Ultrasound Physics
Artifacts
Our Hospital Physics Group
George David, M.S.
Associate Professor of Radiology
Nothing to do with
Anything
Artifacts
• Assumptions can cause artifacts
when assumed conditions are
not true
sound travels at 1540 m/s
sound travels in a straight line
All sound attenuation exactly
0.5 dB/cm/MHz
Distance from Transducer
• Calculation of Distance
scanner accurately measures
time delay between sound
generation & echo reception
Distance = Assumed Speed X Measured Delay / 2
Actual Distance to interface
1380 m/s X 58usec / 2 = 4 cm
Calculated Distance to interface
58 usec
V = 1380 m/s
1540 m/s X 58usec / 2 = 4.47 cm
(Average speed of sound in soft tissue)
4
cm
Distance from Transducer
• Echo positioning on
image
distance from transducer
calculated from assumed speed
of sound
can place reflector too close to
or too far from transducer
can alter size or shape of
reflector
V = 1380 m/s
Actual Object Position
X Position of Object on Image
V = 1540 m/s
X
Attenuation
• For all scanning your scanner
assumes
soft tissue attenuation
» .5 dB/cm per MHz
• Your scanner’s action
compensate for assumed
attenuation
allow operator fine tuning
» TGC
Shadowing
• Clinical Manifestation
 reduction in imaged reflector amplitude
• Cause
 object between this reflector & transducer
attenuates ultrasound more than assumed
Attenuates
 assumed compensation not enough to
more than .5
provide proper signal amplitude
dB/cm/MHz
 intensity under-compensated
• Opposite of Enhancement
Shadowed
Reflector
Shadowing
Attenuates
more than .5
dB/cm/MHz
Shadowed
Reflector
http://raddi.uah.ualberta.ca/~hennig/teach/cases/artifact/noframe/imag2-f2.htm
Enhancement
• Clinical Manifestation
increase in imaged reflector amplitude
• Cause
object between reflector & transducer
attenuates ultrasound less than
assumed
assumed compensation more than
needed to provide proper signal
amplitude
intensity over-compensated
• Opposite 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
Shadowing / Enhancing
• these artifacts not necessarily
bad
• can help in determining nature of
masses “upstream of artifact
which caused shadowing /
enhancing
Scanner Assumptions
• Echo positioning on
image
direction of all sound travel
assumed to be direction that
sound was transmitted
Actual Object Position
X Position of Object on Image
X
Refraction
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 Artifact
• refraction alters beam direction
• scanner places dot in wrong location
along line of assumed beam direction
• can alter reflector shape
Lobe Artifacts
• Side 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 Lobes
same as above except for
transducer arrays
X
Range Ambiguity
• Reflection from 1st
pulse reaches
transducer after 2nd
pulse emitted
scanner assumes this is
reflection from 2nd pulse
places echo too close & in
wrong direction
1
2
Range Ambiguity
• To improve any 1 of
3, at least 1 of other
2 must be reduced.
• many scanners
automatically
reduce frame rate
as depth increases
Lines / Frame
Depth
Range
Ambiguity
Trade-off
Frames / sec
(dynamics)
Scanner Assumptions
Multipath
Artifact
Actual Object Position
X Position of Object on Image
X
Multiple Reflection Scenario
• reflection from reflector
“B” splits at “A”
• some intensity rereflected toward “B”
• Result
1 2
3
A
B
later false echoes heard
scanner places dots behind
reflector “B”
1
2
3
real
false
Artifacts
• Reverberation (multiple
echo) artifact
“comet tail” effect is 1 example
can have dozens of multiple
reflections between
» transducer & reflector
» 2 reflectors
• Mirror Image
common 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
Real
Mirror
http://raddi.uah.ualberta.ca/~hennig/teach/cases/artifact/noframe/imag5-f2.htm
Resolution Artifacts
• Axial and Lateral Resolution
Limitations
results in failure to resolve 2 adjacent
structures as separate
minimum 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 effect
Thickness
Constructive Interference
• 2 echoes
received at
same time
• in phase
• Result
+
higher intensity
=
Destructive Interference
• 2 echoes
received at
same time
• Exactly 180o
out of phase
• Result
flat (zero) wave
=
Acoustic Speckle
• texture seen on image
may not correspond to
tissue texture
• Results from
interference effects
between multiple
reflectors received
simultaneously which
can
 add together
» constructive interference
 subtract from one another
» destructive interference
Mirror Image & Doppler
• Analogous to mirror image artifact
discussed previously
mirrored structures can include mirrored vessel
duplicate image visible on opposite side of strong
reflector
example: bone
• Doppler data also duplicated
flow & spectrum copied from original vessel
Spectral Duplication
• mirror image of Doppler
spectrum appears on
opposite side of baseline
• causes
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
Doppler Artifacts
• Doppler spectrum speckle
• Cause
same as acoustic speckle
random constructive & destructive
interference from sound scattered in blood
Aliasing
• Results in detection of improper flow
direction
• occurs because sampling rate too
slow
• Similar to wagon wheels rotating
backwards in movies
Aliasing
Sufficient Sampling
Insufficient Sampling
Aliasing
• Which way is this shape turning?
#1
#2
#3
Aliasing
Did the shape turn 1/4 turn right
or
3/4 turn left?
1 1/4 turn right?
#1
#2
#3
Aliasing
Does it help to sample more often?
#1
#1A
#2
#2A
#3
#3A
Aliasing
• Maximum detectable Doppler shift
equals half the pulse repetition
frequency
• Sampling rate
Same as pulse repetition frequency
Must be at least twice highest frequency to be
sensed
• Aliasing occurs when Doppler shift
exceeds 0.5 * PRF
Aliasing
• Maximum detectable Doppler Shift
not limited for continuous wave
Doppler
• Maximum detectable Doppler Shift is
limited for pulsed instruments
Maximum Detectable Doppler Shift = half pulse repetition frequency
Coping with Aliasing
• decrease transducer frequency
reduces Doppler shift
shift proportional to operating frequency
• increase pulse repetition frequency
decreases maximum imaging depth
increases likelihood of range ambiguity for pulsed
instruments
77 X fD (kHz)
v (cm/s) = -------------------------fo (MHz) X cosq
Coping with Aliasing
• increase 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 cosq
q
Coping with Aliasing:
Baseline Shifting
• operator instructs scanner to assume that
aliasing is occurring
 scanner does calculations based on operator’s assumption
• scanner has no way of determining where in
image aliasing occurs
Any Questions?