutaneous interventions such as angioplasty,atherectomy, stent placement, and thrombolysis. Techniques available for the diagnosisof peripheral arterial disease include angiography, which is considered the standard ofreference but is invasive, and various noninvasive methods. The noninvasive tests thathave traditionally been performed include segmental pressures, pressure volume tests(plethysmography), and color-assisted duplex sonography. In recent years, magneticresonance (MR) imaging and computed tomographic (CT) angiography have been usedin the evaluation of PAD, with promising results that should only improve withrefinements in technology. Since the type (ie, stenosis vs occlusion), length, location,and number of lesions play an important role in the determination of choice of therapy,obtaining this information before an invasive procedure may be advantageous fortreatment planning. Noninvasive imaging is also useful for follow-up of treated lesionsand for graft survellience.

The noninvasive examination for peripheral arterial disease in our laboratory consistsof ultrasonography (US) and pressure measurements, the latter including ankle:brachialindex and segmental pressures. Although less sensitive than US, this is a relativelysimple and rapid test that provides a global, quantitative, and objective indication ofdisease and complements the information obtained from the US examination. Thepurpose of this presentation is to review the techniques of arterial US and pressuremeasurments for the diagnosis of lower-extremity arterial disease.

HEMODYNAMICS OF STENOSIS

The basis for the Doppler diagnosis of vascular stenosis is the principle of volumecontinuity, which states that the velocity of blood flow through a narrowed portion of avessel will increase if the volume of flow per unit time in the segment is constant. Thevolume of flow Q is equal to the product of the vessel cross-sectional area A and theaverage flow velocity v. Assuming the volume of blood remains constant throughout theregion of narrowing (Fig.1).
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Fig.1 Assuming flow is constant, as the cross-sectional area (A) decreases, the velocity (v)increases.

Q = v1A1 = v2A2;

therefore,

v2/v1 = A1/A2,

and as A decreases, v increases.

As the residual diameter of a stenosis decreases, there is an increase in resistance and,eventually, a decrease in overall flow and a drop in pressure. From a clinicalperspective, a lesion is hemodynamically significant if it causes a perfusion deficit duringrest or exercise. The greater the degree of stenosis and the longer its length, thegreater the associated pressure decrement. The degree of stenosis beyond which asmall increase in severity results in a significant reduction of flow is referred to as a"critical" or "hemodynamically significant" narrowing. This value is generallyacknowledged to be 50% of the luminal diameter in the peripheral arterial system, whichcorresponds to a 75% decrease in cross-sectional area. This number is somewhatarbitrary in that it is strongly affected by peripheral vascular resistance and the status ofthe pre- and poststenotic vasculature.

The major criterion for the Doppler diagnosis of arterial stenosis is a focal increase invelocity (peak systolic velocity [PSV]), but there are several other hemodynamic issuesthat affect the pulsed Doppler waveform and are therefore useful in waveforminterpretation. These are laminar versus turbulent flow, and pulsatile flow.

LAMINAR AND TURBULENT FLOW

The flow velocity profile in a straight vessel with a uniform diameter is known as alaminar profile; it is characterized by a smooth, predictable velocity gradient across the

cross-sectional area (Fig 2),

Fig. 2 Parabolic flow. Flow occurs in orderly, aligned laminae, with the fastest velocity in thecenter, and a progressive decrease in velocity toward the vessel wall.
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with the highest-velocity flow at the center and a gradual decrease toward the vesselwall, with an infinitesimally thin layer in contact with the wall having a velocity of zero.The geometry of this flow pattern approximates a parabola and can be conceptualizedas a concentrically arranged stack of cylinders moving along a smooth path at differingvelocities relative to each other. True parabolic flow exists in the smaller vessels of theabdomen but not usually in the major arteries, where instead there is some flattening in

the middle of the velocity profile, which is known as "plug flow." The pulsed Dopplerfeature of laminar flow is the presence of a clear "window" beneath the spectrum,indicating that the red blood cells are moving in an orderly manner, with similar velocityand direction (Fig 3).

Fig.3 Pulsed Doppler spectrum, clear window. The width of the white tracing indicatesthe range of cell velocities at a given time, and the thinness of this line and the absenceof markings below the line are known as a clear spectral "window." Filling in of thisspace, known as "spectral broadening," occurs when there is a larger range of velocities,such as in turbulent flow. Note that a tracing above the baseline indicates flow towardthe transducer and a tracing below the baseline indicates flow away from the transducer.

Multiple factors in "real" arteries can focally alter laminar flow, such as vessel tapering,curvature, and bifurcations. Disruption of laminar flow can result in a spectrum of flowabnormalities, ranging from "disturbed" to "turbulent" flow, with the precise distinctionbetween the two being somewhat arbitrary. Flow disturbance comprises a continuum of

flow abnormalities ranging from minor irregularities of flow streamlines to completelydisorganized, multidirectional flow vectors (Fig 4).


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Fig. 4 Disturbed flow. There is disruption of the orderly laminae within the region of narrowing(mild flow disturbance), and disorganized, multidirectional flow vectors distal to the stenosis(turbulent flow). The distinction between the two is somewhat arbitrary.

The variables that influence the existence of turbulent flow include vessel radius r, theaverage flow velocity vacross the lumen, the density p of the fluid, and the viscosity h ofthe fluid. With these variables, the Reynolds number (Re) can be calculated by using theequation Re = v2rp/h); a value exceeding approximately 2,000 is generally defined asthe critical value for the transition from laminar to turbulent flow. Since a stenosis isassociated with elevated velocity, turbulence is usually present within and distal to a

stenosis. The color Doppler appearance of turbulent flow is a heterogeneous distributionof different shades of color across the vessel lumen (Fig 5),

Fig.5 Turbulent flow, color Doppler US. The far right of the image, showing homogeneous redcolor, indicates flow in the same direction, of moderate frequency shift and therefore moderate


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velocity. The heterogeneity of the colors elsewhere on the image indicates themultidirectional flow of turbulence and a larger frequency shift (see discussion of"aliasing" in the Doppler artifact section).

since the Doppler angles related to individual cells are no longer identical. The pulsedDoppler appearance of turbulence includes spectral broadening (filling in of the windowof the spectrum), disorganized simultaneous forward and reverse flow, and fluctuationsin flow velocity with time (Fig 6)

Fig. 6 Turbulent flow, pulsed Doppler. There is filling in of the "window" beneath the spectrum,known as "spectral broadening" and indicative of the wide range of velocities present in turbulentflow. The peak systolic velocity is elevated (>300 cm/sec) because of a stenosis. Sometimesturbulent flow may be bidirectional and the waveform contour ill-defined.

PULSATILE FLOW PATTERN

Because of the pulsatile pumping activity of the heart, flow in the arterial system ischaracterized by alternating phases of acceleration and decelerration. The largepressure amplitude produced by the left ventricle is reduced by the receiving arterialbed, the aorta, and the large vessels. These vessels are sufficiently compliant to storesome of the pulsatile energy of the heart, and allow more continuous flow. The degree ofthe continuous flow component is predominantly a function of peripheral vascularresistance, particularly at the arteriolar level. Reflections of the pressure waves withinthe arterial tree also influences the flow velocity waveform. These conditions result intwo basic forms of the Doppler waveform: high and low resistance. Arteries that supplymuscles and skin at rest (extremities, external carotid, penile, and mesenteric whenfasting) have a high-resistance Doppler waveform. Lower-extremity arteries are anexample of this, and typically have a rapid acceleration to and deceleration from peaksystole, a brief reversal of flow in early diastole, and a brief component of antegrade flowin mid-diastole ( "triphasic waveform ") (Fig 7).


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Fig. 7 Triphasic waveform, pulsed Doppler US. There is rapid systolic acceleration anddeceleration, a brief reversal of flow in early diastole, followed by a brief component of antegradeflow in mid-diastole.

Parenchymal organs such as the liver, spleen, kidney, and brain require constantperfusion, in contrast to demand-oriented tissue such as muscle, and have a morecontinuous, low-resistance flow pattern. The characteristic waveform in a vesselsupplying these organs has a signficant degree of antegrade flow throughout diastole,and no reversed flow component (Fig 8).


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Fig.8 Low-resistance waveform, pulsed Doppler US. There is holodiastolic antegradeflow in diastole, with no reversed flow component.

Physiologic or pathologic conditions that alter peripheral resistance can affect thepulsatility and contour of Doppler waveforms. For instance, lowering of peripheralresistance due to a hemodynamically significant lesion or exercise can result in a low-

resistance waveform in the femoral artery (Fig 9).

Fig.9 Low-resistance waveform in the distal superficial femoral artery, distal to a high-gradestenosis.

Similarly, the usual low-resistance waveform in a renal transplant may convert to a high-resistance pattern due to processes that raise intrarenal resistance, such as acuterejection or renal vein thrombosis. These alterations of waveform can be used to detectchanges in peripheral vascular resistance, which may provide important diagnosticinformation (4,7,8,10).
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Fig.10 Doppler angle. The angle of incidence between the ultrasound beam and the estimatedflow direction (parallel to the long axis of the vessel) is the Doppler angle.

DOPPLER ULTRASOUND

Principle

The Doppler effect is a change in the frequency of a wave, resulting from motion ofthe wave source or receiver, or in the case of a reflected wave, motion of the reflector.In medicine, Doppler US is used to detect and measure blood flow, and the majorreflector is the red blood cell. The Doppler shift is dependent on the insonatingfrequency, the velocity of moving blood, and the angle between the sound beam anddirection of moving blood, as expressed in the Doppler equation:

Df = 2 f v cos q ,c

where Df is the Doppler shift frequency (the difference between transmitted and receivedfrequencies), f is the transmitted frequency, v is the blood velocity, c is the speed ofsound, and q is the angle between the sound beam and the direction of moving blood.The equation can be rearranged to solve for blood velocity, and this is the valuecalculated by the Doppler US machine:

V = Df c .2 f cos q

The angle of insonation qis estimated by the sonographer by aligning an indicator on the

duplex image along the longitudinal axis of the vessel, a process known as anglecorrection (Fig 11).

Fig. 11 Angle correction, duplex Doppler US. The line (arrow) within the sample gate is used toestimate the Doppler angle between the ultrasound beam and the blood flow direction.
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Observing the Doppler equation yields several points that are relevant to theperformance of a Doppler examination. First, since the cosine of 90 is zero, if theultrasound beam is perpendicular to the direction of blood flow, there will be no Dopplershift and a potentially incorrect impression of no flow in the vessel. Second, it is evidentthat appropriate estimation of the angle of insonation, or angle correction, is essential forthe accurate determination of Doppler shift and blood flow velocity. The angle of

insonation should also be less than 60 at all times, since the cosine function has asteeper curve above this angle, and errors in angle correction are therefore magnified.

There are several forms of depiction of blood flow in medical Doppler imaging: colorDoppler, pulsed Doppler, and power Doppler. Color Doppler US provides an estimate ofthe mean velocity of flow within a vessel by color coding the information and displaying itsuperimposed on the gray-scale image (Fig 12).

Fig.12 Normal color Doppler US. The mean frequency shift of blood flow is depicted in color, andflow direction is arbitrarily assigned, indicated by the blue and red vertical bar at the right of theimage. Blue-coded flow is toward the transducer, and red-coded flow is away from thetransducer. The deeper, more saturated colors have a lower mean frequency shift.

The flow direction is arbitrarily assigned the color red or blue, indicating flow toward oraway from the transducer, respectively. Pulsed Doppler allows a sampling volume (orgate) to be positioned in a vessel visualized on the gray-scale image, and displays aspectrum, or graph, of the full range (as opposed to the mean velocity, as in colorDoppler US) of blood velocities within the gate plotted as a function of time. Theamplitude of the signal is approximately proportional to the number of red blood cellsand is indicated as a shade of gray (Fig 13).


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Fig.13 Pulsed Doppler US. Velocities are indicated on the scale to the right of the image.Deflections above the baseline indicate flow toward the transducer, and deflections below thebaseline indicate flow away from the transducer.

Color Doppler provides a global depiction of blood flow in a region and may be used as aguide for the subsequent placement of the pulsed Doppler gate for detailed analysis at asite of potential flow abnormality. Power Doppler, which is not routinely used in arterialDoppler evaluation of the lower extremity, depicts the amplitude, or power, of Doppler

signals rather than the frequency shift. This allows detection of a larger range of Dopplershifts and thus better visualization of small vessels, but at the expense of directional andvelocity information.

Artifacts

A detailed overview of Doppler artifacts is beyond the scope of this article, but severalartifacts are particularly important for the performance and interpretation of an arterialDoppler examination. Aliasing is an artifact due to an insufficient sampling rate andoccurs when the frequency shift to be measured is more than twice the pulse repetitionfrequency (Nyquist frequency) (Fig 14).


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Fig.14 Aliasing, pulsed Doppler. There is folding over of forward flow in systole in the reversedirection, below the baseline.

The artifact results in a wraparound of the Doppler spectrum in pulsed or color DopplerUS. Aliasing at pulsed Doppler appears as a "folding over" of forward flow in systole inthe reverse direction. Aliasing at color Doppler US may manifest as a mixture of colorsor as a focus of color in the vessel corresponding to a continuum of colors folded overfrom the normal flow in the opposite direction within the vessel (Fig 15)

Fig. 15 Aliasing, color Doppler US. There is heterogeneity of colors within the vessel lumen,which is one of the color Doppler appearances of aliasing.

Since aliasing is due to a high-frequency shift or inadequate sampling rate or both, itmay be a marker for sites of high-velocity flow,and is therefore a useful artifact fordetection of stenosis. During mapping of the arteries with color Doppler US, if a regionof aliasing is encountered, it should prompt a more detailed analysis with pulsed Doppler
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to see if an elevated peak systolic velocity ratio is present. Aliasing can be reduced byincreasing the pulse repetition frequency or using a lower-frequency transducer, thusdecreasing the Doppler shift (Fig 16).

Fig. 16 Aliasing, pulsed Doppler US, effect of pulse repetition frequency (PRF). Both images wereobtained in the same vessel. The image on the left (PRF = 2,500 Hz) has no aliasing, while the

image on the right, with a lower PRF of 1,515 Hz, shows aliasing.

Another artifact is "bleeding" of color signal from a vessel into an adjacent area withoutflow, potentially masking the presence of thrombus or vessel narrowing. This artifact isdue to an inappropriately high setting of the color gain. It is important to emphasize theimportance of the Doppler angle in interpreting the examination. If the ultrasound beamis perpendicular to the vessel, there may be a spurious impression of no flow(occlusion), or the flow direction may appear to be bidirectional, like a mirror image (Fig17).


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Fig.17 Spectral mirror image artifact. This artifact may occur with a Doppler angle of 90 andmanifests as bidirectional flow, with identical spectra in both directions ("mirror image").

This latter artifact is known as the spectral mirror image artifact and is due to thedivergence of the Doppler beam in two directions along the long axis of the vessel.

Finally, it is important to emphasize that the accuracy of angle correction is essential, asinappropriate estimates can result in spurious velocity determinations and potentialmisdiagnoses (Fig 18)

Fig. 18 Angle correction, pulsed Doppler US. Note the differing velocity readings from the same

location in the same vessel but with different Doppler angles.


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Doppler Diagnosis of Arterial Stenoses and Occlusions

If the lumen of an artery is narrowed, the blood flow velocity increases within thestenosis , as described by the principle of continuity of flow(see the Hemodynamicssection) (Fig 19).

Fig. 19 Calculation of peak systolic velocity (PSV) ratio. The PSVs in the narrowed (or aliasing)portion of the vessel (right) and in an immediately proximal, normal portion (left) are obtained.The ratio consists of the elevated PSV divided by the proximal, normal PSV.

The principal Doppler criterion for the diagnosis of a lower-extremity arterial stenosis istherefore based on detection of a focal increase in


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peak systolic velocity (PSV) with pulsed Doppler (Fig 20).

Fig. 20 Stenosis, pulsed Doppler US. The intrastenotic velocity is elevated (right), compared withthe normal, prestenotic velocity (left).

Detection of a hemodynamicallysignificant focal increase in PSV involves the ratio ofthe PSV within the suspected narrowed segment to the PSV in the immediatelyproximal, nonstenosed portion of the artery. A ratio of greater than 2 is the criterion for ahemodynamically significant (50% or greater) stenosis, and a ratio of 3.7-4 indicates a75% or greater stenosis.

Because of the large variation in velocities within the lower-extremity arteries,depending on location, the use of absolute velocity is less accurate than the PSV ratio,which normalizes for this variability.

The examination of the lower-extremity arterial system is performed by using colorDoppler US to map the vessels and identify sites of possible stenosis, manifest asaliasing or narrowing of the vessel diameter, although the latter feature is oftenunreliable. Any site of suspected stenosis at color Doppler US is then interrogated withpulsed Doppler, which provides a spectrum within and proximal to the possible lesion,allowing computation of a PSV ratio (Fig 21,22)


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Fig 21 and 22 Figure 21. Stenosis with aliasing, color Doppler US. Note aliasing in the commoniliac artery, indicative of a region of high-frequency shift and possibly a stenosis.

Figure 22. Pulsed Doppler US of the color Doppler abnormality shown in Figure 21. There iselevated PSV (>400 cm/sec) with spectral broadening. The prestenotic velocity is 80 cm/sec (notshown), and the PSV ratio is 5, consistent with a greater than 75% stenosis.

The use of color Doppler US in conjunction with pulsed Doppler, or color-assistedduplex US, markedly reduces the examination time, generally allowing both legs to beimaged in 30-45 minutes. In addition to the change in velocity due to hemodynamicallysignificant lesions, the contour of the pulsed Doppler waveform is affected by theirpresence. A normal, resting lower-extremity arterial waveform demonstrates highresistance, having a triphasic form with a rapid acceleration to and deceleration frompeak systole, a brief reversal of flow in early diastole, and a small antegrade flowcomponent in mid-diastole (Fig 23 ).
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Fig.23 Triphasic waveform, pulsed Doppler US. There is rapid systolic acceleration anddeceleration, a brief reversal of flow in early diastole, followed by a brief component of antegradeflow in mid-diastole.

Although the precise relationship between waveform contour alteration and lesion typeand location is not well established, certain generalizations can be made. The spectralwaveform changes within and immediately distal to a stenosis; in addition to increasedPSV, it indicates disturbance of laminar flow and a decrease in pulsatility and loss of thereversed flow component. This disturbance of laminar flow may manifest as spectralbroadening, turbulence, and, in severe stenoses, simultaneous forward and reverse flowand indistinctness of the spectral margin (Fig 24).

Fig. 24 Stenosis, turbulent flow, pulsed Doppler US. There is severe spectral broadening.


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The appearance of the waveform proximal to a lesion is variable and depends on thedegree of collateral circulation formation. The waveform may be normal or the systolicvelocity may be low but the upstroke (systolic acceleration time and slope) unaffectedand pulsatility increased, with absent, reduced, or reversed flow in diastole. With anacute occlusion, ineffectual pulsations may be transmitted to the occlusion, producingnarrow, low-velocity Doppler signals that do not represent true flow (Fig 25).

Fig.25 Pulsed Doppler waveform, proximal to high-grade lesion. There is loss of flow in diastoleand low velocities.

The waveform shape distal to an obstructing lesion often shows a low-resistancepattern, with loss of flow reversal in diastole and abundant antegrade flow in

diastole (Fig 26).


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Fig.26 Pulsed Doppler waveform, low resistance distal to high-grade lesion.

The waveform distal to a lesion may also have a delayed systolic upstroke anddecreased peak systolic velocity and is sometimes referred to as a " tardus and parvus"waveform (Fig 27).

Fig.27 Pulsed Doppler waveform, low resistance with "tardus and parvus" contour. There isdelayed systolic acceleration, and the waveform demonstrates low resistance.

The decrease in pulsatility is probably due to a combination of factors including (a)decreased peripheral resistance due to ischemia, (b) resistance to the reversed flowcomponent related to the stenosis, (c) high level of forward flow throughout the cardiaccycle due to the pressure gradient across the stenosis, and (d) dampening of thepressure wave with consequent reduction of pulse pressure, resulting in less wavereflection and amplification, which normally contribute to the reversed flow component indiastole.

These waveform contour changes are not the primary diagnostic criteria for arteriallesions but are adjunctive and complementary to the PSV changes. Turbulence isalways present within and distal to a stenosis and generally extends a few centimetersdownstream. The development of an abnormal waveform contour does indicate thepresence of a lesion, but it does not accurately indicate the location of the lesion. Distalto a hemodynamically signficant lesion, the waveform often remains low resistancethroughout the remainder of the extremity (Fig 28).


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Fig.28 Pulsed Doppler waveform, low resistance distal to high-grade lesion.

Waveform abnormalities can probably also be detected at varying distances proximal toa lesion. The conversion from a normal to an abnormal waveform when imaging downan artery either means that an intervening lesion was not detected with velocity criteriaor that there is a lesion distal to the point of waveform conversion.

Doppler diagnosis of an occlusion is fairly straightforward and consists of the absence ofcolor Doppler and pulsed Doppler detectable flow in an arterial segment (Fig 29).

Fig.29 Occlusion, color and pulsed Doppler US. There is absence of flow, compatiblewith an occlusion.

It is important to ensure that the lack of detectable flow is not due to technical factors.The Doppler gain should be set at a high level, but short of causing artifacts, and thepulse repetition frequency should be set low enough to allow detection of low-velocitysignal from a subtotal occlusion. The settings for low-flow detection can be normalizedin a given patient by confirming detection of venous flow. Other potential sources of


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false-positive examinations include (a) inability to detect Doppler signals deep in thethigh, such as in the adductor canal region; (b) densely shadowing calcified plaquepreventing detection of Doppler signal; (c) external compression of the vessel; and (d)severe stenosis with very slow flow, below the threshold of detection with the Dopplerinstrument. A false-negative diagnosis can be due to mistaking high-velocity smallarterial collateral vessels parallel to the occluded segment for a patent, stenosed artery

(4,7,9,10,13,14).

Patient Preparation

In our laboratory, we do not require any special patient preparation. Some laboratoriesrequire that the patient fast prior to the test to reduce bowel gas to allow bettervisualization of the iliac vessels. Another possible preparation technique is to have thepatient rest on the examination table prior to the test to allow resolution of any exercise-induced hyperemia, which could affect the Doppler spectral and segmental pressureinterpretation. This preparation is problematic because of scheduling and logistic issues.

US Imaging

US imaging is performed with a linear array transducer, operating from 5 to 7.5 MHz (Fig30).

Fig. 30 Linear array transducer, in typical imaging positions to visualize the popliteal(left) and dorsalis pedis arteries (right).

First, the gray-scale image is optimized, and then the color Doppler image is optimizedby adjusting the pulse repetition frequency (velocity scale), wall filter, and sensitivity toeliminate aliasing in healthy vascular segments and to fill in the entire vessel lumenwithout extension of color signal outside the artery. The velocity scale is typically set at30-40 cm/sec. The iliac, common and superficial femoral, and popliteal arteries areimaged along their longitudinal axis with color Doppler. These vessels are mapped withcolor Doppler to detect any regions of aliasing that may indicate a stenosis, or areaswithout flow, which would indicate an occlusion. Once a suspicious site is identified,detailed analysis with pulsed Doppler is performed. A pulsed Doppler spectrum and
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PSV are obtained within the region of aliasing and several centimeters proximal to thatregion, where the color Doppler appearance is normal. The PSV within the region ofaliasing is then divided by the PSV in the proximal region, and if the ratio is greater than2, a 50% stenosis is diagnosed (Fig 31).

Fig. 31 Stenosis, pulsed Doppler US. There is a focal velocity increase resulting in a PSV ratio of2, compatible with a 50% stenosis.

It is essential to ensure that the angle correction is accurate and that the Doppler angleis less than 60, and preferably as low as possible. Turbulent flow, manifesting as

spectral broadening and an ill-defined waveform contour, is often present within and justdistal to the stenosis. If a vascular segment contains no flow on color Doppler, thisshould be confirmed with pulsed Doppler, and a diagnosis of occlusion can made (Figs32,33).

Fig. 32, 33 Stenosis with aliasing, color Doppler (Fig 32) and pulsed Doppler (Fig 33) US. Notealiasing in the common iliac artery, indicative of a region of high- frequency shift and possibly a
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stenosis. Pulsed Doppler of the same region reveals elevated PSV (>400 cm/sec) with spectralbroadening. The prestenotic velocity is 80 cm/sec (not shown), and the PSV ratio is 5, consistentwith a greater than 75% stenosis.

In addition to insonation of abnormal regions with color Doppler, representative pulsedDoppler spectra are obtained in the iliac, common femoral, proximal, middle, and distal

superficial femoral, and popliteal arteries, and in the posterior tibial and dorsal pedalarteries in the ankle (we do not routinely insonate the entire course of the calf vessels).These are examined for presence of abnormal waveforms, such as a low-resistancewaveform. In the absence of a lesion demonstrated by the principal Doppler criteria(PSV ratio or absence of detectable flow), a conversion from a normal to abnormalwaveform when proceeding distally suggests the presence of a lesion in the interveningsegment or vessel distal to the area of insonation.

This type of information, particularly in conjunction with a segmental pressureabnormality, allows one to suggest the possiblity of a lesion that was not directlyvisualized. For instance, if the popliteal artery waveform is normal and there is anabnormal waveform in the dorsal pedal artery (a branch of the anterior tibial artery) and

a segmental pressue drop in this region, the presence of a lesion in the anterior tibialartery can be suggested.

Sensitivity and Specificity of Doppler Diagnosis of Arterial Lesions

The sensitivity for detection of hemodynamically significant stenoses in the femoral andpopliteal arteries with use of the PSV ratio ranges from 76% to more than 90%, withspecificities ranging from 80% to 99%. The results in our laboratory have been closer tothe lower end of these results. Controversy exists regarding the issue of whether thepresence of multiple lesions affects the sensitivity of stenosis detection. The sensitivityand specificity for detection of occlusions by the criterion of absence of Dopplerdetectable flow are both more than 90%, and results in our laboratory have been similar

to these (4,6,9,13,14).

Normal Doppler Studies

A normal color Doppler study is characterized by the absence of any hemodynamicallysignificant focal velocity increases and a triphasic or biphasic waveform shapethroughout all arterial segments (Fig 34).


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Fig. 34 Normal results from lower-extremity Doppler examination. All waveforms are triphasic,and there are no focal velocity increases to suggest stenosis.

Lower-Extremity Arterial Pressure Measurements

Measurement of systolic blood pressures in the lower extremities is a fairly simple, easilyperformed, and repeatable test that provides a global, quantitative, and objective indexof arterial obstructive disease. Compared with US, it is less sensitive in the detection oflesions and less effective in disease localization, and it cannot characterize lesions as

stenoses versus occlusions; however, it is of value because of its ease of performanceand potential for comparison to prior studies, and because it provides a simple globaldepiction of the disease process. The examination consists of measuring the systolicblood pressure in the arm and at four levels in the leg and interpreting this information asan ankle/brachial pressure ratio and as pressure decrements in the extremity (segmentalpressure). The degree of reduction in the ankle/brachial ratio is proportional to theseverity of disease, and the pressure decrements can assist in lesion localization.
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Principle

Blood flow to an organ is determined by the difference in pressure and fluid energybetween the large arteries and veins, and by the resistance within a given vascular bed.The mean and diastolic pressures normally gradually decrease distally, promoting bloodflow. Systolic pressure normally increases as the pressure wave travels distally, due to

reflection of waves and high peripheral resistance, a process known as systolicamplification. Therefore, the systolic pressure measured at the ankle is normally slightlyhigher than in the arm.

Reduction of the luminal diameter to a critical value by a stenosis results in diminishedpressure and flow distal to the lesion. The systolic pressure is the most sensitiveindicator of disease, as it is reduced earlier than the diastolic pressure. The basis oflower-extremity pressure measurements is the detection of a reduction in systolic bloodpressure along the course of the leg, indicating the presence of an obstructive arteriallesion.

Technique

The systolic pressure at any level in the leg can be measured by placing a pneumaticcuff at the site of interest.The cuff is inflated to a level above systolic pressure, such thatarterial signal disappears, and then the cuff is gradually deflated until flow reappears (Fig35 ).

Fig. 35 Continuous wave Doppler US probe, insonation of ankle vessels.

The pressure at which this occurs is recorded as the systolic pressure. Resumption offlow is assessed by using a continuous-wave Doppler probe, generally at the posteriortibial (PT) or dorsal pedal (DP) artery, although any vessel distal to the cuff can be used.The site of pressure measurement is determined by the cuff position and not the site offlow detection.
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The procedure is performed by placing four pneumatic cuffs at four different positions onthe leg: high thigh (HT), above knee (AK), below knee (BK), and ankle (Fig 36).

Fig. 36 Pneumatic cuffs on legs.

Systolic pressure is determined at each level with the technique described above, withmeasurement for flow performed at either the PT or DP artery. The procedure isperformed separately for each leg. When the ankle cuff is inflated, the pressure isrecorded at both the PT and DP arteries. To calculate an ankle/brachial pressure ratio,which is an index of the degree of disease in the lower limb, both brachial artery systolicpressures are obtained. In the absence of subclavian or axillary artery disease, thebrachial pressure is equal to the aortic pressure, and therefore the ratio is reflective ofobstructive lesions between the aorta and ankle. If the brachial pressures differ, thehigher of the two is used to calculate the ratio for each leg. The examination isperformed with the patient in the supine position. The temperature in the room should bewarm, as cold-induced vasospasm may make arterial signals difficult to detect.

The results of the four leg and one arm pressures are recorded in a table form, and theankle/brachial index is calculated by taking the higher of the two ankle pressures (PT orDP) and dividing it by the higher of the two brachial pressures (Fig 37).
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Fig. 37 Format for recording lower-extremity pressures (in millimeters of mercury), normal study.AK= above knee, BK= below knee, PT= posterior tibial, DP= dorsal pedal AAI= ankle arm[brachial] index.

There are two factors that may result in spuriously high pressure readings andconsequent interpretative difficulties: limb girth and arterial wall rigidity. If a limb is very

large in girth or the cuff inadequate in relative size, the pressure may not be transmittedeffectively to the vessels at the center of the limb and the pressure reading may beartifactually high. This problem is commonly encountered at the level of the thigh. Thisartifact can be minimized by using adequate cuff diameters, generally at least 15 cm. Inpersons without disease, the true intraarterial pressure in the thigh vessels is slightlyhigher than in the arm, because of the phemonenon of systolic amplification. The girthof the thigh results in an exaggeration of this fact, such that the cuff-measured thighpressure in persons without disease (variable depending on thigh size) is considerablygreater than arm pressure, with an HT/brachial ratio of 1.2 or more (Fig 38).

Fig. 38 Segmental pressure chart. Note that the HT/brachial ratio is approximately 1.2.

Exercise Testing

The degree of narrowing at which a stenosis is "critical" is dependent on flow, andpressure gradients that are minimal at rest may be accentuated when flow rates areincreased, as by exercise or reactive hyperemia. The patient is usually exercised on atreadmill at 2 miles per hour on a 12% grade for 5 minutes or until symptoms occur. Twoaspects of the response are evaluated: the degree of immediate decrease in anklepressure and the time for recovery to resting pressure. Our laboratory does not useexercise testing.

Interpretation of Ankle/Brachial Index

Because of the phenomenon of systolic amplification, the systolic pressure in the ankleis slightly higher than that in the brachial artery in persons without disease. Therefore,the normal ankle/brachial index is greater than 1.0, with a mean value of 1.1 (plus orminus 0.1) (Fig 39).
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Fig. 39 Segmental pressure chart, normal values. The ankle/brachial index is 1.0, and there areno pressure drops (>20 mm Hg) either down or across the extremities.

The degree in reduction of the ankle/brachial index also correlates with the degree ofclinical arterial insufficiency and severity of symptoms (Table).

Ankle/Brachial Index and Patient Status

In some persons, rigidity or calcification of the arterial walls can result in the vessel being"incompressible," resulting in extremely high pressures, sometimes greater than 300 mmHg. This occurs most frequently in patients with diabetes but has also been seen in thesettings of long-term corticosteroid therapy, renal dialysis, and renal transplantation.Both of these artifacts can be recognized by the presence of inappropriately highpressures, such as with an HT/brachial ratio exceeding 1.3 or considerable increases inpressures distally.

Patient Status Ankle/Brachial Index

Normal >1.0

Minimal ischemic disease (minimalsymptoms)

0.9-1.0

Mild-to-moderate disease(claudication)

0.5-0.9

Moderate-to-severe disease(ischemic rest pain)

0.3-0.5

Severe disease (gangrene)


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10. Burns PN. Interpreting and analyzing the Doppler examination. In: Taylor KJW, Burns PN,Wells PNT, eds. Clinical applications of Doppler ultrasound. New York, NY: Raven, 1995; 55-98.

11. Wells PNT. Basic principles and Doppler physics. In: Taylor KJW, Burns PN, Wells PNT, eds.Clinical applications of Doppler ultrasound. New York, NY: Raven, 1995; 1-17.

12. Burns PN. Doppler artifacts. In: Taylor KJW, Burns PN, Wells PNT, eds. Clinical applicationsof Doppler ultrasound. New York, NY: Raven, 1995; 99-107.

13. Polak JF, Karmel MI, Mannick JA, O'Leary DH, Donaldson MC, Whittemore AD.Determination of the extent of lower extremity peripheral arterial disease with color-assistedduplex sonography: comparison with angiography. AJR 1990; 155:1085-1089 [Abstract].

14. Allard L. Limitations of ultrasonic duplex scanning for diagnosing lower limb arterial stenosisin the presence of adjacent segment disease. J Vasc Surg 1994; 19:650-657.

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