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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
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