What is Color Doppler?
�Color Doppler flow imaging is blood flow information superimposed in color on the B-mode real-time ultrasound image
�Allows us to assess blood flow within a vessel or organ
�Color Doppler is usually used in conjunction with spectral Doppler to accurately
What is the Doppler shift?
�Created when transmitted sound waves strike moving red blood cells
�Toward the transducer-shift is positive
�This means that the reflected frequency is higher than the transmitted
Why is there a "2" in Doppler equation?
Two doppler shifts during an exam
�First shift-sound wave strikes rbc's
�This reception of the sound wave is sufficient to create a shift
�Second shift-reflections return to transducer and the reception of the reflected sound waves from rbc's by the trans
Machine asks 5 questions with each returning pulse (3 presented here)
Is a Doppler frequency shift presented?
What is the magnitude of the Doppler shift?
What is the direction of the Doppler shift?
What does it mean when the machine asks Is a Doppler frequency shift presented?
If so, the echo is displayed in color, if not then in shades of gray
What is the magnitude of the Doppler shift?
�The magnitude will vary in proportion of the blood flow velocity and angle of insonation.
�Velocity is defined by magnitude and direction
�Doppler frequencies indicate velocity
The Doppler shift is ___related to the velocity of RBC's
directly
What is the direction of the Doppler shift?
�This is displayed in hues of red and blue .
�Is relative to whether flow is towards or away from the transducer
What do the directions of the blood flow mean?
�When blood is moving parallel to the sound beam-entire Doppler shift is measured
�Directly toward or directly away-velocity measurement is 100% accurate
�When an angle exists-the measured velocity is less than a true one
What is non directional Doppler?
�Presence of moving blood cells by detecting a Doppler shift
�Does not distinguish direction
�Examples are fetal Doptones
�Output is sound on a speaker
What is Bi directional Doppler?
�Positive-toward the transducer
�Negative-away from transducer
�Can be displayed by sound only-using two speakers
�Spectrum can be displayed graphically
Transmitted frequency and Doppler shift are related in what way?
�Direct relationship!
�If frequency is reduced by half-the Doppler shift will also be reduced by half
Describe continouse wave
�Requires two crystals in the transducer-
�one transmits
�One receives
�Accurately measures very high velocities
�Unable to detect the exact location of moving rbc's
�Overlap between the transmit and receive causes a limitation known as range ambiguity
Describe pulsed wave
�One crystal necessary
�Alternates between sending and receiving
�Sonographer positions a small marker-called the sample volume or gate
�Machine measures time of flight to the gate
�Transducer will emit a sound pulse at that time
what are the advantages of pulsed Doppler?
�Selection of exact location of interrogation-called range resolution
what are the dis-advantages of pulsed Doppler?
Primary disadvantages
�Inaccurate measurement of high velocity signals
�Aliasing artifact displays as wraparound of the velocity
What is the Nyquist Limit?
�Highest Doppler frequency or velocity that can be measured without the appearance of aliasing
�Low PRF is associated with aliasing
How does transducer selection affect Doppler?
�Higher frequency transducers-higher Doppler shifts, more aliasing
�Lower frequency transducers-lower Doppler shifts but less likely to exceedthe Nyquist limit
What is color flow Doppler?
�A form of 2 dimensional Doppler
�Velocity information is coded into colors and superimposed on the gray scale
�Reports averagevelocities while pulsed Doppler reports peaks
�Reports direction (semi-quantitative)
�Knowledge of angle is less important
What are the Advantages of color Doppler?
�Technical Efficiency:Faciliates the examination
�Instantaneous assessment of flow throughout the entire lumen
-Easy to pick up abnormal flow patterns
�Has the potential for improved definition of smooth or rough surfaces.
�Differentiation between severe
What is the most important advantage of Color Doppler?
Technical Efficiency:�Faciliates the examination
What are the Limitations of color Doppler
�Flow information is qualitative not quantitative
�Low pulse repetition frequencies and frame rates
Flow detection is angle dependent
Flow direction is arbitrary
�Knowledge of anatomy and vascular physiology is essential
Color may obscure vascular patholo
A low PRF gives a ___incidence of aliasing
Higher
�Aliasing occurs when the Doppler shift is twice the PRF
�Aliasing is represented in the color Doppler image as a localized shift in color to represent flow in the opposite direction
-Low Frame rate-visual flicker on screen
What does flow direction is is angle dependent mean?
-Blood flow will not be detected in vessels that are perpendicular to the angle of insonation
-Blood flow is best detected in vessels that are parallel to angle of insonation, but can be seen filling the vessels in vessel that are at 30 to 60 degrees to a
what does flow direction is arbitrary mean?
-Color based on direction relative to transducer
-Can be changed manually by Sonography
�Inverting transducer
�Pushing Invert button
What is color flash artifact?
-Any motion will create a Doppler shift and present as color on the monitor
�Transducer motion
�Cardiac motion
�Respiratory motion
What is the conflict between B-mode and color flow images?
-Grayscale imaging best perpendicular to sound beam
-Color Doppler imaging best parallel to sound beam
What does the color bar do?
�Color assignment is determined by flow direction relative to the transducer and can be arbitrarily assigned to the red or blue spectrum
�Greater frequency shifts indicating higher velocities are assigned a lighter shade of red or blue
�With abdominal and
What are some color Doppler artifacts?
Perivascular tissue vibration
Mirror image artifact
Spectral Broadening (COLOR)
What is Perivascular tissue vibration?
-Arterial pulsations resulting in motion of the perivasuclar tissue causing artifactual color to be displayed outside the vessel
-Displayed as a random mixture of reds and blues
-Similar to the bruit heard through a stethoscope and is more pronounced duri
What is Mirror image artifact?
Mirror image artifact: results from internal reverberations of the Doppler shift echo due to specular reflections from the vessel wall. The resultant delay results in a more deeply situated "mirror image" of the actual object. This artifact can be control
What is Spectral Broadening (COLOR)?
-Occurs in
�Stenoticlesions
�Tortuous vessels
�Kinked vessels
-Demonstrated as brighter hues and in areas of a stenosisas a mosaic pattern
-Can help you recognize stenoticjets and help facilitate your examination when assessing for stenosis, I.e. renal ar
What is Color Aliasing?
�Occurs when Doppler shift is twice the PRF
-Appear as mosaic pattern
-Can help locate stenotic lesions
What is Normal physiologic flow variation?
�Physiologic flow variation can cause confusing flow patterns of flow reversal on the color display.
�Examples of physiologic flow variations are flow reversal at the carotid bifurcation, bi-directional flow within the hepatic veins, triphasic arterial fl
What is Pathophysiologic color flow?
Abnormal flow patterns include "swirl" flow pattern in a pseudoaneurysm, arterialized venous flow adjacent to an AV fistula, bi-directional flow in the vertebral artery (early manifestation of subclavian steal), and reversed (hepatofugal) flow in the port
What are steps to a Successful Color Doppler Examination
�Use appropriate pre-set imaging setting
�Obtain color Doppler images and spectral analysis in conjunction with regular grayscale imaging
�Optimize color Doppler settings
How do you optimize Color Flow?
�Velocity range (scale): decrease
�Doppler angle: < 60 Degrees
�Field of view: use shallow depth if possible
�Color box size: keep it small
�Power and gain: increase
�Dynamic range: increase
�Color priority: increase to write on S.T.
�Wall filter: decreas
Review of Color Doppler Imaging
�Color based on mean frequency shift
�Color superimposed on B-mode image
�Indicates direction of flow relative to transducer
�Angle dependent
�Artifacts
What is Power Doppler?
� Displays amplitude or strength of Doppler
signal
� Allows for greater sensitivity in detecting small
vessels or slow flow
� Usually used in conjunction with color Doppler
or spectral Doppler
Describe power Doppler display
� The color maps for PD are represented by a
single continuous color
� PD does not provide directional information
� PD does not display aliasing
What are power Doppler advantages?
Increased sensitivity
- Displays amplitude or strength of Doppler signal
- Related to number of moving RBC's in vessel being
interrogated
- 3X the sensitivity of color Doppler for detecting
flow in small vessels or slow flow
PD is relatively angle independent, what does this mean?
-Not measuring velocity or determining direction of flow so angle on insonation of little importance
-Flow is detected based on density and movement of RBC's
-Only flow with frequency shifts near zero are eliminated by soft-tissue motion eliminator of ult
What are the comparison between PD & CD
Better signal to noise ratio
-Conventional color Doppler will display noise as random frequency shift
Relatively angle independent
Does not alias
What are the Disadvantages of Power Doppler Imaging?
Motion artifacts
Lacks directional and velocity information
Increased sensitivity may be limited to near field
Cannot distinguish between arterial and venous flow
What are the motion artifacts of PD?
-Any movement whether source or target will produce color.
-Increased sensitivity increases potential for this artifact
-Cannot be used near dynamic structures such as the heart
Can PD be used for directional and velocity information
-Cannot be used in areas where this information is essential
-I.e. Portal venous system
Can PD be used to distinguish between arterial and venous flow?
No. -Must use spectral analysis or color Doppler imaging to make this differentiation
Is PD useful in the near field?
Increased sensitivity may be limited to near field-Can be lost when imaging deep abdominal/pelvic vessels
-Obesity will decrease visualization
What are some applications for PD?
�Renal
�Hepatic
�Testicular
�Neurovascular
�Miscellaneous
What is the future Power Doppler Imaging?
�Contrast agents
�Three-dimensional Power Doppler Imaging
Does Power Doppler Alias?
-Signal does not contain frequency information, therefore it does not alias
-Is superior in demonstrating vessel continuity
Parallel-Perpendicular-Angled
�Parallel-0 or 180 degrees-cosine of 0 is 1 or-1
-measured velocity = true velocity
�Perpendicular-90degrees-cosine is 0
-velocities cannot be measured-will equal 0
�Angle of 60 degrees-cosine is .5
-measured velocity is � the actual velocity
If the red b
Cosine of the angle
�% of true velocity measured is dependant on angle ?
�Measured velocity= true velocity X cos ?
Hz vs cm/sec
Systems measure the frequency difference in hertz between the received and transmitted sound waves
�Doppler equation is programmed into the computer
�Calculates and displays the velocity of the blood
Blood cell velocity
�Doppler shift is directly related to the velocity of rbc's
�Measured in units of frequency (Hz)
�Computer uses frequency info and the Doppler equation to calculate and display the velocity
�0.4m/s makes more sense than 3 kHz