ECHO 2013 / Artifacts of Imaging and Doppler Know Your Physics
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Ultrasound Physics Terminology
Resolution: IsIstheparameterofofananultrasoundimagingsystemthatcharacterizesitsitsabilitytoto detectcloselyspacedinterfacesanddisplaystheechoesfromthoseinterfacesasas distinctandseparateobjects. .Thebetterer theresolution,thegreatertheclarityofofanan ultrasoundimage. .
Axial |
IsIstheabililitytotodiscerntwocloselyspaced |
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Dependence on SPL/ pulse duration ( |
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Resolution: |
objespacedtsinobjectsthe directionin the directionof the beamof the. |
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SPL improves resolution) |
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Goodbeamaxial.Goodresolutionaxial resolutionis achievedis achievedwith |
2. |
Effect of damping ( dampening pulse |
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shortwith shortpatialspatialulsepulselengthslengths(SPL)(SPL). Short. |
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duration) |
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SPLShortareSPLa resultare aofresulthigherof frequencyhigher and |
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Transducer frequency spectrum-relation to |
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higherfrequencydampedan transducershigherdamped. |
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pulse duration ( Freq wavelength) |
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transducers. |
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Bandwidth ( bandwidth pulse duration) |
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Lateral |
IsIstheabililityofofthesystemtotoresolve |
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Dependence on beam width |
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Resolution: |
objectsininaadirectionperpendiculartoto |
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Frequency |
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thebeamdirection. .Gooodlateral |
3. |
Transducer size and focal characteristics |
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resolutionisisachievedwithnarrowow |
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Range (best lateral resolution occurs at the |
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acousticbeams. . |
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near field—far field face) |
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Elevational |
IsIstheresolutionofoftheultrasoundbeam |
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Dependence on beam width |
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Resolution: |
perpendiculartototheimageplaneandis |
2. |
Transducer array and focal characteristics |
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dependentison theonthetransducerelemement |
3. |
Frequency |
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height (similartotobeamwidthabove) |
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Lateral and axial resolution relationship |
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Temporal |
Is the ability to evaluate rapid motion and |
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Pulsing characteristics |
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Resolution: |
is dependent on the frame rate and time |
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Number of lines per frame |
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required to generate one frame |
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Number of focal regions |
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d. Field of view (e.g., sector angle) |
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e. |
Image depth (penetration) |
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Principles of Doppler
Echocardiography
Christian Johann Doppler (1842)
If a source of sound is stationary, then the wavelength and frequency of the sound emanating from that source are constant
If the source of sound is moving toward a receiver, then the wavelength is decreasing and the frequency is increasing
If the source of sound moves away
form the receiver, then the wavelength is increasing and the frequency is decreasing
trains004.wav
Principles of Doppler
Echocardiography
Doppler Shift or Frequency (fd)
Represents the difference between the received (fr) and the transmitted frequencies (ft)
The velocity of blood causing
the Doppler shift can thus be determined
Principles of Doppler Echocardiography
•Cosine
•if angle of insonation is <20° cosine 1
•if angle of insonation is 90° cosine = zero
•At 30°, there is a 17% reduction of velocity recorded
Angle dependency of TDI Strain
Small angle → reliable waveform
0
TDI strain is more sensitive to the angle of-insonation than
regular Doppler
20
-
40
Large angle → mixture of radial + longitudinal strain
Unreliable waveform
Principles of Doppler
Echocardiography
Nyquist Limit is:
Upper limit of the frequency shift that can be detected at a given depth with a given pulsed system
Each cycle must be sampled at least twice to accurately determine its frequency
PRF = 2 x frequency
Nyquist limit = ½ PRF
Vm = Maximal Velocity c = speed of sound
R = range
f0 = insonation frequency
Principles of Doppler
Echocardiography
Inverse relation between maximal velocity detectable and:
Probe frequency
Depth |
PRF with PROBE |
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REQUENCY PRF with DEPTH |
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Definition of Artifact
―an ultrasound image that does not correlate directly with the actual structure being visualized‖
―…is a result of the physical properties of ultrasound waves and their interaction with tissues‖
Hedrick: Ultrasound Physics and Instrumentation; Mosby, 1995
Assumptions of the US Machine
Transmitted waves travel in a straight line from the transducer
Beam dimensions are infinitely small in all dimensions
All echoes originate along the axis of the transducer only
All echoes are derived from the most recent pulse
Each pulse transmitted is received before the next pulse is delivered
Sound waves travel at 1540 m/s in human soft tissue therefore
Distance to a reflector is proportional to the round trip time (13 s/cm)
Amplitude or brightness of the echo is directly proportional to reflective strength
Distinguishing Real from Artifact
Artifact
Crosses Border
Indistinct edges
Motion identical to another real structure
amplitude and frequency at a multiple of the real structure
Real Structure
Respects structures/borders
Distinct edges
Motion independent of surrounding structures
Amplitude and frequency of motion different than surrounding structures (ie: dissection flap)
Not reproduced in an |
Seen in multiple views |
orthogonal view |
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