:

Spectral Doppler

and Color Artifacts1

Myron A. Pozniak, James A. Zagzebski, KatbleenA. Scanlan,

MD PhD MD

Artifacts in spectral and color Doppler imaging can be confusing and lead to misinterpretation of flow information. The authors review these artifacts by considering three main causes: inappropriate equipment settings, anatomic factors, and physical and technical limitations of the modality. Incorrect gain, wall-filter, or velocity scale settings can cause loss of clinically important information or distortion of the tracing. Reflection of the Doppler signal from highly reflective surfaces can create a color Doppler mirror image. Vascular motion can introduce artifactual variation in velocity as the sample volume passes through different velocities in a laminar flow state. Unintentional motion can cause a generalized Doppler shift. Increasing the angle of Doppler interrogation degrades the quality of the tracing and gives the impression of spectral broadening. As angulation approaches 900 directional ambiguity can occur, suggesting bidirectional flow. Grating and side lobes can interrogate areas unrelated to the sample vol. ume and introduce extraneous Doppler information to the apparent area of interrogation. Recognition of these artifacts is essential to proper interpretation of Doppler information and rendering a correct diagnosis. INTRODUCTION

U

Doppler evaluation ence or absence studied with this flow characteristics, interpretation of nical factors that

of hemodynamics provides useful clinical information. The presof flow in a vessel, flow direction, pulsatility, and velocity can all be modality. In addition, color Doppler provides a visual image of the profiling regions of turbulence. With both modalities, correct tracings and flow images requires knowledge of physical and techinfluence Doppler signals. Artifacts caused by physical limitations

of the modality or inappropriate equipment settings can result in displayed flow conditions that may differ considerably from the actual physiologic situation. Understanding limiting factors and appropriately modifying instrument settings should minimize misdiagnosis. In this article, we review artifacts in spectral and color Doppler imaging. We have grouped

Index

them

terms:

I

From

ent ofa requested

1992;

the

RSNA,

(US),

Certificate August

categories:

artifact

those

#{149} Ultrasound

(US),

resulting

Doppler

from

inappropriate

equipment

studies

12:35-44

Department

(M.A.P.,J.A.Z.,

Measurements, ‘

three

Ultrasound

RadloGraphics

53792

into

of Radiology,

K.A.S.), ofMerit 23 and

Middleton,

and the

University Department

ofWisconsin ofMedical

Hospital

and

Physics,

University

for a scientific exhibit at the 1990 RSNA scientific received September 24; accepted September 26. Wisconsin.

Address

reprint

requests

Clinics,

600

Highland

ofWisconsin,

assembly. Supported

Ave. Madison

Madison, (J.A.Z.).

WI Recipi-

Receivedjune 10. 1991; revision in part by a grant from Radiation

to M.A.P.

1992

35

b.

a.

Figure

1.

Images

sonographic

were

gray-scale

generated

of a common

phantom.

Transducer

moved back and forth across the phantom ate a Doppler shift. (a) With proper gain clear

distinct

velocity

manifests

envelope

in a uniform

der the systolic

is displayed.

direction

peak

was

to genersettings, a

with

indicating

Flow

a window

a narrow

un-

range

of

velocities at any point in time. (b) With a moderate increase in gain, the velocity tracing is thickened, that is, the spectrum broadens (*). Systolic window becomes filled in (**). Additionally, a mirror image tracing that suggests reversed flow is also displayed (arrow). (c) Further increase in gain produces

spikes

over

the tracing

is overwhelmed by the meters per second.

settings,

and

those

artifacts

of the

cal examples imaging and U

the detection

higher

velocity

arising

related

limitations

where

from

anatomic

to physical modality.

tive

factors,

and

technical

Phantom

are shown in both spectral Doppler.

DOPPLER

circuitry rn/s =

shifts.

and

color

clini-

flow

SIGNAL

Doppler processing detects phase or frequency shifts in returning echo signals when there is relative motion between scatterers and

the

ultrasound

transducer. The Doppler 2f)Vcos(O)/c, gives the magnitude of the frequency shiftJ) when reflectors are moving with velocity V and angle 0 relative to the ultrasound beam.f, is the ultrasound frequency, and c is the speed of sound in the medium. In addition to an audio presentation, frequency (or spectral) analysis is presented as a tracing of Doppler frequency disequation,f,)

tribution presented

above

36

U

RadioGraphic-s

=

versus

time.

in terms

equation

U

This

display

of velocity

for V. Direction

Pozniak

Ct

a!

by

may

also

solving

the

of flow

rela-

be

to the

transducer,

flow

velocity,

and

im-

portant flow conditions such as turbulence are inferred from this display. Most color flow instruments derive a flow image by applying Doppler signal processing and detection to the echo signal waveform and then estimating the mean Doppler frequency at multiple locations along each beam line (1). Different methods are used by manufacturers for this estimation. All methods require a series ofperhaps eight to 15 pulseecho sequences along each beam line to obtain sufficient data for estimating the mean Doppler frequency within each pixel. This prolonged data acquisition time reduces the frame rate below that of conventional B-mode imaging and introduces trade-offs between color image quality, image size, and temporal resolution. The color image portrays mean flow velocity and direction relative to the transducer for pixels throughout the targeted field.

Volume

12

Number

1

a. Figure the

(a) Color

portal

tained

U

2.

vein with

the

Doppler

represents color

ARTIFACTS

image

the scale

hepatic

adjusted

RELAThD

INAPPROPRIATE

of a portal artery.

b. fails to show

vein False

downward

diagnosis

(arrow)

TO

Doppler Gain Setting Errors Proper gain setting in B mode is critical to an accurate image; it is also critical in Doppler processing for accurate depiction of flow characteristics. If the gain setting is too low, valuable flow information may be lost. The sonographer frequently adjusts the gain to maximize visualization of the spectral tracing. A gain setting that is too high, however, degrades the velocity envelope on the spectral This

mimics

spectral

broadening

At higher

1992

Scale

velocities,

spikes

display

detectability

setting.

tions

of signals.

is

the

ically

of

it

ing

rate

(ie,

the

Doppler

also

increases

veloc-

is set

too

The

phenome-

or

spectral

angle

(which

the pulse

aliased

tracing

effect.

increasing the

This

that spectral Doppler sound beams. If the signal exceeds the onehalf the pulse

a wraparound

shift),

by the

ambiguous

produced.

pler

example,

range

occurs.

frequency), shows

For

velocity

aliasing

sampling

are

ob-

states might not be displayed; a patent vessel may appear (Fig 2). Conversely, when the is set too low for the flow condi.

present,

signals

to

Image

Aliasing

is affected

non is related to the fact and color flow use pulsed frequency of the Doppler Nyquist

and

Errors

When

high, low-flow for example, thrombosed velocity scale

adjacent

(b)

Doppler and color flow imagscale setting is crucial to a

low-flow

repetition

noise project across the tracing and degrade (Fig 1). Gain setting errors in color Doppler are usually obvious. It is appropriate to set the color gain by turning it up until noise is encountered and then backing off until the noise just clears from the image. When the gain setting is too high, the image becomes cluttered with color noise in a random pattern. When the gain is set too low, Doppler shifts are not displayed in vessels, especially those with relatively slow flow.

January

Velocity

Color

considered. flow.

successful ity

and

gives the impression, for example, of poststenotic turbulence. A mirror image spectral tracing can occur if the directional circuitry overwhelmed.

normal

In both spectral ing, the velocity

.

display.

the lumen. was

demonstrates

.

SETfINGS

flow within

of thrombosis

typ-

By increas-

decreases

velocity

Dop-

scale

repetition

(which

frequency),

changing the baseline setting, or using a lower ultrasound frequency, aliasing can be avoided (Fig 3). Color aliasing projects the color of reversed flow within central areas of higher laminar velocity. The key to perceiving color aliasing with no tween

most black

equipment stripe

the

reversed

from

is the

fact

that

the

low-velocity

colors

(Fig

Pozniak

Ct

4)

(2).

a!

U

there

is

filter

be-

RadioGraphic-s

U

37

Figures

and setting, within

3, 4. the higher

(3a) On Doppler image obtained with the velocity scale set too low (arrows), aliasing occurs, velocity systolic peaks wrap around to project below the baseline. (3b) By adjusting the scale systolic component is correctly displayed. rn/s = meters per second. (4a) Blue and green colors

the the central

portion

of the

vessel

suggest

flow

reversal,

but

they

are

actually secondary proper direction.

the higher central velocities. Slower velocity near the wall is displayed in the the wraparound occurs from the top to the bottom of the scale bar and does motion filter (arrow in a); therefore, directions. This is in contrast to the scale, flow direction is now correctly

#{149} Incorrect

Wail-Filter

no black

image

with

stripe

true

U

RadioGraphic-s

U

Pozniak

Setting

Ct

a!

between

flow reversal

the

not go across

display

the (black) wall. different color By adjusting the color

ofthe

(cf Fig 10). (4b)

two

displayed.

Filtration is designed to remove unwanted low-frequency Doppler signals originating from slowly moving soft-tissue reflectors. The cutoff frequency is operator selectable. With filtration set too high, diagnostically significant velocity information can be lost (2). This is especially true in low-flow states and in the measurement of resistive index (Fig 5).

38

is noted

to aliasing of With aliasing,

U

ANATOMICALLY

RELATED

ARTIFACTS #{149} Mirror

Image

Artifact

Mirror image artifacts are commonly observed in conventional B-mode imaging (3). A similar artifact can occur with color Doppler imaging of any vessel adjacent to a highly reflective surface, such as the lung (4). The subdiaphragmatic region of the liver and the supraclavicular region are most notorious for this artifact (Fig 6) (5,6).

Volume

12

Number

1

Figure 5. Spectral Doppler tracing ofa normal arterial waveform. As filtration is increased across this tracing from left to right (arrows), the lower velocity components become obliterated. Increased filtration can eliminate low-velocity diastolic flow and yield an impression

of higher

flow

resistance.

rn/s

= meters

per second.

Figure 6. (a) Image of the supraclavicular region shows two apparent subclavian veins. The more anterior vessel is the true vein. The deeper vessel below the pleura (arrows) is actually an artifactual mirror image reflecting off the lung apex. (b) Longitudinal image of the inferior vena cava. A second vessel is perceived deep to the true inferior vena cava (arrows). Doppler shift is more prominent in the mirrored vessel. As the sound beam reflects from the diaphragm, it interrogates the inferior vena cava flow at less of an angle, thereby undergoing a greater Doppler shift than when it interrogates the true vessel, which is more perpendicular to the ultrasound beam. (c) Schematic shows the course of the true signal (large arrows) and the mirrored signal (small arrows). Increased distance that the mirrored signal travels causes the phantom vessel to project deeper in the image.

January

1992

Pozniak

et a!

U

RadioGraphic-s

U

39

Figure ing

7. was

(a)

Normal

obtained

portal

in a normal

vein

flow

volunteer

is usually during

at a uniform

velocity

suspended

respiration.

toward With

the

liver.

a small

This

sample

portal

vein

volume

trac-

centrally

positioned in the vessel, the spectral Doppler tracing reveals a pulsatile waveform. (b) With Doppler sample volume slightly enlarged, orientation of the transducer relative to the vessel altered, and sample volume positioned more laterally in the vessel, pulsatility has almost completely resolved. This is due to decreased ex cursion between areas of differing velocities within the vessel and a larger cross-sectional sampling. rn/s = meters per second. (c) Pulsatile flow in a flow phantom was generated. Image captures the onset ofthe pulse

waveform across the interrogated region. Propagating wave assumes a parabolic form revealing the laminar flow profile. (d) Positioning a velocity tag (green) in the middle velocity range demonstrates laminar profile of flow in the portal vein. Slowest velocities adjacent to the vessel wall are darker red; middle velocities are green;

and

highest

velocities,

#{149} Vascular-Motion

which

are

artifact

can

pink,

Artifact

When a vessel that is moving relative transducer is interrogated, artifactual ity can be introduced into the spectral This

light

be

perceived

in the

to the pulsatiltracing. portal

vein and its branches. Transmitted cardiac contraction pushes on the liver and tugs on the inferior vena cava. This rocking action of the liver causes slight motion of the vessels with respect to the Doppler sample volume. Even a small degree of motion is sufficient to displace the sample volume from the highervelocity central laminar flow to the slower-

40

U

RadioGraphic-s

U

Pozniak

Ct

a!

are

found

centrally

within

the

vessel.

velocity peripheral flow and back. This can introduce or increase periodicity of the portal venous flow pattern, an appearance suggestive of, for example, tricuspid regurgitation. Increasing the size of the Doppler gate to inelude the entire vessel, imaging other portal branches, or varying the angle may decrease this artifact (Fig 7).

#{149} Color

in Nonvascular

Structures

An area of low echogenicity such as a cyst or a duct is susceptible to color flash artifact. Any motion of a reflector relative to the transducer produces a Doppler shift. Most color flow processors incorporate motion discriminators that separate true flow from random

Volume

12

Number

1

Figure 8. Image of a gallbladder obtained suspended respiration. Transducer motion duced color flash artifact in the image, but tially

Figure

9.

(a) Portal

vein flow 90#{176} perpendicular

dominates

to the Doppler

the

beam.

hypoechoic

Sample

during introit

preferen-

areas.

volume

was placed

over that

portion of the vessel where it passes perpendicular to the interrogating beam. Doppler shift is perceived; however, it is displayed equally above and below the spectral baseline. (b) By altering the angle of insonation away from 90#{176}, the direction of flow is correctly displayed and the velocity envelope is better defined. rn/s = meters per second.

motion lower

of soft-tissue level signals

soft-tissue

regions

reflectors. arising from less

effectively

However, hypoechoic trigger

the the

motion discriminator and do not suppress the resultant color flash. Even minute motion may produce artifactual color signals within dilated bile ducts, cysts, or the gallbladder (Fig 8). This can be erroneously perceived as flow, especially if the color sensitivity settings are high (7). U

INSTRUMENT-

PROCESSOR-RELATED #{149} Directional Directional interrogating 90#{176} angle.

January

AND

ARTIFACTS

they usually and below gain

settings,

manifest the spectral

as a tracing baseline.

directional

both above At higher

ambiguity

is worse

and the velocity envelope on the spectral tracing becomes more indistinct. The ambiguity is easily

corrected

when

the

interrogating

direction is shifted to an angle of 90#{176} (Fig 9). When the sample sitioned

on

a sector

image,

beam

at either volume

care

should

side is pobe

taken to avoid that portion of a vessel that intercepts the interrogating beam at 90#{176}. With a linear transducer, electronic angulation or wedge standoffs can improve the angle of spectral Doppler interrogation (8).

Ambiguity ambiguity can result when the beam intercepts the vessel at a If Doppler signals are detected,

1992

Pozniak

Ct

a!

U

RadioGraphic-s

U

41

.

phantom. minimal

Sector Direction

angulation

color offlow

b

is right

off the

beam

image of a to left. Only axis

relative

to the

direction of flow is required to accurately display the color flow profile. Narrow colorless area in the lumen of the vessel represents absence of Doppler shift

where

flow

is perpendicular

to the

insonating

beam. This is true reversal of flow relative to the interrogating beam. Aliasing does not produce a black area between color display of alternate color direction (cf Fig 4a).

Figure

11.

ing and side main beam.

Schematic lobes

of grat-

relative

to the

(Fig 1 1). Side lobes occur in proximity to the primary beam. Grating lobes, however, can be quite far removed from the central beam (9). The exact configuration of these lobes de-

(Fig 10). With a linear transducer, this becomes a more significant problem, but again, electronic beam steering or wedge standoffs can compensate.

pends on the construction of the transducer, that is, crystal element size and spacing. Tightly curved, convex-array transducers are most susceptible, as are higher frequency linear transducers. If these off-axis lobes of sound strike a highly reflective surface, they can return to the transducer and cause misregistration of an object on the display of the primary beam. This artifact commonly confuses needle biopsy guidance (10, 1 1). Off-axis lobes can interrogate vessels unrelated to the Doppler sample volume. If a strong enough signal is encountered off axis, it may appear on the spectral Doppler tracing,

#{149} Grating-

or Side-Lobe

Electronically

Artifact

focused, phased-array transducers concentrate the primary interrogating beam toward the Doppler sample volume. Because of the spacing of the array elements and the sequence of firing, however, weak secondary lobes of focused sound may interrogate areas unrelated to the primary beam

displaying a Doppler pected (Fig 12).

U

the location sound

When a color Doppler image is produced with a sector transducer, flow perpendicular to the beam usually occurs over a small segment of a vessel parallel to the transducer surface

42

shows

of focused

RadioGraphic-s

U

Poznlak

et a!

shift

where

Volume

none

12

is ex-

Number

1

Figure 12. (a) Transverse trix ofa Doppler phantom, rogating

the

vessel

and

image ofa far removed

displays

Doppler phantom. Cursor is interrogating from the flow in the tube. A grating lobe,

Doppler

shift

on the

spectral

tracing.

(b)

an area

in the gelatin

however, longitudinal

In the

succeeds plane,

ma-

in interthe true

Doppler sample volume interrogates flow traveling toward the interrogating beam. The grating lobe, however, registers flow in the opposite direction as it interrogates an area completely separate from the primary sample volume. (c) High-resistance tracing of the main renal artery in a transplant recipient. (d) This tracing presumably represents a visceral artery interrogated by a side lobe. As the transducer is manipulated to obtain

a better

Doppler

tracing,

sample

a low-resistance

volume

#{149} Spectral-broadening Spectral broadening tics

identified

in stenotic

flow

is squarely

Artifact of the

is one

flow

pattern

in the renal

appears

hilum.

occurs

in pulsed

and

patterns.

There-

continuous-wave

Doppler because of the finite dimensions of the sample volume. A target traveling through the sample volume at a given speed introduces a range of Doppler frequencies centered about the “true” Doppler frequency (ie, the Doppler frequencyf0 calculated with the equation mentioned previously) (12). As the

January

1992

in the background (arrowheads); however, = artery, rn/s = meters per second.

target characteris-

fore, it is an important clinical sign. However, spectral broadening may be introduced artifactually and can cause errors in interpreting peak velocities in spectral waveforms. The artifact

ART

pier

traverses angle

different sition

the

varies.

Doppler of the

target

sample

This

frequency and

volume,

introduces

the

the

Dop-

a slightly

shift

hence

for each

po-

a broadened

spectrum.

Spectral broadening is dependent on the angle of insonation (Fig 1 3) and the velocity of the blood. Spectral broadening generally becomes more severe for angles closer to 90#{176}. Thus, errors can be minimized by using sound-beam pler angle

orientation of less than

that results 60#{176}.

Pozniak

Ct

a!

in a Dop-

U

RadioGraphic-s

U

43

13. (a) Doppler spectral waveform for a flow phantom obtained with the Doppler beam angle at 71#{176} relative to flow direction. (b) Waveform obtained with a Doppler angle of8l#{176}.Note broadening of the spectral tracing, apparent increase in peak velocity, and partial filling of the systolic window for the steeper (8 1#{176}) angle. VEL = velocity, rn/s = meters per second. Figure

Trans

CONCLUSIONS

U

In B-mode imaging, changing the settings affects image quality. Altering the settings may enhance or camouflage important clinical findings. Both spectral and color Doppler imaging are also acutely sensitive to technical settings. Most important, the display of flow information from low-velocity states is dependent on sensitivity,

transmit

focal

scale, and filtration and spectral Doppler must be constantly locities

and

distance,

of insonation

color controls to ye-

if flow

condi-

tions are to be accurately demonstrated. Operators must also be aware of distortions of flow information such as mirroring artifacts that are due to unusual pathways of reflection and transmission in the body. Use of alternative acoustic windows can help identify such conditions. Finally, flow information can be distorted as a result ofequipment limitations. Examples presented include artifacts from transducer side and grating lobes and intrinsic

spectral

broadening.

avoided

when

limiting

factors.

the

Misdiagnosis

operator

can

is aware

be

3.

5.

Color

U

RadioGraphic-s

BJ, Winsberg

princi-

Radiology F.

Echographic

Reading CC, Charboneau JW, Allison Cooperberg PL. Color and spectral mirror-image artifact of the subclavian Radiology 1990; 174:41-42.

7.

Mitchell

DG,

Doppler

artifact

sound Parvey

8.

Burns

P, Needleman

in anechoic

JW, Doppler artery.

L.

regions.

Med 1990; 9:255-260. HR, Eisenberg RL, Giyanani

Color

J UltraV, Krebs

CA. Duplex sonography of the portal venous system: pitfalls and limitations. AJR 1989; 152:765-770.

9.

of these

ZagzebskiJ.

Doppler

Hagen-Ansert

S, ed.

ultrasonography.

Laing sonic

We are grateful to Carrie of the manuscript.

Namekawa

K, Koyano

Real-time

two-dimensional

ing using

an autocorrelation

U

imaging:

5: 227-237.

instrumentation. Textbook

St Louis:

In:

of diagnostic Mosby,

1989;

76-

A, Omoto

blood

flow

technique.

Pozniak

et a!

FC, Kurtz side.lobe

AB. The importance artifacts. Radiology

of ultra1982; 145:

763-768.

R. imagIEEE

12.

.

Laing FC. in clinical

Commonly ultrasound.

1983;

4:27-43.

Evans

D, McDicken

cockJ. mentation York:

44

Doppler

and artifacts.

6.

REFERENCES C,

DG.

Frequency

appearance of the right hemidiaphragm. J Ultrasound Med 1983; 2:243-249. Middleton WD, Melson GL. The carotid ghost: a color Doppler ultrasound duplication artifact. J Ultrasound Med 1990; 9:487493. Kremkau 1W, Taylor KJW. Artifacts in ultrasound imaging. J Ultrasound Med 1986;

4.

11 Kasai

Mitchell

Lewandowski

10.

1.

32:458-464.

92.

Acknowledgment: Poole for preparation

U

Ferroelectrics

1985;

pIes, limitations, 1990; 177: 1-10.

velocity

settings with both imaging. These fine-tuned relative

angles

2.

Ultrasonics

Control

encountered artifacts Semin Ultrasound W, Skidmore

R, Wood-

Doppler ultrasound physics, instruand clinical applications. New

Wiley,

1989;

87-105.

Volume

12

Number

1