Ultrasound Physics

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Ultrasound Physics

• Artifacts

Our Hospital Physics Group
George David, M.S. Associate Professor of
Radiology
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Nothing to do with Anything
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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 exactly0.5 dB/cm/MHz

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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
58 usec
1380 m/s X 58usec / 2 4 cm
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)
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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
Position of Object on Image
X
V 1540 m/s
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Attenuation

• For all scanning your scanner assumes
• soft tissue attenuation
• .5 dB/cm per MHz
• Your scanners action
• compensate for assumedattenuation
• allow operator fine tuning
• TGC

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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
• Opposite of Enhancement

Attenuates more than .5 dB/cm/MHz
Shadowed Reflector
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Shadowing
Attenuates more than .5 dB/cm/MHz
Shadowed Reflector
http//raddi.uah.ualberta.ca/hennig/teach/cases/a
rtifact/noframe/imag2-f2.htm
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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
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Enhancement
Attenuates less .5 dB/cm/MHz
Enhanced reflector
http//raddi.uah.ualberta.ca/hennig/teach/cases/a
rtifact/noframe/imag6-f1.htm
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Shadowing / Enhancing

• these artifacts not necessarily bad
• can help in determining nature of masses
  upstream of artifact which caused shadowing /
  enhancing

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Scanner Assumptions

• Echo positioning on image
• direction of all sound travel assumed to be
  direction that sound was transmitted

X
Refraction
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Refraction Artifact

• refraction alters beam direction
• direction of sound travel assumed to be direction
  sound transmitted

X
Refraction
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Refraction Artifact

• refraction alters beam direction
• scanner places dot in wrong location along line
  of assumed beam direction
• can alter reflector shape

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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
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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
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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
RangeAmbiguityTrade-off
Lines / Frame
Frames / sec(dynamics)
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Scanner Assumptions
Multipath Artifact
Actual Object Position
Position of Object on Image
X
X
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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

2
3
1
A
B
real
1
2
false
3
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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

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Artifacts
http//raddi.uah.ualberta.ca/hennig/teach/cases/a
rtifact/noframe/imag1-f1.htm
Caused by Shotgun Pellets
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Multiple Reflection Scenario
http//raddi.uah.ualberta.ca/hennig/teach/cases/a
rtifact/noframe/imag5-f2.htm
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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

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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
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Constructive Interference

• 2 echoes received at same time
• in phase
• Result
• higher intensity

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Destructive Interference

• 2 echoes received at same time
• Exactly 180o out of phase
• Result
• flat (zero) wave

-

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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

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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

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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
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Doppler Artifacts

• Doppler spectrum speckle
• Cause
• same as acoustic speckle
• random constructive destructive interference
  from sound scattered in blood

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Aliasing

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

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Aliasing
Sufficient Sampling
Insufficient Sampling
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Aliasing

• Which way is this shape turning?

OR
1
2
3
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Aliasing

• Did the shape turn 1/4 turn right or 3/4 turn
  left?
• 1 1/4 turn right?

1
2
3
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Aliasing

• Does it help to sample more often?

2
1
1A
2A
3A
3
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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

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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
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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
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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
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Coping with AliasingBaseline Shifting

• operator instructs scanner to assume that
  aliasing is occurring
• scanner does calculations based on operators
  assumption
• scanner has no way of determining where in image
  aliasing occurs

Yes
No
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Any Questions?