RLTN 3813 Unit 7: Doppler
Ultrasound Physics and Instrumentation I
- Unit 1: Instrumentation Components
- Unit 2: Ultrasound Scanning Modes
- Unit 3: Transducer Characteristics
- Unit 4: Sound Beam Patterns & Characteristics
- Unit 5: Image Artifacts
- Unit 6: Image Quality
- Unit 7: Doppler
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Unit 7: Doppler
- Doppler Physics Principles
- Doppler Instrumentation
- Making Sense of Velocity Profiles
- Doppler Applications
- Terminology
- If you are standing on a street corner with your eyes closed, how do you tell if a vehicle is approaching?
- If you are sitting in your car at a stop light, how do you tell if a firetruck with the siren on is getting closer to or farther away from you?
- If you are watching the stars on a dark night, do you know why some stars twinkle blue and others twinkle red?
- All of these questions can be answered using the Doppler Effect / Doppler Shift Principle as described by Christian Andreas Doppler in 1842.
- The Doppler effect is the perceived change in frequency or wavelength of sound or light when there is relative motion between the source and a receiver.
- For example, if a sound source is moving toward a stationary receiver, the receiver will perceive an increase in the pitch of the sound as the source moves closer.
- Alternatively, if a sound source is moving away from a stationary receiver, the receiver will perceive a decrease in the pitch of the sound as the source moves away.
- In other scenarios, the source may be stationary and the receiver in motion or both may be in motion.
- There is no Doppler effect if both the source and the receiver are stationary, as there is no relative motion between the two objects.
- As a sound source moves, the wavefronts in the forward direction get closer together, while the wavefronts on the far side get farther apart.
- Therefore a receiver on the forward side of the source will perceive the sound as having a shorter wavelength and higher frequency (pitch) than a stationary sound source.
- A receiver on the far side of the source will perceive the sound as having a longer wavelength and lower frequency (pitch) than a stationary sound source.
- The Doppler shift or frequency shift is the difference between the sound source frequency and the perceived frequency at the sound receiver.
- Frequency shift = |Original frequency - Received frequency|
- Note the frequency shift is influenced by the relative motion between the sound source and receiver and by the original (source) frequency.
- As the rate of relative motion (speed) increases, the frequency shift increases.
- As the original frequency increases, the frequency shift increases.
- The received frequency is less than the original frequency if the relative motion between the source and receiver is away from each other.
- The received frequency is greater than the original frequency if the relative motion between the source and receiver is toward each other.
- The Doppler shift calculations are used by medical Doppler instruments to estimate vascular and cardiac blood flow velocities.
- V = blood flow velocity or speed
- c = speed of sound in soft tissue (1540 meters/second)
- delta f = Doppler frequency shift
- 2 = accounts for two frequency shifts which occur as a result of round-trip sound pathway
- f o = original frequency (transducer frequency)
- cosine theta = cosine of angle between the Doppler sound beam and the axis of blood flow being sampled
- The cosine of zero degrees is 1.
- The cosine of 90 degrees is 0.
- The Nyquist Limit is the threshold which determines the maximum detectable frequency shift for pulsed wave Doppler instruments, including color flow.
- This limit occurs because pulsed wave instruments "sample" and therefore do not record "everything" that occurs.
- When the Nyquist Limit is exceeded the Doppler signal will "alias".
- Nyquist Limit (maximum frequency shift) = PRF / 2
- PRF = Pulse repetition frequency is the number of pulses produced per second
- Maximum PRF = c / (2 * c * t)
- c = speed of sound in soft tissue (1540 meters/second)
- 2 = accounts for round-trip sound pathway
- t = time for sound wave to travel to imaging or sampling depth
- For soft tissue, this formula can be reduced to the following.
- Maximum PRF = 1 / (2 * t)
- Note that the Nyquist Limit (PRF / 2) is limited by the imaging or sampling depth and the speed of sound in soft tissue.
- In addition, there are instrument choices, such as lowering the original frequency, that can reduce the likelihood of exceeding the Nyquist Limit.
- The following formula reduces the previous velocity formula by treating c & 2 as constants and adjusts the frequency variables to commonly used units. This formula assumes c = 1540 m/second.
- Velocity (cm/second) =
(77 * freq shift (kHz)) divided by (f o (MHz) * cosine theta)
- The next formula is a rearrangement of the velocity formula to solve for the frequency shift.
- frequency shift =
(2 * f o * V * cosine theta) divided by speed of sound in soft tissue
- The final formula on this list combines two formulas to predict the maximum detectable velocity for a pulsed wave examination.
insert image
- X-axis = time
- Y-axis = estimated velocity or speed of blood flow (red blood cells)
- direction relative to baseline = flow toward or away from transducer
- Z-axis = brightness of screen pixels is proportional to amplitude of returning reflections from red blood cells. The greater the number of RBC's traveling at a given velocity, the higher the amplitude or intensity of displayed signal.
- Some equipment allows the grayscale to be replaced with a color scale, sometimes called B-Color. This IS NOT color flow Doppler, just a change in the "shades" assigned to the signal amplitudes.
- Doppler sound beam is created by a single element or single-group of crystals which send and receive the beam.
- Instrument only displays information received by a operator-controlled sample volume or sample gate.
- The location and length of the sample volume is operator adjustable.
- Spectral display of normal laminar flow typically shows a narrow band of blood flow information.
- Advantages:
- Precise location of sample volume
- Allows calculation of angle theta
- Superior ability to distinquish laminar and turbulent flow
- Disadvantages:
- May be difficult to position sample volume
- Aliasing when exceed Nyquist Limit
- Doppler sound beam is created by one element or set of crystals and received by another element or set of crystals.
- Instrument only displays information received from the region of transmit and received beam overlap.
- The location and length of the beam overlap is transducer specific.
- Spectral display of normal laminar flow typically shows a broad band or filled-in appearance.
- Advantages:
- No aliasing
- Dedicated CW transducers are very sensitive
- Usually easy to locate vessels
- Disadvantages:
- Returning signals are not depth-specific
- Interference from crossing or parallel vessels
- Often no accompanying image
- These instruments use pulsed-wave Doppler methods to create 2-D color-coded real-time blood flow information which is displayed concurrently with 2-D gray-scale real-time images.
- Color flow instruments usually include the capacity to perform standard PW & CW modes and analyses.
- Blood flow velocity and direction are displayed using a spectrum of colors which are operator-selectable.
- Typically a range of red shades are used for blood-flow traveling toward the transducer and a range of blue shades are used for blood-flowing away from the transducer. The sonographer may choose to change this coding pattern.
- These instruments display mean (average) blood-flow velocities. These mean velocities are calculated using a technique known as autocorrelation.
- Linear transducers produce uniform Doppler angles throughout image.
- Sector / Phased-array transducers produce radiating Doppler angles. Therefore the Doppler angle varies across the image.
- Advantages:
- Precise location of color window
- Ability to change shape / depth of color window
- Ability to change baseline
- Ability to change velocity scale
- Ability to change color map assignment
- Superior ability to distinquish laminar and turbulent flow
- Disadvantages:
- slower frame rate
- incomplete spectral information
- display of mean velocities
- angle dependence
- aliasing when frequency shift exceeds Nyquist Limit
- Human Applications
- General Examples
- Identification of abnormal flow patterns
- Following tortuous vessels
- Identifying venous thrombosis
- Identifying arterial aneurysms and associated clots
- Identifying arterial stenosis and/or occlusion
- Identifying pseudoaneurysms and hematomas
- Differentiation of vascular from non-vascular structures
- Identifying vascularity of mass lesions
- Verify patency of known vascular structures
- Verify flow direction of known vascular structures
- Monitoring for drug therapies
- Cerebrovascular Examples
- Pre & Post surgical assessment
- Assessment of stroke or TIA origins
- Carotid artery stenosis and/or occlusion
- Vertebro-basilar stenosis and/or occlusion
- Intracranial stenosis of Circle of Willis
- Intracranial arterio-venous malformation
- Peripheral Venous Examples
- Pre & Post surgical assessment
- Assessment of valve competence
- Clot and/or mass visualization
- Assessment of extremity pain
- Assessment of extremity edema
- Deep vein thrombosis
- Peripheral Arterial Examples
- Pre & Post surgical assessment
- Hemodialysis shunt assessment
- Digital circulation assessment
- Subclavian artery stenosis or occlusion
- Ulcer healing probability
- Abdominal Examples
- Portal thrombosis
- Portal hypertension
- Portal hepatic shunts (TIPS)
- Hepatic vein occlusion
- IVC thrombosis
- Iliac vein thrombosis
- Aortic aneurysms
- Iliac artery pathology
- Celiac axis & SMA pathology
- Splenic perfusion
- Renal perfusion
- Renal hypertension
- Renal transplant perfusion & regional anastamoses
- Pelvic Examples
- Monitoring follicular development
- Iliac vein thrombosis
- Iliac artery pathology
- Renal transplant perfusion & regional anastamoses
- Cardiac Examples
- Valve regurgitation
- Valve stenosis
- Intracardiac shunts
- Typical Instrument Parameters
- Doppler Transducer Frequencies
1 - 10 MHz
- Doppler Instrument PRF's
- Doppler Instrument SPTA Intensities
(mW/cm squared)
|
|---|
| SPTA Intensity (mW/cm^2) |
Pulsed Wave |
Continuous Wave |
| Upper Limits |
389 |
500 |
| Cardiac / Peripheral Vasc |
40 - 1945 |
20 - 2500 |
| Obstetrics |
|
0.6 - 80 |
- Aliasing
- This is a display error in which the Doppler information is displayed on the opposite side of the zero baseline when the frequency shift exceeds PRF/2.
- Autocorrelation
- A technique used for calculating the mean (average) Doppler shift frequencies. These calculations are then converted into mean (average) blood flow velocities. This technique is used in color flow systems.
- Bernoulli effect
- The reduction in pressure which occurs in an area with an increased flow velocity. The simplified Bernoulli equation (pressure gradient = 4 * (velocity squared)) is used to predict pressure gradients across cardiac valves.
- Bidirectional
- A category of Doppler instruments which can detect positive and negative Doppler shifts. Also used to describe waveforms which include flow components in both flow directions.
- Color box position
- A frame which identifies the sample volume from which the color flow information is being gathered. Within this frame, the color flow information is added to the 2-D image. Outside this frame, only the 2-D image is displayed. The larger this frame, the slower the frame rate.
- Color Map
- The scale which indicates the color assignment for blood flow direction and velocity. Or the scale which indicates the color assignment for blood cell amplitude in the power display mode.
- Crosstalk
- This artifact occurs when strong Doppler signals in one directional channel interferes with or is duplicated in the opposite channel. May create a mirror artifact on spectral display.
- Doppler Gain
- A control used to increase or decrease the amplification of the displayed Doppler signal.
- Ensemble Length
- The number of samples taken per flow volume for calculating mean flow velocities for color flow imaging. The larger this number, the longer the dwell time and the slower the frame rate.
- Hue Map
- A color flow map which uses additional colors to indicate higher blood flow velocities.
- Intensity Map
- A color flow map which adds white to the primary color to indicate higher blood flow velocities.
- Packet size
- The number of samples taken per flow volume for calculating mean flow velocities for color flow imaging. The larger this number, the longer the dwell time and the slower the frame rate.
- Pixel
- Picture element. A discrete unit on the display monitor. One color is assigned per pixel.
- Power Display Mode
- A method for displaying the density of moving red blood cells. The greater the number of cells in a sample volume, the stronger the amplitude of the returning signal and the "brighter" the hue assigned to the signal. This mode does not differentiate blood flow directions, but adds all flow signals into a single amplitude (power) scale.
- Saturation Map
- A color flow map which adds white to the primary color to indicate higher blood flow velocities.
- Sector Width
- The width of the color box. As the width increases, the frame rate decreases.
- Spatial Filter
- This is an averaging filter which "smooths" the color image. Small jets are better detected with this filter off.
- Variance
- Each calculated mean velocity is the result of a number of individual velocity samples. The range of velocities (minimum to maximum) sampled at a given interval show variation similar to the spectral width on a spectral analysis of peak velocities. Variance is a statistical value which expresses this spectral range. An additional hue (usually green) is added to the color map to indicate the degree of variance in the sampled frequency shifts (more green indicates a wider range).
- Velocity Filter (Clutter Filter)
- This control eliminates display of high intensity, low velocity signals. Typically, minimal settings are used for carotid studies, while higher settings may be used for cardiac examinations. See also Wall Filter.
- Wall filter
- An electronic circuit which only passes frequencies above a specified level. This is designed to omit strong low-frequency Doppler shifts arising from vessel or heart walls. See also Velocity Filter.
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member of the College of Allied Health. The University of Oklahoma Health Sciences Center is an Equal Opportunity Institution.
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for more information contact
The Department of Radiologic Technology
College of Allied Health, CHB 451
P.O. Box 26901
University of Oklahoma Health Sciences Center
Oklahoma City, OK 73190
Phone: 405/271-6477
Questions or requests for additional information may be directed via internet to:
kari-boyce@uokhsc.edu
Last updated by Kari Boyce on 5/6/96