Ultrasound Physics
Artifacts
Hospital Physics Group
George David, M.S.
Associate Professor of Radiology
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
 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
 assumed compensation not enough to
provide proper signal amplitude
 intensity under-compensated
Attenuates
more than .5
dB/cm/MHz
 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
Attenuates
 object between reflector & transducer
less .5
attenuates ultrasound less than assumed dB/cm/MHz
 assumed compensation more than
needed to provide proper signal
amplitude
 intensity over-compensated
 Opposite of Shadowing
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 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
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 re-reflected
toward “B”
 Result
1 2
3
A
 later false echoes heard
B
 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
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
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