Ultrasound PhysicsArtifactsArtifacts

Hospital Physics Group

George David, M.S.Associate Professor of Radiology

Artifacts

Assumptions can cause artifacts when assumed conditions are not truesound travels at 1540 m/ssound travels in a straight lineAll sound attenuation exactly

0.5 dB/cm/MHz

Distance from Transducer

Echo positioning on imagedistance from transducer calculated

from assumed speed of soundcan place reflector too close to or

too far from transducercan alter size or shape of

reflector

V = 1540 m/s

V = 1380 m/s X

Actual Object Position

X Position of Object on Image

AttenuationFor all scanning your scanner assumes

soft tissue attenuation .5 dB/cm per MHz

Your scanner’s actioncompensate for assumed

attenuationallow operator fine tuning

TGC

ShadowingClinical Manifestation

reduction in imaged reflector amplitude

Causeobject between this reflector &

transducer attenuates ultrasound more than assumed

assumed compensation not enough to provide proper signal amplitude

intensity under-compensated

Opposite of Enhancement

Attenuates more than .5 dB/cm/MHz

ShadowedReflector

Shadowing

Attenuates more than .5 dB/cm/MHz

ShadowedReflector

http://raddi.uah.ualberta.ca/~hennig/teach/cases/artifact/noframe/imag2-f2.htm

EnhancementClinical Manifestation

increase in imaged reflector amplitude

Causeobject between reflector &

transducer attenuates ultrasound less than assumed

assumed compensation more than needed to provide proper signal amplitude

intensity over-compensatedOpposite of Shadowing

Attenuates less .5

dB/cm/MHz

Enhanced reflector

Enhancement

Attenuates less .5

dB/cm/MHz

Enhanced reflector

http://raddi.uah.ualberta.ca/~hennig/teach/cases/artifact/noframe/imag6-f1.htm

Refraction Artifact

refraction alters beam direction

direction of sound travel assumed to be direction sound transmitted

Actual Object Position

X Position of Object on Image X

Refraction

Refraction Artifactrefraction alters beam directionscanner places dot in wrong

location along line of assumed beam direction

can alter reflector shape

Lobe ArtifactsSide Lobes

beams propagating from a single element transducer in directions different from primary beam

reflections from objects here will be placed on main sound transmission line

Grating Lobessame as above except for

transducer arrays

X

Range AmbiguityReflection from 1st pulse

reaches transducer after 2nd pulse emittedscanner assumes this is

reflection from 2nd pulseplaces echo too close & in

wrong direction

12

Scanner Assumptions

Actual Object Position

X Position of Object on Image

MultipathArtifact

X

Multiple Reflection Scenario

reflection from reflector “B” splits at “A”

some intensity re-reflected toward “B”

Resultlater false echoes heardscanner places dots behind

reflector “B”

1 2 3

1

2

3

A

B

real

false

ArtifactsReverberation (multiple Reverberation (multiple

echo) artifactecho) artifact“comet tail” effect is 1

examplecan have dozens of multiple

reflections between transducer & reflector 2 reflectors

Mirror Imagecommon around diaphragm

& pleura

Real

Mirror

Artifacts

http://raddi.uah.ualberta.ca/~hennig/teach/cases/artifact/noframe/imag1-f1.htm

Caused by Shotgun Pellets

Multiple Reflection Scenario

http://raddi.uah.ualberta.ca/~hennig/teach/cases/artifact/noframe/imag5-f2.htm

Real

Mirror

Resolution ArtifactsAxial and Lateral Resolution

Limitationsresults in failure to resolve 2 adjacent

structures as separateminimum image size equal to resolution

in each direction

Section Thickness Artifact

anatomy may not be uniform over its thickness

universal problem of imaging 3D anatomy

in CT & MRI this is known as partial volume effectpartial volume effect

Thickness

Constructive Interference

2 echoes received at same time

in phaseResult

higher intensity +

=

Destructive Interference2 echoes received

at same timeExactly 180o out

of phaseResult

flat (zero) wave -

=

Acoustic Speckletexture seen on image

may not correspond to tissue texture

Results from interference effects between multiple reflectors received simultaneously which canadd together

constructive interferencesubtract from one another

destructive interference

Mirror Image & DopplerAnalogous to mirror image artifact discussed

previouslymirrored structures can include mirrored vesselduplicate image visible on opposite side of

strong reflectorexample: bone

Doppler data also duplicatedflow & spectrum copied from original vessel

Spectral Duplicationmirror image of Doppler spectrum

appears on opposite side of baselinecauses

electronic duplication caused by receiver gain set too high overloads receiver

True sensing caused by too large Doppler angle beam covers flow in both directions

Blood flows toward transducer

Blood flows away from transducer

AliasingResults in detection of

improper flow directionoccurs because sampling rate

too slowSimilar to wagon wheels

rotating backwards in movies

Aliasing

Sufficient Sampling

Insufficient Sampling

AliasingWhich way is this shape turning?

#1 #2 #3

AliasingDid the shape turn 1/4 turn right

or

3/4 turn left?1 1/4 turn right?

#1 #2 #3

AliasingDoes it help to sample more often?

#1 #2

#3

#1A

#2A #3A

AliasingMaximum detectable Doppler shift equals

half the pulse repetition frequency

Sampling rateSame as pulse repetition frequencyMust be at least twice highest frequency to be

sensedAliasing occurs when Doppler shift exceeds

0.5 * PRF

Coping with Aliasingdecrease transducer frequency

reduces Doppler shiftshift proportional to operating frequency

increase pulse repetition frequencydecreases maximum imaging depthincreases likelihood of range ambiguity for

pulsed instruments

77 X fD (kHz)v (cm/s) = -------------------------- fo (MHz) X cos

Coping with Aliasingincrease Doppler angle

Reduces relative flow rate between blood & transducer

Reduces Doppler shift sensed by scanner

77 X fD (kHz)v (cm/s) = -------------------------- fo (MHz) X cos

Coping with Aliasing:Baseline Shifting

operator instructs scanner to assume that aliasing is occurringscanner does calculations based on operator’s assumption

scanner has no way of determining where in image aliasing occurs