1. Artifacts Ultrasound Physics George David, M.S.
Associate Professor of Radiology

2. Nothing to do with Anything

3. 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

4. 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
58 usec 4 cm
Calculated Distance to interface
V = 1380 m/s
1540 m/s X 58usec / 2 = 4.47 cm
(Average speed of sound in soft tissue)

5. 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 X
Actual Object Position X
Position of Object on Image
V = 1540 m/s

6. 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

7. Shadowing Clinical Manifestation Cause Opposite of Enhancement
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
Opposite of Enhancement
Attenuates more than .5 dB/cm/MHz
Shadowed
Reflector

8. Shadowing Attenuates more than .5 dB/cm/MHz Shadowed Reflector

9. Attenuates less .5 dB/cm/MHz
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

10. Attenuates less .5 dB/cm/MHz
Enhancement
Attenuates less .5 dB/cm/MHz
Enhanced reflector

11. Shadowing / Enhancing these artifacts not necessarily bad
can help in determining nature of masses “upstream of artifact which caused shadowing / enhancing

12. 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

13. 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

14. Refraction Artifact refraction alters beam direction
scanner places dot in wrong location along line of assumed beam direction
can alter reflector shape

15. Lobe Artifacts Side Lobes Grating 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

16. 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

17. 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
Depth
Range Ambiguity Trade-off
Lines / Frame
Frames / sec (dynamics)

18. Scanner Assumptions Multipath Artifact Actual Object Position X
Position of Object on Image X

19. Multiple Reflection Scenario
reflection from reflector “B” splits at “A”
some intensity re-reflected toward “B”
Result
later false echoes heard
scanner places dots behind reflector “B” 1 2 3 A B
real 1 2
false 3

20. Artifacts Reverberation (multiple echo) artifact Mirror Image
“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

21. Artifacts
Caused by Shotgun Pellets

22. Multiple Reflection Scenario
Real
Mirror

23. 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

24. 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

25. Constructive Interference
2 echoes received at same time
in phase
Result
higher intensity + =

26. Destructive Interference
2 echoes received at same time
Exactly 180o out of phase
Result
flat (zero) wave - =

27. 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

28. 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

29. 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

30. Doppler Artifacts Doppler spectrum speckle Cause
same as acoustic speckle
random constructive & destructive interference from sound scattered in blood

31. Aliasing Results in detection of improper flow direction
occurs because sampling rate too slow
Similar to wagon wheels rotating backwards in movies

32. Insufficient Sampling
Aliasing
Sufficient Sampling
Insufficient Sampling

33. Aliasing
Which way is this shape turning? OR #1 #2 #3

34. Did the shape turn 1/4 turn right or 3/4 turn left?
Aliasing
Did the shape turn 1/4 turn right or 3/4 turn left?
1 1/4 turn right? #1 #2 #3

35. Does it help to sample more often?
Aliasing
Does it help to sample more often? #2 #1
#1A
#2A
#3A #3

36. 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

37. 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

38. 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

39. Coping with Aliasing increase Doppler angle
Reduces relative flow rate between blood & transducer
Reduces Doppler shift sensed by scanner q
77 X fD (kHz)
v (cm/s) =
fo (MHz) X cosq

40. 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
Yes No

41. Any Questions?