IMAGING TECHNIQUES For Vascular Sonography

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IMAGING TECHNIQUESFor Vascular Sonography

• B-Mode Imaging
• Doppler Spectral Analysis
• Color Doppler Flow Imaging
• Power Doppler

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B-mode Imaging

• Technologist must understand the relationship
  of image quality to power gain
• Increasing power and gain may degrade image
  quality
• Maintain perpendicular incidence as much as
  possible
• Understand controls and their effect on image
• Power
• Gain
• Dynamic range
• Depth, etc.

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Perpendicular IncidenceMaintain perpendicular
incidence
This
Not this!
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DepthKeep vessel of interest as close to the
center of image as possible
Appropriate image depth
Too much image depth
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Dynamic RangeDecrease the dynamic range to
decrease intraluminal echoes
40 dB
60 dB
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Color Doppler
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Retrograde Flow
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Color Doppler Flow Imaging

• Pulsed-wave Doppler
• Color saturation is based on mean flow
  within the vessel
• Uses autocorrelation to determine the mean flow
  velocity
• Evaluate for
• Presence of flow
• Direction of flow
• In relation to the sampling angle
• Quality of flow
• Laminar vs. turbulent
• Relative mean velocity (color saturation)

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Color Doppler - Guidelines

• Understand color orientation
• Red over blue
• Flow towards signal will be red
• Flow away from the signal will be blue
• Blue over red (inverted)
• Flow towards signal will be blue
• Flow away from the signal will be red
• Color baseline
• In most situations the color baseline will be
  maintained in the center
• Positive and negative flow scales will be equal

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Color Doppler - Guidelines

• Know the normal direction of flow in the vessel
  being evaluated
• Artery vs. vein
• Neck
• Artery right to left and vein left to right
• Abdomen extremities
• Artery left to right and vein right to left
• Steer the color box appropriately
• Perpendicular vessel
• Steer the color box and set the color orientation
  properly
• Vessel progressing deep to the right on image
• Steer the color box left to right
• Vessel progressing deep to the left on image
• Steer the color box right to left
• Transverse imaging
• Generally the color box will be center steered

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Color Doppler - Guidelines

• Set an appropriate color scale
• The greater the expected mean flow in the vessel
  being evaluated the higher the color scale
  setting
• Arterial studies will generally have a higher
  scale setting then venous studies
• Doppler gain
• Increase or decrease to show color within the
  vessel which extends to limits of the vessel wall
• Overgaining may mask pathology
• Undergaining may lead to false positive for
  occlusion

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Color Scale Orientation
Scale
0 Baseline - No Doppler Shift
Orientation
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Color Box - Steering
Right to Left
Left to Right
Center
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Presence or Absence of Flow
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Color Doppler ScaleColor Doppler Aliasing
Color scale 19.2 cm/s PRF 2000
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Color Doppler Flow Imaging
Appropriate scale setting
Color Scale 33.6 cm/s PRF 3500
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Wall Filters

• High pass filter
• Spectral and color Doppler
• Allow high level signals to pass
• Filters out low level noise
• Color priority
• Allows for more or less color saturation
• High priority
• Greater color saturation
• Low priority
• Lower level of color saturation
• More gray scale information

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Color DopplerWhats wrong with this?
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Power Doppler

• More sensitive to flow than color Doppler
• Very low dependence on angle
• Based on the amplitude of the returning Doppler
  signal
• Greater the amplitude of the returning signal the
  better the representation of flow
• No directional information
• Flow is encoded one color, independent of
  direction
• Some manufacturers are engineering directional
  power Doppler into their machines
• Subject to blooming of the display due to very
  high sensitivity

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Power Doppler
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Spectral Doppler
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Doppler Spectral Analysis

• Pulsed-wave Doppler
• Uses Fast Fourier Transform (FFT) instead of
  autocorrelation
• Evaluate for
• Presence of flow
• Direction of flow
• In relation to the sampling angle
• Quality velocity of flow
• Laminar vs. turbulent
• Peak systolic velocity (PSV)
• End diastolic (EDV)
• Mean velocity (MV)

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Spectral Doppler - Terminology

• Doppler effect
• Frequency (Doppler) shift
• Doppler equation
• Incidence angle
• Angle correction
• Doppler
• Gain
• Baseline
• Scale
• Wall filters
• Aliasing
• Pulse Repetition Frequency (PRF)
• Nyquist Limit

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Spectral Doppler - Terminology

• Velocity
• Systolic
• Diastolic
• Mean
• A/B ratio compares one velocity to another
• Systolic Velocity Ratio (SVR)
• Peak Velocity Ratio (PVR)
• Diastolic Velocity Ratio Diastolic/Systolic
  Ratio (DSR)
• Resistivity Index
• Pulsatility Index
• Low resistance waveform
• High resistance waveform

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Doppler Effect

• Usefulness in diagnostic ultrasound
• Detect quantify the existence of moving blood
  cells
• Information pertaining to velocity and amplitude
  is displayed in the Doppler information
• Determine their direction
• Toward the source
• Away from the source
• Antegrade
• Retrograde
• Velocity
• Peak systolic
• End diastolic
• Mean

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Spectral Doppler

• Sonographer must understand relationship of flow
  direction to the Doppler incident angle
• Positive or negative, i.e. toward or away from
• Neck vessels
• Doppler incident angle left to right
• Arteries will exhibit a positive Doppler shift
• Veins will exhibit a negative Doppler shift
• Doppler incident angle right to left (inverted)
• Arteries will exhibit a negative Doppler shift
• Veins will exhibit a positive Doppler shift
• Abdominal and extremity vessels
• Opposite that of the neck vessels

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Doppler Spectral AnalysisRECOMMENDATIONS

• Maintain a spectral Doppler angle as close to 60
  as possible.
• NEVER 60
• Over angle correction
• overestimation of velocities
• Under angle correction
• underestimation of velocities
• Angle correct to the vessel wall

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Understanding the Doppler ANGLE

• Doppler equation states that the most accurate
  frequency shifts, therefore, calculated
  velocities, are obtained at 0.
• A 0 Doppler incidence is always assumed for
  cardiac and transcranial Doppler applications.
• Evaluation of vessels at 0 is, in general, not
  practical.
• Vessels which run parallel to the skin surface
  preclude the use of 0.

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Doppler Angle

• Doppler equation states that any detection
  angle, Doppler beam to blood vessel, other
  than 0 will lead to a diminution of the
  returning signal.
• The majority of vascular Doppler diagnostic
  criteria have been assembled utilizing an
  incidence angle of 60.

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Doppler Spectral AnalysisTheta (?)

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Doppler Equation
?FDetected Frequency shift FoTransmitted
Frequency 2Transmit/Receive time c speed of
sound in soft tissue Vvelocity of
blood Cos?cosign Thetacosign of incidence angle
Theta (?) Cosign 0
1.0 60
0.5 90
0.0
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Doppler Effect

• Usefulness in diagnostic ultrasound
• Detect quantify the existence of moving blood
  cells
• Determine their direction
• Toward the source
• Away from the source
• Antegrade
• Retrograde
• Velocity
• Peak systolic
• End diastolic
• Mean
• Q How? A Frequency shift

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Doppler (Frequency) Shift

• ?f caused by motion
• Equal to the received frequency minus the source
  frequency
• For an approaching reflector, ?f is positive, for
  a receding reflector, ?f is negative
• Doppler shifts in diagnostic ultrasound are in
  kHz and are then converted to velocity, i.e. m/s,
  cm/s

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Proper Angle Correction Parallel To Posterior
Vessel Wall
PSV 99 cm/s EDV 32 cm/s
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Spectral Doppler Angle Undercorrection
PSV 69 cm/s EDV 22 cm/s
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Spectral Doppler Angle Overcorrection
PSV 129 cm/s EDV 41 cm/s
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Improper Angle Correction60 IS NOT always 60
PSV 77 cm/s EDV 21 cm/s
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Proper Angle CorrectionParallel To Posterior
Vessel Wall
PSV 92 cm/s EDV 25 cm/s
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60 Spectral Doppler IncidencePSV144 cm/s
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70 Spectral Doppler IncidencePSV188 cm/s
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50 Spectral Doppler IncidencePSV113 cm/s
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Doppler GainUngaining
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Doppler GainOvergained
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Doppler Baseline Baseline is set too high
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Doppler Baseline
Appropriate setting
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Doppler Aliasing
Inappropriate scale setting
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Aliasing The Fix

• Increase the PRF
• Increase the scale
• PRF of pulses occurring in one second
• Typical PRF in diagnostic US, 4-10 kHz
• Aliasing occurs at ½ the PRF, which is the
  Nyquist Limit
• Decrease the sampling depth
• Find a sampling window which shows the target
  vessel more superficially
• Use a lower frequency transducer
• Increase the Doppler angle
• E.g. change from a 50 degree angle to a 60 degree
  angle
• Use a CW Doppler
• Not an option in vascular imaging
• Shift the baseline
• Not a true fix
• Will only work if the baseline was
  inappropriately set

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Doppler Sample Volume1.5 mm
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Doppler Sample Volume 5.0 mm
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Doppler Sample Volume7.5 mm
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Spectral BroadeningDue to vessel stenosis
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OK to determine artery vs. vein only
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OK to determine artery vs. vein only
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High vs. Low Resistance

• The resistance of the vascular bed, which a
  particular vessel is supplying.
• Determined by flow, or the lack of it, in
  diastole.
• Qualities are variable in some vessels.
• High resistance no, or low diastolic flow
• Typical for peripheral arteries
• Changes in response to
• Exercise
• Temperature
• Arterial occlusive disease (PVD, PAD)
• Low resistance flow through diastole
• Internal carotid artery
• Renal artery
• Celiac trunk its branches

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High vs. Low Resistance
ICA Low Resistance
ECA High Resistance
CFA Higher Resistance